Drawing Out the Roll Center For the Race Car and Explanation for How It works! 

Jason, I love your content, you are a great teacher and illustrator. I have watched professional chassis builders go over the same car setups and they might be good car builders, but they are terrible at explaining and illustrating what they are talking about. You are a great help to us just starting out but wanting to build a competitive car. Thank you!

I never really understood roll center until now. I appreciate the demonstration on the board

I’ve always heard others talking about roll center, to be honest, I had no idea what they where talking about. Thanks for show this! I truly appreciate it.

Great video! The most important part of building a car , very informative, a properly built car is most important, not the motor , so many need to learn what your showing!

If you have an idea about how the front tire loads are split during cornering (say 70% outside & 30% inside) then there is an extra level you can take your measurement to, that corrects for any errors in the roll center method. What you would do, is make a vertical line that is positioned between the contact patches, such that it is 70% of the track with AWAY from the outside tire, and therefore 30% from the inside tire. We call this the "resolution line". Then, measure the two heights above ground of the force lines for each side (these are from contact patch to instant center) at the resolution line. These two heights are the anti-roll contributions from the control arms for each tire. Their average height can be used to calculate the total anti-roll contribution from the front end's control arms. This is the more accurate method for creating a meaningful number for front anti-roll from the control arms than using an intersection of force lines (as in your example), or just using the center-line of the car (as some other methods may show). As you can imagine, if the control arms are a lot flatter than your car is, like they are in a lot of race cars, then the instant centers are a long way away and the "roll center" as is commonly shown will be flying all over the place side-to-side, but the cars don't show any strange handling affects from it. The reason being, the commonly quoted "roll center" is not a real thing, and is more of a convenient and lucky rule of thumb measurement for common layouts based upon road cars. In your example of changing the inside top control arm from 10 to 12 degrees for instance, what the resolution line will tell you is that you increased the anti-roll coming from the inside tire via it's control arms (whilst the outside tire stays the same). What this means is that the anti-roll from the front control arms as a whole is increased, which is like saying "you raised the roll center without realizing you did" because roll center geometry is technically wrong. (You also shortened the length of the swing-arm for the inside, increasing camber change and increasing bump-scrub, which you may or may not want to do). The best rule-of-thumb for the resolution-line method is to take the two separate heights on the line and simply average them, ie find the middle, and use this number as what you could call an "anti-roll height" for the front end rather in the same way you do for roll center height in the "crossing the force-lines" method in your demonstration. you can even think of it as a new way of calculating the roll center height, that gives a real number rather than one that could be nearly right, a bit wrong or a lot wrong. I hope that makes sense. Mark Ortiz sets this out in his articles and it is used in the calculations for the Racing By The Numbers software from William Mitchell.

Thanks for replying to my Questions

Jason, I really enjoy watching AND learning from your videos. I quit racing in '04 but still think about it every single day. You are easy to understand and your fabrication is beautiful. Thanks for your videos and Happy New Year.

Best video on roll center I have ever seen! Well done!

Great Job on the videos! been searching for some good information with being a new driver and you have done a great job with explaining everything. hope to see more.

Excellent video! Thank you

Ever thought about building a car for some one else, really liking what your doing and how you explain things is top notch

Learning a lot still don't understand all the lingo but still learning thank you so much for all your videos they are helping me a lot

I loved this video. Been working on race cars for a while. Currently working on "dwarf" cars and trying to dial in front RC and keep "bite" in the car.

Great info Jason!

You should name this car “the professor” once it’s done! Lol

Great explanation. Would love to see you do one on spring table and how it affects roll center migration as well as how rate affects migration. Do you look at where center of mass is in relation to roll center(roll moment) also is there a target roll gradient(deg/g) that you have in mind? I’m thinking of building a stocker for myself and am having trouble deciding on a roll gradient. Thanks for the video!

Enjoy

Hey Jason, setting this roll center, can you do that with stock upper control mounts are is the only for when your installing these after market mounts?

In a previous video about the upper control arms you have them equal in length and even across on the frame. I have a camaro that mounts the upper control arms on the inside of the upper control arm mounts. Then the OEM shock is on the center of the outside of the mount. Since my track rules require OEM upper control arms and control arms mounts. I used offset cross shafts in the RF for more negative camber. My question is I seen someone put the Left control arm on the outside with spacers to give it more positive camber. This is going to change the center of both arms and the angle of the left arm since it is moved out with a short arm in length it reacts quicker. They’re making hard to adjust things while using OEM parts. I seen adjustable ball joints that I might be able to use if this helps. I just didn’t know if the arms not being centered is a issue thanks.

The visual really helps to understand. I run a V8 Warrior/pure stock GM metric. We have to run stock uppers on stock mounts, with any length bolts for camber. Could you recommend a ball joint length for the uppers and lowers? Currently all are 1" longer except the RF lower which is stock length. As I understand this is my only way to adjust the roll center with limited adjustability. Thanks for any help. Car needs to turn better center off. 1/4 mile dirt, slight bank , heavy heats ,dry slick features.

How do you know what the exact ride height is going to be without the car being complete ?

Are your ride heights set

Is this applicable to asphalt racing as well?

👍🏻👍🏻

at the end of the video, did you confuse left and right angles, did you mean 12 on the left drivers side and 10 on the right passenger

Me again. Your like my new toilet watching material KPI equals what scrub radius you have. Only turning left and being rear wheel drive. Would you have a positive scrub radius on one side and a negative scrub radius on the other side. So when you brake the car would pull to the left with the front left toeing out and the front right toeing in? So positive scrub radius on the left and negative on the right. I’m talking about your car in USA turning left. We went racing at the weekend. Got tyre pressures and camber absolutely perfect with temps across the whole surface all within optimal temperature range sent from Hoosier and within 5c from left to right. We did have 11c track temp though. Knocked 0.3 off of a lap time consistently so going in the correct way The roll centre is still well off as its way to the left with a 700lb front left to stop it diving so that’s the next project

What software do you use?