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Old 02-20-2015, 09:24 AM   #1
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Default What causes transmissions to fail?

Sorry if this has been discussed before. I was not able to find anything talking about it.

I'm mostly wondering if transmission break when your shifting gears, or just from torque itself? If it goes from seeing 0 ft lbs to 250 when you are shifting. I'm assuming a transmission will see 0 torque for a second when shifting gears. Then would putting it into gear and pushing with the 250 ft lbs kill the transmission, or is it simply the ammount of torque applied? If you were to do a pull in 3rd gear and never shifted to 4th; would that still kill the transmission?

I'm just trying to get a better understanding of transmissions.
Thank you.

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Old 02-20-2015, 09:32 AM   #2
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Don't know much about this topic but you may want to state which transmission you're interested in. Pretty sure I've heard the 6spd will take more than the 5spd.
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Old 02-20-2015, 09:35 AM   #3
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Just in general. I don't even own a Miata, and don't plan to for a while. I currently own a saab 92x aero with a built ej257.
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Old 02-20-2015, 09:35 AM   #4
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Old 02-20-2015, 09:40 AM   #5
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Old 02-20-2015, 10:03 AM   #6
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The answer is both.
Shock loading, as well as sheer torque, as well as case flexing/shafts separating, etc etc etc.
There is no one answer to rule them all.
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Old 02-20-2015, 10:23 AM   #7
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Do you think they are equally a factor in the cause? Let's say for the Miata's 5 speed.
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Old 02-20-2015, 10:54 AM   #8
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I'm no pro, I think mathewdesigns is the token transmission guy around here, but here goes:

In order of destructiveness, I think shock loading (hard shifting) would require less torque to break the trans than sheer torque in a steady state pull.

My personal experience with a miata 5 sp supports this too: I babied it at 280tq for almost a year, mostly just smooth 1-gear pulls and even when I shifted fast it was always very smoothly and softly, then sold the car and the other guy broke it in a couple months.
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Old 02-20-2015, 11:24 AM   #9
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In addition to the type of breakage mentioned above, I have been told by a transmission guru that the vast majority of "bad syncros", or grinding on certain gear changes, is not from the syncros but from bent shift forks. If the trans is warmed up you can shift very fast as long as youre guiding it in smoothly, not just slamming it into the general direction of the next gear with force behind it.

I often shift fast, but I never shift hard.
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Old 02-20-2015, 11:58 AM   #10
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I would agree with 18psi. If you run enough power it is completely possible to break the tranny with just sheer sustained torque. Bbundy has broken several transmissions during 4th gear pulls on a straight away. The actual cause for failure is usually metal fatigue that results in a gear tooth sheering and ******* everything else up. That or a shift fork breaks.
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Old 02-20-2015, 12:57 PM   #11
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Very useful. Would you say that the estimated 250 for the 5 speed braking was the results of hard shifting from owners or just that these transmitions can't hold the torque in general.

Would you say a turbo bp break the transmition faster than a vortec bp?
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Old 02-20-2015, 01:58 PM   #12
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Power through a gearbox also affects wear and lube temperature. There was a thread on XMSN and Diff coolers, but I don't know who runs them or what they run.

The failures on this forum, however do seem to be more torque than power related.
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Old 02-20-2015, 02:34 PM   #13
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Torque.

High rpm is a factor too but all mine basically have broken when the engine is very near its peak torque RPM in the middle of a pull. The RPM’s of the assembly is a factor in that it is what supply’s the cyclic loading that causes fatigue of the gear teeth. I could drive all around the track in one gear and still break a transmission. And I have never bent a shift fork or even wore out a syncro.
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Old 02-20-2015, 02:47 PM   #14
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Quote:
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Very useful. Would you say that the estimated 250 for the 5 speed braking was the results of hard shifting from owners or just that these transmitions can't hold the torque in general.

Would you say a turbo bp break the transmition faster than a vortec bp?
I'm going to say that the estimated 250 is assuming hard driving. Someone who is ham fisted can break it with less torque. Someone who is good at shifting fast but not hard can make it handle more torque. Then of course there's variance between transmissions, age of trans, etc.

I expect there's more to it as well, such as the spoolup time. Martin broke the 5-speed in Laz after 2 track days at 235 lb ft, but it was a super quick-spooling motor, and he drove it hard. Plus track = higher heat.
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Old 02-20-2015, 05:18 PM   #15
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+1 to pretty much everything already posted.

The only thing I would say otherwise is that a misadjusted clutch is typically the main culprit in wearing out synchros (unless they have a lot of miles on them), not necessarily quick or hard shifting. If the clutch is dragging with the pedal fully depressed then the trans can't be totally disconnected from the engine, and as you are trying to slow the gear on a shaft you have to fight against the engine as well...the engine is going to win and cause a lot of premature wear on the synchro. Once the synchro is worn you start having problems with shifting smoothly at any speed, which precipitates worn/broken forks because of frustration and impatience
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Old 02-20-2015, 05:21 PM   #16
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Is there a way to test if it is dragging?

Wheels up in the air, clutch in, put it in gear?
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Old 02-20-2015, 05:42 PM   #17
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Two tests:

General shifting: Car on a flat, level street or parking lot. With the car running, shift into 1st and give it some gas (1/2 throttle is fine) with the clutch still fully depressed. If the car pulls forward the clutch is dragging.

At speed/WOT: On a highway get the car up to speed. Shift into neutral and leave the clutch depressed, go WOT, bouncing off the limiter, and at the same time try to row through the gears while holding the clutch pedal down. If it fights you in this test then it's still misadjusted but not nearly as badly as if you fail the parking lot test. You either need more adjustment or know that you can't shift cleanly at redline. The larger a clutch's diameter the worse this is, due to flex and vibration at the outer edge where it's farther away from the fixed hub.
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Old 02-20-2015, 05:44 PM   #18
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Time to go test lol.
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Old 02-20-2015, 08:00 PM   #19
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Load changes with rearend gearing as well. As soon as we dropped from 4.10s to 3.63s, the 5spd broke.
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Old 02-20-2015, 10:14 PM   #20
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To add slightly to the miata 5spd discussion:

The 5 speed teeth shear off the ring. Most failures look like this, regardless of which gear failed. The metallurgy of the gears/teeth themselves IS a major issue. Simply, it's not designed to pass such a load through the gear set.

Shock load, total torque, rough shifting, and abuse, are all agitators, of an inherently weak design. Well, weak for a part designed for literally half of the WTQ numbers at witch failure becomes common place.

Another factor is clutch clamping force. A clutch which is slipping slightly will consume energy in that process. Additionally, an aggressive pressure plate and friction material will quicken the process of applying load to the gear set, thus contributing to "shock loading" forces.





I will agree with what's above, most "bad syncros" are issues with the linkage or the forks. Having truly damaged syncros in a modern manual, is fairly rare.
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