Rear shock valving calculations and moment arm

glebulon · 09-09-2022, 05:02 PM

I've been researching aftermarket suspensions, that means reading about flat ride, how the rear spring need to be much stiffer than the fronts, using Andrew spreadsheet...etc.

Here is my question, if I pick a 9k spring in the back and get shocks re-valved for that spring would it simply be for a 9k spring or would it be for a weaker spring because the location of the shock is further away from the pivot point than the spring itself? I hope my question makes sense.

FaRKle! · 09-09-2022, 11:09 PM

You would need to translate the natural spring rate (9k) into the wheel spring rate since it's motion ratio is so different. Then you'd need to do the same for the damper forces since the damper is also inboard of the knuckle. Basically, get the damper and spring forces on the same "location."

glebulon · 09-09-2022, 11:14 PM

Quote:

Originally Posted by FaRKle!

You would need to translate the natural spring rate (9k) into the wheel spring rate since it's motion ratio is so different. Then you'd need to do the same for the damper forces since the damper is also inboard of the knuckle. Basically, get the damper and spring forces on the same "location."

Ok, thanks...that's what I was thinking. I had a pretty long chat with Shaikh on the phone a few days ago and really like the idea of a FCM setup but am not sure if it will happen due to price.

FaRKle! · 09-09-2022, 11:18 PM

Quote:

Originally Posted by glebulon

Ok, thanks...that's what I was thinking. I had a pretty long chat with Shaikh on the phone a few days ago and really like the idea of a FCM setup but am not sure if it will happen due to price.

I really hope you can make it happen! My wagon still puts more of a smile on my face than my M2C (now on Ohlins R&T for testing) and a big part of that is due to how versatile and refined the FCM suspension on it is.

Jvac · 09-10-2022, 11:51 AM

Quote:

Originally Posted by glebulon

Ok, thanks...that's what I was thinking. I had a pretty long chat with Shaikh on the phone a few days ago and really like the idea of a FCM setup but am not sure if it will happen due to price.

+1 for Shaikh's work if you can save up a little longer and plan to keep your f30 long enough to justify the price. I'm extremely satisfied with my custom Fcm suspension and I doubt any of the aftermarket options can even hold a candle to it. Combined with the other mods I've added like lsd and engine tune/bolt ons, the car is an absolute blast and I don't see myself ever getting rid of it even if I get another car.

09-09-2022, 05:02 PM	#1
glebulon Registered 0 Rep 4 Posts Drives: BMW 328i Join Date: Jun 2022 Location: Boston iTrader: (0)	Rear shock valving calculations and moment arm I've been researching aftermarket suspensions, that means reading about flat ride, how the rear spring need to be much stiffer than the fronts, using Andrew spreadsheet...etc. Here is my question, if I pick a 9k spring in the back and get shocks re-valved for that spring would it simply be for a 9k spring or would it be for a weaker spring because the location of the shock is further away from the pivot point than the spring itself? I hope my question makes sense.
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09-09-2022, 11:09 PM	#2
FaRKle! Brigadier General 4043 Rep 3,549 Posts Drives: 328d Wagon, M2 Comp, i4 eD35 Join Date: Sep 2016 Location: Bay Area, CA iTrader: (4) Garage List 2023 BMW i4 eDrive 35 [5.00] 2017 BMW 328dx Wagon [10.00] 2021 BMW M2 Competi ... [5.00]	You would need to translate the natural spring rate (9k) into the wheel spring rate since it's motion ratio is so different. Then you'd need to do the same for the damper forces since the damper is also inboard of the knuckle. Basically, get the damper and spring forces on the same "location." __________________ -328d Wagon Build Log (with helpful reference links) -My YouTube Channel for some of the best DIYs and in depth information Please don't PM me for suspension recommendations unless interested in paid private consultations.
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