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Altering bike gears

Posted: Mon Aug 23, 2010 12:54 am
by ding dong
My Sakura 305 has a 6 speed gear and once my speed exceeds 10mph I find that I have to pedal like fury in order to provide any manual assistance to the motor. Is it possible to replace the highest gear cog with a smaller cog so that I can provide some input at higher speeds and thus extend the batteries range ?

Re: Altering bike gears

Posted: Sun Nov 14, 2010 10:53 am
by ex925
I just looked at a picture of the 305
My best guess is that it depends whether the front chainwheel is detachable
If it is, then possibly a bike-shop could fit a larger chainwheel and a longer chain
If so, you may find the need for a wider-range set of rear sprockets
(so as to still have a sufficiently low bottom gear)

Re: Altering bike gears

Posted: Sun Nov 14, 2010 4:58 pm
by Jeremy
I think the answer is definitely yes, it looks like you'd have to do the same thing as I've just done to my electric folding bike, change the crankset and chain.

The Sakura has a standard cotterless bottom bracket and crankset, but uses a small (around 40 tooth) front chain ring. You can't fit a smaller rear top gear sprocket, as it uses a 6 speed free-wheel, rather than a cassette, and these only come in a very limited range of tooth sizes, 14-28 is most probably what you've got and you'll struggle to find even a 13-28, let alone anything taller. There's also an issue with the rear dérailleur, as it probably won't handle anything smaller than a 14 tooth top rear sprocket which is almost certainly what you have at the moment.

You can't just change the chain ring, unfortunately, as it's not bolt-on type. You can change the crankset for one with a 50 tooth chain ring (about as big as you can get in single speed 3/32" chain rings) though and this will make a significant difference. You will also need a longer chain by around ten links.

I bought a Stronglight 50t 170mm crankset for £28 from this ebay seller: and it seems pretty good. A new 3/32" chain cost another £7 from the local bike shop. It was a ten minute job to swap the cranks over and fit the new chain, but you do need a crank puller tool to get the cranks off the bottom bracket axle. These are only around £5 or £6 , but you could ask if your local bike shop would let you borrow one if you're only going to do this job once.

I think you will find that a big front chain ring will transform the pedalling ability of the bike at speed and allow you to use all the gears sensibly. If your bike is anything like mine, most of the lower gears were pretty useless as they stood, I rode in top gear 99% of the time and even then struggled to pedal fast enough on the flat.