Dusterbd13-michael
Dusterbd13-michael MegaDork
11/27/22 5:59 p.m.

My nb miata is a mutant with a gm v6, t5, etc. Catastrophic rear end failure sureing a burnout with wheelhop in the spiring. Destroyed the problematic ppf delete i built, the housing and the torsen. 

So i have a 3.63 torsen to go in now. I have boxed the housing, and began fab of a clone of the kpower industries ppf delete. 

 

Im trying to figure out what to do about bushings. My housing has good rubber bushings in it. I have some unknown brand delrin bushings from the last setup, but they have a bit of play on the mounting studs. Not sure if they should, they were used when i got them. I had also used the rubber lower piece with the delrin previously. 

The ppf delete will be solid mounted most likely. I know kpower has the bushing, and if theres a compelling reason to do so i will add one, but it makes more sens to me to be solid. 

 

So, what should i do about the mounting bushings?

 

amg_rx7 (Forum Supporter)
amg_rx7 (Forum Supporter) SuperDork
11/28/22 12:40 a.m.

I'd do some poly bushings 

Dusterbd13-michael
Dusterbd13-michael MegaDork
11/28/22 5:52 a.m.

In reply to amg_rx7 (Forum Supporter) :

Full poly, or the inserts? And why poly vs other options?

spedracer
spedracer New Reader
11/28/22 7:31 a.m.

What's the purpose of the car, 100% track, street? Solid mounting is going to add a lot of NVH.

Professor_Brap (Forum Supporter)
Professor_Brap (Forum Supporter) UberDork
11/28/22 7:34 a.m.

I'd get a fresh set of poly or derlin bushings since you drive this on the street. I haven't seen diff bushings with play at the stud. Big red flag for me. 

 

Dusterbd13-michael
Dusterbd13-michael MegaDork
11/28/22 7:38 a.m.

It's a street/autocross car. With the delrin bushings and solid ppf delete of the previous version, i had no nvh complaints. However, if the diff can be quieter id love that.

APEowner
APEowner UltraDork
11/28/22 9:27 a.m.

Not specific to this application but, generally speaking you want the same amount of rigidity in all of the mounts for anything in an application that sees vibration.  If you don't then the most ridged mount is effectively carrying the majority of the load which is a great way to break stuff.

Since this car is driven on the street and the differential is slightly undersized for the application I'd use some kind of compliant bushing in all the mounts to reduce NVH and driveline shock.

Keith Tanner
Keith Tanner MegaDork
11/28/22 10:05 a.m.

In reply to Dusterbd13-michael :

The N in NVH stands for noise :) So maybe you have a little complaint...

What are you doing to cut down on wheel hop? It's not diff movement. I don't think your diff is undersized, I think the failures are a symptom. Those differentials and factory mounts can handle 400 hp for years, but they hate wheel hop.

The way the Miata diff is designed, there's a lot of vertical load on the nose since Mazda was expecting there to be a nice long PPF there to deal with it. If you do put a bushing in the nose mount, it's going to see a lot of hard and rapid loading/unloading which can take them apart pretty quickly - especially if the bushing is vertically oriented. Champion Motorcars made that mistake with their Locost kit, and I think the GRM car made it 20 miles before it failed. 

I'm not sure the side bushings are going to matter all that much with a hard mounted nose. But if you want to cut down all possible movement, Paco Motorsports used to make solid diff mounts.

Dusterbd13-michael
Dusterbd13-michael MegaDork
11/28/22 10:38 a.m.

Wheelhop has been a pretty consistent problem with this car. 

What are your suggestions?

I had planned the front mount to be a solid horizontal 1/2 bolt and tibe to the lower chassis mount like kpower, but can easily put a bushing on there. 

The previous version was a heim joint to a bracket welded to the subframe. It kept knocking the single bolt out of tbe diff end regardless of what we did. 

Is there a better diff mount solution im not thinking about?

Keith Tanner
Keith Tanner MegaDork
11/28/22 11:14 a.m.

I think wheel hop is all about control arms and shocks, not the diff. You need to get rid of those shock loads. If you can find any posts from eliminator (I think that's his username) on Miata.net, he knows what he's doing to make a Miata hook up. IIRC he ended up with a stiff rear sway bar to couple the wheels together.

Just stay away from a vertical bolt with a bushing, that's what was killing the CMC cars. Even a solid vertical bolt might have trouble. Just look at the pivot points of that differential mounting, the diff tries to pivot around the side mounts. On my Locost, I built a short PPF substitute and bolted it to a plate on the transmission tunnel, but that was a different structure than what you have. I think that Kpower design should do the job. It'll just be buzzy.

APEowner
APEowner UltraDork
11/28/22 11:24 a.m.
Keith Tanner said:

I think wheel hop is all about control arms and shocks, not the diff. You need to get rid of those shock loads. If you can find any posts from eliminator (I think that's his username) on Miata.net, he knows what he's doing to make a Miata hook up. IIRC he ended up with a stiff rear sway bar to couple the wheels together.

Just stay away from a vertical bolt with a bushing, that's what was killing the CMC cars. Even a solid vertical bolt might have trouble. Just look at the pivot points of that differential mounting, the diff tries to pivot around the side mounts. On my Locost, I built a short PPF substitute and bolted it to a plate on the transmission tunnel, but that was a different structure than what you have. I think that Kpower design should do the job. It'll just be buzzy.

Keith.  What did you guys do with the V8 cars for diff mounts?  Did you keep the PPF?

Keith Tanner
Keith Tanner MegaDork
11/28/22 11:34 a.m.

No PPF, but also a totally different differential with different mount geometry. We used GM diffs with all their factory bushings. The photos are offline at the moment unfortunately.

Dusterbd13-michael
Dusterbd13-michael MegaDork
11/28/22 12:00 p.m.

So, plan: kpower style ppf delete with a bushing. Poly diff mount inserts. Make sure everything else survived. Then try to find and eliminate wheelhop sources, correct?

Keith Tanner
Keith Tanner MegaDork
11/28/22 12:29 p.m.

Sounds good.

APEowner
APEowner UltraDork
11/28/22 1:29 p.m.
Dusterbd13-michael said:

So, plan: kpower style ppf delete with a bushing. Poly diff mount inserts. Make sure everything else survived. Then try to find and eliminate wheelhop sources, correct?

Good plan.

amg_rx7 (Forum Supporter)
amg_rx7 (Forum Supporter) SuperDork
11/28/22 3:26 p.m.

So... the drivetrain design of the Miata and the FD RX7 are very similar.  THe FD guys have dealt with broken diffs and wheel hop for ages.  Solution (for the fd) is a combination of diff bracing, diff bushings and control arm stiffening.  Not sure if shocks play too much of a role but they do need to allow the rear end to squat and grip.  I suspect the miata will be similar and require some sort of improved diff bracing.

Your original design and the kpower are similar enough.  I think you do need a bushing up front though to absorb some driveline shock.  

Poly would be my choice since it is stiffer than the stock rubber mounts but still allows some vibration damping.

Here's some info on the best FD diff brace for your reading pleasure: https://www.rx7club.com/group-buy-center-69/hpp-2in1-diff-cradle-brace-1100911/

hope that helps

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