is 7 speed cassette obsolete?

General cycling advice ( NOT technical ! )
Tompsk
Posts: 195
Joined: 6 Nov 2014, 9:35am

Re: is 7 speed cassette obsolete?

Post by Tompsk »

Nigel wrote: 11 Apr 2021, 10:15am The sprocket centre-centre distances on an 8 speed is 0.2mm different to a 7 speed (8 speed 4.8mm, 7 speed 5.0mm). Over the six shifts to cover a 7-speed cassette, that's 1.2mm, and getting into the "noticeable" territory on mechanisms, though if set for the centre of the range, and +/- 0.6mm on the extremes it's probably OK.

The bike in question is already using an 8-speed free-hub with a spacer to shift the sprockets outwards. I'll experiment with an 8-speed, thanks for the prompt.
It is (slightly) better than that as the limit screw for the smallest sprocket can be adjusted so the chain is centered over the teeth, the gear cable will just go a bit slack. You can also use the limit screw for the largest sprocket, however the gear cable will go tight as it is trying to shift further than the mech is now set to. This could lead to a max error of 0.4 or 0.5mm. On my bike I have not had any issues and it is an idea I think I picked up on this site from others wo had done similar.
Nigel
Posts: 463
Joined: 25 Feb 2007, 6:29pm

Re: is 7 speed cassette obsolete?

Post by Nigel »

Tompsk wrote: 11 Apr 2021, 10:29am
Nigel wrote: 11 Apr 2021, 10:15am The sprocket centre-centre distances on an 8 speed is 0.2mm different to a 7 speed (8 speed 4.8mm, 7 speed 5.0mm). Over the six shifts to cover a 7-speed cassette, that's 1.2mm, and getting into the "noticeable" territory on mechanisms, though if set for the centre of the range, and +/- 0.6mm on the extremes it's probably OK.

The bike in question is already using an 8-speed free-hub with a spacer to shift the sprockets outwards. I'll experiment with an 8-speed, thanks for the prompt.
It is (slightly) better than that as the limit screw for the smallest sprocket can be adjusted so the chain is centered over the teeth, the gear cable will just go a bit slack. You can also use the limit screw for the largest sprocket, however the gear cable will go tight as it is trying to shift further than the mech is now set to. This could lead to a max error of 0.4 or 0.5mm. On my bike I have not had any issues and it is an idea I think I picked up on this site from others wo had done similar.
Yes, whilst out walking today, I realised the smallest gear would be covered the way you said, by limit screw and slack cable. So that reduces the error if the centering is good. I'm less sure about over-tightening at the top. (There's always a bit of side-to-side movement on a jockey wheel, that's part of the mechanism's design).

Also on the same walk, I realised I have all the bits to hand to try it - a 26in wheel fitted with an 8-speed cassette, so an experiment only needs me to keep count of turns in cable and stop screws to return quickly to the old arrangement.

Looking at 8-speed ratios at Spa and SJS, and ignoring the small 11T, I get 13-28 or 13-30 as common ratios, which is ideal to replace a 13-28 7-speed. This looks like a solution which won't require replacing shifters, brake levers, etc..

Thanks again.


- Nigel
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