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

[@matthewstroud4294]

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.

[@DIRTRACELIFE]

A lot of good info there and mush appreciated. I have come to realize I don't actually care about the roll center. But I must be honest, to me this information is kind of also just a different way to also understand roll center location for the car. What I am learning is, All I really care about is knowing how the car articulates through dive and roll and then how the camber curve of each front wheel is changing as that happens. The lines and locations of imaginary planes are not nearly as important as I once thought. Everyone has chased roll center data for 30 years now but what they really were chasing was camber curves that kept the front tires bit into the track in the turn. So lets just cut the chase and do that. That's where I am at now in my setup logic.

[@matthewstroud4294]

@@DIRTRACELIFE If you know what camber change to go for, then yes. Because the instant-center for a tire tells you the camber curve. Also, by using a method that disconnects the two front tires you are closer to what is really happening. The traditional roll center methods are pretty useless outside of situations where they give the right answer by accident. i like the channel, and I wish stuff like this was about 30+ years ago when I got into oval race cars.

[@jasonlogan3209]

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

[@stevehurley5408]

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.

[@DIRTRACELIFE]

Thanks Steve, I appreciate the feedback.

[@jimmymcmullan1300]

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!

[@DIRTRACELIFE]

Thank you Jimmy, hoping to build the next one even better from what I have learned on this one.

[@marksmith4539]

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!

[@DIRTRACELIFE]

Appreciate it Mark, I try 😀

[@troy4493]

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

[@DIRTRACELIFE]

Troy, I hope to be good enough one day. I'd like to retire from the day job in a few years and hopefully expand my shop, put in a 3 axis cnc and 2 axis plasma table and go to building cars. That's my ideal second career I could enjoy.

[@jasonperry9225]

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

[@DIRTRACELIFE]

Thanks for the feedback Jason, glad you are finding them helpful.

[@gregorygolden1296]

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.

[@DIRTRACELIFE]

Thanks Greg, We're trying!

[@robertwestover2125]

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

[@DIRTRACELIFE]

I appreciate that Robert. Thank you for watching and the feedback.

[@brandonbatton5952]

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.

[@DIRTRACELIFE]

Thanks Brandon, yes we are continuing on with much more coming. I appreciate you watching and glad its helpful.

[@rafaelchavez66]

Thanks for replying to my Questions

[@DIRTRACELIFE]

Yes sir, I hope you do well this year racing.

[@hoovercat8605]

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.

[@DIRTRACELIFE]

Thanks, I'm glad these concepts are helping and useful for applications beyond full auto frame chassis.

[@rollrateguy6109]

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!

[@DIRTRACELIFE]

I sure think you have a firm grasp on this. Yes, I agree you must correlate the roll center versus the center of gravity. For example I set my engine one inch higher than my min clearance but thats part of making sure I am putting my cg well above my rc to promote that migration on entry and hold it middle off. I've been researching and working out my plan for how to explain the springs I am using on the coil build and why. I think where my heads at and yours aren't probably that far apart. I appreciate the feedback and thanks for watching.

[@gregcrowe350]

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.

[@DIRTRACELIFE]

I definitely agree with the +1's on the ball joints. I'm surprised you aren't running a +1 on the RF lower. I would either do it on both or neither for the lowers. Do all you can to get the camber maxed out + on the LF and - on the RF. Ideal is 2 and 6 but you can't get that much on stock "but get what you can". If you're tight middle-off consider adding more rebound in the RF shock or more caster in the setup. Just some wild stabs in the dark there but both are ways to take left rear out by timing on the shock or steering left using the caster.

[@gregcrowe350]

@@DIRTRACELIFE Appreciate it. I will try all 4 ball joints @+1" . I have offset cross shafts and 1/2 nuts on RF bolts to maximize the camber.

[@stevehurley5408]

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?

[@DIRTRACELIFE]

The only difference Steve is that you have to make your changes just in which spindle and what ball joints. A dang good starting point on a bone stock front end is just adding +1 inch upper ball joints.

[@tommyhitt9071]

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.

[@DIRTRACELIFE]

Tommy. I have seen just what you are talking about before. Yes it sure screws roll center up but in my opinion it could be the lesser of two evils because having negative camber on a left front is a disaster. It would move the roll center left but I'm not sure how much. Also the negative aspect is how it is going to make the rc move in an undesirable way during roll. Its going to move the rc hard right and I suspect it may be difficult to keep any positive camber at all in the left front while extended without the longer arm. Heres a tip. You can get a 1/4 inch more arm by switching to a k6136, migging up the holes and the redrilling to get the bigger base k6136 all the way out on the lip of the arm. A 1/4 inch is a world of difference if you have that tight a rule set. Just my thoughts :)

[@rafaelchavez66]

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

[@DIRTRACELIFE]

Your correct Rafael. I'm putting 2 more degrees in the right upper compared to the left.

[@DIRTRACELIFE]

Your correct Rafael. I'm putting 2 more degrees in the right upper compared to the left.

[@rafaelchavez66]

thanks, I'm building a metric chassis. Does it make a difference if I mount the weld in upper A arm mounts back the way they were, Angled, Instead of parallel as long as the caster camber all set to where they need to be?

[@rafaelchavez66]

On the Metric chassis the upper control Arms the ball joint is off set. not exactly triangulated like the camaro front end you built

[@DIRTRACELIFE]

It's complicated for sure Rafael. Parallel uppers significantly simplify the setup but angled upper do give you better control over caster in bump. You have caster loss during bump with a parallel setup and you will have less with the angles. I can map and understand a parallel setup and I can get my bump right so I am using those locations and I don't have an engineeringplan yet for angled locations. Honestly, I think angling can in fact be better but you have to know EXACTLY where to put them. I am hoping by the time I build my next chassis which is metric I have engineered that process.

[@dcooper91g]

Great info Jason!

[@DIRTRACELIFE]

Thanks David

[@dcooper91g]

@@DIRTRACELIFE just found ya. Love the channel! Good luck racing this year! Not sure when we’ll make it to a track but my son and I are working on a couple cars and plan to get back on track sometime this year.

[@jordanwallace5586]

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

[@DIRTRACELIFE]

Thanks Jordan, Yeah I always name them but not sure yet what. It will tell us its name in due time 😁

[@TN-Vols-Fan]

Excellent video! Thank you

[@DIRTRACELIFE]

I appreciate you watching Steve.

[@gochenour7991]

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

[@DIRTRACELIFE]

Well, that's actually pretty easy believe it or not. Since I already have a car (my camaro) on h500s and 8 inch wheels, I just took a measurement for the front center spindle height to the ground with the car set up race ready. then all I had to do was work out the math for that exact spindle height on my new frame with the lower ball joint and inner bolts level to each other. ultimately we are going to drop the rf slightly below that but for initial build we want the frame true level side to side. So now that the frame is exactly right in front I just put the digital level on the frame rails and raised the rear to create 2 degrees of rake. Cut all the blocks and spot weld to the jig, good to go. I will make sure I am showing some more detail on this while mounting the rear end spring buckets. Thank you for watching our channel Rusty and hope you find our videos useful!

[@theracerdude]

Is this applicable to asphalt racing as well?

[@DIRTRACELIFE]

Shawn, the method and logic for how to figure out where your roll center is at is the same. However, the ideal location for where you want to locate that roll center would be very different. So I would say this helps if you have good information on where you want the RC but are trying to figure out how to get there. Thank you for watching and I hope this is helpful.

[@theracerdude]

Awesome! Thank you and thanks for the reply

[@robynwells6230]

Enjoy

[@DIRTRACELIFE]

😊

[@robynwells6230]

@@DIRTRACELIFE I'm going to get back into the racing this year when I retire

[@tandybarnes4826]

Are your ride heights set

[@DIRTRACELIFE]

Yes, they are what I expect from my experience on H500 27.5 tires with these spindles. I could be off by +/- 0.5" . You never are 100% until the car is on the ground though. Thank you for the question and watching!

[@bendoherty7721]

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

[@DIRTRACELIFE]

Ben, In my current camaro, I set my outside tire at zero and my inside tire is set toed out. My bump steer is corrected to avoid gain or loss however the setup does lose Ackerman (correction as you turn in further to avoid scrub). In order to avoid the excessive scrub I toe my inside tire in toward the center of the track by 1/4 inch ( a bit more than 6mm). My calculations show less than half of this would be necessary to correct the Ackerman issue causing the scrub loss however I believe the additional toe-out does assist in making the car more stable down the straight away and in the turn. In my opinion once I go beyond that amount the scrub becomes too great and is detrimental.

[@bendoherty7721]

@@DIRTRACELIFE so Ackerman or anti-Ackerman for asphalt circle short track racing on slicks I believe Ackerman as the same as a tank. The inside would turn more and drag so the outside would drive better. I believe we are entering the corner about 65mph and exiting 60mph. Also for the steering input angle needing to turn the corner of about 10degree the inside needs about 3degree more to be the same around a 0.4mile track to get the correct slip angle. We have a front mount rack now so is anti Ackerman. When talking about scrub radius. I’m talking about where the kpi will intersect the tyre centre contact patch. Positive on one side of the car and negative on the other side so 2 different hub designs would be needed to be designed and fabricated kzbin.info/www/bejne/bGO7ZqaXgbGCmtU

[@Racer7x]

What software do you use?

[@DIRTRACELIFE]

I draw out my motions in Inventor (Autodesk) which is engineering software not racing software. I don't have complete models. I just mock up the sections to figure out how parts work. I appreciate you watching my video sir.

[@bendoherty7721]

👍🏻👍🏻

[@bendoherty7721]

Would you go extreme left to try and get the roll centre as close to the centre of gravity when the chassis is at a 3degree? This explains a lot. I measured a friends uk car and his roll centre is about 2metre to the left. His front left is absolutely cooking and front right is stone cold. Hence why he needs a 800lb spring on a 700kg car for the front end not to fall over

[@DIRTRACELIFE]

My roll center sure wont move that far the way I build but it looks like you have a good understanding. It makes sense that the left tire is doing all the work because it is planted with the roll center causing the rf to just be picked up (right turn racing). I appreciate you watching our channel.

[@bendoherty7721]

kzbin.info/www/bejne/fX7bn2BuasyNl9E Thankyou to you for your help. We completely redesigned the chassis over the past month and look at what we achieved. His first ever race win

[@DIRTRACELIFE]

Excellent!