Porsche's way of measuring ride height vs. the fender method. What gives?
 

Measuring fender edge to ground my car is 25.5" front and 25" rear. Both front are equal and both rears are equal ruling out bodywork variances.

According to the Porsche method (vertical distance of center of torsion bar up to wheel/axle center) I am very far off.

Porsche spec for their method is 108mm for Euro spec, 99mm for USA spec.

My car (with the 25.5" and 25") is at almost 150mm as Porsche measures it.

What gives? I thought 25.5"front, 25" rear was close to Euro spec. Accoding to Porsche, it's 42mm (1 5/8") too low compared to official Euro specs.

Wheels and tires that are taller than the normal Porsche size?

If you then lower your car until the fenders lips are correct this would produce a larger difference between the center lines of the wheel and torsion bar.

Good thought.

My set up is 225/45-17 front and 245/40-17 rear. Overall diameter is 24.75" as measured on the car.

Thanks for being the only reply.

Just ran into the same problem with my 79 SC, with virtually identical tire setup. With my front fender at about 25.5", I found that my measurement "C", or difference between height to center of wheel hub and height to center of torsion bar, is closer to 6", not the 4.25" that porsche specifies. Oddly enough, the rear measurement, with the fender at about 25", isn't too far off from spec. Everything else looks right: 1 degree slope from rear to front, fender gap to tire, tie rod looks about level or sloping slightly down to the wheel (after installing bump steer spacers.

Did you go back and check what tire setup porsche might have been using in their specs, and how that would have effected the height to wheel hub center measurement?

thanks.

How are you guys making the measurement? If memory serves me correct, this is a very difficult measurement to make correctly...

???

-Wayne

There are a lot of things that can make this measurement very difficult to be accurate. The surface that you are measuring from must be level. That's all four corners level. All of the concrete pads I have that look level__arn't. On my new floor in one of the bays, I'm going to secure some metal pads that will be shimmed to be dead level.

Good luck,
David Duffield

If I recall correctly it was center of the torsion bar tube cap to ground subtracted from the center of the wheel to the ground. The torsion bar to ground number was measured at the rear of the front bar...the end with the height adjust. I even ran a plumb from the center of the wheel to the ground to make sure I was square.

The floor being level is definitely a factor. Do you think it can cause this much variation from factory spec?

Also, I think I checked my numbers against spec published in Bentley (for sure) and Haynes (maybe, I can't remember 100%). I did not check any factory published materials though.

The difficulty (getting under the car at static ride height) of making a correct measurement is there but usually human error accounts for 1/8" to 1/4" variation but not the large discrepancy I came up with.

I am puzzled. Any additional thoughts you have would be great.

Okay, since there is some interest in this, I dug out my my Porsche technical specifications book for the 78-81 model year 911 SC. It states that the standard rims on the car were (in front) LM g 6J x15, shod with 185/70 VR 15 tires.

Assuming this is the setup they then use to measure ride height, height to the center of the wheel hub should measure half the wheel diameter plus tire profile measurement, or:

(roughly) 7.5" (19cm) plus 70% of 18.5cm (tire profile), or 13cm, for a total measurement of 32cm, or 12.6".

This compares with my setup: 16 inch rims with 225/50-16, where half the wheel diameter plus tire profile should be:
8", or 20.32cm, plus 50% of 22.5cm, or 11.25cm, for a total height to wheel hub center of 31.57cm, or 12.43"

And the odd measurement question remains unanswered by this small difference. But please, check the process above and see if I've missed something.

What is their spec in the factory book? Does it match what I had listed at the top of this?

Yes, the factory book spec is 108mm(+- 5mm) for the european ride height, or 99mm(+- 5mm) for USA and Canada.

BTW, my Bentley manual has these specifications reversed in its section on front ride height, which further confused my efforts for a while.

FWIW..all Porsche tires from 356 era-on....were about 24.8"-25" overall rolling diameter. Amazing..
---Wil Ferch

I guess a factory method on the rear could confirm a straight tub.

See what I and others say here--> http://forums.pelicanparts.com/porsche-911-technical-forum/199332-ride-height-fender-lip-vs-t-bar-centers-way-out-wack.html

Wil