if you wanted maximum braking, where would you sit?

wle wrote:

if you could position your center of gravity anywhere, to ensure
maximum braking power, where would it be?

obviously over the front wheel is no good, you would flip.

there is a point, leaning either ahead of the front wheel, or
behind the back wheel, that the opposite wheel is off the ground.

clearly those are 2 limits, the answer must lie between them.

if there were very little friction, it would hardly matter.

assume a level road, brakes that can cause a skid no matter what.

ok, so where do you sit?

state assumptions, like coefficient of friction between tire and road,
weight of bike and rider.

show your work.

now for extra credit, make it a function of road slope.

As you still have to hold on to the bars, throwing yourself back off the
saddle is the best you can do. The back wheel still skids but the bike
shouldn't cartwheel if you go back and low enough. Failing that, slide
the bike sideways at 90 degrees to the direction of travel. This may hurt.

Sheldon Brown wrote in message If the angle is shallower than the critical angle, locking up the front
wheel will cause it to skid. This is the case for tandems and many
recumbents.

Have you ever skidded a front tire on clean dry pavement with a
tandem? Or has anyone else?

I have never been able to skid the front tire on my tandem on clean
dry pavement despite practiing emergency stops to the point where I
worried about the fork failing, though I haven't bent a fork or
observed any obvious damage from this maneuver. I know Chalo's bent
plenty of forks but I'd like to see him do that with my Santana.

Bill Putnam, who on second thought wouldn't like to see Chalo bend the
fork on his tandem even if he could.

David Damerell wrote:

It is extremely unlikely that front brake will cause a skid no matter
what. If tyre traction is that poor then that is evidently the limiting
factor on braking, and the CoG may roam freely.

I've done long 2-wheel skids on dry tarmac. A panic snatch of the brake
will get it sliding, after which the friction is greatly reduced.
Steering is *difficult*.

On Wed, 07 Jan 2004 19:54:47 -0600, Tom Sherman
wrote:
I would sit near or below the level of the front axle to reduce or
elminate the tendency for the rear wheel to lift off of the ground, and
I would also be near the rear wheel for more favorable weight
distribution for braking. This describes a recumbent lowracer. Braking
can be improved by adding a second wheel in front (tadpole trike) as
weight transfers forward to two contact patches during braking, and
locking one or both front wheels under braking will not lead to a loss
of balance.

I imagine that all of those conditions would result in fishtailing
under very hard braking. Is that the case?

Tom Sherman - Quad Cities
--
Rick "Sounds like fun!" Onanian

Zog The Undeniable wrote:

It is extremely unlikely that front brake will cause a skid no matter
what. If tyre traction is that poor then that is evidently the
limiting factor on braking, and the CoG may roam freely.

I've done long 2-wheel skids on dry tarmac.

Dry tarmac coated with dust or gravel? Otherwise I find it hard to
believe. A panic snatch on my bikes will result in me going over the
bars.

~PB

Zog The Undeniable wrote:
David Damerell wrote:
It is extremely unlikely that front brake will cause a skid no matter
what. If tyre traction is that poor then that is evidently the limiting
factor on braking, and the CoG may roam freely.
I've done long 2-wheel skids on dry tarmac.

With slick tyres? That seems highly unlikely.

[Note the words "no matter what" copied directly from the original post,
so I think I can specify tyre design.]

If I panic snatch the front brake I get a free flying lesson.
--
David Damerell Kill the tomato!

RE/
ok, so where do you sit?

I can't imagine sitting when braking as hard as I can. I'd be afraid of
endo-ing.

It's more like crouching with my butt somewhere between the back of the saddle
and the rear axle.
--
PeteCresswell

Rick Onanian wrote:

On Wed, 07 Jan 2004 19:54:47 -0600, Tom Sherman
wrote:

I would sit near or below the level of the front axle to reduce or
elminate the tendency for the rear wheel to lift off of the ground, and
I would also be near the rear wheel for more favorable weight
distribution for braking. This describes a recumbent lowracer. Braking
can be improved by adding a second wheel in front (tadpole trike) as
weight transfers forward to two contact patches during braking, and
locking one or both front wheels under braking will not lead to a loss
of balance.


I imagine that all of those conditions would result in fishtailing
under very hard braking. Is that the case?

Fishtailing only happens on the recumbent lowracer [1] when the rear
wheel is locked up, and this takes considerably more braking force than
it does to lock up the rear wheel on an upright.

My trike [2] has two front brakes (Avid mechanical disc on each wheel -
one lever for each brake) and no rear brake. Stopping quickly takes as
much skill as stopping an automobile without ABS and the brakes biased
towards the front.

Subjectively, the stopping power does not feel as great as it is, since
the rider is naturally braced by his/her legs being on the pedals,
unlike an upright bike where bracing is done with the (much weaker) arms.

[1] This does not apply to all recumbent bikes. Some are poorly designed
with a high seat and too much weight on the front wheel, and may have
braking performance that is worse than an upright bicycle.
[2] http://home.mindspring.com/~kb7mxu/i...ragonFlyer.jpg This
is the same model as my trike.

Tom Sherman - Quad Cities

stu wrote:

That can be circumvented. Build a bike that positions your center of
gravity below the front axle. You could lock the wheel up tight and
still not do an endo.

No. The important dimension is where the center of mass is in relation
to the front wheel's contact patch. An endo is possible with the weight
below the axle, although not likely because of insufficient traction.

short version
a locked front wheel does not undo you, you skid and fall of sideways.

As an owner of a bicycle with a 12-inch (30.5 cm) seat height, I can
verify this (at least on a dry, broom finish Portland cement concrete
surface, which is certainly a high traction situation). Something beyond
friction between the tire and the pavement surface (such as wedging the
wheel in a storm drain grate) would have to occur to get the rear wheel
off the ground.

Tom Sherman - Quad Cities

Sheldon Brown wrote:

Tim McNamara wrote:

Under "maximum braking" the front wheel stops dead, the bicycle
flips, and the rider is ejected. If you are talking about keeping
both wheels on the ground that is far less braking force than
maximum.


That can be circumvented. Build a bike that positions your center of
gravity below the front axle. You could lock the wheel up tight and
still not do an endo.


It actually has nothing to do with the axle, since a locked-up wheel
doesn't rotate, so it effectively stops being a wheel.

The critical thing is the angle of a line drawn from the tire contact
patch to the center of mass of the bike-and-rider.

If this angle is steeper than a critical value, locking up the front
wheel will cause an endo. This is the case for typical upright bikes.

If the angle is shallower than the critical angle, locking up the front
wheel will cause it to skid. This is the case for tandems and many
recumbents.

The value of the critical angle depends on the coefficient of friction
between the tire and the road surface.

For vehicles where the angle is steep, the rear brake is useless at
maximal braking.

For vehicles where the angle is shallow, both brakes are needed to
achieve maximal braking.

Note that the _length_ of the line doesn't matter, only the angle.

A good example of this would be an ordinary bicycle, which has a long
line from the contact, patch to the combined rider/bicycle center of
mass that is at a steep angle (from horizontal). It is the general
consensus that these bikes are relatively easy to do "endos" on.

Tom Sherman - Quad Cities