ErnestSw

It seems to me IMHO that calculating ride height from the floor, no matter what your wheel and tire size, should be relatively easy.
Let's take an arbitrary 25" at the wheel well rim as the factory recommended height. Suppose you have 18" wheels and 275/45 tires and the factory recommends 16" wheels and 225/50 tires.
You can calculate the factory recommended RADIUS of the tire/wheel combination.
The factory recommended RADIUS is 1/2 of 16" + (225mm = 22.5 cm divided by 2.54 cm per inch = 8.86" X .50 aspect ratio) = 4.45" + 8" = 12.45" RADIUS.
In this example with 18" wheels and 275/45 tires the RADIUS would be 9" + 275mm = 27.5cm X 2.54 X 45%=4.9" = 13.9" (at proper inflation). So you've raised the suspension 1.45" at the hub and the ride height should be 26.45" at the wheel wheel rim.

Champagne

Unless I am mistaken, the ride height is calculated from a "flat" spot machined on the undercarriage. You measure from that spot to the floor at each corner. This is independent from wheel/tire combos.

Paul

ErnestSw

Hi Paul,
By my example that flat spot would be raised 1.45" off the floor by the wheel/tire combination. It seems to me that it's the geometry ABOVE that spot that you're trying to preserve and therefore you'd need to add the 1.45" measurement to the distance from the floor. I stand prepared to be corrected.

Ed Ruiz

By going to the tire/wheel combination you mentioned, you not only raise the car's height, but you've affected its gearing. The taller wheel/tire combination will throw off the odometer and speedometer, as well as make the gearing taller, which adversely affects acceleration in all gears. Worst of all, the 928 will look like it's being used as an off-road-vehicle. YMMV.

ErnestSw

Ed,
I used that tire/wheel combination purely as an example. I agree that it's not a practical or sensible combination in most applications.

BrianG

One would suspect that factory "ride height" specs are based on factory spec suspension components, including wheels and tires. All things being at spec, ride height defines suspension component location "at rest", as a measure from a defined but arbitrary datum point. Any component change that would alter that measurement without altering the actual rest position of the suspension components would have to be accounted for in the new definition of ride height. Wheels and tires are the only thing that I can think of that would have this singular effect.

NYDolfan

Excuse me for my ignorance <img border="0" alt="[ouch]" title="" src="graemlins/c.gif" /> , I'm sure there's others that know tons more about this than me, but doesn't the tire/wheel radius remain the same on different sized wheels? The smaller the wheel diameter (16") the 'higher' the tire - the larger the wheel diameter (17 or 18") the "lower" height of the tire. This way the speedometer doesn't have to recalibrated?

ErnestSw

Hi Pete,
A 16" wheel has an 8" radius as opposed to a 9" radius on an 18" wheel. The aspect ratio of the tire determines its height. Some tires have very low aspect ratios to compensate for the increase in wheel size, but the calculations will still use the same variables.

BrianG

Yes Pete you are correct. If properly selected, the wheel/tire combo need not affect the measurement datum, but it is the factor of concern in this consideration.