Last week, as the STEMonsters were busy making their own cars with a choice of their chassis and wheel-axle system, a 5th grade STEMonster asked – “Why do some vehicles have tyres of different sizes at the front and the rear?” Haven’t we all seen such vehicles at some point in time but how many of us have ever thought about the why?! This is what makes our STEMonsters special – their power of observation. Now, before you proceed to read any further, think about a vehicle type that has unequal sized-wheels at the front and rear. … Done? Great!
The most common example of such a vehicle is a tractor! A tractor is meant to be used in muddy fields and to carry a lot of load/ weight. The answer to the question about its wheel design lies in understanding the design requirements of a vehicle that can fulfil these actions: high pulling/pushing power of the vehicle, high stability that prevents the vehicle from tipping over, and finally, does not get stuck in muddy areas easily. Most tractor engines are very heavy and are located at the front of the vehicle. Bigger tyres at the rear counter-balance this weight and prevent the vehicle from tipping over. In addition, the rear wheels are the ‘drive wheels’ that are connected to the engine. The front wheels are only for the direction. So, having bigger wheels at the rear prevents the truck from getting stuck or sinking into the mud. Finally, having a higher axle for the drive wheels allows the vehicle to pull more load/ weight with less effort. It all depends on the distribution of mass and Moment of Force, explanation of which is unfortunately, beyond the scope of this article. However, for those of you who are now intrigued enough to understand this deeper, here is an excellent link to understand the concept of Moment of Force and its role in stability of a system.
Of course, there are also instances of unequal wheel size being designed without any foresight and purely for aesthetics. Think about Penny-farthing bicycles (or high wheelers) that became very “fashionable” in the late 1800s without actually serving much purpose!
Image courtesy of the high wheeler – Background vector created by brgfx – www.freepik.com
