Question about front and rear camber

[moneymanager]

Read somewhere that setting front and rear camber close together, say 1.5° in front and 2° at rear, would add oversteer. Does this sound right? Anyone know how sensitive the car is to these settings? Would a 911 set at 1.5f/2.0r feel significantly different on the track than a car set at 1.0f/2.0r? Thanks.

[911st]

Only 'Rule Of Thumb' I ever heard was to set the front a half a point less neg camber.

I do not agree with this ROT. I think it is based on the front having about .5 deg of neg camber gain when the front wheels are turned because of caster. However unless the rear is stiffened way to much the front will lift a wheel such that the front may actually need as much or more than the rear.

I also heard that over about -3 deg on the street can have an effect on handling some.

Camber should only effect handling in that if it is at less than its ideal, that end will not hold as well as it could.

Usually the front of a stock suspension 911 can not get anywhere near enough neg camber to keep the front tires at the best angle to the track. Thus I guess you could say this might make for some additional under steer.

Using your example -1.5 should hold a bit better than -1 and reduce under-steer a little. I do not now that I would be able to notice the difference. However, -2.5 to .3 would probably be noticed and work better.

On my stock suspension that I prepared for the track on my 85 I maxed my front and rear neg camber. I think I got to about -1.25 front and almost -2.5 rear camber. Also ran 205/245's. Car was very drivable and I did not notice an issue with under-steer. However the outside of the front tires experienced most the ware.

It is better to forget the rules and to listen to you tires (heat measurements and ware). They will tell you how much neg camber you need.

Just my opinion.

[Chuck Moreland]

Originally Posted by moneymanager

Read somewhere that setting front and rear camber close together, say 1.5° in front and 2° at rear, would add oversteer.......Would a 911 set at 1.5f/2.0r feel significantly different on the track than a car set at 1.0f/2.0r? Thanks.

In this specific example, yes the 1.5f/2.0r would tend to have oversteer more the 1.0f/2.0r

But as noted above, I wouldn't use camber as a primary tool to adjust over/understeer characteristics. Use tire temps as a guide to determine best camber, to achieve max grip at each end. Use torsion bars/sway bars to establish basic under/oversteer balance.

[moneymanager]

Chuck:
Thanks. I'm trying to chase down a specific problem, almost scary oversteer, which arrived after a realignment and addition of a bump steer kit. Everything checks out, we're running out of things to try. Almost the only thing that changed before/after was the "gap" between the front/rear camber.

[Flieger]

The goal is to get each tire nearly flat to the road at all times for the most consistent traction and contact patch. Struts do not ususally have as much camber recovery as semi trailing arms: the front wheels do not increase their negative camber enough as the suspension compresses to compensate for the body roll. The rear is has more camber recovery, and the short trailing arms on the 930 and RSR have even better camber recovery. This should mean that the rear would not need as much static camber to maintain proper dynamic camber. Less static camber means better stability in a straight line such as braking.

All of this assumes a rigid chassis, which the 911 is not.

What are the circumstances of your snap oversteer? Right at the apex? Upon turn-in? Does it have understeer before hand or is it relatively neutral and then just snaps around?

[moneymanager]

It's a transition thing...just as you start to turn, car feels like it is going to swing around hard. I haven't actually had it come around on the track, it just makes you treat the corner with great respect! On the freeway, the car feels like there is an extra 500 lbs at the rear; changing lanes is more dramatic than it has been or should be.

[Flieger]

Funny, I get some of that, too. I asked for some toe-out on the alignment. I thought it might be too much front roll stiffness.

[moneymanager]

According to the books, I'm no expert, toe-out should make it worse. We went the other way, but it didn't help!

[Steve@Rennsport]

Was the car's bumpsteer measured and adjusted?

Was the alignment equipment in calibration??

Did anyone check the rear spring plate mounts for movement?

Are all the suspension bolts and alignment hardware tight?

[moneymanager]

Yes, no but checked second time elsewhere, yes, & yes! Thanks Steve.

[Flieger]

Yes, toe-out should make it more twitchy. So, I guess I regret that sometimes. Other times, though that quick rotation is nice. It certainly makes for an exciting drive. It is amplified by the tight twisty canyon roads where I need to brake quick and firm, then get off the brakes quickly, and turn in. This is all done so quickly that there is plenty of load transfer on the front tires for grip. On high speed turns/sweepers, it is nice and stable.

My problem is that the alignment shop my mechanic took it to after installing the suspension bearings did not give him a print-out. It was a scheduling thing where he left it there for a day and picked it up later.

I want to get a print-out of the alignment specs sometime.

Anyway, static camber should not be used for oversteer balance, as already stated. You want the most grip from all tires that you can, then use the anti-roll bars to tune the oversteer. (Or, even better, roll centers if you have lots of adjustability on a racecar).

How does the car feel in a high speed sweeper? If the front is stiff, the initial angular acceleration in a tight transition will be very quick, but in a steady-state turn, the car will understeer.

Dampers have lots of effect in transitions. If the front compression damping is too much, it can also cause faster weight transfer at the front, resulting in the same oversteer. Once on a steady-state turn, though, dampers have no effect so it could oversteer as well.

[911st]

Does it do this equally left or right?

Is the car set really low? (Bottoming?)



Thinking out loud, what could make for a sudden shift in handling as described?

First might be something in the alignment that effects grip with a weight change like going from Toe to no-toe or no-toe to toe. This could be in the front or rear.

Next might be something that makes for a sudden change in the front or rear's effective spring rate. Like a bottoming out, some suspension part binding, or a shock failing.

Possably one of the suspension points moving suddenly like a bushing, attachment point, or wheel bearing.

Maybe, something that effects one of the wheels revolution rate like a brake pad hanging up or a limited slip doing something crazy.

I would first do the old shake the wheels with two hands on the top of the wheel to see if anything makes noise or if one is loose in any way. Then put it on a rack and take a big bar to the suspension and see if there is any thing that will move that should not. Then I might think of checking the struts and shocks that they are moving through there full range freely. Maybe mark them with a felt pin at rest and scheck to see how close to there limit they might be.

[Flieger]

I think that the bump steer kit, spacing down the attachment point of the tie rod to steering arm, will mean that the wheel is turned more for each bit of steering input from you. I am not sure how much of a difference in toe the 10 degrees (?) or so of relative inclination will make, but it will be something. This might be what you are feeling- it is like you are turning the steering wheel more/faster.

[Bill Verburg]

Originally Posted by moneymanager

Read somewhere that setting front and rear camber close together, say 1.5° in front and 2° at rear, would add oversteer. Does this sound right? Anyone know how sensitive the car is to these settings? Would a 911 set at 1.5f/2.0r feel significantly different on the track than a car set at 1.0f/2.0r? Thanks.

not usually,
in general having more camber(to a point) increases grip at that end

for A/X they often setup w/ more front than rear to get the car to rotate quicker(oversteer), for track it's usually the opposite

stock 911 specs are
front
camber0*+/-10'
toe 0 (pressed), +15'+/-5'(in) unpressed
caster 6*5' +/-15' try to get = but no more than 30' difference

rear
camber 1*+/-10'
toe +10'+/-10(in)

for use w/ R rubber front camber can go up to 2* more neg front & rear for a total max of ~-2*/-2.5*

[911st]

Info on rear shock bump stop mod: http://www.smartracingproducts.com/pdfdocs/bilstein_rubbershock_rear.pdf

[Flieger]

I am starting to think I might just be full of ****.

Thinking more about it, all the kit should do is shift the ride/bump steer curve- what it is designed for. There can be a bit of flexing with that long lever arm, but that would be understeer.

Originally Posted by Flieger

I think that the bump steer kit, spacing down the attachment point of the tie rod to steering arm, will mean that the wheel is turned more for each bit of steering input from you. I am not sure how much of a difference in toe the 10 degrees (?) or so of relative inclination will make, but it will be something. This might be what you are feeling- it is like you are turning the steering wheel more/faster.

[Flieger]

Originally Posted by moneymanager

It's a transition thing...just as you start to turn, car feels like it is going to swing around hard. I haven't actually had it come around on the track, it just makes you treat the corner with great respect! On the freeway, the car feels like there is an extra 500 lbs at the rear; changing lanes is more dramatic than it has been or should be.

At the instant of turn-in, there is equal loading on the tires, as weight transfer has not happened yet. This would suggest the issue might be in the alignment. Did your specs "drift" before this recent alignment?

To help me understand, If you were to come to a right-left-right chicane in a straightaway, The car feels like it wants to spin at the first right input? Is there any difference if you are already in a large radius turn, then come to a tighter turn in the opposite direction? Is there any change in this behavior with speed?

[moneymanager]

Thanks to all.
Bill Verberg: your comment suggests that more front neg camber relative to the rear is contributing to my problem.
911st: good thinking but there's nothing loose that we can find, I'm not bottoming. And the symptom appears when turning in either direction. I made the shock bumper mod you suggest a while back while chasing another problem.
Flieger: we suspected an alignment problem so a different shop rechecked everything. I am right where I wanted to be with no huge extra rear toe or other "smoking gun" kind of issue. Re your questions about the car's behavior: the problem is with the transitions. Going into turn 2 at Willow, for example, the scary part is the initial turn in, after that in steady state the car steers nicely with the throttle. Coming down the hill at Laguna in the long sweeping left hander, it it the transition at the bottom going right into turn 10 where you feel the big weight shift. Again, all of this is normal Porsche. But I have done something which took the normal feel I have grown comfortable with and magnified it a good deal.
At the moment, based on everyone's comments and my own review of the changes I have made this year, I'm inclined to think that two or three small things may have come together to cause my problem.

[Chuck Moreland]

What are the alignment settings now and before.

What is the ride height measured at the wheel arch peak

What are the shocks

What are the torsion bar sizes

What are the sways

What are the tire size and pressures

[moneymanager]

Thanks Chuck, here's the data...1.5 front camber, 2.0 rear. Earlier, 0.8 and 1.5
Caster: 5.2 before, 6.2 now.
Toe: within .03 of 0 front and rear, before and after
Ride heights: 24.875 in front, 24.5 at rear.
Stock Bilsteins
Torsion bars: fronts 21, rear 26
Stock sways: 20mm front, 18mm rear
Tires are BFG G Force, 205/50 x 15's.
Pressures are normally 35 front/36 rear (no change from earlier practice.)