Title says it all. This thread is not about stance or ride comfort but about performance suspension setup for both street and track. It's my hope we can dump lots of technical data in one spot for both OEM and aftermarket parts so the end user can better assess handling changes using aftermarket performance parts.
As forum members add information, I'll update this post with the latest data and information. Going to build this up over the next few days as I have time so it's very incomplete.
If you're a manufacturer or vendor, please join the discussion or PM me with technical info that might help the end user.
Quite simply, suspension in a nutshell is designed to keep your wheels planted on the road. If the road were always perfectly flat, this would be easy. Springs are designed to support the weight of the vehicle going down the road. Dampers are designed to control the spring's motion up and down but they also dampen the sprung and unsprung weight acting on the spring as well. Every spring rate has a frequency, a rate at which it will compress and extend in a given amount of time. This frequency is affected by the mass at either end as well as the overall suspension design.
If you have an unmodified A5/S5/RS5 and are thinking about modifying the suspension, first take the time to carefully evaluate what you don't like about the way it handles. If you can break it down and compartmentalize different deficiencies, you'll be better able to formulate a plan and select components which will have a positive effect on the car's handling. I usually spend a few months daily driving a car before I decide on suspension (or other) modifications and even take notes.
I'm at a real disadvantage with my RS5 in that I bought it with several modifications already in place. I drove quite a few RS5's prior but when it's your car, you drive it differently. Evaluating the stock suspension gives you a good baseline to work from.
I like to build/tune the suspension around the tire/wheel package with the tires being the most important component. You can have a really expensive, complex suspension setup and if your tires can't take advantage of it, it's wasted money. Every time a variable is introduced, and the car's tire does not stay in contact with the road, you're loosing mechanical grip. Shocks too firm? Loosing grip. Sway bars too big? Loosing grip. Spring rates too high for the roads you're driving on? Loosing grip. Mismatched components that don't work well together? Loosing grip.
Sticky tires will also allow you to run a higher wheel frequency. It'll actually require it.
Tire choice is a really personal thing. I usually prefer a tire with a more compliant sidewall. I don't pretend to understand the magic but driving on a set of Michelin Pilot Super Sports was a major eye opener. I'd gone from Bridgestones and the amount of bump compliance from the Michelins far better, in a different league. Despite high spring rates and aggressive damping, the tires provided a major improvement in compliance over bumps.
At the same time, any loss in steering precision or initial turn-in couldn't really be felt. I did think the Bridgestones had a hair more turn-in response at best. I was sort of shocked by this. There didn't seem to be a downside to the "softer" sidewall and the mid corner stability was just as good as the Bridgestones with better traction on exit.
They were also several pounds lighter each which probably helped reduce the suspension harshness as the damper had an easier time controlling the unsprung mass.
Roll Center
Roll Rate
Natural Frequency-Picture a mass on a spring (with no damper). Push down on the mass and the spring will oscillate up and down at a specific rate until the force dissipates through heat and friction. That's the natural frequency. If you were to drive your car down a road on nothing but springs (no dampers) and hit a bump, the springs would compress and decompress and the car's body would move up and down until the bump force dissipates.
Wheel Frequency-This is measured in Hz and it's your tire oscillating as you drive down the road. A faster rate gives a harsher ride while a slower rate will feel more plush. When you start getting into the 2hz range, you're on the performance end of the spectrum. A luxury sedan will usually have a rate below 1hz and a high performance sports car will have a wheel frequency in the 2-2.5hz range. An all-out circuit race car may have one even higher in the 3.0hz range. So this rate, more than anything, will tell you how the car will ride before slapping on that new coil-over system. I'm guessing (for now) something like the RS5 has a wheel frequency somewhere in the 1.5hz range for the OEM suspension.
f = 1/(2π)√(K/M). f=frequency, K=spring rate, M=mass But mass is the total weight minus unsprung weight which includes everything not supported by the spring such as tires, control arms, wheels, brakes, etc...you can physically measure some of the parts and you'll have to make a guesstimate on the rest unless you're willing to disassemble the suspension. The last time I took my front suspension out, I forgot to weigh it. Next time!
You'll have to corner weight your car to start. From there we have most of the pertinent information assuming you know your spring rate at each corner of the vehicle.
Motion Ratio-The amount of shock travel for a given amount of wheel travel (think of a lever with different pivot points). Theoretically, since the suspension design is the same for all three models, the motion ratio should be the same. You can test this yourself by jacking up the wheel hub assembly an inch (measured at the spindle centerline) and then measuring the change in distance between the top spring mount and the bottom spring mount.
A5-
S5-.628 front, .6 rear
RS5-
Wheel Rate- This is the ratio of your shock to wheel travel. This is your effective spring rate once a spring of a certain force is installed on the car. If you have a 500lb spring and a motion ratio of .5, your actual spring rate is 250lb. This is what you feel as a driver when driving down the road.
Flat Ride Concept-I was introduced to this on another forum and what it essentially revolves around is slowing the front suspension's reaction to a bump and speeding up the rear suspension so that the rebound mechanism causes both wheels to land on the pavement at nearly the same time and vibrate at the same frequency. This supposedly gives you more confidence but I'm not entirely convinced. I think it probably works better on a short wheelbase car like a Miata or AE86 than it does on a longer wheelbase car. The number one goal is to maintain contact with the pavement. I'm sure there's some tradeoff middle ground and a harder rear suspension can send you flying out of the seat. A soft front suspension often doesn't control the spring well enough and introduces more body motion. Greater body motion, shifts in weight, can affect driver confidence and transient response. Damping curves, particularly high speed compression damping, can effectively control body motion to the point where it doesn't impact traction. Flat ride is also supposed to make the ride less "harsh" but it requires a higher rear wheel frequency rate. Generally, the rear is 20% firmer than the front and it does help with different front/rear oscillations, reduces pitch and it feel like the front and rear suspension are reacting to the bump in unison. If the fronts are "vibrating" at one frequency over imperfections and the rears another, this can make the car feel unsettled as the chassis flexes at two different frequencies.
I can see some benefit to this concept on an AWD, front heavy car. A stiffer rear suspension can help the car rotate much like a firmer rear sway bar does. Everything is cause and effect. But as you want more performance out of your vehicle and value performance over outright comfort, you'll move away from the flat ride concept because you'll want a higher front wheel frequency rate.
Spring Rates-This
The factory springs are linear meaning their rate does not change as the spring compresses. Progressive rate springs are just that; as they compress, the spring rate increases but it's not linear. Progressive rate springs seem to be a very popular choice by aftermarket manufacturers for the B8 platform. Racers prefer linear springs as they're more predictable to the driver and easier to "tune".
Spring rate in lbs/in
S5 3.0T-480 Front, 200 rear
RS5
A5
A5 Sport
Sway bars
OEM RS5 sway bars (Solid)
Front-28mm (verified)
Rear-22.5mm
OEM S5 3.0T (Solid)
Front-? (26.5mm guess)
Rear-21.7mm
OEM S5 4.2L
Front-26.5mm (to 1/18/2010)
Front-25.6mm (1/19/2010 on)
Rear-21.7mm
OEM A5 (Solid)
Front-26.5mm (Sport)
Front-25.6mm (non-sport)
Rear-21.7mm (Sport?)
Rear-19.6mm
H&R (Solid)
Front-30mm (adjustable)
Rear-24mm
Eurocode (Solid, both adjustable)
Front-32mm
Rear-25.5mm
034 Motorsports (Solid)
Rear (only)-25.4mm (adjustable)
ST Suspension (Solid)
Front-28.575mm
Rear-23.8mm
GMG (Hollow, adjustable)
Front-28.6mm (35-85% increase in torsional stiffness)
Rear-25.5mm (83-120% increase in torsional stiffness)
Stasis
Rear-24mm
Hotchkins (Hollow/adjustable)-Not sure if these’ll fit the 5 but they do fit the B8 4’s.
Front-32mm
Rear-25.5
More later...
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Originally Posted by S5Toks
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