A forum for owners of Yamaha TTR250 trail and enduro bikes!

Wally Wombat · Member

The dreaded clunk

TTRfan · Super Guru

Hi Robin - how could I forget you! biggrin biggrin biggrin

The sprag clutch runs on a boss on the Gear 3 which sits behind the flywheel. Could the surface of the boss be worn or damaged?

Otherwise you should be OK. Maybe leave the TTR in gear when you shut down and let the clutch out as it stops to prevent the kickback which causes the clunk.

Brian

Wally Wombat · Member

Hi Brian,

I think I may have a solution for this one.

The edges of the sprag are worn, just at the edge, on both sides. Meaning the inner and outer sides but just on the edge confuse

Robins sprag 3.jpg

I couldn't live with the clunk any more. It sounds awful. plus I'm planning another desert trip and want the bike to be right. Didn't you say the clunk could break an internal part if left unattended to? Plus being a 315cc I wonder whether there is more force behind the noise. Anyway, as the sprag clutch was only a few thousand miles old I ordered a new gear 3 in case the boss was worn as you suggested. Once apart I really couldn't see any bad wear but slid on the new part anyway. Then there followed a period of head scratching and looking from different angles.

Robins sprag 1.jpg

After a cup of tea I realised there was a difference between the old and new part. The starter gear from the bike (Blue 04) has a bevelled edge. Right where the sprag connects. Where as the replacement (Yam) part from Totally TTRS had a squarer edge. So the sprag had only been in contact with about 2/3 of the boss. Plus was not sitting flat or square.

Looking again at the boss it is mostly worn just at the edge. I think that can be seen in the photo above.

Robins sprag 2.jpg

I hope that makes sense!

Am confident with the new starter gear (and new sprag), it will be sweet. I also noticed the starter motor splines looked pretty worn. Again only a few thousand miles wear, but perhaps the 'clunk' kicks back on the starter? perhaps upgrading to 'new style' starter gears fixes a couple of problems.

Have ordered a second starter gear to see if it is the same as the last. And to pre-empt on bike #2. Perhaps there were different batches made. Or perhaps Yamaha identified the problem and fixed it on later parts.

Anyway, hope that helps someone.

Cheers

Robin

TTRfan · Super Guru

That does sound odd Robin!

I haven't got any Gear 3s out of an engine at the moment to have a look at but I can't understand why Mr Yamaha would have made them with a bevelled boss confuse

Maybe when fitting a new sprag, owners should check their boss and, if bevelled, order in a new Gear 3 - Steve keeps them in stock here.

Thank you for the research and letting us have the information!

Brian

ttr steve · Super Guru

I can confirm that the used Idler gears I have in stock all have a bevelled edge, approximately 2mm in length and maybe 0.3mm in depth

The new ones have a square edge.

Without opening a sprag clutch I am unable to see if the would run on the bevelled egde, but then why would they have changed confuse

Robin are you able to tell if the sprag is running on the bevelled edge when fitted to the bike? Or maybe easier way to tell is if there is only wear evident on part of your sprag clutch's teeth?

Steve

TTRfan · Super Guru

I think Robin has really hit on something here. I can't believe I have missed this apparent "flaw" over the years.

I have looked back over the photos in my sprag clutch folder and found a couple of photos that support Robin's theory. See below where the sprag teeth are showing obvious signs of wear just on their outside edges evileye

I would appreciate thoughts from Mossproof and any other engineers or knowledgeable owners on here please!

I may have some re-writing of sprag articles to do disbelief

Also, I hope you have a good supply of Gear 3s in Steve!!!! I may need one myself......

Sprag clutch 001.jpg

ttr steve · Super Guru

Think I will be ordering a few in the morning!

Lets hope this is the dreaded clunk causer, because if it is we have all had a result!

Don't think it is something you would have looked into Bri, Tiz only that the new ones are different that this has come to light.

Steve

ttr steve · Super Guru

Oh and Robins diligence wink

mossproof · Super Guru

If the clunk goes away when you fit a new sprag clutch, then it is the sprag that is the problem, but if the teeth don't have contact across the whole width, then that would certainly accelerate the wear. Need some engineer's blue painted on the surface and fit the sprag on, then remove it and see how far on the teeth go. By the sound of it, Yamaha noticed the original part wasn't quite right and rectified it in the design of the replacement parts. More than likely not telling anyone about it in the hope that noone notices!
That's my take on it anyway

Cheers all,
Simon.