930 Coilover spring rates

[j911brick]

I did some research and I cant seem to find the information I'm looking for. I'm building a 930 RSR tribute with 935 suspension upgrades and coilovers. I'm having trouble choosing an educated guess on spring rates and shock rates for the initial setup. The car will be used mostly for street and hill country driving. It will be a semi stripped down car with 930 powertrain and RSR fuel tank. Looks like I have two options for rear coilover shocks, and two options for front inserts (HD & B8. I already have the coilover struts). The car will be running 15" RSR wheels with spacers (If needed).

So what would be a good guess on my spring rates and shock options?

TIA James

[targa72e]

This thread might help.

https://forums.pelicanparts.com/porsche-911-technical-forum/482229-wheel-spring-torsion-bar-rates.html

John

[Duc Hunter]

I think in my 1975 911S steel wide body 3.6 I'm running 250lb up front and 350lb rear. Rides nice. Custom valved Bilstein RSR's to match. With my Eibach swaybars it rides smooth as a street car, very sporty, not harsh. I came to these rates as I believe they are what Clint at Rebel Racing, Steve Wong, and others out in Cali run.

[Bill Verburg]

Originally Posted by j911brick

I did some research and I cant seem to find the information I'm looking for. I'm building a 930 RSR tribute with 935 suspension upgrades and coilovers. I'm having trouble choosing an educated guess on spring rates and shock rates for the initial setup. The car will be used mostly for street and hill country driving. It will be a semi stripped down car with 930 powertrain and RSR fuel tank. Looks like I have two options for rear coilover shocks, and two options for front inserts (HD & B8. I already have the coilover struts). The car will be running 15" RSR wheels with spacers (If needed).

So what would be a good guess on my spring rates and shock options?

TIA James

The thing to know to compare t-bars to coils is the motion ratio that gives the actual wheel rate

this is a slippery piece of information because it isn't a constant so you have to use an average

for a 911 w/ stock suspension arms
front motion ratio on average is 0.894
rear motion ration on average is 1.018
wheel rate =motion ratio^2 * spring rate

spring can be either t-bar or coil

[Tonger]

When Clint Smith did my car, he did 250# progressive springs in the front (300# main plus tender springs) and 500# springs in the rear. My car is a stock 930 weight wise with about 400 rwhp and is also running his digressive shocks and 935 rear spring plates.

My suggestion would be to call Clint at Rebel Racing, he can set you up.

[Duc Hunter]

Originally Posted by Tonger

When Clint Smith did my car, he did 250# progressive springs in the front (300# main plus tender springs) and 500# springs in the rear. My car is a stock 930 weight wise with about 400 rwhp and is also running his digressive shocks and 935 rear spring plates.

My suggestion would be to call Clint at Rebel Racing, he can set you up.

I took am running 935 spring plates, was running the Rebel Racing version and then switched to the Eisenlhor units.

[gled49]

My outlaw weighs about 2400 lbs with 935 type front and 930 rear trailing arms. We raced it with 450 front and 550 rear. After converting back to the street, 200 front and 350 rear. I would describe the ride as fairly harsh.

[stownsen914]

I recall that a 300 lb/in spring in the front is roughly equivalent to a 21 mm torsion bar. Depending on your preference, that might be a little stiff on the street but could be about right if you prefer sporty stiffness. Depends on vehicle weight too of course.

On my racecar, I have 300 front, 450 rear.

[safe]

Its fairly easy and cheep to replace springs, its not like changing torsion bars. You can always go softer or harder later.
I got 400/600 in my track car, its not very stiff for a track car, but you probably want softer for the streets.
My street Porsche 924 (50/50 weight) has somewhere 325/400.

The 250/350 suggested above by Doc is probably pretty nice for the street. I would probably go a little stiffer.

Harshness is mostly in the shocks and bushings. With good shocks you can use pretty stiff springs.

[j911brick]

Originally Posted by targa72e

This thread might help.

https://forums.pelicanparts.com/porsche-911-technical-forum/482229-wheel-spring-torsion-bar-rates.html

John

I had already read that thread but revisiting it was helpful. Thanks

[j911brick]

Originally Posted by Duc Hunter

I think in my 1975 911S steel wide body 3.6 I'm running 250lb up front and 350lb rear. Rides nice. Custom valved Bilstein RSR's to match. With my Eibach swaybars it rides smooth as a street car, very sporty, not harsh. I came to these rates as I believe they are what Clint at Rebel Racing, Steve Wong, and others out in Cali run.

That is helpful, but I'm having trouble getting anyone to give me custom valving so I'm looking at off the self solutions right now. I also dont suspect I'm going to get is right the first time given the variables.

[j911brick]

Originally Posted by gled49

My outlaw weighs about 2400 lbs with 935 type front and 930 rear trailing arms. We raced it with 450 front and 550 rear. After converting back to the street, 200 front and 350 rear. I would describe the ride as fairly harsh.

I was thinking 200/350. You think its too harsh? I suspect this car is going to weigh 2,800.

[gled49]

Weight will help with ride, I’m also on 9 & 11’s x18, 245 40 & 295 35. Sounds like there are others around the same spring rate.

[stownsen914]

Tires do make a big difference in ride harshness for a street car. Low profile performance tires can be pretty stiff.

[j911brick]

Originally Posted by gled49

Weight will help with ride, I’m also on 9 & 11’s x18, 245 40 & 295 35. Sounds like there are others around the same spring rate.

As I'm learning track also has an effect on rates and something I was reading last night was talking about natural frequency but that is getting over my head. As I mentioned I don't necessarily expect to get this right on the first try. Just want to get reasonably close.

[gsxrken]

I wish I could remember half of what I used to know, but when I went down this rabbit hole with Clint one of the recommendations was to keep the approx front and rear spring ratio relative to each other the same as stock. So if the rear was 25% stiffer than the front (not a real example), whatever rates you go with try to keep that same delta.

[j911brick]

Originally Posted by gsxrken

I wish I could remember half of what I used to know, but when I went down this rabbit hole with Clint one of the recommendations was to keep the approx front and rear spring ratio relative to each other the same as stock. So if the rear was 25% stiffer than the front (not a real example), whatever rates you go with try to keep that same delta.

Thanks.