Effect of negative camber on lateral load capability

[burgermeister]

Since there is an ongoing fascination with negative camber, I am curious if anyone has any actual data showing max lateral load capability vs. camber for a typical 50 series street tire? From what spotty "data" I could find in Milliken&Milliken, a degree of camber does not appear to have a humongous effect - a few percent of lateral capability, no more.



My own experiments with accidentally setting the LF camber to +0.5 degrees as opposed to -0.5 degrees (those pesky level indicators read 89.5 either way ) also suggest 1 degree has little effect when using street tires, at least as far as this definitively non-expert driver can tell.

As another thought, according to a=v^2/r, a 6% increase in lateral capability (.85 to .9 g) would only result in a 3% increase in cornering velocity. Not a whole lot for a 60 MPH corner unless my technique was absolutely flawless (it's nowhere close).

Obviously for a race car, every percent matters. It's gotta be optimum.
I would also think the wider the tire and the lower the aspect ratio, the more camber matters as well. No idea how much more or less.

Any thoughts? Am I missing something? Is it more subtle and complicated? Am I just too crappy a driver to notice? I look forward to getting edumacated a bit more...

[surflvr911sc]

Good subject. Practical experience says that it makes a big difference, even on the street. Ten + years ago I had a stock car w/ original suspension set to factory specs, alignment alone transformed that car and 90% of it was increased negative camber.

Submitted mostly just because I like this picture. The tires are 235/40/17 & 255/40/17 RA-1s at -2.5 & -2.0 respectively.

[Bill Verburg]

It's going to depend very much on the
tires
wheels
geometry
track
other use besides track

[Dantilla]

In my experience, the factory settings are great for street driving.

Once I started autocrossing, I was amazed what a difference a competition alignment made, even on the same street tires.

I went from battling for 5th place, to a top contender in the class.

[Flieger]

The main benefit of negative camber on a factory 911 suspension geometry is on the track or when aggressively carving canyons. The negative static camber helps to level the contact patch in the turn when undergoing side loads and suspension/tire deflection. This evens out wear and heat, providing more useful tire life in addition to longer heat cycles and greater grip. Proper warm operating tire pressure also prevents over-working the tire.

Of course, when driving leisurely, the large negative camber does just the opposite.

[burgermeister]

On a stock 911, the contact patch ain't going to be level ... no way enough static camber can be dialed in.

The even wear and heat thing makes sense though - so maybe for a few corners it won't make that big a difference (which I assume is the Milliken graph scenario), but once the tires start overheating on one edge because of continuous high load, high slip angle operation, the difference in wear and adhesion becomes more obvious?

[Chuck Moreland]

The graphs are very short on information framing the tests.

This was likely performed on old school bias ply tires that weren't sensitive to camber the way modern radials are. I don't have my copy of miliken here to check the original publication date, but I suspect it's old enough that bias plys were used.

Further, a few percentage points difference it grip IS a big difference.

[Jack Olsen]

Further still, Ryan's picture is pretty awesome. It ought to be in that 'Essence' thread.

[Peter Bull]

The main purpose of negative camber is to counter the body roll of the car in a corner. When I did the suspension upgrades on my own car I did some calculations on effective camber on the front suspension to see what was necessary to do.

The solid lines in the below diagram show camber variation as a function of suspension travel, and the dash dotted lines show the effective camber. To calculate the effective camber I've assumed that the car rolls symmetrically, so that the suspension extension on the inner wheel equals the suspension compression on the outer wheel. From that I've calculated the body roll of the car as a function of suspension travel. The effective camber is then calculated as body roll subtracted from the static camber. Compression is chosen as negative values of suspension travel, and zero suspension travel is when the car is at rest on the ground.



I've looked at three different front strut geometries.

1) Stock
2) 35 mm raised hub
3) 35 mm raised hub, and 1 degree of extra negative camber added to the hub

Geometries 1 and 2 have about -1.6 degrees of camber at rest, and geometry 2 has about -2.6 degrees of camber at rest.

The black rings mark the amount of effective camber at a cornering acceleration of 1 g, or about 9.81 m/s2 for two different setups

1) 21 mm / 30 mm front and rear t-bars with stock arbs.
2) Leda coilovers and custom made arbs

Setup 1 is used for the blue and red lines, and setup 2 is used for the green line. As can be seen from these graphs, it is almost impossible to achieve negative effective camber in a 1 g turn with a t-bar setup unless you use very stiff arbs or extreme amounts of static camber.

The importance of the effective camber increases with wheel size. A 16" wheel with 50 series side wall is rather forgiving since the side wall will deflect and help keep the contact patch parallel to the ground. On an 18" wheel with 30 series side wall on the other hand, the side wall will not deflect much resulting in a situation where you just use a part of the tire width to generate lateral grip.

/Peter

[burgermeister]

Chuck, I agree that the graphs are short on info. Thus my post, though it looks like noone has anything better.

Fig 2-25 is for a round section bias-ply. fig 2-26 is for a Goodyear Eagle, which I'd assume to be radial.

A few % change in grip might be huge to you - you drive a race car around a track, and are presumably quite skilled and experienced. I drive a street car, and I'm a lousy driver to boot. Would I even notice? As I stated, due to a track day with an accidental misalignment, I couldn't tell the difference between +0.5 (LF) and -0.5 (RF) degrees. My instructor also didn't seem to notice anything amiss...

Peter, I came up with the same conclusion - no matter what I do, I'm stuck with positive camber in a corner. Most of it is due to body roll. (man iti is hard to type with a crabby squirmy tired 2 year old on yer lap!)

Since most of it comes from body roll (camber gain of 0.7 and 0.75 deg/deg Front / rear by my figuring), I see 3 ways to reduce it:
1) Use huge torsion bars to get the car closer to an ox cart with no suspension
2) Use huge sway bars to get the wheels coupled together more like a solid axle (non-independent suspension)
3) Drive a swing axle suspension car, which has near 0 deg/deg camber gain (perfect camber compensation) - but other well know issues, of course.


So far Flieger has offered up a theory why a bit more negative camber might make a difference in actual track usage, and Chuck suggested that a few % is indeed significant (give me a few years - maybe I'll get there! ). But I've heard no fundamental disagreement that the difference is more than a few % ... so I guess I'll keep my occasional DE car close to stock alignment so I don't go broke buying tires.

[rfloz]

No hard tech or fancy graphs, but maybe some practical thoughts.

A stock 911 in good condition is a very good handling car. Any single improvement is going to be fairly small, except for going to R compound tires.

Theory is fine and can be a lot of fun, but what do you want your car to do differently or better? There are plenty of people here who can point you in the right direction.

If you have understeer in slow corners, there are several things you can do (not least would be slower entries); Tail wagging? Same deal. If you have no specific complaint(s), perhaps you are not going fast enough to find them. That's not a criticism. But a car's handling faults don't really show up until you are approaching its limits (Okay, the wife's Infiniti shows much understeer at ridiculously slow speeds, but your Porsche won't).

Can't tell +.5 degrees from -.5? Pffft. Little wonder, -.5 is way conservative. IMO -1.5 degrees in front (with maybe -2.0 in back) would be a good starting point for a street car with occasional track use. That won't kill your tires, unless you drone around the freeways endlessly.

What will kill tires is toe-out. Why would anybody run toe-out? Better turn-in. If your turn in is fine, move on.

Yes, even with quite a bit of negative front camber, most cars, especially those with stock T-bars and sways, will have positive camber at full tilt. But, it will be a lot less that if you did nothing. So, making static camber more negative is useful, up to a point (a tire pyrometer can help find that point).

In auto-cross (my own obsession), wins are all too frequently by hundredths of a second. A change that improves lateral grip by a few percentage points is huge. Since there are no trophies in DE, not so huge there.

Now back to your regularly scheduled theory.

[Flieger]

I like Peter Bull's comment that the 16inch wheels use tires with sufficient sidewall height and flexibility that positive wheel camber can be partially compensated for. Again, I am not saying the contact patch is level to the road, but it is closer than with the low profiles.

FWIW, here are some photos which may or may not have any useful visual information on camber in tight kurves with quick turn-in and weight transfer.

[Dantilla]

Originally Posted by Jack Olsen

Further still, Ryan's picture is pretty awesome. It ought to be in that 'Essence' thread.

+1.

It became my new desktop as soon as I saw it.

[J P Stein]

Caster enters the fray when the wheel is turned off center. The inside tire adds negative camber and the outside tire goes more positive. Porsche designed in an agressive caster for this reason(amongst others). Add that to your charts.

[burgermeister]

Originally Posted by J P Stein

Caster enters the fray when the wheel is turned off center. The inside tire adds negative camber and the outside tire goes more positive. Porsche designed in an agressive caster for this reason(amongst others). Add that to your charts.

I'm a tired and ready for Z's, but I think it might be the other way around - the outside tire adds negative camber, the inside tire gets more positive camber. Alas, I've been wrong often before ...

[Flieger]

Originally Posted by burgermeister

I'm a tired and ready for Z's, but I think it might be the other way around - the outside tire adds negative camber, the inside tire gets more positive camber. Alas, I've been wrong often before ...

Yes, that is the correct caster/camber coupling effect. However, the outside tire is much more important in a turn because it has more grip than the inside tire due to the lateral weight transfer.

[J P Stein]

Ooops. I did say that backwards, didn't I. Oh well, I knew what I meant.

[burgermeister]

Originally Posted by rfloz

Now back to your regularly scheduled theory.

Here in Michigan, theory and a sub-freezing Garage is all I've got for 6 months of the year!

[Chuck Moreland]

Ryan, that is an awesome picture!

If I'm reading it right, Milliken's second graph (the radial one) shows that radials are much more responsive to camber angle. This we know to be true.

For the 800lbs load line, it appears lateral force increased about 10% going from 0 to -2.25 degrees camber.

That is huge.

[burgermeister]

The more interesting and relevant part of the curve is of course to the left of the y-axis (ie, it isn't there), as most 911's are likely to end up with some positive camber on the outside wheel during cornering ...