advice on reducing from 21 speed to 7 speed

[WandsworthRouleur]

^^^^ and rear mech with clutch stops chain coming off and/or bouncing around so no chipping of chain stay.

I'm curious about the benefits a (new tangled!) clutch mech brings.

I'm going to build up The Boy's baby Zaskar as a 1x8 with a 32T narrow wide chainring and a mid cage M900 XTR mech. With a suitably shortened chain, I don't get how the chain could bounce around more, or drop off, compared to ruining in the middle ring on a normal 24 speed setup.

Am I missing something?

 

[konahed]

Can you fit an 8 or 9 speed cassette to a bike that originally had 7 speed?

 

[redzephyr]

Re:

Yes but you should use a 9 or 10 speed cassette and take off a sprocket.... The smallest or largest is preferable.

If you haven't been there before then you should start worshipping at the temple of Sheldon.

https://sheldonbrown.com/speeds.html

https://sheldonbrown.com/k7.html#897

 

[redzephyr]

How come chains are more likely to come off with a single chainring?

I've setup a few 1x's and i don't find they are more likely to come off if you maintain the chain and use a guide.

With a triple you're unlikely to spend a lot of time cranking hard at the end cogs. You'd generally move into the granny or large chainring rather than stay in the middle. On a 1x you don't have those options so you place a lot more strain on the chain and you'll probably find it stretches faster than normal. So the combination of placing greater stress for longer periods in the extremes gears with a stretched chain leads to greater chance of derailment.

It's less likely in a 1x7 because the angles of chain lines are not as great as a 1x9/10/11.

 

[02gf74]

How come chains are more likely to come off with a single chainring?

Not so much as coming off; it is what happens when they do come off - on multi chainring set up the mech keeps it in place on one side, and if it drops off, then there is a chain ring to catch it on the other side.

an alternative is a chain guide- like a front mech but fixed and wide enough for one chain ring to replace the front mech; the clutch mech and narrow/wide ring replaces that.

 

[02gf74]

I'm curious about the benefits a (new tangled!) clutch mech brings.

regular mech keeps tension in the chain using a spring - depending on which gear you are in, the tension varies; the lower the tension the more a chain will bounce over rough ground to the point where it can get thrown off

a clutch mech uses a friction plate so the cage hardly moves thus the chain is taught for all gears.