911 Understeer Issues at Auto-Cross (long)

[rfloz]

So, after all my fears about terminal oversteer, my 1984 Carrera has understeer issues at auto-cross events. Who’da thunk it?

Picture a longish right hand sweeper after a slow right turn. Accelerating hard in second gear as the rear end steps out just a bit; then, a gate followed by a short straight and then a hard left. Making that gate is the problem. Let off the gas too much and the oversteer becomes all too real. Obviously, hitting the brakes is out. Problem is, the front tires are not biting and the front of the car is drifting left. Holding the gas steady means going on the wrong side of the gate.

Or, think of a medium fast slalom – enter fast, brake pretty hard and start to thread through the cones trying to pass each cone at an angle (i.e., “late-apexing” each cone). Then, the front tires just won’t grip enough to make the transitions without slowing more than I would like. The feeling is that the front of the car is sort of floating. Transitions and turn-in are crucial in auto-cross and this is the car's area of weakness.

Or, enter a braking zone a little too hot and the front tires lock up, never the rears. Mintex street pads front and rear with fresh fluid and good brake lines. The car stops better on warm tires, but can still lock up the fronts.

Now, of course, all this can be cured by going slower. I know sometimes you have to slow down to go faster, but, this doesn’t seem like it is the issue. And, the idea, after all, is to go faster. Also, when riding in other 911s, I can feel them execute the above maneuvers without issue going at least as fast as I am going, some a decent amount faster, and braking as late or later.

So, what to do?

I’ve tried setting front toe to zero. This helps with corner entry, but has no perceivable effect on mid-corner understeer or brake lockup. Even so, a little toe out might help turn-in, which is still pretty touchy.

Someone suggested the car might be too light in the front. Now, we all know lighter is faster, right? But, putting the spare tire (all 35# of it) back in seemed to help; would filling the gas tank (another 70-100#) help more?

FWIW, the car currently has a weight distribution of 37.8%/62.2% f/r. Putting the spare tire back in changes that to 38.6%/61.4%. I plan on losing about 45# off the rear with bumper and exhaust modifications. That puts me at 39.2% front w/ the spare and 38.4% without it. Add 100# of gas and the front goes to 40.5%! But, with the spare that’s an extra 135# to lug around. (All figures w/ driver and 5 gallons of gas, unless otherwise noted.)

My rear anti-sway bar can be adjusted stiffer. I haven’t tried this yet as it tends to reduce rear grip when the preferred solution is to gain front grip. In the end, balance may trump grip, though. I’ll be trying this next season.

Apparent non-issues: Tire temps are good across the tread with inner and outer temps within 5 degrees of the center on both front and rear. So, camber (-2.6 degrees f/r) is good, no? The car has raised spindles and a bump steer kit so camber change is about as good as it can be.

The tires are fresh Kumho 710s in 245/315 sizes on 9” and 11 ½” wide wheels (it’s a Turbo-Look). I tried 255/295 A6 Hoosiers and the same problem existed. So, more front tire is no help. Nor does less rear tire seem to help.

Would less front tire help? That may sound strange, but one anomaly I noticed was that the front tires are consistently 20-25 degrees cooler than the rears. Yes, the rears do more work, but they are a fair amount bigger and the front temps are quite cool – 120-125 degrees after three to four 70 second runs spaced maybe 5 minutes apart on asphalt with good grip (El Toro and Cal Speedway in Fontana, for those familiar w/ SoCal venues).

I am thinking more heat in the front tires would be a good thing. I’ve heard from a Kumho engineer that 710s work best at 190-200 degrees, so there is certainly not much danger of over-heating. Would more (or less) air pressure help in the front? Currently I am at 27-28 psi cold in front, 31-32 rear. Pressures go up 2-4 psi f/r except on very hot days (100 degrees), when it may be 6-10 psi and then I bleed them down.

Any suggestions are welcome.

[petevb]

Alignment settings, springs, shocks, swaybars, tire pressures?

Transient behavior is controlled by shocks- what you're describing may be a shock mismatch- the rear stepping in transitions or having the front and rear feel out of sync or symptoms. How does the car feel going through the 2nd cone of a slalom vs the first?

Assuming there are no obvious issues with the above yes I believe putting some more weight up front will help with balance and likely improve times.

[J P Stein]

Possibilities:
I have found that 8 inch wide front (std 911 offset) wheels is the max I can go on my 914 (with essentially the same geometry as the 911). Push led me to this.
AX is not the track. "Stuff" happens when you crank in a lot of lock that track drivers never see....unless they're having a very bad day.

What kind of shocks are you running?
Do you have an adjustable front sway bar? If not, this is a very good addition.

I know a very good 911 AXer who throws it sideways with great abandon. Oversteer is not something to be feared at AX, but used. It takes some practice.
Straights should be made as long as you can & braking done in a straight line till then.

Transferring weight (necessary items) from the rear to the front is a good plan. Adding weight is never a good idea unless it's needed to make class minimum weight.

An adjustable proportional valve with bias tuned to your liking is a worthwhile addition.

As you can tell, I'm big on adjustability. Tuning a car for AX is an art....some would say black art.....but nothing that can't be learned/practiced....with adjustments.
I doubt the inherent challenges of the 911s handling can be tuned out but at least be brought under semi-control. Then it's up to the driver to deal with that pendulum of a back end.

[Steve W]

It sounds like we were at the same event as I remember each one of those turns you described and also had similar understeer all Sunday. But I attributed that to 8 y.o. SO3s I have in front vs 3 month old Direzzas in back. Without knowing much about your suspension, I might suggest softening your front bar if it's adjustable, and dialing 1/8" toe out to improve front end grip. As Pete asks, what are you running for shocks, t-bars, and what is your car's current weight? What was your best time that day?

Separately, I just installed those Mintex street reds with new rotors as you have and I've been getting uneven braking and wheel lockup issues. I've been giving them a chance thinking they need some break in but they don't have as much bite and stopping response as the old OE Textars I used to run so I am likely going to pull them out.

To add, as you know, it was a very cold day that weekend, and you may have needed drop your front pressures further to get them to heat up sufficiently.

[racer]

As someone else asked.. Torsion bar size? sway sizes?

Also, I think a 295-315 rear is complete overkill if your engine is stock (ie 204-217hp). I think part of the problem is too much rear will only make the car understeer. If you went closer to a stock spread (say 225/245 or 245/275) you might get better rotation.

I would also think that you want tire TEMPS similar front to rear. you want to be closer to the manufacturers temp range recommendations.

What tire pressures are you running at cold? hot?

[rfloz]

First, thanks for some thoughtful responses - lots to think about.

Now, as to the current setup:

Torsion bars – 22/29 hollow

Anti-sways – 22/21 (late Carrera) – rear only adjustable and only to stiffer from present setting

Shocks – Re-valved Bils, non-adjustable. These were done to match the current setup, but, after their former shock guru (sorry, the name escapes me) passed away. So, maybe they are not optimal. For now, I am stuck with them (hey, even Roger Penske has a budget). Maybe in a few months I could have them converted to adjustable.

Toe – front zero
rear 1/16” in

Camber - -2.6 degrees f/r

Caster - + 5.6 EDIT - It's + 5.8 degrees

Brakes – Stock Turbo calipers w/ Mintex street pads. It stops pretty well, but, with no ABS and the tendency to lockup the fronts, I tend to apply the brakes a car length or so earlier than other cars can get away with. I may try to find the Textars or some Porterfields. The Hawk HP+ I’ve used before, and liked, don’t seem to be available for this car.

Weight – currently 2550# + 200 (or so) for driver; soon 2500 and the driver needs to lose a few # too.

Engine – stock 3.2 U.S. Carrera – so, 217, maybe

EDIT/ADD - FWIW, the car also has Poly-bronze bushings in the front A-arm and rear spring plate. Allother bushings are new. Also, it has turbo tie-rods. ANd, the spindles are raised (33mm) and bump steer has been adjusted. Finally, the car does have the OEM LSD, 40% I believe. As to condition, all I can say is I have no rear wheel slip out of corners on hard acceleration. Of course, with a stock motor and 315 R tires, there might be no slip w/o the LSD.

Current plan for next season:

Cat bypass, M&K 1 in, 2 out – should take about 15# off rear and add a couple RWHP

Fiberglass rear bumper w/o ugly black things – maybe 30# off rear

A SteveW chip – a few more RWHP (Wish I’d have realized you were at the last event, I want to get a chip to match the new exhaust/cone filter/else stock engine setup)

Try some toe out – 1/8”, maybe

Try a stiffer rear anti-sway bar setting

I have an Odyssey battery from my old car I am thinking of putting in. Good news, about 20# less to lug around; bad news, all off the front.

New tires – 710’s, perhaps 245/275 (I think 275s will stretch to an 11 ½” rim)

A brake proportioning valve is an interesting idea. As I understand it the inline devices are pressure limiting to one end or the other and don’t really act like a brake balance bar in a two m/c setup (which budget, and, I believe, the rules, make the latter an unlikely prospect). Still, the inline one might be worth investigating.

Questions for JP (& racer) – JP -what size tire do you run on those 8” fronts? 710s go straight from 225 to 245 in 17”. I think I could stretch a 225 to the 9” rims – good/bad idea? Current tire pressures are 27-28/31-32 f/r, cold. Would a 225/275 tire stagger be workable, i.e. get temps up and have decent balance? As to an adjustable front anti-sway bar, well the budget for the start of the season is gone (see above), but I do have the stock front bar (19mm ? EDIT - It is 20mm), so, Maybe that’s worth a try (and, it’s free!).

FWIW, the back end feels pretty good. It throttle steers nicely in sweepers, although the front doesn’t want to tuck in as much as I would expect. So, some oversteer is present. I suppose a little more wouldn’t be fatal (at least at auto-cross events). And, it responds well to trail-braking into very slow corners. You know, the ones that make you actually consider trying to force that 915 tranny back into 1st.

Reply to petevb – If I can enter a 5 cone slalom at a decent angle, I pass the first cone on or just off the brakes so, it doesn’t count. The pivot for the next cone seems good, but I can feel a little understeer. After that, unless I back off, the understeer builds. The back end is never an issue through slaloms. A technique I learned from a multi-time Solo 2 National Champion (Tom Berry) is to goose the gas a bit while straight between the cones (as you are passing the cone, really) and then let off. If there is room for this, front bite seems better on turn in due to some weight transfer to the front.

SteveW – Yes, quite cold at the last event (for California – low to mid 50’s, I think), but this behavior seems to occur even on warm days. I did drop my front pressure by 1#, maybe not enough to be effective. My best run was 73.6 IIRC. That was a pretty good run in that the car did what I wanted it to do, although I had to slow more than I wanted mid-sweeper. But, no big understeer moments or brake lockup. I feel like the car could go 1-2 seconds faster if the understeer could be tamed. I will try to have one of the better instructors drive the car at the next event for feedback and a reality check.

Off the wall comment – I used to auto-cross a Mustang GT. Talk about understeer! But, I eventually was able to make the thing quite neutral. It took a couple of years and a lot of parts, but it handled pretty well. It was kind of one of those fast pig deals – still a pig, but it ended up pretty fast. Anyway, the single most effective change I made was to add an adjustable rear anti-sway bar. Lots of other changes contributed, but that bar made tuning the suspension a reality. Of course, there was no oversteer waiting in the wings to bite me in the ass. Not fear, really, just a healthy respect. More practice certainly can’t hurt.

Off the wall comment 2 – I’ve ridden in a couple of students’ mid-engine cars, a Boxster S and a Cayman. Those things pivot better on street tires than my car on R compounds. I love my 911, but . . .

[911st]

Under-steer or push is a normal trait that going slower seems to cure. Many try to go to fast into tight corners on an AutoX.

You mightt try for more front tire. You mightt even be able to use more neg front camber. Mark your front tires or look at them for excess war on the out side.

You could have the front spindles raised. This will give you a better camber curve in the front and gain you about .5 deg of effective camber in a corner with your spring rate. If you have a turbo look, this would make the front a better match the rear's.

A lot of caster makes for changing camber with a turn. Most run about about 6 deg of caster. This can add about -.5 deg of more camber in a tight corner. Thus, if your car holds well in bigger corners and loses more than expected grip in tight corners, it might be because of camber gain from caster. To much neg caber is as bad as to little. It is caster that also make for so much effort to keep the steering wheel cranked in a corner on the track. You could try pulling this back and see if it helps.

I suspect if you have a Turbo Look you should have a clutch type limited slip. If it is in good condition, it can add to push. I tryed autoX in my C2 Turbo one that has 80% lock on throttle lift. It really made my car push.

22/29 is not a bad Auto set up. It is more a DE set up. You mightt drop the front down to 21 or bump the rear to 30. This will help the car rotate.

On normal 911's they seem to run AutoX well on 225 front and rear. If you are running 7&8's you mightt try 225's up front on the 7's. If going w big rear's, try to stay one size less in the front. (Because a 911 has 50% more weight in the rear it is a two and a half wheel car. One in the front, and one and a half in the rear.) If you go with a larger rear stager on the tires, stiffen up the rear.

Toe out helps with first bite upon turn in.

Not an AutoX expert, just some things to think about.

[Flieger]

If I can enter a 5 cone slalom at a decent angle, I pass the first cone on or just off the brakes so, it doesn’t count. The pivot for the next cone seems good, but I can feel a little understeer. After that, unless I back off, the understeer builds. The back end is never an issue through slaloms. A technique I learned from a multi-time Solo 2 National Champion (Tom Berry) is to goose the gas a bit while straight between the cones (as you are passing the cone, really) and then let off. If there is room for this, front bite seems better on turn in due to some weight transfer to the front.

The earlier comment about dampers is a good one to bring up.

The balance of front to rear natural frequencies needs to be considered, as well as the damping ratio. If the front has a higher frequency (less weight, stiffer springs) than the rear, you can get a resonant frequency if the slalom spacing is correct. The drive frequency (steering wheel input) can cause a resonance in the back end if the front responds quickly with a high enough resonant frequency. Steering wheel input is phase shifted by the front's frequency. This can cause a "Pilot Inducced Occilation."

If the rear is quicker to respond than the front, then there will not be a resonance.

You want something closer to the "PIO", without going into a spin.

The response time is influenced by both natural frequency and damping. A high frequency can be slow to respond if overdamped. It sounds like you need to make the front end respond quicker, which means a higher frequency- stiffer sway bars/torsion bars and less weight. It could also be that you need less rebound damping in the front. This will soften the front end and quicken response time. You could also try stiffening the rebound on the rear dampers for balance but this would likely decrease overall grip.

So, softer front rebound damping might help keep both front tires on the ground: getting the front and rear vibrations in harmony through the slalom transitions while keeping most or all of your total grip. It may cause a "jacking up" of the front after many turns due to the decreased damping ratio but I doubt you will feel it much and you should like the turn-in better.

I am only a student, though.

[J P Stein]

Questions for JP (& racer) – JP -what size tire do you run on those 8” fronts? 710s go straight from 225 to 245 in 17”. I think I could stretch a 225 to the 9” rims – good/bad idea? Current tire pressures are 27-28/31-32 f/r, cold. Would a 225/275 tire stagger be workable, i.e. get temps up and have decent balance? As to an adjustable front anti-sway bar, well the budget for the start of the season is gone (see above), but I do have the stock front bar (19mm ?), so, Maybe that’s worth a try (and, it’s free!).


I run 8 X 15 Fuch & 9.5 X 15 X 23.0 Hoosier cantilever slicks up front....but that's not important now... & 10 X 16 rear slicks, but mine is was not a 911. The important thing is the width of your wheel sets one component of your scrub radius....too much scrub=too much push in the tight stuff.

The standard rule is if you push, stiffen the rear or soften the front. You can effectively soften the front by softening your front anti roll bar....you will gain grip at that end. Stiffening the rear will(IMO) make you loose rear grip...not the ideal way to go faster. Without having the ability to do this you're selling yourself short.

I too used Jack French at Bilstein West to revalve my shocks.....I'm sorry for his passing. Jack hit my valving right on the nuts. Short of double adju$table$ I couldn't ask for more. Back in the day, I ran my 19 mm front AR bar full soft in a light car (2000ish )lbs. For the last few years I've run a 22mm AR bar right dead in the middle......that way it can be softened or tightened if it's too lose or pushes.
The car is now way light & more stiffly sprung.

For now, were I you, I go with 225s up front on 8s.....15s get your ride height down without compromising suspension geometry....and they're lighter. Again, an adjustable front anti roll bar will prove itself to be your friend.

BTW, The $hitbox is as quick or quicker that any Porsche we've come across on the West Coast (& we've gone looking for em').....with "the kid" driving. My driving sux.

[J P Stein]

No ABS here. .....

[Steve W]

Originally Posted by rfloz

Apparent non-issues: Tire temps are good across the tread with inner and outer temps within 5 degrees of the center on both front and rear. So, camber (-2.6 degrees f/r) is good, no? The car has raised spindles and a bump steer kit so camber change is about as good as it can be.

You raised your spindles and got 2.6 degrees front camber? How did you do that - did you decamber the spindles 2 degrees at the same time?

Maybe with such wide front tires, and that much camber, you aren't getting as good a contact patch on straight line braking and thus locking up your fronts first. Also I'd think with that much front camber, and little front weight, in quick transitions, the contact patch of the outer front tire may not not be taking a set fast as you may be throwing the car in quick transitions, thus your floaty feeling. Something similar to the the pic below:


Try loosening your front, investigate your front shocks, and maybe dial back the front camber to 2.0.

If you want to find me at future events, I'm car #911.

[petevb]

OK, based on your description I think I've got an idea of how your car is behaving. If I'm right it sounds similar to behavior I've struggled with before in a couple early 911s with big motors.

Point by point:
Torsion bars sound in the ballpark.
Tire sizes, while I haven't used your exact sizes, sound about right. I think you can get the 245/ 315 to work as long as you can get heat into them and they don't stop you lowering the car. Pressures should likely be very low for those sizes (25 lbs or less).
I think you could probably use stiffer swaybars front and rear.
Shocks- if you had adjustable shocks I suspect you could help the issue by tuning them, but as long as your shocks are not far off you should be able to get a handle on the issue using other knobs.
Spindle height- you didn't mention if your car is lowered or if the spindles are raised, but some of your problems can come from a lowered car with stock spindle heights. The change in the roll axis that results is an issue- In general I'd suggest raising the spindles to the max your wheels allow.
Toe- I agree with trying toe out up front. It will also help get heat into the tires.
Brakes- more weight up front or try using a higher friction compound for the fronts. I you go with something very sticky up front (ie pagid gray) you can use an adjustable prop valve for the rear. Don't use it to cut front pressure.
Camber- depends; might play with it if you can't rotate the car when you're done with the other changes.
M&K- I lost power with mine, though the weight was nice.
Front wheel offset- you are running the 1" turbo spacers, so you have a lot of offset to JP's point. This could well be an issue, but the fix isn't easy. Ideally you'd change spindle angle as Tyson did for Jack's Black Beauty. Going to thinner front tires on the same rims doesn't help the offset issue.

I think the easiest way to solve your major issues is still to improve weight distribution, and lucky that's the easiest test: just run more fuel and see if it helps. My friend's been struggling with issues similar to yours for a couple years, and when he finally took my advice fully and weighted up the front last event he took TTOD and won the season in his 911 (of course he does have the better part of 100 horse on you, so the weight isn't a big deal). Of course JP's right here too- ideally you never add weight other than to test... Get everything possible out of the rear- be religious. Then start working down from a full tank and note the difference in handling. More weight up front helps exactly the issues you're have trouble with in my experience, and your car sounds particularly rear heavy.

What the 911 lacks in turn-in it can often make up for in putting power down better. And yes, the fast way to drive them in my experience is with the tail sliding nearly everywhere except through the slaloms...

[rfloz]

Well, lots more info and ideas to ponder.

I edited my 2nd post to reflect some corrections and additions to my setup. I'll mention them below as applicable.

911st - I've made pretty good progress on the slow stuff between adjustments to my driving and to the car so far. It's primarily the push in sweepers and front brake lock that is perplexing. I do have raised spindles (33mm) and a bump steer kit on the car. The caster is +5.8 degrees (not 5.6 as previously posted from my failing memory). Tire temps are pretty good across the tread and, if anything, might suggest a bit more negative camber. I do have the stock LSD, not sure of its condition, but I don't have any problems with rear tire spin.

As to torsion bars, I debated (with myself) between 22/29 and 23/31. The softer setup won to preserve some street manners. That said, changing the rear bars to 30 (or the front to 20) might help.

Max - I met your dad on the Thanksgiving Day Run. He seems a very nice man. I asked him to tell you #84 said hello. Your car is even more insane in person. I hope your studies are going well.

I'm stuck with the dampers I have right now (re-valved Bils), but as noted by JP, adjustability counts, so I will look ahead to something w/ adjustability. I need to re-read your PIO discussion again. it sounds a bit like PIE - Pilot Induced Error.

JP - Slicks! Well that's not fair. Not to start an North coast - South coast thing, but Steve Eguina's GT2 might give you a fair fight, even on the wimpy R tires he runs. BTW, I'd never call a 914 a $hitbox, I've tried to keep up with Dave B of TRE in his 914-6 (the Evil Cock-a-roach) on a couple of our mountain runs. Let's just say I'm glad there are none in my class.

Because the spindles are raised 33mm, I cannot get a 15" wheel on the front of the car (doubling the irony of keeping the utterly useless spare in the car). I currently have 10mm spacers in front to cure a slight interference between the wheels and the upper strut mount sheet metal. That certainly adds to any excess scrub radius. A BFH can be employed instead and the spacers removed. I do have some really ugly 7 1/2" x 17 wheels I could try with say, 225s. Hmmm. I could mount 225s on the 9" wheels and swap them to the 7 1/2" ones if the issue persists. I need to check and see if maybe 16s will clear.

Jack French - right. By all accounts he was a real wizard; the new guy maybe not so much. As I said, I'm stuck for now with what I have, so other solutions beckon. Switching back to the stock 20mm front anti-swaybar from the current 22 is sounding better and better. As is trying a 225/275 tire combo and maybe some toe out. I know, I know, one change at a time.


Anyway, I am feeling like I have a lot more options than I did a few days ago. Now I need to get out in the garage and start moving some parts around.

Thanks to all for the input.

[J P Stein]

Slicks are not better than A 6s.......till you get a bunch of heat into them. This seldom happens on asphalt & there's no concrete around here. The only times I've done it regular like are on skid pads.....at 180 deg the sumbees will hurt you . Till that point I believe it's a wash with the A 6s.....but they are lighter.

[911st]

Very nice car.

More thoughts.

More neg front camber, less caster (for more consistent camber with different radius turns) softer front shocks for turn in (if shocks were done for track work the fronts might be a little stiffer to keep the rear's inside wheel from getting light in transition and rotating but works against getting the front to bite and rear to rotate for AutoX.) More front tire. Apx. 25mm SRP adjustable rear sway bar.

Good luck.

[Flieger]

Here is a good example of PIO: YouTube - f-22 raptor crash landing


The control system electronics had too much lag time (this was a test flight) causing basically constant over-correction. The pilot did a pretty good job of minimizing damage.


So, that was your cool white 911 M472 blasting down the Snake. I thought it looked familiar. I was remarking to my Dad about how great, and fast, it looked flying through the twisties.

He did give me the message. Maybe we will see each other on the New Year's Run.

[petevb]

Originally Posted by 911st

More front tire.

FYI, I don't agree in my experience. More front tire will lead to more front grip. But this needs to be countered with more relative front stiffness (ie stiffer front bar or softer rear) to maintain mid-corner balance, and more front stiffness leads to increased inside front tire lift and issues in transitions/ the slalom with a car with a particularly light front end. The result is much harder to tune on a fairly powerful car.
The square tire setup works very well if you are low on power and don't have adjustable swaybars, but if you can tune the balance then something closer to in-line with weight distribution works better I believe.

[Flieger]

Originally Posted by petevb

FYI, I don't agree in my experience. More front tire will lead to more front grip. But this needs to be countered with more relative front stiffness (ie stiffer front bar or softer rear) to maintain mid-corner balance, and more front stiffness leads to increased inside front tire lift and issues in transitions/ the slalom with a car with a particularly light front end. The result is much harder to tune on a fairly powerful car.
The square tire setup works very well if you are low on power and don't have adjustable swaybars, but if you can tune the balance then something closer to in-line with weight distribution works better I believe.

Hence the advantage of adjusting the front rebound damping softer relative to the front compression damping for a more under-damped, quick-reacting front end. The softer front relative to the rear rebound will keep the front inside tire on the ground more. Quick response time is achieved with an under-damped, high-frequency (stiff springs, light weight) load transfer and roll but this leads to overshoot ("jacking up").

Everything is a compromise, especially on a budget, as I, too, know.

[911st]

Originally Posted by rfloz

...My rear anti-sway bar can be adjusted stiffer. I haven’t tried this yet as it tends to reduce rear grip when the preferred solution is to gain front grip. In the end, balance may trump grip, though. I’ll be trying this next season.

Apparent non-issues: Tire temps are good across the tread with inner and outer temps within 5 degrees of the center on both front and rear. So, camber (-2.6 degrees f/r) is good, no? The car has raised spindles and a bump steer kit so camber change is about as good as it can be.

The tires are fresh Kumho 710s in 245/315 sizes on 9” and 11 ½” wide wheels (it’s a Turbo-Look). I tried 255/295 A6 Hoosiers and the same problem existed. So, more front tire is no help. Nor does less rear tire seem to help.
...

I am now thinking more front tire first.

245/315 is a lot of stagger. I belive BB II runs 275/315. I have also heard that you do not want the front tire to be over 1" narrower than the rears. You are at 9.8" and 13" so you have a lot more rear tire than front. New Porsches do run a lot of stagger but they have more sufficated suspention and the ballance to make it work.

Other wise, the rear needs to be stiffened to get the inside front to do more work.

A 911 has 50% more weight in the rear. I think of a 911 as a two and a half wheel car to handle the 50% more weight the rear has to support. In a corner it takes about one full front tire and one and a half rear tires to carry the weights. That is if there is not much differance in tire stagger. If the rear tires have a contact patch that is much larger than the front, it will take more than one front tire to cary the front's weight. Driveing on three wheels is not going to be the fastest set up in that case.

Making the rear sway bar stiffer takes weight off the inside rear tire and shifts it to the inside front. Thus, it can reduce your rear and increase your front at the same time. Sounds like you tryed this and is did not work for you. Sway bar adjustment is the most effective tool you have to effect your front to rear ballance / weight transfer.

Then go about making sure the front is performing to its potental by verifying proper neg camber and air pressures. Front takes less air pressure than the rear. In autoX it can also require more neg camber than the rear (think of the angle with one front tire lifted)

Again, our running so much caster makes our neg camber vary about .5 deg depending on steering angle. Might try somthing in the 3.5 range and see if it stabilzes your stick between tight corners and sweepers. Most race cars do not run as much caster as we do. This might make the car a little less stable at speed as it is caster that give a 911 most its self centering effect as we do not really run any toe.

If you find the right front wheel is locking up and you had a standard corner ballance you could have a significant enough differance in weight on each front wheel to be the issue.

On my 2900 lb 911 (driver included) my fronts were 610 left front and 548 right front. That is about 10% less weight on one front wheel. Traction is a function of weight. For what it is worth, a 911 has about a 40/60% ft/rr and 52.5/47.5% side to side weight distribuition with a driver.

You could try a CB w a more equal fronts. This will effect handleing ballance side to side. Another thing that you could try is if you have a stock battery (apx 60 lbs) substute a light weight race battery. Better yet, move it to the right front of the compartment. Moving the stock battery would make the most differance and could shift about 50 lbs from the drivers front side to the other.

Again, more front tire and make them do there share of the work.

Just my two cents.

[Ducman]

A lot of options - IMHO the most problematic issue to deal with autocrossing a 911 is low speed understeer. - it seems I really need to get my timing right on the tight turns (more so in a 911 than in other cars) because of the rear weight bias. Extreme trail braking....get the front fully loaded under braking, turn in with the benefit of the extra front grip, then let it rotate.

Not saying I can do it right everytime, but when I do it makes more difference than all the other equipment adjustment combined. Wasn't sure from your post how long you've been doing autox in a 911 - I just think it's very different than any other car. So if it hasn't been that long you might think about not touching anything on the car for a while.