Chain Tension Issue

[tony.mon]

You can check if it's the sprocket by chalking a mark on the tyre at the tight spot, then moving the sprocket round one stud.
Then see if the marks still line up....

[tattoo]

i bumped into him today and he's had it to a machanic,he inspected it and found it to be oval...i'm sure he said they could see the chain lifting so he put a dial gauge on and measured it again just to make sure..

[Wicky]

*Scratches head* How on earth does a CNC lathe/plasma cut an oval sprocket??

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Mmmm You might have stumbled onto something... Are you quicker up hills? legs fresher and pulse decreased?

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[Miztaziggy]

The fact that the sprockets arent right means that they are either made badly, or that they have been damaged since manufacture, maybe they are made from mouldings rather than being plasma cut? Maybe they have been stored with a lot of weight on the them in the wrong direction sort of flattening them a little? Remember that it only takes a tiny change in their shape to cause a tight spot, the effect is multiplied so that a sprocket 1mm out will require 3mm extra chain to go round the tight bit.

[Wicky]

I can''t believe steel sprockets could change shape, get squashed in storage - but then we're back onto a plausible misalignment in the placement of the bolt holes being offset to the whole disk of the sprocket somehow in manufacture. Again with the sprocket off the bike this could be easily identified by tracing the suspect sprockets and rotating it by a 180 and checking the placement/relationship of the mounting holes to the outer circumference/teeth.

[vtryorks]

We're talking two seperate manufacturers too.

I know it sounds unlikely but I've seen it with my own eyes. Its ridiculous. I'll be contacting B&C Express today regarding the Renthal, the mechanic is willing to back me up fully on the issue also.

[Wicky]

Have you confirmed by measurement on one of the two oval effected sprockets off the bike that the bolt holes are offset?

My plan of action is now this:

Measure the sprockets for equal dimensions and check condition for evidence of misalignment.
.....

Seems to be circular then, so only possible explanation is offset centre and/or bolt holes.

[vtryorks]

To, it was measured from the tooth well to bore on opposite sides.

Renthal/B&C Express are working with me on this, since I doubt I'd have any joy from Powerite regarding the first one.

[tony.mon]

Renthal/B&C Express are working with me on this.

I'm glad to hear this, because it damages their reputation and credibility if they just wash their hands of the problem.

As it is I've had nothing but good service from both companies in the past.

[vtryorks]

Alright, we have a conclusion.

Renthals quality control allows 0.3mm variance on this measurement. The measurement being from the bore, to the bottom of the tooth well.

As mine was 0.2mm it was within spec. After measuring some others, B&C had sprockets ranging from 0.12mm to 0.28mm variance.

In my opinion and in that of the mechanic I took the bike to, it is quite a lot, certainly more than he expected. In his words "thats way out". He sells Renthals but had never come across this. The only reason I caught this was because this was the first time I've ever fitted a chain and sprockets. I was massively cautious and quadruple checked everything. A bit anal maybe, but if I had only checked the slack at one point, I'd never have picked up on it. It pays to be anal sometimes.

So, a word of caution to anyone fitting new sprockets. Check your chain slack at several points around the WHEEL rotation. If, like me, you find you have a 10mm variance in chain slack, you'll need to move the sprocket around the carrier one bolt at a time to find the best point. Mine varied from 3mm to 11mm.....hell of a difference. The carriers themselves are likely to be slightly out, which amplifies the effect of the eccentric sprocket.

If your last set was fine, dont assume new ones will be, because the wear pattern will compensate for these differences over time but initially it could be quite dangerous.

Its a pain to check, but imagine having a pillion on, hitting a speedbump with only 18mm of slack when on the stand. It could snap your chain and I dont really want to think about the consequences of that, particularly with a pillion on.

I suspect I am not the only one with this issue. You have been warned!

[Wicky]

This still puzzles me (probably cos I failed maths but passed Technical Drawing O'Level ) - How is it possible to have an effect on the chain of "3mm to 11mm" due to a sprocket out by "0.2mm"?? Are you sure of those numbers in millimeters as 0.2mm is barely the width of a fullstop


This needle's tip's width is three times your stated sprocket error.

The carriers themselves are likely to be slightly out, which amplifies the effect of the eccentric sprocket.

So are your carriers more out than the sprockets?

Its a pain to check, but imagine having a pillion on, hitting a speedbump with only 18mm of slack when on the stand

Hence why the chain slack should be; 30 - 40 mm

Alright, we have a conclusion.

Renthals quality control allows 0.3mm variance on this measurement. The measurement being from the bore, to the bottom of the tooth well.

As mine was 0.2mm it was within spec. After measuring some others, B&C had sprockets ranging from 0.12mm to 0.28mm variance.

In my opinion and in that of the mechanic I took the bike to, it is quite a lot, certainly more than he expected. In his words "thats way out". He sells Renthals but had never come across this. The only reason I caught this was because this was the first time I've ever fitted a chain and sprockets. I was massively cautious and quadruple checked everything. A bit anal maybe, but if I had only checked the slack at one point, I'd never have picked up on it. It pays to be anal sometimes.

So, a word of caution to anyone fitting new sprockets. Check your chain slack at several points around the WHEEL rotation. If, like me, you find you have a 10mm variance in chain slack, you'll need to move the sprocket around the carrier one bolt at a time to find the best point. Mine varied from 3mm to 11mm.....hell of a difference. The carriers themselves are likely to be slightly out, which amplifies the effect of the eccentric sprocket.

If your last set was fine, dont assume new ones will be, because the wear pattern will compensate for these differences over time but initially it could be quite dangerous.

Its a pain to check, but imagine having a pillion on, hitting a speedbump with only 18mm of slack when on the stand. It could snap your chain and I dont really want to think about the consequences of that, particularly with a pillion on.

I suspect I am not the only one with this issue. You have been warned!

[vtryorks]

Because the effect is amplified.

Its very hard to explain. But it isnt just 0.2mm. Its 0.2mm + any other variation which is in "play". Its progressive from the point where the chain touches the sprocket at the top, to the widest point, and back to the bottom. 0.2mm was the measurement for only 1/41 of the sprocket. You're right in that, 0.2mm TOTAL would have very little effect, but the total is far higher.

Think about an oval. Measure at its widest point but to get to that widest point, there is a deformation from circular along a curve.

I am not sure how to calculate it though, some maths boffin will no doubt be along to explain. My brain is basically fried.

It doesnt need much to pick up chain slack. Think about what a quarter turn on the tensioner bolts does.

[Miztaziggy]

Ill measure it and work it out properly, but in effect, circumference of a circle is pie times twice its radius, so if its radius suddely grows by 0.2mm, then the effective circumfrence of that circle just went up by 2 x 3.14 x that 0.2mm so in effect, 6 times the actual amount the sprocket is out. So 0.2mm out will shorten your chain by 1.2mm at its worst, which can quite easily have the effect described.

[vtryorks]

Thanks Ziggy,

2 pie R is indeed the formula. I'm ashamed I couldnt remember that, its elementary maths. Ooops.

I really do recommend everyone checks this when fitting a new sprocket. How it hasnt come up before is beyond me.....

[Wicky]

...will shorten your chain by 1.2mm at its worst, which can quite easily have the effect described.

Nope still doesn't add up to the stated effect of "3mm to 11mm"