Random Transmission Failures

[GregS]

"That's a 3rd gear synchro explosion. Its what happens when you let the clutch out before the 3rd gear slider is fully engaged. The torque is transmitted through the 3rd gear synchro."

So does that mean that we should be patient when shifting up or down into 3rd?

We should be patient on all the gear changes. It's been written about several times.

 

[Steve Thomas]

We should be patient on all the gear changes. It's been written about several times.

Yep, all cars, all the time. A gear change is two smooth deliberate motions of the hand [not one violent shove] syncrnoised with tap dancing. Out of a gear into neutral, out of neutral into a gear. I was taught to think of it like that and to leave a fractional pause while in neutral. And listen to what your hand is telling you at all times.

 

[kmead]

"That's a 3rd gear synchro explosion. Its what happens when you let the clutch out before the 3rd gear slider is fully engaged. The torque is transmitted through the 3rd gear synchro."

So does that mean that we should be patient when shifting up or down into 3rd?

You should always be patient when shifting these transmissions. They don’t like to be rushed and power shifting will destroy something eventually.

You have limited HP, a fast shift is not going to gain you enough to make a difference.

Shifting should only be done using the thumb and index finger, this limits the amount of force you can apply.

 

[Steve Hoelscher]

"That's a 3rd gear synchro explosion. Its what happens when you let the clutch out before the 3rd gear slider is fully engaged. The torque is transmitted through the 3rd gear synchro."

So does that mean that we should be patient when shifting up or down into 3rd?

Yes. Especially when cold. Porsche type synchros in general are well known for this.

I should note that the 3rd gear synchro explosion is primarily due to two factors: 1st: Worn engagement teeth on the gear and sliding sleeve. This is due to poor synchro performance and rushed shifts. It permits the synchro friction surface to engage before the engagement teeth fully engage (due to being worn shorter than spec). 2nd: The force typically used in a competition environment.

Brayden and Mike's failures occured on track during a race. In racing, time is lost shifting. The car's are loud and there is an urgency to the driver's use of the car's controls. When I was competing in my X1/9, I considered the transmission a consumable/wear item. It got reconditioned regularly. I even carried a spare in case I broke one.

The 3rd gear synchro explosion occurs when the driver "misses" a shift. That is either the slider doesn't engage 3rd gear at all, and the driver releases the clutch and applied throttle. Or, he miss-times the shift, releasing the clutch and applying throttle (and therefore drive torque) before the slider fully engages third gear. Either way, if the 3rd gear and its slider's engagement teeth are worn significantly, the drive torque will try to be applied through the synchronizer which exceeds it's designed torque load window by several orders of magnitude. The design functions by trying to expand the synchronizer, and thus increasing the pressure on the friction surface, when torque is applied. But the design is only to apply the torque generated by the unloaded shafts. That's a tiny, tiny fraction of the actual drive torque. So the synchronizer expands with a force hundreds of time greater than it was designed for.

If you have not had the opportunity to observe someone drive a car in that manner I provide the following videos.

The first is my MR2 at the SCCA Solo National Championships. I built the transmission in this car. It is a OE type, synchronized, C52 Toyota 5-speed. It is highly modified but still retains the synchros. Analysis of the data shows that from peak thrust to peak thrust, an up-shift takes about 7/10th of a second and the clutch is used. Downshifts are done without the clutch. This is exactly how I drove my X1/9. I just don't have any in-car video from it.

This is one of our Demo Cardinal track cars. It uses a stock Tremec T-56 Magnum 6-speed manual, as found in a Shelby GT350 (and other similar cars). I haven't done the data analysis on this car to see how long it takes me to shift but from simple observation it is less than a second. Upshifts use the clutch, downshifts don't.

 

[Rupunzell]

Porsche syncros.... Meh...

Overly complex, slow shifting as a design "feature", prone to wear and they do indeed explode by escaping "friction" band. There was a Porsche 901 transaxle in the rotary powered exxe, it was an absolutely excellent example of inept "German" engineering. Slow shifting, not as robust/durable/reliable as Tooted. Adding to this parts cost were absurd. Think a 1/2nd slider for an exxe transaxle is expensive, the same slider for a Porsche 901 cost about $500 U$D.. Porsche 901 transaxles did not last long in the rotary exxe, parts simply wore out under endurance racing conditions.. with the best gear oils used (problems of combined hypoid final drive with syncros puts wonky demands on gear oil).

Adding insult to injury, there is no shift centering spring as delivered, this means drivers are forced to "feel" where the gears are once in neutral.
That "German" engineering is absurd and inept in too many ways.. This designed in flaw was fixed by designing an external shift mechanism to add a neutral centering spring/detent combined with a cable shifter. Worked good.

Porsche syncros were "so" good, Porsche gave up on them going to Borg Warner syncos in their later transaxles.. which has a different set of problems like sealed bearings that expire causing a long list of problems and 2nd gear disasters which Porsche does not offer replacement parts..
IMO, if you're gonna do a real race car. do a proper straight cut, dog drive gear box with all interchangeable gears to allow proper setting up of gear ratios for what is best for the engine dyno curve and track/driver needs.


Bernice

 

[MikeHynes]

Brayden and Mike's failures occured on track during a race. In racing, time is lost shifting. The car's are loud and there is an urgency to the driver's use of the car's controls. When I was competing in my X1/9, I considered the transmission a consumable/wear item. It got reconditioned regularly. I even carried a spare in case I broke one.

The trans in my post above didn't explode because I was "speed shifting".

I take my time shifting. Clunk, clunk. No hurry, I try to be very relaxed about shifting. I understand the syncros are a weak point. I don't want to damage the trans, and I sure don't want to DNF. That's a lot of time and money down the drain. Not to mention that watching from the sidelines is not as much fun! Besides, I don't see a huge benefit to "speed shifting". Maybe it would take a couple of tenths off of my lap time, but the trans in this vid is NOT a dog ring box, and I didn't want to change it - again. I no longer bring a spare to the races with me. If it breaks now - I go home.

BTW - in this vid (Father's Day 23) I started at the back of the pack because I ran out of gas on the last lap of the qualifying race. Doah! Plus the pace lap was at pretty much race speed because the pace car driver must of thought he was racing too.

Anyway, no speed shifting here...

 

[Big D]

I want to thank everyone who took the time to respond. Based upon the videos, I'm a "feel my way" into gear driver. While I haven't raced this car since it was new, I still love to drive her enthusiastically. It was helpful to learn that I'm shifting in a way that shouldn't cause harm. (I double clutch the first dozen shifts, keep her under 4,000 until the engine is up to to temperature and avoid shifting into first unless she's hardly moving.) One of the things I enjoy most is practicing how closely I can match engine speed to the transmission when shifting up and heal and toe'ing down. Steve, how did you learn to down shift without using the clutch?

 

[Steve Thomas]

In racing, time is lost shifting

Not nearly as much as when you get that no drive feeling. Very good point about feeling urgency. I suggest most amatuer race drivers have far more to gain under brakes and in terms of lines, smoothness etc than they will ever get out of gearshifts.

 

[Kevin Reay]

Steve, thanks for sharing this.


That sucks.

View attachment 29347
This picture of the broken case reminds me of what happened to one of mine.
I typically wouldn't do this but due to circumstance I used the clutch slave cylinder mounting tab as one of the anchor points to lift the engine/trans assembly off a stand. Got it a few inches up when this happened:
View attachment 29348
I guess a cast aluminum housing can be brittle.



That's surprising. I would think the hardened bolt would strip the threads out of the aluminum case before it would bore a hole in the steel speedo drive.

I have rebuilt (welded) many housings similar to these on machinery, earth moving and such. from the minor repairs i did on my X ,the castings weld quite well. So if you have most of the parts and something to bolt it to to ensure the bolt holes ect are in the correct place repair is possible even if some machining is required after.
Kev