Ratio question- Mtbr.com

1. ## Ratio question

Had a discussion this morning. Is riding a 32/16 the same as 34/17 or 36/18 ? Is the feeling the same...all being 2:1? And then, wouldn't it be the same as 42/21? Would the rider feel any difference?

2. I've been wondering the same thing, will it feel the same?

3. ## I lost a bet on this one once before

as long as the ratio is 2:1 it's all the same. In my mind I had to think it would not be the same with a 40:20 instead of a 34:17 but anyway you look at it one turn of the from crank makes the back wheel go around twice. In SS 32 and 34 tooth rings are much easier to come by than 40, hence 34 x 17 and 32 x 16 are very popular ratios.

Now if you want to complicate the mix add in 180mm cranks vs. 175mm !!!!

4. According to Sheldon Brown's site, all of the 2:1 ratios have the same gain ratio and the same gear inch measurements. I didn't buy it at first, but it does make sense when you break it down. Brown's site is pretty neat if you haven't checked it out. All kinds of information.

5. ## tire diameter

chainging the tire diameter say 29" to 26" chainges the gain ratio also.

6. ## So.......

If you applied a set force on the crankarm and ran 32f/16r then switched to 16f/32r the force at the rear wheel would be the same? I read something once written buy an engineer that the order of ratio has an effect on the required force per output. Just makes you think.... hummmmm.....

Eric

7. ## torque is a lil different...

given the same crank arm length, lets assume 175mm for standard SS crank arm,
if you use a smaller diameter chainring (32 tooth)
and apply force (pedal stroke) to it, the force will be greater on this smaller chain ring
than say..a larger chain ring (42 tooth).

if the gear ratio is the same, yeaa you will essentially end up w the same work, 32:16 vs 42:21

SOO you may notice w a smaller chain ring that you may have to tighten up the bolts that attach it to the crank arms more often, reason why..
there is more leverage being applied to it.

8. ## Use some logic

All things considred (eg crank length, chain-stay length), if the gear ratio remains the same, there will be no effect as it regaurds the rider. Am I the only one who believes this?

9. A 40x20 feels smoother to me than a 34x17. I do not go any faster, just feels different.

If you applied a set force on the crankarm and ran 32f/16r then switched to 16f/32r the force at the rear wheel would be the same? I read something once written buy an engineer that the order of ratio has an effect on the required force per output. Just makes you think.... hummmmm.....

Eric

Well, being an engineer, I can tell you, that's complete ********, in the first example, you have a 1:2 ration, in the next one, you have a 2:1 ratio, different.

11. Am I the only one who believes this?

Nope

I tend to opt for slightly bigger cog and chainring if I can - typically 36:18 - because it reduces the tension on the chain, wears slightly slower and there's more wrap at the cog. The ratio's (obviously) exactly the same though.

12. Originally Posted by Tillers_Rule
Well, being an engineer, I can tell you, that's complete ********, in the first example, you have a 1:2 ration, in the next one, you have a 2:1 ratio, different.
Exactly...like he said it is different.1-2 vs 2-1
Just screwing around, I was bored.....

Eric

13. Originally Posted by shiggy
A 40x20 feels smoother to me than a 34x17. I do not go any faster, just feels different.
I also noticed a different feel switching from 36/18 to 32/16. It felt that there was more low speed torque. Although, at the same time, actual distance traveled (measured by actual crank revolutions) was exactly the same.

14. Originally Posted by loonyOne
I also noticed a different feel switching from 36/18 to 32/16. It felt that there was more low speed torque. Although, at the same time, actual distance traveled (measured by actual crank revolutions) was exactly the same.
There should be a little difference in drivetrain efficiency, maybe that's what you're feeling.

With larger rings and cogs the chain has to bend less to go around them, thus there is less friction in the drive train and more efficiency. A very minor difference, but it might be noticeable.

15. ## Size Matters!

The same ratio ideally is the same ride but in practice it's not the same. As AndyArmstrong pointed out there is greater tension in the chain with smaller rings and cogs. What does the tension do? It pulls the BB back and the axle forward. All the bearings in the BB and axle have greater stress and so more friction. The rollers in the chain have greater friction too. There's more tension to flex the frame.

As you downsize the chainring the same torque produces greater tension. With small enough chainrings and cogs (with same ratio) you could produce enough tension to break the chain, axle, bend chainstays, etc. before the bike even rolls.

16. There is less drivetrain friction if you have the same ratio, but use larger cogs. This is because the load is spread over a greater number of teeth (greater surface area). As Andy said, that gives longer chain life and a side benefit of having more chain on the cogs means it's less likely to slip.

Sam

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