I'm running 34 x 21 on 26c - what rear sprocket would be the equivalent when running 700c?
Gear Conversion Please!
Tue, 01/05/2012 - 3:41pm
#1
Neil
Gear Conversion Please!
Assuming you have 1.5 tyres and 165 cranks your GI=40.3~
For 700x32 you would need : 32x21-ish or 34x23
that was just scribbling on paper though and there is probably a better answer.
http://software.bareknucklebrigade.com/rabbit.applet.html
That^ is always useful. But Drew is rightish, 30x20 is pretty close too.
I inadvertently started working it out in the middle of writing about DiGeorges syndrome and had to start again as there where large numbers in the middle of the page.
Assume I have 175mm cranks and 1.5s please... Oh, and once we've done that mathematically, can you tell me how it will feel...??
Crisp, like walkers.
If you use use 35c tires with 34x23 you most likely won't notice any difference at all while pedalling. And I can't vouch for ride quality/comfort though.
So the answer is 'add 2 teeth' to compensate for the increased wheel size? What if I only have a 700c drive wheel and keep a 26" front - just the one tooth?
The front wheel size doesn't matter much for the gearing if you want to keep your GI the same. So 2 teeth.
This^ penny farthings are all about the drive wheel, the other one just rotates at whatever speed is needed to compensate.
What has this got to do with a rabbit?
Very little.
Cranks length is irrelevant to gear size but does give you a different power to torque conversion.
By by calcs, 34 x 21 on a 26" with 1.5" tyres is a 40.5 inch gear.
32 x 21 on 700c rims with 32mm tyres makes 41.15 inches.
As courts are tending towards larger sizes for higher-importance events, I'd welcome the extra 1.6%.
Have a play with this:
http://www.gear-calculator.com/#
You can select wheel & tyre size then just slide cogs and chainrings to get the GI you're after. It's by far the best one I've found out there.
Thinking about two bikes with the same theoretical gear inches, but one having a smaller front/larger rear sprockets and the other a reverse - would there be any difference in feel or pedal rotation?
If it's the same GI then no, it should just affect chain/sprocket wear.
There will be a difference in the torque applied to the driven wheel. The combination of smaller chainring large cog does produce more torque on the driven wheel. My physics and maths are not really up to giving the full explanation but Wiki makes a start at it ( http://en.wikipedia.org/wiki/Gear )
Think of the old favourite of starting off in second gear in your car when driving on snow or ice; it's the torque at the wheel that makes the difference in effective traction. On a bike the best way to feel this is granny gear on an MTB versus using something in the middle of the range when riding up a modest climb on wet grass. You tend to get more slippage in granny as the torque requires more grip from the tyre.
In terms of wear Rob is correct, if for example you had 32x20 and 20x30 with a variation in wheel size that made the effective GI the same then there would be no difference in chain or sprocket wear rates as the total tooth count would stay the same. If however the chainstay length varied, which it would need to in order to account for the wheel diameter variation, the combination that gave you less chain would see an increase in wear on the cog and the chain.
Quite how this would relate to feel for playing polo is a little subjective, I'd suggest that the higher torque option would feel slightly more snappy of the blocks. It's a similar effect to using longer crank arms for the same GI but without the impact on 'spinability' that longer cranks seem to give.
Thanks for that great explanation. So if I swapped the combinations around my performance might not change, but with the smaller front cog I'm asking for more grip from the rear in the first wave of power, hence the feeling of snappiness?
Would it be fair to say that most chain derailments come from running a smaller front cog set up (leaving aside SS tensioners)?
If you change front or rear then you'll feel a difference, if you change both to bigger or both to smaller but keep the ratio and GI the same then it's just mainly a wear issue. 30/20 would feel the same as 27/17 or 24/16 as they are all 3/2 ratio. As for the derailments it would make some sense that having the force spread over a bigger cog leads to less as there is a bigger contact point, although maybe having the force in a smaller part of the chain could mean a certain amount of more "grab" if that area is holding on harder. I'd of thought a decent chain line and tension should mean no problems either way.
Larger cogs and chainrings wear less and distribute the load over a greater area. All things being equal then yes, larger cogs and rings should give less risk of derailments, but as Rob says chainline and tension play a bigger part in this than a couple of extra teeth. I'd also add clean chains that are not worn make a difference to derailment. Some people would also tell you that 1/2 link chains drop less, frankly I consider them the spawn of Satan and will never allow one to touch my bike again.
But they look cool, I thought they wore things down a bit quicker as they tend to wear one side of the teeth more.
They weigh more, they increase friction, they stretch faster. Using a single half link to slam your wheel or for magic gear situations is cool but otherwise you're just a slave to fashion and probably the sort of tosser that uses excessive amounts of hair gel as a substitute for buying a helmet.
Ha
Off topic but have you seen these...
http://www.bikerumor.com/2012/04/19/soc12-randomness-coyles-wood-helmets...
agree with the half link chains, chewed up my chainring quickly, i wont use them again