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Old 11-15-2018, 04:28 PM   #1
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Spring Rates - Koni Yellows

Hello all,

I currently have oem M-tech springs in my m20 powered 323i and i have just installed slightly short stroke koni yellows (80-2522sport rear, 8641-1210sport front.

You may ask why i put $1000 worth of shocks in with stock springs. Well i'm not made of money and the shocks were worn out. So I got the shocks with the intention of more things later. I had the shocks dyno tested and the adjustment only affects the rebound. Compression damping is not changed at all with the adjustment. Adjustment is very fine. 1/12th of a turn makes a massive difference.

Installation was fine, and I am very impressed with the shocks. really transformed the car.

Front feels really good, happy as larry. My only comment would be that i seem to have an annoying "bounce" in the rear. Hit a bump and you feel the suspension compress and the body oscillates annoyingly. Upping the damping in the rear (remember rebound only) seemed to help but i could definitely feel the suspension was over damped. Over damped springs give a distinct sensation that is very uncomfortable.

From all the googling it seems my current spring rates are 150lb (~2.6kg/mm) front and 240lb (4.1kg/mm) rear.

Is the e30/mtech springs known to have "too soft of springs in the rear" ? Because it certainly seems like the rear spring is too soft compared to the front?

All of the aftermarket springs seem to have much stiffer springs in the rear than the front (when compared to the ratio between F/R on oem). eibach race is apparently 8kg front and close to 14kg rear.

Car is just a street driven fun car. Though i do intend on converting the front to coil overs at some stage (to get some front cambers and ease of swapping springs).

Side note, I have tried to calculate this all using a desired ride frequency of 2.4hz for the front, and 2.5hz for the rear. using corner weights from the internet (unconfirmed...). and i came up with 7kg/mm front and 3.5kg/mm for the rear, which is softer than oem in the rear! (yes i took into account rear motion ratio).

lots of rambling here, comments welcome.

Last edited by e30davie; 11-15-2018 at 04:41 PM.
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Old 11-18-2018, 08:07 PM   #2
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No takers in my rambling discussion?

I'm going to try backing off the rebound damping considerably and see what its like.

It is currently at 1.75turns, it was at 1.5 turns and the bouncing was worse so i don't have much hopes.

I feel like slightly upping the rear spring rate, while keeping the fronts the same might be good way forward.
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Old 11-19-2018, 07:51 AM   #3
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To address your comment about spring rate front vs rear, you have to remember that the wheel rate is what matters. The front multiplier is like .88, the rear is around .45 . The spring is located half way between the wheel and the fulcrum in the rear. So your rear springs need to be twice as stiff to provide the same wheel spring rate at the wheel in the front.

That said, the ideal setup is not to have the same wheel spring rates front and rear. Most of the time, you want a softer rear springs. e30's are generally front weight biased. So even though the actually spring rate of an aftermarket rear spring is stiffer than the fronts, it'll end up being a bit softer in the rear when you calculate the actual wheel rate.

As for your bouncing, I'm not sure, it sounds like you have the rebound too soft, but that would be surprising on a Koni with a stock spring. I'd just get proper springs anyway, no point in trying to tune such a mismatched combo. Stock springs are terribly soft for any type of performance driving. H&R race would be well matched. The spece30 guys recently had a rule change and the market is pretty flooded with cheap H&R race springs currently.
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Old 11-19-2018, 01:48 PM   #4
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yer i know about the wheel rate, mentioned it in post #1. rear is 0.67 front is 0.94.

H&R Race springs used to be the spec e30 required,and they are 5.6kg (315lb) front and 10.1kg (570lb) rear. And at the moment are around US $383.96
Approximately AU $525.

The new Ground control "coilover kit" is the new spec e30 as you mention. it has all kinds of hardware included to make the front and rear adjustable. it has rates of 9.6kg (535lb) front and 12.5kg (700lb) rear. for $489 usd or $670AUD


Rears aren't that stiffer on the HR vs GC (10.1 vs 12.5) but 5.6kg to 9.6kg is a significant jump in the front!

I was reading this and he suggests the H&R are still a bit soft for a race car.


That GC kit might be the go from a value for money point of view.
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Old 12-03-2018, 03:51 PM   #5
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Well i ended up getting a set of "king springs" for a bargain and threw them in. $200 AUD for a set of "lows"

King springs is an ausi company, so i dont think they sell them in USA.

I have never had much respect for kings as generally they are too soft compared to most. But in this case i was not after a Motorsport spring. Just after something stiffer that was still comfortable.

I rang up kings and they said the fronts were progressive from 220lb up. and the rears were progressive from 360lb up

sounded pretty reasonable compared to stock. Chucked them in an the annoying bouncing as per opening post was still there. I eventually dialed back the rebound damping and im fairly sure i was experiencing "jacking down" in the suspension because at a certain point the bouncing ceased completely and it just soaks up the bumps nicely now. good learning experience for me.

But anyway, for reference essentially i am very happy with the setup at the moment. quite compliant for street use but stiff enough to not bottom out. and when you get into it it really grips very nicely and generally handles very flat. Anyway, no more ranting, here is a photo. it looks way better now too
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Last edited by e30davie; 12-03-2018 at 07:08 PM.
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