More out of control Brompton modifications
I call this a "BirdyBrompton" and this is an experiment. It looks even stranger than a bona fide Birdy, but rather incredibly it works, more or less.
The Brompton is the perfect bike for London, which has smooth roads and few hills, but on broken surfaces such as canal towpaths, cobbles and Edinburgh's usual roads, it's a dreadfully hard ride on the hands, even with Ergon grips. Memorably, ten or fifteen miles along the Thames path to Kew Gardens was almost more than I could stand, which was why I returned to Kings Cross via the roads.
Why shouldn't a Brompton be re-engineered to have a ride quality a bit more like a Moulton? Why not indeed. Headtube suspension a la Pashley and older Moulton AM-series is impossible owing to the short steerer tube and quill stem. An AMP fork (parallelogram action plus either headtube or external springing) exists only for ISO406 wheels, but could be shortened by TIG welding. A Noleen/Cross Link/Girvin Vector fork with upper and lower steerer tube location, which is otherwise similar to a Moulton Flexitor fork, would prevent full rotation of the stem as required when folding. The Burrows suspended monoblade as used on the ISO349 front wheeled Ratcatcher 9 recumbent is no longer in production, but would otherwise be a near-perfect solution, and it also provided for a disc brake. Other suspension formats are available, such as mid-fork blade rubber torsion units as used on Radnabel recumbents, or mid-fork blade leaf springs as used on the early Dahon Jetstream, or trailing links as used on the Pacific Reach. Or as Steve Parry did, incorporate a modified suspension seatpost into the stem, on the notion that because the rider is much heavier than the bike, it's the rider who needs the suspension. I've done my time with a Girvin Flexstem and I disagree.
With no facility for metal fabrication, my only remaining option was the Riese & Müller Birdy folding bike, whose suspension fork at least is designed for an ISO355 wheel which is as near as dammit the same size as a Brompton. The Birdy fork does not use a threaded steerer tube; in fact it's an upside down Aheadset arrangement with the fork clamping to the bottom just as a stem would clamp to the top. In order to maintain compatibility with a Brompton, you need a special threaded steerer tube with an extra bit on the bottom (and Ben at Kinetics included the post for the stem clip, too). Look closely and you'll see a non-standard front wheel in there too. The Brompton hub has a between-dropouts width of 74mm, and an 8mm axle. The Birdy fork uses a 100mm hub but uniquely with the flange spacing of a 74mm hub, and a conventional 9 mm axle. A longer 8mm axle couldn't be found for my Brompton hub. My solution was to take a standard width Halo 24-hole small flange hub and lace it radially to the basic cheap 28-hole Alesa/Brompton rim. 24 into 28 does go if you think about it, with small variances in spoke length. For equalising the circumferential stress I opted for 20 spokes in four groups of five rather than 24 spokes in two groups of 12. Once tensioned the lacing is almost radial but not quite. Radial spokes were necessary because the small but wide hub flanges required inbound spokes only; any outbound spokes would interfere with the fork tubing.
A Birdy fork is longer—much longer—than a Brompton fork. This raises the front of the frame and slackens the head angle, which has the effect of increasing trail from 18mm to about 53mm. This results in a heavy, tippy feel to the steering as the height of the centre of gravity lurches noticeably as the wheel is turned away from the straight ahead position. You would become used to the handling within an hour or so, but it's not pin sharp like the Brompton. It can be ameliorated to an extent by raising the rear of the bike, most simplistically by lengthening the suspension block and compensating for the remaining slacker seat tube angle by moving the saddle forwards and tilting it down. This sharpens the steering somewhat. The main alteration that cannot be compensated for is that the frame of the bike now sits about two inches higher, which means it becomes practically impossible to touch the ground when on the saddle.
The other effect of a longer fork is that the front wheel axle is no longer coincident with the chainstay when the bike is folded. This can be overcome with a new stay for the hook. With that in place, the folded package sits slightly taller than before, and slightly wider, but not greatly so.
The Brompton is such a well thought out package that is all this crazy talk about suspension forks worth it? Yes it is, and no it isn't.
The Birdy fork is a linkage with nylon bushings and only one (well, a pair) point of location, and so it flexes slightly left and right under power. With weight hung on the front of the bike—because it's a Brompton, and that's what you do—it flexes more. But on poor surfaces the fork performs remarkably well, especially high frequency defects such as cobbles. It screws up the compact dimensions of the folded package, and makes it heavier to carry. But it looks cool, and it's probably unique.
Edit (16 Oct 2016):
Looks like this modification really hit a nerve with some viewers! There is some crazy commentary from Spain ("Se me han erizado los vellos al ver esa horquilla en una B!). Hello to the Brompton blogger in Singapore; hello to the Brompton rider in Seoul who called it "awesome and ridiculous", and who has experimented with elastomer handlebar grips; hello to Kiyo, the Brompton blogger in Japan who said it was "interesting, but it's not beautiful"; and hello too to the French Brompton fans who picked up on it. You were all correct, too: it was an ugly looking thing.
As you can see in more recent photographs, I put the original fork back on my bike. Luggage on the front mounting block and a Birdy fork (or at least, this old and flexible Birdy fork) was a recipe for shimmy if I rode one-handed, such as when signalling. At low speeds it was merely annoying, but at anything above 10mph it became positively dangerous.
Original DSC_0882
More out of control Brompton modifications
I call this a "BirdyBrompton" and this is an experiment. It looks even stranger than a bona fide Birdy, but rather incredibly it works, more or less.
The Brompton is the perfect bike for London, which has smooth roads and few hills, but on broken surfaces such as canal towpaths, cobbles and Edinburgh's usual roads, it's a dreadfully hard ride on the hands, even with Ergon grips. Memorably, ten or fifteen miles along the Thames path to Kew Gardens was almost more than I could stand, which was why I returned to Kings Cross via the roads.
Why shouldn't a Brompton be re-engineered to have a ride quality a bit more like a Moulton? Why not indeed. Headtube suspension a la Pashley and older Moulton AM-series is impossible owing to the short steerer tube and quill stem. An AMP fork (parallelogram action plus either headtube or external springing) exists only for ISO406 wheels, but could be shortened by TIG welding. A Noleen/Cross Link/Girvin Vector fork with upper and lower steerer tube location, which is otherwise similar to a Moulton Flexitor fork, would prevent full rotation of the stem as required when folding. The Burrows suspended monoblade as used on the ISO349 front wheeled Ratcatcher 9 recumbent is no longer in production, but would otherwise be a near-perfect solution, and it also provided for a disc brake. Other suspension formats are available, such as mid-fork blade rubber torsion units as used on Radnabel recumbents, or mid-fork blade leaf springs as used on the early Dahon Jetstream, or trailing links as used on the Pacific Reach. Or as Steve Parry did, incorporate a modified suspension seatpost into the stem, on the notion that because the rider is much heavier than the bike, it's the rider who needs the suspension. I've done my time with a Girvin Flexstem and I disagree.
With no facility for metal fabrication, my only remaining option was the Riese & Müller Birdy folding bike, whose suspension fork at least is designed for an ISO355 wheel which is as near as dammit the same size as a Brompton. The Birdy fork does not use a threaded steerer tube; in fact it's an upside down Aheadset arrangement with the fork clamping to the bottom just as a stem would clamp to the top. In order to maintain compatibility with a Brompton, you need a special threaded steerer tube with an extra bit on the bottom (and Ben at Kinetics included the post for the stem clip, too). Look closely and you'll see a non-standard front wheel in there too. The Brompton hub has a between-dropouts width of 74mm, and an 8mm axle. The Birdy fork uses a 100mm hub but uniquely with the flange spacing of a 74mm hub, and a conventional 9 mm axle. A longer 8mm axle couldn't be found for my Brompton hub. My solution was to take a standard width Halo 24-hole small flange hub and lace it radially to the basic cheap 28-hole Alesa/Brompton rim. 24 into 28 does go if you think about it, with small variances in spoke length. For equalising the circumferential stress I opted for 20 spokes in four groups of five rather than 24 spokes in two groups of 12. Once tensioned the lacing is almost radial but not quite. Radial spokes were necessary because the small but wide hub flanges required inbound spokes only; any outbound spokes would interfere with the fork tubing.
A Birdy fork is longer—much longer—than a Brompton fork. This raises the front of the frame and slackens the head angle, which has the effect of increasing trail from 18mm to about 53mm. This results in a heavy, tippy feel to the steering as the height of the centre of gravity lurches noticeably as the wheel is turned away from the straight ahead position. You would become used to the handling within an hour or so, but it's not pin sharp like the Brompton. It can be ameliorated to an extent by raising the rear of the bike, most simplistically by lengthening the suspension block and compensating for the remaining slacker seat tube angle by moving the saddle forwards and tilting it down. This sharpens the steering somewhat. The main alteration that cannot be compensated for is that the frame of the bike now sits about two inches higher, which means it becomes practically impossible to touch the ground when on the saddle.
The other effect of a longer fork is that the front wheel axle is no longer coincident with the chainstay when the bike is folded. This can be overcome with a new stay for the hook. With that in place, the folded package sits slightly taller than before, and slightly wider, but not greatly so.
The Brompton is such a well thought out package that is all this crazy talk about suspension forks worth it? Yes it is, and no it isn't.
The Birdy fork is a linkage with nylon bushings and only one (well, a pair) point of location, and so it flexes slightly left and right under power. With weight hung on the front of the bike—because it's a Brompton, and that's what you do—it flexes more. But on poor surfaces the fork performs remarkably well, especially high frequency defects such as cobbles. It screws up the compact dimensions of the folded package, and makes it heavier to carry. But it looks cool, and it's probably unique.
Edit (16 Oct 2016):
Looks like this modification really hit a nerve with some viewers! There is some crazy commentary from Spain ("Se me han erizado los vellos al ver esa horquilla en una B!). Hello to the Brompton blogger in Singapore; hello to the Brompton rider in Seoul who called it "awesome and ridiculous", and who has experimented with elastomer handlebar grips; hello to Kiyo, the Brompton blogger in Japan who said it was "interesting, but it's not beautiful"; and hello too to the French Brompton fans who picked up on it. You were all correct, too: it was an ugly looking thing.
As you can see in more recent photographs, I put the original fork back on my bike. Luggage on the front mounting block and a Birdy fork (or at least, this old and flexible Birdy fork) was a recipe for shimmy if I rode one-handed, such as when signalling. At low speeds it was merely annoying, but at anything above 10mph it became positively dangerous.
Original DSC_0882