Spring Rates Discussion: Stiffer Front or Stiffer Rear or Even/Square all around???
 

I am aware spring-rates only play part of a role in coilover & spring/shock combinations and designs; and that shock valving plays a huge role as well. But, I think we would all love to hear your opinions, experience and insight from both professionals & track/auto-x-hobbyists in regards to spring-rates. Specifically, which end should be stiffer, front or rear, or square/even both front and back.

I notice F&R ratio stiffness varies greatly with all the different manufacturers of coilovers and lowering springs. Anywhere from even square rates all around, to 2kg stiffer in the front, to 2kg stiffer in the rear.

Insight would be great as to why ...and also recommendations on what would be the ideal set-up for certain track/driving situations; and also the most ideal setup for universal/general use that can be used globally and for everything (of course with tradeoffs I'm sure).

ie: maybe stiffer in front to neutralize the car's natural oversteer, or stiffer in the rear for less rear sway or more oversteer, or square/even all round because maybe the car is already well balanced and maybe to have a broader free range and ability to tune the adj valving to the users liking, etc., etc.?

This will be good info for those wanting to buy, or experiment with, custom spring rates for their coilovers (whom offer custom spring rates; ie: Tein, Ohlins, etc.) and also Ground Control coilover sleeve springs (custom spring rates) or their CO kits.



A better coilover (Tein SRC or MFlex, Ohlins, KW, etc.) would defo be nicer; but those are out of the price range for a lot of people. Which in turn, makes it very attractive to go for the GC springs/sleeves with B8's, or perhaps even the GC/Koni kit; as these are the most affordable spring/shock combo that offers real stiff/custom rates along with high quality, heavy duty valving and sound like the most performance/bang for the buck.

I read past posts that GC has a recommended F6.6kg R7.7kg for their kit. Would this specifically be because of the Koni's particular valving? Would the B8's be vastly different and be better with diff rates?

I am interested in a 6/6, 7/7 or 8/8, or maybe a stiffer front 8/6 or 9/7. Would this work well with the B8's or are these too stiff for the B8's to handle or are they completely not valved to handle a square or stiffer front set-up? ...I am sure this will be hard to answer without testing these rates first-hand and/or a deep knowledge of how the B8's valving work and were designed, he he.

One thing I see all the time and is incorrect is people assume a BRZ with 5 kg/mm rates front and rear will have the same exact balance (but different limits) as a car with 10 kg/mm rates front and rear. Increasing both ends by the same amount may alter the balance of the car. Like all cars, the front and rear are very different obviously so things change in different ways at both ends.

For us on the softer side of things we like to keep the rates even. We want to keep the car off the bumpstops up front especially to keep handling natural and linear as well as avoid sudden loss of grip. This also helps keep the rear end in check. Additionally, keeping the front end happy geometry-wise means you still have enough front grip to keep the car from understeering. It can be fun, fast, and easy to drive.

For very firm set-ups with fewer compromises we sometimes go with a firmer rear, but it depends on the system. Cars at this level are usually less bumpstop-active and may run with smaller swaybars. Roll is limited enough that large dynamic changes are less of a concern (for a well set-up car). There is still some room for driver preference for balance.

There are still other variables affecting balance...alignment of course but ride height is an often overlooked factor.

- Andy

Heavily shortened by me.
If you have 4k springs all round the difference is only 1.76k but if you are running 12k all round the difference is 5.28k.

There's the motion ratio but also things like camber curve, roll centers, bump travel, etc that are different and change differently front and rear. So there's a whole bunch of stuff to look at. :)

- Andy

Ah, I see. Nice info on the motion ratio! The steeper curve in F/R ratio difference as you increase rates makes perfect sense. I was wondering why so many coilover makers (KW, GC kit, Tein, RSR, etc.) have much stiffer rear rates. But now at the same time, I still wonder why others have a much stiffer front (HKS, Apex, Blitz, etc.). Did the latter take into account this motion ratio and those other things, or did they just guess and slap on anything for testing? But, I think this would be common sense for these engineers though; as many many cars have very similar setups (ie: front strut at hub, rear shock slightly inwards on lca).

Maybe the coilovers that took long to develop and were released very late (as opposed to very early and right when the car was available) would have the most R&D and might be the best choices. ie: Ohlins Road and Track, Bilstein, etc. only just got recently announced (obviously a lot of R&D went into these)...vs, HKS and other coilovers (which were announced only a few months after the car was released?? lol)

Companies like Ohlins and Bilstein did actual first hand R&D on the exact FRS/BRZ platform...and perhaps, maybe, companies like HKS, Tein, etc., didn't and based it more on their past data/experience with other similar cars?

Bunch of throw away lines coming up:
Everything's a compromise.
There is no one correct way.
Different people want different outcomes/results.
I like chocolate.

It's difficult to know what every company did and why and how long they were working on it. I wouldn't assume that companies that came out with products later did any more R&D than companies that had stuff available earlier. Ohlins and Bilstein for example may have just had more on their plate when the car was released and didn't get around to jumping in until later.

Bilstein more likely was waiting to see what the market would be like, especially for their struts. That's a huge up front tooling cost for them, a lot more than coilovers. They didn't have an application for Subaru Impreza at all for a long time. They do now because they make them exclusively for our sister brand GTWORX. :)

That said, it wouldn't surprise me if a lot of companies just made lower mounts and camber plates that fit, added the same damper bodies with the same valving they use on other cars, and picked spring rates out of a hat. :lol:

But like Captain Snooze said, there isn't 1 right way and different applications call for different set-ups. Some companies have certain tendencies (building for safe understeer) while others don't, or even go back and forth. Some are building for street guys/gals while others are going for more track oriented set-ups.

I've thought about this a lot actually and I've sort of given up trying to figure out why certain companies do certain things. When the GR Impreza came out with a completely new rear suspension, a lot of companies used the same spring rates that they used on the older GD Impreza. I never got a good explanation as to why. Eventually I saw many of them switch to different rates.

The EVO 8/9 was interesting too. Most lowering springs (like OEM) were rear biased because of the rear motion ratio being lower than front. But most coilovers were front biased for....well no one knows why. Some would say it was the JDM way of doing things but no one knows what the time attack EVOs were doing in Japan and anyone who was fast here and knew what they were doing certainly weren't running front biased rates. I think most companies making lowering springs just took the OEM rates and added 10%. Companies making coilovers were just looking at weight distribution. When the EVO X came out, the market was more educated, and a lot of companies changed their line of thinking.

- Andy

It is a very complex problem with more than one correct answer.

The key to satisfaction when upgrading suspension (and brakes) is to deal only with reputable manufacturers and parts suppliers who have done their homework properly.

Factory suspension engineers have huge resources to design and manufacture these systems. The same big aftermarket manufacturers of springs, dampers and brakes also supply the car makers. Key to this is the car manufacturer relies heavily on their suppliers to do some or sometimes all of the engineering on different sub systems in the cars.

It would be good to be able to read or see a video comparing the various aftermarket suspension set ups. However, to be useful you would need to have an idea about what you expect from your planned modifications. There are so many ways to set up a car that you need to develop an idea of how you want yours to behave before you go shopping.

This.

Certainly a personally preference. My car is under going a turbo conversion and the new found power is making me rethink my suspension setup since rear end traction will have a new found importance. Also I will be driving the car differently with the power. Ultimately I've just jumping in and will change it up a few times before finding what works for me.

Linear vs Progressive is another point that needs to be considered. I like linear myself.

Yes...and this is what I am worried about (and I am sure everyone else too) and why I mentioned r&d time and how could some companies release coilovers a few weeks after the frx/brz just hit the dealers (a bit tooo fast, no?). No one wants to spend their hard earned money(which isn't exactly just a few hundred dollars) on a half-@ssed engineered, nor lazily/hastily designed, $2-3k coilover system, where the "well-known named suspension company" just reused the same dampers/valvings from their other older models + a spring rates that sounds like it'd work , etc., etc. Makes me wary how Tein, HKS, etc. could release coilovers so quickly. I really hope they aren't doing this..lol.




Yes, I am wondering why so many JDM rates are so front bias, while US/Europe brands are the opposite.

But, after reading many threads on here with track experience from diff people and diff coilover/spring/strut setups ..I see some similarities in what people want and it might make sense. ie: I read some wanting to get a stiffer front sway bar, or others wanting to ditch the rear sway bar completely, and others wanting to get softer rear spring rates for more grip because stiffer rear gets too playful/squirmy under acceleration, etc... which are all reasons gravitating towards, and having a similar result, to going for and having a stiffer front spring rate. So maybe the JDM company's front rate bias tendencies have something going for them and are saying something?

Already mentioned, bears repeating: Wheel rate is what is important. Spring rate is a *means* to get the desired WHEEL rate. It is the wheel rate that determines natural frequencies and handling characteristics.

Given the motion ratios, I can't imagine that stiffer front springs would be a good idea.
Front motion ratio is ~.95, rear is ~.75.
That means that front wheel rate is (.95)^2 or 0.90 x spring rate, while rear wheel rate is (.75)^2 or .56 x rear spring rate.

To have equal wheel rates, rear springs would have to be (.90/.56) = 1.6x stiffer than fronts.

A set of springs balanced for the car's 54/46 weight distribution would suggest a front wheel rate 1.17x stiffer than rear wheel rate.

To achieve that you'd still need stiffer rear springs, 1.37x stiffer than fronts.

I would think that same stiffness springs all around would be too much front bias. My preference would be to start with rear springs significantly stiffer than fronts.

Even "weird" setups can be made to work, but in my experience I've generally ended up with the wheel rates biased to the rear relative to the weight distribution, and balanced with stiffer front sway and sometimes even removed or disconnected the rear sway.

How does the changing roll centers when lowering the car play into this?

Short answer: It doesn't play into front/rear frequencies and ideal desired front/rear wheel rates. It does affect the handling balance of the car, though. You *could* take the approach of "correcting" any unwanted effects from different changes in front vs. rear roll center heights by changing spring rates, but I'd be more inclined to try to fix the root cause of the problem (if it's really even a problem at all).
Outside of that, I would probably address it with changing front/rear roll stiffness with sways rather than go with what I consider wonky front/rear wheel rates.

I don't have specific knowledge of the effect of lowering the car on front and rear roll centers. I might assume that the rear roll center would be affected more than front, but that would make the rear effectively *softer*, which would imply you'd want *stiffer* rear springs to compensate. I could be off on that, though, so anybody with the geometry please share!

You also should consider that the suspension designers probably designed the suspension to be optimal for maximum handling performance at a ride height setting LOWER than stock. That's what I would have done...

Lowering the roll centre increases roll at that end of the car.

Height of CG relative to roll centre (technically the roll axis as the two roll centres can't be separated) determines the moment acting on the road springs and bars.

Lowering the car lowers the CG, that's how roll is reduced by lowering.

Changing camber can change the roll center but not by much. Roll axis is pretty much fixed. McPherson struts will develop a slightly different roll centre if you install and use camber plates.

There are obviously a lot of opinions on this and arguments can be made either way - what I do notice is that most fast setups that I know about seem to have spring rates front and rear within ~1 kg/mm of each other - front biased, rear biased, or even.

The fastest STX twin (autocross) was running KW V3's with the front and rear springs swapped at for 400/340 F/R, in addition to a stiffer front bar.

RCE and CSG seem to prefer even rates or more rear-biased, depending on stiffness it seems (what Andy was talking about keeping the frequency bias the same vs the spring rate bias).

Here's some numbers I put together for spring rates, wheel rates, bar rates, and how it all contributes to frequency:
http://www.ft86club.com/forums/showp...&postcount=107

Yes, it's just one piece of the puzzle as there are so many other factors (ride height, travel, alignment, DAMPING), but the numbers are still useful. This should at least help if you are looking to keep the NF "balance" the same as you go stiffer overall.

And also it will really depend heavily on driving style and talent.

That's because if you want to be competitive, an AutoX setup and a track setup are mutually exclusive.

AutoX priorities:
- lightning fast weight shifts and small direction changes (less than 30 degrees). This is for slaloming.
-- This is achieved by REALLY high front spring rates, and a giant front sway
-- This also makes it so that under sustained cornering (sweepers), the car pushes like CRAZY, so that's why AutoX cars go deep, brake hard, and power out, rather than make a more rounded arc.

Track priorities:
- Sustained cornering speed, and being able to put power down at corner exits
-- This is achieved by getting a nice static balance.
-- Keep in mind that under power, you're unloading the front, and loading the rear.
-- Track prepped cars will sustain more raw cornering Gs, but not change direction and shift weight as quickly.


For example:

AutoX prepped FRS
- 14k/9k spring rates
- Blade type front sway, minimal rear sway

Track prepped FRS
- 10/12k spring rates
- Sways to fine tune balance.

Each car has its strengths and weaknesses. The AutoX car will likely slalom ~20% faster (raw mph), but the track prepped FRS will hold more speed on a skidpad (raw mph).

^I think another contributor is torque - in autocross, you're almost exclusively exiting corners in 2nd gear, where there is still a lot of torque amplification and can power oversteer pretty easily. In higher gears there might not be enough torque to break the rears so there's still some grip availability and you would want a more oversteering attitude.

If you're cornering hard enough, and the car's loading is balanced to allow it, the rear will still come out.

Fast forward to 4:25, and you'll see this FRS break the rear end loose at close to 127mph or so.

[ame="http://www.youtube.com/watch?v=HCWz5ymLKrI"]Scion FRS vs Porsche GT3 86Fest II - YouTube[/ame]

Also very true! Definitely application specific. Driver preference plays a roll too.

- Andy

*Raises hand*
Why are you squaring the motion ratio?

Thank you for this post. I was thinking exactly this as I was going through the thread.

This is why highly developed, track oriented coilovers (a la Ohlins R&T) often run a rear biased spring setup. I ran that setup in my e46 M3 and the rates were 400F/630R. It was definitely a track vs autocross setup.

This also seems to be the case with the GC springs setup, and frankly to a lesser extent the FRS stock setup. My .02

When you shift gear the drop in rpm is so low and steady... Great video.
I have to improve my shifting much more, even if I have driven manual for 7 years I still can't shift and accellerate in such a smooth way like you were doing (at least with this car)

Counterpoint: On my maximally-lowered E36 M3 race car, we're running

Moton 3-ways
1599F/343R
Adjustable 38mmF/non-adjustable 32mmR hollow sways
Toe-outF/toe-inR.

Turn-in is exceedingly crisp.

FWIW, the spring rates are uncommon on these cars, yet they work quite well.

While I'm no expert in these matters, I've developed the opinion that there is more than one way to skin this cat.

Absolutely agreed, and I'm no expert on any of this either. What we can do is speak from our various concrete experiences and let folks who stumble on this information deduce what they may.

Of course a three-way Moton is a highly specialized race damper that probably has the flexibility and capacity to compensate for a low-rated spring to a certain degree which coupled with super thick bars is working for you. For comparison, if I'm recalling correctly, my e46 M3 stock front swaybar was only 25mm.

I also remember hearing something about the unique peculiarity of e36/e46 chassis front suspension geometry that allows them to run huge front bars and little spring compensation needed in the rear. Could be a buncha baloney :iono:

Without factoring motion ratios, spring rate bias is pretty useless. Not saying you're wrong with the theory, just that if the motion ratios are different front vs rear on the M3 like they are on the FRS then you need a higher spring rate out back just to give equal wheel rates. That's not taking any handling bias into account either.

The car was built by Jim Bell for Grand-Am GS II, and I think the seemingly way-out-of-the-box springs and bars were chosen for the pro driver (Terry Borcheller) and the tracks they were running on. I had to add serious aero to it to make it work for me.

When rebuilding the shocks for that car, Moton was given the spring rates and the corner weights. I don't know if they altered their normal rebuild process to suit those factors. The plots can be seen here: http://s253.photobucket.com/user/Fas...20Dyno%20Plots

That's awesome. So you picked up a fully prepped pro racer! One of my track buddies did that with a 997.1 Carrera. The thing is a beast. Formerly raced in GS as well; can't remember which team. Got an absolutely killer deal. That's the way to go.

Hey, I got a killer deal, too - only $25K. Seriously. What's even more serious is how much money these things eat - stacks and stacks of Benjamins, and with alarming regularity. Dead seriously! :-/

I've been at the track 40-50 days a year with the car and I have a pro shop on it all the time, so that's most of the reason why. It's actually very reliable in the conventional sense.

Other than the expense, it's been a great experience, and a route I recommend to anyone who is really serious about tracking a car. As we all say (and either know or learn later), it's cheaper to buy than it is to build.

So as to stay on topic, here's a video of the car with its 1600# front springs and 1½" front bar shot at the Glen during aero testing - not much roll here! lol BTW, it's running the AP Competition Kit - those brakes are very capable. [ame="https://www.youtube.com/watch?v=skBtBia6b6g"]AP-2:04/3/2 - YouTube[/ame].