jphughan

I just noticed the stock alignment specs for the GT4 as specified in the manual, and if I'm converting between degrees and inches correctly, the rear toe is alarming. Here is what the manual specifies:

Front axle
Total Toe +2' +/- 2'
Camber -1°30' +/- 3'

Rear axle
Per Wheel Toe +10' +/- 2'
Camber -1°30' +/- 5'

(UPDATE: Distinguished total vs. per-wheel toe above according to the manual.)

Fwiw, the 991 GT3's specs are identical except the rear camber tolerance is +/- 3' rather than 5'.

Anyhow, since I'm used to seeing toe in inches rather than degrees, I found this site to convert from the former to the latter. I looked at the rear axle first, so I entered 5 degrees for the one-wheel toe value and 27" for tire diameter based on MPSC2 specs on Tire Rack. That site gave me a total rear toe-in of 4.7 inches!! I'm used to seeing toe-in ranges like 0 +/- 1/8" max here on the M3 side of the fence, so this spec is nearly 40x more. Is this website horribly wrong, or do Porsches somehow benefit from a ton of toe-in on the rear axle?

I checked the front toe-in with the correct tire diameter, and 2' seems to work out to just under 1" there, which still seems pretty significant, though granted 0 toe is within tolerance there -- although so is 2"....

Any insight from those more knowledgeable than myself here would be most appreciated.

ExMB

Toe is listed in minutes, 60 min = 1 deg

Dr.Bill

So, doing the math correctly, that would be 0.03" toe in for the front, and 0.16" toe in for the rear. Seems about right.

jphughan

..........

I'm an idiot. I interpreted the ' symbol to be degrees, when obviously it is minutes. Sorry everyone.

user1029

Yeah how much you can shave off your lap time or the life you take off of your tires everyday you daily drive it on the street

4carl

same toe as the 981cs . more neg camber front less rear than 981CS. After i put 500 or so miles on mine to let the springs set ill set it up. -2 F&R 0 toe F .062 R toe. The attached link is a great toe conversion chart a lot simpler than doing the math. carl
http://www.smartracingproducts.com/s...able-cw-v3.pdf

hccyong

You should also note that the front toe value is overall (both wheels), while the rear toe is specified per wheel!




Does anyone know what the caster and steering axis inclination numbers should be at nominally?

4carl

interesting.carl

jphughan

Good call out, updated my original post to reflect this. I think that while I may increase camber to -2 on both axles as many GT3 guys seem to do even though they have virtually the same alignment guidance as specified here, I'll keep the rear toe. Maybe this suspension design results in toe-out while unloaded such as under heavy braking and this spec is designed to compensate for that and provide rear stability? Still, I wonder why Porsche wrote the toe specs differently for the two different axles. Could it have to do with alignment machines measuring toe differently on the two axles, or is it more critical to get each individual wheel right in the rear whereas at the front you can have one wheel slightly toed out as long as you have the other toed in enough to compensate? I'm guessing the latter, since all that would happen on the front in that scenario is your steering wheel wouldn't be perfectly centered when you wanted to go straight and you'd of course be able to countersteer to cancel that out, but the tires should still wear correctly relative to each other, whereas you can't steer the rear wheels since we didn't pony up for a GT3.

As for caster, is that even adjustable in this car? Not sure what you mean by steering axis inclination.

4carl

10' on a 20" wheel is .058" per wheel(see chart) a total of .116"= little less than 1/8th of an inch(.125). carl

jphughan

Taking the width and aspect of these tires into account, the correct value for 10' is 0.081", but actually using 0.065" for 8' on the low end of the range converts more readily to fractions, since that's almost exactly 1/16" per wheel, and thus 1/8" total toe-in on the axle. On the high side of the tolerance at 12' per wheel, that works out to a hair over 3/32" per wheel, or 3/16" total.

orclean1

Is the chassis set up from the factory set for driving on public roads or track?

jphughan

The manual says it's set up for a happy medium between the two, which I take to mean sway bars in middle positions and max ride height, both of which would be consistent with GT3 deliveries, and I'll bet that the car is configured in the lower-downforce setting if for no other reason than it's easier to just leave the diffusers in there than stash them in the cargo compartment for delivery. As for alignment, I would assume it's aligned according to these specs. You may definitely want to increase camber for the track (as I wrote above, -2 for both axles seems popular), but the toe spec is already pretty good IMHO given that 0 toe on the front is within spec and the rear toe is pretty minor.

hccyong

I'd need to see a better drawing or photo of the suspension to see the compliance steer effects of this suspension, but generally most good modern designs will be tuned to provide stable toe-in on the rear from braking torque and lateral forces. Some may provide toe-out under roll though to counter understeer.

It is somewhat common to spec front and rear toe with different conventions for the reason you mentioned. You want rear toe to be symmetric or else the car will crab or have a wrong thrust angle. The front asymmetry will be countered by an off-center steering wheel which is generally obvious and a final step for alignments.

Steering axis inclination is the front view equivalent of caster. On a strut suspension, it's the angle between the top strut mount and the lower ball joint (or where the two lower ball joints would theoretically meet). I wouldn't be surprised if neither caster nor steering axis inclination is adjustable; I just thought Porsche would list it here. But then again, I just noticed that it's not an alignment spec page but a "chassis setup" page, so it make sense to list only what is really adjsutable.