Rear racing springs - what spring rate?

[sackracing]

I'm going to upgrade to Wolfe adjustable spring perches and the narrow rear racing coils. Does anyone know about what rate rear spring a GN will need? Is there a formula? I don't have a true weight, I believe the car is about 3400 lbs with the battery still in the front and a glass rear bumper. Any info on which height spring to order would be helpful too.

Thanks

 

[87GN_70GS]

For the spring rate you want front and rear wheel rate somewhat matched. So take the front spring rate, and divide by 3. That's approximately the square of the front arm suspension ratio (1.75). The rear spring rate is the wheel rate, there is no geometry to consider. Then factor in front/rear weight distribution.
Example: Front springs are 500 #/in. Front wheel rate = 500/3 or 167 #/in . Frt/rr weight distribution is 55/45 %. 167 x .45/.55 = 136 #/in . So rear spring rate should be roughly 140 #/in. A bit more if you carry groceries on the trunk or haul stuff.

For height, that's personal preference. But heigth at the expected rear load is what is specified.

 

[Mike E]

It's a balance between power/torque, spring rate and shock damping ability. You want the lightest and longest spring you can put in the car and still get to your desired ride height. But not so light that your shocks can't control the extension and compression. With the Afco D/A shocks you can go lower than you would with a cheaper shock. I was reccomended a 125lb. for my car and I run a 10" to get ride height I want. If your near stock ride height I would start at 12".

14" springs with the spring adjusters all the way down. I could have put some 35's under it like this.

10" springs with 1/2" of added height in the adjusters. 275/60 hoosiers

 

[johnplogii]

I'm running 12-110's & would probably recommend 12-130's. My adjusters are all the way up.

 

[tsm3089]

12 inch 120 hypercoils here. About 5/8 up on the adjustment.

 

[sackracing]

Mine is going to sit about like your middle picture, with 275s. I'm going to look up the hypercoils. Should be about stock weight in the rear, except for the glass bumper.
I'm going to use Afco double adjustables in the rear.

 

[sackracing]

and I do love a WH1

 

[sackracing]

who makes adjusters?

 

[sackracing]

12 inch 120 hypercoils here. About 5/8 up on the adjustment.

Any problems with the small diameter coils aligning with the rear end to frame?

 

[Mike E]

Trz makes some adjusters. I made my own and offset them 3/4 inch foward so the springs would line up.

 

[sackracing]

Trz makes some adjusters. I made my own and offset them 3/4 inch foward so the springs would line up.

You need my address to send me a set of those ones? Please?

I think I'm going with larger diameter springs to avoid the alignment issues. Maybe.

 

[lrayzer]

What kind of alignment issues are you guys running into with a small diameter spring ?, they should create more clearance, I assume you are not using factiry location mount points.

 

[Mike E]

The offset problem is going to be there no matter what size spring you use, it's just more noticeable with small springs. Even with stock parts the spring mounts are offset.

sack - I'll send you a PM.

 

[lrayzer]

Sounds like you guys are using factory pick up points and not a crossbar, got a picture to share ?

 

[Mike E]

pics of the adjusters here.
http://www.turbobuick.com/forums/fabrication-no-solicitation/332851-fabricated-parts-my-t-type.html

 

[lrayzer]

Ok, I understand now you are talking about using a factory upper point, and adapting the bottom, but this photo shows a coil over and a shock combo ?
I am more interested in the coil over set up to make some more room under the car.

Looks like you have a mill too.

 

[Mike E]

The adjuster under the spring could go on the shock to turn it into a coilover. I just cut threads in a piece of steel tube to match the shock body and cut it to fit the axle tube. It has all the adjustability of a coilover, but everything is in the stock locations. In my opinion its as adjustable as you can get and still be "stock suspension".

 

[Scott Atk]

Here's some good info on the subject:
http://www.turbobuick.com/forums/ge...y-kits-strange-moser-etc-fit-w-o-fabbing.html

 

[tsm3089]

Wolfe adjuster moved a little forward. Works fine.