Keep snapping belts!

Clutches, Chain & Belt Drives

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alexanderfoti
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Keep snapping belts!

Post by alexanderfoti » Mon Oct 05, 2015 5:17 pm

Right, this is the third one now, and I have reached the end of reasoning.

This is the belt that drives my rear wheel. 22 teeth on the front sprocket and 162 on the rear.

This is what the belts look like:

Image Image

This matches the shock load failure type on this page:

http://sdp-si.com/web/images/Belt_failu ... _Guide.pdf

Any thoughts as to how to get around this. I Already run belt tension to the point where I can just about twist the belt 90 degrees. I think the shock loading is coming from the CVT when it changes through the ratio from slipping to fully engaged when just moving off.

There is no more space for a bigger belt unfortunately, as it either rubs the frame or rubs the rear shock absorber. To swap the drive to chain would mean a huge rear sprocket that is custom made and would run £90 odd. I would like to keep the existing drive if possible?

gilburton
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Re: Keep snapping belts!

Post by gilburton » Wed Oct 07, 2015 7:30 am

I've not checked but do bikes that run a rear belt have the pivot point of the swinging arm in line with the drive sprocket/pulley??
This is the reason that chains need some slack as the chain is continuously tightening and slackening when the swinging arm moves up and down.
If your belt is breaking it would seem to me it's too tight but I recognise that you can't slacken it off as it would then jump.

You need a strong belt tensioner that would give a little.
I've no doubt something would be available from an industrial supplier??
A car timing belt tensioner could be used mounted on the lower run on the swinging arm but it would need to be on a short lever free to pivot with a strong spring rather than a fixed tension like a cam belt.
Possibly Tim could help you out??

Either that or you could get a hardtail :D :D :D

gilburton
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Re: Keep snapping belts!

Post by gilburton » Wed Oct 07, 2015 7:48 am

Ok just checked the Kawasaki 305 belt tension and it would seem that they tension it in the same way as a chain i.e. an up and down slackness.
They specify 9.5-15mm slack or about 1/2" which is the average chain slack.

The method you are describing i.e. twisting is for cam belts which need to be tight or the car engine can suffer damage if the belt jumps .

I assume your rear wheel is aligned properly and the belt is not rubbing on one side of the rear pulley or the other?

Here is the link give it a try

http://www.gpz-305.de/English/Tips/Technical_data.html
Last edited by gilburton on Wed Oct 07, 2015 7:55 am, edited 1 time in total.

alexanderfoti
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Re: Keep snapping belts!

Post by alexanderfoti » Wed Oct 07, 2015 7:54 am

Hmmm Interesting

I have tried varying the tension on it, starting with a small amount of tension to save the bearings stress, but it jumps very easily with that. It rounds off the teeth as well and I can feel it jumping.

It seems as though it only takes 2-3 occurrences of the jump to cause a very large amount of damage to the belt. Considering the belt is £38 a pop its getting expensive.

The jumping seems better with more tension. I also worry that the belt isn't big enough for the power output, its right on the limit of the power rating for RPM/pulley size.

I have a sprocket and platewheel on its way so am going to experiement with putting a chain on it I think, all the bits for the conversion to chain have cost me less than I have spent on belts so far.

tappy
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Re: Keep snapping belts!

Post by tappy » Mon Nov 02, 2015 10:24 pm

I think the failure in your picture actually looks closer to the fatigue failure, which would kinda tie in with you having the belt go around such a small front sprocket - the tight angle and short wrap distance will be putting a lot of load on the belt.
It would also suffer more fatigue if the pulleys are even slightly out of line, non-parallel etc.

Bearing in mind that the "power limit" for that belt will be dependent on all the other things being perfect (tension, alignment etc) so if you're on the limit then you're going to need to make sure that tension, alignment, wrap angle, length between centres etc are all favourable.

Shock loading is probably a part of the problem, and it's worth noting that bikes like the Buell Firebolt have a jockey-wheel tensioner that keeps the belt at the right tension regardless of swing-arm position.
Have you done any kinematic analysis of the swingarm movement relative to the front sprocket position?
With such a large rear pulley could you not incorporate a cush drive to help absorb shocks?

alexanderfoti
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Re: Keep snapping belts!

Post by alexanderfoti » Mon Nov 02, 2015 10:26 pm

I thought the same, but the tooth roots are caved in, exactly as per the shock loading one. Maybe it's a bit of both. I gave up on the belt side of things now, I have converted it to a 428 o ring chain. Hopefully it's a bit more reliable.

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