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TR2/3/3A TR3A 3-Synchro Gearbox Rebuild

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I can understand making all the teeth asymmetrical to improve fast upshifts at the track. I have trouble understanding only making 2/3 of the teeth asymmetrical though. Maybe the engineers figured 2/3 of the shifts are up, with only 1/3 down?? Maybe Triumph wanted to emulate the German idea, but were non-commital?? If it improves upshifts, then why not make ALL the input shaft dog teeth on the input shaft asymmetrical, since they only see upshifts??

Boggles me.
 
I found a technical paper on asymmetric gear teeth design that does a good job explaining their purpose in the introduction:

"In propulsion gear transmissions the tooth load on one flank is significantly higher and is applied for longer periods of time than for the opposite one. An asymmetric tooth shape reflects this functional difference. Design intent of asymmetric gear teeth is to improve performance of the primary drive profiles at the expense of the performance for the opposite coast profiles. The coast profiles are unloaded or lightly loaded during a relatively short work period. Asymmetric tooth profiles also make it possible to simultaneously increase the contact ratio and operating pressure angle beyond the conventional gears’ limits. The main advantage of asymmetric gears is contact stress reduction on the drive flanks, resulting in higher torque density (load capacity per gear size)."
DIRECT DESIGN OF ASYMMETRIC GEARS: APPROACH AND APPLICATION*Alex Kapelevich, AKGears, LLC, 316 Oakwood Drive, Shoreview, MN 55126, USA

Based upon my understanding of the above, I would guess they used 22 asymmetric teeth to increase the load capacity for the gears during drive, but kept 8 teeth symmetric to maintain an acceptable level of performance during coast. If that's the case I think the asymmetrics are a better gear, but also probably more expensive to manufacture (which may be why they switched to symmetric gears at a later date).

Joel
 
Interesting article Joel. So the asymetrical is to reduce loading in drive, but at the expense of coast. I’m not sure I would rate that as a “better” design, but more of an attempt to prolong a gear that is designed at the edge of it’s limits. In other words, it sounds like an attempt to stretch a stock box enough to hold it together in performance applications.

I used to build rock crusher muncie M22’s behind 1200HP motors. That was a box designed from the ground up for performance. All teeth were symmetrical, and we even ground off every other tooth to speed the shifts. We effectively doubled the design loading on the dog teeth, and then hit them with 3 times the stock horsepower.
 
It would be interesting to interrogate (think bright light or water boarding) a Triumph engineer that was responsible for the design of some of the components used on the TR6. Lack of replaceable cam bearings, 180 deg thrust washers on the crankshaft, fine thread studs that hold the hubs to the aluminum trailing arms,lack of drain plug for the diff,no tap on the later radiators, and a change to symmetrical gear on later transmissions. I suppose many of the decisions were related to cost.
Berry
 
Oddly enough, everything you mention except the fine thread studs is a change from earlier cars. So I'm guessing you are exactly right about trying to reduce costs.

Some of those decisions obviously weren't the best; but it's worth remembering that we are trying to keep these cars running long after their design lifetime is over. The factory's primary goal was to keep them running past the warranty period (!) and ten years was a pretty good design lifetime back then. Especially for a "cheap" car.

One other point, I know from experience that such decisions are very rarely made by engineers. Every engineer would rather make the best product possible. Cost reductions always come from management (often in the form of people whose only purpose is to cut costs). Engineers are much more often on the "do this or get fired" side of the equation.

Symmetric gear teeth are stronger and, as mentioned in the TRF article, that change came in when the V8 Stag gearbox was "rationalized" with the TR6 (again primarily with the goal of reducing costs). The Stag still kept it's more expensive roller thrust bearings on the countershaft, but most of the other parts were made the same between the two.
 
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