Help me understand SS gear ratios- Mtbr.com

# Thread: Help me understand SS gear ratios

1. ## Help me understand SS gear ratios

My SS runs a 34 x 17. My buddy runs a 32 x 16. His point of view is we are equal as they are both 2:1. By take is we are probably close but not exact as my front ring is larger.

If the front ring size does not matter as long as the rear makes the ration 2:1 why would folks not run a bigger ring in the front to sceam on the

2. ## hit the button a bit too quickly

If front ring size does not matter why would you not run a 40 x 20 and still have a 2:1 ratio. Thanks in advance for the help.

3. ## why not run a 40

-oddish size, harder to find than 32 compact or 34 standard
-bigger ring more likely to be bent (on a compact or standard crank)
-no standard bash ring made for a 40 (I think), if you want to run one
-less flexibility in gearing, can't go less than 2:1 very easily (but can easily go higher)
-for the weight-weiners, it's minutely heavier

4. 34:17 = 32:16 = 2:1

Each time your pedals go around once the back wheel goes around twice. Your front ring is larger (by 2 teeth) but so is your back ring (by 1 tooth - 2:1 again, see?).

I'm sure some people do run 40:20. It depends on what is easiest for you to find for your rig (or what fits when running vert. drops without a tensioner). The most common front ring sizes are 32 and 34 - thats why most people use them.

5. Originally Posted by sea otter
My SS runs a 34 x 17. My buddy runs a 32 x 16. His point of view is we are equal as they are both 2:1. By take is we are probably close but not exact as my front ring is larger.

If the front ring size does not matter as long as the rear makes the ration 2:1 why would folks not run a bigger ring in the front to sceam on the
First, you can use the edit button to update your post if you need to. No need to reply to your own post.

Second, on gearing: 2:1 is 2:1 no matter how it is achieved. Personally I like bigger rings/cogs because they fell smoother/easier to me and they should wear longer. My usual setup is 36 x 21. A 2:1 setup is too high for my local riding. I might use 41 x 21 for Sea Otter.

Reasons not to use big chainrings include:
• Less log/rock clearance
• Easier to bend big rings on logs/rocks (I ran a 44 x 26 for a while. Kept tweaking the ring riding over logs)
• Less frame clearance
• Harder to use easier overall gearing
• Harder to find big compact rings (94 mm BCD)

6. ## Are all 2:1 ratios the same?

Second, on gearing: 2:1 is 2:1 no matter how it is achieved.
What about the mechanical advantage given with a larger diameter chainring? I remember reading something along those lines not so long ago. Can any of our riding engineers attest to this?

djy.../

7. Originally Posted by dodjy
What about the mechanical advantage given with a larger diameter chainring? I remember reading something along those lines not so long ago. Can any of our riding engineers attest to this?

djy.../
From my understanding, there is no difference in the mechanical advantage if the final ratio remains the same. There may be less friction loss with bigger gears - 40 x 20 vs 32 x 16 (less bending of the chain and more teeth engaged to distribute the force).

Longer cranks have a greater mechanical advantage, as does a lower drive ratio whether it is achieved with a smaller chainring, larger rear cog or a small rear tire.

8. ## 34:17; 32:16 and 40:20 are all the same......

Gear ratios 34:17; 32:16 and 40:20 are all the same. One revolution of the front ring, produces 2 revolutions of the rear.

When you pedal your bike, you apply force to the crank arms which in-turn transfers the force to the front chain ring. Mechanical advantage or torque is force times distance. So, for the same force, longer cranks produce MORE mechanical advantage on the cranks. More advantage on the cranks means easier pedaling for YOU. So, now you're providing MORE torque to the cranks with less effort on YOUR part. Sounds ideal...... but..

Now, when the load gets transferred from the crank to the chain ring (through the chain ring bolts); now with larger chain rings are meeting with the chain further away from the BB. This mechanical advantage now requires you to have a beefier chain ring because now more torque is being applied (The distance from the chain ring bolts to the chain).

Another thing is the overall CHAIN LENGTH between 34:17 and 40:20 is different. It 34:17 has a shorter overallchain length, thus less wt. Also, as someone else said, smaller front chain rings provide slightly more ground clearance.

In a nutshell, on 2 different identical bikes with the same crank arm lengths, although 32:16 and 40:20 are the same gear ratios, the 40:20 will be heavier on your bike.
Make sense?

9. ## Not to split hairs...

Originally Posted by 34x16
Now, when the load gets transferred from the crank to the chain ring (through the chain ring bolts); now with larger chain rings are meeting with the chain further away from the BB. This mechanical advantage now requires you to have a beefier chain ring because now more torque is being applied (The distance from the chain ring bolts to the chain).

Actually, the net torque imposed on the larger and smaller chainrings remains the same as long as the final drive ratio remains the same. Using the same set of large or small cranks on a 34:17 and a 32:16 (for instance) will result in the same torque/power being transferred by both chainrings. The only difference will be that the larger chainring is more flexible (if it is made of the same thickness and type material) and hence is more prone to flexure as a consequence of impact or torque induced distortions from existing (permanent) deformations.

And although I haven't seen any bicycle specific scientific studies on this, I know that sprocket-chain drive systems will experience a diminution of efficiency if you try to shrink one or both of the sprockets too much. There's a limit to how small you can make a drive or driven sprocket, and if you exceed that limit the efficiency will drop off precipitously (as well as causing structural problems from the chain trying to wrap and traverse a sprocket with only a few teeth). Although I suspect the efficiency difference between commonly used SS ratios like 32:16 vs 36:18 is slim to nil.

10. ## Ratio is the same...but

the number of Teeth that are engaged at any one time for a 40X20 is greater than a 32X16 is greater than 20X10 (that would be wacky)... you get the point. Surly has a good spew about this on there web page.

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