Tuesday, 29 May 2012

Little wheels = hard work?

***WARNING***  There now follows a technical article.  I apologise in advance for inadvertant yawning, dribbling, sleeping on computer keyboards or "grandmothers being taught to suck eggs".  Pour yourself a strong coffee, forgive me my  geeky nature and read on...

"I'll bet your bike is hard work with those little wheels!"

If I had a pound for every time I've heard that since commuting on the Dahon, well I'd have quite a few quid, that's for certain.  And the simple, truthful but stock answer is, "No it isn't at all.  It feels like a big bike, not a kids bike."

The less simple bit is explaining why, but usually my inner engineer just won't let me get away without trying.  I think sometimes the point sinks in prior to them glazing over...

It's all down to gearing you see.  There are a few measures for bike gearing but the one I'll refer to is called "gear-inches".  It is calculated by working out the ratio of your front to back sprocket and multiplying that by your wheel diameter in inches.  So for example, on my single-speed, fixed-gear mountain bike it works as follows:

Front sprocket = 42 teeth
Rear sprocket = 16 teeth
Wheel diameter = 26 inches

Therefore I am running (42/16) x 26 or 68.25 gear-inches.  In meerkat-speak, "simples".

Conversely my 700c-wheeled hybrid has a range of about 28 to 100 gear-inches through its 21 gear ratios

It's a bit of a weird measure as it doesn't exactly tell you the distance moved along the road per pedal revolution, which it would if wheel circumference were used instead.  It does however provide a simple and comparative measure between one bike or gearing ratio and another taking differing wheel sizes into account.

Still with me?  I'll get to my point.

The Dahon tech stuff in the catalogue states that my bike has a range of 48 to 72 gear-inches in it's three gears.  They are pretty evenly spaced and so the actual gearings are something like 48, 60 and 72 gear-inches.  That's roughly equivalent (I know because I have the spreadsheets to prove it) to the spread of gearing on my hybrid when on the middle chain-ring.  The key difference being that the hybrid has that range with 7 gears whereas the Dahon has only three.

That not withstanding, for one rotation of the pedals in an equivalent gear, my Dahon and my hybrid (and any other bike) will both have the same number of gear-inches and will therefore move the same distance along the road.  Therefore, to all intents and purposes, the gearing makes it the same as riding a large bike and not the hard work that people imagine.

Sure, the Dahon's wheels will have gone round more times, but the pedals won't which is where I'm putting in the effort.  Indeed the fact that the Dahon has smaller wheels mean less rotating mass and thus marginally quicker acceleration (it does also mean that they have less inertia when rolling too but let's not split hairs).

I do find that having a range of more closely spaced gears makes it a little easier to ride faster.  With the larger gaps between the Dahon's three ratios you tend to end up either spinning in one gear or heaving in another.  It takes a little effort to "get on top of" the higher gear, if that makes sense?  With more ratios it is easier to tune the gearing to the amount of effort you can comfortably put into the pedals.  It also means you spend more time faffing with gears too which is why single speed bikes are so blissfully simple to ride (definitely another post for another day!)

Interestingly (I'm sure you'll agree) is that the derallieur-geared 7-speed version of my bike has a much wider range of gears.  I think something like 34 to 92 gear-inches which would be really useful as I find the 3-speed's maximum of 72 a little low when going downhill.  It would be nice to have a higher gear or two to be able to power down hills rather than spinning or coasting.  It's a small thing really though and not worth losing any sleep over.

So there you go.  I have it off my chest.

Folding bikes are not harder to ride by dint of their small wheel size and anyone still awake knows exactly why.

No comments:

Post a Comment