Spring rates for Coilover system
Spring rates for Coilover system
(OP)
Hi,
I have a small car that I am designing, with double a-arm suspension. I know the approximate weight, shock ratio, and a few other parameters,and want to calculate what poundage of spring that I need.
Thanks
I have a small car that I am designing, with double a-arm suspension. I know the approximate weight, shock ratio, and a few other parameters,and want to calculate what poundage of spring that I need.
Thanks





RE: Spring rates for Coilover system
However for a circuit car the approach is different. There the ideal would be to use the softest rates possible that don't ground out.
If you've got aero then the rules are different again.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Spring rates for Coilover system
We are working a little more simply than that. We have alot of information on the basics, such as suspension motion, angles, ratios,etc, but the more indepth info we haven't gone into. What we have basically is the vehicle weight, spring rate of 1.67,( overall leverage ratio of the suspension arms, angles, etc.) and we want to find the pounds per inch for whatever spring will be a good starting point. We also know the shock info, such as oal, travel, and damping. The rest will be up to tweaking. This is an experimental car.
Any help is appreciated.
RE: Spring rates for Coilover system
And to be honest selecting a shock before the spring is a bit odd.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Spring rates for Coilover system
Thanks
RE: Spring rates for Coilover system
So, working in SI, you've got 650 kg, split roughly between each wheel, so that is 160 kg. Less 30 kg unsprung, that is 130 kg
for the front wheel, 1.3=1/2/pi*sqrt(k/130)
k will be in N/m, you'll have to convert back to pounds/in. That is the rate at the wheel (wheel rate), you then use the usual rule to get the spring rate.
Now work out what the wheel force is at full jounce. Is that enough for your application? If not then you can think about jounce bumpers, spring aids, or non linear springs or stiffer springs.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Spring rates for Coilover system
Thanks for your repeated help.
RE: Spring rates for Coilover system
This is where theory and practical application diverge. There are many race cars that I have helped design and/or worked with over the years that use substantially higher spring rates to work around a problem or to allow a driver to control the vehicle properly.
Some of the issues that require higher than theoretically optimal rates are:
Excessive roll, pitch or squat
This can cause issues with camber gain, toe changes, power delivery.
Roll or pitch rate.
A car can not negotiate a series of corners (chicane) if it is still recovering from the first turn .. Increasing the rate at which a chassis goes through a cycle can improve the driver's ability to deal with it. The same logic holds true for anywhere fast response required...be that cornering, braking etc.
Vehicle "balance"
Especially in FWD cars where the rear is lighter relative to the front. Often the rear of a FWD car is going to have a higher wheel rate (& freq.) than the front in order to help the car rotate and to help keep the front of the car "flat". Rear rates over 1000 lb/in are not uncommon.
A BMW factory designed 330i touring car we recently worked with came with 500# rear springs and 1600# (yes 1,600) fronts.
Now my question for those reading this is... how do you choose a wheel frequency for a race car? I know that modern damper technology has allowed the use of these high rate springs, but what is too high? Perhaps a new rule of thumb is to use the HIGHEST rate spring one can and still dampen it effectively ??
RE: Spring rates for Coilover system
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Spring rates for Coilover system
One of the things that I've always had trouble with was the natural frequency ratio from front to rear. Two cars that I have been working a great deal with are opposite in their front and rear natural frequencies. One is a toyota supra and the other is a nissan 240sx. On the supra the rear has a higher natural frequency that than the front, but on the 240 the front is higher than the rear. Having them this way was the only way to get the TLLTD to work out the way I wanted without having to run too much of an anti-roll bar. I'm from the school of thought that doesn't like a big arb on a race car.
Now one of the reasons, that I didn't concern myself too much with the actual ratio of the front/rear natural frequency was these cars have a greater amount of damping than most, I'm running 70-80% of critical in both rebound and compression in the low speed range of the damper. So having a much lower damper natural frequency than it would if using less damping, I figured things would work out alright. I basically shot for natural frequencies in the 2-2.5hz range and tried to get the desired load transfer distribution and that was my baseline for both setups.
So how far would be safe to deviate from the rule of thumb on natural frequencies. I know that too stiff of a car is no good and neither is too soft. I mean just based on natural frequencies, I've designed setups for the 2-2.5Hz range for the 240 and the supra with no aero package and for the racetrack, but I've also done a FSAE car with frequencies much closer to 3Hz and also no aero package.
Am I putting too much stock into looking at natural freqencies and is still as much a good estimate as it used to be? Or are there just too many things to consider and that it less of a rule of thumb and more of just another variable to consider when putting together a suspension system?
Tim
RE: Spring rates for Coilover system
But when it comes to transient handling, I'm not at all sure that ride frequency/frequencies - even damped ones - are easily related to any sort of frequency in roll and the time it takes for that to approach "equilibrium".
I know one autocrosser who has enjoyed considerable success over quite a few years by not running particularly high spring rates (and is adamantly opposed to them). And of others within that same competitive arena who have gotten faster every time they swapped in stiffer springs. Maybe a smoother driving style works better when mechanical grip is favored, and a point-and-shoot driver needs something stiffer?
Tim - similar front with relatively different rear geometric roll center heights between the two cars (or similar rears/different fronts)?
Norm
RE: Spring rates for Coilover system
It is nice that you can compare two different cars with different suspensions, springs and weights with natural frequencies, but what does that really get you that you can't get through other, possibly more important, variables? I mean I am looking at this from the race car suspension side of things and I'm sure things are different in production cars.
Tim
RE: Spring rates for Coilover system
For a race car that would not be an inappropriate goal.
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Spring rates for Coilover system
Perhaps for a competition car it's more useful as part of the logical approach than as a set of "hard" numbers to shoot for (assuming that other things in the ruleset aren't actually driving the design, a la NASCAR or certain SCCA autocross categories). By involving just one type of motion and one elastic component, it provides a starting point from which to iterate, and helps you decide what you're going to trade off as you go through it all. It's something that you can use to help make "judgement calls", assuming that you don't have access to a larger database than what most peoples' own personal experience represents. And even though the driver's perception of ride quality isn't perhaps very important during competition, ride and pitch as motions still have some effect on performance.
Norm
RE: Spring rates for Coilover system
I guess, thinking about all of this a little more, everything is just a guideline or essentially a starting point. There's no hard and fast rule for anything with suspensions.