RX-7 Rear Suspension Project

[Alvin]

I am excited! This has been on the backburner for several years now.
Problem: The current, stock 3.9 open rear end is holding up fine, but with years of drag launches and open track events it's showing signs of failure.
-Lighting up the inside tire at the dragstrip or turns isn't the best way to put power down
-The roadster LSD is simply too expensive and should be left to real race car drivers.
-I also keep braking the torque rod mounted to the frame/axle.
-If I ever wanted to add more power(think 300rwhp) through a turbo there's is no way the stock setup will last.
Goal:
-Design and install a robust 4-link setup that replaces leaf springs and shocks with rods and adjustable coilovers.
-LSD and disc brakes.
-All must fit in front of stock roadster gas tank.
-Will try to keep current 15x6 Panaport wheels but eventually want wider fitment.
At first I considered the 8" Ford setup since ratios/LSD's were cheap and available. Scrapped that idea after weight/bulk concerns.
The RX-7 rear end we are using is setup out of a '85 GSL-SE model. I picked it up along with a my new front end components. It comes standard with a LSD and disc brakes!

I will upate this post with pics. I'm off to the shop to take measurements for narrowing the axle...considering how we can fit a wider tire under stock fenders, mini-tub?

[fj20spl311]

Alvin,
keep us posted as I am sure several of us are going to be installing a stronger rear axle.

I am still leaning toward speedway ministock housing.

Phil
FJ20DET project

[nomadtrash]

how we can fit a wider tire under stock fenders, mini-tub?

You can gain a couple of inches on the inside by removing the leaf springs and rolling the frame lip. The inside of the wheel well can be moved a little also. You may be able to cut the wheel well along the seam and widen it a little by welding in a strip. You will never be able to go very wide without notching the frame or cutting the fenders.

I paid Moser $110 to cut and respline my RX7 axles. A local race shop narrowed the housing because Moser didn't want to mess with it. You are going to have to cut around 10 inches off to get it the same width as the roadster axle. You will need to work on the brakes too. The porportioning will be off.

[Alvin]

Considerations:
- We are considering shortening the axle by 2". This will give us room to play with wider wheel width/offset. My goal was to fit at least 225/50/16 tire on a 16x7 wheel.
Once we get the axle linked, then further frame notching may/not be considered.
-Debating on link setup, with a 4-link we can have 2 parallel upper/lower links or 2 triangulated upper links. With 2 parallel upper/lower links a panhard bar is needed.
-Shocks, double adjustable or simple. There isn't much travel.
-Exhaust routing, with triangualted upper links.
Here are pics of Guy Selle's setup(thanks Gordon), upper/lower parallel links and a panhard bar, but he pretty much fabricated a new frame from the doors back:

[nomadtrash]

Perhaps you should consider parallel lower links with a long torque arm under the housing that connects to the X-member. You would need a panhard bar. Griggs Racing has made some Mustangs handle quite well with this setup.

I would go with single or double adjustable shocks. More exotic shocks aren't needed.

[nomadtrash]

I would also suggest attaching the panhard bar to the frame on the passneger side and to the axle on the drivers side.

[Alvin]

Thanks for the suggestions Andy.
-housing is ready to be jigged and shortened
-calling Moser to get axles shortened/replined, get price quote.
-Researching link materials

[nomadtrash]

http://www.stockcarproducts.com/rsusp1.htm

[datsunrides]

Any news?

Mark

[JFBrink]

Alvin,

Given your usual fastidiousness, I'm looking forward to your weight analysis of the components, once the RX-7 rear is cut down. I'm curious to see how it stacks up against the Roadster rear. I've moved both around in my wife's station wagon and can only say that they are both damn heavy. Also, will a bunch of bars for a Panhard setup, or whatever you choose, end up lighter than a pair of leaf springs?

Keep us up to date as you progress!

Jesse.

[Alvin]

Updates to come!
I'm assuming the the whole setup will weigh more than stock with the coil-overs, links, and disc brakes.

I'll weigh the car again to get a figure for sure!

Also searching wheel/tire combos to fit with the new space available.

I'm willing to trade off a few pounds in the back for durability, handling, traction , etc.

[nomadtrash]

I beleive I weighed all the parts and have it written down somewhere. The links and coil overs are much lighter than the leaf springs and shocks. The RX7 rear axle with disc brakes is lighter than the stock axle with drums. I would estimate the overall weight savings at least 75 pounds.

[Alvin]

I beleive I weighed all the parts and have it written down somewhere. The links and coil overs are much lighter than the leaf springs and shocks. The RX7 rear axle with disc brakes is lighter than the stock axle with drums. I would estimate the overall weight savings at least 75 pounds.

Well that sounds like good news! I didn't think the alum drums weighed more than a disc rotor and caliper combined...we'll see.

[Alvin]

Any ideas on rear spring rate, coli-over suggestions
QA1's?

[dbrick]

If you are able to find the spring rate of your front springs, I would go close to that. The car is close to 50/50. Any big difference in front rear rate will cause it to rockinghorse. If you get the specs of the original leaf spring rate , remember there's alot of friction added with the leaf setup, so the actual rate is higher than the leaf sping numbers.

Looks like it will be nice when it's all done.