Chainring/Sprocket combo

[GrasB]

When you're gearing a single speeder is it better to go for a larger or smaller chain ring & sprocket. I am thinking that 52/16 with 650x23C tyres gives me the gearing I'm riding on now with loads of scope for gearing changes right now I'm thinking 49/15 is good but would 46/14 or 52/16 be better options?

 

[Will1985]

That's a fairly big gear (~80" for those who don't want to check). Bigger is generally believed to be more mechanically efficient.

 

[alecstilleyedye]

i've heard that one sprocket higher than you'd use for an equivalent freewheel gear is a reasonable rule of thumb.

 

[Joe24]

i've heard that one sprocket higher than you'd use for an equivalent freewheel gear is a reasonable rule of thumb.

How does that work
On a freewheel bike you change gears normally, on a singlespeed yes you freewheel, but you cant change gear, so how do you knwo what gear to be in
I was always told to go for around a 65" gear. I started on that, and slowly upped it to 72", or a high 90" on the TT bike.

But going by what gear you normally sit in on a geared bike is just strange You wont be sat in that gear when you get to hills.

 

[Young Un]

How does that work
On a freewheel bike you change gears normally, on a singlespeed yes you freewheel, but you cant change gear, so how do you knwo what gear to be in
I was always told to go for around a 65" gear. I started on that, and slowly upped it to 72", or a high 90" on the TT bike.

But going by what gear you normally sit in on a geared bike is just strange You wont be sat in that gear when you get to hills.

I think he means like if you had a single speed, then changed to fixed.

 

[brodie]

The OP's not asking about what gear ratio to use. The question is what's the difference of using:

bigger ring/bigger sprocket

or smaller ring/smaller sprocket

for the same gear ratio. Eg, 49x19 and 44x17 are the same gear.

From what I've heard, the bigger ring/bigger sprocket combo is more mechanically efficient, there is less wear on the chain as it's under less tension. The smaller ring/smaller sprocket combo is bit lighter, easier to accelerate.

 

[Will1985]

I answered in my reply....just not concisely.

I went for a 55t on my TT bike partly because the ring was stiffer and partly because I heard about the mechanical efficiency - mated with a 13-23 rather than the more common 12-21 (or 23).

 

[GrasB]

Yeah, gearing is set at around 79", I've been riding at that length for 6 weeks now, & I'm definitely using 650C rims as there's to much peddle/wheel overlap with 700C wheels. So the general feeling is 52/16 may be a better option,

 

[D4VOW]

A smaller ring/sprocket combo will be lighter but a larger combo will provide less wear. There is also the skid patches to think about too. 52/16 will give you 8 skid patches whereas 49/15 will give you 30 leading to better tyre life if you intend on mainly leg braking.

 

[skwerl]

The OP's not asking about what gear ratio to use. The question is what's the difference of using:

bigger ring/bigger sprocket

or smaller ring/smaller sprocket

for the same gear ratio. Eg, 49x19 and 44x17 are the same gear.

From what I've heard, the bigger ring/bigger sprocket combo is more mechanically efficient, there is less wear on the chain as it's under less tension. The smaller ring/smaller sprocket combo is bit lighter, easier to accelerate.

why would the chain be under less tension? tension comes from how tight you set the chain, not from your gear ratio

the reason bigger is better is because there are simply more teeth to wear out. Chain wear is no different. Every link still gets worn, although there'll be a few more links on a bigger ring/sprocket setup.
Also better to run an odd number at one end to increase skid patches

 

[brodie]

why would the chain be under less tension? tension comes from how tight you set the chain, not from your gear ratio

Obviously, we're talking about the chain when the bike's being ridden.

It's similar to using shorter cranks, all else being equal, eg pedal revs, gear ratio, bike speed etc, you need to apply a bigger force at the pedals with shorter cranks.

 

[skwerl]

Obviously, we're talking about the chain when the bike's being ridden.

It's similar to using shorter cranks, all else being equal, eg pedal revs, gear ratio, bike speed etc, you need to apply a bigger force at the pedals with shorter cranks.

No. with shorter cranks you have to apply more force to produce the same amount of torque. The torque at the chainring is the same and therefore the force on the chain is the same

eg. to produce 1Nm torque to the chainring you have to apply 5.7N force on a 175mm crank or 6.1N force to a 165mm crank. the resulting torque in both cases is still 1Nm.

 

[brodie]

No. with shorter cranks you have to apply more force to produce the same amount of torque. The torque at the chainring is the same and therefore the force on the chain is the same

eg. to produce 1Nm torque to the chainring you have to apply 5.7N force on a 175mm crank or 6.1N force to a 165mm crank. the resulting torque in both cases is still 1Nm.

Imagine a chainring the same size as the crank, ie the chain is at the same distance from the axle as the pedal is from the axle. You apply a certain force to the pedal, and the chain is under a certain tension; equivalent to the force at the pedal.

Now change to a shorter crank and corresponding smaller chainring. To get the same torque. you need to apply a bigger force at the pedal, ie the chain is under higher tension; equivalent to the force at the pedal.

It's the principle of the lever, in this case it's the diameter of the chainring.

 

[skwerl]

ah. but on a bigger crank/sprocket combo the chain will be under that lesser force for a longer distance (ie a longer arc). when you factor in the distance spent under that force the two combinations should equate to the same. ie chain wear will be the same
I s'pose a big/big combo means less chance of a chain snapping if the same torque is applied vs a small/small combo. But we're talking chain-wear here

 

[Matty]

When you're gearing a single speeder is it better to go for a larger or smaller chain ring & sprocket. I am thinking that 52/16 with 650x23C tyres gives me the gearing I'm riding on now with loads of scope for gearing changes right now I'm thinking 49/15 is good but would 46/14 or 52/16 be better options?

Doesn't a small rear increase chance of chain slip? Sure a LBS told me that 14 was the sensible limit for a fixed.