Chainset swap 7 to 9 speed

[Altissima]

Could anyone advise please if we can change a 7 speed Shimano Tourney chainset for a 9 speed Thorn?

The reason for the swap is primarily to get shorter cranks (150mm) for my OH who is 4 ft 11 tall. The spin off would be a better quality chainset. The bike is a Specialised Sirrus Women's v-brake bike in XS size.

I am assuming we would need a new BB and I would like to save a few £s by keeping the 7 speed cassette and 7 speed shifters, at least until the cassette wears out. Does this make sense or would it be better to go 9 speed cassette and shifters with the new chainset?

Many thanks

[hamster]

Should be fine, I've happily run 9s chainsets with 7s. As you say, it may need a BB change.

[Jdsk]

The reason for the swap is primarily to get shorter cranks (150mm) for my OH who is 4 ft 11 tall.

I'd include crank shorteners and fettling the existing cranks in the options.

https://www.sjscycles.co.uk/search/?term=crank%20shorteners

Jonathan

[RickH]

The reason for the swap is primarily to get shorter cranks (150mm) for my OH who is 4 ft 11 tall.

I'd include crank shorteners and fettling the existing cranks in the options.

https://www.sjscycles.co.uk/search/?term=crank%20shorteners

Jonathan

Bear in mind the Cyclo ones are only for narrow steel cranks. The thorn ones are good, I have some, but are more fiddly to fit as they use a replacement bolt on on the BB to secure one end. If you ride with significant heel in pedal action they can have a tendency to be "ankle bitets" as you lose the taper towards the BB of modern cranks. On the plus side the effective crank length is almost infinitely adjustable within the range of lengths.

A more compact version for alloy cranks is available, but generally more expensive still, such as these. You may be able to find the same, or similar, cheaper elsewhere I haven't looked extensively.

[Brucey]

the other thing you can do in many cases is to have the extant cranks shortened. A few different people offer this service and it is about £30 IIRC. Not every crank is suitable for shortening, not every new length is possible and in some cases it simply "isn't worth it".

FWIW going 150mm is a big change; you might find 165mm is suitable (at her height this would be 'normal') and this would widen the range of available cranks. It makes sense to try a few different lengths if you can do before purchase.

cheers

[Altissima]

Hi, thanks very much for all of your replies.
I didn't include this detail before but my wife has a bespoke built bike with 150mm cranks already and loves it. She has had that bike for 7 years and we have had several thousand happy miles touring.

The only problem is that when we moved house (just before lockdown) we had to put our bikes into temporary storage (so we thought) and now can't get at them. Unfortunately , off the peg bikes although much cheaper come with, in many cases, one size fits all components.

You are right Brucey, the extra 15mm on crank length was immediately noticed by Ruth and to save her joints she would like the 150mm cranks, and 44/32/22 chainrings on the same Thorn chainset that she has on her Dave Yates tourer....Just wondered if they would work with 7 speed cassette and 7 speed chain?

I don't know enough to answer and reading the technical advice on other forums is making my head spin!

[Brucey]

if 150mm is known to work then I can see why you want the same thing. Do bear in mind that the BB spindle length may well need to be different with any new crank. Other than that I can't see why it shouldn't work nicely.

cheers

[Altissima]

Thanks very much Brucey, that's really helpful