New chain on old block

Rayb · Post by **Rayb** » 21 Apr 2008, 6:56pm

Hi Forum

Recently fitted a new chain to an old 6-speed block. The block was not particularly worn so was hoping I could get away with just changing the chain (otherwise I will have to change complete backwheel). The chain is an 8-speed chain and appears to work well 99% of the time - I have the old 'friction' gear shifters rather than SIS. I have ridden about 70 miles with the new chain and I get the occasional jump in a couple of gears, but it doesn't appear to be consistent with pressure or position of pedals.

I have two questions: with the 8-speed chain, do I have to be more precise with the position of the gear shifter when I change gear and, as the chain is brand new, will it 'bed in' with the block or will it always skip ?

Appreciate people's opinions

thirdcrank · Post by **thirdcrank** » 21 Apr 2008, 7:14pm

Rayb

1/ Although an 8 speed chain is narrower than a 3/32" 5/6 speed chain, the reduction in width is achieved by flush side plates and rivets, rather than having less space between the plates and thinner sprockets. I think it should run just like a 3/32" would.

2/ Don't know about bedding in.

A nosey parker question from me:- Why cannot you fit a new six speed block? They are still quite easily obtainable and not expensive.

Steve · Post by **Steve** » 21 Apr 2008, 8:24pm

Yes, I think you'll find that the chain will "bed in" with time, that is to say as it begins to wear it will be a better match for the worn sprockets.

Post by **Mick F** » 21 Apr 2008, 8:31pm

I think that what is being said, and I'm not beating about the bush, is that the chain should have been renewed before the sprockets became worn.

If chains are changed soon enough, the sprockets will last and last and last and last ....

DaveP · Post by **DaveP** » 21 Apr 2008, 11:43pm

Its not going to get better!
Sprocket wear IIRC is concentrated on the trailing edge of the teeth. When all the teeth are a little worn you have a sprocket that would fit a chain with a pitch just a bit longer than the half inch of a new chain.
The power transmitted by your new chain will not be evenly shared between the teeth of the worn sprocket. The ones doing all the work will wear even faster and the problem will get worse. The chain is unlikely to catch up because chain wear is mostly internal and those new surfaces, protected by the factory grease, just arent going to wear as fast.
TBH most people give up trying before this becomes an issue - all it takes is one unluckily timed skip to cause a gonad/crossbar incident...

If you change the freewheel now you might get away with reusing the chain

Rayb · Post by **Rayb** » 22 Apr 2008, 8:56pm

Thanks for everyone's postings - been a great help for me to understand the parameters involved. The opinions expressed suggest I might be lucky and the teeth are not too worn but the occasional jumping is unlikely to get better and I will have to replace the block. Forutnately it would seem that it is still possible to get 6-speed blocks, which I will pursue before considering replacing the back wheel.

CJ · Post by CJ » 23 Apr 2008, 11:41am

DaveP wrote:Its not going to get better!
Sprocket wear IIRC is concentrated on the trailing edge of the teeth. When all the teeth are a little worn you have a sprocket that would fit a chain with a pitch just a bit longer than the half inch of a new chain.
The power transmitted by your new chain will not be evenly shared between the teeth of the worn sprocket. The ones doing all the work will wear even faster and the problem will get worse. The chain is unlikely to catch up because chain wear is mostly internal and those new surfaces, protected by the factory grease, just arent going to wear as fast.
TBH most people give up trying before this becomes an issue - all it takes is one unluckily timed skip to cause a gonad/crossbar incident...

If you change the freewheel now you might get away with reusing the chain

I think DaveP is being a little pesimistic. In my experience (of occasionally fitting the new chain a little too late) the jumping does get better. How many miles it takes depends on how bad it is to start with. If it's only a couple of sprockets only occasionally, I'd expect that to cease after a hundred miles or two.

The new chain does not concentrate wear on one tooth - that's what was happening with the old worn chain, that made the teeth slightly hooked. The new one bears on the teeth more evenly, despite them being hooked. But it won't make them any less hooked. Next time you will need a new freewheel, so you might as well run this chain until it's well and truly worn out!

DaveP · Post by **DaveP** » 23 Apr 2008, 2:53pm

CJ wrote: I think DaveP is being a little pessimistic....

The new chain does not concentrate wear on one tooth - that's what was happening with the old worn chain, that made the teeth slightly hooked. The new one bears on the teeth more evenly...

Nowt o't soart! That tay cup is clearly 'alf empty...
Well. Ok, Just because it never worked for me doesnt mean it will never work for anyone else!

However, I dont see how the new chain could bear on the teeth more evenly.
My understanding is that as the teeth wear the point of contact climbs up and into the load bearing face of the tooth, so that the chain rides further out from the axis. The chain is now trying to engage a sprocket with an effective pitch greater than that of the chain, thus inevitably reducing the number of teeth by which the driving force is transmitted, which in turn will increase contact pressures and the rate of further wear. To make matters worse, the pitch of the sprocket will be somewhat variable, as its shape results from wear and tear rather than a machine tool.
A new chain will have a pitch slightly shorter and more regular than the old one. Therefore, when fitted, it will be an even worse fit. So much so that it can slip off the teeth, or skip. When it remains engaged the pitch mismatch will be worse than before, no of working teeth even lower and a high rate of wear will occur from the outset.
As I stated earlier, that wear is likely to be onesided. I cant see any factors which would help chain and sprocket to bed in together, short,possibly, of extreme wear.

If you have the time and energy to elucidate, I'm listening enthusiastically

late edit for greater clarity

Post by **Mick F** » 23 Apr 2008, 3:03pm

I'm with you, Dave. My pint pot is always half empty!

If something is worn, it should be renewed. Fitting a new chain is a waste of a good chain, and will, according to CJ (and I'm sure he's right) take time to wear in.

A waste of a good chain, and a waste of time.