911 Understeer Issues at Auto-Cross (long)

[Randy W]

Originally Posted by rfloz

I seem to recall an Excellence article featuring your #51 car and you were running a 245/275 setup IIRC. Have you gone to a no stagger setup since (it was a while back)?

Yeah, that was a while a go! I started in ASP and had some success with that stagger (true slicks were not allowed). In XP I'm somewhat limited in tire sizing so no stagger is what I have to work with - it's really not an issue once the sway bars are reset.

Originally Posted by rfloz

While the fronts wouldn't get as warm as the rears due to the rears carrying more of the load and power application, especially with no tire stagger I would guess, I don't have a good idea what that difference should be.

I wouldn't worry too much about tire pressures - those are for fine tuning and for giving you something to do between autocross runs! . A 2 to 4 lb. difference is about right . I have not used Kuhmos on a 911, so I cannot advise you about a baseline. Hoosiers are generally considered to be better for lighter cars (about 36/38 hot for R compounds on a 911). Lower tire pressure give you more "feel", higher pressures give you more grip (to a limit of course).

Originally Posted by rfloz

FWIW, I have gone through a couple of the EVO schools and have (finally) put "looking ahead" and "go slow to be fast" (in the tight stuff) to good use. I am hoping your 911 specific techniques will help me build on that.

Thanks again for the input. It is appreciated.

You are most welcome.

[rfloz]

Originally Posted by J P Stein

While I agree that a very good driver can overcome a poor handling car, why should he & what would he do with a car that handled well?
Heroic driving is fun to watch and but very hard to do consistently.....beyond most, me thinks.

A balanced car is a lot more fun to drive and your butt time will translate to any well balanced car. I have seen Evo School instructors flummoxed by a poor handling cars......going slower than the owners. Put these same folks in a balanced car and they will lay waste to an owner that has been driving the car for years. Nothing personal, that's their job.

I've watched Dave Schotz wrestle an F Stock Mustang around. Having owned one, I can testify that thing was an absolute pig. Dave did stuff that frankly looked impossible. We mere mortals want/need something a little more balanced.

A few years back Tom Berry was instructing in my then ESP Mustang (a pretty well setup car, if I do say so myself). He put a bit more than 2 seconds on me the first time he drove the car! I watched and asked what he did and found 1.5 seconds, but that last half second was still out there.

[rfloz]

Originally Posted by Randy W

Yeah, that was a while a go! I started in ASP and had some success with that stagger (true slicks were not allowed). In XP I'm somewhat limited in tire sizing so no stagger is what I have to work with - it's really not an issue once the sway bars are reset.



I wouldn't worry too much about tire pressures - those are for fine tuning and for giving you something to do between autocross runs! . A 2 to 4 lb. difference is about right . I have not used Kuhmos on a 911, so I cannot advise you about a baseline. Hoosiers are generally considered to be better for lighter cars (about 36/38 hot for R compounds on a 911). Lower tire pressure give you more "feel", higher pressures give you more grip (to a limit of course).



You are most welcome.

XP - a fun class for tinkerers, as well as drivers.

The A6s are a bit faster, both in the time and the wear departments. I have had trouble adapting to them, which is my issue, not Hoosier's. The Kumhos have nearly as much grip and seem a litttle more forgiving (better, or at least easier, for the less adept). The Hoosiers seem to have a quicker drop off in traction when you get over the edge and, to me, have a weird kind of squishy feel to them. My thinking is that more expert drivers can adapt better to those kinds of issues.

Hey, at least I have the f/r tire pressure delta right.

[rfloz]

I am sure everyone has been waiting with bated breath for my schedule of changes to rid my 911 of the dread understeer/front brake lockup (real or perceived).


And so, without further ado (more or less in the order I intend to make them):


Take more weight off rear of car

Work on getting a “set” in sweepers

Remove front wheel spacers

Practice threshold braking

Try less air pressure – 24/28 f/r?

Look ahead farther

Try Textar pads or PG-97s

Practice trail braking in slow corners

Add some toe out – 1/8”?

Try left foot braking (again)

Stiffen rear anti-sway bar

Maximize throttle at apex

Swap in smaller front anti-sway bay

Work on getting a “set” in sweepers

Add some more negative camber

Reduce caster

Practice slalom turn in on cones 2, et seq

Try smaller front tires

Compare temps and turn in

Try smaller rear tires

Work on anticipating corrections, esp. in sweepers

Add weight in front

Take up knitting


Those still awake will notice driving improvement techniques liberally spread amongst the setup changes. You have Mr. Wells to thank for that.

Thank you Mr. Wells.

On reflection, not only am I well aware my driving could use improvement, but also that my driving errors may well be contributing to the perceived issues.

Thanks to all who have contributed.

[Jeremy Cottrell]

Originally Posted by 911st

Yes! Dwight in ASP. It usually took a 3.5 914-6 with pro built hight butterfly RSR-MFI w huge sticky meats and a national pro level driver to beat him for TTOD at local events in that little 911 w no oil cooler and about the smallest brakes Porsche ever put on a 911.

Sounds like Ron Mistak's wicked 914-6.

[Jeremy Cottrell]

Originally Posted by rfloz

A few years back Tom Berry was instructing in my then ESP Mustang (a pretty well setup car, if I do say so myself). He put a bit more than 2 seconds on me the first time he drove the car! I watched and asked what he did and found 1.5 seconds, but that last half second was still out there.

Watching guys like Tom, Steve Abbott and Jason Isley can be a great learning tool. After taking a ride with Dave Palmquist once, I realized the biggest problem with my car was the nut behind the wheel and I've been working to improve that nut ever since. It was one of those, "Oh, that's how you do it!" moments.

I also agree with Evan Fullerton regarding the fact that maybe we need to make some bigger changes and then ratchet it back a little. Evan won our "Top Gun" award this year, and is obviously an excellent driver and a good guy to gain knowledge from. My old Targa was near impossible to drive with huge amounts of understeer. I turned it into what most would call a real tail wagger, but it was about 5 seconds faster on a 60 second course. Most of that was accomplished by a larger rear swaybar, tire pressures, aggressive toe, and yes, leaving that spare tire in and filling her with gas.

[Dave at Pelican Parts]

Originally Posted by Jeremy Cottrell

Sounds like Ron Mistak's wicked 914-6.

Tom Provasi. Mistak is rather further south, the Mitchells and Provasis were (are?) in the Bay Area/Sacramento areas, and they used to run against each other pretty frequently.

--DD

[ttweed]

Originally Posted by rfloz

I am sure everyone has been waiting with bated breath for my schedule of changes to rid my 911 of the dread understeer/front brake lockup (real or perceived).


And so, without further ado (more or less in the order I intend to make them):

[snipped]

Remove front wheel spacers

I am a little puzzled by this idea. If the problem is lack of front grip (understeer), why would you want to reduce the front track? In general, larger track = greater grip. The decrease in scrub radius will not offset the loss of grip from decreasing track width, IMHO, if that is what you are trying to achieve with this move. Reread Evan Fullerton's post above. Increased scrub radius will effect steering effort and stability, but will not really contribute to understeer, AFAIK.

Try smaller front tires

I am with Evan on this one as well. I would go up in size and put a 275 on the front instead of a 245 before I would try a smaller tire. At the low speeds and short durations of autox runs, with 5-minute waits between runs and the 40/60 weight distribution of a 911, the fronts are never going to get up to temp well. The widest possible contact patch is going to help with maximizing grip in turns when it is the purely mechanical keying/deformation/tearing components of friction at the tire/ground interface doing the work, before the "adhesive" quality of sticky, heated rubber compound comes into play. This is especially important on cold/wet surfaces, and is one of the reasons to try lower pressures under those conditions (as well as the fact that flexing of the tire helps build heat quicker), even if you find the resulting steering response a bit mushier. I have always found the Kumhos liked lower pressures than the Hoosiers or Toyos I have run anyway.

Add some toe out – 1/8”?

I would put this at the top of the list. Toe-out may not be the best thing for steering stability in high speed track work, but for autox, it can transform the turn-in and cornering of a car.

You mentioned that your car has a limited slip diff. Is it the factory ZF clutch type? They are great for road racing and give outstanding stability under braking, but the fact that they are engaged under deceleration is going to inevitably result in understeer on trailbraking in corner entry. A Torsen type diff that is only engaged on acceleration is usually preferred for an autox car that needs to be rotated easily in tight, slow corners. Since this is quite a big, expensive change, I would put it very low on the list.

Good luck,
TT

[J P Stein]

I switched from 10 inch wheels & 10 inch R35 slicks to 8 inch wheels with 9.5 inch
R35 slicks up front. My understeer left the building in all but the tightest corners....and then it was manageable. The only difference was a 1 inch reduction in scrub radius....from a positive 3 inches (approximately) to 2 inches.
That is AFAIK from here. I would advise all my competitors to ignore this, after all, I may be drawing the wrong conclusion.

I do agree with the TB diff for AX.....which is why I bought/installed one.

[911st]

Originally Posted by Dave at Pelican Parts

Provasi...
--DD

Yep!

I think Dwight & Linda may have retired to Florida.

[ttweed]

Originally Posted by J P Stein

I switched from 10 inch wheels & 10 inch R35 slicks to 8 inch wheels with 9.5 inch
R35 slicks up front. My understeer left the building in all but the tightest corners....and then it was manageable. The only difference was a 1 inch reduction in scrub radius....from a positive 3 inches (approximately) to 2 inches.
That is AFAIK from here.

Wow! 3" of positive scrub? You 914 guys are a crazy bunch!

Suspension tuning is a black art, and I certainly don't have all the answers, but are you sure that the scrub radius was the only difference in that change, JP? Did the rear end stay the same as before, and all you changed was the front? Was there any change in height (rolling dia.) of the 9.5" slicks vs. the 10" slicks which might have changed the rake of the car? Was there not a difference in sidewall flex due to either the construction of the tire or the lesser support offered by the narrower rims with nearly the same width tires, which could change the contact patch deformation under load (obviously in a good way, since it improved your handling balance)? I don't know specifically which tires you're talking about, but "R35" seems to indicate a Hoosier slick. I don't see a 9.5 and 10" slick in the current Hoosier lineup with the same overall diameter, unless you also changed from 16" rims to 15", in which case you changed to the FA C3000 construction (for lighter cars) from the GT2 slicks for heavier cars. These definitely have different sidewall construction, regardless of the difference the rim width change would make. Did you use the same pressures and alignment with both setups? Were the tires in the same stage of "freshness" at both ends when you compared them? There is nothing that will increase grip instantly as well as fresh rubber.

There are a lot of variables at work even in a seemingly simple change. When you say the scrub changed by 1", are you including any difference in the offset of the 8" rims vs. the 10" rims (which would change the placement of the wheel's centerline) as well as the change in tire width?

There are a couple of circumstances when a decrease in track may help front grip and decrease understeer that I can think of. One is that the caster of the strut increases the amount of camber on the outside wheel when steering, and a large positive scrub radius can slightly reduce this caster-applied camber. If your setup is "camber-challenged" already, perhaps reducing the scrub would help. If you're running bias-ply slicks, though, they don't need a lot of camber and this wouldn't matter much. Also, a higher track width means the difference in arcs the front wheels will be taking while turning will increase--hence more Ackerman would be needed. Ackerman is not easily adjusted on our Porsches, so additional toe-out may be needed to compensate for the lack of Ackerman. With no change in toe, less track width will mean less Ackerman needed to improve turn-in and cornering performance.

It's great that you solved your handling problem, but I'm not sure if it was from reducing the track or scrub radius. For sure, I will not tell your competitors. I would hint to them instead that your improvement was due to your mastery of left-foot trailbraking. That should keep them totally screwed up for a few events trying to add that skill!

TT

[rfloz]

Originally Posted by J P Stein

I switched from 10 inch wheels & 10 inch R35 slicks to 8 inch wheels with 9.5 inch
R35 slicks up front. My understeer left the building in all but the tightest corners....and then it was manageable. The only difference was a 1 inch reduction in scrub radius....from a positive 3 inches (approximately) to 2 inches.
That is AFAIK from here. I would advise all my competitors to ignore this, after all, I may be drawing the wrong conclusion.

I do agree with the TB diff for AX.....which is why I bought/installed one.

Well, since I'm certainly not one of your competitors . . .

This seems to go back to my original question of - is the understeer a product of the front tires not getting enough heat in them? I.e., did the lessened scrub radius and the 1/2" less tire width (10" to 9.5") help the tires heat up?

I'm not trying to be argumentative here; just trying to see what will work. FWIW, I have removed the 1/2" front wheel spacers and gone from zero front toe to 1/8" total toe out for my next event. So, I've reduced scrub radius a bit and helped turn in with the toe change. I'll try more rear anti-sway bar next, I think.

There are just so many variables and most interact. TT's description of suspension tuning as a "black art" certainly rings true to me.

[J P Stein]

The 9.5 inchers are 1 inch taller than the 10s, .5 in. on the radius. I've run the same alignment specs for years and have it checked once a year.
Sure I'm sure, *no other changes*.

The scrub radius is a WAG. The stock wheel were 5.5 inches in width. The 10s added 2.25 inches of SR. The backspacing of the 2 different wheels were about 5 inches each. I figure that was .5 to .75 positive there to start out with stock.....prolly damn close to what an older 911 has for SR.

We 914 guys are a bit off center, but I wasn't crazy enuff to leave it that way....tho I suffered thru a season trying to get rid of the push. Never did, but got me some hellacious over steer in the sweepers tho.

Cantis' side walls are not as stable as straight side slicks. I used the 8 inch wheels in an attempt to improve the turn-in of this breed.

All the wheels have the same backspace....about 5 inches. The extra width goes outboard changing the centerline of the wheel.,,,in this case, the extra 2 inches changes the center line by half that. The tires are just along for the ride.

I spent a lot of time skulling this thing out before I made the change. The problem with the 10s was at large steering angles only. I noticed in the pits that the tire wanted to shuffle sideways with a lot of steering lock....not to mention the very high steering effort. When the tire gets too far ahead of the pivot point is when things go to hell.....it no longer wants to roll. I doubt it would be any problem for a track guy. Put them on a 15 pace slalom tho or a 180 around a single cone........

[ttweed]

Originally Posted by J P Stein

The problem with the 10s was at large steering angles only. I noticed in the pits that the tire wanted to shuffle sideways with a lot of steering lock....not to mention the very high steering effort. When the tire gets too far ahead of the pivot point is when things go to hell.....it no longer wants to roll.

Just a wild-ass guess from a distance, but with this description of the problem it sounds to me like the tire construction might be making a bigger difference than the reduction in scrub radius, though both may be contributing. The sidewall deformation with the cantilevers may be allowing the tread to stay on the pavement instead of "skipping" or hopping sideways at high steering angles, which definitely would have decreased grip with the 10" wheels. Your situation was an extreme one, to say the least, though, putting 10" wheels on a suspension designed for 5.5" stock rims, with most of the width outboard of the SIA.

TT

[911st]

I am not a suspension expert but I am a thinker, have a lot to learn about suspensions, and have some thoughts I would like to bounce off someone.

Changing scrub radius dose not seem to directly effect grip like camber dose.

It reduces forces that might be felt at the steering wheel. Reduced kick back or force at the wheel makes a car easier to drive.

If a car is set up from the factory so that at stock SR both wheel perfectly follow a natural arc in a turn, and one makes a significant change in SR, the wheels will no longer follow a perfect arc reducing grip.

The angle of the steering arm off the strut seems to be what effects the arc of each wheel.

Is this something that one should consider adjusting when playing with SR?


Reducing caster also reduces forces at the steering wheel. I think its main purpose seems to be to create self centering force.

However, a byproduct of this is changes in negative camber with increased steering angle. I think I measured as much as a half of a deg of or more of increased neg camber with turning of my front wheel on a stock 911.

Seems that reduced caster would make for more stable camber setting at different cornering attitudes. There might be some benefit to increasing camber change with a turn. This can put the inside wheel at a better camber setting to the ground. It can also make for a lower camber setting in a straight line for better braking and still get an increased camber setting when cornering.

I think stock 914 & 11's run about 6 deg of caster. I think many race cars run much less.

Thus, is it a valid goal to reduce caster to achieve more stable camber values so neg camber is more likely to be closer to its ideal in both long and short radius corners?

[Flieger]

More toe-out should be added when increasing positive scrub radius if one wishes to maintain correct Ackerman steering geometry.

Alternatively, toe-in means that the outside wheel is more turned-in than the Ackerman angle to a curve and the loaded wheel dominates in high-G turns so toe-in can be good for the track.

Zero toe is a good compromise.

[J P Stein]

I'm going to run my MR2 in SCCA C Stock. Getting specs from guys that do this, I found that they jump through the legal hoops to get -3 camber up front (it involves "crash bolts"). I got to asking why so much and got an answer. With stock springs they get a lot of body roll....even with stiff front AR bars. They complain about disappearing negative camber.

It turns out that the caster is non-adjustable and is set at about 2-1/2 deg.....ah, now it makes sense. The 6 deg (approx) of a 911/914 will give you another -1 negative camber at 15 deg steering angle...or damn close to that.

BTW, a half inch spacer behind the front wheel will give you another 1/2 inch of positive scrub radius.....but, as I said, I am not absolutely convinced that excessive SR will cause a push at large steering angles.....but I'd bet that way. It seems that there are too few data points to prove or disprove my assumption. My high school physics covered force vectors, but that was 40+ years back.

[911st]

Flieger,

Interesting thing on the toe out. Is that why it helps with turn in? It gets both tires on a more correct angle / radius to the turn being made?


Again not an expert on this but I do not see how increases in SR would effect much except this Ackerman curve stuff and as Flieger noted, the inside front wheel is usually off the ground anyway in a fast 914/911.

Should more effect steering effort and kick back under braking should one wheel lose traction. I think this might be why the C2 has greatly reduced or shall we say neutral SR. To work well with ABS and not twist the wheel out of ones hand.

[ttweed]

Originally Posted by 911st

Changing scrub radius dose not seem to directly effect grip like camber dose.

It reduces forces that might be felt at the steering wheel. Reduced kick back or force at the wheel makes a car easier to drive.

Yes and no (as usual with these kinds of questions, there are compromises involved). Some amount of scrub is a good thing--cars with zero scrub suffer from "squirm" in cornering and have no "road feel" or feedback from the steering wheel, which does not inspire confidence in the driver.

If a car is set up from the factory so that at stock SR both wheel perfectly follow a natural arc in a turn, and one makes a significant change in SR, the wheels will no longer follow a perfect arc reducing grip.

Not sure what you mean by a "natural" arc. Due to track width, the inside wheel must turn in a tighter arc than the outside wheel--this is what is "natural" for a car to corner well, and is also the reason for a differential on the rear wheels (which do no steering). The problem with the front wheels that are doing the steering are far more complex, and even more difficult when the front wheels are also the driven wheels (something we don't have to worry about with Porsches, luckily) and one must consider "torque steer" forces. Large changes to scrub radius will increase steering effort and also changes the track width, which will effect the steering arc. Such a change will need to be accounted for by alignment settings and/or steering geometry changes or both to maintain the "factory" handling balance, which may or may not have been ideal by design for your particular use.

The angle of the steering arm off the strut seems to be what effects the arc of each wheel.

Yes, that is how Ackerman is usually adjusted in race cars, by moving the location of the steering rack fore and aft or changing the location or length of the steering arm on the upright. Not an easy prospect in a Porsche unibody with MacPherson strut suspension. Adjusting toe on the front also effects this arc and is much easier to accomplish.

Reducing caster also reduces forces at the steering wheel. I think its main purpose seems to be to create self centering force.

Yes. The forces are still there, the caster just does some of the work to return the wheel to center.

Thus, is it a valid goal to reduce caster to achieve more stable camber values so neg camber is more likely to be closer to its ideal in both long and short radius corners?

Caster is not the only thing contributing to the camber change curve. What you are forgetting here is that camber also changes due to the suspension moving through its range of travel. In a turn, the car leans and the outside suspension compresses, which reduces camber. When a car like my early 911 is "camber-challenged" (in that with stock components I cannot adjust for more than about 2 degrees negative when modern radial race tires may want 3+ for optimum performance) then I want all the caster I can get in order to offset the positive camber change in suspension bump. It's all a compromise. If we had double wishbone race car suspensions with more perfect camber curves, we could get away with less caster. If we use very high spring rates and huge ARBs, then body roll and suspension movement decrease, which will allow less caster, but that brings a whole new set of problems with compliance and ride comfort, especially for a car that is also used on the street.

As usual, YMMV,
TT

[rfloz]

Okay. Somehow I seem to have missed this post from TT, as well as Evan Fullerton's. Not ignoring you guys, just missed it.

Originally Posted by ttweed

I am a little puzzled by this idea. If the problem is lack of front grip (understeer), why would you want to reduce the front track? In general, larger track = greater grip. The decrease in scrub radius will not offset the loss of grip from decreasing track width, IMHO, if that is what you are trying to achieve with this move. Reread Evan Fullerton's post above. Increased scrub radius will effect steering effort and stability, but will not really contribute to understeer, AFAIK.



I am with Evan on this one as well. I would go up in size and put a 275 on the front instead of a 245 before I would try a smaller tire. At the low speeds and short durations of autox runs, with 5-minute waits between runs and the 40/60 weight distribution of a 911, the fronts are never going to get up to temp well. The widest possible contact patch is going to help with maximizing grip in turns when it is the purely mechanical keying/deformation/tearing components of friction at the tire/ground interface doing the work, before the "adhesive" quality of sticky, heated rubber compound comes into play. This is especially important on cold/wet surfaces, and is one of the reasons to try lower pressures under those conditions (as well as the fact that flexing of the tire helps build heat quicker), even if you find the resulting steering response a bit mushier. I have always found the Kumhos liked lower pressures than the Hoosiers or Toyos I have run anyway.


I would put this at the top of the list. Toe-out may not be the best thing for steering stability in high speed track work, but for autox, it can transform the turn-in and cornering of a car.

You mentioned that your car has a limited slip diff. Is it the factory ZF clutch type? They are great for road racing and give outstanding stability under braking, but the fact that they are engaged under deceleration is going to inevitably result in understeer on trailbraking in corner entry. A Torsen type diff that is only engaged on acceleration is usually preferred for an autox car that needs to be rotated easily in tight, slow corners. Since this is quite a big, expensive change, I would put it very low on the list.

Good luck,
TT

So, as to the 1/2 "spacers. Yes, I put them in to try and see if increased track would help. Also, the tire was rubbing a bit on the underside of the shock tower. Solved the latter and didn't seem to help the former. That said, I guess I got caught up in JP's scrub idea. They are off the car right now, so I will see where it goes. Easy enough to change back.

As noted, my initial idea was that lack of heat was the cause of the problem. I'm not wedded to that. It sounds ilike a larger tire will help overall front grip despite the heat issues. Yes, a 275 will fit, but I may need a 1/4" spacer as the 245s rub a bit on the outside now when the spacers are on the car. And, lower pressures will be tried with the 245s (which have some life left) and then the 275s when they go on.

Trying a smaller tire on the back as Evan suggests is problematic as the next smaller size (from 315) in the 710s is 275. I'm not even sure that will fit an 11 1/2" rim. I suppose I could try Randy's "square" setup with 275s front and rear, But I think I will try to sort other stuff first.

Toe-out is now 1/8" total. Hey, at least we all agree that should help.

I do have the factory ZF (clutch type) LSD. At 70,000 miles you'd think it would be pretty worn out (My old Mustang would go through the LSD clutches after 7-10 events! And then it would really light up the inside rear tire coming out of a turn) But, I don't get any inside wheelspin coming out of hard corners in the 911, so maybe it is still hooking up. A Torsen would be sweet, but it's not in the cards for now.

What else? Oh yeah, rake. The car is about 3/4" lower in front, measured at the lower body (not at the top of the wheel opening) just behind the front wheel and just before the rear. If necessary, I could raise the back a bit, but the front is as low as it's going to get (see rubbing issues, above).

I can see how making big changes to really feel the effect make sense. So, once I see what I've got, I will put the rear anti-sway bar to full hard (there are only three total positions possible with just two holes on each side) and see what happens. Again, easy to move it back a notch.

Thanks again for all the information and suggestions.