dyi front camber mod

[insite]

the max camber a given tire can use is proportional to the tire's grip capability. if you use too much camber for the tire, you won't be using the entire contact patch & you'll lose total grip.

imagine zero camber: when you turn, the tire rolls over onto the sidewall. the result is that you're using the outside half of the tire too much & the inside half of the tire not at all.

imagine correct camber: when you turn, the tire rolls, but because of camber, the tire actually rolls ONTO the contact patch, giving you 100% of the tread contact with the road.

too much camber: while straight, the tire rides on the inside shoulder. when you turn, the tire rolls onto SOME contact patch, but the outside half of the tire is still not making good contact with the road.

the result of too much camber will be that the insides of the tires wear too fast. also, ultimate grip in corners will be reduced because you've actually reduced the size of the contact patch.

keep in mind that CASTER affects mid-corner CAMBER, so while you may only have -0.8 dialed in with the wheels straight ahead, when the wheels are turned & the suspension is compressed, you will have significantly more (-2.5 or more). it's easy to go too far w/ front camber on street tires w/ the boxster because the car has a LOT of caster designed into it.

to understand caster, it helps to illustrate extreme scenarios:

zero degrees caster: the rotational axis for the steering is completely vertical. turning the steering wheel ONLY turns the wheels right & left.

90 degrees caster: the rotational axis for the steering is completely HORIZONTAL (i.e. straight out in front of the car). turning the steering wheel tilts the tires left and right, but does not turn them at all.

SOME caster: turning the steering wheel BOTH turns the wheels AND tilts the tires.

as to braking, less camber generally means better braking distances. that's not to say that great braking doesn't exist w/ heavy camber, only that great braking gets even BETTER with less camber.

that having been said, the reason i say -1 may be too much for street tires has more to do with wear & grip than braking sacrifice. you will wear the insides of the tires down.

[Pat]

insite~
Would cornering on a road course be different than cornering in AX? One can generate big lateral Gs on a road course around a very long radius corner if driving fast enough. That would mean the caster is almost insignificant, due to minimal steering input. In AX, a LOT of steering input would be required to generate the same lateral load due to the slower speeds, therefore making the caster a much more significant variable.
Thoughts?

[insite]

modern porsches are designed with a LOT of caster in order to maximize the effect at smaller steering angles. as a rule of thumb w/ our cars, every degree of steering angle adds almost a tenth of a degree of camber.

keep in mind, though, that this is not the only mechanism through which the suspension geometry adds camber. the car is ALSO designed to gain camber as the suspension compresses. the rates at which this occurs have to do with the inward angle of the struts & the length of the lower control arms. i don't have camber curves plotted for this car, but i CAN say that suspension compression of 1" will result in approximately -.7 degrees of camber gain. therefore, other suspension changes will modify the camber gain w/ certain amounts of weight transfer. example: soft suspension will compress a LOT, & gain significant dynamic camber through kinematic changes to suspension geometry. stiff suspension will move less & gain LESS dynamic camber, therefore more static camber should be dialed in (comparatively) to compensate.

when someone tells you that their camber is set to -1 or some such number, that is the STATIC camber. the measurement, though, is truly a dynamic parameter.

if you hit the track, you'll want to set camber based on pyrometer readings & treadwear, amongst other things. if you auto-x, the tires never really heat up & your tools will be chalk marks and tire wear.

[baxster]

I'm surprised there weren't more responses about the actual modification. I'll let you know how much camber I was able to get with the mod.

Thanks again insite and harryrcb!!

[Klimo]

This is a great read. Thanks insite. I am in the process of getting my car ready for track days, no auto x. I do about 6 track days in the summer. I am not worried about giving up some street driveablitly or having to go through tires every 10k. I use Michelin PS2s. It sounds like -.8 in the front might be enough to get what we want out of the car, but if it isn't what are my option for increasing camber on the stock suspension. I don't want to be drilling holes or anything into my suspension like what's being talked about on here. Do I need new struts? Can i jsut add camber plates? Thanks.

[insite]

Klimo -

what size are your tires? depending on stagger, you have some options.

if you need more static camber, the best place to start IMO is to lower the car. this automatically adds camber at all four corners. after that, the best options are either camber plates or GT3 style control arms.

[hcj986s]

Insite as usual has got this exactly right. The only thing I'd add is that 1 degree isn't necessarily too much for street tires (or any other tire for that matter) if your suspension is really soft. As the car rolls, the suspension compresses and the geometry changes. As the suspension compresses, camber typically decreases (more negative) (although this is not the case with typical MacPherson struts). Let's say that the stock setup adds 1 degree of negative camber for every 2 inches of compression, which occurs at 1 g.

If the stock setup has 4 degrees of roll per g, then the outside wheel might develop positive camber despite the suspension compressing (you've seen pictures of Boxsters with positive camber on the outside wheel). After all, -1 degree plus 4 degrees of positive roll gives +3 degrees of camber.

Stiffen the suspension to 2 degrees of roll per g and set the car up with 2 degrees of camber static and the story changes: at 1 g, the outside tire is now vertical to the pavement, giving an optimal contact patch. Lower the car, and the roll center falls, increasing the roll rate further.

One final note: static camber is properly set by measuring tire temperatures at the track. If the outside edge is running hotter than the inside edge, then you need more negative camber.

My 50% stiffened 944 Turbo ran 0.8 degrees of static camber in the front with a 1.8 degree per g roll rate. I have yet to get the Boxster out on track.