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FB rear end setup - School me

Old 04-04-14, 10:01 AM
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FB rear end setup - School me

**had this in 1st gen section but no bites!

OK guys, So I have used the search function and have been reading a lot as I know this had been covered but please help me out here.

I have an 84 gsl-se. It's a racecar so not worried but about petty practicalities like noise and comfort

I need to fix the back end though. It comes around way too easy and way to fast! But I need to do it on a budget and while I have a buddy who can weld I don't want to ask too much of him, lest he not get involved in any of my projects ever again.

I understand the main issue is binding caused by both the watts linkage and the end links.

So in order to mitigate these problems without running out of "friend-welder capital" I have come up with the following: please tell me why they shouldn't work.

1. Ditching the watts for a panhard BUT keeping the stock 4-link setup. It seems like everyone does panhard and tri-link or nothing. Why can't I just do panhard and stock 4 link?

2. Heim joints on all 4 links. It seems like people only do the bottom. Aren't the upper zig-zaged ones the main problem?

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Old 04-04-14, 07:57 PM
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Just replied in the other section.
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Old 04-09-14, 06:49 PM
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The uppers bind. Heim joints or not.
Low buck fix (helps but doesn't really fix it) is Freash rubber in new parts with the pb&j method.
Pb&j = drilling holes in the upper bushings and leavening all the pivot bolt lose (almost snug) with lock nuts so they don't back out.

Or just leave the wats link alone. (Though it does give you some bump steer it's not to bad)

Get rid of the uppers (the biggest problem) Get a torque arm and get "7s Only's" A smetrical wats link (or panardbar).
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Old 07-28-14, 10:54 AM
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Old 07-28-14, 10:55 AM
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just spent 30mins on a reply to get chucked out!
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Old 07-28-14, 11:47 AM
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I race a 1981 Group 1 12A RX-7 here in UK that has to run close to the standard suspension setup.
I know exactly what you face. My car has spent its entire racing life trying to kill me!!

The Problem:
Mazda employed a youth opportunities scheme candidate to design the rear suspension of their new sports car.
From the 60s when coil sprung rear suspensions with live axles started to appear at least in Europe it was realized that
longitudinal control was required now the leaf springs had gone.
Trailing arms was a economic solution compared with wishbones that required independent rear ends.
The ideal is to have equal length upper and lower trailing arms of at least half a track long.
The difficultly comes with packaging with the upper arm encroaching into the rear passenger space, as does the swing room for the same arm.
The solution to the packaging problem is to use shorter upper arms.
The designers realized that if they left the arms on the outer ends of the axle; during roll the compressed side would roll forward; introducing rear steer;
and also result in mechanical mind as the upper and lower arm geometries conflicted. In the RX-7 this occurs at about 3deg of roll.

The solution implemented every vehicle of this design I'm aware of; except the RX-7, is to place the arms at angles around the top of the Diff casing to provide some lateral control too. The binding is taken away; the roll steer is reduced and the outer ends of the axle are supported by the spring and damper.
This is not a good example but might help illustrate; on the a standard road car these top links would be on the diff and shorted closer to centre

The Watts linkage is unconventional but works; I've played around with different linkage/pivot lengths/positions without any real return.

Before you make any changes to a race car consider whether you are covered by any technical regulations.
A> Assuming you have to remain largely standard:-
1. Stiffen up the Rear Roll. This can be done in 2 ways roll/stabiliser bars or springs.
The former has a tendency to make the car oversteer; albeit more predictably and lift the inside driven wheel. A strong LSD and fat sticky tyres required. RX has tendency to break half shafts and rear wheel bearings so strengthening may be required.
The latter (400lb++) makes the car a bit like a skate board with no load transfer rearwards under acceleration.
Dampers need to be high quality to cope with the high frequency motion.

2. Accept the limitations but reduce the binding effects.
Replace the lower arm bushings with spherical bearings (Rose/Uniball joints). One end could be Urethane but NOT both.
Replace one end of the upper arms with spherical bearings (Rose/Uniball joints).
Drill 4/5/6 3/8" holes with a wood borer drill (oil & water lube) slowly to reduce their stiffness. Some only do this on the loaded side.

Note: If you lower the car with the standard setup the top linsk will cause the Diff nose to droop below its optimum 15deg.
Lengthen the top links to put it back (weld in new 0.5" id water pipe), or create offset bearings when doing the Uniballs.

B> Modifications
You need to create a proper geometry 4-link. Parallel Links.
They all could do with being longer than standard; but leaving the lower arm as is but with Uniballs is a reasonable starting point.
If keeping the lower as standard, lower arms may be used as top links too and manufacture new backets within the rear passenger area and create the compartment for the arm to swing.
Taking the opportunity to link to the safety cage for additional rigidity and strength.
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