[ProjectMayhem]

Read this on another forum "Also the gears they push, basically with smaller wheels and bikes well designed for small people they are putting less force thru the knee and this is very important for immature humans"

Now I don't know much about gear ratios etc but in my experience it takes more force to pedal a bike with smaller wheels over the same distance/terrain, than one with larger - thinking of the difference for my son on 12" compared to 16".

The question on the forum I was reading was whether someone should get a 16 or a 20 for their small 5 year old. I agree that 16 would probably do him best as it is a first proper bike, but wasn't sure that the "less force through the knee on smaller wheels" is correct.

Can someone more knowledgeable agree or disagree and explain why?

 

[smilinsteve]

Bigger wheels wheels need a lower gear to have the same gear inches. Same gear inches will mean roughly the same force, discounting larger rolling resistance for the smaller wheel.

 

[ProjectMayhem]

Thanks, so you are saying less force is required on larger wheels?

 

[vmps]

The wheel size doesn't matter for this purpose, the gearing is what is important. You can have the same effective gear ratios with any size wheels, using different gears.

 

[smilinsteve]

The short answer is exactly what vmps said, it depends on the gearing, not the wheel size.

This example might help you understand it better.

Suppose a bike has the same gear teeth on the front chainring as on the rear cog (24x24 or whatever). That means that 1 turn of the crank causes 1 rotation of the wheel.

If the wheel is bigger, the bike travels farther for each rotation. This extra distance is not free. It takes more energy to turn the big wheel 1 rotation. (and more force on the pedal).

But to go the same distance, you only have to turn the bigger wheel 4/5 of the way around (In you example, the 16 inch wheel is 4/5 of the 20 inch wheel)

So if you change the gearing on the bigger wheel bike to 16 teeth in the front and 20 teeth in the back, then 1 rotation of the crank results in 4/5 of a turn of the wheel.

So when the 16 inch wheel has 1:1 gearing and the 20 inch wheel has 3:4 gearing, they will require the same energy to turn the crank and go the same distance for each crank rotation.

If you are talking about kids single speed bikes, I assume they take this into account with gearing, but rather than focusing on the physics of it, its probably best to have your son test ride, and seeing if it is easy enough to pedal.

With any kids single speed bike, the kid will go up a hill and require more force and he will give it all his strength, and fail to make it up the hill. That is nothing to worry about. Force on the knee is part of riding and part of walking. The exercise will do him far more good than harm.

 

[smilinsteve]

On the other hand, they might have been talking about the size of the bike causing stress on the knee, not the size of the wheel. If you put a small kid on a big bike with long crank arms, then the position of the knee over the pedal is compromised, which can cause additional stress.
So you want the bike to fit reasonably well, although I know parents, incluidng myself don't want to buy a bike the kid will outgrow in 1 year, so tend to go for a larger size to grow into.

 

[ProjectMayhem]

Thanks, the poster went on to explain that they were referring to the gearing on larger wheel bikes being more suited to larger children, ie if you have a bike to suit 6-10 year olds then put a 5 year old on it they would be pushing higher gears.