Friday, 21 October 2011

Riding the legend - the day I hired a Sinclair C5

The Sinclair C5 - photo from Sinclair Research

Click here for Sinclair C5 specifications

A while back - I guess it must have been about 15 years ago, I went on a holiday to the seaside.
I think it might have been to Weymouth.
Anyway, there was a chap there who was hiring out Sinclair C5s, the legendary, and odd, product of Sir Clive's fevered imagination.
So I hired one for a day...

The first thing that I noticed was the steering position - the handlebars were underneath my legs! (rather than above them, on a "normal" bike).
then I noticed the "feet forward" pedalling position, as I sat flat in the C5.
Having got the hang of that, off I went.

I fitted pretty reasonably in the C5.
I am about 6'3" (circa 1m90) tall, and back then I weighed about 80 or 85 kg, so I was perhaps best described as being of "light to medium build".
I could travel by pedalling or using the electric motor, or both.
To get a decent range from the battery, it was recommended at the time that one started off pedalling, then used the motor to maintain one's speed.

Problem is, the C5 has only one gear, so I had to lean a bit on the pedals to get going, and my little legs are going round like billy-o by the time I had reached a decent speed!
  • The C5 (according to the original marketing brochure) had a gear ratio of 42:20, driving 406mm wheels with 2 inch rear tyres - this implies that an overall gearing for the pedals of about 44 "gear inches".
  • By comparison, my three-speed folding bike has gears set at about 38, 51 and 68 "gear inches".
So the (only) gear on a C5 is just above the lowest gear on my bike.

But why was it hard to pedal from a standstill if the gears were that low?

  • The poor old C5 was cursed with short cranks for the pedals - in the region of 130mm length, rather than the typical 170mm cranks fitted to most adult-sized bikes.
  • Longer cranks act like longer levers - your feet move round in a bigger circle, but less effort is required to push the pedals (a similar effect is observed using long handled wrenches/spanners, compared to short handled ones)
  • So that 44 "gear inches" feels like about 57 "gear inches" on a "normal" bike, but only gives the speed of 44 "gear inches".
So you get the speed of my first gear, but with more work than my second gear!
Presumably, fitting "short" cranks allowed the front bodywork to be more compact, and the user's feet to be closer to the ground.

Looking at the numbers just reinforces the conclusion I had already come to from actually driving (or should that be "riding"!) the C5 - it badly needed a two- or three-speed gearing system for the pedals!

Also, no doubt exacerbated by the hammering that hire bikes (in general) can get, sometimes when I reached maximum speed with the pedals and engaged the motor, the chain would come off the rear cog.

  • The problem is often encountered with a very long chain run, such as that on the C5. 
  • The pedals (and chainwheel) were quite a way in front of me, while the rear cog was behind me, giving a chain length of 181 links (according, again, to the original marketing brochure) - compare that to the chain on a typical bike which is 92 to 116 links long, and you can see why problems emerge that are not found on "regular" bicycles.
Two solutions come to mind:
  1. use a two-part chain run, with a relay cog or gearbox between them (as used in several tandems and tricycles) - the first chain would go from the pedals to the pedal gearbox (Sturmey Archer sell a suitable three-speed one, with both "input" and "output" sprockets), and the second chain goes from the pedal gearbox to the rear freewheel.
  2. more simply, to use a decent chain tensioner (apparently the C5 HAD a chain tensioner, but it was clearly not very effective on the machine I used!)
But, other than that, an entertaining little ride.
It just needed some development, which the project never got - a few simple improvements such as 
  • a Sturmey-Archer 3-speed hub (or similar) and a decent chain tensioner
  • a three-speed tricycle-style gearbox mounted partway down that long chain run
would have made the Sinclair C5 MUCH better!

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