B8 S4/S5 Suspension Thread

B8 S4
#1

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.

First up:

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
PSS10: 540f/298r
RSS: 686f/343r
Prokit: 336f/171r
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)
Hotchkiss: 32f/25.5r
H&R: 30f/24r
GMG: 28.6f/25.5r
RS5: 22.5r
Stasis: 28r

Ride Height
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:

Front: 27.3l/27.4r
Rear: 27.8l/27.5r

Most B8’s are around 26.8 out back.

Alignment
Below is the stock alignment #'s in the outer end of the range (ie: most aggressive accepted).
Camber: -1.08f/-1.33r
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 :frowning: 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:
http://dreamingin302ci.blogspot.ca/2013/11/suspension-tuning-primer-1-figure-out.html
http://dreamingin302ci.blogspot.ca/2013/11/suspension-tuning-primer-2-look-at-big.html

Why good shocks matter:
http://dreamingin302ci.blogspot.ca/2013/08/shocking-behavior.html

Be considerate of your ride height & roll center:
http://dreamingin302ci.blogspot.ca/2013/06/flckle-roll-center.html

#2

Great info, thanks for sharing.

#3

Great post. Would be nice to be able to track down the effective spring rates for the sway bar options and also add EC’s bars to the list.

#4

I can calculate the bar rates if anyone would be so kind as to:

1: Measure the length of the swaybar
2: Measure the length of each arm

So if the swaybar looks like this: |___________| I need the measurement of the ends ( | ) also, the measurement of the total width of the bar ( ____ ). I can plug it into a calculator and update this thread.

#5

Cool stuff indeed! I’m definitely a noob when it comes to suspension, but I’d really like to learn more.

One question which repeatedly comes up is “will lowering springs paired with stock shocks created a bouncy ride”? I wonder if we could somehow use your research as a building block for modeling the frequency response of the suspension system under different conditions (ride height for instance)? And of course understand what it means for the ride to be “bouncy”. Also would be interesting to understand what we want for track driving (but I understand this can be the subject of entire textbooks).

I’ve been pretty happy with my H&R springs paired with the ADS dampers set to ‘comfort’ on the street. I put it in ‘dynamic’ on the track, likewise impressed with results. But I wonder how much better it could be with proper coilovers ;D

I’d be fine with doing some bode plots or whatever means people perform this analysis.

#6

I dint want to clog the thread but this is a great post. lots of information. Can you maybe go in depth on what adding each part will do body control wise.

#7

Your findings are spot on with regards to ride height.

I remember having issues with my rear H&R OE springs after install looking too tall. After some investigation, some visits to the dealer, I found out that Adaptive Dampening optioned cars came with a plastic spacer at the top of the spring (0.5" exactly). Once removed the car sat level front to rear.

I also had Stasis & Bilstein coil overs, and the Stasis were definitely the best performing option – as in provided the best front end feel and rear end flick ability. However, the stiffer linear spring was harsh on every day expansion joints, and performance wore out quickly (stasis serviceability was questionable). They were non adjustable.

The PSS10 bilsteins had single adjustable 10 click rebound/compression settings. I staggered mine 6 on the front and 4 in the rear to compensate with the weight bias and for a neutral setting. More stock like compliance, but the progressive spring was unpredictable in high speed cornering, giving sometimes snap over steer in the rain.

#8

drob23 - it’s good that you are having good experiences with the OE sport + ADS dampers!
In past experience with other applications, throwing on lowering springs on a stock length strut yields a few problems–even if the strut has sufficient valving for appropriate rebound control, the biggest issue was lack of bump travel.

The whole bump travel thing will vary between cars, as some cars are just poor from factory. For example, the WRX (GD generation) was horrible with bump travel–from factory, you had like 45mm bump travel to play with…so imagine you drop the car 24mm (1 inch). Not only do you drop the roll centers (which by the way, makes the car more susceptible to roll–yes, lowering your car will make it more prone to body roll…all things being equal), but you greatly sacrifice the available suspension stroke. To further add to the problem, you have bump stops. Typically, a car feeling “bouncy” with lowering springs is actually the car smashing into the bump stops on compression.

Most often, a lowering spring is just a progressive spring with a very soft initial rate (softer than stock in most cases), and then it loads up to a firmer rate. Sometimes when you pair a lowering spring with a stock length strut, your available suspension stroke is only able to load up the spring to it’s initial softer rate. This causes the car to run into the bump stops regularly, causing bouncing–because essentially, you are riding on a softer than stock spring and smashing into rubber bump stops. Your OE sports probably worked out bc the B8 simply has sufficient suspension stroke to make it work. Not to mention that the ADS rear end is on a spacer…suggesting that the shock stroke is shorter than usual. So when you lower the car + remove the spacer, you are not compromising stroke too much.

#9

notarmed - not suprised on your findings there. I’m actually kind of disappointed with the available aftermarket suspension offerings for this car. No matter how “track focused” they want to make you think they are, they really aren’t. The spring rates seem to be old carry overs from previous platforms (ie: the same coilover on a B7 has the same rates), and on paper they seem to be more configured towards attaining excellent ride quality rather than improving chassis dynamics. Sure, a good strut is going to be a huge improvement over stock regardless (which is why the PSS is so good). But the spring rates do little for making the car drive better. Yes, the car will be more composed with a higher front spring rate and lower CG. Yes, the car will turn in quicker with the higher front rate. No, the car still won’t rotate off throttle. A box stock Civic Si rotates better than our cars–and none of these aftermarket springs/coilovers attempt to improve on this–with the exception of Stasis/Ohlins (and custom JRZ).

justincredible - it’s not really easy to go into detail regarding what each part does, bc there are almost always dual effects. That is, it’ll improve one area while sacrificing another. But if I were to be super general, and omit any driver input factors, and omit differences in corner entry vs mid corner vs corner exit behavior, then:

Shocks: Shocks with higher rear-end rebound will reduce understeer and rear-end rise under braking. Shocks with higher front compression reduces understeer and reduces brake dive. The inverse is true (ie: more front rebound = more understeer, and more rear compression = more understeer). Again, this is a major generalization.

Springs: Whichever end of the car is softer is the end that will have more traction. So in general, a stiffer rear spring will reduce understeer.

Swaybars: These tie the left and right wheels together. Think of a swaybar as a horizontal spring. As one side compresses, the sway is twisted, and it lifts the opposing end up. This is how a swaybar reduces bodyroll. So the stiffer the swaybar, the greater the effect of lifting the opposite wheel up (because it is able to exert more force on the opposing end). So like springs, whichever end has the stiffer bar is the end that will have less grip. However, the bigger the swaybar you run, the less “independent suspension” action you get…because the bridge between both ends of the car (left and right) is stiffer.

#10

So Boro, are you saying because Stasis went with such a high rear spring rate this helps the car rotate off throttle?

I’m interested in emailing H&R to inquire what their highest rate spring is and might throw it on my RSS setup.

I’ll be honest, road carving in my current setup is ridiculous, the car is unbelievably solid and predictable. I havnt experienced understeer since I dialed it in.

#11

Yep - the high rear rate is to help with off throttle rotation.
FWIW the RSS should be able to handle some pretty crazy rates. Instead, I’d inquire on what range of spring rates that can be run out back…and then buy a hyperco or eibach or whatever you want off the shelf spring. In general though, you wont experience push on the street in most situations unless you are driving like a complete maniac. I did however immediately notice the lack of neutrality on corner entry with the B8 S4 on my first test drive. It’s quite the difference coming from even an archaic solid axle car (hint: the solid axle car was wayyyyyyyyy more neutral) :slight_smile:

#12

Hmm… Please share your findings Jones2012s4.

I’m (finally) getting the RSS+ on my RS5 next week if all goes well.

#13

Will do, the guy I have been corresponding with is John Filipak.

I’ll let you know what he says!

#14

I emailed John this week.
He seems to know his stuff from what little contact I had with him–though he was totally unwilling to disclose OE sport spring rates other than stating they are same as factory rates.

FWIW bimmer guys change the PSS springs to custom rates (to like #700 out back)…and Bilstein makes the H&R struts on the coilovers. So probably a similar thing.

#15

Thanks for the explanation. I have always loved the suspension on my S4 I drove mustangs for a long time and suspension wise the S4 is more stuff and confident. I really enjoy how my car reacts and feels. I have not noticed any fade in how the car feels as it still feels great and stuff. I wanted to make sure I had an idea on how you see each part effect things so if I do feel the car getting less stuff I will know more of what I should concentrate on.

#16

These are the springs they mated to my JRZ RS1 kit. Eibach 7" springs, 2.25" diameter, 500 pound front, 650 rear.

http://performanceshock.com/index.php?main_page=product_info&products_id=1075

#17

West,

Did they use 7" springs for both front and rear? Currently, my H&Rs are 180mm in compressed in the front and 160mm in the rear.

Also, the fronts have helper springs that sit above the main spring and nestle into the isolator pad between the top hat.

#18

Yes, they did do a helper spring:

175.225.150

Which I’m guessing is in the format length.diameter.load_rating

#19

What does a helper spring actually do, relative to not using one? Like instead of a 7" spring with a 1.5" helper, why not just use 8.5" spring?

#20

Found this

http://www.swiftsprings.net/products/universal/assist-helper-springs.html