The size of wheels and castors

In the early 1960s, Dr Alex Moulton launched the bicycles that are known by his name.  They were a revolutionary design - for well over half a century, bicycles had (almost) invariably a diamond frame and wheels about 26inches in diameter, and no suspension.  Moulton bicycles had suspension, small wheels (17inches in diameter) and an open frame.  The smaller wheels meant that the tyre pressure had to be higher than was then normal, and the suspension was essential.  It was about 25-30 years before suspension became common on "traditional" sized bicycles.  Modern small-wheeled bicycles have slightly larger wheels (20inches is common).

There continues to be debate about the comfort and ease of riding bicycles with different sized wheels.  There is not much choice, because they are standardized.

Not so castors and wheels on trolleys used for moving goods around shops.  The first vendor that I found in the UK offers castors with diameters: 50mm, 75mm, 80mm, 100mm, 125mm, 150mm, 160mm, 200mm, 250mm, 260mm, 330mm, 370mm and 400mm (and I may have missed some out!).  If you want, you can have castors made to measure.

An assortment of castors
Bicycle wheels need to match the other components of their vehicle.  However, castors can be chosen for the purpose.  They don't need to match mudguards and frames.

The cleaner in the shopping centre this morning was struggling with his trolley, when I stopped to chat.  The shopping centre is paved with small slightly rough flagstones, about 300mm square.  The trolley was not designed for use on such a surface.  It had 100mm castors, and they didn't cope.  Actually, I think they were a little too small for use on a hard surface indoors, because of the load being carried.  Some designer had economised - someone who didn't have to move the trolley around that shopping centre - and there was no feedback to suggest a change in the design.   For the sake of a few pounds, the cleaner's work was being made harder.  Now, if the designer had applied a little operational research "What if?" analysis, things might have been much better.

Later in the day, I found a trolley of similar dimensions, with larger wheels, which was much easier to handle.  It was in a department store, and moved bags of clothing across the smooth floor with ease.  Well done, that designer!


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