A little more gear inchery

***GEEK ALERT***GEEK ALERT***GEEK ALERT***

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!

By jove, I've finally cracked it! (part 1)

I've generally been pretty happy with my Dahon bike since buying it.  However, there have been a couple of things that have niggled at me persistently throughout that time.  First among these was the need to carry my worldly goods in a rucksack with the attendant achy shoulders (on heavy days) and sweaty back that it caused.
I have always looked with envy at the front mounted luggage available to Brompton owners and wished that something similar existed for my Dahon.  There used to be a Dahon bracket listed in their website to mount bags and baskets on the front of the frame but it seems to have been discontinued (think it might still be available in the US).  I did consider modifying a Brompton one to fit the mounting holes on my bike but at over £100, the bags are a bit pricey had it not worked!  Pricey but with a super reputation for good quality, I might add.

Panners would be good too - I know plenty of cyclists who swear by them - but the standard Dahon rack sits too close to the floor and if anything sticks out beside it (as panniers would) you tend to hit your heels on every pedal stroke.  Not great.

Eventually however, and very randomly, I found an online link the Rixen and Kaul "Klickfix" range of products and was more than pleasantly surprised!  They do such a huge range of bike luggage and mounting options that it would be hard not to find a solution to most problems.  The website and downloadable catalogue is here.  They do a front mount that fits the Dahon and also a seatpost mount - all with a common interface to a range of bags and baskets.  The rucksacks that they make look great but I was concerned about the weight limit of 5kg quoted in the catalogue.  On a "heavy" day recently, with my own rucksack full of laptop, charger, change of clothes and lunch, I weighed it.  Unfortunately it came in at over 7.5kg and some of the roads I ride are a little uneven to say the least.  I had visions of snapping the bag from it's mount in the middle of a ride and, given the price of the kit, wasn't willing to fork out to try it!  Am I coming across as a cheapskate?

However, I did really like the idea of mounting luggage on the seatpost, out of the way behind me. 

Sooo, a short Google later, I found a number of seatpost-mounting luggage racks compatible with lots of standard panniers.  Even the good quality ones (with plenty of positive reviews on Amazon) were relatively cheal and so fewer than forty pounds later, I became the owner of a luggage rack and panniers.

I've had them on for a couple of weeks now and have to say that they are excellent.  It is so nice to ride unencumbered by a rucksack and although I still work up a sweat, it is nothing like before.  When heavily laden it does sway a lttle and make the bike feel odd - usually when standing up out of the saddle to pedal up hills.  It's nothing too bad though and I think would be improved if I fix a way of securing the bottoms of the panniers.  They have straps but I want something that will release more quickly to get on and off trains.

Both of the items I bought were "Avenir" products which I think are made by Raleigh:

Simple, strong and lightweight.  You can get them with removable sides
but those were wither expensive or had poor reviews.  This one is excellent.

Simple but good quality and most importantly my laptop (just) fits in.
A netbook or tablet would be no problem whatsoever.

And here's a (poor quality) picture of it all installed.  Not sure why the back tyre looks flat in the picture
as it wasn't!  Just the angle or a trick of the light I reckon.

The only slight issue I've had is that the Dahon has such a fat seatpost!  Most of the seatpost mounted luggage state that 31-32mm is the maximum diameter that they will fit.  The Dahon's is about 33.5mm.  However, although this means that the quick release link doesn't reach, by replacing it with a longer bolt and packing out any gaps (between seatpost and clamp) with rubber pads It is fitted safely and securely.  This set up has endured a couple of heavy days without any problems at all.

So the upshot of it all is that I am now a much happier bunny with a less sweaty back and less achy shoulders. 

And the second thorn in my side?  Well that'll have to wait until I have a bit of time to write it up!

A massive milestone (just not for me)

Last weekend, my youngest child learned to ride without the help of stabilisers (training wheels in the US)!  I am so pleased as it is a really important milestone in my view marking the start of a lifetime of cycling, one way or another.

Now, current wisdom dictates that kids learn to ride much faster if they can ride a balance bike first (a bicycle without pedals that you sit on and push along with your feet) rather than learning to pedal and then having the grief of balancing (or not) when the stabilisers are removed!  We bought a balance bike from friends a couple of years back but could never get the youngest to ride it.  She'd tootled around on her bike for several months with stabilisers attached and resited all attempts to ride with them removed.

The other week, a friend's child came to play who had only ever riden a balance bike.  Both my wife and I watched amazed as he picked up a "proper" bike and pedaled away with no problems and very little wobbling!  He literally had it down on his first go! 

Quick as a wink, I removed the crank from my daughter's bike and told her that if she wanted to ride it, it was as a balance bike or nothing.  Tough love!  Within a couple of days she was scooting along on it, clearly able to balance in motion with her feet off the floor.  We took the plucge and refitted the crank.

Once again, I was stunned.  With a minimal effort, she got on the bike, set it rolling and then pedaled away!  As easy as that and she hasn't looked back since.

So my message to you is, if you have a youngster learning to ride a bike, don't go anywhere near a set of stabilisers!  Get a balance bike (or take the crank out of a regular one) and leave them to it.

This week I shall mostly be a proud Dad! :-)

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.