Gear mechanics

aquanaut

I was just walking the dog and thinking about how gears work (as you do) and it all slotted into place for the rear gears, but I cant for the life of me figure out why the front smaller ring is easier - can anyone enlighten me? Thanks

Morgan

Nobody knows for sure. Just accept it and keep pedalling.

macnab

Smaller ring equals smaller circumference equals less work done per revolution.

Beasty

Gear ratio = number of front cogs divided by number of back cogs

If you have a 52 on the front and 13 on the back the ratio is 4:1 - A high gear

If you have a 39 on the front and 13 on the back, the ratio is 3:1 for the same cadence you go 3/4 the speed on the small ring compared to the big ring, so it's easier

bikenut

I remember describing this in an interview 10+ years ago

It's all to do with number of times your pedals rotate per wheel rotation. Assuming a certain gear on the back, then it takes maybe 6inches of chain to rotate the wheel a full rotation. Using a small gear on the front, , your pedals rotate more for 6 inches of chain (1 wheel revolution) than using a bigger gear.

Hmm - that sounds more complicated than it should be

aquanaut

Yeah, but less teeth, less length of chain. I hope I dont wake up at 3.30 thinking about this!

Lumen

aquanaut wrote: »

Yeah, but less teeth, less length of chain. I hope I dont wake up at 3.30 thinking about this!

No, the chain length doesn't change. The derailer takes up the slack.

aquanaut

Lumen wrote: »

No, the chain length doesn't change. The derailer takes up the slack.

Yeah - drunken typing mistake - think im getting the idea now with the ratio's. Thanks all

Shadow78

If you had a 20 tooth cog on the back
20 tooth small ring 40 large ring at front
Wheel circumference of 3m

On the small ring one revolution drives the rear cog one rev = 3m travelled
On the big ring one revolution drives the rear cog 2 revs = 6m travelled

blorg

Yes of course but doesn't explain why it feels harder. Also, going up a hill feels harder, even using the smaller ring at the front. You would think then that since small=easy the easiest combination would be small-small, like the more small the better, right? But it's not. I think this is due to cross-chaining.

Shadow78

if you had a 10 small cog and 20 large cog on the back
other assumptions as in post above
so a 20 rear cog and 20 front ring gives 3 m (20/20 = 1 rev of wheel)
10 rear cog and 20 front ring gives 6m (20/10 = 2revs of wheel)
20 rear cog and 40 front ring gives 6m (40/20 = 2 revs of wheel)
10 rear cog and 40 front gives 12m (40/10 = 4 revs of wheel)

The bigger ring at front drives mores chain and smaller cog on back takes less chain to turn 1 rev
the smaller ring at front drives less chain and bigger cog at back takes more chain to turn one rev

blorg

Not possible I'm afraid, freehubs are too large to take a 10T; 11T is the smallest you can go. 20T at the front is pretty difficult to get to too, maybe you can do it with a MTB crankset but then the knobbly tyres are slowing you down. All that drag! It's like riding in treacle.

niceonetom

You can get 9 tooth freewheels for BMX hubs I think so you could climb like billy goat with one of those. Right?

Speaking of freewheels, how do they work? I'm assuming wizardry. It's wizardry, right? And what about coasting backwards? You can't explain that.

BostonB

blorg wrote: »

Yes of course but doesn't explain why it feels harder. Also, going up a hill feels harder, even using the smaller ring at the front. You would think then that since small=easy the easiest combination would be small-small, like the more small the better, right? But it's not. I think this is due to cross-chaining.

Don't really understand how you are thinking about this. I don't do a fraction of the cycling you do. But this is how I think about it.

Maybe I'm being simple, but I thought its simply what does the least work, or which turns the rear wheel as little as possible, and which thus makes you move the least distance. If the front gear was half the size of the rear, you'd move the rear wheel only half a rotation, for every full turn of the pedals. Decrease the front gear size, you move the rear wheel less. Increase the rear gear size, you move the rear wheel less. The less you move the less work you do, the less effort.

The smaller front gear shouldn't feel harder, as in you need to push down harder. But you will need to pedal more often. Which you might not prefer. So maybe harder is not the word. Less comfortable maybe. I often adjust the gears so I pedal slower, it just feels more comfortable. I'm only commuting and then slowly, so maybe I'm talking complete drivel. I'd mention cadence but it doesn't seem to apply my level of cycling.

aquanaut

actually this does lead me to a question that may be going off the original question, but don't want to get into bad habits. I have a compact double. When I see a hill coming up, lets say im on the big gears and I know I need to go to the smaller front gears, if i just shift the front gears I know i'll end up spinning like crazy, so i go 2 gears up on the back and then drop the front staight after (this seems to result similarily as just going 2 gears down on the back) - I probably do the same going the other way (ie from an uphill to a downhill). I hope this makes sense and my question is, is this the best technique / stange techique or worst - if so, what is the best method? thanks in advance

blorg

No, that makes perfect sense, particularly on a compact where there is such a gap at the front. It is a lot easier to shift further at the back when you actually start up the hill.