cuyd

I was using 53x18T on singlespeed bike and now I can swap to 50x17T. Gear Ratio is nearly same, am I going to feel any difference?

I've checked the forums and there's a lot of rumor around this but no proper data, like some people claiming that bigger gears are more efficient or cause less wear while others claiming that smaller gears are lighter and bike is more responsive.

zacster

You won't feel any difference. The weight difference between them is negligible. I tend to stay in my big ring even though I'm more likely to use the lower gears in the small ring vs. the high gears on the big ring. I mostly stay in the section where there it overlap anyway. With 2x10 or 2x11 or 2x12 you have so many gears it doesn't much matter as long as you are at a comfortable cadence.

maddog34

2.944 vs 2.941... meh.

I wouldn't bother with the swap, unless the new 50t ring looks a bunch better.

now swapping just the chainring might be advantageous... most SS bikes i see run 46/17 or 18..

SurferRosa

A bigger ring usually looks better.

FBinNY 

Since there won't be ANY appreciable difference, there's no point to changing, until/unless the ring is worn out.


FWIW larger sprockets always help reduce wear in any chain drive. The improvement comes 3 ways.

1- Increased torque for the output sprocket means lower chain tension, saving wear on both chain and sprockets.
2- More teeth = less chordal action, or smoother performance.
3- More teeth means less time each tooth is in the wear zone (the arcs of engagement/disengagement.

So, if no other reason not to, ie. low gear considerations on multigear bike, I would opt for the larger combination. That said, small differences produce comparably small effects.

Troul 

I'd just swap in the cassette & not change the crank.

cuyd

Thanks, what's behind my story is that large 53T chainring got wrecked and must be replaced. As current chain and sprocket are also worn out they will be replaced as well. I would normally go with exactly same setup but I can get nearly mint 50T for free so I thought about 17T sprocket to keep gear ratio intact.

choddo

Given the price of (and lead time on) high quality chainrings, the idea has merit.

KerryIrons

4. More teeth engaged means less force per tooth and therefore less wear.

FBinNY 

With respect, this is a very common fallacy regarding chain drive.

Two conditions must be met simultaneously to cause wear; load and movement.

The only place you get wear is where the chain is winding onto and off the sprockets at 6 and 12 o'clock. Everywhere else the chain is moving with the sprocket, not relative to it.

Even better, a new perfectly matched chain causes near zero wear to sprockets even at the tangents, because the teeth are cut to allow it to swing freely without touching as it comes on and off. Once the chain wears and it's pitch increases it will bump and rub the next tooth as it winds on and off.

That's the beauty of chain drive. Minimal friction between chain and sprocket and only occurring one tooth at a time.

DiabloScott

All true - but more teeth on the wheel means each tooth will be engaged less frequently (assuming all else is equal), and that means less wear. Almost certainly not worth thinking about unless we're talking industrial machinery.

OP said SS,not FG - otherwise skid patches might figure in the ring and cog choice.

FBinNY 

Nope, if all else is equal, then every tooth on the freewheel is engaged in the wear zone once per wheel revolution. Likewise with chainring teeth. What changes is the length of the arc.

But, yes this is a very minor consideration. The advantage of larger sprockets relates mainly to chain tension and flex.

choddo

Surely the frequency of chainring teeth being in the wear zone per mile of travel will vary by chainring size, assuming the cog doesn't change and the rider applies a little more torque?

FBinNY 

Yes, the frequency of chainring tooth engagement per mile varies. It's tied to cadence, while cassette tooth frequency is tied to wheel revolutions.

This is part of the reason aluminum chainrings outlast steel cassette sprockets.

However you call it, just about all the wear on both the chain and sprockets happens at either end of the upper loop. (The lower loop doesn't factor because there's near zero load there).

DiabloScott

Correct, for cassettes it's not the frequency of tooth engagement that differs but the duration. If we keep speed constant for our comparison, the wheels will be spinning the same RPM and each tooth on a 28t sprocket will be in the wear zone for 1/28th of a revolution and each tooth on a 14t sprocket will be engaged for twice that time.

For chainwheels, at the same speed the smaller ring will be spinning at higher RPM so each tooth gets engaged more often.

If you keep crank RPM the same for the comparison and let the speed change, then the opposite is true.

Conclusion - you can justify any gear combo you want if you select your assumptions properly.

zacster

99.9% of cyclists never get to the point where any of this will matter anyway.

KerryIrons

This theoretical analysis ignores the fact that chains are not perfect in their pitch and that even as a chain wears, the rollers contact the chain ring teeth at different points as the chain engages and then disengages as it goes around.

FBinNY 

Yes, but chains are amazingly precise when new, though obviously things get much worse with wear. However, even with extreme wear, there's near zero to zero load on any teeth save those approaching 12 o'clock. If you care, have a friend stand on a pedal, and gently lift the chain from the chainring at 3 o'clock, then work your way back to 12. In most case you can get to within 2-3 links from the top before feeling resistance, except with extreme wear where you'll start feeling increasing load at 4-5 links.

So, yes there's some movement, but everything is fully settled beyond the short zone near 12. (or before it in back)

BTW- as folks who obsess may confirm, chain wear isn't linear. It's on a curve happening very slowly, then as wear causes slop accelerates. So it may take 1,500 miles for a chain to stretch 1/4%, the next 1/4 will happen more than twice as fast.

Road Fan

All the data you need is to divide 53/18 = 2.944 and 50/17 = 2.941. The difference between them is 0.11 % which I can't imagine being able to feel. If you had a multispeed bike the rest of the geartrain in the two cases would come under consideration.

Another point is that larger gears are generally more efficient than smaller gears, all other things being equal.

Jax Rhapsody

A bit faster acceleration and slower top speed is the difference.

spclark 

Same applies if you've never had more than ten I think (or twelve if your DO spacing can take a 6) but the BIG/SMALL combo makes it plain that flat terrain is different than where there are hills. Low rise's easier to overcome than a long, steep mash w/o a suitable ratio to put to work.

79pmooney

Yes, bigger chainrings and cogs run more efficiently and last longer. That said, I run my fix gears on 42 or 44 tooth chainrings and cogs around 17 teeth. Why? Well, if I want to go up serious hills, I do not have to change chainrings to get the lowest possible gearing using common velodrome equipment. (42-24. 42 being the smallest a 144 BCD crankset will run and 24 being the biggest 1/8" cog available without looking under rocks.)

Chainrings last me probably 10,000 miles. Cogs a long time. I replace the 1/8" Isuzu chains at 1/16" "stretch" but even that takes a while. So my less than optimum cog sizes add up to such a small handicap that it isn't worth jumping through any hoops to do things differently. Now, if I was never going to do those climbs or I reverted back to my 20s, I might go 48 tooth and a 19 or so. And enjoy a ride that is just a little nicer.