scook94 said:
Jimbo, as usual you're way too technical for me! the bike is going to be based round a new Ti frame, so hopefully it'll be nice and light, lighter than my current steel frame anyway.
I'm not great at climbing, but I'm making good progress at improving. I don't really want to jeopardise that too much by having too big a granny ring, but on the other hand I'll (at some point) want to really start doing some serious climbs.
I'm currently doing 45 mile rides over hilly and undulating terrain so there's not much cruising to be done.
I'm thinking that if the proposed set up is going to be too different I'll stick to 50/34 on the front and maybe go with 11-26. Perhaps sticking with what I know up front will make it easier to get the cassette that would suit me?
He's too technical for most of us !
Seriosuly though.... your existing 12-25 cassette, is that 9 speed or 10 speed? Do you know how many teeth on each sprocket?
Let's assume it's 9 speed and 25mm tyres - so from the table I linked earlier your current gear inches are as follows
50 Chainring:
12T 110 GI
13T 102
14T 94
15T 88
17T 78
19T 70
21T 63
23T (58)
25T (53)
On your 34 chainring:
12T (75) GI
13T (69)
14T 64
15T 60
17T 53
19T 47
21T 43
23T 39
25T 36
The ones in (parentheses) are ones you don't use - right?
Now - which of those do you use MOST?
It doesnt matter if you have completely different chainrings on the new set-up but you'll want similar gear inches (or maybe slightly higher numbers if it's a whizzy Ti job and lighter than your current steed) so you need to pick your cassette accordingly.
You can multiply those gear inches by MacB's formula if you want mph for a given rpm - if you find that easier.
but on my commute home there's one hill I normally hit my top speed on, generally 36-37mph, I've never needed to go beyond 105 GI's to do this and can do it at 90 GI's if I pedal like mad.
