Saddleless bikes and the law

[jtaylor]

"Richard" <[email protected]> wrote in message
news:[email protected]...
> Richard Bates wrote:
> > I'm guessing that having two brakes on the front wheel may allow
> > better control. The point between a brake providing maximum
> > deceleration and locking the wheel is quite fine. Let's call this the
> > "critical brake point". Sharing the force between two brakes, both
> > more distance from the CBP could perhaps give better deceleration with
> > less risk of wheel-locking?
>
> An interesting point. And I think the answer is no.
>
> If the two brakes are totally independent of each other and (for the
> sake of argument) approximately identical, and the rider is experienced
> enough to reach critical point on both more or less simultaneously, you
> will have twice the braking force (and hence twice the deceleration).

No. The "critical point" is is determined by the weight and geometry of the
cycle/rider combination, not by the number, size, type, colour, price,
etcetera of the brake(s).

 

[Richard]

jtaylor wrote:
>>An interesting point. And I think the answer is no.
>>
>>If the two brakes are totally independent of each other and (for the
>>sake of argument) approximately identical, and the rider is experienced
>>enough to reach critical point on both more or less simultaneously, you
>>will have twice the braking force (and hence twice the deceleration).
>
>
> No.

No to what bit?

> The "critical point" is is determined by the weight and geometry of the
> cycle/rider combination, not by the number, size, type, colour, price,
> etcetera of the brake(s).

Where did I suggest in the above what determines the critical point (ie,
just before locking) of braking?

In fact, it has nothing to do with weights or geometries, but is
entirely determined by the materials of the brake block and the braking
surface.

R.

 

[jtaylor]

"Richard" <[email protected]> wrote in message
news:[email protected]...
> jtaylor wrote:
> >>An interesting point. And I think the answer is no.
> >>
> >>If the two brakes are totally independent of each other and (for the
> >>sake of argument) approximately identical, and the rider is experienced
> >>enough to reach critical point on both more or less simultaneously, you
> >>will have twice the braking force (and hence twice the deceleration).
> >
> >
> > No.
>
> No to what bit?

No to twice the braking force.

>
> > The "critical point" is is determined by the weight and geometry of the
> > cycle/rider combination, not by the number, size, type, colour, price,
> > etcetera of the brake(s).
>
> Where did I suggest in the above what determines the critical point (ie,
> just before locking) of braking?

In suggesting that by having twice as many brakes you would have twice the
braking force and twice the deceleration. The maximum braking force you can
have occurs when the wheel locks. One brake is sufficient to lock the
wheel.

>
> In fact, it has nothing to do with weights or geometries, but is
> entirely determined by the materials of the brake block and the braking
> surface.

Wrong. The force at wheel locking is not determined by the brakes, by but
geometry & weight (we're assuming speed and road/tire friction are held
constant). Get out your pencil and draw the vectors.

 

[Richard]

jtaylor wrote:
> The maximum braking force you can
> have occurs when the wheel locks.

The maximum braking force is NOT when the wheel is locked; this provides
significantly less braking force than when the wheel is still rotating,
but heavily retarded. This is why lorries jackknife, as the (locked)
rear wheels try to overtake the (unlocked, but braking) front wheels.

R.

 

[jtaylor]

"Richard" <[email protected]> wrote in message
news:[email protected]...
> jtaylor wrote:
> > The maximum braking force you can
> > have occurs when the wheel locks.
>
> The maximum braking force is NOT when the wheel is locked; this provides
> significantly less braking force than when the wheel is still rotating,
> but heavily retarded. This is why lorries jackknife, as the (locked)
> rear wheels try to overtake the (unlocked, but braking) front wheels.
>


At the point of locking the maximum braking force is reached - then of
course it is somewhat reduced as the wheel starts skidding. That's why I
said "when the wheel LOCKS", not "when the wheel is LOCKED". Nobody was
arguing that - instead it was the idea that somehow having more than one
brake would allow a greater force.

This force is m * g * cot(theta), where theta is the angle between the
c.o.g. and the horizontal - exceed that force and theta increases suddenly.
I'd be interested to see how you think a term describing the number of
brakes on the wheel could be inserted into that relationship.

 

[Jack Ouzzi]

On Thu, 13 Jan 2005 13:01:00 +0000, Richard
<[email protected]> wrote:


>a) if the coefficient of friction between the front tyre and the ground
>is sufficiently high, the back wheel will start to lift. A reasonably
>skilled rider will let off the front brake at this point to avoid going
>over the bars. Going over the bars is not inevitable in any case; the
>energy contained in the system has to be sufficient to lift the mass of
>you + bike over the apex of the inverted pendulem. BOTE calculation: a
>70 kg rider on a 10 kg bike with a 0.5m top tube has a centre of mass
>normally about 1m above the ground. To go over the handlebars with a
>locked front wheel requires 80 kg to be raised 0.5m, ie 400 Joules of
>energy. This can come from a velocity of just over 3 m/s or ~7 mph.
>Less than that, and you'll simply lift off the ground at the back, but
>then be deposited back again as it fails to get "over" the hump. A
>significant weight shift backwards is hard - you'd probably only get a
>few cm change in position of your CoM. On the other hand, it'd be easy
>to weight-shift forwards and reduce the speed required to go over the top.
>
>b) if the coeff. of friction between the front tyre and the ground isn't
> high enough for (a), the front wheel will skid. Again, best practise
>is to release it pronto before you fall off.
>
>If the ground isn't solid (and is, say, loose gravel) you can sometimes
>stop faster by locking everything up. The wheels act as ploughs and the
> energy is dissipated in shifting gravel into a heap in front of the
>tire. But, again, with both wheels locked you lose most of your ability
>to balance and all of your ability to (effectually) steer.
>

Hell, have I got to remember all this when I go out training on my
fixie (with back brake) :-|

 

[soloriens]

En message <[email protected]>,
dkahn400 <[email protected]> ecrit
>soloriens wrote:
>
>> No. Fixies must have a front brake. A cycle [of a defined size]
>> cannot be used on the road unless
>>
>> "... if the cycle is so constructed that one or more of the
>> wheels is incapable of rotating independently of the pedals, it is
>> equipped with a braking system operating on the front wheel"
>>
>> for bikes made since 1984 the saddle height is the definition of
>> size. Before that it was the wheel size (Mr Moulton spoiled that
>> rule).
>
>What is the specified saddle height, and how would that relate to, say,
>a Trice Micro?
>
....every bicycle or tricycle the height of the saddle of which is 635
millimetres or more ...

the height of the saddle is ... the height above the ground of the part
of the seating area of the saddle which is furthest from the ground when
the cycle to which the saddle is attached is vertical and the saddle is
raised to the fullest extent compatible with safety and the tyres on the
wheels of the cycle are fully inflated.

and what's a trice micro (goes for quick google) Oh, I see, a recumbent
trike. Nice looking bit if kit. Not thought of in 1983 though so I
dunno how these rules relate except that the brakes rules apply to all
cycle with four or more wheels whatever the height of their wheels or
saddles. Trikes must obey cycle rules but with brakes on at least two
of the front wheels. ( a quadricycle or 5 wheeled cycle with three front
wheels would need brakes on only two of them!)

I don't write this stuff I just have to work with it.
--
Soloriens

 

[Richard]

jtaylor wrote:
> At the point of locking the maximum braking force is reached - then of
> course it is somewhat reduced as the wheel starts skidding. That's why I
> said "when the wheel LOCKS", not "when the wheel is LOCKED".

OK - you meant "the moment in time at which the wheel begins its state
of being locked".

Nobody was
> arguing that - instead it was the idea that somehow having more than one
> brake would allow a greater force.

Look at it from an energy perspective; the kinetic energy of the vehicle
has to be dissipated somehow, and that's mostly in heating up the brake
blocks and surfaces via friction. The more brake blocks you have (for
a given surface), the more energy you can dump into them in a given
amount of time - or, conversely, the more rapidly you can expend the
given energy, and thus quicker you can brake to a stop.

> This force is m * g * cot(theta), where theta is the angle between the
> c.o.g. and the horizontal - exceed that force and theta increases suddenly.
> I'd be interested to see how you think a term describing the number of
> brakes on the wheel could be inserted into that relationship.

First tell me how you can have an angle between a point and a line.

R.

 

[jtaylor]

"Richard" <[email protected]> wrote in message
news:[email protected]...
> jtaylor wrote:
> > At the point of locking the maximum braking force is reached - then of
> > course it is somewhat reduced as the wheel starts skidding. That's why
I
> > said "when the wheel LOCKS", not "when the wheel is LOCKED".
>
> OK - you meant "the moment in time at which the wheel begins its state
> of being locked".
>
> Nobody was
> > arguing that - instead it was the idea that somehow having more than one
> > brake would allow a greater force.
>
> Look at it from an energy perspective; the kinetic energy of the vehicle
> has to be dissipated somehow, and that's mostly in heating up the brake
> blocks and surfaces via friction. The more brake blocks you have (for
> a given surface), the more energy you can dump into them in a given
> amount of time - or, conversely, the more rapidly you can expend the
> given energy, and thus quicker you can brake to a stop.
>
> > This force is m * g * cot(theta), where theta is the angle between the
> > c.o.g. and the horizontal - exceed that force and theta increases
suddenly.
> > I'd be interested to see how you think a term describing the number of
> > brakes on the wheel could be inserted into that relationship.
>
> First tell me how you can have an angle between a point and a line.
>

The point of the angle is the contact patch.

Now how about the number of brakes?

 

[[email protected]]

Mark Thompson wrote:
> > What about on a recument *bi*cycle
>
> Yeah, it would be almost impossible to go over the bars, whilst those
with
> under seat steering are used to being over the bars anyway. Reason
#34 for
> buying a recumbent: So I can investigate the limits of front wheel
dual
> braking in perfect safety.

When I tried out a kmx-x and slammed on the brakes hard it endoed and
the back end started to swing around as soon as it left the ground.
There may be 'bents on which it is possible to brake as hard as you can
without worrying about handling but certainly not on all
best wishes
james

 

[Jon Senior]

bugbear wrote:
> Beat me - my main-means-of-transport fixed has cantilevers
> front and back (it was built in the 80's before discs were common)

Mine has long drop dual pivot front and a Weinmann centrepull rear
(Whose pads may just reach the rim!). I left the rear one on when I
built it because I was paranoid. I've only tried to use it once in the
year I've been riding it, and it did nothing at all. The only reason for
using it was I was trying to do too many things at once and had a
handful of water bottle as I reached a junction.

I think it'll probably remain, if only because the new (Very old
actually, but new to me) aero levers I've got, don't have springs in
them and rely on the brake to pull them taut.

Jon

 

[PiledHigher]

From the Australian road rules!

. Riding a bicycle
The rider of a bicycle must—
(a) sit astride the rider’s seat facing forwards (except if the bicycle is not
built to be ridden astride); and
(b) ride with at least 1 hand on the handlebars; and
(c) if the bicycle is equipped with a seat—not ride the bicycle seated in any
other position on the bicycle.
Penalty: 1 penalty unit.

 

[Jon Senior]

Mark Thompson wrote:
> Yeah, it would be almost impossible to go over the bars, whilst those with
> under seat steering are used to being over the bars anyway. Reason #34 for
> buying a recumbent: So I can investigate the limits of front wheel dual
> braking in perfect safety.

Pah. My homebuild (approx equivalent geometry to a Streetmachine) has
demonstrated two things.

1) On a recumbent, it _is_ possible to lock the front wheel on a dry,
smooth road.
2) In other conditions (I was going down hill when I discovered 1), it
is possible to lift the rear wheel.

In controlled conditions (Riding the length of the warehouse at work!),
I've managed to stop so hard that the rear not only lifts, but throws
you and follows you up, so you end up with the bike pointing nose down
at the floor.

[Mandatory Nanny State Disclaimer: Don't try this at home! Or at someone
else's home. Or at work. Or anywhere else. Forget you even read this post.]

Jon

 

[Mark Thompson]

> In controlled conditions (Riding the length of the warehouse at
> work!), I've managed to stop so hard that the rear not only lifts, but
> throws you and follows you up, so you end up with the bike pointing
> nose down at the floor.

Yay, reason #35!

 

[dkahn400]

Jon Senior wrote:

> In controlled conditions (Riding the length of the warehouse at
> work!), I've managed to stop so hard that the rear not only lifts,
> but throws you and follows you up, so you end up with the bike
> pointing nose down at the floor.

Could you work on this to get a stunt dismount so that you finish
standing with the bike on its end and one hand casually resting on the
seat?

--
Dave...

 

[Dave Larrington]

Richard Bates wrote:

> As some bicyclists are sufficienty skilled to pronounce the rear brake
> pretty useless, would there be any benefit gained by having two brakes
> on the front wheel. With modern equipment, the two brakes could be,
> say, a cantilever and a disc?

Common practice on (upright) trikes.

--

Dave Larrington - http://www.legslarry.beerdrinkers.co.uk/
World Domination?
Just find a world that's into that kind of thing, then chain to the
floor and walk up and down on it in high heels. (Mr. Sunshine)

 

[Danny Colyer]

Jon Senior wrote:
>>In controlled conditions (Riding the length of the warehouse at
>>work!), I've managed to stop so hard that the rear not only lifts,
>>but throws you and follows you up, so you end up with the bike
>>pointing nose down at the floor.

and dkahn400 wondered:
> Could you work on this to get a stunt dismount so that you finish
> standing with the bike on its end and one hand casually resting on the
> seat?

It's certainly possible to do it so that you finish standing with the
bike near vertical on the front wheel and both hands holding it up (I've
done it, although not intentionally).

Standing on end wouldn't be a great idea with a SWB bent, because that
basically means standing it on the chainring (probably not what you
meant ...). And (at least with a Street Machine with a pannier on the
back) holding it up casually with one hand just ain't gonna happen.

--
Danny Colyer (the UK company has been laughed out of my reply address)
<URL:http://www.speedy5.freeserve.co.uk/danny/>
"He who dares not offend cannot be honest." - Thomas Paine

 

[jtaylor]

"Richard" <[email protected]> wrote in message
news:[email protected]...

> Nobody was
> > arguing that - instead it was the idea that somehow having more than one
> > brake would allow a greater force.
>
> Look at it from an energy perspective; the kinetic energy of the vehicle
> has to be dissipated somehow, and that's mostly in heating up the brake
> blocks and surfaces via friction. The more brake blocks you have (for
> a given surface), the more energy you can dump into them in a given
> amount of time - or, conversely, the more rapidly you can expend the
> given energy, and thus quicker you can brake to a stop.

Not if the limit is determined by the geometry and weight - more brakes will
not increase that limit.