aftermarket custom spring rates for JRZ suspension

I’m working with a couple of vendors on sourcing a JRZ RS1 suspension setup from the Netherlands. I’m decided on the dampers but I’m not firm yet on which spring rates I should choose, or even which spring manufacturer top of the line. I’ve heard good things about Swift; I know less about Hyperco. All in this is going to run me about $3600 plus CA sales tax and freight from Europe.

From the expert:

“I am thinking something around a 500 front and 650-750 rear would be a very nice package. This will have satisfactory ride and enough platform for track days. We can make the dampers and hardware such that OE ride height and 2” lower can be achieved."

Any idea why so much in the rear? My RSS+ from H&R uses 686 front rate and 343 rear.

Like wise the Bilstein PSS10’s use 540 front and 298 rear.

I know these are not full blown top of the line track suspension parts but still brings up the question as to why the difference.

My only reference point is what the BMW guys are doing. For the 1M coupe:

“Spring rates used on the BMW 1M JRZ RS1’s are Swift 336 lb/in (6kg) front and 672 lb/in (12kg) rear”

The BMW’s have less weight in the front, not to mention RWD (load transfer is important to put power down) and have a macpherson suspension in the front vs our double wishbone (I believe it’s a variation to be precise).

Not saying I’m an expert, but I wouldn’t base numbers off BMW applications, at least look into what high end options for our cars run, like what Jeff suggested or the ohlins that come from Stasis.

How do you like the H&R RSS+? Mine should be arriving this month (ahead of my trip to Laguna Seca).

The 1M with a driver in it it 54% over the nose on a corner balance scale, so it’s a spot on comparison (other than weighing 34xx pounds to our 38xx).

The quality of the damper determines what spring rate you can do. A high rate in the rear “increases oversteer” in the same way that a stiff rear swaybar would. I plan to set my eurocode setup to soft/soft when the dampers are doing the work that they’re supposed to.

http://www.dynocomp.com/data/detail/big_363.jpg

What are the over all end goal with the suspension? Have you upgraded the sway bars? I’ve found that really stiffened up the car while making the car more nimble. Too heavy in the back makes for a very uncomfortable ride no?

GMG told me to run 800 front 1000 rear when I asked for a quote on the JRZ RS Pro’s.

Like I was saying, the swaybars are kind of a hack. The dampers are really what control the car. Spring rates can be proportional to how fantastic the damper is. I think JRZ is going to provide me with the optimal balance of top-of-the-line quality without the frequent rebuilds of the Ohlins kits like Stasis Challenge. JRZ has a simple 1-way design.

By keeping most of the ride height, it should be extremely driveable.

OK suspension nerd chiming in here.
I feel that this entire question is misguided. Or perhaps, it can be asked in a more insightful way.
First, the technical:

Spring rates cannot be compared from one application to another with ease. The thing that one must take into consideration is motion ratio. A 500lb spring on 1 application is not going to be a 500lb spring on another. To clarify, what you are after is wheel rate. That is, what the actual spring rate is at the wheel, after all the leverage of the suspension arms, pick up points and suspension design has been considered.

In a BMW, the front end is mac strut. In general, you will get 90 to 95% of the spring rate at the wheel…so you are only losing 5 to 10% of the spring’s stiffness at the wheel (ie: a 500lb spring on a mac strut car is more like 450lb at the wheels aka the wheel rate is 450lb/in).

With a bmw, you have a mix of suspension designs. You have max strut front, and multi link rear. On an E36, you are getting 94% of the spring rate at the front wheel, and about 60% at the rear. That is why the spring rate is greatly biased towards the back–it’s to compensate for the suspension geometry (lever forces)!

So with that out of the way, you first must figure out what the motion ratios are for the B8 before figuring out what spring rates would make sense. I write more about this on my blog, along with the mathematics of the calculations here: http://dreamingin302ci.blogspot.ca/2013/10/wheel-rates-vs-spring-rates.html

Technical aside, just what are you trying to address with the spring rate change?
Specifically, on your car, what did you want to improve? How does it not suit your driving style, which parts of the corner did you want it to react better to…and how? Provide examples, and under what sort of driving inputs the car is behaving a certain way in… All of these aspects come into play when determining how to tune your suspension.

Phenomenal response. You definitely have the sounds like he knows what he’s talking about award for this week. Do you own an Audi? Also you mentioned wheel rate. How do you go about finding out what wheel rate is?

Yes, I do have an Audi :slight_smile:
I just picked up a '15 S4 3 weeks ago.

Wheel rate can be measured. It’s easy (though tedious). I’m curious to know the wheel rates myself…and may be tempted to one day do the measurements. A how-to can be found here: http://alison.hine.net/cobra/tweaks/motionratio2.htm

I suspect the front is around 0.7 and the rear is around 0.65. The rear, being a low profile multi link, is similar to how bmw’s are packaged. And they are generally around 0.65 out back. The front is firmer, because the strut is placed closer to the knuckle (ie: pivot point is further outward), effectively making the front end geometrically stiffer.

If we assume those numbers are true (they aren’t! But I bet they arent that far off), then taking the H&R RSS as an example, the wheel rates would be:

336lb/in front, 145b/in rear.

I personally would not like those rates for how I’d want my car to drive :slight_smile:

Update!
Edmunds concurs that the front motion ratio is 0.7 :slight_smile:
http://www.edmunds.com/car-reviews/track-tests/2009-audi-s5-suspension-walkaround.html

boro92, great information and thanks for sharing… Oh and welcome to the forum… We are in the process of putting together what we hope is a comprehensive post for the B8/B8.5 S4 from a performance standpoint… If over the next few months you could contribute some of your knowledge on the suspension front it would be appreciated… I know the road race guys would love it… Here is the post… Once again welcome to AR!

http://audirevolution.net/forum/index.php?topic=2656.0

This is very enlightening and thank you for sharing. Would the motion ratio change with a specific alignment, like if you dialed in an extra degree of caster? Or if you compared a car with OEM unsprung weight vs. 100 pounds lighter from brakes/tires/wheel/suspension replacements?

I really just want the tire to make more contact with the road, without slamming the car. It’s important that I can drive off track without losing my front end, that I can drive on rough pavement on the edge of the track without upsetting the car, and that I can clear some of the steep hills and approaches in SF.

Thus far I have 034 upper control arms, and spherical bearings in the front lower control arm (2 places), and a spherical bearing in the front lower link. I’ll likely revert the sphericals in the front lower control arm when I do the JRZ kit, as they’re a bit noisy and I’ve put about 15k miles on them so they don’t have much life left. They’ve done a fantastic job of keeping the static alignment true when the car is moving. You can even see it when the car is under hard braking - it dives/squishes less.

On a related note, would an RS5 front lower control arm drop into a B8 S4? Or is that part specific to the wider tracked RS5?

Great response and addition to the thread boro92!

I’m paying close attention too as I will be starting suspension mods soon (H&R RSS+ arriving soon).

I can take better pics of the front suspension of the RS5 next time I have it up on a jack. It would be interesting to compare.

I have some pics on my thread below:
http://audirevolution.net/forum/index.php?topic=2372.0

Thanks everyone for the welcome. I’ll contribute what I can.

Westwest - changing alignment (camber, caster) via conventional methods (shortening upper control arms) will not change motion ratio. Nor will lighter wheels. Motion ratio has to do with where the suspension points are mounted on the chassis, and the location of the spring/shock location in relation to all the suspension arms.

If you consider the lower arm alone and think of it as a lever, having the strut attached further inboard = softer suspension. Moving the strut outboard towards the outer edge of the lower arm (where the knuckles mount) = stiffer suspension. It’s simply adjusting the length of the lever to change the acting forces on the spring. This is in essence what wheel rate is.

So in general terms, spring rate matters bc you buy springs based on their rate. But the end goal is the resulting wheel rate after you’ve installed the springs in your car.

There’s a whole TON of other things at play here that affect handling balance (alignment, roll center, anti squat/dive, ackerman etc etc). But if you’re at a point where you want to change the handling in a big way with springs alone, we’ll need to know what you’re after–that is, how you want the car to drive.

IE: Maybe you want the car to work a certain way off power, and then work a specific way on power.
Or perhaps you want to enhance mid corner grip so that you can get on power sooner. That sort of thing. Those specifics will detail out both alignment changes, geometry changes (even ride height!!!) along with spring rate.

To go along with this, wouldn’t changing offset via spacers and/or aftermarket wheels/tires also affect suspension performance?

Absolutely.
Changing offset affects ackerman. So in effect, if you go with a wider front offset, you are actually reducing corner entry response. The car will not want to turn in as rapidly as before, bc the front wheels are fighting each other. Think of it this way: If both front wheels are steered, like so: / /

Then the outer wheel is travelling a longer distance than the inner wheel due to the distance between both wheels (ie: the inner wheel is turning a smaller circle). However, both wheels are angled the same amount!!!

Ackerman is where the inner wheel turns sharper than the outer wheel, so that they account for the differences in distances traveled while cornering. Alternatively, toe out does a similar thing. This is why a car with more front toe out yields better turn-in response.

You can also play with offsets to affect weight transfer during cornering. Whichever end is wider is going to be transfering more of the suspension loading to the end which is narrower. Think of a car setup like this from top view:

| |
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The above represents 4 wheels. The top is the front, the bottom is the rear. Imagine the car cornering. Now the suspension is in a “load” situation, where one side is loaded due to body roll. The wider end actually transfers some of the roll to the narrow end, as it’s easier to tilt something on a narrow platform than it is to tilt something on a wide platform. So in the above scenario, the rear of the car will want to roll more than the fronts would.

So as you can see, even playing with offsets alone can affect handling to some degree (although mostly negligeable, unless we’re talking putting fat spacers in).

Here’s the crazy complication in the scenario above (wide front, narrow rear): Putting wheels further outboard will make your motion ratio slightly lower (geometrically softer). However, putting wheels further outboard also changes how much weight is transferred per corner during cornering–in this case, the front end rolls less, despite having a softer wheel rate than it originally had. This is offsets at play.

It’s a given and take situation in all cases. Often times, we’re talking about small changes, so the net result wont be huge. But there are many factors all the time.

Ugh, i realized my post was confusing, and i can’t find the edit option :slight_smile:

To better answer the question:
Wheel offset changes ackerman. Which, when the front wheels are steered, each wheel is carving out 2 different sized circles. So yes, absolutely, offset affects handling.

Changing offset to a wider offset alone would slow down turn-in response. The greater the distance between the 2 front wheels, the greater the issue. This must be corrected with ackerman or toe out.

More on this topic in my suspension tuning blog post: http://dreamingin302ci.blogspot.ca/2013/11/suspension-tuning-primer-2-look-at-big.html