So I called around quite a bit and got a bunch of spring rate information from various vendors.
In addition to all of this, I also found some other interesting tid bits. Of course, information is not helpful if it’s not shared! So I’ll share here.
Motion ratios for B8 Platform:
A motion ratio is essentially the net effect of forces imposed on the spring via the design of the suspension geometry and layout. Specifically, a spring is compressed on a car via mechanical leverage imposed on it by control arm(s), linkage(s) and the wheel itself. Basic physics tells us that the longer the lever, the more leverage one has. For example, a mac strut design exerts little leverage on the spring (as there is essentially no lever outboard of the spring), so the motion ratio will be high. Conversely, the motion ratio on a double wishbone car will be lower.
The motion ratios for the B8 (S4 and S5) are 0.628 front, 0.6 rear. Note: This was reverse calculated based on known data. I obtained the spring rate of the Stasis Ohlins coilover, and also through some extra digging found the wheel rate.
Using Motion Ratio to obtain Wheel rate:
Wheel rates are calculated by motion ratio. Wheel rates are essentially the resulting “spring rate” after all suspension forces and leverages have been imposed on the spring. In other words, a wheel rate is the “net spring rate” after a spring of a specific stiffness has been installed on a car. IE: You may have started with a 800lb/in spring. But if your motion ratio is 0.5, then your actual spring rate at the wheel is roughly half (400lb/in)! This is why you cannot compare spring rates from 1 application to another. If 1 rate is known to work well on 1 car, it does not mean it’ll work well on another! If you want to do a more direct comparison, you should be comparing wheel rates. There is a formula to calculate the wheel rate:
(Motion Ratio)^2 x Spring rate
Based on the known stock spring rate of 480f/440r, the wheel rates for our cars is actually 189f/158r
B8 Spring Rate Chart
Stock S5: 480f/440r
Solowerks Sedan: 690f/250-275r
Solowerks Avant: 690f/320-350r
KW HAS: 542f/286r
H&R Street: 450f/450r
ST Coilovers: 690f286r
Stasis post motion ratio: 296f400r
Stasis pre motion ratio: 750f1100r
H&R OE Sport: 480f/440r *According to tech I spoke with. He stressed that OE sport = factory spring rate. But this may only be the rate at static compression. It may ramp up due to it being progressive (vs factory springs are linear)
Sway bar sizes
Stock: 26.5f/21.7r (or 19mm for ADS)
B8’s ride approximately 0.5 to 3/4 inch lower than a B8.5, based on comparative #s between my car and other B8 cars. These are based on fender to ground measurements. What’s interesting is that if you check ETKA, there are approx 5 to 6 different variations for springs at each end of the car. Each spring has a different load rating (and thus spring rate and different spring free-length), resulting in a change in ride height and ride feel. With that in mind, it’s actually not easy to compare cars to each other–unless they are optioned identically the same. For the record, I have a 2015 S4 base model with only sport diff as the option. My corner heights are:
Most B8’s are around 26.8 out back.
Below is the stock alignment #'s in the outer end of the range (ie: most aggressive accepted).
Toe: 0.33deg total f/0.33deg total r
Hopefully this info helps others in making purchase decisions for their car…either based on intended ride qualities, or to alter handling balance for attacking the track. My take away on all this is that 95% of all aftermarket suspension stuff for the B8 platform is woefully inadequate to address the inherit balance “issues” of the chassis. That is, this car doesn’t rotate off throttle, and exhibits a lot of corner entry push. Pretty much none of the basic/more affordable aftermarket springs and coilovers improve on lack of off-throttle rotation. However, most address the corner entry push to a degree (via increasing front roll stiffness greatly – see PSS10, RSS, Solowerks, KW, ST, Stasis). It seems that no one other than stasis focused on improving the overall balance of the car (combining improved corner entry with boosting lift-throttle rotation).
What else is really interesting is that it turns out the RS5 rear spring, though stiffer than the S4, is actually shared with the A6–they are the same part number! Kind of funny. As well, it seems that the RS5 shocks in some instances use the same shock as the S5 cabrio.
Now I wish I can edit posts, bc I am sure I need to go back and update this post as I uncover more technical data Oh well! Hope this helps somebody.
More reading on wheel rates can be found on my blog @ this post: http://dreamingin302ci.blogspot.ca/2013/10/wheel-rates-vs-spring-rates.html
If you are considering tuning/adjusting the car, some food for thought:
Why good shocks matter:
Be considerate of your ride height & roll center: