I need help with my suspension baseline—Tuning Shocks at the Track

Lance writes:

Need help with an SCCA EP 240Z with custom Moton 2 way remote shocks on all four corners. McPherson front so motion ratio = 1? 350# front springs 225# rear. Would like the car to turn in better and it is loose on exit. First year with car could not get the shocks dialed in (compression, rebound, can PSI). Is there a good baseline setup to start with on the shocks givin a spring rate? Most of the other 240Z cars in class running 450# front springs with a big sway bar. On a test day what technique can I use to dial in the suspension. Does the can PSI really impact the spring rate?

Hi Lance,

Yes we can help. (BTW thanks for running Moton!). Re: baseline, yes there is. What I prefer to do is calculate your critical damping slopes and to do so we need:

  • Corner weights
  • Unsprung weights
  • Motion ratios
  • Spring rates
  • Weights both with and without driver
  • stated fuel load, include max capacity just to get big picture

Dyno-spring-shock-deltaveeThis allows us to establish baseline valving, though we can still tune and tweak if there’s even more data like wheel position histogram data for example.

Baseline critical damping slope (force vs. velocity slope more specifically) is more a function of corner weights and less so a function of spring rate, meaning that at risk of generalizing, for non-ground effect cars one can use quite a range of spring rates before requiring a revalve, especially with a damper like a Moton with a large adjustment range. Spring rate change does effect damping but not as much as many racers think it does.

As for what other guys run, I do prefer (but not mandate) the soft spring, big ARB philosophy, but notice my word choice; this is the fun/ frustrating challenge of motorsport competition, for every ‘rule’ or ‘best practice’ there’s something else to try and to learn that may work better.

Re: test day method to “dial in” settings, perform a shock scan, running through each range of settings, basically never completing a full lap, come in for another shock change, go back out immediately. Log your thoughts immediately via crew guy, voice recorder, in-car notebook, etc.

Rebound adjustments will be felt much stronger vs bump adjustments in general.

The most important thing to remember and one that most racers don’t understand is that you treat the dampers as a timing device that controls what-happens-when to the car as you enter and exit the corner. So yes, you have a shock issue in that you have entry push and exit loose. If you’re pushing or loose mid-corner, then it’s a mechanical spring/ ARB issue, not dampers (other factors too…).

Re: nitrogen canister pressure (also read this post) the gas does offer a springing effect. You can measure the ‘nose force’ kinda crudely on the car or off the car with a scale, more accurately on the shock dyno, and you can also calculate the nose force by knowing canister pressure, area of the piston minus area of the shock rod. This is why some say you can quickly test for spring rate change by quickly increasing or decreasing nose force via canister pressure changes (of course it’s not exactly the same as a spring change and everyone has a different opinion on this technique). Canister pressure can impact spring rate, but it’s all relative:

  • What’s the wheel rate?
  • What’s the tire’s spring rate(s)?

For canister pressure you must run at least enough to avoid fluid cavitation (the de-gassing crowd is a bit confused, though likely helping themselves slightly more than hurting—at net gain), for most guys and most high pressure monotube dampers it’s 150psi, but you can go as low as you want until you feel or data log a lag in responsiveness or degradation in handling. Conversely, high canister pressure reduces the responsiveness of the damper due to such high nose forces.

Please let me know if this helps you and if you’d like more specific guidance on set up and at-track techniques for dialing in handling.

I’d be happy to help you, we can dyno and freshen your shocks, check baseline and offer dyno graphs at different adjuster settings; call me when you can, 888-407-5122.


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