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

[ZDan]

Originally Posted by u/Josh

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...

[Suberman]

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.

[Wepeel]

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.

[CSG Mike]

Originally Posted by Wepeel

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).

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).

[Wepeel]

^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.

[CSG Mike]

Originally Posted by Wepeel

^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]

[Racecomp Engineering]

Originally Posted by CSG Mike

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).

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

- Andy

[Captain Snooze]

Originally Posted by ZDan

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.

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

[YodrOne]

Originally Posted by CSG Mike

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).

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

[Victor Draken]

Originally Posted by CSG Mike

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.

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)

[dradernh]

Originally Posted by YodrOne

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.

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.

[YodrOne]

Originally Posted by dradernh

...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

[wparsons]

Originally Posted by YodrOne

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.

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.

[dradernh]

Originally Posted by YodrOne

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.

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