FB Rear Suspension Geometry Problems/Options/Solutions
Joined: Mar 2001
Posts: 31,796
Likes: 3,210
From: https://www2.mazda.com/en/100th/
I don't need to have them to say they will bind - THEY WILL BIND.
You can see this for yourself. Set the car up on jackstands, remove the springs, support the axle in the middle to the car's normal ride height, unbolt the rear of one of the uppers, and loosen the other bolt. Now get another jack under the axle end and articulate the axle.
You will notice that at some points there will be over an inch of disparity between the through-hole in the upper link, and the axle mount. At different ride heights, this disparity can be as much as two inches.
Mazda got around this by using huge swaybars (18mm '79-80, 15mm '81-85) to keep the suspension from wanting to articulate in the first place, and large compliant bushings. They had to make the geometry like this in order to fit a rear seat into an already cramped chassis.
These are the geometrical issues that tear the upper link mounts off of the body on hard-driven cars.
These issues will get INCREDIBLY WORSE if you stiffen up the bushings so they can't move, and especially if you lower the car.
Y'know, I actually have a stock rear suspension car again, I just may make an educational video demonstrating this.
You can see this for yourself. Set the car up on jackstands, remove the springs, support the axle in the middle to the car's normal ride height, unbolt the rear of one of the uppers, and loosen the other bolt. Now get another jack under the axle end and articulate the axle.
You will notice that at some points there will be over an inch of disparity between the through-hole in the upper link, and the axle mount. At different ride heights, this disparity can be as much as two inches.
Mazda got around this by using huge swaybars (18mm '79-80, 15mm '81-85) to keep the suspension from wanting to articulate in the first place, and large compliant bushings. They had to make the geometry like this in order to fit a rear seat into an already cramped chassis.
These are the geometrical issues that tear the upper link mounts off of the body on hard-driven cars.
These issues will get INCREDIBLY WORSE if you stiffen up the bushings so they can't move, and especially if you lower the car.
Y'know, I actually have a stock rear suspension car again, I just may make an educational video demonstrating this.
Car is 84 gsl-se chumpcar racer. We hit a wall at Summit Point and had to do a major rebuild of the car. I was not so experienced in car setup and thought "stiffer poly bushings are always better for racecars"
So I put poly bushings in all suspension arms and watts linkage and tightened the crap out of all of them.
The following video documents the results at the next race we entered. VIR north course in the rain.
No, the mounts were all fine, but there was a lot of damage to the rear body work so we just removed the axle and everything in the back to make repairs easier.
When re-assembling, I put the entire (ST? maybe?) polyurethane bushing kit on the trailing arms and the watts linkage.
On top of that, I wanted the car to be "tight" so I really torqued down all the bolts for the trailing arms.
The result was the rear was totally bound up and so the car just slid... well, you see the video. Combine that with rain and the car was literally undrivable.
Anyone looking to set up a first gen should read this entire thread from page 1. It contains all the best advice and information available.
I now run Poly lower links, barely tightened stock rubber upper links, and a custom panhard bar.
Car handles like butter.
Learn from my mistakes.
When re-assembling, I put the entire (ST? maybe?) polyurethane bushing kit on the trailing arms and the watts linkage.
On top of that, I wanted the car to be "tight" so I really torqued down all the bolts for the trailing arms.
The result was the rear was totally bound up and so the car just slid... well, you see the video. Combine that with rain and the car was literally undrivable.
Anyone looking to set up a first gen should read this entire thread from page 1. It contains all the best advice and information available.
I now run Poly lower links, barely tightened stock rubber upper links, and a custom panhard bar.
Car handles like butter.
Learn from my mistakes.
Old [Sch|F]ool
Joined: May 2001
Posts: 12,848
Likes: 563
From: Cleveland, Ohio, USA
Making the bolts extra tight should have zero bearing on the suspension. The bolts load in tension to make the inner sleeves one with the body. The inner sleeves should never have any relative movement with the body.
I've heard of some guys who used to use poly-locks to leave the bolts loose. This scares the bejeezus out of me, because it is putting all suspension loads through the bolts, in shear. That's not the way bolts work!
I've heard of some guys who used to use poly-locks to leave the bolts loose. This scares the bejeezus out of me, because it is putting all suspension loads through the bolts, in shear. That's not the way bolts work!
Yeah. Because recently while installing another diff (toyota8"), and using poly bysh (superpro)top and bottom, was having a play with the articulation. The arms when bolted firmly to thr car but not thr diff move freely up and down and pretty good side to side.
With the diff in place (no shocks or springs panhard not watts link) the articulation is fine altough some serious deformation is occurring at the upper bushes. Those little guys were getting tortured.
Previous did lose top right arm bracket once, i put ig down to crap welds and some hp (turbo). More recently discovered rhs upper arm cracking at the welds like its trying to pull itself straight.
Have replaced all arms w oversize cromolly (stock fitment, same type of superpro bushes) and now im worried im going to rip the mounts out of the car.
Never experienced actuall binding w the poly bushes (that im aware of) though.
Im wondering if extending upper mount further backwards on the diff and using longer arms would help?
Possibly do the opposite on the lowers?
With the diff in place (no shocks or springs panhard not watts link) the articulation is fine altough some serious deformation is occurring at the upper bushes. Those little guys were getting tortured.
Previous did lose top right arm bracket once, i put ig down to crap welds and some hp (turbo). More recently discovered rhs upper arm cracking at the welds like its trying to pull itself straight.
Have replaced all arms w oversize cromolly (stock fitment, same type of superpro bushes) and now im worried im going to rip the mounts out of the car.
Never experienced actuall binding w the poly bushes (that im aware of) though.
Im wondering if extending upper mount further backwards on the diff and using longer arms would help?
Possibly do the opposite on the lowers?
Senior Member
Joined: Oct 2013
Posts: 749
Likes: 34
From: Clemmons, NC
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