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Old 08-07-2009, 02:41 PM   #1
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Default Why do gears break on the stock transmissions?

So on FlyinMiata's website, they say "the truth is that the teeth on the gears will rip right off because of the torque." In my mind, there are two things that come into play: gear material/design and movement of the clusters.

If the gears are structurally unsound for high torque, then the only option to keep the 5speed is to replace the gears.

If the clusters are flexing and moving apart, the gears spread apart and moves the contact area on the teeth from the inside (stronger, thicker) to the outside (thinner/weaker), then can something be made to keep the gear clusters together?

Am I totally out in left field with my train of thought?
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Old 08-07-2009, 03:01 PM   #2
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I recall FM's experience with Quaife suggested that flex was not an issue and the gears were just weak. You're welcome to search miata.net for those threads where they discussed it. Or call Keith at FM.
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Old 08-07-2009, 03:32 PM   #3
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small teeth = weak teeth
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Old 08-07-2009, 03:35 PM   #4
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I believe some of the issue was poorer gear design on the older boxes. Basically not allow the teeth to fully mesh with each other and putting higher loads through less contact area and eventually shearing it all off.
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Old 08-07-2009, 03:58 PM   #5
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To OP: yes your train of thought is more or less correct in terms of the gears, however flex in the shafts shouldn't be am issues. You're forgetting one factor: helix angle. Mazda put a fairly high helix angle (load will be transferee parallel to shaft, not perpindicular) to keep gear noise down. Good for noise and 90hp, but not for strength and 200+ hp. Smallar teeth does not always mean weaker teeth, as again it depends on helix angle, as well as other factors. Teeth also do not mesh in the root of the tooth, but more towards the middle, called the contact patch, which is located on the pitch circle. The gears FM sells by qualfie are made no doubt from a better material, using modern hobing and grinding machines (more precise) and have less of a helix angle for more strength. I couldn't tell you whether or not they changed the hardeness however.
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Old 09-14-2009, 12:31 PM   #6
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Can anyone tell me if the 6 speed box is more durable than the 5 speed? I'm on my 4th five speed box now in approx 16,000 street miles at 220 whp. If I knew that a 6 speed would solve the problem I'd shell out the extra cash for one, but 5 speed boxes here can be bought for £50 ($80).
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Old 09-14-2009, 12:38 PM   #7
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Quote:
Originally Posted by GrahamC View Post
Can anyone tell me if the 6 speed box is more durable than the 5 speed? I'm on my 4th five speed box now in approx 16,000 street miles at 220 whp. If I knew that a 6 speed would solve the problem I'd shell out the extra cash for one, but 5 speed boxes here can be bought for £50 ($80).
have you been living under a rock? Yes, its stronger because we don't blow them up in general. Don't expect anything to last if you treat it like a *****.
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Old 09-14-2009, 12:38 PM   #8
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Geez.. is it really that bad at only 220whp?
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Old 09-14-2009, 12:45 PM   #9
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Quote:
Originally Posted by GrahamC View Post
Can anyone tell me if the 6 speed box is more durable than the 5 speed? I'm on my 4th five speed box now in approx 16,000 street miles at 220 whp. If I knew that a 6 speed would solve the problem I'd shell out the extra cash for one, but 5 speed boxes here can be bought for £50 ($80).
You're supposed to use the clutch when you shift. Damn.
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Old 09-14-2009, 01:21 PM   #10
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I've not really looked into fixing the problem, but after the 3rd broken one I'm getting a little pissed off replacing them. And I do use the clutch

I'll look out for a 6 speed then.
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Old 09-14-2009, 01:44 PM   #11
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Quote:
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Can anyone tell me if the 6 speed box is more durable than the 5 speed? I'm on my 4th five speed box now in approx 16,000 street miles at 220 whp. If I knew that a 6 speed would solve the problem I'd shell out the extra cash for one, but 5 speed boxes here can be bought for £50 ($80).

What the heck are you doing with the damn thing?
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Old 09-14-2009, 01:53 PM   #12
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Quarter Mile 12.96s @ 110.6 mph - Crail Raceway 28/06/09


he's living a 1/4 mile at a time so it seems....
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Old 09-14-2009, 01:55 PM   #13
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a 12.96 at 220 whp is pretty impressive in my book. Must throw a hella 60 ft there.
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Old 09-14-2009, 01:57 PM   #14
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have you been living under a rock?
He's from Scotland so chances are good. Hell, they call their piles of rocks "castles" over there.

Frank
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Old 09-14-2009, 02:44 PM   #15
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Originally Posted by magnamx-5 View Post
Must throw a hella 60 ft there.
Which could explain the tranny explosions. Surprised the rear end hasn't gone too. Guess the tranny is the weaker link and it's saving the rear gears. Are the 6 speeds gearing worth a damn for drag racing?
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Old 09-14-2009, 07:30 PM   #16
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Which could explain the tranny explosions. Surprised the rear end hasn't gone too. Guess the tranny is the weaker link and it's saving the rear gears. Are the 6 speeds gearing worth a damn for drag racing?
I have broken 6 of the 5 speeds. Driving style has very little to do with it. 300+ ft-lbs of torque and R-compound tires kills them in a short amount of time. Iíve broken 2nd twice, 3rd, twice and the gear that drives the secondary shaft and all gears but 4th twice. Gears stripped clean off.

I think I will try and find a cheap 6 speed until I can save the money for the Quafe. I still hate the gearing on the 6 speed. The problem I am having is I keep on getting 5 speeds for $75 or less and I havenít found a 6 speed for less than $700.

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Old 09-14-2009, 07:41 PM   #17
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Old 09-14-2009, 09:00 PM   #18
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Any real concerns about mileage on a 6 spd. If a car runs and shifts Ok are 100K mile units an OK deal for $500 bucks since I got snaked on that $400 one in FLA.
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Old 09-15-2009, 12:16 AM   #19
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It all depends on maintenance and use, just like a lot of other used car parts. It could of been beat to hell at the track, or it could have near new 1st-5th gears and a very worn 6th gear after 100,000 freeway miles. Of course the track'd transmission could be better, sine hopefully a driver skilled enough to track a car knows how to shift, opposed to the 16 year old who's daddy bought her a 6spd miata before teaching her how to drive it. 100,000 isn't a whole lot and although I'd probably trust it, I am on a 5spd with 120,000 fairly abusive miles, so anythings better. Any more info?
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Old 09-15-2009, 12:31 AM   #20
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Quote:
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Any real concerns about mileage on a 6 spd. If a car runs and shifts Ok are 100K mile units an OK deal for $500 bucks since I got snaked on that $400 one in FLA.
6 speeds seem to grind even if cared for. M.net has a poll on grinds in the 6 speed; you may want to check that out. Seems like a crap shoot. Aisin made the s2k trans as well, but corrected the grinds in later models. Mazda didn't.

It seems like the grinds start early, according to the poll results.
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