why Brompton forks have such low trail?

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Freddie
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why Brompton forks have such low trail?

Postby Freddie » 31 May 2018, 1:46pm

I know they are made to, when carrying a load, primarily carry a load up front, but why is the fork trail so low? I haven't ridden a Brompton, but what with the tiny wheels, it must be impossible to ride no hands and keep a straight line. You might say keep your hands on the handlebars, to which I would reply much of the extra input necessary to hold a line remains, hands on or off.

It seems common for small wheels bikes to have a fork that looks like one on a larger bike, providing low trail to a wheel size that already has a tendency to fall in on itself. Do these manufacturers know or care anything about bike handling, because they seem to have little concept of how to achieve normal handling when the wheels are made smaller.

Is it just a case of most people thinking small wheels must equal twitchy steering, so Brompton don't care to fix the problem. Surely these folding bicycles could be made eminently more ride-able, if some concern was shown for fork geometry.

Randy_Butternubs
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Re: why Brompton forks have such low trail?

Postby Randy_Butternubs » 31 May 2018, 3:06pm

The first time I rode a Brompton I found it comically unstable but I got used to it in a few minutes and started to appreciate it for weaving around cars. I didn't find it required any particular effort to hold a line but I didn't try riding without my hands. Having some weight on the front carrier supposedly calms them down a lot too.

If it does have particularly low trail I'm guessing it's down to a combination of a) helping with the compact fold, b) being more suitable for carrying weight on the front, and c) it being intended for use mainly at lower speeds in heavy traffic.

Re: the tendancy of the wheel to fall in on itself: isn't that more a question of the slackness of the steering head angle? I found the Brompton very easy to push along by the saddle without the front wheel flopping to one side. I can't say the same of my full-size bikes.

There seems to be some debate over whether low trail actually causes twitchy steering or not. Jan Heine in particular finds that it does not, but I can't comment. I suspect there are multiple contributing factors anyway.

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Re: why Brompton forks have such low trail?

Postby UpWrong » 31 May 2018, 4:13pm

I sold my Brompton because I couldn't confidently steer it whilst signalling. In contrast my Tern Link is very stable.

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[XAP]Bob
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Re: why Brompton forks have such low trail?

Postby [XAP]Bob » 31 May 2018, 4:29pm

It's the only (upright) bike I've ever been able to ride no hands...
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Freddie
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Re: why Brompton forks have such low trail?

Postby Freddie » 31 May 2018, 9:02pm

Randy_Butternubs wrote:Re: the tendancy of the wheel to fall in on itself: isn't that more a question of the slackness of the steering head angle? I found the Brompton very easy to push along by the saddle without the front wheel flopping to one side. I can't say the same of my full-size bikes.
I do believe it is a function of wheel size (not exclusively, but smaller wheels will deviate/fall more so than larger ones). Take the old children's hobby of hoop rolling; it is far easier to roll a large hoop and have it stay upright, than it is a small one. I think this might be gyroscopic effect in action:

http://sheldonbrown.com/brandt/gyro.html

The link also explains why pushing a bike by the saddle and riding one, with regard to wheel flop, are two different things.
UpWrong wrote:I sold my Brompton because I couldn't confidently steer it whilst signalling. In contrast my Tern Link is very stable.
You might interested in something I dug up regarding fork rake as applied to a Dahon (different bike, similar low trail though).

http://www.johnforester.com/Articles/Bi ... /dahon.htm

In particular the section near the end of the page entitled 'Stability of the Dahon bicycle', as it seems to mirror your experience with the Brompton.

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Re: why Brompton forks have such low trail?

Postby Randy_Butternubs » 31 May 2018, 10:36pm

Freddie wrote:
Randy_Butternubs wrote:Re: the tendancy of the wheel to fall in on itself: isn't that more a question of the slackness of the steering head angle? I found the Brompton very easy to push along by the saddle without the front wheel flopping to one side. I can't say the same of my full-size bikes.
I do believe it is a function of wheel size (not exclusively, but smaller wheels will deviate/fall more so than larger ones). Take the old children's hobby of hoop rolling; it is far easier to roll a large hoop and have it stay upright, than it is a small one. I think this might be gyroscopic effect in action:

http://sheldonbrown.com/brandt/gyro.html

The link also explains why pushing a bike by the saddle and riding one, with regard to wheel flop, are two different things.


That's interesting, but I think the gyroscope effect is a red herring. Brandt himself notes that at speed the effect is significant, but small. At low speeds it becomes insignificant, which is odd given that he goes on to claim it as a significant effect when pushing the bike.

My understanding is that the dominant self correcting force on a bicycle comes from the trail, which is what your second link talks about.

What I was getting at regarding the steering angle is that there is also a de-stabilising force acting on the bike. If you turn the bars from straight-ahead to 90 degrees left or right the front axle will drop slightly. By the same mechanism, weight on the front of the bike will cause the wheel to try to point either dead left or dead right. This is why the stationary bike you've left propped against a wall will defy you by suddenly swinging the front wheel to one side and falling over. It is also why bicycles with low-trail geometry are recommended for carrying a heavy front load.

To visualise this more easily, consider the extreme case of a fork that points horizontally forward from the body of the bike. With the wheel straight the front axle will be a distance from the ground equal to the wheel radius. With the wheel turned 90 degrees to one side the axle will almost be touching the ground. The slacker the head tube angle and the larger the wheel, the greater this effect will be.

This probably explains why I found the small-wheeled Brompton (and my dad's Moulton) easy to push by the saddle: when stationary or at low speed the effect of trail is non existent or very small and so the (lack of) stability of a bike may be dominated by this steering angle/wheel size effect.

This might explain why a small-wheeled bike like the Brompton can get away with low trail. It also might explain why the Jan Heine-esque (French constructeur) bikes handle well with low trail geometry since they also use the smaller 650b wheels (although with fatter tyres the difference is pretty small).

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531colin
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Re: why Brompton forks have such low trail?

Postby 531colin » 31 May 2018, 10:44pm

Jan Heine wants to have his cake and eat it......he finds that low trail steering allows him to flick the bike easily around potholes, and yet the same bike will take him safely home when he is shattered. I find those 2 characteristics are diametrically opposed.

Theres some stuff about wheel flop here..https://en.wikipedia.org/wiki/Bicycle_and_motorcycle_geometry#Wheel_flop
....good luck with that!

I have no experience designing small wheel bikes, and no desire to learn. With full size wheels as long as you follow the "sweatmarks" made by generations of framebuilders, you won't go far wrong.

Steve Mundie http://www.steve-mundie.co.uk/ can build small wheel bikes that steer properly. I rode one of his up the road, and came back no hands. It can be done, if you set about it in the right way.

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Re: why Brompton forks have such low trail?

Postby Brucey » 31 May 2018, 11:13pm

FWIW low trail works best if you have a slightly steeper head angle and relatively heavy wheels.

I have never been able to easily ride any mass-produced small-wheeled bike (with wheels 20" or smaller) no hands. Most other bikes, no problem.

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Re: why Brompton forks have such low trail?

Postby pjclinch » 1 Jun 2018, 10:28am

One person's "twitchy" is another's "responsive". While I can put in short no-hands stints on my Brom and (slightly longer) on my TSR I'd certainly not consider e.g. changing a jacket riding along. On the other hand, they're a lot nimbler in tight spots.

I've never really found the Brom awkward, and in fact have used it to teach complete beginners (adults) as the ease of saddle height adjustment and a low stepover are otherwise very handy. On one occasion I picked up a friend at the station, riding down in the 8 Freight with the Brom in the back. At the sttaion friend took the Brom and her case went in the 8 Freight. She reported the first few hundred meters felt a bit twitchy but after that she dialled in and had no further problems.

A fellow Bromptonaut who never had any problems with a Brom reported that a Cresswell Micro was the twitchiest bike he's ever ridden. A Strida was not so much twitchy as squirmy...

For an... interesting no hands the 8 Freight is the one. It has a practically vertical, straight fork. It doesn't jackknife, but it just wanders back and forth as you pedal.

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Re: why Brompton forks have such low trail?

Postby Brucey » 1 Jun 2018, 11:05am

the 8 freight is that rare thing, a bike where the wheels are nowhere near in line with one another by design. If it had more trail in the steering, it wouldn't wander so much but it would lean more to one side, all the time, I think.

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Re: why Brompton forks have such low trail?

Postby pjclinch » 1 Jun 2018, 12:43pm

Since it turns out I don't feel the need to pedal around no-hands with a crate of shopping the 8's rather odd natural steering dynamic is in the realms of the Entirely Acceptable! (as with my 'bent, a pinkie on one side is all that's needed for completely predictable handling, but I'm not going to be letting go completely (the occasional careful experiment excepted).

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Freddie
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Re: why Brompton forks have such low trail?

Postby Freddie » 1 Jun 2018, 2:13pm

531colin wrote:Steve Mundie http://www.steve-mundie.co.uk/ can build small wheel bikes that steer properly. I rode one of his up the road, and came back no hands. It can be done, if you set about it in the right way.
They seem to have almost dead straight forks, so obviously have a much higher trail than most other 20" wheeled bikes.
Brucey wrote:I have never been able to easily ride any mass-produced small-wheeled bike (with wheels 20" or smaller) no hands. Most other bikes, no problem.
Which suggests they don't know what they are doing and are making a fork with a curve because 'bikes have forks with a forward curve'.
pjclinch wrote:One person's "twitchy" is another's "responsive".
Within a range. If you refer to the second link in my last post, it seems if you have too low a trail the bike does not respond adequately to lean input, hence why it is very difficult to ride these bikes no hands and more attention must be paid when signalling (which requires removing one hand and leaning the body more than with both hands), so much so that 'UpWrong' got rid of his because of problems attempting to signal.

A bicycle that cannot be adequately steered by leaning by a reasonably skilled, regular rider, is one that is poorly designed. Given that the Brompton is targeted at people who probably don't do 50+ miles cycling of a weekend, they are less likely than most here to be reasonably skilled, regular riders.

If Brompton extended the main frame 1" and straightened the forks by a similar amount, I imagine it would probably ride perfectly sensibly and akin to how a typical bike rides.

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Re: why Brompton forks have such low trail?

Postby Mick F » 1 Jun 2018, 2:59pm

Dunno what the Moulton's trail is.
I have tried in No Hands and it's no easier or harder than Mercian.

I'll have a go at measuring them both later.
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Re: why Brompton forks have such low trail?

Postby pjclinch » 1 Jun 2018, 6:37pm

Freddie wrote:
531colin wrote:Steve Mundie http://www.steve-mundie.co.uk/ can build small wheel bikes that steer properly. I rode one of his up the road, and came back no hands. It can be done, if you set about it in the right way.
They seem to have almost dead straight forks, so obviously have a much higher trail than most other 20" wheeled bikes.
Brucey wrote:I have never been able to easily ride any mass-produced small-wheeled bike (with wheels 20" or smaller) no hands. Most other bikes, no problem.
Which suggests they don't know what they are doing and are making a fork with a curve because 'bikes have forks with a forward curve'.


I think you may be drawing too much from anecdotes. No-hands riding is very much about confidence allied to technique and is about far more than the bike itself. So while I can ride a unicycle (not brilliantly, but I can), I'm nowhere near as good a no-hands bike rider as my wife, and she's quite happily ridden kilometers on Dutch hire-bikes while I'm really not happy for more than 50m on an identical model. I used to ride no-hands a lot as a teen, until I crashed one day, and then lost the nerve and couldn't do it all for years. Now I can do it again, on some bikes better than others so the bike is important, but it's not the case that if e.g. 531colin can do it on a Brand X Model Y then anyone can.

Freddie wrote:
pjclinch wrote:One person's "twitchy" is another's "responsive".
Within a range. If you refer to the second link in my last post, it seems if you have too low a trail the bike does not respond adequately to lean input, hence why it is very difficult to ride these bikes no hands and more attention must be paid when signalling (which requires removing one hand and leaning the body more than with both hands), so much so that 'UpWrong' got rid of his because of problems attempting to signal.


Riding one and no-handed don't really have that much to do with one another. My experience with a Brom is it is no trouble one-handed. My experience teaching people to ride one-handed is that, a bit like no-handed, much of it is a mind-game. A lot of people get unsteady one handed not because of the design of bike but because they're too tense and won't let the bike and the steering combine to do what's wanted. This effect is particularly noticeable in cases where folk think the steering is a problem because it's not what they're used to. My other bike that exhibits this is my 'bent tourer, which I absolutely will not let go of the bars while in motion but on which I can ride with confidence at speed with just one finger on one side. Quite a few people can't ride it at all without a bit of practice, and the death-grip they have on the bars is a lot of why that is.

Freddie wrote:A bicycle that cannot be adequately steered by leaning by a reasonably skilled, regular rider, is one that is poorly designed.


No, it's just not designed for riding no-handed. And as lots and lots of people are quite happily riding them using their hands on the bars that's a bit of a moot point. I can see why pro-riders want to be able to change in and out of rain capes while at speed but for most riders, most of the time it's a bit of a non-issue. And given the inherent compromises in folders, if Brompton did a different model that didn't fold quite as neatly but allowed you to ride no-hands just like a road bike I don't think it would add to their sales.

I think I'd pay quite good money to see you try and take Mike Burrows to task about the 8 Freight being "poorly designed" for the above reason :wink:

Freddie wrote:Given that the Brompton is targeted at people who probably don't do 50+ miles cycling of a weekend, they are less likely than most here to be reasonably skilled, regular riders.


I take it that "reasonably skilled, regular riders" are sort of people that have some particular need to ride no-handed on a typical journey?

Freddie wrote:If Brompton extended the main frame 1" and straightened the forks by a similar amount, I imagine it would probably ride perfectly sensibly and akin to how a typical bike rides.


The frame was extended 30mm back in 2004 when they re-engineered the mainframe hinge. Mine predates that so I don't know what effect it's had, but they claim a better ride. If you stretched it again it wouldn't fold as neatly because the mainframe length outlines the fold.

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Re: why Brompton forks have such low trail?

Postby fossala » 1 Jun 2018, 6:48pm

Mick F wrote:Dunno what the Moulton's trail is.
I have tried in No Hands and it's no easier or harder than Mercian.

I'll have a go at measuring them both later.

I was thinking this last night, riding Moulton with no hands is easy. I also had a Brompton for a bit, couldn't ride it without hands even for 2 seconds.