Why I don't think you should undercut gear dogs

From my recent findings in my transmission, which I never would have found had the weld not broken on my star cam, I have formed some opinions about the common "fix" of undercutting the dogs to keep a gear from being able to jump out.

The issue is that the engaging surfaces of the dogs exhibit wear over time, and the corners round off or even form an angle. Here's a diagram of what I found on my 4th-gear set. The wheel with the dog pins is on the right, and moves to the left to engage 4th gear. That gear has holes in its wheel to accept the dog pins. On mine there was severe wear showing an angle on both the dogs and the edges of the holes.



The effect of that angle is the production of an axial force to the right, resisted only by the shift fork. The result of that was:



It's a self-feeding failure, as the worn shift fork means that the 4th gear engagement gets weaker and weaker over time, using less and less of the surface of the dog pins, wearing them down harder and creating more axial force into the shift fork, wearing the shift fork even more, ad infinitum.

I had the issue on 2nd gear as well, enough so that I had the second gear skip issue that's been discussed several times elsewhere, and that's what made me decide to actually go into the transmission when this happened, rather than just replace the shift drum where the star cam broke. My 2nd gear showed similar wear but nowhere near this bad. In use, 4th gear never skipped or jumped out of gear, but I got a false neutral when shifting to 4th several times. Another thing that just feeds the wear rate.....

So the "common wisdom" is to either machine the dogs on the parts, or get new parts and machine those, so that an angle is intentionally placed the other direction, effectively locking the engagement while under thrust. Quick-and-dirty diagram:



Now the angled cut of the dogs pulls the wheels together, preventing skipping and jumping out of gear. Eureka!!!

But wait...... hold on a minute there........

When a gear is engaged, those facing surfaces of adjacent wheels don't actually touch. There is still space between the wheels, and the only touchy-feelie parts is where the dogs and holes mate. Undercutting the dogs will pull the sliding wheel towards the other wheel, as intended, but what stops that axial movement first? Do the wheels touch, or does the moving wheel run against the shift fork first and start grinding there????? Basically the same issue I had but on the opposite side of the fork.

Motorcycle transmissions are extraordinarily simply things, mechanically speaking. Compared to manual car transmissions there are no synchronizer rings, the dogs are on the gear wheels themselves rather than separate collars, the gears are straight-cut so there are no thrust bearings needed.

That last piece is the problem. When you undercut the dogs, you introduce axial force on the shaft and something has to become a thrust bearing. It's probably going to be a shift fork.

Thanks for bumping my understanding forward another notch with that clear articulation.

Maybe you need to do a lot of engine braking and wear the opposite side of the dogs. I've never looked at one up close. Don't know how much larger the slot or port is versus the dog. It must be quite a bit larger otherwise it would very difficult to complete a shift.

It's reasonable that the 1st to 2nd shift and the 3rd to 4th shift may be the most diffcult to perform. You're moving two forks and two gears in both cases.

Neutral to 1st -- one fork, one gear.

1st to 2nd -- two forks, two gears.

2nd to 3rd -- one fork, one gear.

3rd to 4th -- two forks, two gears.

4th to 5th -- one fork, one gear.

I've never seen an AE shift but I suspect the servo shifter attacks the shift drum with gusto and speed. The shift rod appears to have a built-in spring or shock absorber to offer relief when the dogs and slots don't align perfectly.

Great post. Thanks.

And then there are those proponents of 'speed shifting', aka clutchless shifting.

Damn. Transmission PhD. Thanks once again.

Constant Mesh said:

Maybe you need to do a lot of engine braking and wear the opposite side of the dogs. I've never looked at one up close. Don't know how much larger the slot or port is versus the dog. It must be quite a bit larger otherwise it would very difficult to complete a shift.
It's reasonable that the 1st to 2nd shift and the 3rd to 4th shift may be the most diffcult to perform. You're moving two forks and two gears in both cases.

Neutral to 1st -- one fork, one gear.

1st to 2nd -- two forks, two gears.

2nd to 3rd -- one fork, one gear.

3rd to 4th -- two forks, two gears.

4th to 5th -- one fork, one gear.

Click to expand...

The wear doesn't really come from use, it comes from failure to shift cleanly. Too much speed difference between the mating wheels and they will hit pretty hard. That's for those folks who spend a lot of time on the clutch between gears, or give a bit of a blip between gears. The engine speed shout NOT increase during an upshift, but a bit of a blip during a downshift is useful. Those guys who barely ease off the throttle during shifts, and shifting slowly, are banging their gear dogs hard.

Clutchless shifting, if done properly and with neutral throttle, is just about the smoothest shift you can get. Doing it wrong can be quite bad, though. Racers do it, so why not us? Well, racers have crews whose job it is to tear down bikes. ALL THE TIME!!!!!.

Shifting "gently" is actually pretty rough on these gearboxes, too. People are taught with manual cars not to rush the shift, let the synchronizers do their work. Well, these boxes don't have synchronizers, and going gently will encourage some skipping before the dogs catch hard enough to stay in place. You want to be firm enough to catch the dog in the first hole it sees, and not barely catch it, either.

The point of my post, though, is not what causes the wear so much, as what NOT to do about it. I've seen so many posts, here and elsewhere, about undercutting the dogs so the gear won't skip, but I don't think that's any better for the transmission than the issues from premature wear. You introduce forces along the shaft (sideways to the gear rotation) which these type of transmissions DO NOT WANT.

Dang it! Something else to think about as I ride!

I turned the slick shifting trans on my FJ1200 into a stubborn mule of a box by having the cogs undercut...they don't disengage easily..

Great post!

Thanks wfooshee

Constant Mesh said:

I've never seen an AE shift but I suspect the servo shifter attacks the shift drum with gusto and speed. The shift rod appears to have a built-in spring or shock absorber to offer relief when the dogs and slots don't align perfectly.

Click to expand...

Yes, they do shift with gusto and speed, but the force is obviously limited to whatever the shift motor can give. It's certainly adequate (I can't remember any of my three AS (AE) bikes ever missing a shift while moving (I've had a very occasional baulk shifting from neutral into 1st when stationary).

So, it's not like a foot stamping on the shift lever, but it is positive and quick.

Have any YCC-S bikes ever needed their gearboxes overhauled? (I'm not talking about actuators, I mean the internals of the box.)

I'll maybe take a video showing the shift just out of interest. (I do stupid stuff like that sometimes.)

mcatrophy said:

...I'll maybe take a video showing the shift just out of interest. (I do stupid stuff like that sometimes.)

Click to expand...

Ok, stupid stuff.

Explanation first.

• Bike on centre stand.
• I'm wearing Wellington boots, not riding boots.
• I've stuck a piece of masking tape over the rubber boot of the actuator arm so that it shows up, it's near the top, slightly left of centre. You'll see it when it moves.
• Although I almost never use my foot shift lever, I use it here so you can see cause and effect.
• Characteristics of the YCC-S come into play, the rpm has to be low to engage 1st, then the speed has to be raised to change up through the gears (it won't change up if it thinks the engine will lug in the higher gear). I reach fifth. (I might even have tried for sixth
  
  .)
• I then put the rear brake on to slow the rear wheel, and let the Gen III YCC-S change down for me to 1st
  
  .

Click on image below to view the movie (hope my server is man enough):