when do you stop adjusting shocks/bars and start adjusting sways?

[obrut]

Hi all,

I have a 69E race car running in a (tightly regulated) historic race series in Australia.

The basic car specs are:

* 2.0 litre MFI engine/901
* 1000kgs dry
* 15x6 wheels all round (can't go wider)
* 205/60 R tyres (can't go wider or lower profile)
* 23mm solid front T bars
* 31mm hollow rear T bars (= a 30mm solid ??)
* 15mm front sway bar
* 15mm rear sway bar
* Shocks Koni adjustable

The shocks have recently been rebuilt to suit the rest of the suspension by a Koni shop. Reinstalled and run on full soft both ends as a starting point with terrible results (also had some braking issues that weekend).

Next round we set the rear shocks to halfway between soft and hard, left the fronts full soft. This made a HUGE improvement. (Brake issues sorted as well). Car was easier to drive and got faster and faster all weekend with a final full 2 sec improvement (over a 1.14 lap).

The car feels pretty well balanced front to rear and doesn't seem to have any strong under/over steer tendencies. It is still a 911 though and does have a little slow speed understeer and some high speed oversteer can be provoked via throttle.

However, it leans more than I would like which can lead to some instability when transitioning between left and right etc. A suggestion from the pits from a long time 911 racer was he thought I should stiffen the shocks both front and rear by the same amount.

So, what is a good next round of adjustment?

a. stiffen rear shocks a few more turns
b. stiffen front and rear shocks
c. go up a sway bar size?

etc etc

Would appreciate any guidance you can offer. Side question, when do you stop tuning with shocks and move to sway bars?

[Flieger]

Dampers are felt most in transitions- chicanes.

Since you say you need more low-speed oversteer and more high speed understeer, it sounds like you would want to stiffent the front anti-roll bar. If the total body roll is too much, stiffen both but stiffen the front more than the rear. You want to shift the roll stiffness distribution forward which should accomplish both of your handling goals.

You may need to slightly alter the damper settings after the anti-sway bar adjustment, depending on how the car feels to you. Since you are increasing your effective spring rate when rolling in a turn, that means you will be a little under-damped in those situations compared to what you were before. If the car feels bouncy and does not settle down fast enough after a chicane, then stiffen both dampers equally. The car may feel fine without adjustments, though.

[esotoracing]

IMO and experience as a set up guy, shocks are for dampening (controling the up and down movement of the suspension, not stiffening or act like a spring. If it was my vehicle, i would go bigger sway bar or if the are adjustable, go to a stiffer setting.

Also forgot to mention what air pressures your running. . .that is important as far as fine tuning and finding those precious tenths & comfort level.

[obrut]

Thank guys

Tyres are D01J's running about 33/34 psi hot, although I think I will also try 31/32 hot at the next round.

Suspension wise we're trying to do one change at a time and will try stiffer rear shocks (which I can readily undo if its not an improvement) and I will also try and get hold of a bigger rear sway to try as well.

[Flieger]

I would stick to your plan of only changing one thing at a time. I would try the anti-sway bar first. Can you adjust the bar stiffer?

[rfloz]

Originally Posted by esotoracing

Also forgot to mention what air pressures your running. . .that is important as far as fine tuning and finding those precious tenths & comfort level.

Yes, indeed. Dropping the front tire pressure from 28 to 24 (R tires for auto-cross) cured my low-speed understeer issues.

[zioo]

Effectively, the softer tire pressures also lowered the front end a bit. A mm or two of lowering the front end might help and allow 2 lbs. per square inch back into the tires.-zioo

[rfloz]

Originally Posted by zioo

Effectively, the softer tire pressures also lowered the front end a bit. A mm or two of lowering the front end might help and allow 2 lbs. per square inch back into the tires.-zioo

Well, I can't speak for the OP, but my car is low enough that I don't dare attack my quite normal driveway straight on and I can high center on parking lot humps. So, I think I'll stop there.

I was just trying to point out that a relatively small (and free) change can have significant results. My car was no fun at all in low speed corners and now is much better. Likewise, if he already has adjustable anti-sway bars, try adjusting them. You can always go back.

[RaceProEngineer]

Originally Posted by obrut

However, it leans more than I would like . . . . So, what is a good next round of adjustment?

a. stiffen rear shocks a few more turns
b. stiffen front and rear shocks
c. go up a sway bar size?

. . . . . Side question, when do you stop tuning with shocks and move to sway bars?

Adding to, or elaborating upon, what the others have advised: Chassis pitch and roll are controlled PRIMARILY with the springs and anti-roll bars. You have already done the hard part, by analyzing what the car needs. Now, just apply the obvious solution.

While it might be EASIER to adjust (adjustable) dampers than "fixed" anti-roll bars, that is not always the correct solution. We do all "coarse tuning" with springs, then anti-roll bars. In the absence of aero adjustments, we use the dampers as a "fine tuning" mechanism.

Ed

[Randy W]

Originally Posted by RaceProEngineer

While it might be EASIER to adjust (adjustable) dampers than "fixed" anti-roll bars, that is not always the correct solution. We do all "coarse tuning" with springs, then anti-roll bars. In the absence of aero adjustments, we use the dampers as a "fine tuning" mechanism.

Ed

This is the order in which I understand suspension to be best adjusted as well. Springs chosen for primary purpose of car (street, street/track, track) or a particular race circuit. Then sway/anti roll bars and tire pressures (most easily adjusted on on a skid pad). Shocks adjusted for the layout and condition of circuit, as well as track surface conditions. Rainy weather will also require softer suspension settings and tire pressures.

In answer to the original question, I would stiffen both front and rear bars.

[obrut]

Thanks Randy/Ed,

I agree entirely. I have sourced a 18mm rear bar (roughly twice the stiffness) for the next round but suspect I will end up going bigger both ends.

The "one change at a time" approach is a little frustrating but I think it will ultimately prove a good idea.

[911st]

The fastest narrow body I know ran 22/28 torsion bars and only a front 22mm sway (no rear sway bar on the track).

I suspect you are going to be fastest with a front stiff set up. Other than being front stiff you are probably not going to find the car responds much to fine tuning of the sway bars. In my opinion you will be fastest in a corner with three wheels on the track but with the inside front wheel not to far off the track. Any time the inside front tire is working it will be taking weight and traction off the inside rear tire.

Thus, in the short run I might try putting a 22 or 23mm adjustable sway bar on the front and play with it. I would probably leave the 15mm on the rear.

Just like not trying to use shocks as a substitute to stiffen your spring rate, I would not use sways for much other than balancing the car. If it needs to be stiffer there are bigger torsion bars.

Stiffer torsion bars will not only reduce your lean but also dive under braking.

Going stiffer would help some in wheel to wheel racing where quick maneuvers are important but probably will not make you any faster in a corner.

Your biggest challenge as to spring rates is your tall tires. At some point the tires are going to have more travel than your suspension and it is undampened spring rate to boot.

Shocks can effect initial turn response but are best used to dampen the tires over undulations on the track and keep the tread in contact with the track as much as possible. I suspect your shock adjustment is just for rebound so you do not have a great amount of control over your shocks.

Just my opinion, I am not a suspention expert.