Suspension Tuning Question Options

[yeahmag]

At this past PCA event a fellow competitor was watching my car and mentioned that the front looks nice and planted, but the rear was wallowing around a bit. I'm considering adding the rear bar back on and upping the front anti-roll bar setting to balance the car out and settle the rear down. Is that the correct approach?

Tires: Kuhmo V710

Front:
21mm torsion
19mm sway - urethan bushings
Max caster/camber with stock plates
Stock rubber bushings in the rest of the front end
Turbo Tie Rods
Koni Sports (set full hard during autocross)

Rear
180lb springs
Full Urethane
Koni Sports (set full hard during autocross)
*NO* rear bar

-Aaron

[SirAndy]

Is that the correct approach?

Maybe ... (IMG:style_emoticons/default/biggrin.gif)

Yes if you have a limited slip.

No if you don't have a limited slip.


In my (limited) experience, adding a rear bar will help keeping the rear flat but also promote the inside wheel getting "light" in tight corners resulting in wheel-spin on cars without a limited slip.


If you don't have a limited slip, the rear body roll actually helps keeping the inner (light) wheel planted ...

(IMG:style_emoticons/default/popcorn[1].gif) Andy

PS: If you go with a limited slip and rear bar, it really helps getting the rear shock-towers tied into a full roll-cage to eliminate any body flex in that area.

[yeahmag]

Interesting... I don't have an LSD, but I've been feeling some lack of stability in the rear. Maybe it's time to step up to a higher spring rate back there.

-Aaron

[Borderline]

Before you go spending $$, try softening your shox. I run 22mm front Tbars with 225# rear springs. I run Koni race up front and Koni sports in back and leave them set at full soft. The stiff rear shocks may be making the car a little tail happy in the transitions. Softening the shocks will let the suspension work. Going to stiffer springs in back or adding the rear sway bar will tend to make the car oversteer even more, assuming what you're feeling is oversteer. FWIW (IMG:style_emoticons/default/biggrin.gif)

[pcar916]

(IMG:style_emoticons/default/agree.gif) I'd soften the front sway bar and play with tires pressures first. No rear bar until like Sir Andy said, you run an LSD and/or a full cage.

If the back is loose then soften the rear shocks and/or lower your rear tire pressures first. Then you can soften the front as well to promote turn-in.

Remember!!!!!

Do one thing at a time and drive it.

If your front is plenty stiff then soften the front sway bar. For AX the front should be far less stiff than a road-course or street car to promote turn-in. Here's what I run and it's NOT competitive (AX) unless I run loose on the front sway bar and dial softer on the rear shocks. Don't worry about the spring/torsion bar rates since I have a six.

Front
23mm torsion bars
22mm sway bar
Bilstien Sport shocks
Poly-bronze bushings
Big Brakes

Rear
200# adj. coil springs
Poly-bronze bushings
Koni-adj shocks

I have a rear sway bar I'll be puttin' in soon but the limited slip (clutch-type) doesn't make for a wonderful AX turn-in until the rear is looser and the front is run soft. For AX only a Torque-Bias'd diff is (usually) better since it goes "open" on decelleration.

Air pressure in the tires is also critical and the more stiff your car is the more critical it becomes. I can modulate a lot of over/understeer with tire pressures alone.

Good Luck! (IMG:style_emoticons/default/smilie_pokal.gif)

[yeahmag]

What started all this is that I had been running the car fairly loose:

Front:

Mid way on the Koni's
Front bar full soft

Rear:

Mid way on the Koni's
No rear bar

I ran the Koni's up to full hard and the car did everything better and I cut a second off my time (mid 60's average course) with no other changes.

[SirAndy]

I've been feeling some lack of stability in the rear.

Tighten the front sway bar OR drop the tire pressure in the rear OR soften the shocks in the rear OR tighten the shocks in the front.


As has been said above, one thing at a time ...
(IMG:style_emoticons/default/beerchug.gif) Andy