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PostPosted: Wed Apr 18, 2018 11:38 am 
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Hello again, I inquired here a few months ago and found the responses very helpful. I thought I would post again for some more advise. I have a 1978 Civic I am converting from a thrashed old race car to something more suitable for vintage. The car was originally run in gt3 in the mid 80's and I bought it off the internet from Canada about 12 years ago. Its a tub based car so I think its more like a production car than a GT class. Once it arrived I quickly found it was mostly worn out and unsafe. So the car was completely torn down and a lot of the brake parts and suspension parts went to the trash can. Its been mostly sitting for many years and recently I have been spending some time to move the project forward.

I have the chance to redo the suspension geometry. The original pickup points were altered but in the end it was not real well thought out. The front control arms were three inches shorter than stock due to a poor knuckle choice, the rack and pinion was welded into the rear cross member and impossible to service. Right now a lot of the original suspension is very rusty (Canadian car) so I have to build new parts anyway. I do have Mitchell's wingeo3 software that I purchased about the same time as the car and have figured out how to make it run on my newer computer. I was wondering if there was a certain range of roll center height I should be shooting for? I have read a lot on the internet and there are so many conflicting answers, I figured I would ask here. I want to keep the suspension style correct as the car was built in the 80's, so no double a-arm conversion etc. I do think relocating the inner mounting points upward again is a good idea as the camber gain is actually negative if the factory mounts are used at the current ride height.


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PostPosted: Wed Apr 18, 2018 12:14 pm 
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I don't have a number but you don't want it very low. One oft cited rule of thumb is to keep the lower a arm (ie the line from ball joint center to a arm pivot roughly horizontal. When I first got my VW I had no Mac strut / fwd experience so I dropped the nose down quite a bit. Looked good but handled horribly until I brought the nose back up.


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PostPosted: Wed Apr 18, 2018 12:53 pm 
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Raising the inboard control arm mounts helps.
Dropping the outboard ball joint has the same effect, and is generally easier.
Put the longest drop pin you can fit inside the wheel setup, and see where that puts the control arm angle.

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PostPosted: Wed Apr 18, 2018 9:13 pm 
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Thank you for the tips!


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PostPosted: Thu Apr 19, 2018 9:23 am 
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Also consider what happens to bump steer - keeping the arm (or lower inner pickup to outer lower pickup) horizontal at rest allows the lower outer pickup to travel in roughly the same arc in bump/rebound. If the arm is below/above horizontal at rest, as the body rolls in a turn the wheels are steering differently as the respective arms are going through different portions of the arc (one knuckle could be pushed outwards and one being pulled inwards) and bump steer changes radically.


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PostPosted: Thu Apr 19, 2018 9:52 am 
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On that note, it's worth mentioning that *most* cars have very little bumpsteer from factory, so if you relocate lower ball joint by 2", then you should in theory relocate the outer tie rod by roughly 2" in same direction.

I made that mistake on an old McStrut street/track car, and the steering was pretty wild. fortunately, the body roll would 'exaggerate' the steering, so if I was turning to the left and the body leaned right, the left side would toe out and right side would toe in as the body rolled. The only way to be 'fast' in that car was to throw the car into the corner and get all of the body roll done at turn in. otherwise if you tried to use slow hands and roll the car into the corner, it would just keep turning tighter and tighter on you as the body rolled and took a set. crazy stuff, and very NOT confidence inspiring!!

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PostPosted: Fri Apr 20, 2018 5:20 am 
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Thank you all for the advice.


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PostPosted: Mon Apr 23, 2018 9:32 am 
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I spent the last week off and on looking through forums with not many specifics. I found this on a DSR forum and it made sense, so I thought I would post it here incase someone else starts looking in the future.

Quote:
I have a few comments on this issue.

1. The term roll center is a misnomer in that the vehicle does not necessarily roll about a geometric position that is defined by the geometry of the suspension.

2. The commonly accepted geometric roll center as defined by the control arm geometry is a very reasonable way to estimate the weight transfer characteristics of the suspension in question. Is it extremely accurate? I think that it is very reasonable approximation. I have experience, through extensive laboratory forced based measurements and additional track testing, that really indicated that it is a very good approximation of weight transfer through the suspension points. Forced based measurements that I have conducted have always indicated that the "force based roll center" is ALWAYS higher than the "geometric based roll center"

3. I think that we all understand that higher roll centers transfer more weight through the geometry than do lower roll centers. This is certainly true & what is equally important are the actual & real jacking forces associated with these higher roll centers. The thing to remember is that once the roll center is below a reasonable level, that is best related to track width & COG height, then jacking forces become negligible & other factors are far more important

4. Does asymmetrical suspension geometry such as used in oval track race cars affect the weight transfer? ABSOLUTELY it does. I can guarantee you that the transfer forces do change due to asymmetry. Then remember that EVERY car has asymmetrical suspension once it hits the ground & starts moving around a race track.

5. Does "geometric lateral roll center migration" affect weight transfer? Yes it can, but ONLY when the roll center is fairly high above the ground in the 1st place. Think of it this way. "Geometric roll center migration" can be huge when the geometric roll center is very close to ground level. This is simply due to the fact that the geometric lines are very close to parallel to the ground & the intersections move all over the place. This is essentially meaningless.

6. Here are my rules:
a. define your tires. This means that you need to understand what are the real optimal camber / lateral load / slip angle characteristics are. This does not mean that you need a full blown tire model. Just find out what the most commonly used tires are run at & get some idea of what the real world needs & work. Get the tire data if it is available then try to understand it.
b. define what your optimal track width needs to be. Sounds simple but is much more complex than 1st looks might indicate.
c. define your static camber and approx. camber gain
d. define where you want to have your roll center height. Limit the lateral migration if possible (& it usually is possible).
e. think about all the other variables all the time. Pay attention to caster, mechanical trail, scrub radius, anti-dive/squat etc.
f. Do not go crazy obsessing about the details. Make sure that you have enough adjustment built into the design then build it & go testing, then test some more. Do not be afraid to make changes. PAY ATTENTION TO TIRE TEMPS every single session the track, record everything you can think of.

enough Happy Time. Build it & go racing.

Thanks ... Jay Novak


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PostPosted: Wed Apr 25, 2018 6:24 am 
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Lowering the outer pivot is the easiest way to have some dynamic camber gain and keep the Roll center . You still have to move the rack or the tie rod joint to reduce bump steer. Just worry about the bump at suspension slightly compressed , (The turn in angle).
The most important areas of concern IMHO, designing the A arms/balljoints, so that you have at least 1.5 degrees dynamic camber mid turn on your most important corner / surface.
Take pictures of your car working . Take tire temps .
Be carefull adding castor, watch the bumpsteer.

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Mike Ogren/Protech Racing, http://www.FWDracingguide.com http://www.ogren-engineering.com/ 352.428-8983 mogren@tampabay.rr.com


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PostPosted: Wed Apr 25, 2018 9:10 pm 
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Location: Portland, OR
Quote:
I have a few comments on this issue.

1. The term roll center is a misnomer in that the vehicle does not necessarily roll about a geometric position that is defined by the geometry of the suspension.

2. The commonly accepted geometric roll center as defined by the control arm geometry is a very reasonable way to estimate the weight transfer characteristics of the suspension in question. Is it extremely accurate? I think that it is very reasonable approximation. I have experience, through extensive laboratory forced based measurements and additional track testing, that really indicated that it is a very good approximation of weight transfer through the suspension points. Forced based measurements that I have conducted have always indicated that the "force based roll center" is ALWAYS higher than the "geometric based roll center"
]


1. Roll center is an form of instant center. It’s only good for that instance. As soon as the vehicle rolls there is a new roll center. So, no, it doesn’t roll around the roll center for all of the roll angle.

2. The geometric roll center isn’t inaccurate, it is just effected by the compliance’s of all the chassis and suspension parts. This is why the force based measurements vary. Force based measurements will vary as well depending on how the forces and boundary conditions are applied. Force based roll centers can be lower than geometric. Hotchkiss suspensions (without panhard) and possibly the twist beam suspensions can have force based roll centers at the axle level as the axle is the only thing that truly transfers lateral force across the car without roll.

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