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I am getting an absurdly low theoretical spring rate for my E30 for the rear?

Suspension Tuning & Optimization

Relevant Module: Practical Skills > Choosing Springs

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So I pulled up some numbers from the internet. A 316i E30 weighs 1086 kgs, has the weight distribution of 53% and 47%, front to rear respectively. From this source : https://strictlyeta.net/technical/specifications/

Then I found someone who had calculated the motion ratio and got 0.94 front and 0.67 rear for an e30 m3.

From this source : http://e30sport.net/tech_articles/eff_rate/eff_rate2.htm

I chose 2 hertz.

I got around 42 lb/in spring rates in the rear. Which is absurdly low. But the front is fine at around 190 lb/in? But springs rates for e30s are much higher for the rear at 300 lb/in or so.

(((((1086*0.47)/2)-50)*4*pi^2*2^2)/1000)*0.67^2 then converted to lb/in to compare with h&r springs but yeah I think I am doing something wrong. I am sure my sources aren't the best but it can't be that off. Its by a magnitude of 10!


Our apologies for the delay in response. I've been working Pikes Peak Hill Climb and Andre is on vacation. Andre will return next week and I've left him a link to this thread so he will be in touch soon.

Thank you for your patience,

From our man Sam at HPA:

"Motion Ratio is often misused interchangeably as a blanket term to cover both installation ratio and motion ratio. The meaningful difference between the two appears when you go to calculate Wheel Rate. The Wheel Rate calculation we’ve used (Kw=Ks/MR^2) correctly references the Motion Ratio, if you prefer to work with Installation Ratio however you must also invert your Wheel Rate calculation (Kw=Ks x IR^2).

Either way the two calculations come to the same result, the trick is understanding which convention is actually being used (Installation Ratio or Motion Ratio) regardless of the potential misuse of the terminology.”

I've also attached some helper files with more info.

Attached Files
  • Motion-Ratio-vs-Installation-Ratio-1.jpg
  • Attachments may only be downloaded by paid Gold members. Read more about becoming a Gold member here.

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

Hi Pankaj, apologies for the delay in getting back to you. How did you get on with the information mike posted above? Do you still need help on this?

I will add that you 'can' get yourself in some trouble often when you take details or specifications that you've found on the internet for your car. The problem is that there's an inherent assumption made that whoever has posted the information got it right and sadly more often than not I find that's far from the case. It's always better to actually take the measurements yourself so you know you're dealing with accurate data.

We usually reply within 12hrs (often sooner)

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