Suspension Design

I have been looking at the way various manufacturers set up their suspensions and have been trying to figure out why I am having so much problems with my front suspension.
I have copied the corvette C5 suspension geometry exactly but this makes the front of the lower control arms very low (2" below bottom of the pan). When I look at the way every other replica is set up, the control arms appear to be almost perfectly parallel to the ground. The C5 both upper and lower control arms are angled away from each other at the front by about 2" each from highest to lowest points.
What is the purpose of the arms being at that angle? I always thought it was anti-dive but if no one else has it, how do you deal with anti-dive. If I decrease that angle at the front it will make me alot more comfortable but I also want the handling to be the best it can be.
If I just raise the whole front suspension, I run into fitment issues.
Any one have any thoughts? Changeing to the older style suspension is one I have considered but dont really like the idea. I have read some books and looked on the net and even have an analyzing program that I downloaded a demo for but trying to determine real world effect is hard.
thanks for any help

""I have copied the corvette C5 suspension geometry exactly but this makes the front of the lower control arms very low (2" below bottom of the pan).""

If it's outboard this is ok because when the wheel bumps up it bumps up also, if it's inboard this is a clearance problem.

""When I look at the way every other replica is set up, the control arms appear to be almost perfectly parallel to the ground. The C5 both upper and lower control arms are angled away from each other at the front by about 2" each from highest to lowest points.
What is the purpose of the arms being at that angle? I always thought it was anti-dive but if no one else has it, how do you deal with anti-dive.""

It's anti-dive in front and anti-squat in rear. Sometimes the correct amount is not discernable by eye and may seem to be flat, you have to measure. 2" seems like too much, and is probably for the front heavy Vette.Some of the replicas have no anti dive/squat although it is desirable.

""If I decrease that angle at the front it will make me alot more comfortable but I also want the handling to be the best it can be.""
If I just raise the whole front suspension, I run into fitment issues. Any one have any thoughts? ""

Suspension is designed from the wheel/tire inward, the later Vettes have a lot of outer offset on the wheels, look at the outer rim it's flat faced with no lip. Replica Halibrands and BRM's have more inner offset which causes problems when used with Vette suspension, mostly scrub radius issues.

Vette's don't have optimum geometry until the most recent generation, one of the biggest faults being too much king pin inclination which is fixed unless you use a different upright.

Ask yourself what are you building the car for, all out competition or just having fun, you may have to compromise somewhat with Vette pieces.

""Changing to the older style suspension is one I have considered but dont really like the idea.""

Agree, they have worse geometry.

""I have read some books and looked on the net and even have an analyzing program that I downloaded a demo for but trying to determine real world effect is hard. thanks for any help""

I always thought it was easier to do cardboard cutouts or just drawings, but computer is better just a steeper learning curve. The "to win" series by Carroll Smith is good, or "Chassis Engineering" by Adams. If you analyze to every small detail it can get quite complex. I always try to hit the important stuff like Ackerman, bump steer, camber gain, scrub radius and squat/dive.

Since my suspension is on the ground right now, here are the measurements of the DRB setup. These are of the drivers(read left) side.







Here is the bottom right.





Bottom left




It is set the same way as the stock 87 Vette. The lower arms are rather big and you would have to add shims to line it up as needed. The ride height is set with the coilovers and spring settings. Unless you are really into the engineering of the front suspension, the stock setup is a good way to go. Can't explain the offset. The arms swivel through the ball joint and the upright rotates on it's axis and does not vary no matter what angle the upper arm is at. I'm sure the guys at Mid America could help you out. Used to have a number for them. It is probably as you said for anti dive. Some more knowledgable than I would have to chime in.
Bill

My upper arms are similar in the angle you have showing there, I cant tell for sure, but the mounts for the lower arms look parallel to the ground, is this correct? I have played with different positioning in an analyzer program and have basicallly found that if I lower the rear mount to the same height as the front one then raise the whole front end to the bottom of the pan, I go from 57% antidive to 25% with not alot of changes elsewhere. Thanks to Kalun pointing out the obvious weight distribution and amount difference which I completely overlooked, I am happy with that. My only concern is whether or not there is an issue of the balljoint not being 100% perpendicular to the control arm. It doesnt seem to cause any binding as far as I can tell, and I suspect it will work. If anyone else has any thoughts, I would be glad to hear them.

Eric
Kalun makes a great point when he says stick to the basics, that anti-dive you have on that boltup which looks like the upper wishbone looks really excessive. But considering the front end weight and weight transfer on braking it was really necessary, and a compromise at best. Also keep in mind that if you set up that much anti dive in a chassis that doesen't require it that will also cause handling problems. The anti dive is meant to keep that front end as close to ride height as possible, and I think you need far less, that would simplify your mounting points.
Good luck
Phil

Yeah, the Vette is 3,500 #'s with weight bias toward the front.

Too much anti dive causes the suspension to lock up, rigid. 25% sounds good.

Ball joint sounds OK, it doesn't have to be perpendicular as long as it doesn't bind.

before final weld take the spring off the shock and run the supension from full bump to full droop and steer max to both sides and check all clearance/interferance.

When building a scratch frame always weld the suspension mounts last to avoid weld warp alignment problems.

The bottom mounts are parrallel to the ground. The upright will rotate though its axis at "any" tilt angle and even if the vertical axis is not 90 degrees(no shock hardware etc mounted). The DRB GT40 is designed for 15"/10" spring damper units with 1/2" diameter spherical ends. These allow for a front ride height of 90mm-110mm and rear 120mm-140mm. Front springs are 180# and rears are 200#.
Bill

Bill

I am looking for the measurements that you posted, don't seem to be able to open the pictures. Can you help?

Peter

Anti dive is built in by angling the upper a-arm mount where it mounts to the frame. The rear end of the cross shaft will be lower than the front. If the a-arms are not parallel to each other (when viewed from the front) that affects the roll center height. The inner ends should be closer together to raise the roll center height. If the a-arms are parallel with each other, and the ground, the roll center will be at ground level. Most performance suspensions will have the lower a-arm parallel with the ground and the upper a-arm will have some angle down on the inner end to raise the roll center.

peter jannings said:

Bill

I am looking for the measurements that you posted, don't seem to be able to open the pictures. Can you help?

Peter

Click to expand...

Bill I cannot open the pictures either??

Those are 'old' posts, is it a change of server problem.

Jac Mac

Probably so, I will see if I can find the pics elsewhere. If not you will have to wait til at least weeks end. All suspension pieces sould be at home by then(I hope), and will remeasure.

Bill

are you using the C5 uprights? or did you copy the geometry and try to translate it to a different set of components?

either way, the roll center for the corvette is nowhere near where you will want the roll center for a mid engine car (if it is so).

the CG and roll centers along with the roll couple is all wrong for mid engine. If you use the C5 geometry, it will never be right.

cheers
Eric

Guys, I am at the same point with my project, and i'm not sure for a mid engine layout weather the rear roll centre should be above or below the front, does any one know the answer, the front and rear lower arms angle up slightly.
I have scoured the net for the answer but no luck so far, i have come across a set of factory diablo manuals with drawings of the suspention, but its a lot of work to draw them up to scale ,to hopefully come up with an answer,any thoughts.

Best regards
Lambo

Lambo,

For most purposes the roll centre wants to be generally parallel to the roll axis which in a midengined car will usually mean that it is higher at the rear than at the front.

FWIW I'm running 13 mm at front 50 mm at rear with a 1.5m effective swing axle length. That's my choices for my application. Not everyone will agree on suspension requirements, so regard this as just an indicator for you. No doubt people more knowledgeable than me will add their bit.

Cheers

Russ

"For most purposes the roll centre wants to be generally parallel to the roll axis which in a midengined car will usually mean that it is higher at the rear than at the front "

Think about this statement Russ

Haha.. True Trevor.

I should have said parallel to the mass centroid. Thanks for picking up that blue.

Cheers

Eric D said:

are you using the C5 uprights? or did you copy the geometry and try to translate it to a different set of components?

either way, the roll center for the corvette is nowhere near where you will want the roll center for a mid engine car (if it is so).

the CG and roll centers along with the roll couple is all wrong for mid engine. If you use the C5 geometry, it will never be right.

cheers
Eric

Click to expand...

Eric,

Would you be kind enough to explain why the C5 geometry is all wrong? Also, what are the guidelines for roll centers for mid-engined cars vs. the C5.

Thank you,

Andy

Russ,
How about

" Parallel to the median gradient of the mass centroid "

but there exists differing schools of thought in this regard as to whether or not this is the ideal

Ok Trevor, I'll go with that.

I was about to rush off to work when I read your post and didn't have time to check with my reference books to get the correct nomenclature. The roll axis thing was just a moment of brain fade after an exhaustive day at work!

I enjoy your challenges. Keep it up.

Thanks