gearing 2

ARob Posts: 143
edited April 2008 in Road beginners
its a very simialar question to one asked recently so very sorry about that.

there must be a formula for gear ratios that says what size you need for a given cadence at a given speed but its beyond me to work it out!

the question however is, i have a 42/28 lowest gear. on the steep hills round here (20-25%) my cadence drops to around 50 which is hard going for me. Assuming the same speed what gear combo would give me a cadence of around 80-90? i only have 8 speeds on my bike so dont want to spread the rear ones out too much so i guess that means the back one would be around 26 or 28 so does that suggest a chain ring of 34 or 39 is needed at the front?