Running staggered tires in different diameters

B8 S4
#41

Front motion ratio is 0.65
rear actually has a motion ratio like a mac strut. Look at where the spring is located - it’s attached to the knuckle.
The 0.65 number came from reverse calculating Stasis provided #'s, but Stasis messed up on their calculations :frowning:
If you look at the design of the rear suspension, you’ll see why it’s nearly 1:1

pic for reference

http://i.ebayimg.com/00/s/MTIwMFgxNjAw/z/wUsAAOSwo8hTnzov/$_1.JPG

See the rear spring holder? It’s the same part as the knuckle (which holds the wheel bearing). So the wheel is acting with direct force on the spring. It’s on the same pivot point. another pic below for clarity:

http://www.airsociety.net/wp-content/uploads/2013/12/accuair-b8-audi-air-suspension-ride-e-level-bagged-airsociety-011.jpg

At best, the rear motion ratio is between 0.85 to 0.95. Regardless, it’s very close to 1:1 and shows why spring rates for coilovers on this car have such a front bias. But if you factor in the motion ratios, the wheel rates end up being as you’d expect them to be.

For example, the R8 rear suspension, although a different design, has a hard mounting point also right on the knuckle. The motion ratio of that car has been calculated to be 0.875:1. So anyway, it’s safe to say our car is in the 0.9 range. I use 0.9 to calculate my wheel rates for this car

#42

I asked one of the pro’s up here in the bay area who does a lot of in house Audi development including parts on my car:

[quote]rear motion ratio is definitely not 1:1. I have not measured it, but the spring is way inboard, probably closer to .6. I should take the time to measure it someday.
[/quote]

#43

I thought the same thing as your suspension guy until i took off the wheel and actually looked at the suspension. It isnt a matter of spring location…cause that looks misleading. What is affecting the motion ratio is the fact that the spring is attached to the knuckle. That impacts everything.

What I really should say on the topic is this.
Motion ratio is the difference in spring movement to the knuckle movement.
You only get a ratio if the movement of one is faster/different than the other–thus “motion ratio”.

If the spring mounts to the knuckle, as it does in this case, then they would move at the same rate. Thus, a 1:1 motion ratio. It’s not a matter of how inboard a spring is from the knuckle (though in many cases, that is an indicator), but what the hard mounting points are!!!
In this case, the knuckle extends outward to capture the spring, and as the knuckle moves, the spring moves at practically the same ratio. Thus, my calculations for wheel rates on this car for the back is always using an assumed 0.9 motion ratio. but in all honesty, I see this as fairly accurate. Your suspension guy will agree if he sees that the spring is mounted to the knuckle.

For this same reason, if a sway bar end link is attached to the knuckle, then the motion ratio is 1:1 also. It doesn’t matter at which part of the knuckle–as long as it’s on the knuckle, the movement of the sway bar will be identical to the movement of the knuckle.

more views

http://services.edmunds-media.com/image-service/unversioned-ed/ximm/?quality=85&image=/img/track-tests/2009-audi-s5/800-s5-sus-rr-oa-rr-b.jpg

http://services.edmunds-media.com/image-service/unversioned-ed/ximm/?quality=85&image=/img/track-tests/2009-audi-s5/800-s5-sus-rr-oa-under-2-b-thumb-555x373.jpg