Re: Near Miss from Trying to Signal

[Elisa Francesca Roselli]

Simon Brooke wrote:

> Dynamic steering wobbles are usually caused by too much lateral
> flexibility of the frame. They can be made worse by weight forward of
> the steering pivot (e.g. a heavy bar bag or bar-mounted basket). They
> are extremely dangerous - as you have found - and on the whole if it
> can't be traced to an unusual amount of weight forward of the steering
> pivot I would be inclined not ot ride that bike any more.

Well, Behemoth does have a front basket, but there was nothing in it that
afternoon, so it shouldn't have been especially heavy, and there was a
heavy pannier on the rear rack.

As for abandoning the bike, this has been open for discussion before. She's
a beautiful Dutch bike bought only last November
(http://www.bikkelbikes.com/images/bikes/bikes_groot/popup_groot/neerhem-d.jpg).
In April I went to the UK to look for a replacement but didn't have much
luck finding something certain to be better. I cannot tell without a
longish trial ride, but am often too nervous to manage those on unfamiliar
bikes, and too shy to ask them of dealers, so its a vicious circle.

What could cause lateral flexibility of the frame? It's a sturdy, and very
heavy, aluminium monotube. The steering has been odd from the beginning,
but as I am a very inexperienced beginner I thought it might be me.

Several other people who've tested her for me have remarked that she
squirrels, especially at lower speeds. I am due to take her back to her
dealer for a "contrôle technique" and will raise this issue of the
steering. But this in itself requires riding her through central Paris,
which would also require some decent weather and a day off work since I
sure as hell wouldn't risk the Saturday zoo, so it's quite a project.

 

[Simon Brooke]

in message <[email protected]>, Elisa Francesca
Roselli ('[email protected]') wrote:

> Simon Brooke wrote:
>
>> Dynamic steering wobbles are usually caused by too much lateral
>> flexibility of the frame. They can be made worse by weight forward of
>> the steering pivot (e.g. a heavy bar bag or bar-mounted basket). They
>> are extremely dangerous - as you have found - and on the whole if it
>> can't be traced to an unusual amount of weight forward of the
>> steering pivot I would be inclined not ot ride that bike any more.
>
> Well, Behemoth does have a front basket, but there was nothing in it
> that afternoon, so it shouldn't have been especially heavy, and there
> was a heavy pannier on the rear rack.

Weight on the back won't make a lot of difference in this situation (it
could make it worse but that's quite complicated dynamics). What can
happen when you have weight in your basket is that that weight can act
as a pendulum, accentuating any steering wobble you have. As Steve
points out, bent forks can be implicated in this sort of problem. Has
Behemoth ever been ridden hard into a kerb, wall or vehicle?

> As for abandoning the bike, this has been open for discussion before.
> She's a beautiful Dutch bike bought only last November
>
(http://www.bikkelbikes.com/images/bikes/bikes_groot/popup_groot/neerhem-d.jpg).

Yup. This sort of frame design is the worst for axial flex, and quite
bad for lateral flex. On a conventional bike frame the crossbar joins
the top of the head tube with the top of the seat tube helping to
prevent both from flexing. Behemoth doesn't have one. This is OK in
Holland which is mostly flat and where bikes like this are typically
pedalled slowly, but as speed increases so does the energy in all parts
of the system, and a bike which will trundle happily along all day at
eight miles per hour may experience dynamic problems at sixteen.

> What could cause lateral flexibility of the frame? It's a sturdy, and
> very heavy, aluminium monotube. The steering has been odd from the
> beginning, but as I am a very inexperienced beginner I thought it
> might be me.

If you look at the design the main tube and seat tube join at the bottom
bracket and are braced by one short brace just above it. This means
there's a lot of unsupported length of both tubes and it is this
unsupported length which can flex. Flex in itself isn't necessarily a
bad thing, but if the harmonic period of the flexion of the frame is
similar to the harmonic period of the steering they will tend to excite
one another, leading to the violent steering oscillation you
experienced.

Note, of course, that I haven't ridden the bike and that this is just
one possible explanation - but it's the one which seems to me to best
fit the symptoms you describe.

--
[email protected] (Simon Brooke) http://www.jasmine.org.uk/~simon/

[ This .sig subject to change without notice ]

 

[Just zis Guy, you know?]

On Mon, 10 May 2004 17:31:14 +0200, Elisa Francesca Roselli
<[email protected]> wrote in message
<[email protected]>:

>(http://www.bikkelbikes.com/images/bikes/bikes_groot/popup_groot/neerhem-d.jpg).

Have you, perchance, raised the handlebars up on the adjustable
headset thingy?

Guy
--
May contain traces of irony. Contents liable to settle after posting.
http://www.chapmancentral.co.uk

88% of helmet statistics are made up, 65% of them at Washington University

 

[Peter Clinch]

Simon Brooke wrote:

> Yup. This sort of frame design is the worst for axial flex, and quite
> bad for lateral flex. On a conventional bike frame the crossbar joins
> the top of the head tube with the top of the seat tube helping to
> prevent both from flexing. Behemoth doesn't have one.

etc.
Nor does my Brompton, mind, and that's further compromised by a couple
of hinges in the tubes and a handlebar stem so long it positively
discourages any sort of pulling on it, but the handling's still okay if
you don't visit Honk City.
This doesn't prove that there's nothing wrong with Behemoth, but it does
demonstrate that such a design isn't intrinsically awful in a city bike.
From reading Elisa's posts my first feeling was "something wrong with
bike", but when you add in similar problems found on other bikes on a
recent visit to Cambridge it becomes harder to be sure all the problems
are with the bike.

Though having said that, I would get it thoroughly checked out.

But having said /that/, there shouldn't really be any stresses on the
frame freewheeling down a very gentle incline so it would be unlikely to
be flexing in such a situation. If pedalling and the resultant forces
can be eliminated by rolling down a gentle hill like that and the bike
then becomes unstable simply by removing one hand from the bars (where
the other is still in place to hold the bike steady) that suggests to me
(not conclusively, but more likely) that the problem is more unconscious
input into the steering than the bike being wibbly.
My thoughts concentrate on this because I've had related problems
getting to grips with steering on some 'bents, especially tiller
steering on low bikes. That felt dangerously twitchy at first, like I
was constantly in danger of being thrown, but once I learned to relax
and put no power through the bars at all everything suddenly got okay.
And I've seen this happen with others too: putting any sort of force
into the bars on many 'bents really buggers things up, and it's easy to
do even when you know that, and even easier when you're thinking "I
MUSTN'T PUT ANY FORCE THROUGH THE BARS OR I'LL WOBBLE!".

So my suggestions are get it given a thorough inspection, including
riding by the mechanic doing it, and learn to relax on the bike. And,
yes, I know that's easier said than done.

Pete.
--
Peter Clinch University of Dundee
Tel 44 1382 660111 ext. 33637 Medical Physics, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net [email protected] http://www.dundee.ac.uk/~pjclinch/

 

[Elisa Francesca Roselli]

Peter Clinch wrote:

> This doesn't prove that there's nothing wrong with Behemoth, but it does
> demonstrate that such a design isn't intrinsically awful in a city bike.

My other bike, Myrtille, has a similar design and an even lower
step-through, and handles like a dream. In any case, I won't use a high step
through. I find it very uncomfortable in other ways and it's incompatible
with my skirt habit.


> From reading Elisa's posts my first feeling was "something wrong with
> bike", but when you add in similar problems found on other bikes on a
> recent visit to Cambridge it becomes harder to be sure all the problems
> are with the bike.

True, I couldn't signal on the Cambridge bike either. But the steering _was_
easier. I think it's two separate issues. I cannot signal on any bike
(Myrtille included), but some bikes are much easier to steer than Behemoth.

By the way, "shimmy" has happened to me before, on Myrtille. As John Senior
said, it's a phenomenon that so far has not been explained. Perhaps it's
just a local ocurrence. Myrtille hasn't shimmied again since. Possibly she
had a soft tyre. So this too might be a separate issue.

> My thoughts concentrate on this because I've had related problems
> getting to grips with steering on some 'bents, especially tiller
> steering on low bikes. That felt dangerously twitchy at first, like I
> was constantly in danger of being thrown, but once I learned to relax
> and put no power through the bars at all everything suddenly got okay.
> And I've seen this happen with others too: putting any sort of force
> into the bars on many 'bents really buggers things up, and it's easy to
> do even when you know that, and even easier when you're thinking "I
> MUSTN'T PUT ANY FORCE THROUGH THE BARS OR I'LL WOBBLE!".

I've heard this about bents and semi-bents. What about the Brompton? Is that
twitchy, with those small wheels?

> So my suggestions are get it given a thorough inspection, including
> riding by the mechanic doing it, and learn to relax on the bike. And,
> yes, I know that's easier said than done.

We concur, as this more or less sums up my battle plan.

EFR
Ile de France

 

[Peter Clinch]

Elisa Francesca Roselli wrote:

> I've heard this about bents and semi-bents. What about the Brompton? Is that
> twitchy, with those small wheels?

Small wheels don't necessarily equate to twitchy steering, it's usually
more related to the amount of trail, and though small wheels can affect
the amount of trail it isn't a given that you won't have any.

I don't find the Brom twitchy at all. First time riders report they
think it's a bit twitchy but after a few hundred meters they're usually
dialled in no problem.

I am by no means a frequent or capable no-hands rider, but I can ride it
no-hands for 100m or so. On the no-trail Streetmachine you know that
letting go completely just isn't an option, though it's steady as a rock
at high speed with just fingertip pressure on one bar to keep it
straight. The 'bents are only twitchy if you put in more than you need:
once you've learned to put in only what you need then the steering is
light, easy and responsive. This is true to some extent on a very
upright bike, like a Dutch roadster... you only need very light pressure
on a single bar to stop the bike swerving or jack-knifing, but with your
body weight divorced from the steering it's easy to swing the bars
wherever they'll go. Before you're used to it, a bad thing, once you
are used to it, great!

Pete.
--
Peter Clinch University of Dundee
Tel 44 1382 660111 ext. 33637 Medical Physics, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net [email protected] http://www.dundee.ac.uk/~pjclinch/

 

[Roos Eisma]

Peter Clinch <[email protected]> writes:

>I am by no means a frequent or capable no-hands rider, but I can ride it
>no-hands for 100m or so. On the no-trail Streetmachine you know that
>letting go completely just isn't an option, though it's steady as a rock
>at high speed with just fingertip pressure on one bar to keep it
>straight. The 'bents are only twitchy if you put in more than you need:
>once you've learned to put in only what you need then the steering is
>light, easy and responsive. This is true to some extent on a very
>upright bike, like a Dutch roadster... you only need very light pressure
>on a single bar to stop the bike swerving or jack-knifing, but with your
>body weight divorced from the steering it's easy to swing the bars
>wherever they'll go. Before you're used to it, a bad thing, once you
>are used to it, great!

Most of the steering is about body weight and leaning rather than
handlebars. When riding handsfree my control comes from pressure on the
pedals and saddle. Maybe the OP with steering issues should focus on her
body rather than the handle bars for a while? Lean into corners?
(surprisingly difficult to realy know what you're doing when riding a bike
once it is routine!)

Roos

 

[Peter Clinch]

Roos Eisma wrote:

> Most of the steering is about body weight and leaning rather than
> handlebars. When riding handsfree my control comes from pressure on the
> pedals and saddle. Maybe the OP with steering issues should focus on her
> body rather than the handle bars for a while?

Yes. I've been trying to emphasize not really doing anything much with
the handlebars but in doing that have been going on and on and on about
handlebars, perhaps the wrong approach...

Pete.
--
Peter Clinch University of Dundee
Tel 44 1382 660111 ext. 33637 Medical Physics, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net [email protected] http://www.dundee.ac.uk/~pjclinch/

 

[Jacques Moser]

>
> Several other people who've tested her for me have remarked that she
> squirrels, especially at lower speeds. I am due to take her back to her
> dealer for a "contrôle technique" and will raise this issue of the
> steering. But this in itself requires riding her through central Paris,
> which would also require some decent weather and a day off work since I
> sure as hell wouldn't risk the Saturday zoo, so it's quite a project.

As others have confirmed that the bike is not stable, and as you admit
that your sense of balance is not the best, I would suggest you look for
another bike. Even if you want to stick to a "lady's" bike, there must be
some with a more rigid geometry.

Jacques