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Author Topic:   Moment Center/Roll Center
Rook78
Dirt Maniac

Total posts: 106
posted January 13, 2005 02:01 PM  
I have begun the quest...the search for knowledge and understanding of Roll Center. I have been reading, and now I am extremly dangerous...Here are some questions I have. Thanks in advance.

1)When running hoosier tires as opposed to Americn Racer, how would that affect where to posistion the roll center? Hoosiers having a taller, more flexiable sidewall than the AR tire will make a difference, correct?

2)I run a flat tight track and also a high bank wide track, do I need to adjust roll center from one to the other? I have no adjustments on my upper conrtrol arm mounts, are there ways to adjust without cutting and welding?

3)If I understand correctly the lower the angle in the upper control arm, the lower the roll center, and conversly, higher angle, higher roll center. Many cars in my area are designed with a lot of angle in the upper arm, why? Does this not mean they have a high roll center?

4)Can roll center be too low? What is desired height for 3/8mile tacky high bank and flat quarter mile kind of sandy/dusty?

5)Does it matter which side of center roll center falls on? If so, what is recomended.

I appreciate the time you guys, I plan to purchase a roll center program, but I need to understand some of the cause and effect scenarios involved with this topic and also want to confirm that I am comprehending what it is I am reading.

Thanks again
Rook

[This message has been edited by Rook78 (edited January 13, 2005).]

wfoondirt
Dirt Forum Champ
Total posts: 505
posted January 13, 2005 02:36 PM  
First off there is a pretty big debate in the performance industry about the effect that the geometric roll center actually has on an asymetrical car, there are some good white pages on the SAE website that expand on it.

1.) The difference in height of tire has little effect on the roll stiffness from the roll centers since it is based off a moment arm to the CGH. Basically a taller tire will raise the RC and the CG equal amounts so the moment arm stays the same. As far as sidewall stiffness that is a totally different animal and will only muddy a RC discussion.

2.) Other than changing the pivot points you will need to either change spindles or ball joints to change the roll center. As far as which direction you would want to change, it is totally dependant on what you want the car to do. Generally to acomplish the same about of body roll as the banking increases you will need to lower the RC.

3.) Before you go too much farther into RC's I would pick up a RC program or learn how find the roll centers manually. Most people find it easier to understand by being able to make changes and see the results on paper. To understand why someone would have a high roll center you first have to understand that there is no single perfect front end geometry, everything in suspension design requires a compromise. In the case you mention they may be compromising the high roll center for camber control.

4.)Yes it can be two low, if the roll center gets to be below ground there are alot of jacking forces that come into play that should be avoided. Again there is no perfect RC but typically modifieds are in the 3-6" range.

5.) The static lateral location is not as important as the RC migration in ride/roll.

Personally I recommend learning to draft the roll centers by hand first so you gain more understanding versus just plugging numbers into a computer. The computer programs are a great tool once you have an understanding of what it is actually doing.

Also if you really want a true understanding of these concepts I'd recommend getting a copy of Race Car Vehical Dynamics by Miliken/Miliken. It's definately not light reading but it gives you the basis for the concepts.

[This message has been edited by wfoondirt (edited January 13, 2005).]

NJantz
Dirt Forum Champ
Total posts: 755
posted January 14, 2005 06:26 AM  
1)When running hoosier tires as opposed to Americn Racer, how would that affect where to posistion the roll center? Hoosiers having a taller, more flexiable sidewall than the AR tire will make a difference, correct?

Tire height differences won't effect the static roll center, but it will factor into body roll changes which would have an effect on the dynamic roll center.

2)I run a flat tight track and also a high bank wide track, do I need to adjust roll center from one to the other? I have no adjustments on my upper conrtrol arm mounts, are there ways to adjust without cutting and welding?

Adjusting the roll center from one track to another could be a good tuning tool. For that flat track, you might move the static rc further left in the car so you create a larger lever arm which will make more dynamic roll.

In stead of cutting your top brackets, can you drill another hole in there to alter the a-arm angle?

Coleman sells a threaded ball joint that allows you to adjust your front roll center.
Slick little unit. I highly recommend these. The ones I bought I could alter my RC by about 2" if memory serves me right.

3)If I understand correctly the lower the angle in the upper control arm, the lower the roll center, and conversly, higher angle, higher roll center.

To answer that properly, what do you mean by lower the angle? If you raise the upper a-arm balljt height and or lower the inboard mounting points, then you have lowered your roll center height.

4)Can roll center be too low? What is desired height for 3/8mile tacky high bank and flat quarter mile kind of sandy/dusty?

Yes, it can be too high and too low. Every thing I've read said to put it 3-5" above ground on the dirt. Some people will tell you 4-6". My modified was at 3" below ground before I first started tinkering with it. I'll tell you that it didn't work!

5)Does it matter which side of center roll center falls on? If so, what is recomended.

Yep, the further left of centerline the longer the lever arm you generate which loads the RF more on entry. It will definitely pinch the RF down under the car the further left you go.

In my opinion the only way to do this roll center redesign is with some CAD program or with a scaled model. In the past I've made models out of PVC pipe and cardboard. That is the only way to really learn it. The RC software would be good after you have learned the basics.

Also, just so you understand that when you go making RC changes you also change everything else like toe, caster, camber. And as WFO mentioned these suspensions are all one big compromise. You have to pick and choose what number is important.

I don't know how experienced you are with front ends but make sure you have an advanced understanding of caster,camber, bumpsteer, and toe curves. You might even plot some graphs of that data.

Some other good sources are the Carroll Smith books. Engineer to Win, Tune to Win, and I can't remember the 3rd one.

Good luck, its a satisfying avenue that most people haven't explored on our level.



boas51
Dirt Maniac

Total posts: 190
posted January 14, 2005 06:39 AM  
i worry more about my camber curves, and secondly about rc. the rc will almost always fall into the 3-6" range if you work solely with correcting camber curves. the left upper can be tweeked afterwards to correct the rc left or right. generally the left upper will be mounted slightly higher than the right, on the frame.

WPP
Dirt Freak

Total posts: 451
posted January 14, 2005 07:09 AM  
If the rollcenter is left of center line should you run more front spring and if the rc is to right should you run a softer spring .I am talking about spring rate on the right front here.

Rook78
Dirt Maniac

Total posts: 106
posted January 14, 2005 07:45 AM  
Nate, thanks for your detailed reply.

"To answer that properly, what do you mean by lower the angle? If you raise the upper a-arm balljt height and or lower the inboard mounting points, then you have lowered your roll center height."

I will have to reread what I thought I understood regarding upper arm posistioning, I thought the greater the angle downward from the ball joint the higher the roll center. I currently have very little angle in my arms downward from b-joint to inboard mounting posistion. And if I understand you correctly, this would raise the RC? This being true, would explain why the car has the tendancy to be a little pushy?

"In my opinion the only way to do this roll center redesign is with some CAD program or with a scaled model. In the past I've made models out of PVC pipe and cardboard. That is the only way to really learn it. The RC software would be good after you have learned the basics."

I would like if possible to get some more detailed info on how you constructed this model. If possible, could you PM me or e-mail me or even fax me a diagram?

I appreciate the input from everyone, thanks.

Also Nate, good luck with the Late Model deal.


kevinwardracing
Dirt Full Roller

Total posts: 35
posted January 15, 2005 06:11 AM  
Good Luck...

[This message has been edited by kevinwardracing (edited February 17, 2005).]

Rook78
Dirt Maniac

Total posts: 106
posted January 17, 2005 03:49 PM  
Thanks Kevin, I am going to a swap meet at the end of the month and will be looking for these adjustable B-joints. What is the price tag normally for both the Colemans and the Howe? I dont see them in Speedways catalog or website.

I run the screw in upper ball joints with the adjustable A arms, do they come in that style?

FlyNLoIMCA17
Dirt Forum Champ
Total posts: 753
posted January 18, 2005 04:01 PM  
Yes, you can get the screw-in adjustable mono-ball upper ball joints. I seen some the other day, I can't remember where though. It might have been at www.afcoracing.com If not just search the web, they are out there. The one's I seen were $59.95 each.

FlyNLoIMCA17
Dirt Forum Champ
Total posts: 753
posted January 19, 2005 03:51 PM  
I checked on that and I was wrong. Afco doesn't have them. But Stock Car Products and A-1 Racing do. SCP's are $59.95 and A-1's are $58.05.

here's SCP's http://stockcarproducts.com/susp7.htm

A-1's website is www.a1racing.com
Their # is 1-800-669-8442

Hope that helps you!


happy
Dirt Full Roller

Total posts: 73
posted January 22, 2005 06:06 AM  
We ran into the same thing, the car was deigned pretty much to run the pucks and when we put the hoosiers on it we really struggled trying to get the car set. with the extra tire height when you try to set the ride height you really throw the angles off from what our base set up was suppose to be and in turn through the roll centers off.

WPP
Dirt Freak

Total posts: 451
posted January 22, 2005 06:46 AM  
If you set your ride heights set them with the lower control arm angles with the smaller tires and put the bigger tire and go back to what you had on angle with the smaller tires it should be close you just can not take ride height setting from the ground it will be off i went threw the same **** with mine i was takeing ride heights from the ground.

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