FC3S Differential Pinion Brace (By Ray Platek)

[bikedad]

So I broke my pinion mount once again. Busted the tab on the cradle that holds the differential mount.
Twice now when doing hard launches and getting a little wheel hop I've slammed the pinion into the floor board.



I have the S4 TII differential. Pinion snubber installed and the pinion mount is boxed (made solid)
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Ray (Rayminator) Platek had a nice basic pinion brace that he installed on his LS1 FC. He put 40,000 miles on his car with untold amount of drag launches.
His car was in the mid 11's with a consistent 1.6 sec 60ft and he never had a problem. I gave him a call and he offered to give me a hand.
This is a very basic but bullet proof installation. You will need access to a welder and have some skills in welding.

I pulled my entire cradle out of the car. Ray came over and helped me weld the pinion brace in to the cradle.

2 pieces of 5/16" flat stock cut 7" each.  In one piece a 9/16" hole drilled 3/4" centered from one end.
The other piece a 1/2" hole also drilled 3/4" centered from one end. (The difference in hole sizes is to allow about 1/16" play in pinion up down movement.)
A 1/2" Grade 8 bolt with locknut and washer will hold them together at the pivot.
Bolt the differential into the cradle and align the 2 pieces. Clamp and mark to weld the 2 pieces as shown.

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Remove differential and proceed to weld the pieces (on edge) to the cradle and diff mount.
We used a Hobart 130 mig welder with the help of a acetylene torch to preheat the metal for penetration.
The piece on the diff mount could be bolted if desired.

(Sorry for the camera phone pictures)
Bottom of cradle
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Bottom of cradle
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Diff mount
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Welded up and bolted together
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At the pivot it's greased and not tightened to allow movement
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Lock nut and washer to allow movement
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[Rayminator]

One hole is 1/2", the other hole is 9/16".......grade 8 bolt is 1/2".

[bikedad]

One hole is 1/2", the other hole is 9/16".......grade 8 bolt is 1/2".

Fixed

[BlackSeven]

I had a similar setup welded in my OLD FC, and it ripped the sheet metal right off the subframe where the brace was welded in. I launched the shit outta the car one night trying to impress an old girlfriend and BANG....

[Rayminator]

I had a similar setup welded in my OLD FC, and it ripped the sheet metal right off the subframe where the brace was welded in. I launched the shit outta the car one night trying to impress an old girlfriend and BANG....

What materials did you use? Launching on slicks with a prepped track, will surely be more of a load, than launching on the street.
Mine is still intact, on the car I sold, and has been daily driven. It was also taken to Road America, Autobahn Park, and Blackhawk Farms, for track day sessions..........no sign of failure, or fatigue.

[BlackSeven]

It was made out of crap materials and crap welds. I estimate the wheel hop/pinion bouncing ripped it more then a launch on stickys on the track. I bought the entire sub-frame with an S4 diff attached to it so I just bolted the entire thing in since it seemed like a good design at the time. It actually made the wheel hop die down quite a bit, but I think the better way to mount it would be a bolt through design with some inserts to strengthen the attachment point on the rear sub frame. Similar to the way hinson uses inserts on his FC pedestal mounts. That way you wouldn't risk tearing the sheet metal off.

I think your design is definitely the right direction to go for the FC pinion mount, but I was just letting you know about the experience I had with welding directly to the sheet metal on the subframe.

[Rayminator]

It was made out of crap materials and crap welds. I estimate the wheel hop/pinion bouncing ripped it more then a launch on stickys on the track. I bought the entire sub-frame with an S4 diff attached to it so I just bolted the entire thing in since it seemed like a good design at the time. It actually made the wheel hop die down quite a bit, but I think the better way to mount it would be a bolt through design with some inserts to strengthen the attachment point on the rear sub frame. Similar to the way hinson uses inserts on his FC pedestal mounts. That way you wouldn't risk tearing the sheet metal off.

I think your design is definitely the right direction to go for the FC pinion mount, but I was just letting you know about the experience I had with welding directly to the sheet metal on the subframe.

There are a few reasons that can cause the lower / horizontal bar to tear away from the crossmember.

- If the bar is too short, and does not reach back far enough, to provide the leverage that is required to keep it from "teeter - tottering", on the front edge of the crossmember

- If the bar is not welded all the way around, including the front and rear edges. It can cause a stress riser, which could easily start a tear at that point

- If the weld was undercut into the stamped metal, causing it to be thinner at that point

- if you had the above things going on, and your original pinion mount was doing little to nothing, to hold the front secure.

3fingerwilly ran one that was nowhere near as beefy as this one, and never had any issues. Granted, he has an automatic, and minimal wheel hop.

There you have it, a few different executions of the same idea. Some functioned correctly, while another failed.
We have had endless threads on controlling FC wheel hop.

One of the best solutions, if not the best, is the AWR lateral rods ( not sure what they are called in the Mazda manual) that AndrewB was running on his car.

[digitalsolo]

FWIW, my diff is braced similarly (though I have a Dana 36 and the brace on mine runs backward instead of forward).      Instead of using a slightly larger hole to allow for movement I installed a poly bushing and mounted it tightly, so just the deflection of the bushing allows movement.    It mostly just keeps some noise isolation.

My testing has shown that tightening up the rear diff mount movement helps a TON in controlling the overall diff movement.

[Rayminator]

FWIW, my diff is braced similarly (though I have a Dana 36 and the brace on mine runs backward instead of forward).      Instead of using a slightly larger hole to allow for movement I installed a poly bushing and mounted it tightly, so just the deflection of the bushing allows movement.    It mostly just keeps some noise isolation.

My testing has shown that tightening up the rear diff mount movement helps a TON in controlling the overall diff movement.

I wanted a solid positive stop when under extreme load, but the stock set-up, when driving normal, this does that.
When running several stock "rubber" pinion mounts vs the welded one, there seemed to be little to no difference in vibration etc.
So I just left the welded one in place

Off topic.....Have you run your car with the new converter? Never saw any updates on that.

[Andrew R]

If I run into problems I'll prob do something like this -looks good.

[digitalsolo]

Off topic.....Have you run your car with the new converter? Never saw any updates on that.

I've been knee deep in other people's cars the past few months, so unfortunately, no.    I quite literally got it running and the exhaust redesigned to get around the tranny (and about 7" more ground clearance vs. my old setup) yesterday.   I'm working on a V6 (4.3/700r4) setup for Indyparts right now, once that's done, I'm switching the cooling system to AN fittings, since Zac's cooling line failure concerns me.    It should be out and making some runs this spring.  Thanks again to all involved in the converter. 

[fjm57]

Great write up, post.

Jim

[mech-head]

Don,

Thanks for posting this.  Update us on how it feels.

[3 fingered willy]

I built mine from 1x.25 in flat iron from the hardware store. Yes my auto trans wont abuse the pinion near as bad as the T56, however. I built this when I had my NA rear in without the poly bushings and 929 axles. The first month my swap was done I blew the pinion mount. All because of wheel hop. You can use light steel the way Ray has with this style so long as you use it correctly. If you look a little closer at the picture, you will see that my brace is built somewhat like a truss. There are 2 mount points, one is simular to Rays way, but I have a brace that connects to the middle part of the crossmember so it stiffens the bottom brace. in my case I used the OEM pinion mount holes and a 8MM bolt to hold it together.

Beefier is ALWAYS better tho. I was really concerned about weight! :-P HaHa!

[bikedad]

WOOT! It works!

Got everything back together late last night. Couldn't test drive it until today. (Way too loud exhaust for the neighborhood)
Went home today for lunch and decided to take the RX7 back to work.
Stopped on a secluded country road on the way back to work and proceeded to do a nice drag launch. Wow! It hooked quickly. barely any noticeable wheel hop and it kept pulling hard through the gears. No squat between shifts. Very smooth power transfer. There is a slight whine from the pinion shaft but it's very negligible. If I were to put a bushing in the place of the 3/8" bolt that would probably go away.
The tranny even seems smoother. Probably because the drive shaft isn't moving up and down like before.
While driving It's much quieter and smoother. The problems I was experiencing before was the extreme pinion angle causing vibration and noise.
I replaced the whole diff cradle as well. I installed an S5 convertible cradle which has a beefier pinion mount anyhow. I  Pretty much used stock parts with the exception of some Energy Suspension bushings in the sway bar and end links.
The camber alignment is very close. I measured from the cradle front mount pins to the lateral link bolt (like it shows in the manual) and it was only off by about an 1/8". I tweaked it a little to line it up. I'll still take it in for an alignment though. I'm just waiting until I replace a bad tie-rod on the passenger side of the steering rack which I'll do when I change out the steering rack brace. (the aftermarket one I put in when I rebuilt the car last winter)

All in all it feels very good. Well worth the effort. Especially the price. practically nil if you have the tools.
I would even recommend doing this with an NA Open Diff as well.

Is there better options? Maybe, but I went this route especially after driving Ray's car with this setup. He put over 40,000 miles on his swap with a lot of drag racing. It's very simple but strong. With the way we welded the pieces there is no way they're coming loose. The cradle would have to literally split in half before the welds break. My car is only about 300 RWHP so I believe I'm good to go.