Brompton 10-speed! Sturmey 5-speed hub, Brompton 2-speed shifter and adapted Highpath sprockets
Brompton Gear Range ‘Could the Brompton be ‘easily’ redesigned to take wider,more conventional hubs? The rear drop-out spacing is under 115mm,whereas ‘normal’ hubs are 135mm (with 145mm normal in the USA).As the Brompton folds to the right, the extra width might only be 10mm or so. Could the hinge be widened by 5mm? Are there other design elements that would have to change?’ Simon Avakian, Palo Alto,California USA
‘I have a new 2×6-speed S-type Brompton with a Schlumpf Speed Drive which I am very satisfied with. But the gear range is only just acceptable – my wish would be gears from 2m to 10m (25″ to 125″).I can’t help dreaming of a Brompton with a Rohloff Speed Hub without widening the rear frame. I have seen a German MTB-site where the Rohloff hub gear is mounted in the frame instead of the rear wheel.’
Jon Dreyer Rensmoen,Norway
Several engineers have stretched the Brompton rear frame to accept wider hubs, such as the Nexus 8speed and even the 12-speed Rohloff.There’s no need to alter the hinge,and as Simon suggests,the bike emerges little wider than normal.It might even be possible to fit the hub elsewhere,but these are heavy and expensive conversions,and for most purposes it’s debatable whether either would be worthwhile.Surely we can come up with a lighter, cheaper and more elegant solution?
Let’s look at the proprietory equipment that can easily be fitted to the Brompton and try mixing and matching these bits and pieces to produce a widerange system.The widest range hub that can be fitted without serious engineering is the 5-speed SturmeyArcher,which is now back in production, although not fitted as standard by Brompton,as it once was.This hub has a limited 225% range,but it’s light and cheap,and we can extend the range by fitting dual chainrings (which don’t work well on the
..Is this the way Brompton will go? A10-speed would certainly satisfy the critics…
Brompton) or dual sprockets,which do.Fitting Brompton’s own 2-speed derailleur to a Sturmey 5-speed widens the range to 260%,which is useful,but hardly ground-breaking. For the really big gears we need something like the Highpath 12/18 tooth conversion.This is designed for use on the roomier SRAM 3-speed hub,but with a bit of machining it is possible to squeeze one onto a Sturmey 5-speed.
This combination gives a gear range of 337%;more than any hub gear except the expensive Rohloff,and without the weight and complication of fitting a crank-mounted Speed Drive.The idiosyncrasies of gearing mean that the bike is really a 7-speed,because three ratios are almost identical: it’s best to think of the conversion as a 5-speed with two extra gears at the bottom (or the top).Using the Brompton 44-tooth chainring,the gears emerge as: 41″,49″, 62″,79″, 93″ in the high range,and 27″,32″,41″,52″ and 62″ in the low.A smaller 40-tooth chainring would give gears of 25″ to 85″.We haven’t produced Jon’s 500% range,but we have done quite well for a weight penalty of only 200g or so.
To my surprise,the prototype system works more or less without grumbles or adjustment.In practice,one tends to leave the bike in the low range or high range for quite long periods, using the 2-speed changer only when a more extreme gear is needed.
The bad news is that a 10-speed would be quite difficult to make at present. Highpath has wisely opted not to produce a thinner and weaker sprocket,and it’s unlikely Sturmey will offer to produce a wider ‘driver’ on its 5-speed hub!
Is this the way Brompton will ultimately go? A 10-speed would certainly provide enough range to satisfy the critics,and the company could introduce one very quickly.
A to B 55 – Aug 2006
