Rear Spring rate difference between true coilover and stock setup

[stevehifi]

Hi Guys,

Just wondering if anyone has the actual conversion ratio between a true rear coilover setup and the stock rear spring mounting option.

For instance what would an 800# spring rate on the rear stock control arm location equate to on a true rear coilover?

I've heard about 0.55 when going from stock to true coilover but I wanted to hear from the forum members if there is an actual real ratio (calculated)
noted somewhere by a suspension company.

Thanks,
Steve

[mindspin311]

This thread should get you going in the right direction - https://f80.bimmerpost.com/forums/sh....php?t=1573051


You need to know the motion ration of the suspension, then apply that to the spring rate.

[stevehifi]

This doesn't really answer my question. The thread you describe is for the stock spring location on the rear and corresponding motion ratio.

I need to know the motion ratio for the true rear coilover setup.

[RevNev]

The M4 GT4 race car has rear coil overs and lists 3 optional spring rates of 150, 170, 190lb. Significantly softer in the outboard shock location compared with the stock spring location.

Front spring rates list 180, 200, 220lb and that's about right with stock spring rates.

[M3SQRD]

Originally Posted by RevNev

The M4 GT4 race car has rear coil overs and lists 3 optional spring rates of 150, 170, 190lb. Significantly softer in the outboard shock location compared with the stock spring location.

Front spring rates list 180, 200, 220lb and that's about right with stock spring rates.

The GT4 spring rates are in N/mm, not lbf/in.

[RevNev]

Originally Posted by M3SQRD

The GT4 spring rates are in N/mm, not lbf/in.

Their running 1200lb springs in the front of an already stiff car with stock springs of 185lbs. That's double the spring rate found in a McPherson strut cast iron engine V8 race car that's nose heavy. You sure it's N/mm?

[robbo mcs]

Originally Posted by RevNev

Their running 1200lb springs in the front of an already stiff car with stock springs of 185lbs. That's double the spring rate found in a McPherson strut cast iron engine V8 race car that's nose heavy. You sure it's N/mm?

I am pretty sure he is correct.

Here are the part numbers and specs

https://www.realoem.com/bmw/enUS/sho...diagId=33_2166

Ohlins is a european company, and would exclusively use metric measurements. They would not use imperial measurements, and certainly would not mix metric and imperial

For example 170-60-150. Is 170mm length, 60mm inner diameter and 150 is the spring rate. That has to be 150 n/mm, which is plausible, being 856lbs. That is the softest rear, the stiffest 190 is 1084 lbs. Seems about right to me, based on what I know people are running on other marque GT4 cars on smooth european racetracks

[M3SQRD]

Originally Posted by RevNev

Their running 1200lb springs in the front of an already stiff car with stock springs of 185lbs. That's double the spring rate found in a McPherson strut cast iron engine V8 race car that's nose heavy. You sure it's N/mm?

Yes. You have to remember they’re running effective aero (downforce) packages which require higher spring rates. 180-220 lbf/in would be soft for a professional race car with aero and, if those are truly the only three options and the unit is lbf/in, then there’s really zero range of suspension adjustment for different tracks, conditions, etc. There are Ohlins TTX setups on stock street cars running higher rates.

I’m running F/R 600/900 lbf/in springs (non-coilover R) with MCS 2WR dampers and it’s amazingly comfortable on the street. My setup is biased toward the street. My R56 Mini Cooper S with Ohlins R&T dampers (coilover setup F and R) runs higher spring rates than 180-220 lbf/in (300 lbf/in) and it’s on the softer side.

[RevNev]

Originally Posted by robbo mcs

I am pretty sure he is correct.

Here are the part numbers and specs

https://www.realoem.com/bmw/enUS/sho...diagId=33_2166

Ohlins is a european company, and would exclusively use metric measurements. They would not use imperial measurements, and certainly would not mix metric and imperial

For example 170-60-150. Is 170mm length, 60mm inner diameter and 150 is the spring rate. That has to be 150 n/mm, which is plausible, being 856lbs. That is the softest rear, the stiffest 190 is 1084 lbs. Seems about right to me, based on what I know people are running on other marque GT4 cars on smooth european racetracks

You're probably right with n/mm but the softest front spring at 1027lb is massively stiff for a McPherson strut setup. Cast iron front heavy V8's with aero on slicks we'd run 650/750lb. Having said, the Porsche GT3 race cars bounce around as if they're way over sprung and the M4 GT4 may be similar.

I'd say racing an M4 on slicks on Australian tracks about 400lb in the front would be good start. I can't see optimum lap times in the GT4 front spring rates on our race tracks.

[M3SQRD]

Originally Posted by robbo mcs

I am pretty sure he is correct.

Here are the part numbers and specs

https://www.realoem.com/bmw/enUS/sho...diagId=33_2166

Ohlins is a european company, and would exclusively use metric measurements. They would not use imperial measurements, and certainly would not mix metric and imperial

For example 170-60-150. Is 170mm length, 60mm inner diameter and 150 is the spring rate. That has to be 150 n/mm, which is plausible, being 856lbs. That is the softest rear, the stiffest 190 is 1084 lbs. Seems about right to me, based on what I know people are running on other marque GT4 cars on smooth european racetracks

Another way to confirm the unit of the GT4 spring rate is not lbf/in is to look at the spring length. For example, one F spring option (140-60-180) is 140 mm long which is 5.5”. Assuming the sprung weight at a F corner is 800 lbm and the spring rate is 180 lbf/in, the car would compress 800/180 = 4.4” under static weight alone - a 5.5” long spring doesn’t have 4.4” of travel so the F spring would coil bind before it’s able to fully support the F corner static sprung weight. The GT4 spring rates are NOT defined in lbf/in...GT4 spring rates are in N/mm.