27 replies to this topic

[dapleb]

Makes sense. Never had that prob but obviously some have.

[dandywarhol]

Makes sense to me too but I removed an original gearbox sprocket on a mk2 by just on doing the nut - it certainly wasn't locking compounded on

[ChrisG]

I always assumed the shoulder it clamps up against was part of the output shaft, but looking at that microfiche it appears not. Definatly does seem to be a good explanation for the problem.

So the follow on question is does genuine yam vs aftermarket sprocket make a difference, or is it all in the mind as all bikes that have had the problem have done enough mileage to be on thier second sprocket, and we're all cheapskates and tend to buy pattern ones?

[lemonentrails]

Its just my theory on why so many on here have had issues with the gearbox sprocket nut / output shaft.

Makes sense to me,when i first started reading about this problem on here i couldnt understand why so many were having problems, I aint heard of it before as i have always used loctite out of habit (because I work on brit bikes !).

So the follow on question is does genuine yam vs aftermarket sprocket make a difference, or is it all in the mind as all bikes that have had the problem have done enough mileage to be on thier second sprocket, and we're all cheapskates and tend to buy pattern ones?

I aint had any problems with good quality aftermarket sprockets (JT or Renthal) and more often than not have had to tap the sprockets onto the splines. so IMO its not an issue.

[robelst]

I aint had any problems with good quality aftermarket sprockets (JT or Renthal) and more often than not have had to tap the sprockets onto the splines. so IMO its not an issue.

The JT front sprocket on my MK2 doesn't have the slightest free-play so I guess not all after market ones are bad.

My theory of loosening sprockets was possible overtightening of the sprocket nut; it needs around 70 Nm which is far less than the sheer size of the nut would suggest. I have seen workshop-monkeys using their (often impressive) body-weight for undoing / fitting sprockets .

But this is a more founded and even more worrying theory

[ChrisG]

My old sprocket which fell off was a JT, not sure how well it fitted when it went on but there was quite a bit of free play when it came off, could have been due to it rattling aorund for a few thousand miles though. The replacement Yam one is tight fit. Definatly didn't overtighten the old one as my torque wrench only goes to 70NM

[dapleb]

Even if you could get a nut to tighten to 200Nm of torque it still would stop the rockining back and forth of the sprocket.

==================================================^not

[Readmarx]

maybe I'm reading this wrong....

I've had sprockets come loose on XT & TDM motors only. The best solution is cleaning the shaft threads, new nut and washer and Loctite and a smooth sweep to the correct torque setting. Even still, the nuts can eventually come loose. They come loose because - I assume - the lateral load on the drive sprocket i.e. the huge twisting forces of a chain at 80/90mph when cornering or driving hard allow the tiny tolerances between the sprocket and splines to matter. They allow the load to transfer into movement and thus stress on the nut/shaft threads. Personally I think it's a fault on thread depth/pitch on the shaft and the nuts - that they're maybe just a bit too shallow. On the MKI motor I've just fitted to my XT frame I thought I'd bought the wrong sprocket - the fit was that tight.

The XT & TDM motors have shoulders on the shafts at the base of the splines to seat the sprocket. This is the same on my triumph, aprilia and suzuki tl1000r trackbike which has the SV motor (in it's original hooligan form anyway ). The DRZ set-up sounds odd to me.

If you take your time and do the job properly and monitor the front sprocket for movement every 500 - 750 miles. It should be OK.

[dandywarhol]

Never heard of any bike needing the sprocket checked every 500 - 700 miles Even my race Yams never needed their sprockets checked and certainly never were loose if I changed the front sprocket to change gearing

[robelst]

Never heard of any bike needing the sprocket checked every 500 - 700 miles

That'd mean checking 3 times a month for me
If a bike would break without doing that, I think I'd rather break it, helping it out its mysery

Loctite sounds like a better alternative

[Readmarx]

Check the sprocket that often only if it's come off on you before (a girl I know quite likes that..) or you're nervy about it happening. I wouldn't say to do that regularly as, say, checking your tyre pressures.

Graham, I'm confused. I could swear (feck!) - see! - that the splines that the sprocket sits on are finished against a shoulder which is an unmachined section of the shaft. The pressure of the nut then rest against this shoulder, not any bush? When they've come loose on me before, the nuts have een wobbling about on the shaft and only held on by the bent washer.

[ChrisG]

I'm pretty certain GG is right, look at the shoulder on the bike, then look at the shaft on microfiche, that shoulder's not there and looks suspiciously like the oil seal spacer.

[stormy]

But I think that you're missing the point here.

Whilst the nut does pinch up against sprocket, which pinches the oil seal sleeve, which then pinches up against the bearing inner race, which pushes on the second gear hub face (the bearing sits inside this bit, well does on the one I have in my hand at the moment), which presses against the washer which is pushed against the shoulder on the shaft,.... it is the job of the bloody locking washer to prevent the damned nut coming off, not a huge amount of torque! The torque merely seats the components together.

When I first joined Carpe, there was a thread about this wassher, and how peeps could save money. For a mere £0.80p (I think) it makes no sense to try and cut corners by reusing these things. Correctly installed, with a correct front sprocket, i.e. the ones with the in built hub boss, and with the tab correctly bent over, the nut won't come undone.

mike

[ChrisG]

Both times my sprocket has come loose the tab washer was still neatly bent over against the nut, stopping it from comming undone. The problem seem to be not the nut comming undone, but components wearing and therefore removing the compression and allowing the sprocket to rattle around. Once the tension is off the nut it can move slightly within the confines of the tab washer and as it's made of soft metal (presumably to ensure the nut gets damaged not the shaft) it wears and the whole thing gets looser.

[stormy]

Both times my sprocket has come loose the tab washer was still neatly bent over against the nut, stopping it from comming undone. The problem seem to be not the nut comming undone, but components wearing and therefore removing the compression and allowing the sprocket to rattle around. Once the tension is off the nut it can move slightly within the confines of the tab washer and as it's made of soft metal (presumably to ensure the nut gets damaged not the shaft) it wears and the whole thing gets looser.

Chris,

are you saying then that your gearbox is wearing out that fast? Which components do you believe are wearing to produce this reduction in the "pinch"?

Does your front sprocket have the raised centre boss? If you cast your mind back to when I finished the rebuild on my TDM, I asked the question about the front sprocket, as it was loose on the shaft when the nut was torqued up.

This turned out to be because the front sprocket didn't have the raised centre, so effectively the sprocket was allowing the nut to reach the end of the threads before touching the sprocket faces. As a temporary "fix", I installed a locking washer (one flattened) on the inboard side of the sprocket, and a normal one on the outboard side. This simulated the raised centre hub and no loose sprocket as a result.

The genuine Yamaha front sprockets have this raised section to make sure the nut does not run out of thread before pinching the sprocket. I can't help wondering about peeps who have had problems with this area in the past, if it isn't down to using the incorrect type of front sprocket, i.e. a thin one.

I'm happy to be proved incorrect on this, but I find it difficult to believe that gearboxes wear so badly (i.e. along the length) to cause this problem. It took 5 tonnes of hydraulic pressure to release the shaft from the bearing and oil seal sleeve on mine on Tuesday, in order to replace the un-replaceable bearing. Believe me when I say it took a similiar amount to re-assemble it again. So torquing up the nut to 100lbs feet or 5 elephant hectares ain't going to move anything along the shaft to take up any wear.

mike