Chain length wrong

[gerry84]

I was recently kicked off of my bike due to the chain being too long, I think. I got 3 broken ribs.
When I got home, and calmed down a bit, I calculated tha the chain should have been 6 inches shorter, ( ie chainstays only 15 inches long) whereas I hadn't taken any notice of this.
Is this longer chain doing harm? The chain got wrapped round the bottom gear sprocket, and fouled the seat stay, and the bike stopped dead.

[tatanab]

The chain got wrapped round the bottom gear sprocket, and fouled the seat stay, and the bike stopped dead.

I imagine you mean the top gear sprocket i.e the little one since bottom gear is a long way from seatstays. I also imagine you were in the small chainring and the chain was hanging slack. Small/small is not a good combination.

I have a couple of machines where the chain would hang slack if I were to use such a combination, but I have done that for a purpose and cannot imagine using that combination anyway.

I also imagine you probably hit a bump to cause the chain to be able to jump off.

Advice - don't bother trying to calculate chain length with respect to chainstay length, simply set the length according to the manufacturer's instructions for your rear mech.

[slowster]

How to correctly size your chain:

https://www.parktool.com/blog/repair-he ... gth-sizing

How to adjust a rear derailleur, including setting the limit screws, e.g. to prevent the chain coming off the smallest sprocket and jamming in the gap with the seat stay:

https://www.parktool.com/blog/repair-he ... adjustment

[Lookrider]

This chain lengh scenario can be a little contrasting at times
Now I'm only just learning last few months about maintenance of my own bikes
So I watched tbe park video using the big big method and I'm not at all going to challenge them as I'm out if my depth
The mathematical way is another format but I have tried both and they do not end up same lenght??
I have recently done a 2 day course on my own bike where it was stripped bare by me and rebuilt by me under the mechanics watchful eye and advice
He showed me another way of small small through tbe derailiuer and pull chain till it's a 10mm gap between chain and bottom jockey wheel

My qwery are
Park tool method and mathematical do not take into account the derailiuer which can be short medium or long cage so it cannot be correct for all of them
The small small through the derailiuer seems to have the chain longer and it is looser than the other 2 methods
So to a layman like me who is over curious ?? Which method do most mechanics use

[Jdsk]

So to a layman like me who is over curious ?? Which method do most mechanics use

Sheldon Brown's version of the big-big method:
https://www.sheldonbrown.com/derailer-a ... html#chain

I'm an amateur, but my guess is that this is what professionals use first.

Jonathan

[531colin]

With modern "magic links" its so, so easy.
Bung the chain on as it comes.
Put it on "big-big"
Double over the bottom run of chain to see how many links you need to take out so that it will (just) get on "big-big" .....and more importantly, get OFF big-big.

[slowster]

Park tool method and mathematical do not take into account the derailiuer which can be short medium or long cage so it cannot be correct for all of them

Not so. The difference in lengths of cage is not significant, i.e. sizing the chain by using big/big and adding two extra links as per the Park Tool/Shimano method will accommodate small, medium and large cages.

The small small through the derailiuer seems to have the chain longer and it is looser than the other 2 methods

As the Park Tool video states, this is the sizing methodology recommended by Campagnolo. It is probably no coincidence that Campagnolo mainly makes road components. The big/big + 2 links method increases chain tension, which has the advantage of reducing chain slap when riding off-road.

[Lookrider]

Thats a fair explanation
However the through the derailiuer method that I was shown was for a rd bike and yes theres a lot of chain slap ...so much if you stop pedalling the momentum will have the chain hit the stays ?? But it does ride ok and changes gear ok