Chainring and gears

I need to replace the centre chain ring on my bike, and I see they're
supplied as (1, 2, 3) and (7, 8, 9). Out of curiosity, what's the
difference?

I can see it with chains, needing a narrower chain for more gears makes
sense. And I can just about see why number of chainrings could be a
factor. But the rear cassette - why would that matter?
--
Cheers, Rob

On Tuesday, 13 September 2016 08:50:47 UTC+1, RJH wrote:
I need to replace the centre chain ring on my bike, and I see they're
supplied as (1, 2, 3) and (7, 8, 9). Out of curiosity, what's the
difference?

I can see it with chains, needing a narrower chain for more gears makes
sense. And I can just about see why number of chainrings could be a
factor. But the rear cassette - why would that matter?
--
Cheers, Rob

You can get a much wider ranger of gears by using a triple chainset and 9 speed rear cassette.

My max is 30 gears or 3x10.

http://guides.wiggle.co.uk/chainsets-buying-guide

On 13/09/2016 08:50, RJH wrote:
I need to replace the centre chain ring on my bike, and I see they're
supplied as (1, 2, 3) and (7, 8, 9). Out of curiosity, what's the
difference?

I can see it with chains, needing a narrower chain for more gears makes
sense. And I can just about see why number of chainrings could be a
factor. But the rear cassette - why would that matter?

I doubt whether it matters for 7, 8, 9 because they are combinations of
rings and sprockets of the same thickness. On the cassette the gap
between sprockets changes to accommodate the different number so the gap
at the front is probably also changed to reflect this. I can confirm
that an old 2x6 freewheel can be upgraded to 2x8 cassette and 9sp chain,
without changing the front.