Roll Baby Roll

Meera spent the better part of last week, researching joggers and umbrella strollers. One consistent theme I noticed was that joggers/strollers with bigger wheels got better reviews. People commented that bigger wheels absorbed the bumps better and I was left pondering the point. With a pen and paper and some diagrams, it became obvious why. So I thought I will share my thoughts  in this blog.

             Fig 1                                  Fig 3

          

Fig 1, shows a small wheel going over a bump in the pavement. Here the bump is approx. equal to the diameter of the wheel and so the wheel dips by an amount equal to its radius. Baby feels the bump obviously. Fig 2, shows the same gap in the pavement. But this time a wheel with a much bigger radius rolls over it. Here obviously only a small portion of the arc of the wheel dips. Its clear that bigger the wheel, lesser the dip. Ideally a wheel so big that its curvature is almost flat would have no dip at all.... but that would be a really really big wheel.

The other scenario is when going over the edge of curbs. If the height of the curb is approximately equal to the radius of the wheel, like in Fig 3.  Then it becomes clear that a horizontal force will not be enough to make the wheel roll over the curb. You need a bit of vertical force too. But the bigger the radius of the wheel w.r.t the height of the curb, the easier it is to apply a horizontal force and have the wheel roll over it.

Its now pretty obvious why the joggers have huge wheels- the ones we finally got has 20" rims (diameter) compared to 3" rims on the regular stroller. But everything cannot be peachy, there has to be disadvantages to big rims. One comes pretty fast- the bigger the radius then lesser the rate of curvature per unit length. And lesser the rate of curvature per unit length then lesser the strength of the wheel. In other words if you go fast over a huge bump, the wheel could bend. Of course this is also a function of the inherent strength of the material used to make the wheel.  But all things being equal- a smaller wheel holds its shape better than a larger wheel. The other negative I could think of- bigger the wheel, more the metal needed to make the wheel. Which increases the weight. So the need to go in for alloys to reduce the weight while retaining the same strength, which of course increases the cost.