Braking Distance of Cycles

On Tue, 17 Aug 2010 09:09:53 +0100, Roland Perry
wrote:

In message
, at
17:05:48 on Mon, 16 Aug 2010, thirty-six
remarked:
There are no figures, each machine and person is individual, the
advice is to never go so fast as you cannot stop within the distance
you can see to be clear on your own side of the road. This takes
account of machine, rider and conditions which are all VARIABLE.

Including whether it's uphill or downhill; which by observation has a
much more significant effect upon cycles than motor vehicles.

Indeed - I wonder what the approx stopping distance would be for a
cyclist coming down a hill at 40mph in the wet.


--
Latest accident figures from DfT Q1 2010
Killed or Seriously Injured:

Pedestrians DOWN 8%
Car Users DOWN 8%
Motorcycle users DOWN 8%
Cyclists UP : 4%

Spot a theme?

In article ,
Alan Braggins wrote:
In article , Roland Perry wrote:
In message , at 09:32:56
on Tue, 17 Aug 2010, chris French
remarked:

IME, it doesn't make a lot of difference, if anything, a child in a
rear mounted seat improves matters as you have more weight where you
want it over the rear wheel. Yes, it is high up, but it's also further
back, which makes a big difference. Even just moving your weight back
by slipping off the back of the saddle helps.

I wasn't really asking about the physics - more the psychology.

But presumably the physics of a child sat between the saddle and the
handlebars is worse than if they weren't there?

I'll bet you've never seen a cyclist with an occupied child seat doing
anywhere close to the 20mph under discussion, which will have a more
significant effect on the braking distance than the weight shift.

I have. I used to do 15+ MPH with one. But those were child seats
on the rear carrier, of course, though I have once seen a cyclist
doing c. 18 MPH with two children, one on a seat on the top tube.


Regards,
Nick Maclaren.

In message , Nick Maclaren
writes
In article ,
Paul - xxx wrote:
On 17/08/2010 09:32, chris French wrote:
Even just moving your weight back by
slipping off the back of the saddle helps.

Actually that's a well known technique for stopping more quickly than
'normal'. Used a lot in off-road mountain biking .. the sudden weight
transfer to the rear helps maintain balance and control and
significantly shortens the braking distance.

If you learn to use the technique correctly and quickly, it also
delivers benefits on-road too.

Well, yes and no. It's hard to do unless you have your saddle
low, which has other problems - that's not a problem for off-road
'mountain biking', as people do use low saddles for that.


I do it on most of my bikes, which ave the saddle set at the appropriate
height. With the cranks horizontal standing up gives plenty of clearance

There is also the point that, while you are doing it, you are
accelerating the bicycle hard forward, and so have to delay braking
or risk losing control. Even at 0.5 seconds, that's 5 yards at
20 MPH.


Well yes, for really emergency braking it's probably not of much use as
there would be the time. But useful in of other situations.

A more serious problem is that, if you crash, you will be rammed
hard in the abdomen by the saddle, with the consequent risk of
rupturing your spleen or doing similar damage. That could turn a
survivable accident into a fatality.


Probably depends on the extent to which you do it. On road I generally
just move back, and maybe down a bit, but so much so as to be in a
positron to ram the saddle into my abdomen.

Off road people do sometimes take it to more extremes, but in those
situation, probably just more likely to fall off in heap than ram front
first into something
.

--
Chris French

In article ,
chris French wrote:

Even just moving your weight back by
slipping off the back of the saddle helps.

Actually that's a well known technique for stopping more quickly than
'normal'. Used a lot in off-road mountain biking .. the sudden weight
transfer to the rear helps maintain balance and control and
significantly shortens the braking distance.

If you learn to use the technique correctly and quickly, it also
delivers benefits on-road too.

Well, yes and no. It's hard to do unless you have your saddle
low, which has other problems - that's not a problem for off-road
'mountain biking', as people do use low saddles for that.

I do it on most of my bikes, which ave the saddle set at the appropriate
height. With the cranks horizontal standing up gives plenty of clearance

THE appropriate height? There is a range, depending on your riding
style and other factors. There is also the issue of how wide the
saddle is. Anyway, it's hard to do for many people, whether or not
it is for you.

I could do it with some setups but not others.

A more serious problem is that, if you crash, you will be rammed
hard in the abdomen by the saddle, with the consequent risk of
rupturing your spleen or doing similar damage. That could turn a
survivable accident into a fatality.

Probably depends on the extent to which you do it. On road I generally
just move back, and maybe down a bit, but so much so as to be in a
positron to ram the saddle into my abdomen.

I like the idea of sitting on a positron :-)

Yes. That doesn't improve the braking much, but doesn't cost anything,
either, so is a winner. It might change 0.35 g into 0.4 g, perhaps
even 0.45 g, but no more.


Regards,
Nick Maclaren.

Nick Maclaren wrote:
In article ,
Tony Raven wrote:
I have used drum brakes, which are largely unaffected by wet. As
Bicycling Science points out, an upright cyclist will go over the
handlebars at 0.5-0.7 g, depending on the bicycle, rider and style.
However, that DOESN'T mean that you can brake at that, safely,
because a bicycle becomes uncontrollable when the weight on the rear
wheel reduces enough that it starts to skid. A maximum safe
deceleration of 0.35-0.5 g is a reasonable figure, comparable with
the Highway Code level of conservatism.
Not true, a bicycle is perfectly controllable and you can brake right up
to the point where the rear wheel has lifted clear of the ground.
Cyclists can do it and you will see motorcyclists doing it too (known as
a stoppie)
http://en.wikipedia.org/wiki/Stoppie

Nonsense. Some cyclists may be able to, but many (almost certainly
most) can't.

If I can then most that are likely to be riding at 20mph can. It
doesn't take much practice to get the grip of it.

Also, a lot of emergency stops are when part-way
through a turn, which is MUCH harder, and I have seen a lot of
cyclists coming unstuck while trying to do that.


That's moving the goal posts. We were talking about maximum braking
which is taken as in a straight line. Try emergency braking a car when
partway through a turn and you won't get maximum deceleration either
without something untoward happening.

Brake turns are HARD to get right, and most cyclists are likely
to come unstuck doing them. Also they are fairly likely to
side-swipe the wheels from under a cyclist parallel to them, and
I have seen THAT happen, too. They are NOT something that should
be done in traffic, except in EXTREME emergency.

More goal post shifting from the point under discussion. But ditto cars.


The Highway Code figures are for an average driver
and a reasonable margin of safety; comparing them with an extreme
athlete and a merely probable avoidance of a crash is ridiculous.


And the Highway Code distances are in a straight line braking. But its
nice to be thought of as an extreme athlete at my age :-)

Tony

Roland Perry wrote:
In message , at 10:59:47 on Tue, 17 Aug
2010, Tony Raven remarked:
But presumably the physics of a child sat between the saddle and the
handlebars is worse than if they weren't there?

Probably about the same difference as if you were/weren't wearing a
rucksack.

Do you wear rucksacks on your belly?


No but the difference between having one on your back and not on your
back is probably about the same as having or not having a child sat on
the crossbar.

Tony

chris French wrote:

Off road people do sometimes take it to more extremes, but in those
situation, probably just more likely to fall off in heap than ram front
first into something


Usually when going downhill and it has the added advantage that if it
starts to go pear shaped you can let the bike go out from underneath you
leaving you standing (?) and the bike to crash on its own.

Tony

JMS wrote:
On Mon, 16 Aug 2010 08:55:59 -0700 (PDT), thirty-six
wrote:

On 16 Aug, 12:44, JMS wrote:
On Sun, 15 Aug 2010 20:12:53 +0000 (UTC), Mike P
wrote:

snip

Thanks

Is it neccessary to read it to be a safe rider, if one is already
in possesion of common sense? - a serious question..

So we have the following stopping distances:

Car - 20 mph dry 12 metres
Cycle : 17 metres

Car : 20mph wet 14 metres
Cycle : 41 metres

Ho ho, these figures are way out unless the machine is sub-standard
and the rider/driver is drunk or otherwise compromised.


So you can quote some more authoritative figures can you?

Give them a little time & someone will make some up for you.


--
Dave - intelligent enough to realise that a push bike is a kid's toy, not a
viable form of transport.

On Tue, 17 Aug 2010 17:48:58 +0100
Tony Raven wrote:

Usually when going downhill and it has the added advantage that if it
starts to go pear shaped you can let the bike go out from underneath you
leaving you standing (?) and the bike to crash on its own.


Sheldon Brown recommended this as a way of dismounting from a fixed.
Never tried it myself, but I suppose it's very similar to dismounting
from an "ordinary".

http://www.sheldonbrown.com/fixed.html#mounting



Mike

DavidR wrote:
"JMS" wrote
On Sun, 15 Aug 2010 20:12:53 +0000 (UTC), Mike P
wrote: snip

Thanks

Is it neccessary to read it to be a safe rider, if one is already in
possesion of common sense? - a serious question..

So we have the following stopping distances:

Car - 20 mph dry 12 metres
Cycle : 17 metres

Looks like Cycle craft have completely cocked up (or MattB can't
read). The Highway Code makes that from 6m "thinking distance" plus a
vehicle with brakes of 67% efficiency. Probably optimistic for a car
starting from 20mph but if starting from a higher speed (so that
brakes are fully applied and warmed) it should be able to cover the
last 20mph in about 4-5m.
The cyclist "thinking distance" must be the same ......


Judging by the cyclists around here it would be much longer...



--
Dave - intelligent enough to realise that a push bike is a kid's toy, not a
viable form of transport.