Brompton Gears, How Comparable with a Full Size Bike?

[simonhill]

I'm about to buy a new Brompton. Other posts on here have helped me decide the spec, but I am wondering about the gears.

I plan on getting the 6 speed, but even going for the 12% reduced chainwheel, the lowest gear is still 29.1", which seems quite high to me, for what I see as an urban run-about. I am comparing it with my normal (use for everything) bike, a 26" LHT.

This got me wondering, can you do a direct gear inch comparison with such a small wheeled bike?

PS, I will be able to try out the different gearings, but like to know the theory before I do the the practical.

[horizon]

The small wheels as you know change the gearing but I cannot think of anything else that is different in terms of getting up hills on my folder (a Tern Node 16 speed 24" wheels). It is what it says on the tin.

[mercalia]

ah I have done that comparison as been toying with the idea my self

I came to the conclusion that the 6 speed with reduced 12% gearing was similar with gaps to my 26" Dawes 1-Down with 11-30 cassette using just the middle 36 front ring. So not bad for a general run about but no good for steep hills

There is a guy in Scotland who does a Rohloff conversion with 14 speed. if you have the money about £2500
in fact he can supply other hub geared versions as the page below describes Alfine 8/11 or Nuvinci at lower prices

http://www.kinetics-online.co.uk/folding-bikes/brompton/rohloff-brompton-order-form/

He can supply the complete converted bike or just the Rofloff with new rear traingle and controls

[JakobW]

Unless you live somewhere ludicrously hilly or carry large loads, a sub-30-inch gear should be fine for an everyday bike; consider that a traditional 3-speed roadster had a normal gear in the high 60 inches and a bottom gear of 50" or so; even if top gear is chosen to be a normal cruising gear the bottom is still about 40".

[Geoff.D]

A gearing comparison is independent of wheel size.

A "gear inch" is a way of indicating how far a bike will travel forward for a single rotation of the pedals. If it travels further, more effort will naturally be needed. This is said to have a higher gear. However, this historical British system avoids actually calculating the forward distance (avoiding the use of "pi" in the calculation). It says that the gear inch of your bike is the diameter of a notional penny farthing wheel that gives the same forward distance in one rotation of its cranks as you get in one rotation of your own bike's cranks. So, a gear of 60" says that a 60' diameter penny farthing does the same job.

The calculation is - multiply the wheel (outside) diameter in inches by the number of teeth on the chainring and divide the result by the teeth on the rear sprocket. I can explain why this is so, if necessary.

Two bikes with the same gear inch will feel the same in terms of effort. They may feel different in terms of handling, or tyre choice, or stiffness of wheel, or reaction to road surfaces, or frame geometry, etc........but in terms of the effort to accelerate, hill climb and ride into the wind they're the same.

[horizon]

Unless you live somewhere ludicrously hilly or carry large loads, a sub-30-inch gear should be fine for an everyday bike;

I have wondered if the manufacturers' expectations of their bikes are lower than the prospective owners' - the bikes really are intended as local runabouts and not to be put through what many members of this forum might expect - going up ludicrous hills and carrying large loads, generally at the same time.

[mercalia]

Unless you live somewhere ludicrously hilly or carry large loads, a sub-30-inch gear should be fine for an everyday bike; consider that a traditional 3-speed roadster had a normal gear in the high 60 inches and a bottom gear of 50" or so; even if top gear is chosen to be a normal cruising gear the bottom is still about 40".

hmm but you were 16 when you had one of them with all the excess energy and strength

[rfryer]

I ride a Brommy 6 spd with reduced gearing around Edinburgh.

On flats, I use 4th, with a bit of 5th if I'm feeling keen. 3rd is for headwinds, or easy hills. That leaves 2nd and 1st for climbing, and they're very, very rarely pressed into use on this terrain.

I think I'd find the gearing a bit high if I was regularly tackling hills of 10% or more. But for hills less steep than this, I think the Brompton gearing system is excellent.

[recumbentpanda]

I would like to point out that ‘gear inches’ is nothing to do with how far the bike rolls forward. It is a measurement originally devised to help cyclists get their heads around this new fangled idea of having a drive chain, and with it, the possibility of ‘gearing’ through having different sized sprocket wheels at either end of said chain.

What everybody was used to of course was ‘ordinary’ bicycles, where the pedal cranks were affixed directly to the axle, and the degree of mechanical advantage was of less importance than having a reasonably slow cadence at normal cruising speeds. This usually meant going for the largest possible wheel that would fit between your legs while still leaving enough room for a saddle! The crucial dimension of a bicycle therefore, was the diameter of the front wheel in inches.

When the peculiar contraption called a ‘safety bicycle’ came along, with its same size wheels and chain and sprocket gearing, what everybody wanted to know was ‘what’s the equivalent of that in old money?’ Eg: what would be the equivalent front wheel size on a penny farthing have to be to get the same mechanical advantage as a particular gear on a safety bicycle was giving you.

At first, people were presumably asking for this info because they had some idea what it felt like to ride different sizes of wheel on ‘ordinaries’. It stuck around long after the era of the ‘ordinary’ bicycle, because it was a handy way of reducing lots of confusing ratios to single numbers for easy comparison.

Curiously, misconceptions about gearing persist in a different form with the Brompton. People often used to ask me, ‘don’t those small wheels mean you have to pedal very fast?’ Well no, thanks to the mathematical magic of ratios, the cadence is the same as for a normal bike, and consequently, so is is the effect of the gears offered by the hub. Mine has a three speed old school Sturmey Archer, same ratios as many a 27” wheel ‘sit up and beg’ roadster.

[mercalia]

cllick to enlarge

[thirdcrank]

I've only ridden a Brompton once and that was for a only short distance when a satisfied user was keen that I should give it a go. I have a Birdy, which is ridden less now than was once the case.

If I've got your query right, I think you are asking more about the riding - especially climbing - characteristics of this type of bike than for an explanation of gear ratios.

IME, it's not really ideal if you have to struggle, especially standing on the pedals, when climbing with a small-wheeled bike. OK, it's never ideal if you have to struggle like that but it seems worse to me on small wheels. It may also be something to do with straight bars not giving an ideal position for steep hills. Since it tends to be only idiots who ride around looking for steep hills to climb, it's not a problem for most people. In the same vein, you are perhaps less likely to be riding a folder with a heavy load. (Cue for somebody to post a pic of somebody shifting a fridge up the north face of the Eiger on a Brompton. )

I think it's better to have a tad lower climbing gear for a folder than a big-wheeled bike. Mine came with 36" bottom gear (based on the assumption that 18" wheels are near enough to that diameter to work in the formula.) When I replaced the cassette, I changed from a 28T big sprocket to 30T, just because it was available at that time, rather than a necessity. That lowered the gear 3-4 inches I think, which doesn't sound much and it is around 10%.

I'd suggest that as you can have a test ride, try riding it up one of your typical hills.

I've concentrated on climbing because that's where it's most important but mine has always had a top gear of something like 92". A Birdy will scoot with the right rider.

[Brucey]

tell us what chainrings and sprockets you have on your LHT and a gear comparison chart can be constructed. This is a guesstimated one that compares the BWR 6s gearing with a derailleur setup, but the chainring and sprocket sizes I've used may not be correct.

http://www.gear-calculator.com/?GR=BBWR&KB=44&RZ=13,16&UF=1325&TF=90&SL=2.6&UN=MPH&GR2=DERS&KB2=24,36,46&RZ2=11,12,14,16,18,21,24,28,32&UF2=2050

In this comparison the Brompton gearing range is about the same as what you would get using the middle ring alone of the derailleur setup.

BTW you may wish to trade the rubber buffer in the rear suspension of the Brompton for a firm one; when climbing the rear suspension moves around and this can feel like it is not efficient.

cheers

[Mick F]

I bought a 20" Moulton TSR a couple of years ago, and the gearing was quite inadequate IMHO mainly due to the small wheels.
I fitted a wider ratio cassette and a bigger outer chainwheel, but still I wasn't satisfied, so fitted a Sturmey Archer 3sp to up the ratios and also to lower them. It works a treat, though I have a terrifically wide set of gears now.

I know it doesn't help the OP with the Brompton, but I am well aware of the problem with small-wheeled bikes. I get the impression that the only available gearing for them is really aimed at "normal" bikes and there's nothing out there that is small-wheel specific. Brompton try, but they are limited in that they need narrow dropouts for the folding system.

[softlips]

I rode Flanders the other week and was surprised to see two guys doing it on Bromptons. I’m looking to get one so asked what gears they had and both were std six speeds.

So while in London last week I tried a six speed from Brompton Junction. Rode it for quite a while and only really used 4-6th. I tried 1st and would say it would get you up most hills if reasonably fit.

[horizon]

So while in London last week I tried a six speed from Brompton Junction. Rode it for quite a while and only really used 4-6th. I tried 1st and would say it would get you up most hills if reasonably fit.

softlips: for your assertion to have any meaning we would need to know the gear inches of that lowest gear, the total weight the person was aiming to carry, the gradient of the hill and the ability/fitness of the rider.

A quick go on a bike on the flat unloaded will give the impression of being able to cycle up the side of a building but the reality is very different.

(BTW although lower gears may be desirable it still doesn't mean the rider can use them.)