if you wanted maximum braking, where would you sit?

Bill Putnam wrote:

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 locked up both front wheels on my recumbent tadpole trike. It
should be noted that I have Avid mechanical disc brakes that are
designed for use on wheels of about 27" (~68 cm) in diameter on wheels
that are about 20" (~51 cm) in diameter.

Tom Sherman - Quad Cities

You won't be able to get butt far back enough and still reach the bars on
a road bike to skid the front wheel (on clean dry road).

True, but braking on a road bike isn't usually as nearly effective as on an
MTB because you can't really pull big leverage while on the hoods.

--
Phil, Squid-in-Training

"S o r n i" wrote in message
...
Cockpit of a 747?

Bill "landing, that is" S.

Interesting choice. Maximum braking forces occur during taxiing before
takeoff and have been known to put a wrinkle in the forward fuselage
crown skin panels. Braking forces on most commercial airplanes are
limited by system design but generally this is the load case
(compression buckling) that determines the skin gage of the upper
panels.

Phil Holman

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?


Rick "Sounds like fun!" Onanian

No, it isn't the case on any of my trikes. Yes, it is fun.

--
Ted Bennett
Portland OR

We skid our front wheels all the time on our icebikes ( studded
tires).. lock up the front wheel and continue to pedal.. the rear
wheel has more than enough traction on ice to move the bike forward
with the studs scratching the ice while being locked up.. But it takes
practice keeping the front wheel straight... a little movement in the
handlebars and down you go. Wheelies on ice is another way to put some
excitement in your life.

charlie

Sheldon Brown wrote in message ...
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.


I think you are correct because the OP misstated the problem. In
reality however Tim M is correct. In practice, when we go over
the bars it is not [usually] because the front wheel stops and
the bike and rider pivot around the contact patch. More often
we go over the bars because the frame and fork and rider and rear
wheel pivot around the front axle.

If you build a bike like Tim says, you are right that it could
still endo around the contact patch. However, such a bike would
not endo around the front axle. I have seen riders endo after
dropping the front wheel in a pothole. Tim's bike would not do
that. The rider would come off the seat in an even more painful
way.

Doug

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.
wle.


The bikes two tires will have their maximum adhesion sum if weight unde
braking is evenly distributed on front and rear tires. The more weigh
on the front, the more front braking, the less rear braking and vic
versa. The endo issue occurs when the moment caused by the decel of th
bike+rider about the front contact patch exceeds the product of distanc
the combination cg trails the front patch times the mass- a point a
which the weight is entirely on the front tire. If selecting positio
for maximum tire adhesion, you’ve already removed endo as an issue

The front/rear weight distribution of the bike under constant speed i
not the same as the weight distribution under deceleration. Unde
decell, a moment is created which is the product of the height of th
combined (bike+rider) cg above the road times the mass of th
rider+bike. That moment will shift weight rear tire to front tire b
the moment/(cg distance behind the front contact patch). The lower yo
are, the less forward shift in weight under braking. The further bac
you are, the more weight on the rear to start with and you ca
distribute the weight more evenly under decel to a point. Beyond tha
point, you have too much weight on the rear and the front tire i
underutilized in braking

The optimal point will vary on a given bike on level ground vs
downhill (although your posted question specified level ground)l, an
since the traction varies from surface to surface, the optimal poin
will vary as well. In fact, the optimal position is not a constant on
given bike as you have to factor in the aero drag of the rider at sa
30mph vs. 10 mph

If you neglect the air drag, since it will be a minority component i
the calculation, the optimal position for a max G decel will be
continuous series of points along a path going back and upwards
although lower and further back than typical seat positions

I started to do some calculations assuming a bike decels perhaps twic
as fast as a street car but calculated endos at much lower g’s tha
that. So before proceeding further, does anyone know typical best dece
rates on bikes in g’s? or times or distance from 20-0mph


-

(Bill Putnam) wrote:

Sheldon Brown wrote:
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.

The implication is that you *can* use even more braking power than
that before absolute limits apply. Tandems can often stop much harder
than singles, though, and shorter stops (than you can get already)
might not be very useful.

I face this issue sometimes, where the real limitation on my braking
is how much I can stomach the fork bending back down there. More
often, polluted braking surfaces or less-than-optimal brakes impose a
limit first.

I took my rain bike on my recent holiday vacation, where I discovered
that stopping on dry downhills steeper than about 10% causes me to max
out the drum brakes and call for divine intervention. I would not
mind if _those_ brakes taxed the fork's strength a little more!

Chalo Colina

On the road I've skidded my front wheel in a panic stop in the following
postion...

Right foot clipped in
Left foot out front lower than the front axel
No appreciable rear brake, the rear wheel about 1ft off the ground
Front brake locked
Rear of truck in front of me going from 45mph to zero, blue smoke as it starts
to jack-knife
Me encountering the rear liftgate at less than 1mph, no damage to me or the bike

Clearly my CofG had most of my weight pressing straight down on the front wheel,
my left foot lower than the axel provided additional downward force to
counteract a total flipover.

My skinny slick sew-ups provided the maximum friction and slip enabling me to
skid, stop and not flip over.

JeffP....

"wle" wrote in message
om...
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.

wle.

RE/
We skid our front wheels all the time on our icebikes ( studded
tires).. lock up the front wheel and continue to pedal.. the rear
wheel has more than enough traction on ice to move the bike forward
with the studs scratching the ice while being locked up.. But it takes
practice keeping the front wheel straight.

That sounds like good practice for surviving front wheel washout in regular
riding.
--
PeteCresswell