Gear Selector Trouble

[Signal]

Gentlemen we have been having a lot of problems with the gear selector mechanism on our classic sidecar.

The spring (first picture) has broken numerous times. We thought we had fixed this by fitting a heavier duty spring.

We also have had problems with the actual selector mechanism breaking (second picture).

This is on a pretty hot 750 rephased long rod motor that is taken to 8500 rpm at times.

What we believe is that the long rod and rephase is setting up some sort of harmonic that is causing the mechanism to vibrate excessively.

We had a DNF yesterday due to an ignition failure but before that the gear selector had also broken again. I don’t know the exact cause yet but my money is on the spring.

I know sidecars break things that would never fail on a bike but has anybody else had these problems and what was the fix.

 

[gggGary]

Hmm do you think it's trying to kick back out of a selected gear causing shock loads on the shifter? Do you still have the shock springs in the clutch hub? what about a rubber "vane" rear sprocket mount?

 

[Signal]

Thanks for the reply Gary, yes running springs in the clutch hub and XS special 16 inch rear with no cush drive.

To be honest we don’t know what is doing it, and what is really puzzling is that other XS powered outfits are not having these problems. They also are only using the clutch hub cushons but running wider 6 inch wide tyres.

The stresses on sidecars during racing are incredible, in the first race we broke the oil feed tube to the head at the banjo. We were told by the experienced guys that stuff will break on these things that will never break on a bike.

I will ring MMM and talk to him in the new year and ask if he has struck this before.

 

[TwoManyXS1Bs]

Hey, Signal. An interesting conundrum. I'll throw in with the suspicions on the rephase. As posted by Mrriggs, the rephase introduces a "rocking couple":

http://www.xs650.com/threads/277-degree-rephase-experience-good-or-bad.10920/

http://www.xs650.com/threads/what-is-the-difference-between-a-277-and-270-degree-re-phase.19680/

This "rocking couple" could induce lateral oscillations in various waggily things, like the long, thin shifter arm.

If you've ever monitored handlebar vibration, you'll notice that the vibration displacement magnitude is small at the handlebar clamps, larger at the handlebar ends. This displacement magnitude increases as the force path lengthens.

This rephase "rocking couple" would be a foreign phenomena on a 360° engine, and the factory wouldn't have made considerations for its effects.

For this "rocking couple", the force path would start at the crank centerline, and move outward to the shifter boss holes in the case. From there, it increases in length as it travels up thru the shifter arm, to the shift claw. Not hard to imagine the shifter arm waggling in resonance at certain rpms.

With this in mind, I'll share some rambling thoughts and pics, some excerpted from Hans Pahl's engine cutaways.

 

[TwoManyXS1Bs]

Not much space behind the clutch for the shifter arm and its spring.

Even though the shift claw spring is outside the clutch's rotation edge, it'll move inboard, to behind the clutch, during a downshift or downward depression of the shift lever.

Also, note that the clutch rotates backwards from the crank.

You'll want to check for sloppy/loose fit of the shift claw's riveted pivot.

 

[TwoManyXS1Bs]

Yeah, looks pretty close in there.
Imagine that thin shifter arm waggling in there.

 

[TwoManyXS1Bs]

Don't know what, if any, work was done to your clutch basket and dampers. Might want to ensure that nothing is protruding excessively back there.

Might consider a slight rounding/deburring of the primary gearteeth edges.

 

[TwoManyXS1Bs]

You may be able to stiffen the shiftarm, like adding weld beads along these blue lines. I can't tell you precisely where, you'd be the pioneer here.

The dashed red line represents the clutch primary arc.

 

[TwoManyXS1Bs]

The shift claw's spring anchor points could be bent inboard a tad, just to get the spring away from the basket's primary gear.

 

[TwoManyXS1Bs]

Setting the shiftshaft's lateral play.

Using special thickness shims, I've set my XS1B's shiftshaft lateral play to 0.020" (0.5mm). The differential expansion of the aluminum case and the steel shiftshaft, from ambient to full hot engine temp, would take up to 0.010" (0.25mm) away from any lateral play. That leaves at least 0.010" (0.25mm) on my bike, and works fine.

 

[TwoManyXS1Bs]

Setting the shiftclaw position.

While pressing the shiftarm inboard, fully seated, I like to have minimal clearance of the shiftclaw's backside to the shiftdrum's inboard groove face.

This is done oldschool, by bending into submission.

This way, the shiftclaw should remain centered (laterally) in the shiftdrum's slot, behind the starwheel, and not catch/drag on the sides.

 

[TwoManyXS1Bs]

No pics for this one. Another option would be to add a second 1mm shim washer behind the clutch, spacing it out a bit. A matching shimout would need to be done on the crank's primary gear, to keep the primary gears in 100% engagement.

*gasp* - end of ramble...

 

[Signal]

2M Thank you, thank you , thank you for such a full reply.
We had a look at it tonight (just got in from the work shop) and we are thinking pretty much along the same lines as you.
Adding to the rocking couple forces will also be significant lateral forces due to cornering.

We are the only XS outfit running a rephase and long rod combo and the only ones having this problem. It goes like stink and really flys off the line at the starts so we have no thoughts of returning to standard rods or phasing.

What we are going to try is a little hard to explain so I will post photos when its done. The idea is to limit the lateral movement of the shift claw with a couple of machined "washers" in front and behind it holding it in position on the drum.

There is damage to the cases on the bearing boss below the shift claw where it has been rubbing and we have re relieved the bolt heads on the clutch basket after a cush spring replacement.

Our next outing is in early Feb so we have a little time up our sleeve.
Will keep this thread updated.

 

[Signal]

After some thought and advice from 2M this is what we have done to try to do to fix the problem.

It involves modifying the shift star to have a washer both sides of the shift claw, a thicker shift claw and reinforced shift shaft.

The washers and shift claw are made from 4130.

We have two meetings in February both on bumpy tracks so hopefully this will hold together, time will tell.

 

[TwoManyXS1Bs]

Wow, pretty wide stiffening rib on that shift arm. There's enuff room for that?

If I may, a couple suggestions.

Trim back the blunt upper section, angled so that there's no chance of catching anything on the reverse-spinning clutch basket.

Run a thick weld bead down to the shaft boss, to tie the shaft to the stiffening rib. Of course that'll warp the arm, and will need to be re-straightened.

And, those long shift claws always bother me. You need that long protrusion to prevent overtravel during upshifting. But, I see nothing wrong with trimming a little off the lower corner, reducing some dangly weight.

 

[Signal]

Thanks 2M will recheck clearance.

 

[Signal]

Last weekend we did a practise session and 2 races on a fairly bumpy track. The gear shift worked as it should for all three sessions. Today we pulled the clutch off to check for any signs of contact or impending failure, it all looks good.
Fingers crossed we may have fixed the problem.
It was really nice to finish a meeting with a running bike.

 

[TwoManyXS1Bs]

That's great, signal!
Will remember this thread in case other rephase shifting problems are reported...

 

[Signal]

Had our second test of this mod on Sunday, all went well with the gear selector. Haven't stripped the bike for inspection yet. I will post a link in the video thread to coverage of our races. The DNF was due to a broken fuel pump earth, we had a great start but it was all for nought.

 

[TwoManyXS1Bs]

Glad to hear about the shifter.

Sorry to hear about the fuel pump.

... stuff will break on these things that will never break on a bike...

I reckon so.
We oughta start a pool on what's next...