77 911S track dimensions?

[TT Oversteer]

I just completed my home alignment and discovered something strange while taking measurements off of the strings.

My front track is WIDER than my rear track by a quarter inch per side for a total of one half inch. Wheels and tires are the same front and rear (16x7 with 205/50s) but I measured from the mating surface of the brake hub to wheel just to confirm.

I recently rebuilt the rear suspension with polybronze bushings and monoballs, etc and everything measured out correctly so it's not a bent rear trailing arm and I don't think anything is screwed up in the rear suspension.

The front suspension was done by the previous owner with plastic A-arm bushings, new balljoints, new Bilsteins.

Could the front wheel hubs be from some other application, maybe to allow bigger brakes? Are there different width front hubs that will fit this car? It doesn't look like there are spacers installed. All alignment specs adusted fine, I just find it weird that the front track is a half inch wider than the rear.

So........what I'm looking for is info on track dimensions for the stock '77 (front vs. rear), a width dimension for the front hub, and any ideas you all may have on this one

[Flieger]

Many racing cars have a wider front track than rear track. It is because the front wheels follow a wider arc than the rear wheels do while turning. The front wheels pivot around the inside rear wheel in addition to going around the curve so they have more distance to cover. The rears just go around the curve. This can cause the rear wheels to clip cones on the apex of tight autocross turns. Therefore, many Formula SAE cars run a narrow rear track- they just say "it goes around turns better". On a high speed circuit, a wider rear track helps stabilize the rear end and control oversteer in faster curves.

The wider the track, the less lateral load transfer due to lateral accereration in a turn. This allows that wider track to be more evenly loaded so both tires are closer to optimal grip/slip angle efficiency in their linear range of operation. Your tires last longer and grip better and there is less body roll.

Many Porsche racing cars had a wider front track than rear track up until the really high horsepower required really wide rear tires. Those wider rear tires needed the wider rear track. The wider tires also had wheels with different offset so the hubs moved around, complicating matters of track comparison. I think the early 917 had a wider front track but it soon transitioned to a wider rear once power developed.

So, don't worry about it. I would be glad to have a wider front track.

[Bill Verburg]

w/ 6" ET36 wheels v/h( f/r),
v 1369mm
h 1354mm

changing width or ET can also change track

[TT Oversteer]

So according to your data, Bill, my car is correct. I never would have guessed that some 911's are supposed to have a wider front track than rear. The difference between your front and rear track specs is 15mm which is approx. = to 1/2 in. give or take a mm. This checks to within a millimeter against my measurements with strings and a tape measure.

I have the tech spec for '78 SC and it shows that car running a wider REAR track than front: 1361mm F and 1367mm R. I assume this is just due to the staggered wheel fitment on the SC with 7's in the back and 6's in the front. The Euro SC spec shows an even wider track and larger difference between F and R: 1369mm F and 1379mm R. which, again I assume is tires.

I never imagined a car with substantial rear weight bias would have a narrower rear track than front. Interesting what I've learned working on this car. Thanks for the info guys.

[Bill Verburg]

Originally Posted by TT Oversteer

So according to your data, Bill, my car is correct. I never would have guessed that some 911's are supposed to have a wider front track than rear. The difference between your front and rear track specs is 15mm which is approx. = to 1/2 in. give or take a mm. This checks to within a millimeter against my measurements with strings and a tape measure.

I have the tech spec for '78 SC and it shows that car running a wider REAR track than front: 1361mm F and 1367mm R. I assume this is just due to the staggered wheel fitment on the SC with 7's in the back and 6's in the front. The Euro SC spec shows an even wider track and larger difference between F and R: 1369mm F and 1379mm R. which, again I assume is tires.

I never imagined a car with substantial rear weight bias would have a narrower rear track than front. Interesting what I've learned working on this car. Thanks for the info guys.

Changing track is one of many tools available to change handling characteristics
wider front track makes it easier to turn ie more over/less under steer
wider rear track does the opposite

wider front wheels work similarly to wider front track, wider rear wheels work similarly to wider rear track

[Walt Fricke]

The track specs for a '77 are 1369F, 1354R.

Surprised me when I read this long ago.

Easily varied with spacers, rim offsets, radical camber, etc., of course.

Walt

[Damon in STL]

Anyone know where to find the rear hub to hub dimension for different trailing arms? Or, the rear track as described above?

I'm trying to find out what the differences are between a turbo trailing arm and a narrow body trailing arm.

[Bill Verburg]

Originally Posted by Damon in STL

Anyone know where to find the rear hub to hub dimension for different trailing arms? Or, the rear track as described above?

I'm trying to find out what the differences are between a turbo trailing arm and a narrow body trailing arm.

The turbo trailing arms push the rear wheels out 60.mm per side more than 911 trailing arms

[Damon in STL]

Perfect....that was the info I needed!

[Bill Verburg]

60.5mm per side