# What is an advantage/disadvantage of difference wheels size in front and rear of a bicycle?

Suppose there is a vehicle with difference size of wheels in front and in rear. Specifically bicycle, There would be 2 cases that front wheel larger than rear wheel and vice versa

I would like to know what is advantage and disadvantage between each setting. From normal setting to larger front and to smaller front. While rear wheel would be a driven wheel and front wheel would be a control. If control wheel smaller or larger which would become harder? Or which setting would be less efficient?

• What have you found so far - this is not a free-homework completion site... Commented Jan 13, 2018 at 8:46
• @SolarMike This is not homework just curiosity and so far I have no clues even how to use word to describe it. Searching for word asymmetric wheel given only asymmetric rim Commented Jan 13, 2018 at 8:55
• Many vehicles have different size wheels. For example a tractor. If the front wheel has a different duty then a different size may be better. I don't get two cases of front larger. Commented Jan 13, 2018 at 14:15
• @Paparazzi Thank you but I would like to know engineering terms and equation that would be related. At least for stability and efficiency Commented Jan 13, 2018 at 14:45
• One obvious item with small front and large rear is a wheelchair... Commented Jan 13, 2018 at 16:13

## 2 Answers

In the case of a bicycle a larger diameter wheel rolls up and over stuff like a root easier. Since the rear is power you can power over the root. You see some 29' front and 26' rear but the inconvenience of two size of tube and tire is a major drawback.

There are several that come to mind.

Drag is due to rolling resistance at the contact patch, bearing resistance, and wind drag. These all operate with different scaling laws, both with respect to speed, and size.

Inertia is very important to the feel of a bike. Getting the caster and inertia right for the bike has a huge effect on steering. Wheel size affects weight, and therefore inertia.

Wheel size affects the gearing needed. This is the big one for conventional chain drive bikes. If you want reasonable speeds, you need a decent sized rear wheel because of the constraints on single stage chain drive gear ratios.

Propulsion and braking. These often aren't equally distributed, and you need to provide acceptable tire wear. You can do it with width or diameter. But in the case of a farm tractor, width is constrained by the space between rows.

As a kid, I owned the only bike in town with a 24" rear and 20" front tire (thanks dad), and I got made fun of over it. I was not aware of any offsetting benefit at the time. Although the practice I got from being chased probably helped me to become a competitive racer later on.