why Brompton forks have such low trail?

General cycling advice ( NOT technical ! )
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pjclinch
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Re: why Brompton forks have such low trail?

Postby pjclinch » 2 Jun 2018, 12:37pm

Freddie wrote:
pjclinch wrote:That different people interact differently with different bikes is hardly a surprise. So it's no surprise that just as you and 531colin don't get on with Brom handling, there's plenty (like me) that do. And for those that do get on with a Brom, that a Tern may be more stable is a bit of a moot point. A trike is easier to balance than any of my bikes, but if I don't really have any trouble balancing my bikes then the extra point of balance stability on a trike is really neither here nor there for me.
What you seem to suggest is that there is no such thing as objectively good handling and that all handling is dependent on the individual riding the bicycle.

Is that really true? Whilst there are people who can't ride no hands whatever the bicycle and even people that can't ride a bicycle at all, this doesn't discount the fact that some bicycles are more stable than others and there is a range in which a bicycle provides normal handling and outside of this range the handling is sub-optimal.

Why should somebody have to relearn how to ride a specific bicycle? Doesn't that suggest the design is flawed from the get go (save things like fixed gear, which is a different kettle of fish).


Your assumption is that stability no-hands is of itself a Good Thing that trumps any other Good Things (like it folds up and goes in a very small space, or can turn on a sixpence), and that's not a safe one. I think "relearn how to ride a specific bicycle" is hugely over-egging the pudding. IME this process typically consists of get on, off you go, "oh, this feels a bit odd", couple of minutes getting used to it, "oh, it's okay after all", The End. But even if you weren't over egging the pudding, why should you learn something different? Because you might get something worthwhile that works well for you better than what you had before (so I put in the time to learn to ride on a recumbent, and now I'm more comfortable so enjoy my touring more, even if I can't ride it no-hands).

All bicycles are compromises at some level, and are thus intrinsically flawed at some level. You show me Freddie's Improved Brompton which is more inherently stable to ride no-hands and I'll show you a spot in traffic it's rather harder to pop through than my Mk. 3. There is no such thing as "objectively good handling", because different people want different things from their handling. The world speed record holding bike needs to be rock-steady at 80 mph with a rider on the verge of passing out, and yes, you really want stability there. I'd guess the turning circle is something akin to a jumbo jet, OTOH...

You'd probably get consensus (as opposed to objectivity) on what constitutes very poor handling. I don't think I've ever heard a good word about the original Bickerton's ride, but while the Brom does have people who find it over their personal line the amount of happy customers who have no trouble suggests it's in, or at least very close to, "a range in which a bicycle provides normal handling" for most of the people, most of the time.

Pete.
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Re: why Brompton forks have such low trail?

Postby fastpedaller » 2 Jun 2018, 1:22pm

I'm surprised at the comments, as I had an earlier Brompton c1987 and found it to be no problem at all. The only bike I've had a problem with was the Spa tourer WITH A TAPER ROLLER HEADSET - once I changed the headset to a (cheap) ball bearing one (with good quality grade 10 balls) it changed from very unnerving to great!
Whether a Brompton design change (or less likely manufacturing tollerances) have had a huge impact compared to my early one, I've no idea?
Worse cycling decision I made was to sell the Brompton.

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Mick F
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Re: why Brompton forks have such low trail?

Postby Mick F » 2 Jun 2018, 2:39pm

Mick F wrote:Still not measured the trails ..............
Just done it with Mercian and Moulton, though it was difficult to be dead on accurate with sticky tape and string whilst leaning the bikes against a wall and eyeballing the string to be straight.

Mercian, I'm fairly happy with the measurement, but Moulton less so though I'm not far out I'm sure.

Mercian = 3" trail
Moullton = 2" trail

Even if I'm a tad out, there's still a factor of about 33% here.
Mick F. Cornwall

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Re: why Brompton forks have such low trail?

Postby Mick F » 2 Jun 2018, 4:01pm

Hang on a sec here.

If you take the argument to extreme, the larger the diameter of the front wheel, the longer the trail?
The smaller the wheel, the shorter the trail?
If you had a zero diameter front wheel, you would have zero trail?

........... or am I wrong?
Mick F. Cornwall

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Re: why Brompton forks have such low trail?

Postby Randy_Butternubs » 2 Jun 2018, 4:16pm

Mick F wrote:Hang on a sec here.

If you take the argument to extreme, the larger the diameter of the front wheel, the longer the trail?
The smaller the wheel, the shorter the trail?
If you had a zero diameter front wheel, you would have zero trail?

........... or am I wrong?


If the wheel had 0 diameter and no offset/rake then it would have no trail yes.

Bicycles and motorcycles always (?) have some offset though. Normal offset with the wheel moved forward reduces trail so a normal bicycle frame with tiny wheels would have negative trail.

This doesn't mean small-wheeled bikes have inherently low trail though as you can change the steering angle and rake to compensate.

Edit: I've taken it as read that Bromptons have particularly low trail but does anyone know if that is actually the case? I can't find any specs for their steering geometry.

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Re: why Brompton forks have such low trail?

Postby pjclinch » 2 Jun 2018, 6:30pm

Head angles and trail... This from the VeloVision review of the 8 Freight...

The 8 Freight is a pleasure to ride when it’s loaded. But when you first ride it, the steering can feel quirky. I suggested to Mike Burrows that this ‘quirkiness’ – a ‘steering the bus’ sort of feel – was the result of the steep, 80 degree head angle. This configuration makes steering very
‘immediate’. I mentioned my idea of a more laid back angle and was quickly put to rights. My mistake apparently.
I was referred to several esteemed design publications (the 1896 book Bicycles and Tricycles by Archibald Sharpe, and another book of motorbike design theory) which suggest that in fact, a full 90 degree head angle is ideal! I was also told that I would have understood this better if I had properly read the related chapter in Mike Burrows’ own book on bicycle design (Bicycle Design: Toward the Perfect Machine). Besides all that, Burrows has himself experimented with myriads of head angles over the years.
OK, I now accept that a two metre long bike is going to have some idiosyncratic handling characteristics and that it’s got nothing to do with the head angle. I just ride the things, perhaps I should know better...


The more I look in to bicycle handling the more I get the impression that there's a bit more to it than the assumptions we tend to tae for granted!

Pete.
Often seen riding a bike around Dundee...