Undercut 1st gear

Ratranger

XS650 Junkie
Top Contributor
Messages
841
Reaction score
2,613
Points
243
Location
Oklahoma
Ok, so I know others have had a similar problem with a bike popping out of 1st under power. How most bikes keep this from happening is to have "undercut" drive dogs. The use decides the angle, and many modern bikes will use about 5*. The way the undercut works is the dogs that engage each other have matching angle cut that cause the gears to pull together tighter under power.

The problem is 1st gear on an XS is not undercut, the dogs were straight when new. Mine had about a 1* angle the wrong direction. This means under power the gears push apart. This slipping out of gear will cause more wear and destroy shift forks. With good stones you can bring things back to being flat, or go one step further and undercut them. I'm going to show how I did it.

For the 4th gear slider I used my lathe with a milling attachment and a flex shaft grinder in a tool post mount. For stones I used 5/16 chain saw sharpening stones that I dressed with a diamond nib. Now I don't want a large angle because theae bikes can be finicky to get into neutral as is, so I chose a 1.5* undercut. This translates to about .0075" taper over .25". I used a test indicator in a mag base to dial this in with an 1/8" pin in the grinder chuck.

Once the tool was dialed in, I used the same indicator to dial in a dog on the lathe. I got them within .0005" of being flat in relation to the tools movement. So each dog was checked to keep accuracy good. I also used a new grinding stone for each dog, this kept wear from causing the angle to go off. I took off just enough to bring the entire face of the dog to the same angle.

The process was repeated on the opposite side of the dogs next, which mean redialing in everything, as the grinder needed to cut a taper the opposite direction.

Now 1st gear I'm still working on, I did the rough cut the same as above but due to it having slots instead of dogs I couldn't cut the entire surface. So the edges are being finished with hand stones and diamond laps.
 

Attachments

  • 20190414_170841.jpg
    20190414_170841.jpg
    163.8 KB · Views: 285
  • 20190412_194624.jpg
    20190412_194624.jpg
    129.3 KB · Views: 242
1st picture here is the wear on a 1st gear and then a picture of the one I'm undercutting. And then a picture of a cleaned up dog on the 4th gear slider.
 

Attachments

  • 20190414_183053.jpg
    20190414_183053.jpg
    67.5 KB · Views: 213
  • 20190414_183113.jpg
    20190414_183113.jpg
    86.9 KB · Views: 209
  • 20190414_182926.jpg
    20190414_182926.jpg
    83.7 KB · Views: 205
Pretty serious project you've got there, Ratranger.
Following with interest.
Getting the dogs to simultaneously engage the slots, to distribute the loads equally, reminds me of the meticulous fitting and lapping of Mauser bolt locking lugs. Not for the faint of heart.

Saw something in your first 2 pics that you may want to investigate.
Notice how your 1st gear bushing appears to be offset, protruding toward the slider, recessed at the other end.

1stGearBushing01.jpg 1stGearBushing02.jpg

This would reduce the penetration of the 4th slider's dogs into the 1st gear's slots.

Have a look into this thread, discussing 1st gear problems and corrections. In post #27, I found my spare 1st gear bushing with a similar offset bushing.

http://www.xs650.com/threads/first-gear-issue.49825/
 
Good catch, thanks. I'll take a look at that before install, the wear pattern in the slots shows the dogs were only half way engaged. Between that and the slight taper the wrong way it would be easier for it to pop out.
 
It was fine. It didn't protrude, and the width was .630 measured with a mic. So I left it alone. The bike does stay in 1st even giving it a fairly hard launch now.
 
Back
Top