When calculating wheel rate, do you subtract the unsprung weight from corner weight when determining springs?
Wheel rate calculation question
When calculating wheel rate, do you subtract the unsprung weight from corner weight when determining springs?
-John Allen
Tacoma, WA
'82 Royale RP31M
(‘72 Royale RP16 stolen in 2022)
To calculate wheel rate you don't need mass at all. Just spring rate (and tire if you're doing the whole corner) and motion ratio. Now, if you're calculating natural frequency then yes you need sprung mass, which is corner mass (as measured on the scale) minus unsprung mass.
I should have been more clear. I’m trying to calculate target values for the springs on my car. I have figured out the motion ratio and wheel rate, but my question is should I use total corner weights or unsprung corner weights when determining a starting point for springs. I’m inclined to start with frequency and go from there, but I’m new to some of this (my old car was easy to figure out what worked).
my new RP31 has inboard shocks with outboard on rear both with drastically different motion ratios. Steve Roux has said he used 600# front springs and 325# rears on his RP31I believe mine is current sitting on 350#/250# (I will be measuring them soon). From what I’ve calculated his effective spring rate at tire contact is approx total corner weight. Is that a good starting target? Since the springs are not suspending unsprung weight should I factor that into the corner springs?
thanks for any input!
-John Allen
Tacoma, WA
'82 Royale RP31M
(‘72 Royale RP16 stolen in 2022)
Generally, I always start with a target for ride frequency and then work my way back to wheel rate (using a given tire) and then back out the spring required to reach that target. Ride frequency is a way to normalize it across multiple platforms. For FF to FF where the mass and distribution are usually very close that isn't as critical.
Thanks , that is the type of info I’m looking for. Do you have a frequency that works better for you? I would be looking for a range appropriate for FF and running treaded Avon’s (if that matters)
-John Allen
Tacoma, WA
'82 Royale RP31M
(‘72 Royale RP16 stolen in 2022)
Target Ride frequency
What is your target for ride frequency? Front? Rear? % difference?Originally Posted by 2BWise
How do you decide on the target? Type of car? Aero?
David
So I will preface this with the fact that there is no hard and fast rule to any of this.
That said, for a non aero car on treaded tires I'd likely aim for between 2.0 hz and 2.5 hz on the high end. Generally, targeting an initial split of roughly 10% front-to-rear for a non aero car (aero cars with significant platform requirements throw that out the window). I'd start at 2.0 hz and work your way up. In my experience anything above 2 hz, on street tires (assuming the tread Avons act similarly) doesn't add much in the way of additional performance.
% split
Higher % to the rear? I have seen different ways to calculate the % split, can you share yours please.
The rule of thumb is higher rear ride frequency. But that targets vehicle without any aerodynamic considerations. Once you require additional platform support its not uncommon to have the front higher. The calculation I use is [ ((rear ride freq / front ride freq) - 1) *100 ], which provides the percentage increase of the rear greater than the front.
Can someone help a newbie? What is the exact formula for calculating spring rate at the wheel? Also T style sway bar rate both in the bar and at the wheel.?
Thanks in advance!
Last edited by airindy; 11.20.22 at 9:38 PM.
Wheel rate = (Spring rate) * (Motion Ratio ^2)
Wheel rate => springs in series => 1 / wheel rate = (1 / spring 1) + (1 / spring 2) + (1 / spring N) for N number of springs
Could you include examples for those of us that this doesn't come naturally to?
*Numbers are made up*Wheel rate = (Spring rate) * (Motion Ratio ^2)
Say you've got 400 lb/in springs and you're bellcranks provide a ratio of 0.75" spring travel for 1" wheel travel.
Spring rate = 400 lb/in
Motion ratio = 0.75
400 * (0.75^2) = 225 lb/in wheel rate
Now, lets say you tire has a rate of 300 lb/in.
Your overall wheel rate will be calculated as springs in series
1 / wheel rate total = 1 / tire spring rate + 1 / spring wheel rateWheel rate => springs in series => 1 / wheel rate = (1 / spring 1) + (1 / spring 2) + (1 / spring N) for N number of springs
= 1 / (1 / 300 + 1 / 225)
==> 129 lb/in wheel rate
If you were to include the anti-roll bar you'd use the above calculation but add in ARB wheel rate contribution to the spring wheel rate contribution. If there were any other springs in series you'd include those. For spring in parallel its additive to the overall rate.
1 / wheel rate total = 1 / tire spring rate + 1 / (spring wheel rate @wheel + ARB rate @wheel)
Last edited by 2BWise; 11.21.22 at 6:08 PM.
An old man's setup
After a few years of working on car setups, I have found that there are no magic numbers. What is most important in a setup is what makes the driver comfortable and what works on the stop watch for lap times. Setups are both driver and car dependent.
It is real easy to over think setups.
What has worked for me is to keep stuff simple. So I look at the weight on the front and rear tires. I start with a spring rate that gives me a wheel rate that is equal to the weight on the tire, or in some relation to that weight. Say my front corners are 220 pounds and my motion ratios at the front is 0.7 inches of spring movement to 1 inch of wheel movement. For that corner I might start with a 450 pound spring. That is 220 / 0.7^2 = 449. If the motion ratio was 0.5, you would use 880 pound spring. It may be that your experience shows that a spring rate larger or smaller that the corner weight works better.
For the rear of the car, I would do the same calculation.
The spring frequencies for the front and rear should be some what close with the front likely to be a bit stiffer. What you want to avoid is big changes in the pitch of the car as it encounter's track undulations or as the car deals with cornering forces. Keeping the frequencies of the front and rear help balance the chassis movements.
The very first thing I change to tune the car handling is the rear ride height and I follow that with spring changes at the rear. When I change springs, I always adjust the ride height so that I keep the ratio of the ride height to spring frequency, plus the tire spring rate frequency, constant. The calculation is a bear but that is what Excel work sheets are for.
I spent a lot of years racing oval tracks where very small changes have big effects. On ovals you tune ride height in 1/16" increments.
Wheel rates
Steve- The old man method is tried and true. It always works!
Would not the spring rate and sway bar rate be in parallel and then that calculated rate be in series with the tire rate???
john f
Yes it would you're correct. In my hurry I knew I'd slip up. Thanks for the correction.
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