Gearing dreams and reality (another one for epicyclists)

Sometimes, you do not fully appreciate why something is the way it is till you try to do it differently (as a writer and a lover of books, I hope that may be the lasting effect of e-readers such as the Kindle:  they will make people appreciate just what a clever piece of technology a book is – but that is by the way). My Dream Roadster, as its name suggests, is an attempt to realise an ideal form of the Imperial Roadster bicycle by retaining its desirable features while overcoming its shortcomings, principally in gears and brakes.

108_2464

In practice, I have found that the Dream Roadster has served mainly to deepen my appreciation of the production Imperial Roadster, in particular the two versions of it that I own, a pair of Royal Sunbeams. The matter of braking systems I will consider another day (the Dream Roadster has drum brakes, where the standard Imperial has rod-operated rim brakes) – but my excursion earlier this week ,  which involved the strenuous ascent of Necessity Brae on my 1934 Sunbeam,

necessitybrae

revived a question I had asked before, how many gears does a man need?.  The Sunbeam’s low gear is around 54”. I had managed to climb the hill in that, though it took a deal of effort and determination – at several points I had thought about stopping, but had willed myself on.  Evidently, then, a lower gear would make things easier, but how much lower should it be? That was the question I set out to answer the next day, when I repeated the same route using the Dream Roadster.

The Dream Roadster has a five-speed rear hub (SRAM/Sachs P5) coupled to a two-speed bottom bracket gear, a Schlumpf Mountain Drive. On the present set-up, this gives nine distinct gears (two are duplicates) – a normal range of (approximately) 47”, 58”, 75”, 96” and 118” with a lower range of 19” 23” 30” 38” and 47”.  My guess, from previous experience, was that the lowest useful gear would be the 30” one so I resolved to put that to the test. Before setting out I made a couple of trial ascents of hills near by, Glenlyon Rd

Glenlyon Rd Craigie

and Quarry Rd

108_2697

which confirmed my suspicions: the 19” is effectively useless – though it offers virtually no resistance, the speed at which it must be turned to make even the slightest headway requires a far greater effort than walking, for less return. The next gear up – 23” – is only marginally useful as it still requires to be spun at a higher rate than I find comfortable to achieve a forward progress less than walking pace.

So I set out to repeat the trip of the day before (map 4 here) with a minor variation at the start – a new secret way such as Craigie (the district of Perth where I live) abounds in

108_2699

My aim was to use 30’’ (the mid-gear of my lower five) as my lowest gear. The ascent of Necessity Brae was still strenuous,

necessitybrae_2

but less so than the day before, and at one point where the slope lessens I actually changed up to the 38” gear for a time. Though I already knew I could ascend the hill in a substantially higher gear, my aim was to find the most practical lowest gear for the Dream Roadster, and the answer to that appeared to be around 30”.

This was borne out by the fact that I continued without dismounting, save to record the occasional rarity –

red phone box

and some wild flowers here and there:

blue flowers

purple crocuses

(it’s nice, with the hedgerows so dominated by red, to find a touch blue and purple)

I even managed the sharp ascent to Craigend which had undone me the day before, though this time I was expecting it. I returned as I had previously, with occasional stops for blackberries and to record the onset of Autumn,

108_2709

(a sight that recalled a line from Eliot:  ‘the communication of the dead is tongued with fire beyond the language of the living’)

and so home via another secret way:

a narrow walled lane

The one casualty of the trip was my mileometer, which I must have shed somewhere along the way (I did have to stop at one point to adjust the front wheel, which had slipped, so I think it was some time after that – I retraced my route on foot next day, but to no avail).

lucas cycle odometer

before setting out…

bicycle wheel sans odometer

…after returning home

So, the conclusion to be drawn (besides the obvious one of checking the tightness of all fixtures before setting out) is that around 30″ is as low a gear as I need. However, once I fell to my calculations, I found a familiar problem. An earlier version of the Dream Roadster had coupled the Mountain Drive to a 7-speed SRAM/Sachs hub, giving 14 gears with a spectacularly wide range of 763%. This setup was arrived at before I ever built the bike and sprang from a couple of simple-minded comparisons – the fabled (and fabulously expensive) Rohloff Speedhub had 14 speeds but a range of ‘only’ 526%, so an arrangement that was cheaper but offered the same number of gears over a wider range had to be better, surely?

In practice, I soon encountered the difficulty I describe above – the lower end of the gear range was something like 17, 20 and 24 inches – of which only the last was even marginally useful. This led me to conclude that the SRAM/Sachs 7 speed hub offered an adequate range in itself, so I swapped it for the SRAM/Sachs 5 speed I had installed in my daughter’s bike, giving us both (I hoped) a more useful range of gears.

However, setting 30” as a bottom gear with the P5/Mountain Drive combination and a 28” wheel implies a direct drive  of some 119”, not only ridiculously high in itself and giving an unfeasibly huge 188″ top gear, but requiring a gear ratio (front to back) of 4.25:1, meaning that a 48T chainwheel would need an 11T sprocket (impossible to obtain for a hub gear – 13T is the smallest I have come across). With 13T at the back a 55T chainwheel would be required to maintain the same ratio.

The upshot is that I have ordered a 14T sprocket which, with the present 48T chainwheel, would give a gear ratio of 3.4:1, meaning a direct drive of c95” and a lowest gear of c24” – still barely useful. The full range would be (approximately)

lower: 24, 30, 38, 49, 60        upper: 60, 75, 95, 122, 150

I look forward to testing that – the long-striding twelve-and-half foot top gear should be fun* but I have to admit that I am already toying with the notion that for my purposes six gears might be enough – using the same 14/48 set up with a typical 3 speed would give me approximately

lower: 28, 38.5, 52        upper: 70, 96, 131.

That gives a bottom gear much closer to my tested useful minimum and makes an interesting comparison with the fabled six-speed Sunbeam of 1908, which offered 49, 66, 72, 88, 96, 129:

Sunbeam A6 crop

(photo by kind permission of Leon Wise)

Some might object to the increasingly wide gaps in my upper range but I think that is a matter of taste and cycling style. The justly-celebrated Rohloff offers 14 evenly-spaced gears (at 13.6% intervals) but for me that is an epicyclic hub brilliantly conceived to do the same job as a derailleur, only better – it’s all about maintaining cadence, keeping the same input and varying the gear to suit.

Riding a roadster bicycle is about varying input to suit the conditions: you are prepared to labour up a hill, or even dismount and walk, knowing that eventually you will reach the top and be able to freewheel down the other side; and on the flat, there is no more lordly feeling than to sweep along at great speed by turning a tall gear at a dignified, leisurely pace. Large steps between high gears are not a problem as you only use them when you are already travelling at speed; it is in the low gears that you want to avoid the jarring shock of too wide a gap.

Around 70 inches was the common single-speed gear for the Edwardians, who liked to calculate at 10 gear inches for each inch of crank, and regarded 7” cranks as the standard. I reckon that a future 6-speed Dream Roadster with the set-up above would give me in one bike the equivalent of two three-speeds, one well-suited to the hill country, the other formidable on the flat.

I have to say I’m sorely tempted…

* 13 yards on the road for a single turn of the pedals, or better than 26mph at a modest 60 rpm – though I expect wind-resistance would be a factor.

4 thoughts on “Gearing dreams and reality (another one for epicyclists)

  1. >> but I have to admit that I am already toying with the notion that for my purposes six gears might be enough <<

    Personally, I find anything greater than six gears to be a distraction. The most I ever need is six – and quite honestly, 95% of my riding is done in one, with occasional forays into a higher or lower gear. (Obviously, we each have our own comfort zones when it comes to spinning a crank.) Interesting experiment!

  2. My famous ‘Tiger’ machine was built for essentially one purpose – getting up hills, so as to come down them at maximum speed. For strength, I eschewed the 3/16″ chain and stayed with 1/4″ This limited me to four cogs at the hub, so I bought the largest (23T I think) and the smallest (13T, again from memory) and a couple of intermediate sizes.
    Later experiments indicated that the maximum I could achieve at the pedals was 180rpm, which in bottom gear was about 19mph on a 26″ wheel. Extrapolating from there suggested a maximum on the level, with a fully compensating tailwind, of about 45mph.
    That I did once see an indicated 40mph, coming down Broxden (approx 1 in 24) with a stiff breeze behind, suggests my calculations were well in the ballpark. Sad to say, 45mph tailwinds are few and far between, so I never had a chance to test it on the level.

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