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Originally Posted by ShaikhA
Hey bbnks2, I fully agree that all crucial variables need to be considered to have a fast, responsive, predictable car. However, my experience differs re: 'springs being least important of all.' In fact, I'd say they are #1 in importance to set up a 'cancelling / Flat Ride' relationship between front and rear suspensions. From there, all the tuning is MUCH easier and more natural since the springs aren't fighting you where the driver feels a need to increase damping to add more 'control.'
The biggest revelation I ever had was a test ride in an Audi S4 Avant that had DEAD dampers but air springs that created Flat Ride. It absorbed small and medium bumps quite well, and though bounced more on larger bumps it settled more rapidly than I expect. Plus, it was able to 'slalom' quite well. It obviously needed (and received) more damping than zero but as a test, running just Flat Ride was very eye-opening!
Your car has Flat Ride, with the 6k/16k setup (~2.0 Hz front, 2.1 Hz rear ) so perhaps that what you mean by neutral / balanced spring rates? A Flat Ride car isn't fighting itself, vs. one with pitch. It naturally wants to stop oscillating. This means it's easier to run softer damping (and improve grip + comfort) with Flat Ride.
Yes .. provided you're staying with Flat Ride vs. going into pitch territory. I'm going to create a public version of the spreadsheet I made for Chris' 128i so users here can put in variables for their car and compare different setups.
Totally agreed - it's hard to compare one setup to the other without knowing many variables, but with at least a few key ones known it's possible to speak reasonably about how a car would handle with one setup vs. another. You also didn't mention damper behavior in there, which I'd put next to springs and sways in terms of determining the car's handling (and ride) behavior.
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I think I meant more "spring rates" being least important, as in, running 350lb/in vs running 450lb/in. That small change is not going to do much for you at all if spring rates are all that you are looking at. Is suspension travel even F:R when motion ratio is accounted for? Bump stop length appropriate for the motion ratio of the struts? Does the car over-steer because you dialed in 1.5" of rake?
I was more trying to say that people end up tweaking things unnecessarily because of purely self-induced handling issues. Like, running stock-like rear spring rates which puts the car down onto the rear bump stops almost instantly in roll or acceleration...
Can't agree with tsk94 post. Not going to reply in-line as Shaik explained things well. Flat ride is not about having balanced roll couple distribution. That is a big misinterpretation. You can have springs tuned for flat ride and still build yourself an under-steer oriented car. That is what is great about starting out with neutral balanced springs lol How would you even maintain, or determine, balanced roll couple distribution if your springs aren't balanced? The springs are a component of roll couple distribution lol...
Also, race cars do indeed use flat-ride. There are tons of technical article out there explaining about how everything from dirt rally to oval racing using spring frequencies as part of their suspension tuning strategy. It's literally what every OEM in existence does as well but on an even more complex scale than what we as enthusiasts try to recreate on a 2D scale. There is a lot of good information in the post but I just think he is looking at it all wrong. Tune a cars springs for flat ride, or close to it, and then use a million other things to fine tune its balance to your liking. Like, an adjustable sway bar. Cheap, quick, easy and can be done at the track to fine tune the car on the spot. Can't swap springs at track side and most race cars can't change springs at all as they are homulgated to run a certain spring rate.
An example I've broken down in great detail in other posts is the M235iR. At first glace it doesn't seem to be tuned for a "flat-ride." But, it actually is pretty close. It is homulgated to run 16K/16-18k springs in race form. But, it has a true real coilover and a 54/46 weight distribution. When you crunch the numbers, the car is very close to having neutral spring rates in the ~3hz range. That is on a purpose built race car with aero running Pirelli slicks.
Since Shaik mentioned the E46, that is another big sticking point for me. Everyone wants to run E46 spring rates on these cars, including TcKline who produced some of the first kits for the E8x/e9x chassis (450/700). But, the rear motion ratio and suspension is much different. So, why would the same spring rates that work well on an E46 work well on an E82? Without looking at any math or frequencies you can call BS on it. Actually crunch the numbers and again you see that to get the same exact ride that 450/700 produces on an E46 you need to run closer to 400/900 on an E82. I didn't actually crunch the numbers... just going off the top of my head from the comparison that I did a long time ago and the E46 having a higher .46? motion ratio. All numbers I crunched for people back in some autocross argument threads lol Complain about under-steer but refuse to believe the front-end is relatively stiffer than the rear. "But all the fast E46's run these rates."
Quote:
Originally Posted by ShaikhA
Another benefit of Flat Ride tuning is that a higher rear ride frequency than front creates a more rapid initial yaw moment (rotation / turning) of the chassis.
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Yes, when you actually read SAE whitepapers on suspension theory it holds true that the rear should be relatively stiffer than the front so that it can respond to inputs at the same time, to put it simply. People get obsessed with big front sway bars and lots of front roll stiffness but I don't see any scientific or suspension theory to support it. Just anecdotal experience and the "feeling" of better transient response.
I am half-way through reading Dennis Grant's book. He "autocrossed and won" in a car no-one thought could be competitive. And, he did so tuning around basic suspension theory and not the "monkey-see-monkey-do" approach.