Front Roll Centers

Messing around with some front roll center stuff and trying to understand where the trend is for roll centers and what effect they have on the car, and ultimately what types of track conditions favor the location. My application is with a dirt modified, but I come from the dirt latemodel world and I "assume" the location would apply to either application.
So my questions are:
1. Would the front roll center height vary between an aluminum motor, steel block aluminum headed motor, and an all steel engine and would setting the roll center height matter for those combinations?
2. What effect does the roll center have from left, to center, to right? I make the assumption that having the roll center to the left is going to make the RF softer and make the car lay down on the rf more and not pick up the left front as much, would that be true or false, and for what type of tracks is it best located? Left for dry slick, in the center as neutral, or where do you "think" most roll centers are being located?
3. Does having the roll center located from right to left have to work in conjunction with the caster or does it even matter, and if it does what direction would you go with the caster based upon roll center left to right location?
4. Does the spring combination matter with the roll center right to left?
5. Is there anything to the front to rear roll center location?
6. Where do you think the roll center is from right to left on most latemodels and most modifieds?

I know I can run the program or figure out where the roll centers are, I just want to understand the effects or just some ideas of how I attack the front roll center. I never gave it alot of thought because those things were figured out by chassis builders, but I find myself wondering...
Thanks,
Chris

I would scrap all of those question about roll center and focus on understanding camber relative to the race track given a certain amount of body roll and suspension travel.

As someone who works in the top level of auto racing and has a decent understanding of vehicle dynamics, let me save you the time. Roll center is a fictitious thing that does not correlate on the track. The roll center on some cup cars is a half mile in the infield and a half mile below ground. It is a meaningless thing. Anyone who thinks it matters is just hitting their jacking and camber values correctly and not understanding what is important.

Well, what is important then?

Are you advocating force based roll centers?

What is jacking forces?

Camber values would come from static and gain, gain would be from A arm length and beginning angle?

I have always thought roll centers, are maybe not meaningless but only part of the equation. Plus there has been the argument about force based roll centers, which is interesting and seems to have a lot of merritt.

The biggest influence on a race car would be CG location, fwd to aft and vertical. Your basically swinging 2350 lbs thru a turn, on a banked dirt track. I would think CG and the Polar Moment of that weight would be critical.

So what is being said is I should just throw any height ball joints in the car because it has no bearing on anything? I make an assumption that locating the lower and upper control arms on the frame has an influence on the car and not just because of arm lengths? I am not asking the question because I am looking for the magic weekly bolt on secret, all the pieces have to work together, but what is being said is roll centers up and down and side to side do not factor into anything? I would think they still affect motion..

Quote:

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Originally Posted by racinlm

So what is being said is I should just throw any height ball joints in the car because it has no bearing on anything? I make an assumption that locating the lower and upper control arms on the frame has an influence on the car and not just because of arm lengths? I am not asking the question because I am looking for the magic weekly bolt on secret, all the pieces have to work together, but what is being said is roll centers up and down and side to side do not factor into anything? I would think they still affect motion..

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Balljoints affect plenty that matters. Camber gain, Caster gain, static camber, bump steer settings etc

I will give you the age old line i get all the time, ready?

Call your Chassis Builder and do whatever he tells you to and or use. If he says jump around on one foot, and then hand him $500 bucks just do it... LOL

Anyway, rant off.

I spent the weekend viewing data from a DRP Pull down rig, i can tell you that upper A arm locations matter.

Roll Centers they way we calculate them are just a recording device to me. It tells you where you are currently, so you can get back to the same place changing parts etc. You can change it, move it etc and doing the calculations only tells you where it is again. Does that mean its going to work, or be better that is dependent on a lot of other variables but it can help indicate what direction you need to go IMHO.

Read up force based roll centers, its interesting and the argument is pretty valid.

Nobody is saying that it doesn't matter. We're saying that there are a bunch of other things on the front end of your car that play a WAY larger role in the overall vehicle dynamics.
Historically, people altered roll center in stock car racing because they were starting with "stock cars" that had sub-optimal roll center location for a racing application. So this was a key area you could find cheap speed. Suffice to say a lot has changed with the dynamics of a modern dirt late model (induced body roll being top on the list in my opinion) that require a closer focus on other things that affect how efficiently you can get traction out of a given tire. So much so that the gains to be had FAR outweigh the perceived negative affect of a kinematic roll center change.

The most extreme example would be in Formula 1. The engineers there spend NO time figuring out where roll centers are. Everything is focused on controlling car attitude and tire location to maximize aerodynamic downforce and control outwash. Yes it's an extreme example but if you don't think it is directionaly important to a conversation around vehicle dynamics in dirt late model racing, you've had your head in the sand for the last 15 years.

Krom,
Yes it is a recording device. The point attempting to be made is that it isn't worth recording. To extend on the music analogy, you wouldn't be concerned about how an instrument is tuned if the song you're are recording is being played by other louder instruments that almost completely drown out the sound of the un-tuned instrument. Just let it play along.

Ok, i see your point. At the end of the day your tire loading against the track, at a given attitude would be paramount.

I guess thats something that i need to look at closer, camber and caster at a dynamic condition, along with bump steer.

New ways of thinking i guess.

Yup. Lots to consider. Camber gain, caster gain, bump steer, dynamic caster affect on weight jacking with steering input. Some cars now are running both control arms angled uphill when viewed from the side. How that affects bump steer and front-to-rear wheel location during suspension travel is pretty interesting.
You think about how today, when correcting for bump steer, we usually only account for the angle of the tie rod when viewed from the front. But think about the angle change when viewed from from the top as the spindle moves fore and aft...then things get more complicated.

Ah roll centers. One of all time my favorite quotes is "Roll centers has the biggest (words typed on message boards) to (actual relevance to lap time on the track) ratio of anything in racing"

The "kinematic roll center" diagram that everyone loves to use (intersection of lines drawn from tire contact patch to their respective IC points) is flawed immediately from the get go because: it assumes tires are solid, and it assumes the springing is symmetric. IE, if I put a 10,000lb spring in the RF, where is the roll center? It's right where the tire touches the ground. That's the point that the car will flip over around because the suspension is in-compressible for all intents and purposes (a literal "roll" center)...... kinda like when you are on a bumpstop.

Looking at jacking forces and anti-[roll, squat, dive etc] is important..... kinda.. I wouldn't say calculating it is a good use of time, but knowing how it works is important. All any of those things do is effect the amount of weight transferred elastically (through the springs) vs kinematically (through linkages). It does not effect **how much** weight is transferred.. But, as Mark Bush would say, of a pair of weight receiving springs, the stiffer of the 2 gets the weight first.. and anti-____ can make a suspension act stiffer. As Matt said, lots and lots of things to consider. But as far as priorities I would put roll center location pretty far down on that list.

Quote:

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Originally Posted by Austin34471

Ah roll centers. One of all time my favorite quotes is "Roll centers has the biggest (words typed on message boards) to (actual relevance to lap time on the track) ratio of anything in racing"

The "kinematic roll center" diagram that everyone loves to use (intersection of lines drawn from tire contact patch to their respective IC points) is flawed immediately from the get go because: it assumes tires are solid, and it assumes the springing is symmetric. IE, if I put a 10,000lb spring in the RF, where is the roll center? It's right where the tire touches the ground. That's the point that the car will flip over around because the suspension is in-compressible for all intents and purposes (a literal "roll" center)...... kinda like when you are on a bumpstop.

Looking at jacking forces and anti-[roll, squat, dive etc] is important..... kinda.. I wouldn't say calculating it is a good use of time, but knowing how it works is important. All any of those things do is effect the amount of weight transferred elastically (through the springs) vs kinematically (through linkages). It does not effect **how much** weight is transferred.. But, as Mark Bush would say, of a pair of weight receiving springs, the stiffer of the 2 gets the weight first.. and anti-____ can make a suspension act stiffer. As Matt said, lots and lots of things to consider. But as far as priorities I would put roll center location pretty far down on that list.

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Thanks for the answers, you all have been very helpful.