RX-7 Rear Suspension Project

[Alvin]

hmmm...I just dug through my e-mail and Gordon Glasgow said that the rear NISMO leafs are rated at 207 lb/in.
If fronts are 860 lb/in than what do you think?

[fj20spl311]

IMHO you want the effective spring rate to be close front to rear.

The springs in the front have a ratio of something like 3:1 because the act on the A-arm, so the effect spring rate would closer to 290 lb/in.

Phil

[Alvin]

Thanks!!
I didn't know about the lever effect, so mounting the coil-overs at different angles will change spring rate?
What is the best angle, Guy Selle's (on first page) looks liek they are mounted upright.

[Alvin]

edit:
I found this spring rate calculator for mounting coil-over angle:
http://www.type1r.com/misc/calc/springrate/index.html

So we should be shooting for a wheel rate close to 290lb/in?

[fj20spl311]

IDN what will be the best spring rate.

I would use the "Spring Rate Calculator for Independent Suspension" on the web site you referenced to calculate the effective spring rate for the 860 lb/in comp springs. I also remember seeing a spring rate calculator for front to back using the wheelbase as one of the parameters. I think the HP also has some bearing on the rear spring rate...more HP requires a higher spring rate.

You do the work and we will all gain. LOL

Phil

[nomadtrash]

I measured my front wheel rate at 190 lb/in. I am using 860 pound comp front springs. I added a bunch of weight on the cross member and then measured the amount of sag in the ride height. Although the roadster may be about 50/50 static you will have some dynamic weight variables that make a front rate higher than the back preferable. Most notable is the roll front roll center which is lower than the rear. This causes the rear to lift a tire in the corners because the rear rolls more than the front. If you monkey with the roll centers in the front or rear you can end up with some funky handling issues. I think Bill and Elliot had issues with a front tire that would come off the ground around corners. That is what can happen with a rear roll center lower than the front (panhard bar too low), soft rear springs, and a really stiff front swaybar. Bill and Elliot did win a lot of races so that may be the fast way around the track.

[dbrick]

Phil is correct, I should have said "effective" spring rate, or wheel rate. There is a book by Fred Puhn, "how to make your car handle" Old school, pre computer, has all the math and explanations. Changing the spring rate at one end effects the other, as the front suspension is sort of the "pivot" for the rear and vice versa; this is where the wheelbase comes into play. If you have multiple positions to mount the coil overs, you can get a variety of different wheel rates from the same spring by varying the leverage. All the roll center stuff, while important, gives me a headache. Best thing I can say is have a big open parking lot and a light breakfast.the first time you push the suspension to the limits, may be a whole different driving car.

It's alot of trial and error. I'm with Phil, when you guys have the answers, I'll try it

BTW,Is it colder at the polar moment of inertia?

[64roadster]

why not go for broke?

http://www.bryanf.com/510/rsusp.htm

I know its a 510 but with a little help it would work.

[RC240z]

Andy my car is set up that way, and for a relatively "stock" suspension it is a fast way around the track, as I have picked up 5 seconds over my previous best at California Speedway.

Softer rear springs, Panhard rod, and the removal of the aftermarket rear swaybar. The other part of the equation is a "locker" style differential.

[Alvin]

Update:
In-progress pics
still need to install coil-overs and re-do exhaust(clearance is tight now!)
RX-7 axle before narrowing



Fuel Tank modified for in-tank pump:








Narrowed axle with brackets mounted:







Removing stock suspension, welding in brackets for upper/lower links:









Sold NISMO leafs and stock axle assembly:


Links hanging:



Links mounted:

[nomadtrash]

You should seriously consider a stronger upper frame mount that supports both sides of the bar that the heim joint engages. A four link changes the load direction from up and down to push pull. There will be an enormous amount of load on that bracket.

[kogi]

You do the work and we will all gain. LOL

Phil

WOW!!!! that's wicked
nice project alvin... so do you have a test dummy to work out the bugs

[Alvin]

quick video showing the links moving though it's travel:

[dieter ka24de]

What springs did you end up with ?

[Alvin]

QA1 street coilovers,
getting exhaust done tomorrow will update with pics