i have a 42x18 set up, and i'm thinking about making the ratio a little higher.. does it really matter whether i decide to go higher in the front or lower in the back? i guess i want my ratio just a little higher, so i don't spin out as easy

Hi, I would go with a larger chainring. The smaller the cog the less chain contact you have. Your chain is only on 1/2 of the cog at any time.

Tony Arms.

cogs are generally cheaper than chainrings, and a 16T cog is plenty secure in that situation.

it also depends on the BCD on your cranks. if it's 130, you don't have many choices for single speed rings, although TA makes a few different sizes. basically, if you want to save money, get a slightly smaller cog, as it won't really have much of an effect until you get down to like, 11 or 12T cogs.

You can do 42/17 or 45/18...which will give you almost identical gear inches. But you can still easily spin out with these combos. I personally like 42/16, 45/17, & 48/18 as my fixies are currently set up that way. I want my knees to last.

George

An addition to banjo bob's question: I am constantly wearing out the same spot on my rear tires from skid braking. Am I correct in assuming that by moving from a 48-16 to a 48-17, this will alternate the position of the rear wheel every time i stop?
thanks

you are correct, sir!

you can also get the same effect from just taking the chain off and rotating the wheel, so the same spot you're skidding on would be a different spot. it would be 2 spots if you can skid with either foot forward.

larger chainring = less torque

go with the smaller cog

I understand the leverage/torque aspect here, similar to a longer crank giving you more torque. BUT.....if you're running IDENTICAL gear inches, what difference does it make whether you're pushing a 42 or a 45 ring? I know this issue has been discussed extensively on this forum before.

George

I can't describe with technical jargon, but its the ratio thats important. Its harder to generate torque with a larger chainring, but with the larger cog (to maintain similar gear inches) requires less torque to turn. So its a wash.

That said, I'd replace the cog as they are cheaper.

maintaining the same gear inches or ratio, but using different sized chainrings isnt a wash. the more torque you generate with the smaller chainring definately has an effect. thats like saying going from 165mm cranks to 175mm cranks is a wash, anyone who has done this will tell you that the 175's make accelerating seem easier. while the ratio is conserved, the torque is affected. the more torque you have the easier it will feel when accelerating from a standstill. on the smaller chainring bike it will feel easier to get up to speed, and will be easier to decelerate.

I'm not sure I understand. Are you saying running 50/25 gearing will feel different (from the pedaler's perspective) than 40/20 gearing? Even though the gear rations are the same, and given the same crank length and wheel diameter?

The torque will be affected by the crank length only. The size of the chainring makes no difference as long as the ratio stays the same. Wheel size has an effect at the other end. ie 650C vs. 700C at the same number of gear inches due to the rotational weight. Oh, well. You say potatoe I say potato

i say yes, they will feel different

the torque will be affected because there is a diffence in distances between the cranks and the chainrings. this difference has an effect in calculating the moment arm of a system, and torque is similar to a moment arm in that respect (the longer the distance from the fixed point, the higher the torque/moment). the difference between a moment arm and a torque is that a moment arm is a "torque" on a non-rotational fixed system.

so, what im saying in simple words is that the difference in length between the edge of the chainring to the center of the pedal axle on the crank arm of the 2 different sized chainrings will affect the torque.

now, whether this difference is negligible or not, i dunno....but theoretically there is a difference.

Thats what I was trying to say:
I've never noticed it. But I'm not a physicist.

Legalize it is wrong. As long as the ratio is different then there will be no change. The whole chainring/cog/chain system is simply a torque miltiplication system and it doesn't care if you're at 50/25 or 40/20 or 30/15. Torque multiplication of the system is identical. If you change crank lenght or wheel size then you change the input or output moment arm of the system, if you change the gearing you only change the torque mulitplier.

Here's practical proof. As stated in my earlier posting above, I ride 3 different bikes, 3 different chainrings, but all very close ratios. I centainly cannot feel a difference riding from one to the other....

George

FWIW, I was curious about it. I plugged all the values into Sheldon Brown's gear calculator. Given a wheel size of 700x32, a crank length of 170mm, 40/20, 50/25, and 30/15 front/rear gears work out the same as in gear inches and gain ratios (which takes into account crank length).

54 gear inches and gain ratio of 4.0 for all three combinations.

Unless there's some other factor I or Sheldon overlooked, I can't see how they'd feel different.

that settles it then. my housemate and i were trying to figure this out, but we werent definate. hes a civil engineering student and im an electrical engineering student. we need a mech. eng in the house i guess!

legalize it, where are you studying?

to get back to banjo bob's original question: you can change the cog or the chainring; it won't matter much which you change regardless of the argument that precedes this, if only because you're not making a big change.

as for the price question, it depends on whatcha get. a dura-ace cog tends to sell for more $$$ than a rocket-ring (which will fit 130mm cspiders, btw), but generally, the rings cost more'n cogs.

hth.

-rob

dura ace cogs are like, $15-$20...EAI cogs, on the other hand are usually around $30.

temple university

right....the gear ratio is all that matters, the size of the chainring relative to the cog (in teeth)...do i need to whip out the old engineering text to start quoting?....i didnt think so :-) jk