Suspension frequencies

[noogie]

This thread will be used to discuss suspension frequency theories.

Common suspension frequencies:

• 1.0 Hz - passenger cars
• 1.25 to 1.75 Hz - sports cars
• 2.0 to 2.5 Hz - autocross and racecars with low downforce
• 2.5+ Hz - high downforce racecars

To calculate the GR Supra's factory suspension frequency, you'll need the motion ratio, corner weight, and spring rate.

Calculator:
http://farnorthracing.com/autocross_secrets16.html

Factory spring rates:
Front: 168 lbs/in; 3.0 kg/mm
Rear: 515 lbs/inch; 9.2 kg/mm

Motion ratio:
1 / lever ratio = motion ratio
Front: 1 / 1.18 = 0.85
Rear: 1 / 1.69 = 0.59
source: https://www.hks-power.co.jp/en/product_db/hipermax/db/31743

Corner sprung weight:
Corner weight - unsprung weight
Front: 395 kg - 45 kg = 350 kg
Rear: 367 kg - 50 kg = 317 kg
Note: the unsprung weight is an estimate. 45kg and 50kg are rule of thumb with the higher number on the drive axle.

This gives you a factory suspension of about:
Front: 1.2 Hz
Rear: 1.51 Hz
Note: The factory suspension utilizes the bump stops for additional spring rate. The rates above don't include that and is actually softer than the real rates.

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[noogie]

Ohlins Road and Track coilover rates:
Front: 90 N/mm ; 9.1 kg/mm
Rear 160 N/mm ; 16.3 kg/mm

Suspension frequency:
Front: 1.98 Hz
Rear: 1.95 Hz

 

[noogie]

Glad you started this. After seeing this HP video, I was planning on doing my own calculations to figure out what the ballpark should be for coil over spring rates. No one ever gives justification for why they run what they run. Trust but verify.

Forgot to subtract the unsprung weight. Fixed the original post.

I think you'll typically see non-aero street tire cars aim for 2-2.5 Hz. The "black art" seems to be the +/- 0.5 balance between front and rear.

I think most OEMs aim for the "flat ride" concept, which promotes comfort.

Perhaps with the relatively front heavy Supra, you want more spring to mitigate the nose from diving too much.

 

[noogie]

Copied spring rates from another post.

You can see most lowering springs have similar front to rear spring ratios. It's most likely due to the stock damper valving, which restricts your spring rate options. When you have a custom shock, you can valve your springs to whatever rates you want.

BRAND	FRONT (LBS/IN)	REAR (LBS/IN)	FRONT (KG/MM)	REAR (KG/MM)	F/R Ratio
STOCK	168	515	3.0	9.2	0.33
EIBACH	205	705	3.7	12.6	0.29
HKS	241	666	4.3	11.9	0.36
RSR	198	683	3.5	12.2	0.29
SWIFT	269	672	4.8	12	0.4
TEIN	202	588	3.6	10.5	0.34
H&R	?	?	?	?	?
Ohlins R&T	514	914	9.1	16.3	0.56

 

[Thraxbert]

Really great to see this thread. So many suspension threads devolve into "idk feels ok to me." Which, you know, valuable, but not terribly informative to others.

On the topic of H&R springs, I cannot find hard specifications anywhere. However, they do state (in some listings) that the rates are 10-20% higher than stock. Based on that, I think we can approximate them at: 201 lbs/in front, 566 lbs/in rear for a 0.36 F/R ratio. I made the assumption that the fronts got OEM*1.20 and rear got OEM*1.10.

 

[noogie]

Updated the motion ratios. Looks like HKS makes these numbers available.

 

[i3igpete]

because i got tired of plugging things into dennis's website

 

[i3igpete]

Refactored with calculator for gas pressurized shocks, prefilled by eyeballing some MCS/JRZ dimensions on google. added damped frequency and settling time eq's as well.

to use: only need to fill in white cells in a given column. grey and color cells are calculated.

to do: get estimates for motion ratio on non-divorced rear setups

won't do: raw shock inputs

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