Potomac-Greg

I just did the t-bar delete and went with 6" 600# springs on Escort Cup coilovers. To be honest, I think 7" springs would have been better. With the 6"ers I'm right up to the top of the threads, and that's with the "slinky" helper springs taking up at least 1/2".

johntorg

Van, I thought the factor was .65, which I used in my table. I think that's supposed give you the differential between the front and rear. am I missing something here?

Greg, good information. what ride height are you running and what's the diameter of your tires?

Potomac-Greg

Where do you measure ride height? I've found that the best measure is from fender lip (top) to the center of the wheel. That takes out the tire/wheel size variable. That said, I have not measured my ride height since I just put it all together last night. It's low.

I use 15" phone dials with 225/50/15 tires.

Potomac-Greg

Where do you measure ride height? I've found that the best measure is from fender lip (top) to the center of the wheel. That takes out the tire/wheel size variable. That said, I have not measured my ride height since I just put it all together last night. It's low.

I use 15" phone dials with 225/50/15 tires.

Van

There's no guarantee that the 2 fenders are manufactured/spot welded on in the same place. An accurate ride height measurement is taken from the ground to the axis the torsion bars pivot around.

Johntorg, for the rear spring/wheel rates, the formula is as follows:
-The "motion ratio" is about 63% - that means, if the wheel moves up 1", the shock compresses 0.63".
-The formula to calculate wheel rate is "motion ratio squared, times spring rate".
-That would be, (0.63)^2 * 550 lbs/in
-Which equals 218.3 lbs/in

johntorg

Van, OK I guess we were looking at two different thngs. Anyway, I found out that my front springs are 500 lb/in so I will go with 750 lb/in in the rear as a starting point. The better 944s I looked at, running 15 Inch wheels, were all around 23.5 to 24 inches measured from the ground to bottom of the wheel arch at the wheel centerline. I will use that as a starting point.

Greg, I have some 225X50X15s I can measure and subtract the radius from my ground up measurement. The reason I would like to know is that I am afraid that I will run out of adjustment on the 6 inch shocks if yours are very near the top at a ride height (measured from the ground of 23.5 to 24 inches. If you ride height is higher, I should be Ok with the 6 inch springs.

Potomac-Greg

You also need to know the range of the spring perch adjustment. That varies by manufacturer. I have Escort Cups, and about 5/8" of "helper" spring, and then 6" of real spring, and I am just about at the top of the threads on the coilover with a low ride height. I would have been better off with 7" springs, and I'm damn lucky I got the helper springs.

johntorg

Greg, Thanks, I have the same shocks and so I guess I should go with 7 inch springs. I wasn't planning on using helper springs. Any thought on that? BTW do the Escorts use 2.25 or 2.5 inch springs. i haven't removed mine yet to check.

Charles A. Toupin

If I can add my 2 cents, most of the guys I know usually keep a max of 200 lbs difference between front and rear. I currently have 400 and 600. I was suffering high speed oversteering, so I will reduce to 550 at the back. I even tried 450 at the back and it was fine, but a bit too soft.

C.

Larry Herman

Since most don't know the wheel rate for the front of our cars and the effective spring rate of McPherson struts is about 1, it is easier to just work off of the effective spring rate, which is .63 * 550, or 346 lbs. We have 500 front springs and 650 rear (effectively .63 * 650 or 410 lbs) in our 951. The car is very well balanced.

Plus, in your formula you forgot the sin of the spring angle. http://www.miracerros.com/mustang/t_wheel_rate.htm

Charles A. Toupin

What is the weight of the car Larry?

Tx

C.

Van

No, I didn't forget the sin of the spring angle. I got that 0.63 number from very careful measurements in a controlled environment with dial indicators and calipers. I'm telling you, for exactly 1.000" of wheel travel, the rear shock compresses 0.630". That is the motion ratio - the difference between wheel movement and shock displacement.

The front motion ratio is 0.91.

Also, remember, it's not motion ratio times spring rate, it's motion ratio squared times spring rate. 0.91 squared is 0.83 (not much different) - but 0.63 squared is 0.397.

Van

P.S. that's for a late offset 944 - YMMV

Larry Herman

I agree with you and accept your measurements.

Motion ratio squared X spring rate * sin of spring angle = wheel rate

Motion ratio X spring rate = effective spring rate. Maybe not as informative for suspension engineers as wheel rate, but good enough comparison for us common folk.

Larry Herman

3100 lbs