What undercut gears look like

rPGoatBoy · Jan 25, 2014

Sent a couple of new cogs up to A.P.E. in Rosamond, CA with the hopes they've got the solution to my FJRs inability to remain in 2nd. Here's what their work looks like; not too shabby!

You can see the difference in the angle of the undercut between stock and the shiny A.P.E. machining, it may look subtle in the picture but it's more noticeable with eyeballs on it.

The corresponding dog pockets below:

Silent · Jan 25, 2014

Yup. I did that to an XS1100 by hand with a dremil. The results were excellent. It's in the link in my sig line if you're interested in how it's done.

a.droid · Jan 25, 2014

Silent said:
Yup. I did that to an XS1100 by hand with a dremil. The results were excellent. It's in the link in my sig line if you're interested in how it's done.

yup, I clicked the link. Thats a hell'va project. Retired?

Fred W · Jan 26, 2014

I was just wondering...

Does having the dogs undercut make up-shifting without the clutch more difficult? Not that this would be a deal breaker to me. I almost never do that now and tend to at least fan the clutch when power shifting. It just seems like maybe with the more positive engagement of the dogs they wouldn't want to slide out to release for the next gear as easily if there is even the slightest load on them.

kolonial72 · Jan 26, 2014

Fred, I would think that clutchless shifting can still be accomplished without too much fuss. One may need to be a bit more precise with respect to the shift point (rpm), but I doubt most would notice the difference. Disclaimer - I have not had this done on my bike, so it's mere speculation.

Silent · Jan 26, 2014

a.droid said:
Silent said:

Yup. I did that to an XS1100 by hand with a dremil. The results were excellent. It's in the link in my sig line if you're interested in how it's done.

Click to expand...

yup, I clicked the link. Thats a hell'va project. Retired?

No sir. I'm a working stiff. Still another 20 years tip I can retire. It was a labor of love.

08FJR4ME · Jan 26, 2014

Was there obvious damage or wear before having this done? Would have been nice to have a before picture. I take it the whole bike has to be torn down and the cases split in order to get to these parts?

Hope if works,

Dave

rPGoatBoy · Jan 27, 2014

Fred W said:
I was just wondering...
Does having the dogs undercut make up-shifting without the clutch more difficult? Not that this would be a deal breaker to me. I almost never do that now and tend to at least fan the clutch when power shifting. It just seems like maybe with the more positive engagement of the dogs they wouldn't want to slide out to release for the next gear as easily if there is even the slightest load on them.

I can't say as I haven't installed these yet, but I'm imagining that even shifting with the clutch should present more resistance at the shifter with the increase in the undercut angle of these gears. I've never shifted without the clutch though. I will report back in a few weeks.

08FJR4ME said:
Was there obvious damage or wear before having this done? Would have been nice to have a before picture. I take it the whole bike has to be torn down and the cases split in order to get to these parts?
Hope if works,

Dave

Yes, I've got my motor split on my workbench right now. I'd have started a thread about it, but I already did that a couple of years ago (can't search the forum for topics I've created more than 1 year back for some reason), and Streethawk is currently undertaking a very similar project, with pictures galore. BUT, here's a before & after of the first time I did this project a couple of years ago. In both pictures, the gear on the right is as is new from Yamaha. As this is the 2nd time I've encountered this problem, both of the gears featured on the right now look much like the gears on the left:

Same with the associated shift fork, they both look like this:

08FJR4ME · Jan 27, 2014

Oh Boy, that isn't very good.

I hope I never have to do this. Is there any warning signs when this starts to act up?

Dave

RaYzerman19 · Jan 28, 2014

A buddy of mine has some symptoms.... looks like I might have to get my hands dirty. I'm not an expert at splitting cases or what the undercutting achieves.... what if I just replace the offending parts (dog gears, mating gears, forks)?

08FJR4ME · Jan 28, 2014

RaYzerman19 said:
A buddy of mine has some symptoms.... looks like I might have to get my hands dirty. I'm not an expert at splitting cases or what the undercutting achieves.... what if I just replace the offending parts (dog gears, mating gears, forks)?

It looks like to me they increased the angle slightly so that you get a more positive engagement. This would help from slipping out I imagine. I guess if this has happend twice I would most likely try this also. If it is just a normal wear situation or a real bad mis-shift swapping out the offending parts would most likely be suitable enough for most folks. That's my opinion for what it worth.

Dave

Silent · Jan 28, 2014

The natural tendency of a straight cut gear dog is to push apart slightly. This is because the gears can twist slightly on the shafts. If the gears are in good shape, this is not an issue as the loads generated are not enough to separate them. However, when the gear dogs or slots wear and the gripping edge wears, then the tendency to push apart is increased to the point that it actually pushes them apart under heavy load. This results in popping out of gear.

By undercutting the dogs, you are changing the tendency of the gears from pushing apart, to pulling together, thus preventing the possibility of separating under heavy loads. The only drawback is it takes a more positive shift to change gears. They use the straight cut gears because they shift a bit smoother than the undercut ones.

ionbeam · Jan 28, 2014

Agree with and adding to Silent and other posts in this thread. The amount the gears need to be cut is actually very small. A 3% to 5% cutback is liveable for street use. I have ridden several V-4s with cutback gears done to different percentages. The shift feel ranged from slightly notchy to annoyingly notchy but in every case there was a distinct feel of a POSITIVE shift and in one case a definite reluctance to up shift without unloading the transmission.

Undercutting involves cutting/grinding the drive dogs and slots to precise angles that force the gears to lock together under acceleration. A gear dog that is 0 degrees lets the corners round over, causing the gears to disengage from one another. This is really all the gear dog needs to be cut back. The cut does not need to go to the base of the dog because the gears do not engage that deep.

Sometimes people will cut back the shift drum to take pressure off of the gear while it is engaged.

The following pictures show dramatic angles to illustrate the problem and the fix. Note the angle difference between a worn stock gear and the opposite angle of a undercut gear.

wheatonFJR · Jan 28, 2014

ionbeam said:
The following pictures show dramatic angles to illustrate the problem and the fix. Note the angle difference between a worn stock gear and the opposite angle of a undercut gear.

Thank y'all fer da picherz, dey heps me unnerstan now...

(Seriously)

rPGoatBoy · Jan 28, 2014

08FJR4ME said:
Oh Boy, that isn't very good.
I hope I never have to do this. Is there any warning signs when this starts to act up?

Dave

Nope. If you should have the misfortune of this problem, one day, you'll notice what initially seems like a double or triple misfire a second or two after a 1-2 shift. Just a momentary hiccup of power accompanied by a noticeable clunk. Then a week or two later, it'll happen again. And then again, becoming more frequent (but only in 2nd gear under moderate or more power). It's the transmission popping into a false neutral for a fraction of a second until the gear dogs align again and pop back into place.

The good news is this problem isn't pervasive; this forum would be rife with examples of this issue if that were the case (see: BMW rear gear). So chances are you're not going to be affected. I'm just lucky, I guess.

RaYzerman19 said:
A buddy of mine has some symptoms.... looks like I might have to get my hands dirty. I'm not an expert at splitting cases or what the undercutting achieves.... what if I just replace the offending parts (dog gears, mating gears, forks)?

You'll probably be alright. Is your buddy's bike also an FJR? It's a problem that happens mostly with big(ger) displacement high(er) torque bikes. Apparently this can happen with more than one gear with 'Busas; APE sells entire undercut transmissions for that Suzuki 1300, mostly to racers. It's not a difficult job to do, there's no sorcery or wizardry behind it; it's just time consuming.

I spent the $200 on the machining/tax/shipping because I don't want to do this a third time. I'd rather ride it.

RaYzerman19 · Jan 29, 2014

Thanks, guys for all that. Yes it is an FJR....

PhilJet09 · Jan 30, 2014

I have an '83 xl250r that jumps out of second gear more and more. Looks like I'll be making this my winter project. New gears aren't available any more and the used ones that show up occasionally on fleabay are probably the same or worse than what I have.

Silent · Jan 30, 2014

PhilJet09 said:
I have an '83 xl250r that jumps out of second gear more and more. Looks like I'll be making this my winter project. New gears aren't available any more and the used ones that show up occasionally on fleabay are probably the same or worse than what I have.

Check out the link in my sig. It shows how I undercut the dogs and slots on the existing gears in an XS1100. It's not as hard as many believe it to be.

rPGoatBoy · Feb 8, 2014

Update: After assembly, I checked the rotation of the gears and noticed that the machine work was done to the wrong side of the dogs and pockets. Back to the machine shop they went.

So the first three pictures I posted are inaccurate! If you send gears off to be undercut, make sure they do the correct side!

What undercut gears look like

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