Wednesday, 17 October 2012

A little more gear inchery


My technical geekery knows no bounds – truly it doesn’t. 

I posted a while ago about my bikes gearing and the science of gear inch calculations.  Well it seems that I had some duff information and my calculations were wrong.

On a three speed hub gear, one gear is a direct drive and the other two are ratios of that one.  I had always assumed that, since first gear on my bike was the quiet, non-clickety one, that it was by logical deduction, the direct drive.  However, the maths behind this never stacked up.  With a 38t front sprocket and 16t at the back, a direct drive on 20 inch wheels would give 47.5 gear inches ((38/16) x 20 = 47.5).  However, I also know that the Sram iMotion 3 speed hub-gear has a total spread of 186% from lowest to highest gear.  Therefore my theoretical top gear, if the original assumption was correct should have been 88.35 gear inches (47.5 x 1.86 = 88.35).  However this is a pretty high ratio and the Dahon always felt considerably lower geared than that!

So eventually, curiosity got the better of me.  I turned the bike upside down and in each gear rotated the crank one revolution.  By counting the number of turns on the rear wheel, I got the actual drive ratios in all three gears.  With its original gearing (38/16) I found that it was actually the Dahon’s middle gear that was the direct drive, giving as it did 38/16 or 2.375 turns of the back wheel for one turn of the crank (or thereabouts).  The hub gear’s spec sheet also stated that the high and low ratios were 36% up and down from the middle one (which ought to have been a clue as to which was the direct drive, but there you go.)  And so, that yielded the following results for the Dahon’s gearing:

Gear 1 – 34.9
Gear 2 – 47.5
Gear 3 – 64.6

All of which felt much closer to the truth in practice.  I have said before that I found these gears far too low in actual use – first was rarely ever employed, second some of the time and third most of the time.  It seems that the gearings mentioned for this bike on the Dahon webste (42 – 78 gear inches IIRC) are actually for the seven speed version and not the three speed.  Anyway, I started to look at options to raise the gearing. 
The simplest and cheapest option by far was to install a smaller sprocket on the rear wheel and so I found one (14t) on eBay and, for the princely sum of about three quid, ordered it.  It arrived pretty quickly and installation was a simple matter of removing the rear wheel, taking off the spring clip which holds the sprocket and then the sprocket itself before reinstalling the new item.  Owing to the 14t sprocket’s smaller diameter it has meant that things are now very “snug” around the rear gearing.  The chain runs pretty close to the bearing cover next to the sprocket but this does not appear to be causing any damage or other problem.  The ratios since installing this modification are as follows:

Gear 1 – 39.9
Gear 2 – 54.3
Gear 3 – 73.8

Which is better but still not ideal.  From my experiments into single speed riding, I have found a gear in the mid to high 60’s to be the optimum for me.  Low 60’s is acceptable but sometimes too low for the down hills, however with the higher gear option afforded by an internal hub-gear, that would not be a problem.  Having a lower but usable first gear would also be a bonus.  So having worked things through ad nauseum, I have a couple of options.  A 48t front sprocket and 16t rear would be the best compromise all round and would require the purchase and installation of a 48t crank.  This combination would give:

Gear 1 – 44.0
Gear 2 – 60.0
Gear 3 – 81.6

However, I do have a 52t crank in the garage from my ongoing fixie project and if installed on the Dahon would give gearing of:

Gear 1 – 47.8
Gear 2 – 65.0
Gear 3 – 88.4

I do think that this would be a good combination and, as I have the necessary hardware, may give it a go first.  

I read a comment about touring on a Brompton which said that six gears are enough as long as you have the right six.  I’d go as far as to say three is enough for most people but again, it needs to be the right three.  The key to it all, as I am finding, is to have a middle gear that you can use for most of your cycling with the higher and lower gears as added options.  The middle ratio above might even be so close to my ideal that to all intents and purposes I end up riding it as a single speed.  The lower gear 1 for “knackered thighs, end of the week riding” will be really useful though as will the higher one for bombing down hills!  What I’ll actually have, on reflection will be the original gears two and three as gears one and two plus an even higher option.

It could all be tinkering over nothing but I’m an engineer and a geek so I like messing around with machinery.  There are few things that I own which I have not considered modifying in some way.  Why should the Dahon be any different?

Besides, it is great fun and through all of that spannering, you do learn an awful lot about your machine!

1 comment:

  1. There are many things to consider when choosing a bike like for example the type of riding you do and the distances you will cover whenever your using it. Other things to add into the mix for you to think about are gears, wheels, bars, pedals, shows, saddles, tyres.