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Transmission brace INCREASES g'box case flex?

Old 10-13-2018, 05:15 AM
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Default Transmission brace INCREASES g'box case flex?

It seems there are a few folks out there with 400+hp miatas who are trying to preserve the stock gearbox by bracing it in addition to the PPF, or doing a PPF delete, and mounting the rear of the stock box in a way similar to the Kmiata mounts. I'm not talking about bracing the PPF itself, a la the Goodwing powerbrace thingy, but trying to reduce case flex in the stock box by bracing the rear of it. If anyone has a picture of theirs, it would be great to see.

So, I fired up the welder and started mucking around, and came up with this:
Brace with the factory PPF bracket sitting on top








I had almost finished the brace, and was about to drill holes in the floor pan, when it hit me - if the bellhousing end of the gearbox is twisting as the engine revs, then wouldn't bracing the rear of the gearbox to the chassis actually INCREASE the torque and therefore the amount of flex put through the gearbox casing???

I mean, there isn't much engine movement with solid engine mounts anyway, but it's the theory of the thing I am trying to get my head around. If you are wanting to reduce the gearbox flexing down its length, why would you bolt one end to the chassis? Wouldn't it make more sense to actually make TWO gearbox braces to chassis - one at the front, using the bottom 2 bellhousing bolts, and the other at the end of the gearbox? Something like this:

Does anyone with direct personal experience want to weigh in at this point, and perhaps upload a pic of their stock, braced gearbox?

For those who are thinking "NVH will be off the charts", doesn't matter because track car. Do Kmiata put the poly bushes in there to reduce NVH, or to allow a little movement because of the issue raised above?

And, yes, I know that if you have a 400+hp miata then the Kmiata trans swap should be done, and I plan to - but I need the stock box to last till then. The Kmiata stuff is awesome, but it is over $8K landed in Australia. And before other internet import experts howl that down, the homework is thus (in $AUD):
- Kmiata Gearbox kit = $4248
- Kmiata rear diff mount = $492
- clutch = $403
- shipping = $808
- GST & Customs** = $1,844
- 2nd hand BMW gearbox = $1000 - 1800
Total = $8795 - $9595 AUD
**Where VoTI (value of taxable import = CV (customs value) + 5% Duty (Car Parts) + Shipping = 31% of CV.
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Old 10-13-2018, 09:00 AM
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You're running down the wrong path here. No amount of bracing of the tail shaft will do anything for you. The flex everyone is talking about is the main shaft and Jack shaft are trying to push themselves apart due to the nature of gear teeth angles. So you would need a brace that kept the trans case from expanding radially. Just compare the 5 speed to the 6 speed cases and you'll see all the extra bracing on 6 speed case for this very reason.

Last edited by Bronson M; 10-13-2018 at 10:49 AM.
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Old 10-13-2018, 10:34 AM
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Ahh, interesting - because of the curved teeth, the gears are pushing themselves apart? So you are talking about “banding” the gearbox, like the steel hoops around the old-fashioned wooden barrels. That would be damn hard to fabricate.
You reckon the gearbox case flexes outward, rather than flexing longitudinally...
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Old 10-13-2018, 10:43 AM
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All spur gears push themselves apart, regardless of tooth shape or angle. The flexing is internal to the gear box; the bearing pockets each shaft ride in are trying to move away from each other. The case itself needs the bracing, not sure how you'd DIY that effectively. Running wooden barrel bands around it wouldn't do much if anything IMO.
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Old 10-13-2018, 01:46 PM
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you wanna keep the shafts together.
kinda like this:
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Old 10-13-2018, 07:01 PM
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awesome, thanks guys for helping me grasp the concept. I guess the only feasible way to make sure the box lasts is to just turnnthe boost down until I can go Kmiata!
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Old 11-16-2018, 10:43 PM
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Originally Posted by jspeed.713 View Post
What exactly is the black bracing component doing?
That brace is keeping both sections of the transmission together. Think of it this way, as you increase power through the transmission the gears begin to push away from each other. They normally push away from each other when accelerating but extra power causes excess stress. When that occurs the shafts inside the transmission begin pushing against the case/bearings and begin to wear. Enough of a push and then you have a case crack/catastrophic failure of the trans.
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Old 11-17-2018, 08:51 AM
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I don't think that would help if the shafts are being forced apart within the box. I envision two shafts bowing out at the point of gear contact (looks like two jump ropes in double Dutch).

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Old 11-17-2018, 12:14 PM
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That guy bumped a bunch of old threads just to get post count and post in classifieds. I don't think he actually cares.
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Old 11-17-2018, 12:32 PM
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Originally Posted by wackbards View Post
That guy bumped a bunch of old threads just to get post count and post in classifieds. I don't think he actually cares.
Fixed his ***. Thanks for the heads up.
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Old 11-20-2018, 11:50 AM
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Eh? If “that guy” refers to me, then Wackbards you have made an unsubstantiated claim, bud. Genuine question about a real issue.
Also have never made a single post in the classifieds - show me where I have.

Sixshooter, may I ask precisely what “fixing my ***” constitutes? I haven’t noticed anything invasive as of yet...
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Old 11-20-2018, 12:14 PM
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sixshooter You are correct in regards to wanting to post in classified, but I do care. posted in topics I care about and I am interested in. I was picturing both shafts in bending as your image depicts. The forces are similar to what is shown in section 12.1 is the "y" axis.(forces in parallel axis gear mesh) from khk gears technical references: https://khkgears.net/new/gear_knowle...ar_forces.html

I did not understand how the black bracket in the image posted would resist or reduce the bending moment on the gear shafts. Any axial moments created on the shafts generating thrust at the two longitudinal ends of the trans case should be substantially smaller than the forces on generating the shaft bending moment.

I have not designed any gear rotating equipment myself but this is my 2 cents and is why I wanted to understand how that brace/bracket worked.
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Old 11-20-2018, 06:43 PM
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Originally Posted by ninerwfo View Post
Eh? If “that guy” refers to me, then Wackbards you have made an unsubstantiated claim, bud. Genuine question about a real issue.
Also have never made a single post in the classifieds - show me where I have.

Sixshooter, may I ask precisely what “fixing my ***” constitutes? I haven’t noticed anything invasive as of yet...
Not talking about you. There's posts you cannot see because they've been deleted.
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