I understand that i'll go faster using the larger cog on the front and the smallest cog on the rear.
Hmmm... Not quite sure on this one, mate. You will go further for each revolution of the pedals in a high gear (large at front, small at back) but your actual speed will still be proportional to the speed you push the pedals around. If I want a burst of speed, I tend to go to a lower gear as that lets me spin up faster.
Anyway, I'm off topic. Here's gears for you.
Imagine you have no choice of gears, and that you're always in the 28 on the front and the 14 on the back. (28/14). Every time you push the pedals around, 28 chain links will get dragged around the front ring (there's one link for each tooth, see?).
Down the back of the bike, there are only 14 teeth, but there's still one tooth per chain link, so the only way the extra 14 links can be handled is for the back wheel to spin twice as fast as you're pedalling. Now it'll eat up chain using 2 x 14 teeth, so 28 links, which is what you had at the front, so all is good.
As it happens, you'll travel about 7 feet every time the rear wheel goes around, so that'll be 14 feet travelled for each rev of the pedals. If you want to go faster, you have to pedal faster. To get up to top speed, you'd have to pedal ludicrously fast.
Problem with humans is that they can only really exert a decent amount of power within a narrow band of pedal speeds. For me, its centred around 110 revs per minute, and that won't be fast enough.
This is where gears become useful, as they let us vary our rate of travel without changing the speed we're pedalling at. By switching to the 38 cog at the front, I'd be pulling another 10 chain links for every spin of the pedals. That would take me about 19 feet along the road. If I'm still able to pedal at the same speed, I'll be moving a fair whack faster.
Assuming I'm still pedalling slower than I want to, I switch up to the 48 at the front. Now the back wheel is going around 3 and a half times for every pedal revolution, and I'm travelling 24 feet every time I push the pedals around. Now I'm going some.
So far, so good. Now the road starts to head up hill. Its requiring more effort to keep the pedals going around, and I can't keep up. My cadence (the speed I'm pushing the pedals around at) drops to about 70 revs per minute. This is too slow for me to deliver power effectively, so I slow down even more. Just like a car, I'm going to stall.
Instead of grinding to a halt, I drop back down to the little 28 cog. As I'm travelling less far per pedal revolution, it fees a lot easier on my legs. I can actually keep going. The ground levels out, I switch back up, and carry on.
On your bike, you can vary the gear at *both* ends of the chain, but the principle still holds. If you repeated the above using the 34 tooth rear, you'd find you went less than 6 ft for each full pedal revolution on the 28, and less than 10ft on the 48. Chances are, you'd never run out of power, but you'd almost certainly find you couldn't pedal fast enough to make decent progress.
When in doubt, shove it in the middle at the back and use the front to do the big changes. Or shove it in the middle at the front and use the back to make smaller in-ride adjustments. You can break things by running nasty combinations (small and small, or big and big, for example) so the middle is always safe ground to run to.
Andy.
