I currently ride a really old mountain bike (1998 schwinn mesa), and I'm planning on getting my first fixed gear. I always right 42/15 on 26in x 1.95inch wheels which has a GI of 72.5. For my fixed gear, I want to use a 48/17 on 700c x 25c wheels which has a GI of 74.6. Now I'm not completely sure but the fixed gear is probably going to be much lighter than my mountain bike, I'm going to use the cinelli vigorelli frame just to let you know. Does the weight affect the GI in anyway? I would think so because when I ride my mountain bike, I feel a difference in pedaling when I ride to school with my backpack full of stuff, as opposed to riding regularly with nothing on.

I always thought GI had to do with how far you will travel with one full rotation of your crank. So no I don't believe it does have anything to do with weight. Maybe slightly, since theoretically your tires would be slightly less "tall" with more weight on them - which slightly decreases the circumference of the circle (wheel). But probably not enough to matter.

Anyone know for sure?

Weight may affect the amount of effort it takes for you to ride, but it does not affect gear inches.

The gear-inch measurement is independent of weight, just like the slope of a hill is independent of weight. But it's still easier to push 100lbs up a 30 degree incline than it is to push 200lbs up.

Since you are moving tween wheel sizes you should use gain ratio.

https://sheldonbrown.com/gears/

And of course weight is a factor.

I'm not convinced. If weight changed gear inches, it would probably be an entry field on Sheldon's calculator, no?

Bat56 is saying that weight is a factor in GAIN ratios, not gear inches.

[chainring] / [cog] * [rear tire effective diameter measured in inches] = [gear inches]

This function is not dependent on weight.

Weight isn't a factor in Gain Ratios either. These are all just measurements of mechanical advantage. Maybe the best way to think about it is that it is a measure of how fast you would be going at a certain cadence. If you spin at 100 rotations per minute at 48/17 you will be going 22.2 mph. It doesn't matter what the bike weighs and it doesn't matter what you weigh.

Does weight affect the gear-inch calculation? No. However, weight may have an influence on preferred riding style. I think skinnier riders are more prone to spinning, while heavier riders are more likely to mash. So if you're trying to decided between two GI choices and you're heavy, go with the higher gear inches; if you're lighter, go with the lower gear inches.*



*Based on my impressions only; would be interested in what others think.

Does eating with a really small fork increase the amount of food on my plate?

deep.

Oddly, a smaller fork doesn't seem to make a difference for kids but a smaller plate absolutely convinces a 3 year old they have more mac and cheese.

Dude, stay on topic. I didn't ask about the size of the plate.

No, I am saying two things:

1. Let's get off GI because everyone around talks about inches when we should be talking about gain ratio. Do we all use the same length cranks? No. And especially when someone comes in talking about two different size wheels we need to talk about ratio.

2. I am saying that if you want to calculate what gear ratio you need to make it up a hill (for example) weight is a factor.

Ek = 1/2(m)(v)^2

in this formula: Ek is energy. m is mass. v is velocity.

This means that you need Velocity rims and Formula rims for this to apply to your situation.

Yes, weight is a factor.

Think of the bike as a machine. It has 2 levers (the crank arms). These levers (combined with gears) move the mass.

More mass? Given the same gear ratio, the levers will need to be pushed harder to move the mass.

Less mass? Given the same gear ratio, the levers will need to be pushed less hard to move the mass.


That being said, there are lots of other factors in play, too:

- Comparing a 26" wheel to a 700c (nearly 27") wheel. This is signficant
- Going from knobby tires with 60psi to narrow tires with 120psi
- Crank length

So, in short: You are just going to have to test.

I suggest 48/18, not 17. 48/18 will be such a ratio that will allow you to start and stop easily. You may not be able to cruise at high speeds, but that's the trade-off that you must make for the sake of safety. Just learn to spin faster.

Gain ratios don't do anything to make gearing more portable between wheel sizes than gear inches. For all we know, the OP could be using the same length cranks on both bikes.

As for the original question, and partly as a counterpoint to Doohickie's post , if your weight is high enough to slow you down, you may consider a lower gearing for that reason. Similar to the way that heavy studded tires will lop off speed and strain your knees if you try to use the same gearing as otherwise.

So... in other words...

ftfy

Both weight and gear ratio affect acceleration (including going up hill). Weight doesn't have anything to do with steady state (assuming drag is the same).


Speaking from experience, I stick with basically the same gear ratio in both cases. The mountain bike is actually geared lower due to smaller diameter tires, which makes up for the greater rolling resistance of low pressure fat knobbies vs skinny efficient road tires.

I agree with Carleton however. 48-18 is a good ratio for FG street riding. That is what I use.
Yeah, you could do 48-17. Your top speed would be higher, but your average speed and acceleration would be SLOWER (and it would take longer to get to school)

Ah, but the extra weight, as applied to the cranks, produces a higher crank torque and actually speeds you up.

Not correct. Wheel size and crank length are independent variables.

I am genuinely interested in what you are getting at. What did you mean by "portable?"