Help! Converting 24 speed to 8 speed - chain issues...

choon

Ok first post. Here goes...

I have an '04 model Specialized Sirrus (hybrid) which I have been tinkering with for some time.

My plan was to convert it from 24 to 8-speed by removing the two smaller cogs from the chainset and removing the whole front derailleur system, leaving only the 48t cog on the outer position of the spider. This all went very smoothly - I got the cranks and chainrings off, got some single chainring bolts and initially all seemed well.

After a few runs, however, I was having problems with the chain coming off (inwards towards the bike) when I was on the largest rear sprocket so I moved the large chainring to the middle position on the spider. This solved that problem but now the chain intermittantly falls off in the other direction when on the smallest sprockets - not all the time but enough to be annoying. The cassette is a Shimano HG-50, 8-speed, 12x25t with an HG-53 chain. It's had almost daily usage commuting for the past 3 years.

Will I need to buy a single-speed specific chainring (if so what do you recommend)? Or does anyone have any suggestions to keep my chain in place?

Any help appreciated...

ratty2k

Still need an 8 speed chain, single speed ones tend to be wider and wont fit. Might well be just worn out, time for a new cassette and chain methinks.... or possibly the limit screws on the rear mech?

choon

Thanks for the reply. Having re-read my post I see it was a bit misleading. The chain was falling off from the chainring not from the cassette. I think it may be that the largest and smallest rear sprockets cause the chain-line to move too far off centre and this derails the chain from the front chainring. I think....

The cassette and chain could certainly use replacing anyway.

nicklouse

re fit the front mech and adjust the stop screws to hold it above the chain ring.

hoathy

A chainring that is not ramped will make a difference to! to help the chainline slightly you might take the ring off of the outer position on the chainring and move it to where the middle ring used to be, this makes it more central.

nicklouse

Hoathy wrote:

to help the chainline slightly you might take the ring off of the outer position on the chainring and move it to where the middle ring used to be, this makes it more central.

he has allready done it.

hoathy

nicklouse wrote:

Hoathy wrote:

to help the chainline slightly you might take the ring off of the outer position on the chainring and move it to where the middle ring used to be, this makes it more central.

he has allready done it.

good point, my mistake.

ratty2k

Front mech as nick has said will stop the chain coming off, the other thing is the chainring you have is ramped to allow shifting. COuld be worth chainging it to a "proper" single speed one as the tooth profile is taller and not ramped- chain wont come off so easily and it'll look neater than with the front mech on.
Also, (not sure about this) can you shorten the chain any? Would of thought it could be shorter as it hasn't got to cover the amount of gears it did previously?

nicklouse

ratty2k wrote:

Front mech as nick has said will stop the chain coming off, the other thing is the chainring you have is ramped to allow shifting. COuld be worth chainging it to a "proper" single speed one as the tooth profile is taller and not ramped- chain wont come off so easily and it'll look neater than with the front mech on.

not a true single speed one as the teeth will be too wide as it still needs to be 8sp chain compatible.

there are many out there.

hoathy

you can't really shorten the chain as when you are in your new smallest gear you will be running the biggest rings back and front.

When i initially built my single speed i bought a TA Specialites chainring, it wasn't ramped, and it was compatable with a geared chain (which is what i was planning to used at the time).

ratty2k

nicklouse wrote:

ratty2k wrote:

Front mech as nick has said will stop the chain coming off, the other thing is the chainring you have is ramped to allow shifting. COuld be worth chainging it to a "proper" single speed one as the tooth profile is taller and not ramped- chain wont come off so easily and it'll look neater than with the front mech on.

not a true single speed one as the teeth will be too wide as it still needs to be 8sp chain compatible.

there are many out there.

Fair point nick, was thinking of the DMR ones I have knocking around that have no ramping... didnt realise they aren't SS ones....
As to chain shortening, if its only got to cope with the cassettes range of gears, a short rear mech could be used, and therefore a shorter chain?

choon

Many thanks for all the input.

I think I may have to go down the single-speed crankset route. I was planning to buy a new one anyway and a single-speed specific one may just solve my problem.

Can any of you guys recommend a nice single-speed crankset or chainring that is good with an 8 speed cassette? It would need to be 48t (or thereabouts), 5 bolts and 170mm cranks(I'm a short person).

If that doesn't work I'll seriously consider putting the front mech back on....

One other thought. Would the amount of usage on the chain cause it to become looser over time? Would a more tight fitting chain help?