Un-confuse me about ride height please.
 

I'm looking at the RS specs for example and I cant help but think that if the tire is a bit taller it throws off the measurement completely, resulting in the suspension geometry being off. Is there some ratio or calculation between the pivot point on the lower inner arms and lower outer arms? That would seem to make sense and would put the suspension geometry right where it needs to be. The reason I'm asking is I run a taller tire and would in essence have to lower the coil-over all the way down to achieve the speck ride hight which would put my a arms further up under the car compared to a stock tire size.. Or am I just over thinking this?

There is a spec. I can't remember if it is torsion bar axis to ground or torsion bar bar axis to ball joint in the front. In the rear it is from torsion bar axis to hub axis (axle height).

If it is to ground, then just add half the difference in tire overall diameter for your car.

I was looking into this over the last week because i want to check mine. What i gathered from the threads on the subject is that you measure center of torsion bar to ground and subtract center of wheel hub to ground. You wind up with the difference between hub and tb. That should account for tire diameters, no?

From a previous discussion

http://forums.pelicanparts.com/uploa...1287974422.gif

Edit I see what you are saying. If tire diameter increases that measurement stays the same but the actual ride height is higher. Ill let someone with more experience can offer their wisdom on the matter

Yes, that is independent of tire size.

Since I posted that pic, I'll chime in

The important geometry to maintain has nothing to do w/ height per se, What is important to keep control of is the angles of the steering arm wrt the A-arm. That relationship determines the bump steer characteristic of the car.

Additional factors that affect the geometry of the 2 are steering rack height and spindle height.

Note that tire height doesn't factor in at all, but 911s were designed around a tire that is 25.5" tall +/-

actual oe tire heights have ranged from ~23.1 to 25.5"but the street spec has has been the same for all yrs from at least '72 except for the years when US politicians inserted themselves into the design process
http://forums.pelicanparts.com/uploa...1341923396.gif

http://forums.pelicanparts.com/uploa...1341923352.gif

http://forums.pelicanparts.com/uploa...1341923377.gif

I now get it fully. I will have to find those specs for my TB-less 91 and figure where they measure from. I am thinking all the talk about ride height and how not to go below a certain height is estimates based on the assumption that you are using stock height tires. Or it's just for looks.
Thanks everyone for chiming in.

FYI I just got me some pss9s and was pondering the best height to shoot for.

Any ride height measurements that reference the fender are dependent on tire diameter. Best is to measure the way the factory called for. Most service manuals should have the factory spec and the proper measuring points listed.