Winfried

Hello

I'd like to check something about gear inches.

According to BikeCalc, the meters of development on my current mini velo (9.2m) is almost identical to my previous road bike (9.6m).

I find it hard to believe that 20" (37-451, actually) wheels can go as fast as 700c simply by using a smaller cog. Is the calculation correct?

Merci.

PS: OTOH, just riding 100km on the mini velo left me totally aching while 150km on the road bike is no problem. I don't know if something can be done about it.

hueyhoolihan

c'est vrai.

Steve B.

This likely correct. Gear Inches is essentially the same thing as Development (according to Sheldon). Using smaller rear cogs and different front rings is common with bikes using smaller diameter wheels (Triathlon bikes, Bents, etc...), to keep the development similar to a 700C wheeled road bike.

Sheldon Brown's Bicycle Gear Calculator

Coluber42

It is theoretically possible to get the same gear inches with the small wheels as with the big wheels. But that will not be the complete story on how efficient the bike is to ride or how you feel afterward.

By the way, gear inches are not quite the same thing as meters of development. Development, or rollout, is the distance you travel on one pedal revolution. Gear inches refers to the size your wheel would have to be in order to give you that rollout if your gear ratio were 1:1. An easier way of thinking of it, and the origin of the (admittedly sort of stupid) unit of "gear inches" is that if you didn't have a chain drive at all and only had cranks attached directly to a wheel, the only way of getting a higher gear would be to use a bigger wheel. That's why pennyfarthings have a gigantic front wheel. At first, people rode velocipedes, which had two wheels and no pedals and you'd kick off the ground. Then they started putting pedals onto the front wheel, but with a wheel that size, they'd spin really fast and not go that fast. So they made the wheel bigger. The diameter of that big wheel is equivalent to gear inches; the circumference is equivalent to rollout, or development (in other words, one revolution of the cranks is also one full revolution of the wheel).
Using a chain drive with different sized gears in the front and back allows you to get more than one revolution of the wheel out of one revolution of the cranks (or less than one, if you want). So development, or rollout, is really a more logical unit because gear inches refers to the giant front wheel on a type of bike that is a wee bit dated by now. But at least in the USA, most of us use gear inches anyway, because we're used to it, because that's what everyone else uses, etc. But in any case, (pi)*(gear inches)=(rollout in inches) and (rollout in inches)/(pi)=(gear inches).

That said, there are various other reasons a small-wheeled bike might be less efficient. All else being equal, a smaller diameter wheel has more rolling resistance because the size of the wheel is closer to the size of the bumps. You may not have as efficient tires on it either, because there are often not as many tire choices, depending on wheel size. A smaller diameter wheel also has more friction in the drivetrain and in the bearings, because at a given speed over the ground, the parts have to spin faster. You're using smaller-diameter cogs in back to get the same gear, so the chain flexes more to get around the smaller cogs. Small wheeled bikes are often not as stiff, and you may lose more power in flexing the frame around. Also, small-wheeled bikes are ironically often heavier because they're often made to fold. Basically, you often give up something in efficiency, riding position, and weight for a bike that can fold. Doesn't mean it can't still be a comfortable bike to ride on all day, but compromises have to be made somewhere.

cplager

The world's fastest bike has 20" (406) wheels. Yup, gearing buys you a lot.

My 20" (406) and 26" (559) bikes have identical gearing 17"-106". The difference between the two depends more on which tires I have on the bikes more than anything else.

Winfried

Thanks much for the explanation.