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28 Aug 2018
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Front sprocket came lose
I just had a wild idea to go from 15 front sprocket to a 14 because why not and I mostly drive off ...
When I tried to unscrew the nut holding the sprocket I was surprised that the the nut came lose with my fingers !!!
The safety retaining washer was in place (sort of) but how can this happen ? I've seen the person who tighten it with an air wrench and I said it will never cause problems that was maybe 5k km before maybe less.
I've changed the sprocket the shaft was in pretty good condition and I've tighten it with a good wrench that I have and I've bent the safety locking washer.
Is this a nut that I have to check regularly ? I don't want to mess up my gearbox shaft because of this ...
Really intrigued what could have happened ? It wasn't tight in the first place ?
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28 Aug 2018
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the nut just keeps the sprocket in place axially speaking, it doesnt help transmit any of the torque, the splines do that. other bikes have some times just a slotted plate keeping the sprocket in place, not even a nut pressing...
as long as you used the locking washer so the nut cant unscrew, you are good.
on my race bikes where i change sprockets often, i have the nut drilled so i can use safety wire instead of a new washer every time. i snug them up with a wrench but the nut is always hand tight when i have to remove it.
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28 Aug 2018
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I'm no expert but I would say it wasn't done correctly in the first place, it's the only logical explanation. Tight nuts and a lock washer / tab washer or similar is pretty trouble free, when done correctly. Just because you see someone use an air wrench doesn't mean its tight. Torque wrench and manual brut force is the best way to check. I would bet you will never have this happen again, you will make sure 
Of course I may be wrong and happy to be corrected...
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28 Aug 2018
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In theory, there are two basic designs here.
1: Big nut with a locking tab to absolutely hold the sprocket tight and in place
2: loose sprocket, with a bit of metal and two bolts to keep the sprocket from coming off.
My experience has been that if you have type 1, the nut should be TIGHT always, as the shaft splines will wear excessively if not. With type 2, I believe the shaft is harder, and the sprocket heat treating may be less in the center, so that it wears and not the shaft.
To get the big nut tight, first step is to make sure that it's tight up against the splines by rotating the rear wheel backwards, so that under acceleration the sprocket is tight against the splines so LESS likely to TRY to loosen the nut. Less force under deceleration. Then make sure the nut is tightened to FACTORY SPEC torque not just an air gun which may or may not do the job. Then bend the tab washer over thoroughly, tightly, and it should be tight forever.
Others may well have different experiences!
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29 Aug 2018
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Quote:
Originally Posted by Grant Johnson
To get the big nut tight, first step is to make sure that it's tight up against the splines by rotating the rear wheel backwards, so that under acceleration the sprocket is tight against the splines so LESS likely to TRY to loosen the nut. Less force under deceleration. Then make sure the nut is tightened to FACTORY SPEC torque not just an air gun which may or may not do the job. Then bend the tab washer over thoroughly, tightly, and it should be tight forever.
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This is what I've done myself today ! I've tighten the nut as hard as I could with a 30 cm long key then bend the tab washer over it.
I will check it after a while to see how it is !
I think the air gun is was bad or not adjusted properly for torque since I had this problem on the rear axle also ... same gun same guy ...
I was missing the safety wire in the rear axle nut and the bolts at the end of the swing arm.
I will test all important bolts and nuts tomorrow because I don't want to have any problems.
L.E. I just re-read your post and figured out that the nut might be in the wrong way placed !?
The nut has a groove on one side and is flat on the other side. I put the nut the way I've found it with the groove towards the sprocket ?! and now it holds the sprocket in place ... should it lock it self on the axle or on the sprocket ?
The design seems to be no.1 from your example and I think it's correct the way I've mounted it.
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29 Aug 2018
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I've changed plenty of front sprockets over the years and never found one that was loose. However I can certainly see how it could work loose even if it was properly tightened at initial fitment. If there is excessive clearance between the sprocket and the splines, either because the splines are worn or the sprocket is hardened at the teeth only, then it would be possible for the sprocket to slip a few degrees round on every acceleration and braking. This will wear the abutment faces causing the sprocket to become loose, and given that there is very little elasticity in the bolted-up assembly, even a tiny amount of wear will result in looseness. That's why you should always have a secondary locking method such as a serviceable tab washer installed and properly fastened.
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29 Aug 2018
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Its not a problem when you torque it 110nm and use lockwasher all ears.
Last edited by xtrock; 30 Aug 2018 at 00:28.
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31 Aug 2018
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Quote:
Originally Posted by xtrock
Its not a problem when you torque it 110nm and use lockwasher all ears.
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exactly, who tightens nuts with air wrench...  
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31 Aug 2018
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Quote:
Originally Posted by tremens
exactly, who tightens nuts with air wrench... |
you'd be amazed, that one especially - holding it is difficult, so an air wrench is quick and easy and SHOULD be extremely tight - though no idea what the torque is set to!
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31 Aug 2018
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Quote:
Originally Posted by Grant Johnson
you'd be amazed, that one especially - holding it is difficult, so an air wrench is quick and easy and SHOULD be extremely tight - though no idea what the torque is set to! |
exactly, you don't know what torque is applied, bad idea.
I changed many time xt660 and ktm front sprockets. You need to have torque wrench for this, and it's not that difficult, just sit on the bike and press the rear brake while tightening the nut. You really don't want that sprocket come off during ride...
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1 Sep 2018
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Quote:
Originally Posted by tremens
exactly, you don't know what torque is applied, bad idea.
I changed many time xt660 and ktm front sprockets. You need to have torque wrench for this, and it's not that difficult, just sit on the bike and press the rear brake while tightening the nut. You really don't want that sprocket come off during ride...
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Thanks all guys for answering I will check it after a few more kms I hope with no problems... I just can't check it before each ride it's annoying 
I applied quite some force that I was turning the engine will have to have someone help me with the brake.
I just don't want it to come off ... during 
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1 Sep 2018
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If it's turning the engine and you have NOT got the brake on, it's nowhere near tight enough.
You can do it alone.
I believe the sprocket is on the left side on your bike. Instructions assume it is.
- Put in 1st gear.
- Get the socket and bar on the nut, and stand on the OTHER side of the bike,
- step on the brake pedal,
- reach across and tighten.
- REALLY tight, or better with a torque wrench!!
You may need to reposition the wrench a time or two as it all "winds up" as you put pressure on it.
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1 Sep 2018
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additionally helps pushing bike's front wheel against the wall while on brake.
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1 Sep 2018
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A piece of wood through the spokes braced against the swinging arm always worked for me. If it's in top gear it won't bend the spokes.
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