Roll couple and rear tire spin Options

[yeahmag]

Just got through my first autocross after the rebuild and the car was great! With a good friend and hot-shoe behind the wheel the car was the 2nd fastest aircooled car and #7 PAX adjusted in a pretty big field (PCA OCR).

I'd like to get better control of the rear, inner tire. We were spinning it up quite a bit. I'm thinking more rear spring is the correct approach. Current set up:

Front:
21mm torsion bars
19mm sway bar set 3/4 hard
Rebel Racing Bushings
Monoball camber plates
19mm raised spindles
Bilstein Sports
2.5 degrees camber
5 degrees caster
1/16" toe out

Rear
225lb springs
Rebel Racing Bushings
No rear bar
Koni Yellows (full soft)
2.0 degrees camber
1/16" toe in

Tires are fairly used up Hankook C71 Z214 in a square set up (205/50-15) run at about 22lbs hot.

Granted replacing the tires would help, but even when they were new it would spin these up. I have 275lb springs sitting around that I can swap in. Am I heading in the right direction?

Me

https://www.youtube.com/watch?v=hdMUYqusga4


Evan (damn he's fast)

https://www.youtube.com/watch?v=mI6O_gVzPuA

[Krieger]

I think your going in the wrong direction. Try 180#, 200 might be too stiff. For an experiment, since you already have the 275# springs, you could give them a try and see how much less traction you have. Whatever you do, make sure you do not use an impact wrench to disassemble the rear springs.

[yeahmag]

The idea is to keep the wheel with the weight on it from compressing too far. This works opposite of a roll bar that will tend to pick up the inside wheel.

There is some fairly good discussions on this and my guess is it really only works if you don't have an under/over steer problem (and likely to only work if excluding the roll bar from the equation - which I don't run on the rear).

The car is fairly neutral and rotates ok, but feels "wallowy" in the rear.

One big change is that I'm now Rebel Racing Teflon bushings in the rear. I've heard rumors going from poly/delrin to a better bushing (Rebel/Elephant) you need to increase your spring rate to counteract the lack of drag from the lesser bushings.

[ChrisFoley]

Tighten the front bar and try that before making any other changes.

[stownsen914]

I agree with your thought to use heavier rear springs instead of the rear swaybar. Try Chris's suggestion. Also, I see you mention having the rear Koni's on full soft. I'm trying to reason in my head whether additional rebound damping (that is what is adjustable on the Koni's) would help or hurt wheel spin. You mention that the rear feels wallowy. Adding rebound damping will help alleviate this. It may be worth trying since it's easy to try.

The sure fire fixes would be wider tires and/or an LSD of course (IMG:style_emoticons/default/smile.gif)

Scott

[yeahmag]

Chris, the bar is set to 3/4 hard, so I only have "1 spot" to go. I'll have to see if I can even move it to that hole. If I remember right, the crappy bar I have has "RSR Like" adjustments and I seem to remember that last hole being a bit difficult to get the heim joint in. That being said, I'm building my own end links from scratch and that may help...

Scott, if I can't do Chris' recommendation due to my crappy bar hardware, I'll toss those 275lb springs in for the next event.

Now to go sell some more of my stash so I can buy tires...

[ChrisFoley]

Stiffer rear springs isn't going to help the rear end stick better in my opinion.

Do you have adjustable shock absorbers? If so, soften the rear shock setting.
Edit: never mind. I just read that your rear shocks are full soft already.

Then drop the rear spring rate to 200 if you can't stiffen the front bar further.

[yeahmag]

Yup. Full soft (Koni Yellow). I'm not really having what I would call a traditional traction problem. I'm unweighting the inside wheel, which is why I think a heavier spring (with no rear bar) in an autocross might be worth trying.

[ChrisFoley]

The idea is to keep the wheel with the weight on it from compressing too far.

That would just hold the unloaded wheel further from the pavement.
What you need is to stiffen the front relative to the rear.

[brant]

I'm also wondering if you have a cage in this car?

they do flex a lot, twist and squirm

[stownsen914]

Sorry if I missed it, but what kind of horsepower are you putting down? Depending on how much you have and how aggressively you drive, this may be a challenging problem. 914s have a high roll center in the back, which promotes jacking in the turns - exactly what you're seeing.

[sixaddict]

Once Chris speaks ---I listen !

[yeahmag]

(Let me know if you get sick of talking about this...)

Chris, That seems counterintuitive to me. The heavier springs (8" coilovers) can't hold the wheel up. A sway bar certainly can and would (on the same axle). The spring can ONLY push DOWN (or hold the car up depending on how you want to discuss it). Is the idea to use the diagonal to hold the opposite corner (on diagonal) down?

Brant, It has has a Mad Dog stiffening kit and some suspension stiffening - no cage.

Scott, probably only around 130HP, but I've not dynoed it.

[ChrisFoley]

Is the idea to use the diagonal to hold the opposite corner (on diagonal) down?

Yes.
914s are inherently rear grip limited so doing anything which takes away grip at the rear is usually a bad choice. Stiffer springs take away grip.

If you can't increase your anti-sway bar stiffness, can you install 22mm front torsion bars? IMO, 225lb rear springs are not that well matched with 21mm fronts anyway.

[yeahmag]

Chris, I'll see if I can "get" that last adjustment on the roll bar. I'll also see if I can hunt down stiffer front torsion bars. Right or wrong, I'm one to want to mess with spring rate and just fine tune with the front bar (vs. using it as a major component of the suspension).

Do you have a preferred set up for an aggressive autocross car?

[brilliantrot]

I thought Aaron's car felt pretty darn good and i'm not sure I would change anything without putting on some new A7s first as I suspect they would cure any and all complaints with grip.

With the low friction suspension mounts in there, removing all that stiction does free up the suspension a little to the point of needing stiffer springs then I think many have used in decades past. Other then lighting up an inside tire a little too much, it was incredibly balanced and I would be hesitant to upset that too much.

For what it's worth I ran 200lb rear springs on my old 914 with the elephant poly bronze trailing arm bushings and always considered going up to 225 because the rear felt pretty soft with some roll oversteer but I don't remember having inside wheel spin issues as much. I also ran pretty fresh tires on the car all the time so it's hard to say. Maybe tender springs are the answer to keep some down pressure on the inside wheel under droop? Not sure we were getting enough roll to unseat the springs but I always wanted to add some to my 914 to try it but got lured to the dark side of 911s before that ever happened.

If I had driven a 914 at all in the last 9 years or not let 4 years lapsed since doing any real performance driving, the car would have had fastest aircooled on some really old, not exactly spectacular tires even when they were new so I think the car works pretty darn good as is.

Evan Fullerton

[ChrisFoley]

...
Do you have a preferred set up for an aggressive autocross car?

Not really. I've always worked with what was available at the time, trying to get the spring rates close enough that the front anti-sway bar could be adjusted to get the balance close.

In the case of your car, I would prefer either 1mm bigger front torsion bars or 25lb smaller rear springs. I would also prefer a larger anti-sway bar.

Ideally the anti-sway bar adjustment range would be enough to induce oversteer, or understeer at the extremes.

[Cracker]

I haven't seen this...LSD or TBD in the car now? If neither, sell some more stuff and invest...

Tony

[ChrisFoley]

I haven't seen this...LSD or TBD in the car now? If neither, sell some more stuff and invest...

Tony

Not much point in spending on a limited slip with only 130 hp.
Seems like a small adjustment to the balance is all that's desired based on comments from Aaron and Evan.

[Cracker]

One of my race cars has 80 ft lbs torque and has an LSD...works brilliantly! (IMG:style_emoticons/default/poke.gif)