Raw unedited rant: Why a 21" wheel off-road is better than a 19" or 17" rim

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This rant was posted 2 years ago for Patreon supporters...
based on a resource given to me on the Patreon comments from Zak here is my question...
Looking at that it is assumed that a larger diameter wheel would have more stability at the same speed of rotation at the axis. Ok...
Now the question is since we all feel the larger diameter wheel being more stable off road and feel it self correct when deflected. (of course rake and trail are factored into this) why.
If the smaller diameter wheel spins faster producing a higher tangential speed to achieve the same tangental velocity and it is assumed that at that same tangential velocity a larger diameter wheel would rotate slower at the axle but based on distance would have a greater tangetal speed in relation to its axle but still be less than the smaller diameter wheel with its faster rotation. Why is the 21" more stable?
ok people... make me smart!!!

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1 май 2023

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Комментарии : 183

@advmotorcycletravel 3 года назад

My 1979 Honda XR250 had a 23” front tire... man I loved that motorcycle! Riding in the woods I could pop right over logs and never went over the handlebars like when my previous 19” tire fell into a rotted out stump hole LOL.

@CurtisBrandt Год назад

Exactly! I was just going to say something about those 23" wheels on earlier XRs!

@mikemerrill175 Год назад

I had an 80 XL-500 with the 23” front. It worked really well except that you could feel the gyro effect resisting quick transition back and forth turns. It was a fun bike except when it would occasionally kick back when starting.

@CurtisBrandt Год назад

@@mikemerrill175 Those were the days!

@twowheelsabove Год назад

23" rim + 3" (80/100) front tyre it's about 66cm _(~61cm for 21")_ of total outer diameter, for bicycle world measurements it's just nothing, we have 29" or 29+" wheels, which are 74cm and ~78cm accordingly 😉

@johnnyblue4799 Год назад

Linear Velocity (VL) = speed of the bike in the straight line; Angular velocity (VA) = the rotational speed of the wheel; R = radius of the wheel. VL = VA * R (m/s, rad/s and m respectively) One full circle is 2*pi rad. If both wheels travel with the same linear speed the bigger wheel (21") makes approx 0.904 rotations for each rotation of the smaller (19") wheel. Hope this helps.

@Wintersdark Год назад

Roughly 10% slower. That's a much bigger change than I'd guessed. Very interesting thanks!

@stavrozinio Год назад

this is why same tire will wear much faster on 17" compared to 21". I have been using same tires on my svartpilen and norden and although norden is much heavier, svart was munchin through rubber faster.

@rogerclark9285 Год назад

@@Wintersdark Going from a 19 to a 21 gives about 10% more radius so that should be expected.

@davidruggles996 Год назад

Great talk Brett. Coming from a mountain bike background, 26" wheels gave way to 29" wheels for the same set of riding advantages.

@Hallnadventure Год назад

Exactly, I had the same experience. Reluctant at first to change from 26” to 29” wheels. Now my full suspension mountain bike has a 29” front and 27.5” rear. Noticeable difference with 29” front tire to ride over obstacles and through holes.

@davekarl2256 11 месяцев назад

For taller Mtn bikers the 29” on both front and rear is better in most every scenario, the manufacturers have engineered out most all of the longer wheel base disadvantages while still giving the 29” rear for better roll over etc.

@gers3985 Год назад

It might help to clear up the nomenclature a little bit. The "Tangential" velocity is the speed of any point on the wheel perpendicular to the radius from the wheel's center of rotation (the axle) to that point. So for two wheels of different diameter, rotating about an axle at the same revolutions per minute, a point on the outer edge of the larger diameter/radius wheel will have a greater "Tangential" speed than a point on the outer edge of the smaller diameter/radius wheel. The axle has the same velocity as the motorcycle. That velocity is, when not running over an obstacle, horizontal to the ground and called "Translational" velocity. The wheel itself has "Rotational" velocity, usually given in revolutions per minute around the axle. The rotational velocity of the wheel mutiplied with the wheel's inertia produces a gyroscopic stabilizing effect. The larger wheel by virtue of its larger diameter/radius will have more inertia, so even if you manage to built two wheels with the same weight, the larger diameter wheel will have the greater inertia and therefore be more stable. With more rotational inertia the larger wheel will also be able to maintain its rotational speed going over obstacles, which try to slow both the motorcycle's translational speed and the wheels rotational speed. Hence, larger diameter wheels are more stable rolling over obstacles.

@BretTkacs Год назад

Thanks for the comment 👍, it was a good read

@mash988 Год назад

Get S: Thanks for answering my suggestion / question on larger diameter gyroscopic stabilising effect!!

@iwanv1625 Год назад

The 21 inch is also harder to break down the speed. The Gp race bike has a 17 inch front wheel for easyer brake down and cornering because of the lower gyroscopic force.

@konstantinkolaxazov3471 Год назад

As always, the best educational and scientific way to tackle and explain the difference. Thank you for the great content.

@jimkonugres8454 Год назад

Great explanation of all of this makes perfect sense I always wondered about. To answer some of your questions I think this sounds like another perfect collaboration with Ryan!

@JosepsGSX Год назад

I had to ditch my previous definition of "rant". Until today a calm, articulated, casual display of information was not one of the options. I quite like this kind. Thanks for sharing!

@enduromotorradtouren Год назад

Great tech talk, Bret. Lots of food for thought. Cheers, Günter/Nürnberg

@malcolmmckechnie769 3 года назад

Hi Bret. Great video. Thank you. Really thorough assessment of the 21” versus 19” (or smaller). Three comments from me... 1. I think taking the additional mass of the 21” wheel into consideration would be useful to discuss. 2. I think you’ll find that the resultant outer diameter of the larger will be greater than 2” if you consider the proportion increase in tire and tire profile ratios. 3. The last question regarding the velocity of the different wheels.. I don’t think matters. Because the wheel speed is dictated by its contact and connection with the road. The larger wheel simply spins slower.

@BretTkacs 3 года назад

#1, there isn't really as gain in mass since the rim and tire are narrower. I haven't done weight test yet but from handling them I am going to guess the 21" is actually lighter. I will share the results when I do the conversation review #2, the net gain was 20mm #3 yes the wheel spins slower but the Tangental rotation is higher for the same tangental velocity.

@dommotos Год назад

@Bret Tkacs I think larger wheel gives more "torque" as larger sprocket gives.

@drewt.4353 2 месяца назад

The exact video I was looking for. Thanks

@JagLite Год назад

Very interesting! I don't have a scientific understanding but I was amazed at the difference replacing the factory 19" front wheel with a 21". My brother and I bought Honda Sl100's (1971?) and after changing my wheel we took turns riding both bikes and it was a huge difference offroad. My brother sent his hub to get a 21" wheel the next day. I would never have thought those 2" would make much difference but it sure does. Thanks for the technical discussion to help understand it.

@JasonBWSBKMotoAmerica Год назад

Thanks for making remember what it feel like to be in Geometry and science class

@remy5418 Год назад

I also ride bicycles, and I feel like this is the same exact conversation that we had several times about MTB wheel sizes. For downhill bikes, the standard used to be 26 inches. Later, it became 27.5 inches. And now 29 inches. The explanation is the same: larger wheels roll over obstacles easier, and bikes are faster, more inertia, etc.

@kevinw6282 8 месяцев назад

Had the exact same thought. Have ridden MTB for many years, gone from 26" to 29" wheels. Virtually all the same factors apply. 29 just rolls over technical stuff with more ease but can have the same issues with handling or responsiveness. For mountain biking, I don't think I'd ever stray from 29" wheels.

@Petesmotoadventure Год назад

Excellent presentation. Have to respect the chalk board, low tech but highly effective. Thank you as always for the information.

@doctoroptical952 Год назад

Hey thanks for still making videos bro. Your videos made me a better rider.

@ignacioyusnovella2588 Год назад

Giroscopic effect is proportional to Angular momentum. Formula (simplified): AM=mass x radius x speed Two wheels in two bikes at the same speed, one with 19 and another 21, wheel speeds are the same, only changes mass and radius. The bigger the radius, bigger giroscopic effect.

@schweiberholzfaller8597 Год назад

For your question…I wonder if the answer is in the calculation of surface speeds. Lathe operators calculate the surface speed to know the feed rate for a given material type and the diameter of the piece at the moment it’s being cut. A quick google search landed this.. The following equation is used to calculate spindle speed: rpm = sfm ÷ diameter × 3.82, where diameter is the cutting tool diameter or the part diameter on a lathe in inches, and 3.82 is a constant that comes from an algebraic simplifica-tion of the more complex formula: rpm = (sfm × 12) ÷ (diameter × π)

@miro_s Год назад

Hey Bret, not sure about your final question, you had the formula right on your blackboard: VT = wR. For a fixed VT the 21” will have have slower angular velocity w, around 0.9 of the angular velocity of the 19” wheel, that is 9 full turns against 10. Dually, for the same angular velocity w, the contact patch of the 19” wheel will travel 0.9 the distance of the 21” in the unit of time, and so will be 10% slower. This may change if the tire profiles differ, consider that a 21” tire with 90/100 tires is actually 28” and covers 88” each rotation.

@murraehaynes3182 Год назад

Always a pleasure…nice physics/mechanical lesson

@MrSharkyJR Год назад

Answering the question at end, if there is no slip (which is usually the case in the fr tire) then the speed at this point is the same as the road so if you are traveling at a 100km/h then both size tires should have the same speed at the edge which is the 100km/h and the rpms will be different to compensate

@allthingstwowheeled 3 года назад

Good info here. You should ask Ryan at Fortnine. He seems to be good about the math and physics stuff as it relates to riding.

@L--Z Год назад

Ryan F9 once made a cameo appearance in a Bret Tkacs video to explain the physics personally. (Even better is Bret's cameo appearance in Fortnine's Ural video.)

@Naptime1953 Год назад

Great info! Would love to hear about the difference in road handling since most of my mileage is on road.

@theravengeo Год назад

Great video.Very informative, great explanation on the advantages. I'm so glad my KTM 1290 SAR has a 21 inch front wheel. I keep saying my bike is way more capable than I am! I also like that you draw circles as badly as I do! LOL

@pezia 3 года назад

Did not really get the question at the end. The speed of the point on the "edge" is the tangential velocity to my understanding. You can think of the hub moving as the linear velocity of the wheel (the actual speed of the bike). Tangential velocity scales linearly with "r", the distance from the hub. So twice the distance, twice the speed.

@James-ww4mc Год назад

Man I miss you at PSS!! Hope all is well and you’re enjoying life!!-Jim

@BretTkacs Год назад

I miss you guys too. My exit was not what I wanted. I wanted a glorious final get together to say thank you and goodbye. Promises were made and broken 😐

@johngill7219 9 месяцев назад

Great video, I have 3 bikes and interestingly they have 3 different front tire diameters, 17,19, 21. I choose the bike to use by the trip terrain and it is governed by front wheel diameter. Now I know why.

@adstryker5084 Год назад

The tangential velocity at a given radius is the product of the angular velocity and the radius of interest -- therefore, for a given angular velocity, the ratio of the tangential velocities at two different radii is simply the ratio of those radii. Distance traveled is the product of the radius and the angular displacement (i.e., the angle traveled). For a given distance traveled, the angle traveled is simply the ratio of the distance traveled to the radius. For a given speed (tangential velocity) the angular velocity is the ratio of the speed to the radius. Angular velocity is the number of degrees (or radians) the wheel rotates through in a given time period (usually given in units of radians/sec or degrees/sec). Note, there are 360° (or 2*pi radians) in a complete circle (i.e., 180/pi degrees per radian, or about 57.3° per radian). Calculations are most naturally done in radians -- if you want to talk about the angles in degrees just find the answer in radians and convert radians to degrees. There are a number of calculators you can find by searching for "angular velocity calculator" -- it's probably worth playing with a few of them to see which ones best meet your needs.

@edward_plays_outside8013 Год назад

Let me try to help with your last question. If we assume the wheel is rolling without slipping then the tangential velocity of the outer edge of the tire is the same as the linear velocity of the bike. It helps to think about reference frames when discussing this type of problem. For an external observer the part of the wheel I contact with the ground is stationary. The axle is moving at the same speed as the bike and the top of the tire is moving twice as fast as the bike. If you were a bug on the axle moving with the bike you would see the bottom of the wheel (the part in contact with the road) moving backwards at the same speed as the bike and the top moving forward at the same speed. In both cases the diameter of the wheel does not matter. In short the tangential velocity of the outer edge of the wheel is always equal to the linear velocity of the bike since the wheel is rolling without slipping. The smaller wheel will have a higher RPM since each revolution covers less distance.

@BretTkacs Год назад

Thanks for the explanation...

@virtusferrum7708 Год назад

RyanF9 is the math guy. Send it over, he probably would love a good chalk board trip down formula lane

@dtrex392 Год назад

Draw a circle and put 3 points on it - the center of the wheel (hub), the contact point at the bottom of the wheel (contact point), and the top of the wheel. If you’re going 60mph, the hub of the wheel is going 60mph. Draw an arrow representing that. The bottom of the wheel is going 0mph. That’s right, it’s going 0mph relative to the ground. (When that’s not the case it’s a skid or a slip.) The top of the wheel is going 120mph. Draw an arrow that is 2x the length of the 60mph arrow at the hub. Draw a triangle. What you have is actually a lever, with the pivot on the ground. So as the tire approaches the contact patch, the speed of the tire is approaching zero. I don’t think the wheel size between 19” an 21” will have much of a significant difference with regards to the speed of the wheel at the circumference. Both are approaching zero. Take that lever-arm idea to the next step. Draw your step as you hit a, say, 4” tall step. At that contact point, the speed of the wheel relative to the ground is zero. Make that the fulcrum of your lever-arm. Draw a line to the hub of your wheel. Make a little arrow…..let’s make it more reasonable for off-road and say it’s 25mph. Draw another arrow that is perpendicular to the lever-arm. This is the way your hub will have to move when the wheel hits the object. Do that for both 19” and 21”. You will see this arrow is more horizontal on the 21” wheel - it’s going more forward than upward, compared to the 19” wheel. Now, the question really is whether this is significant and noticeable or not. Again, it’s important to note that the velocity is zero at the contact point with the ground, or the object. Don’t think about the speed of the perimeter of the wheel relative to the hub. Wrong ‘frame of reference’, we would say. Treat the wheel as literally a lever between that object contact point and the hub. This is essentially showing you what you already know from your first analysis. The hub of the 21” wheel is going more forward and less upward, when you make your little reaction arrow, compared to the 19” wheel (lever-arm.)

@johnhenry3536 Год назад

The hub is not going 60 mph - if that were actually true, the contact patch would be going MUCH faster. This would then be reflected on the speedometer, which is calculated / programmed to measure contact patch speed - not hub speed. Nor is the "top of the wheel going 120 mph" - that's utterly ridiculous. The top of the wheel is going the SAME speed as the bottom of the wheel, both are traveling FASTER then the hub. You failed at math apparently.

@dtrex392 Год назад

@@johnhenry3536 Its simple free body diagramming that any mechanical engineer like myself knows, and is fundamental to any BSME degree. The hub is traveling at the speed of the bike. The bottom of the wheel is STATIONARY. If it wasn’t, you’d be skidding. The top of the wheel is moving at double the speed of the hub of the wheel. You’re using the wrong frame of reference. The GROUND is the reference, not the bike. Every time the wheel rotates, it goes from 0 mph at the contact patch to 120 mph at the top of the wheel, then back to 0 mph. Any first year mechanical engineering student knows this.

@SheenHunter-SeattleFreeze 3 года назад

This looks good.

@AF-O6 Год назад

This information is so good it makes me wish I wasn't such a miser! But being old, feeble, and spending 90% of my time on road, I find a 19 is perfect balance for potholes and traversing grooves and edge traps.

@BMRTV Год назад

Switching from my Versys (17) to KLR the difference is greater than I thought it would be. Yes, there are lots of differences, but stability in crap and ability to just go over ruts and other stuff is so nice.

@mikemerrill175 Год назад

I considered putting a 21” on my GS. What stopped me was how to get a fender to fit if I did. Buying another bike is the simple but more expensive approach.

@danielsolomon9915 Год назад

I understand that changing the tire diameter also will change the geometry of the bike and therefore it can make a difference to the handling. My question is: How or where on the motorcycle does one measure to determine if the bike is traveling at an angle front to back or is level? Do you measure rake angle????

@alanprice9938 Год назад

Hopefully somebody else has already said this, but the exterior of the tyre, the circumference, is travelling at the same speed on both the 19 or the 21…… it’s dictated by your travel across the surface (dirt or tarmac). So the outer stays the same, but the speed at the spindle reduces, as the diameter increases.

@PetrolJunkie Год назад

I think you have a clear explaination of the points that matter as to why a 21 is better off road on why the physical size matters but I think it's also important to think about what types of tires are available. One clear disadvantage to the typical 17 and 19 fronts is limited tire selection and availability at both extreme ends of on road and off. (Had to wait nearly a month for a front tire because knobbies became scarce over the winter.) It's hard to find a truly aggressive knobbie in the smaller sizes, but it's even harder to find less aggressive 21 inch tires for mixed ridding. Tire choice more than anything made me decide not to convert my GSA. I ride on 70/30 tires most of the time and a comparable 21 is a crapshoot on whether I can find them. Again, if you are going to focus on riding dirt this may not matter. I have plenty of pavement between home and the trail heads to cover. I also don't strictly need an aggressive knobbie, some times a 70/30 is actually a better choice.

@superflycatchermtb Год назад

Are you in the US? There are many large ADV bikes now with 21" fronts that I've had no problem finding options for my GS with 21" conversion. Trail Max Missions for one, is a great handling tire both on and off road. Same with Motoz GPS or Dual Venture. I really like the 21" on my GS....but it would be wasted money if I wasn't riding lots of loose and technical terrain.

@PetrolJunkie Год назад

@@superflycatchermtb First, let me define "find." Can I go to my local motorcycle dealer or tire shop and find the tire on the rack? Most of the local dealers stock OEM tires and a hand full of alternates. The 21 fronts are mostly on dedicated dirt bikes so aggressive knobbies are fairly easy to find in stock. Good luck finding anything more road oriented. The there is no local KTM dealer, for example, so the fronts that KTM dealer would normally keep on hand for the 1290 ADV are not available without special ordering them. So most of the dealers do not have ADV tires for a 21 inch rim in stock because it's very rare to see a KTM 1290 ADV. There is a Triumph dealer, but they rarely sell Tigers and carry OEM tires only, and keep a very thin supply. The BMW dealer keeps just one GS on the show room floor and it takes months to sell it. ADVs are not popular bikes in this area. Even the dealer doesn't keep a much of an inventory on parts, most everything needs to be ordered, preferably two to three weeks in advanced. I live in HD land, literally 90% HD, 6% crotch rockets, 2% other for palatable road going bikes. As a result the local tire shops have loads of tires that fit HD motorcycles, HD clones, and other cruisers. Anything else they have to special order. But, I have a BMW R1250 GSA with a 19 front. There are dirt bikes with a 19 front and I can cheat a little and get some of those tires to work. Otherwise I have to special order tires that are an exact fit because BMW's rims are not the same width as common 19 front dirt bike tires. (why? why?) I'm going to respoke a set of dirt bike wheels with BMW hubs to make that life easier. Like I said before I run 70/30 tires most of the time anyway because I can actually find those locally without having to special order them. What's the big deal about special ordering? If I pick up a nail I have to keep a spare tire in the garage otherwise I'm not riding for three weeks. That's not an option. Having rubber sitting around just in case is not a great idea, rubber ages. I can rotate my stock but it still takes up garage space. Anyway, this has gone on long enough. I'm sure you get the point by now.

@dalejones6627 5 месяцев назад

Plus the a-arm suspension on a GS with more vertical wheel travel the tire leaves the ground less often and duration.

@lavransmathiesen9099 Год назад

Rim speed is the same as ground speed, larger wheel has slower axle speed; that is, fewer rotations. Large wheel has more rotational mass: it’s heavier, so harder to start, and harder to stop. I’d say reasonably, that rotation plus the change in rake & trail caused by larger tire is a big part of the change in rebound and handling. Try the changes on a bicycle- not humping hundreds of pounds of frame & motor shows tiny changes in head angle, rake, and trail has more effect that wheel size alone.

@dandersonjr Год назад

From reading the comments I didn't realize there were so many mathematician's that rode adventure bikes.

@BretTkacs Год назад

😂

@louisbezuidenhout7614 Год назад

The top of the 21 moves forward at twice the speed of the axle, so does the 19. The contact point of the tyre on the ground is standing still. Any specific point on the circumference makes half acircle as it rotates.

@0HARE Год назад

For the same reasons you’ve covered here, I’ve always bought the largest diameter tires I could get for my 4x4 vehicles. Simply said, a larger diameter wheel/tire will roll over obstacles easier, and with less shock to the suspension.

@briangc1972 10 месяцев назад

I think a much larger factor than the diameter is the mass of the unsprung weight, the fork angle, the rake, and amount of trail; all of which are changed when you switch from 19" to 21" front wheel size.

@dalejones6627 5 месяцев назад

May not change rake on the GS.

@not_so_novice Год назад

Angular velocity is radian per unit time. A particle on tyre will take less time to complete one revolution on 19’’ than 21’’. So for particle to cover 90 degrees, it will take shorter time in 19’’ than the same particle on 21’’. Thus angular velocity would be higher on 19 than 21 inch for a given angle.

@JMartinhomestead Год назад

Ive done something similar to my 2018 NC750X. comes factory with 17" wheels front and back, which leaves no good off-road options. using lots of math and research, I used an oversized rear tire, shorter links that lifted it for more ground clearance, and then a very oversized rear tire for the front that gave me a mostly level bike with the exception of needing to extend the front forks 1". every little inch, half inch or quarter inch effects the bike handling significantly, however as it sits it rides very well, predictable and stable. it is now trail proven and was keeping up with dual sports on some nasty rocky terrain. in the future I'd really like to find a suspension/wheel combo that leaves me with a 19" wheel so I can have good options for tires and reduce my sprung weight. BTW the MITAS E07 is a fantastic tire for street and trail, thanks for recommending it!

@gnarlydog Год назад

I use an XSR700 for dirt-roads riding (as I love the light weight of that moto compared to some chunky Adv bikes). I have extended my forks by 1.5" for a slacker steering angle and mounted a 120/90-17 up front (instead of the OEM 120/70). Apart for some rotation weight disadvantage I have so much more rubber between rim and obstacles that pretty much eliminated rim damage if compared to a typical 90/90 21" tyre. While not full-on enduro the riding on forest tracks is indeed rather pleasant with that set up

@matthewtomaszewski3269 Год назад

When is the midterm and will be multiple choice?

@hugonunezb.4676 Год назад

Another factor that determines the handling is the tire width. 19” tires typically have a wider footprint vs 21” ones. When riding on lose sand, the 21” will dig better than the wider and thus, it can make a real impact on the overall performance

@hazcat640 Год назад

Also if you have a gear driven speedometer changing the wheel size will affect the accuracy. It will read one speed, say 55 but you'll be doing 60 or vis versa.

@yorkchris10 Год назад

Honda had a 23" MX tire that no-one else in the industry wanted to adopt. I had a Honda Nighthawk with a 16" front wheel that didn't make me faster getting to the cafe and it wasn't particularly bad on gravel. It's well known in MTB circles that a 29" (tire diam.) rolls over obstacles better than a 26" tire. Getting a 29" tire rolling requires more effort. The large GS is the only large ADV without 21" option now? Everyone else has done testing and validation - I take it? Beer & Johnston for relative velocity.

@quantum_kiwi Год назад

So the real pro trick is to do what a lot of the desert riders in the American Southwest do: Put a 18 inch rim on the front, with a fatty (4-18) trials tire on it. The overall diameter comes out about the same as a regular dirtbike tire on a 21 inch front wheel and you have gobs more sidewall and flotation in soft sand.

@jeanmascret3211 Год назад

Thank you men. Why would I switch from thin 21" to a large 19" ? On the sand I feel my front is more floaty it dig less just like on a TW200, But I didn't realize that the 21" was digging out of the sand better !!!

@danielgouws7850 Год назад

Thanks for a norther excellent video! On the GS how did the ratio change between the front and back wheel effect the ABS and Traction Control?

@BretTkacs Год назад

Of course its affected but not enough to notice.

@matthewoakley6081 Год назад

What about the increase of trail with 21inch giving more stability ?

@getskillschannel2172 Год назад

There is another factor… the contact surface change , so the brake performance and friction surface increase between 19” and 21” mmmm 😊

@mike.boysen Год назад

Difference in Wheel Speeds. Model: Default (GPT-3.5) User What is the formula for calculating the difference in outer rotational speed of a 19" motorcycle wheel versus a 21" wheel? ChatGPT The outer rotational speed of a wheel can be calculated using the formula: v = rω where v is the linear speed of a point on the wheel's circumference, r is the radius of the wheel, and ω is the angular velocity of the wheel. To calculate the difference in outer rotational speed between a 19" and a 21" motorcycle wheel, we need to first determine the radius of each wheel. Assuming that the wheel sizes refer to the diameter of the wheel, we can find the radius (r) by dividing the diameter (d) by 2: r19 = 19/2 = 9.5 inches r21 = 21/2 = 10.5 inches Next, we need to assume that both wheels are rotating at the same angular velocity (ω). In other words, the motorcycle is traveling at the same speed and the wheels are rotating at the same rate. With these assumptions, we can find the linear speed (v) of a point on each wheel's circumference using the formula: v19 = r19ω v21 = r21ω Finally, we can calculate the difference in linear speed (Δv) between the two wheels by subtracting the speed of the 19" wheel from the speed of the 21" wheel: Δv = v21 - v19 = r21ω - r19ω = (r21 - r19)ω = 1ω So, the difference in outer rotational speed between a 19" and a 21" motorcycle wheel is simply ω, or the angular velocity of the wheels, which is assumed to be the same for both wheels.

@pokkenosotto5523 8 месяцев назад

The tyre profile and presure would also change the bump angle.

@carletto_au Год назад

How about the sealed road behaviour ? Are 21 inch wheel less safe due a reduced contact patch because of being narrower?

@narrator-timothymckean Год назад

Thanks Bret! I've been thinking of making this conversion on my Honda N700. It has a 17" front wheel from stock and am hoping to change to a 19". Do you think I will experience the same degree of change in behavior you are describing between the 19 and 21? How do you ensure that the new wheel will still fit correctly to the brakes and other existing parts on the bike?

@BretTkacs Год назад

Yes it will help but you will have yo do your homework to ensure everything else fits. If you contact a company like Woody's Wheel Works they may have already worked out those other details

@gnarlydog Год назад

a "cheat option" is to mount a 120/90-17 tyre on the OEM rim. Works great for me on a XSR700

@Heheyelaaa Месяц назад

Hey, was just wondering, i noticed that the video is for offroad usage. What if the bike i am riding ktm 1190 adventure r is for purely for commuting on asphalt, would like to change the front wheel from 21” to 19” Reason: more contact patch for cornering and at the same time, i am 5’6 What are your thoughts on this?

@dommotos Год назад

Obvious thing explained cientifically. I love those. ❤️

@gordonblackley8863 Год назад

The outside of the wheel in terms of rotational velocity is identical to the smaller wheel at a given road speed, however, at the wheel bearing, the velocity is slower on the 21.

@stevemartinez4180 Год назад

Stacking tolerances matters. Well don,,

@EastCoast2wheel Год назад

I think what you're looking for is called surface speed. It's used in machining and you can find any number of calculators for it on google.

@RideWithRen Год назад

For the last question, all you need is to calculate the diameter of the wheel. A wheel that has a diameter 10% shorter will rotate 10% faster at the axle.

@take5th Год назад

You also slightly change the rake, trail, and wheelbase when increasing changing tire sizes, lowering/raising suspension, etc. every point of geometry and articulation is part of a single system once underway.

@BretTkacs Год назад

With the suspension change rake doesn't change but trail does get longer... Good point

@take5th Год назад

@@BretTkacs now go have some pi.

@davekarl2256 11 месяцев назад

Well done Brett, but the related question I was hoping you would address is how much of a disappointment you should expect from a larger 21” front ON ROAD? If OEM stock from factory the bike is supplied with that larger diameter it is typically also longer travel as well. In my experience that makes for better compliance and ON ROAD comfort in every paved scenario. Yes the smaller diameter front wheels, with their associated wider width front tires, offer better grip for aggressive on road and track day riding but that level of grip is beyond what most anyone would ever ask of their bike especially if loaded up with panniers and camping gear. Just came off a KTM 1290 SA with a 19” front and got a 2023 Triumph Tiger Rally Pro and the new bike’s suspension is so much better for relaxed and spirited touring on road, and better too on any dirt. Want to hear your thoughts on the negatives you have experienced with the 21” front when riding ON PAVEMENT please.

@BretTkacs 11 месяцев назад

I hate to say but the negatives are light to none. In reality I prefer the OEM KTM1290 with a 21" over the 19". Much has to do with the tires and other geometry. In fact my last track day included the Ducati DesertX and it was a total blast. Knee dragging was no problem.

@rogerclark9285 Год назад

Adding 2 inches to the diameter of the rim will add 6 1/4 inches to the circumference if the tires are the same height. To answer the question in the comment, on the bigger rim the ground contact point will be farther behind the axis of rotation of the forks , i.e. greater trail. The increase in that distance gives more leverage to the wheel to bring it back to the center position.

@user-nw2vy5ob8s Год назад

Самое полное и понятное объяснение. Брет лучший 👍

@superflycatchermtb Год назад

Hi Bret, like you I run a 21" front on my '19 1250GS Rallye with sport (GSA) suspension. I was at your last clinic in Oregon at the ranch near Milican but on my 890 at that time. I had a talk with Casey at Ted Porter's BMW about what options I had for a lower front shock. He suggested that the newer generation of ESA on my bike should, in theory, level the bike with it's auto preload and leveling and therefore shouldn't need to swap the front shock to a lower one. Curious if you've investigated this, and I'm curious if you might know how to check that theory? Another question is about rim width...my bike has a 1.85" width rim from Woody's that is recommended for a tube set up only. They sell a 2.15" wide rim that can be set up tubeless but takes some modifications to avoid spoke to brake caliper contact. I'd really like to switch to the 2.15 so I can run tubeless tires which tend to also measure out a bit wider even with the same 90/90 aspect ratio and also have great load and speed ratings. Do you have any experience with a 2.15" x 21" rim on your GS? Any other thoughts on this topic? BTW: I find the 21" so much more confidence inspiring in mud, loose rocks and sand than the 19, and very little impact on road handling.

@BretTkacs Год назад

I sold the 1.85 that was on my bike for a 2.15. I wanted to keep my tire monitors. ESA controls ride height with preload. I should test that. I just need to measure level under the bike then switch back to OEM rims and measure again after ESA adjust. It's not the height that I would be checking but level.

@superflycatchermtb Год назад

@@BretTkacs Thanks for the reply, I think I'm going to order a 2.15 rim from Woodys for the ability to run the tubeless tires and TPMS's. Question: how did the wider 2.15 rim do with "spoke to caliper" clearance....did you have to file a lot of material off to make it clear?

@jamesfairmind2247 9 дней назад

No question a 21 inch is the best front wheel size for serious off-road but frankly, the average adventure rider will not be encountering very many cases in the lifetime of his bike ownership which it will justify giving up the considerable dynamic advantages of a 19 inch on tarmac. Please remember that every First World Record established by motorcycle riders has been on a machine with a 19 inch front wheel, yet todays showroom salesmen will try to tell you that a 21 inch wheel is essential for adventure riding. Here are just a few examples of 19 inch wheel record holding riders in date order. 1935 Florence Blenkiron and Theresa Wallach, first humans to cross the Sahara on any form of motorised transport (Panther). 1964 Steve McQueen, first Hollywood actor to win a sliver medal at the ISDT (Triumph). 1973 Ted Simon around the world and write a bestseller about it (Triumph). 1980 Elspeth Beard first woman around the world (BMW).1994 Dave Barr, first man around the world with 2 artificial legs (Sportster). 2004 Ewen McGregor and Charlie Boorman on "the long Way Around" (BMW). 2019 Henry Crew youngest person around the world (Ducati). A tiny sample but there are hundreds more unofficial records all achieved with 19 inch front wheels, I may even hold a couple myself such as the 7,500 mile TAT in 27 days.

@TheGravelBadger Год назад

Oooh! Science! 😳

@stevelawrie9115 Год назад

I don't know if this question is relevant to this discussion but as an aside, is there more air leakage from a bigger bead?

@Dr.Twisty 3 года назад

If your GS has auto-leveling suspension, would you need to manually change the front suspension when you change to a 21” front? If you wouldn’t have to change it, could you then conceivably change between 19” and 21” fronts depending on what type of riding you’re going to do? My biggest resistance to changing to a 21” front is that I still love sport touring on my GS and I wouldn’t want to sacrifice its asphalt performance. Thanks for such an interesting discussion!

@BretTkacs 3 года назад

I do have auto leveling but auto leveling only adjust rear preload and assumes the front has the stock ride hight. It does not sense tilt or attitude only squat

@eric3810101 Год назад

i have an 18" front with a road tyre for wet city commuting. the stock 19" front with a 70/30 tyre, a narrow 19" with a 50/50 and a 21" with skinny 90/90 - 21" full knobbly for wheel for off-road....swapping between them takes 15-30min.......,.the difference in axle height helps not hinders each change

@davekarl2256 11 месяцев назад

I’m not feeling a lot of sacrifice on a 21” front, seems like as easy trade off if I can only have one diameter given how much better it is off road. It’s an ADV machine we are not talking about a track bike.

@larsihle9256 Год назад

Hey Brett, having 30 years experience as an automotive master mechanic I'm curious if your tire size change affected abs finction. It certainly does with cars but Im not sure if motos use a front to rear comparator when activating abs.

@eric3810101 Год назад

on the 2013> bmw R1200/1250GS, and i presume other models, the computer has an "adaptation" system where it learns the the difference between the size of the wheels and accounts for it. presumably through the abs system. it can account for difference in wheel radius by +/- 25mm....on my 2013GS i have a range of front wheels from 18-21" and 3 rear 17' wheels with different height tyres on them....all of them work ok in all combinations after the computer applied a correction factor......can go into more detail if anyone needs it

@briangc1972 10 месяцев назад

Some motorcycles, Ducati and Aprilia are two, have a calibration mode that we are to use eery time we buy new tires. The dash instructs us to ride at 20 mph in 2nd gear for a 1/4 mile. If we do as the dash instructs, then it updates the data used for calculating ABS. (If we don't, it makes us do it again) Many newer motorcycles have 6 axis ABS, that makes them far more advanced than most automotive ABS systems.

@dbmiller5881 Год назад

Not that it really matters... but no one seems to talk about actual "ground speed" - or velocity of the motorcycle. It seems to me at any given speed, regardless of tire size, the velocity of the outer edge of the tire, has to be the same? The rpm of the wheel would change, based on diameter (or radius if you prefer) but not the feet per minute of the tire surface (because 30 mph is always 30 mph, regardless of wheel diameter)

@ZacZarei Год назад

Simple. For two bikes going same speed, front tires rotational speed is proportional to the diameter of the tires. The reason is bike speed is ralated to tires circumference. So comparing 19" vs 21" front tires, when bikes going at the same speed the tire of 21" rotates %10.5 slower. Here is the calculation: 1- 21/19= 0.105

@RideWithRen Год назад

Given the same motorcycle frame, the 21 inch wheel will also give a longer wheel base, a steeper fork rake, and longer axle caster, all that will enhance the motorcycle stability too, and give a longer leverage going up obstacles. The increase is very minor, but with all the changes you already mentioned it all makes the 21 inch a better choice for off road riding.

@demezon6572 Год назад

Engineer here. You are wrong on so many counts that i do not know where to begin. In short: NONE of what you said is true. Trying to understand you i'd say you mixed different CONSTRUCTION bikes with different wheels.

@superflycatchermtb Год назад

@@demezon6572 I believe, Ride with Ren is correct, but in your mind then what would be the effect of a larger diameter wheel on Wheel Base, Fork Rack and Axle Caster? Could you elaborate?

@whitedrguy6503 Год назад

I guess there is a reason why MX and enduro bikes use 21” WHEELS and Moto GP, super bikes and so forth use 17”. A 21” gives more straight line stability at the expense of quick steering due to the gyroscopic effect of the larger diameter, a 17” has the opposite effect, sharper steering,less stability, a 19” is a compromise between the two. On rough terrain you need stability on smoother roads quicker steering is preferred, back in the day larger diameter wheels were used on the front because the roads were rougher than today and motorcycles used mostly 19” on road bikes, CB 750, GSX 1100, XJ 1100 , then 16” were all the rage on sports bikes.

@twowheelsabove Год назад

having 18" on both😏

@zyonsdream Год назад

Wouldn’t the better solution to adjusting the attitude of the bike be to replace the rear shock to lift the bike up an inch. This would raise the ground clearance an inch. This could be done with dog bones, longer spring, overall upgraded shock. If you use lowering springs in the front you just compromise the front suspension. I’m also not a fan of sliding forks up in the triple tree.

@chopshopchopper 10 месяцев назад

I think he wants a larger wheel, because they roll over objects better.

@Kawasuki636 Год назад

The outer speed of the wheel is the same for 19" and 21". They have to cover the same distance in the same time = same speed. But yes, the angle speed is higher on the smaller wheel - to compensate the shorter circumference. Otherwise you would go faster or slower.

@BretTkacs Год назад

A smaller wheel has to rotate more times to cover the same distance. To maintain the same forward speed would require the smaller diameter wheel to rotate faster.

@winmosser Год назад

@@BretTkacs in physics the frame of reference is critical. If you choose the ground as your ref, then the wheel is stationary when it contacts the ground and is travelling at 2x the axle velocity at the top, regardless of diameter. If you choose the axle then the larger wheel has a smaller angular velocity than the smaller one, in order to satisfy the conditions stated in the first statement. Also I think you missed a point in that changing to a larger wheel will increase the caster ie the distance between the tire contact patch and the extension of the fork pivot will be increased, which will make the steering less twitchy.

@ryanmalone2681 Год назад

How does one just reduce the suspension on a GS?

@narrator-timothymckean Год назад

One way is to slide the whole thing up higher in the triple clamp. Essentially lowering where the bike is mounted on the suspension tubes.

@sagaronwheels 4 месяца назад

Tangential Velocity will be the same for both wheels for same distance covered. V = R.W, here W = @/t, When you solve it the end result will be V21 = V19. So tangential velocity is the same. If stil not clear I can share video explanation with you.

@BretTkacs 4 месяца назад

Tangular velocity would have an equal speed for the same equal distance from the axis but since the point of contact on a 21" rim would be farther from the axis than that of a 19" rim the 21" rim would have a different tangential velocity at the contact patch

@demezon6572 Год назад

Tangential velocity of a 21" wheel is HIGHER than when riding a 19" front wheel (at the same linear velocity). Please correct. The mistake appears in the 5th minute.

@darraghchapman Год назад

Thanks for the vid, very good info as always. Re: the question at the end of the vid, have a play around with Wolfram Alpha (should be the first result if you google that). Very comprehensive site that now accepts AI powered natural speech inputs, so you don't have to be a maths professor to use it :)

@bryanfulmer4823 Год назад

For equations search internet for “rotational equations of motion”. You’ll find what you’re looking for.

@michiganmoto7687 Год назад

I’m curious as why you would lose suspension travel? Did you actually shorten you fork tubes? Why wouldn’t you simply loosen the triple tree and slid the tubes up a touch to account for the height and rake angle?

@BretTkacs Год назад

The specific bike I modified was a BMW R1200GS with telelever suspension. The only way to chamge ride height is to change preload or the entire front or rear shock.

@narrator-timothymckean Год назад

@@BretTkacs Ah.. right... telelever.

@dw8931 Год назад

2 * π is (roughly) 6.28, not just 4.

@alexbastow7915 Год назад

If you squash a small tire you get small contact patch easy to turn front tire faster but les traction and stebility larger tire squeshed larger contact patch more grip feel and stebility but slower to turn.

@charlescaldwell5043 Год назад

Ask your abs pulse rate.

@UncleWally3 Год назад

At 70+ years old I own an Africa Twin but, what I really want now is a “pipe ‘n slippers” NT1100. Sadly however, Honda doesn’t like North America so, that’s that. Out of curiosity, could an AT be refitted to use a 19” front wheel and, if so, would that make it easier for me to pretend it’s an NT1100?

@kengarcia3262 Год назад

I am 67 , have a 30" inseam, and a Husky 701 Enduro. The stock 21-18 rims make the bike a bit too tall for me, gives the bike a bit of a nervous weave on the freeway, and feels heavy in the bars while in the twisties. I had a second set of wheels made up in 19-17 that solved a bunch of issues for me on the street and makes the bike a much better street Adventure tourer but still outperforms any 550+ pounders in the dirt sections. I switch back to the 21-18's for trips where we will be doing hard technical dirt sections however.

@narrator-timothymckean Год назад

I'll trade you my NC700x. ;)

@chopshopchopper 10 месяцев назад

I just looked up that bike. That is a good looking ride...

@orangelion03 Год назад

Recently performed the more expensive option for the 21" modification to my GS: I sold it and bought a Tiger 900 Rally Pro =D The offroad handling improvement is considerable (I realize that there are weight and suspension differences as well). I'm not an experienced off-roader as only started it in 2015, but have had enough miles and training to know that the switch to a bike with 21 has provided more confidence and improved feeling of control. Are there specific reasons why BMW has never offered a 21" option on their R series GSs? You'd think it would be something they would at least offer on the Rallye editions. Limitations of the Telever suspension? Cost?

@BretTkacs Год назад

I think the majority of GS riders are road only (maybe some unpaved roads) and this might cut into their own 850 sales. I would love to see the Rallye come with a 21" and the standard and GSA come with the 19"

@johnnyblue4799 Год назад

My question is, how does switching to a bigger wheel affect the calibration of the ABS?

@Wintersdark Год назад

You need an abs sensor wheel that's modified for the new wheel size.

@johnnyblue4799 Год назад

@@Wintersdark I thought it should be done at software level in the ABS module, or a device similar to a speedohealer...

@josmith4531 Год назад

The ABS works by checking to see if the wheel has quit spinning, then pulses the breaks, to keep the wheel spinning. As apposed to locked up, not spinning and just sliding.

@johnnyblue4799 Год назад

@@josmith4531 I know that. But a bigger wheel spins slower, therefore fewer pulses. ABS will likely react differently.

@gnarlydog Год назад

@@josmith4531 correct. I have swapped from 17" to 19" and ABS still works fine

@TheTurbomoose Год назад

I'm not sure I understand the question in the end or it's too simple to answer. The speed of the outside diameter of both wheels need to be the same if they travel in the same speed. But the difference in revs, is the ration between the sizes of wheels. The smaller wheel spinns 10,5 % faster.

@TheTurbomoose Год назад

Not faster but 10,5% more revs.😀

@johnnyblue4799 Год назад

@@TheTurbomoose It's faster. We're talking angular velocity, not linear. Angular Velocity To Linear Velocity Formula The following equation is used to covert angular velocity to linear velocity. VL=VA∗R Where VL is the linear velocity (m/s) VA is the angular velocity (rad/s) R is the radius (m)

@2smoking.and.4stroking Год назад

Sine and cosine. As simple as that.

@wesb123 Год назад

I honestly expected Bret to be able to draw a whole lot better than this haha ✍️ 😆 🎈

@ElDuderino999 Год назад

One important thing to add to that _size_ discussion: We are NOT talking about 19 or 21 inch ‚wheel‘ size, but about ‚rim‘ size. It is important to understand that simultaneously varying rubber dimension ratios is important about just as well - in essence it could impact rim dimension changes in both positive and negative ways if you select a bigger or smaller tyre height ratio in addition. Conclusion: Always consider the *total* diameter change rather than just the rim size difference.

@Ironhawx Год назад

I’m confused. For the simpletons like me here, won’t the 21 inch rim fit on a 21 inch tire and so on?

@Hansengineering Год назад

@@Ironhawx with tires you always have to question "what part is being measured?" There are *five* different and completely incompatible bicycle tire sizes all called "26 inch", for example. That 26" is measuring the OD of the mounted and inflated tire. A motorcycle tire, 90/90-21, is 90mm wide, 90mm tall, and 21" rim diameter. So the total OD is 21" + 90mm + 90mm or 28" overall. Then your _rollout_ diameter is with the vehicle fully loaded, you measure how far one rotation takes you, and divide by π, and that's the effective diameter. Then you'd do the same thing for your 19", and tires other than 21" come in multiple widths and heights. Then you can compare axle height between the two wheels.

@dunes2dust216 Год назад

I believe you actually find the tyre sidewall height for a 90/90-21 tyre as 90% of 90mm (width). Double that and add to the 21”. Works out to be 695mm or 27.36”. Similarly a 120/70-19 sidewall height is 120% of 70mm. Gives an OD of 651mm.

@Hansengineering Год назад

@@dunes2dust216 you right

@ElDuderino999 Год назад

@@dunes2dust216 exactly: and if all that juggling around with inches, millimeters, ratio percentages, doubling and halving is too complicated (and error prone) -> there are neat online wheel diameter calculators to help you out. Just enter the values and it lets you double check your theories within seconds. So, always make sure you know your final overall wheel diameter and radius BEFORE doing ANY change to suspension!

@georged9631 Год назад

I guess a 21” front wheel would be also an advantage on crossing rivers, water or flooded roads…🤔

@BretTkacs Год назад

Yes

@SheenHunter-SeattleFreeze 3 года назад

But wait! There's more! Lol

@donmartin9567 Год назад

So if 21" is better than 19", then why not 23". Why not 25", 27"???? Well, other than they aren't made which creates the question, "if the larger diameter tires are better then why don't we have really larger diameter tires"? And, while there are multiple answers, there are other compromises. I don't pretend to to know the answers but I suspect there are multiple reasons for running 19s over 21s. And, again, these are Adventure Touring bikes so that means they are a compromise. It also depends upon what kind of riding one does.