I am trying to get my car as low as possible. For minimal weight transfer, but without screwing up something else.

I May be there now. BUT by no means is my combination optimal.

The car measures shop floor to fenderwell lip between 26 and 26 3/8 inches on all 4. I have not done the measurments on a dead level surface yet.

I am concerned about maintaining some suspension travel. Now there is very little. I have no noticible bump steer, but have not measured it with a dial gauge. I checked it with pulmb bob and Chalk Lines at drop and compression within the expected travel limits. I do not have any tie rod adjustment for bump steer.

Rear trailing arms are set to make the car push on compression, ( ie. outside trailing arm will get shorter) so as not to get loose from rear steer.

The car is almost on the bumpstops now and may hit on compression. BUT, this has not caused problems.

Alignment
2 5/8 Camber.
5.5 Caster
Zero Toe.

The ball joints and the arms are stock. Front Springs are 9 X 5.5 Afcoils with adjustable space. rears are 6 X 5 afcoils with adjustable spacer.

Bushings are Del a Lums front a arms. rear control arms are heims. Panhard Bar has been lowered apromimatly 4 inch and uses heims and is adjustable.

Shock and Struts are Koni Yellow.

Front Sway Bar is the G92 / 1LE
Rear Sway bar is the smallest I could find. Anything bigger and the car will swap ends.

Tires TOYO RA1 275/45/17
Operating Pressure 39 lbs hot
Wheels Vintage Wheel 9.5 X17

Engine Stock w/ t5
Weight 3464.

To me car handles exceptionally well. Mario Andretti may have another opinion. I dont have a skidpad. Or one of those G toys so I can not hypothise a current G reading.

Any comments or suggestions for improved performance or better suspension geometry are welcome and would be highly appreciated.

Thanks.

 

You have everything right and have obviously done your homework.

Not knowing your stiffness (spring rate) from not driving the car myself it is impossible to give and better setup than what you are trying. Ride height is based on A-arm geometry and suspension travel as you have figured and I suspect that your V8 car still has adquate travel to invert the A-arm geometry at full compression travel with your rates. So unless you alter your A-arm mount points, you are about where it should be with the weight and travel range.

I pesonally sit quite a bit lower on the nose at 24 7/8"- 25 1/8" and at 26" in the rear. Yet I run very stiff springs with a normal travel range of 1" on everyday driving (most cars are 2 1/2" - 3")So even thought I do start out just slightly inverted A-arm angle, my final compression max is about the same as others due to the lighter front end I have. And I don't get the massive brake dive the V8's get. I also run the same caster (5/ 5.5) but have bumpsteer extenders to help eliminate my already worse starting position of the spindle.

I run much less camber though (well I normally do) I jsut ran it down finally to have the frontend reset this morning after installing my brake setup and bushings about 1 month ago. I wanted everything to settle in and the brakes to fully season in assurring no engineering problems before I had the alignment reset (just in case I had to make any changes. Anyways, I really didn't realise I was that far off but the initail setting was -2.2/-2.3.and the toe was 1/32out. I set it back to the .8 and 1/8in (final specs were -.80/-.76 cam and 1/8" toe in.
It was nice to see I have massive potential neg camber if wanted in the future, it would go to -3 easy on both sides.

Back to subject, The roll centers look great for a V8 (you say 4" down and I assume thats the axle mount which gives you a level 2" roll center drop.) If your springs are stiff enough, I would suggest trying to take all 4 corners down another 1/2" and add bumpsteer adjusters without touching anything else. That will effectively slightly raise the rear to from axis of inclination on the roll axis, but its low enough now (much more than stock that the minor raise may not hurt, including the a-arm angle, and the panhard angle if again the spring rates are on the stiffer side to limit travel.)

From what I've read you post though, you for the most part sound like you don't really need advice and know enough that you hae to jsut drive it to tell, and then make the apropriate tweeking to fine tune it.

The only thing from here is how muh money do you want to spend lightening your unsprung weight and reducing you polar weight via removing and relocating various crap.

Great job.
Dean

 

Thanks,

Panhard bar sits level as mounting point on chassis has been moved south also.

Spring rates are...
front 900
rear 300

By roll center inclination I assume that you mean the rear roll center is higher than the front.

What are the merits of changing this inclination?

Thanks

 

Thats is what I mean on inclination.
The stock roll axis is not level (lower in front to start with on a stock height car) so when you lower the car, the front goes down at a faster rate then the rear per same lowering measurement of ride height.

I don't have a roll center model to be specific, but to give an example, say you lower the front 1" the front roll center may go down 1"- 1 1/4".
Lower the rear ride height 1" and the roll center goes down exactly half that equaling 1/2"

If you have already dropped the rear roll center 2" from adjustment, Plus the aprox 1" it went down by lowering the car itself, that puts the rear roll center down about 3" from stock. 300 lb springs are great to control the new roll force leverage especially when running a very light bar. That is on the very stiff side though hence why the car has loose tendencies with a larger bar. Most run a 225 at highest but would use a slightly larger bar like a 19mm-22mm.

Now the front being lowered about 1 1/4" would lower the front roll center I am aproximating about 2" (there is a progressive increase that is hard to calculate without a computer model of the front suspension points). This would set the roll axis I would think about level with the rear down 3 and the front down 2 from stock.

Now if you tried to lower the front & rear just a tad more(1/2"),The front would go to about 3 1/2" and the rear to about 3 1/2" putting it back to aboout stock roll axis inclination at most.
Thats why I stated I would try it and see what the results are when driving it because every little bit you can get the center gravity down, then every little bit the car will corner better if you have the adjustment to fix the geometry. Since you have more adjustment to go in the rear roll center, you could then try and drop the rear roll center another 1/2" or 1"( I would go 1" since you have very heavy rate rear springs) You could alway make your rear bar adjustable with a kit to then fine tune the roll stiffness so you can lay down the power.

 

Thanks again,

I will go read my books and try to digest this information.

Do you have any thoughts on a decoupled torque arm? This may be my next experiment.

 

This is the ultimate chassis book. It's dry reading but if it's not in this book, it's not required to know. Everything you could possibly want to know about chassis design. What does what and why. Mostly geared for drag racing but the theory will work for all sorts of racing.

http://speedtalk.com/doorslammers_drag_chassis.html

 

I know the 4th gen cars experience wheelhop under hard braking. thus why they have tried to develop one. Its a non-issue for me with how my car handles and feels under throttle or braking.

If you want to try and build one, I would recommend trying to incorporate a torque absorber in the decoupling side of the arm so there is always tension under the arm under braking. This way it does not "freewheel" so to speak when going from throttle to brake, or brake to throttle. I would not like that uncontacted no mans land to bounce off the stops on either end over bumps while braking and slam engage on uneven surfaces while trialbraking. A torque absorber would allow the arm to drop angle progressively and return under tension as brake pressure is lessened or completely released making an automatic return instantaniously without throttle slam jamming it back.You would not need a braking decouipled snubber, the spring tension could be adjusted for the limit of decouple. Even under uneven pavement acceleration, A non-tension Tqarm could bobble the nose of the decoupler and bang it against the lock snubber causing acceleration loss of traction.