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Author Topic:   front roll center
racnfool
Dirt Forum Racer

Total posts: 89
posted August 26, 2005 12:36 PM  
I have noticed my front roll center location really influences needed spring rate on the right front. Moving roll center to the right makes a 600# spring react as though stiffer than a 800# with the roll center more centered. My question for you guys is which setup is more desireable for dry slick? Both can work but when someone asks a spring rate question there are always 20 opinions on the same front clips. Would this be the reason?
Thanks,
racnfool

Kromulous
Dirt Forum Champ
Total posts: 796
posted August 26, 2005 01:08 PM  
The newer trend i been hearing is lighter springs, down to like 550lbs. This is on a chevelle clip.

Now thats just a trend, i dont know the exact science behind it, i can only speculate. Maybe some others can correct me if i am way off.

Anyway i believe its got alot to do with how high the RC is, in relation to the CG. As well as the roll couple (thats what i call it) to the rear RC. I beleive they should be more level, looking at them from the side of the car. Why have the front at ground level and the rear 14" above ground? doesnt make since, and it would seem to not let the chassis work, front and rear to work together.

I rasied ours (front RC) 1" last week and the car instantly became more smooth, and easier to drive. The car had near perfect balance, and it actually felt like the front and rear were working together, was headed to the front till we burnt to pistons and had to shut it down.

I also think there is something to making the moment arm shorter, when your raising the RC. I wouldnt know how to explain it, but i tend to believe that if the moment arm is shorter, the car reacts better.

Like i said maybe somebody can explain it better then me, i may be way off as well.

Krom.

wfoondirt
Dirt Forum Champ
Total posts: 505
posted August 26, 2005 02:38 PM  
quote:
Originally posted by Kromulous:
I also think there is something to making the moment arm shorter, when your raising the RC. I wouldnt know how to explain it, but i tend to believe that if the moment arm is shorter, the car reacts better.

Reducing the moment arm makes the car more responsive by reducing body roll. Alot of body roll makes the car feel sluggish. Look at it as a buffer between the driver inputs and car response. For a good smooth driver this will make the car feel better and usually make him faster, for a so-so driver it can make the car twitchy. This is one reason I don't like factory built cars. They tend to be dumbed down to make it possible for anyone reguardless of skill be able to do soso in the car. The downside is a good driver can have problems with car feel due to excessive body roll, weight transfer, and chassis flex.

What you saw Krom is one goal of making a chassis handle and is what most people try to achieve whether they realize it or not. The key is matching roll angle front to rear. Basically triing to make the front and rear suspension roll an equal amount without one end triing to make the other end roll. If you look at it mathmetically you want to treat the front and rear as if they weren't connected, then find the natural roll angle (ammount it will roll given the lateral g's) of each end of the car. You then work to make those angles as close as possible. Essentially that is what everyone is triing to do by changing springs/rc's etc. The more rigid the chassis, the more important it is.


CUSTOMPERFORMANCE
Dirt Forum Champ
Total posts: 1172
posted August 26, 2005 08:50 PM  
3.5 inches above ground and 2 inches offset to the left works good for the chevelle stub.

badfastjr
Dirt Maniac

Total posts: 193
posted August 27, 2005 08:46 AM  
what kinds of things can you do to change front roll centers and where are they usually at on a metric chassis with all stock components

hoosier transplant
Dirt Maniac

Total posts: 104
posted August 28, 2005 08:22 AM  
I was hoping that this thread would take off and have some more input. This is a very important setup area that most don't bother with because it is time consuming to get all the info to know where your roll center is located. I understand it will take a good four hours or more to measure for accurate readings, but will pay off in the long run. I kind of understand some of it but not all. I'm getting ready to start building a mod for next year and I WILL take the time to find both my roll centers, but was hoping this thread would give me some more help. Some of you guys are really sharp on this and the rest of us need you input. Please inform us. Thanks, Donavon

Kromulous
Dirt Forum Champ
Total posts: 796
posted August 29, 2005 06:47 AM  
Front roll centers can be raised a couple of different ways. The upper arm angle, and the lower arm angle is the major influences.

Usually you will notice the left side has more angle than the right, thats what biases the RC from side to side, if both are at the same angle it will be in the middle, or on the CL of the chassis.

On our Ford clip, i run the lowers flat, level, and the uppers at 15* down angle and the RC comes out real good. Left side upper has about 2* more angle in the upper arm. This helps bring my RC over towards the driver about 2".

Also i use the coleman monoballs for the upper ball joints and some small chrysler screw ins in the lowers. So everything is adjustable.

racnfool
Dirt Forum Racer

Total posts: 89
posted August 29, 2005 06:47 AM  
I sure didn't think we needed to be so wound up about roll centers and was beginning to think the articles were hype to sell software and books. One small change to my ride heights took my roll center from roughly center and 3" above ground to left near the left contact patch of the left tire. The car "buried" the right front tire and carried the left front 2 feet. Also felt like it could roll at any time. The following week I pulled my computer out and found all my original settings to get back to centered and 3" above ground. Car returned to being nuetral and easy to drive. Point is I had people saying RF needs more spring, left rear is driving too far under, etc, etc. Roll center is important and we need to explore more for more speed. Some of the "old timers' have what appears to be roll centers nearer to the right side by mimicking what they see on late model chassis and applying it to the stock stub. Some of these have extreme short right side uppers and huge angle in the upper arms.

Thanks for all the input guys this is interesting.
racnfool

[This message has been edited by racnfool (edited August 29, 2005).]

aggressive
Dirt Full Roller

Total posts: 33
posted August 29, 2005 02:32 PM  
It seems everyone thinks the place to have your roll center is left of centerline, but I think most are in agreement on height,so I'm going to give my 2 cents worth on why the right side works better.Since most tracks have become a freight train around the bottom come feature time the trick is from the center off not to push and if u are clamped up there is only one way that we have found to keep the push out of it and that is soft spring in the right front.And as stated in a earlier post to do that u must move the roll center to the right.We won this past fri.with a 550 in the RF on the cushion at one end and on the bottom at the other,and this is a chevelle clip and it didn't bottom it out, and thats 22 wins with the roll center moved to the right so we are real happy about the results we have had,so if your situation is similar u might look into trying it,and if u race somewhere that the Pierce cars are winning take a look at the angles in there top a-frames pretty easy to tell that they are way to the right.....so thats my 2 cents worth

Kromulous
Dirt Forum Champ
Total posts: 796
posted August 29, 2005 02:44 PM  
Good points, moving the RC to the right should allow the weight to transfer on to the RF faster as well. Plus coming out of the corner the softer spring should help get the car on the bars.

Its really a tricky deal, lets say you got a 1000lb RF and the RC is 4 to the left, and then you mathmaticly figure out if you move the RC 8" to the right what spring would provide the same wheel rate, lets say that comes out to be a 600lb. Will it handle the same?

No, is my thoughts but i am unsure why. At least i dont know exactley why, but i am guessing it has alot to do with the moment arms length vs the CG of the front suspension.

Aggressive, have you notice any response difference when you chnged this? I would assume the responsivness of the car would be much better.

I'm just learning alot of this stuff myself, but i often find the old way of thinking is going by the wayside quickly. Thats why i am always on here reading and asking ignorant questions.

Maybe Wfoondirt has some more thoughts on this, and about Agressives post.


Kromulous
Dirt Forum Champ
Total posts: 796
posted September 08, 2005 08:09 AM  
Aggressive, could you elaborate more on moving the ront RC to the right? How far and whats it feel like when you drive it etc.

Thanks, Krom.

FlyNLoIMCA17
Dirt Forum Champ
Total posts: 753
posted September 08, 2005 07:37 PM  
Krom,

Heres my opinion on your post. You talked about calculating the wheel rate and moving the RC if it would handle the same....you said NO. Your right and heres why. Wfoondirt said it in his post. When you take the RC from 4" left and move it to 4" right and calculate the wheel rate to end up with the same rate the reason it doesn't handle the same is because of the balance. Lets say that with the front at 4" left that it is closely matched to the rear RC placement, not neccesarily dead on but close enough. Then you move it 4" to right of CL. Now the front and rear aren't matched anymore. As to wether you like it one way or the other I guess is up to you but it is a proven FACT that a balanced setup can be faster. I'm sure driver feel and experience has a lot to do with it just like anything else on a racecar.

Also just matching the placements of the front and rear isn't always enough. you have to match something else the Wfoondirt also mention, the roll angles. When everything is matched, the car will be balanced and handle much better. Also you will notice the tire temps drop considerably. I done a good bit of work on the RC of my car over the winter. Theres a lot still to be done but I made progress. Last year I remember running a race where the track was extremely dry-slick and smooth. I'm not the best driver in the world LOL! so what happened is I blew the rear tires off it. I started w/ a new RR and in 20 laps it had NO tread left on it!!! Now this year, I have ran those same track conditions and after the race my tires feel like they haven't even warmed up yet!! So far this year I have used about 2 or 3 tires!! Thats it! And those weren't new either they were slightly used(2 races to be exact) (I was getting used tires from another racer for a while) So you can see the difference. And my car isn't quite completely balanced yet, still a lot to be done and I mean a lot!!

Kromulous
Dirt Forum Champ
Total posts: 796
posted September 09, 2005 07:25 AM  
Well the reason i ask is i spent the weekend at the Big Dawg watching D. Schwartz. I can see there running a light RF spring, with that clamped up LR set up. Its hard to explain.

I watch his car and its plain to see that there spring rates are light, front and rear. As well i can watch the car roll over in the corner and it rolls over evenly, not more biased to the rear. As he comes out of the corner the LR corner picks up and the car looks like its driving the RF into the ground coming down the straight.

If i am thinking right, there running a really light RF spring, with the RC moved over to the right side. Then that will keep the car from bottoming out the RF from body roll, but will allow the LR to compress it alot so they can get the drive in the car.

I got our car balanced, finnally this year. Its good and drives well but our spring rates are high, and theory says that a heavier spring reacts slower than lighter spring rate. Which is fine, for now, but i want to try and get a handle on what there doing with the light spring set ups and the front RC, so when i build our second car this winter i want to set it up like that and try it, while we leave the other car alone.

Its looking like something i am just going to have to put on and try it, and try to find it thru trial and error.

Krom.

doghouse racing4
Dirt Forum Racer

Total posts: 91
posted September 13, 2005 09:32 AM  
It would be nice to hear more on this subject. Can you just look at a car and tell approx. where the front roll center is? Chevelle stub 68-72, what angles are the uppers to get roll center close or is it that easy?

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