Regarding discussion of motion ratio from 18:15 to 19:00, you missed it! When you move the spring inboard to D/2, now you need TWICE the reaction force in the spring to react the moment from 200 lb. load out at the end of the arm. So now the force in the spring is not 200 lb., but 400 lb*. So deflection *at the spring* has to be 2". But 200 lb. Tyree is way out at the end of the arm and sees *twice* that deflection, the end of the lever moves 4", not 2"! The *wheel rate* in the 1st case is the same as the spring rate, 200 lb/in. With the spring moved to D/2, the new *wheel rate* is NOT 1/2 the original, it is only 1/4! The WHEEL RATE is equal to the SPRING RATE multiplied by the MOTION RATIO *SQUARED*. With the spring out at the end, motion ratio = 1, and the wheel rate = spring rate = 200 lb/in. With the spring at D/2, the motion ratio = 0.5, and the new wheel rate is spring rate 200 lb/in multiplied by (0.50)^2, or 200 lb/in * 0.25 = 50 lb/in. That's the big thing that gets missed, the relationship between spring rate and wheel rate is not linear with motion ratio. Wheel rate varies with motion ratio *SQUARED*!
@@AKracecars Great video though, *very* interesting suspension geometry... I'm a tarmac/road course guy, but for sure the dirt-track competitors have to know a *ton* more about how suspension works to get power down to both rear wheels despite wildly asymmetric loading! Fun...
Big thing I remember going to the LR spring behind/ double shock is slower corner center, but more traction out, longer straight acceleration. Bad for us under motored guys who depended on momentum for keeping the RPM's up. Everyone now was deeper in the corner, turning the car harder, up on the bars and hard on the throttle on lesser track. We had to go to it because cars could pass us going in but we had to stop to prevent hitting them in the center of the corner, killing our corner speed. We could qualify as fast, but not race with them. And remember sitting in the garage staring at the 4 link for hours trying to visualize how everything reacted to travel and axle rotation, axis rotation, location, angles and locked, unlocked brake and shock mounts. Add spring, shock rates, lead, torque arm position, 5th coil rate ,angles and position of each component without a computer !! And then compare it to other suspensions like Rayburn swing arm we also used in that time period.
Ur content is exactly what I want to watch about dirt raceing. I check this RU-vid for new content all the time. If u wanna make a video talking about any part of a dirt late model and how it works, I would watch it.
What a cool story. How many countless thousands by that point had seen/touched/built/tuned the LR of a 4-link suspension and not had the insight Skip Arp had. It is too bad Skip didn't get at least a few solid months to kick everyone's butt! A competitive advantage borne out of one's intellect and creative spirit deserves some longevity. Ah well. Skip Arp is a humble HOF legend and gets some well-deserved recognition and respect. The technical piece on the backend helps demystify and educate. More rabbit holes! Who is God even?
I grew up around when late models first started really blowing up late 80s early 90s and I think the cars in the early 90s were so nice. My old man built engines for his buddies car who owned a body shop and his car was so slick I loved the shape of it all hand made by them.
Absolutely tremendous content Mr. Kirkpatrick. Documenting one of the greatest innovations in the history of competitive motorsport. I remember listening to Mark Bush telling a story of a few days before that HAT race at WVMS, Mark Richards calling him to ask him what he thought of putting the LR spring/shock behind the rear end. So, somebody at GRT or whoever was involved in manufacturing the birdcage bracket had loose lips lol. I also always heard rumors that a modified in east TN had mounted his LR spring behind. I think enough time has passed to kill that rumor because you would think if it was true we would have heard about it. I've also laugh at the theory that moving LR spring back ahead would "solve" modern DLM racing's issues on track. From people that for sure know better too. It's one of those things when there's a problem, people don't want to say they don't know how to solve or really understand the issue to begin with, they just pull something out of their ass that they know people want to hear. It's much easier than putting in the effort to properly analyze it.
he ran the atomic weekly with scott bloomquist, Ronnie Johnson, Billy ogle jr, Jimmy Owen's,Chris madden, randy weaver he had very tough competition just at the atomic speedway
I Was at the track the first night. Skip got in a car. I'm old... He was Good. From the Start. One of The Best in my book. As the Family... Down to Earth Folks...
i love folks tinkering. your front end video was amazing. my kinda stuff. my line of work deals with a lot lof inkages so the throttle pedal idea is right up my alley. thanks keep up the good work
Enjoyed it man! After this, I know there were different things guys were doing in front of the axle on the left rear. You might see a chain, chain with spring or dummy shock. Sometimes they would run nothing at all especially on an under rail car.
We have a local dirt track and I went to the races a lot but then life happens and didn't go for many years. Then I went once in the early 2000's and holy hell! What a difference! Left rear tire moving forward, jacking the car on corner entry like nothing I'd ever seen before. I had to go find a buddy that's raced there forever and I was like what in the hell man?? And he explained the changed rear setup and told me "Yeah it's been this way a while! you have to run it, everyone does, it's faster and better traction. Not sure I'll ever get used to that corner transition when it goes crazy wonky, but damn, they can drive em!
The spring behind helps it get on the bars quicker, when fully extended the left rear spring does not much of anything. Clamp cars have instant traction but the amount of time you have to wait to get back in the gas is what kills it. But what else helps is the soft right front and the amount of rear stear gained.
I ran the spring shock behind 4bar on the lr and a swingarm on the rr with a long top bar at a short track here in IL. One night the lr topped out hard and broke the shock eyelet allowing the spring to fall out I ran ten laps with no lr shock/spring due to never really being out of the gas it didn't hurt performance that much but as soon as I got clear out of the gas for a caution flag the lr dropped and the race was over for us.
I tend to think the affect of the altered motion ratio, had more beneficial results, as related to the shock, then the spring rates. Funny that Skip related it to drag racing, as shock tuning is key, in getting a car to hook up, and the left rear naturally would be the more problematic wheel to get to hookup on a dirt car.
In your example about the arm, moving the spring inward and still maintaining 1 inch per 100Lbs. That is not accounting for spring deflection right? in theory the further in the spring goes the more load you see on the tire because with the arm it takes more force to make it move?
Skip is a good un. One of the shows I remember him winning was a UDTRA event at Atomic in 2002. He started in the 18th position. There were a lot of good ones from the east Tn area he raced with. Scott Bloomquist , Ronnie Johnson, Scott Sexton, H.E. Vineyard, Freddy Smith, Duayne Hommel, Billy Ogle.... etc. Those are just a few that came to my mind.
I swear the beat came outta the atomic I remember when jimmy Owen's and Chris madden started there now there big time in Lucas an world of outlaws I remember bloomquist pulling in the atomic with the first tractor trailer rig I ever seen I always wanted billy ogle jr to make it
That’s interesting mate. Any info on pinion angle on these set ups? Seems you need to land the front, point and accelerate and the more you do the less and longer your steering will be at your control. Not sure just wondering. Regards Ken
Great video. I'd love to collaborate with you on a build, that wouldn't be a 4-link, but would have way more bite that has ever been seen. I've been doing it for years. Y'all take care.
I really appreciate what you put together here. One thing to point out with your bar swap myth bust: you can't change the instant center that wildly and expect the car to react the same way. While the axle housing action with respect to the shock is the same, the housing reaction to pushing the car forward will be completely different. You would be better off using the same moment and bracket ratios to look at the difference in shock valving required by the new position since it can have more control than the spring.
“The housing reaction to pushing the car forward will be completely different” ….will it be? Rear steer and axle thrust are coupled. If the axle path through its range of travel is the same, the thrust/IC and its migration will be the same. And since since our cars ride around on a limit strap the whole lap anyways, nothing really matters on the Lr suspension all that much. Lol
@@AKracecars the limit strap is the only part that matters in that. The instant center is different because of the bar position, but if it's at the limit the whole time than even the shock/spring position become irrelevant. The bar position sets forward bite on that side of the car, which in this case also controls when and how aggressive the rear steer will be. Short and high (front of the bars close and high) gives a very short pivot that's easy to drive under. Bars separating at the front gives an infinite length that might even have a static fixed position under load. Dirt cars are extreme but the principals are still at work.
@@willrobb9520 love the technical discussion, but I’m gonna push you a little on this one. Let’s say you have perfectly equal length and parallel 4 link bars, oriented generally like in the video (up hill to front on a decoupled bearing suspension mount on the rearend tube) Technically there would be no instant center because the extended dashed lines running through the links never converge. Now let’s say you move both front mounts closer together by .1inch. Now the “instant center” would be 1 mile up and forward of the car. Now let’s take the front mounts and separate them by .1inch (from parallel). Now the instant center would be a mile behind the car somewhere under ground. Despite these changes having extremely negligible effects on the motion ratio and the axle path through its range of travel, you’re saying these 3 set ups would handle wildly differently?
No, I'm not saying they would be wildly different. This case is extreme and you're aware of that. I was pointing out what looked to be an inch for inch swap in the bars. The angle of your original position versus the next setup, where the original looks like an approximate 40 inch center. Yes, that have more anti squat than the diverging bars which as you point out have no center. With respect to how it would handle in the case of your 1 mile centers, no there wouldn't be a static change. In the case of going from a 40 inch IC to an infinite center there should be a big difference unless the physics are so poor in left field that it doesn't. I'm not ruling that out since the chassis already attempts to lift the left rear as the axle tries to drive under the car during the drive through the turn. In other words, the whole transition is different from something symmetrical with a four link. But you knew that. I'd like to see you plug in the forces in the motion study to replicate the housing pushing the car.
We had a 97 mastersbilt leaf car that we put 4 bar set up in leaf brackets had 15’ rods on bottom 18’ on top with shock in front best race car we ever had Dewayne Hughes drove it skip knows this car led every lap of 100 lap hava tampa at north ga broke some teeth on gear and freddy smith got by us wound up being a major mistake selling it to Lamar Scoggins should have kept it.
Great video… I’m not sure how much it has to do with the spring in the early days every was way way off the spring at full droop. You can see arps spring is off the cup in the 20:44 clip. I follow you in what’s currently going on though.
@@AKracecars not really, they more sat and drove off the Lr most of the shock in front days as in this video. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-LxhccZDkA10.htmlfeature=shared If you saw a car with any real movement pre shock behind it was because the lr front shock and spring was on a clamp bracket… those cars were wild….
Only boomers with very little comprehension of suspension and geometry make stupendous claims like the 4 bar ruined the racing or that moving the shock and spring ahead will fox racing. The rules itself and lack of independent variables, experimentation and true innovation are what’s destroyed the racing. Come racing will never yield good results. In fact of the same people in charge of dukes today had been In charge years ago we would all still be in stacked leaf springs. There would be no bar cars. It would have been banned instantly. We don’t need common sense, math or simulations to know that moving the spring forward won’t fix the racing. We ran spring behind in the 90’s and early 2ks and the racing was absolutely AMAZING. Granted most only ran a dead valve “dummy” shock with the rear spring. But many often still do today too.
