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More contact patch grip = More spring rate?

Suspension Tuning & Optimization

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Discussion and questions related to the course Suspension Tuning & Optimization

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I am having a tough time trying to figure out what spring rates to run on the Gen-1 BRZ. --- (1,211Kg x .55 to figure front weight distribution divided by (2) minus NSM. 1,211Kg x .44 to figure Rear divided by (2) minus NSM.)

Equation used to find F & R spring rates: [K = m x 4π^2 x 2^2] --- (I implemented proper conversions 'N/m' > 'N/mm' while multiplying the known (MR) of the ''Target Spring Rate @ Wheel'' to figure the accurate ''Spring Rate''.)

Results yielded: 6.2Kg/mm (F) spring rate and a 1.9Kg/mm (R) spring rate.

Q: My biggest concern is that I will be racing on 40tw Hoosier R7's which produce loads more of Lateral G over a 100 or 200tw street tire. How do I compensate the spring rates for the extra level of lateral and longitudinal G forces sustained?

Q: My next concern is dealing with too much understeer mid apex through exit. My instinct is telling me its the 22mm front sway bar restricting the roll and adding more friction the applied outside contact patch while lifting the inside tire a good amount off the ground under peak G's. After taking the online course I have figured that the front bar is either TOO STIFF, or the 9.8Kg front springs in relation to the rear 10.7Kg springs are forcing understeer characteristics. (End of day I need help here)

Q: Also I found that you guys are getting a rear (MR) of 1.3 but I consistently find my rear (MR) equates to (0.73 - 0.77). Any ideas if this is the correct way to write the result of (MR) as it pertains to the rear of the 2015 Subaru BRZ?

The 2015 Subaru BRZ is setup for USA - NASA TT5 competition:

• 1,211 kg (competition weight on track)

• 225/40/17 Hoosier R7 tires (contact patch size = ~ 250mm)

• Currently running MCS 2WNR suspension (F = 9.8Kg & R = 10.7Kg)

• Front and Rear aero

• Factory FA20 all the way back to the factory 4.10 rear end

Attached Files

Hey Nicholas, first off the car looks awesome!

I think the first thing I would clarify here would be the understanding of Motion Ratio, and this might influence the rest of your questions.

The rear Motion Ratio you've calculated is relevant but more commonly called installation ratio in suspension setup terms. It is essentially the inverse of Motion Ratio as we defined it in the course, and I've added an image below to help describe the difference. The inverse of 0.77 is 1.3 which is how we got our Motion Ratio.

This detail is commonly misunderstood and causes problems when you have a McPherson front suspension and multilink or other rear suspension, because both the front Motion Ratio and the Installation Ratio are close to 1 in a McPherson front so the spring rate emerges relatively reasonable when you run the calculations, but then the rear suspension throws out weird numbers as the Motion Ratio extends above 1.0 the Installation Ratio extends inversely below 1.0 making for a stark difference in the calculation output if you use the wrong measurement convention (Motion/Installation).

I've created a 'Spring Stiffness (Given Target Ride Frequency)' Calculator spreadsheet which has an Installation Ratio to Motion Ratio converter on the right side, which you can fill out to make the calculations easy. Given your target ride frequency is 2Hz, fill that spreadsheet out with the Motion Ratio as described in the course and that should give you some more realistic spring stiffness starting points and might change your understanding of the handling balance issues you're having.

Hopefully that helps, let me know how you go.

Motion vs Installation Ratio

Attached Files
  • Spring-Stiffness-given-Ride-Frequency-Target-v2.xlsx
  • Attachments may only be downloaded by paid Gold members. Read more about becoming a Gold member here.

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