Cheap bright tail light

wrote:
SLIDING BACK is more dirt motion than road grip.


It is about not going over the bars when you have to reach into the
front brake hard.


... With road contact
motion to side, inside best but go with the flow. Adding a sideways
weight vector should stay a trip over bars into at worst a fly by.


A sideways slide? That sure would be impressive, coming to a stop in a
plume of blue smoke on the road, but I wouldn't dare. Plus a tire might
come off. On mountain bikes it may be needed, like with a friend whose
front brake faded out during the last 100ft of a steep downhill section.
He set it sideways and came to a stop in an impressive dust plume.

If you meant while still going in a straight line that would just make
you sail off the bike a few inches lower but you'd still go off and the
bike will likely endo as well. You can't get much of a CG shift this way.


Last endo on asphalt was at speed after a fine day cycling up and
down a canyon, with cliff, from inexperience with the setup grabbing
brake levers to lift front wheel over a speed bump.


Some European bikes had (have?) the front brake on the right. That can
scare the daylights out of a rider not used to it.

--
Regards, Joerg

http://www.analogconsultants.com/

On 9/24/2014 12:37 PM, Joerg wrote:
Frank Krygowski wrote:
On 9/24/2014 10:26 AM, Joerg wrote:
James wrote:
On 24/09/14 10:55, Joerg wrote:
Phil W Lee wrote:

[...]

You can't aquaplane a bicycle tyre in any circumstances short of fully
streamlined downhill record setting, so a slick tyre is always better.
All the gaps in the tread achieve is to allow it to squirm around and
reduce traction.

Of course, getting your weight back and down will help with ultimate
stopping power as well, so good luck outbraking a recumbent,
particularly if it's a lowrider design.


If I slide behind the seat I could unless the guy has disc brakes.
Today
I was behind a recumbent for the first time. Couldn't pass because his
bike was so wide but he rode at a pretty good clip.


Disc brakes may provide better modulation, but cannot alone prevent the
rear wheel from passing the front wheel.


That's why mountain bikers instinctively slide past the rear seat and
then downward. An instinct often developed in lots of steep downhill "Oh
s..t!" situations.

For those interested in quantifying the benefit:

Once semester before I retired from teaching, I decided to assign some
class exercises to bicycle problems. Some of them involved bike braking
systems. To that end, the students had to calculate the center of
gravity position of a bike+rider. Here's the method we used:

After measuring various bike dimensions, we used two bathroom scales on
the level lab floor to measure the tire loads. That allowed us to get
the X coordinate of the center of gravity. To get the Y coordinate, we
repeated but with the front wheel's scale elevated a bit, something like
a foot or so. I donated all my class notes to those who took over my
job, so I can't double check, but IIRC the COG turned out to be quite
close to the nose of the saddle. (BTW, that puts the COG to front tire
contact patch pretty close to 60 degrees above horizontal.)

The next part of the exercise was to do the belly-to-saddle position
Jeorg mentions, to see how much difference it made. As I recall, the
difference was not dramatic, and the improvement in maximum possible
deceleration was pretty slight.


Any experienced mountain biker will attest to the contrary. Have you
ever ridden a mountain bike hard? By hard I mean with emergency stops,
wildlife jutting out of the bush right in front of you, a few "Oh dang!"
moments after misjudging a diwnhill section steepness, and so on.

Even experienced road bikers slide behind the seat in an emergency
situation:

http://janheine.wordpress.com/2013/0...-on-a-bicycle/

Yes, I've ridden a mountain bike hard, and I've used the technique.
I've also taught cycling classes, which included practice at moving back
over the seat during hard braking. What I'm saying is that one can
quantify the benefits; and they're not as large as some people seem to
believe.


The only way to find out is actually doing it, attempting maximum
braking with and without sliding behind the seat. IME the effect is
stunning.

What's "moderate" for some may be "stunning" for others, I suppose. But
"The only way to find something out is by actually doing it" sounds like
a recipe for a trial-and-error approach to everything in life. There
are ways to apply more intelligence.

I'll work out some numbers later.

--
- Frank Krygowski

Frank Krygowski wrote:
On 9/24/2014 12:37 PM, Joerg wrote:
Frank Krygowski wrote:
On 9/24/2014 10:26 AM, Joerg wrote:
James wrote:
On 24/09/14 10:55, Joerg wrote:
Phil W Lee wrote:

[...]

You can't aquaplane a bicycle tyre in any circumstances short of
fully
streamlined downhill record setting, so a slick tyre is always
better.
All the gaps in the tread achieve is to allow it to squirm around
and
reduce traction.

Of course, getting your weight back and down will help with ultimate
stopping power as well, so good luck outbraking a recumbent,
particularly if it's a lowrider design.


If I slide behind the seat I could unless the guy has disc brakes.
Today
I was behind a recumbent for the first time. Couldn't pass because
his
bike was so wide but he rode at a pretty good clip.


Disc brakes may provide better modulation, but cannot alone prevent
the
rear wheel from passing the front wheel.


That's why mountain bikers instinctively slide past the rear seat and
then downward. An instinct often developed in lots of steep downhill
"Oh
s..t!" situations.

For those interested in quantifying the benefit:

Once semester before I retired from teaching, I decided to assign some
class exercises to bicycle problems. Some of them involved bike braking
systems. To that end, the students had to calculate the center of
gravity position of a bike+rider. Here's the method we used:

After measuring various bike dimensions, we used two bathroom scales on
the level lab floor to measure the tire loads. That allowed us to get
the X coordinate of the center of gravity. To get the Y coordinate, we
repeated but with the front wheel's scale elevated a bit, something like
a foot or so. I donated all my class notes to those who took over my
job, so I can't double check, but IIRC the COG turned out to be quite
close to the nose of the saddle. (BTW, that puts the COG to front tire
contact patch pretty close to 60 degrees above horizontal.)

The next part of the exercise was to do the belly-to-saddle position
Jeorg mentions, to see how much difference it made. As I recall, the
difference was not dramatic, and the improvement in maximum possible
deceleration was pretty slight.


Any experienced mountain biker will attest to the contrary. Have you
ever ridden a mountain bike hard? By hard I mean with emergency stops,
wildlife jutting out of the bush right in front of you, a few "Oh dang!"
moments after misjudging a diwnhill section steepness, and so on.

Even experienced road bikers slide behind the seat in an emergency
situation:

http://janheine.wordpress.com/2013/0...-on-a-bicycle/

Yes, I've ridden a mountain bike hard, and I've used the technique. I've
also taught cycling classes, which included practice at moving back over
the seat during hard braking. What I'm saying is that one can quantify
the benefits; and they're not as large as some people seem to believe.


Weird. Maybe you live in an area where the laws of physics are different
than in California.


The only way to find out is actually doing it, attempting maximum
braking with and without sliding behind the seat. IME the effect is
stunning.

What's "moderate" for some may be "stunning" for others, I suppose. But
"The only way to find something out is by actually doing it" sounds like
a recipe for a trial-and-error approach to everything in life. There
are ways to apply more intelligence.


Nope, it's the real "rubber-meets-the-road" test. It is intelligent to
apply tests as realistically as possible, ideally "in situ". Guess why
car manufacturers have test tracks?


I'll work out some numbers later.


But do it on the road, not in the garage. It's the only way to really
find out.

--
Regards, Joerg

http://www.analogconsultants.com/

Frank Krygowski writes:

On 9/24/2014 12:37 PM, Joerg wrote:
Frank Krygowski wrote:
On 9/24/2014 10:26 AM, Joerg wrote:
James wrote:
On 24/09/14 10:55, Joerg wrote:
Phil W Lee wrote:

[...]

You can't aquaplane a bicycle tyre in any circumstances short of fully
streamlined downhill record setting, so a slick tyre is always better.
All the gaps in the tread achieve is to allow it to squirm around and
reduce traction.

Of course, getting your weight back and down will help with ultimate
stopping power as well, so good luck outbraking a recumbent,
particularly if it's a lowrider design.


If I slide behind the seat I could unless the guy has disc brakes.
Today
I was behind a recumbent for the first time. Couldn't pass because his
bike was so wide but he rode at a pretty good clip.


Disc brakes may provide better modulation, but cannot alone prevent the
rear wheel from passing the front wheel.


That's why mountain bikers instinctively slide past the rear seat and
then downward. An instinct often developed in lots of steep downhill "Oh
s..t!" situations.

For those interested in quantifying the benefit:

Once semester before I retired from teaching, I decided to assign some
class exercises to bicycle problems. Some of them involved bike braking
systems. To that end, the students had to calculate the center of
gravity position of a bike+rider. Here's the method we used:

After measuring various bike dimensions, we used two bathroom scales on
the level lab floor to measure the tire loads. That allowed us to get
the X coordinate of the center of gravity. To get the Y coordinate, we
repeated but with the front wheel's scale elevated a bit, something like
a foot or so. I donated all my class notes to those who took over my
job, so I can't double check, but IIRC the COG turned out to be quite
close to the nose of the saddle. (BTW, that puts the COG to front tire
contact patch pretty close to 60 degrees above horizontal.)

The next part of the exercise was to do the belly-to-saddle position
Jeorg mentions, to see how much difference it made. As I recall, the
difference was not dramatic, and the improvement in maximum possible
deceleration was pretty slight.


Any experienced mountain biker will attest to the contrary. Have you
ever ridden a mountain bike hard? By hard I mean with emergency stops,
wildlife jutting out of the bush right in front of you, a few "Oh dang!"
moments after misjudging a diwnhill section steepness, and so on.

Even experienced road bikers slide behind the seat in an emergency
situation:

http://janheine.wordpress.com/2013/0...-on-a-bicycle/

Yes, I've ridden a mountain bike hard, and I've used the
technique. I've also taught cycling classes, which included practice
at moving back over the seat during hard braking. What I'm saying is
that one can quantify the benefits; and they're not as large as some
people seem to believe.


The only way to find out is actually doing it, attempting maximum
braking with and without sliding behind the seat. IME the effect is
stunning.

What's "moderate" for some may be "stunning" for others, I suppose.
But "The only way to find something out is by actually doing it"
sounds like a recipe for a trial-and-error approach to everything in
life. There are ways to apply more intelligence.

I'll work out some numbers later.

About a decade ago I carefully measured my center of mass
both on the saddle and behind the saddle, on a road bike,
then used this to compute the effect.

I see someone archived it on the web:

http://www.industrializedcyclist.com...-pitchover.pdf

The computed max declerations on level ground
are 0.63*g and 0.83*g. The difference is not insignificant.

--
Joe Riel

On 9/24/2014 5:41 PM, Joe Riel wrote:
Frank Krygowski writes:

On 9/24/2014 12:37 PM, Joerg wrote:
Frank Krygowski wrote:
On 9/24/2014 10:26 AM, Joerg wrote:
James wrote:
On 24/09/14 10:55, Joerg wrote:
Phil W Lee wrote:

[...]

You can't aquaplane a bicycle tyre in any circumstances short of fully
streamlined downhill record setting, so a slick tyre is always better.
All the gaps in the tread achieve is to allow it to squirm around and
reduce traction.

Of course, getting your weight back and down will help with ultimate
stopping power as well, so good luck outbraking a recumbent,
particularly if it's a lowrider design.


If I slide behind the seat I could unless the guy has disc brakes.
Today
I was behind a recumbent for the first time. Couldn't pass because his
bike was so wide but he rode at a pretty good clip.


Disc brakes may provide better modulation, but cannot alone prevent the
rear wheel from passing the front wheel.


That's why mountain bikers instinctively slide past the rear seat and
then downward. An instinct often developed in lots of steep downhill "Oh
s..t!" situations.

For those interested in quantifying the benefit:

Once semester before I retired from teaching, I decided to assign some
class exercises to bicycle problems. Some of them involved bike braking
systems. To that end, the students had to calculate the center of
gravity position of a bike+rider. Here's the method we used:

After measuring various bike dimensions, we used two bathroom scales on
the level lab floor to measure the tire loads. That allowed us to get
the X coordinate of the center of gravity. To get the Y coordinate, we
repeated but with the front wheel's scale elevated a bit, something like
a foot or so. I donated all my class notes to those who took over my
job, so I can't double check, but IIRC the COG turned out to be quite
close to the nose of the saddle. (BTW, that puts the COG to front tire
contact patch pretty close to 60 degrees above horizontal.)

The next part of the exercise was to do the belly-to-saddle position
Jeorg mentions, to see how much difference it made. As I recall, the
difference was not dramatic, and the improvement in maximum possible
deceleration was pretty slight.


Any experienced mountain biker will attest to the contrary. Have you
ever ridden a mountain bike hard? By hard I mean with emergency stops,
wildlife jutting out of the bush right in front of you, a few "Oh dang!"
moments after misjudging a diwnhill section steepness, and so on.

Even experienced road bikers slide behind the seat in an emergency
situation:

http://janheine.wordpress.com/2013/0...-on-a-bicycle/

Yes, I've ridden a mountain bike hard, and I've used the
technique. I've also taught cycling classes, which included practice
at moving back over the seat during hard braking. What I'm saying is
that one can quantify the benefits; and they're not as large as some
people seem to believe.


The only way to find out is actually doing it, attempting maximum
braking with and without sliding behind the seat. IME the effect is
stunning.

What's "moderate" for some may be "stunning" for others, I suppose.
But "The only way to find something out is by actually doing it"
sounds like a recipe for a trial-and-error approach to everything in
life. There are ways to apply more intelligence.

I'll work out some numbers later.

About a decade ago I carefully measured my center of mass
both on the saddle and behind the saddle, on a road bike,
then used this to compute the effect.

I see someone archived it on the web:

http://www.industrializedcyclist.com...-pitchover.pdf

The computed max declerations on level ground
are 0.63*g and 0.83*g. The difference is not insignificant.

Right, that's 32% better acceleration.

I didn't use your equations (which I don't remember seeing before) but I
worked it out for the numbers I gave earlier, with the assumption that
the center of mass is at 38" or 96.5 cm. That's higher than yours, and
admittedly it's based only on what I remember from the class exercise.
However, my measurements are for hands on hoods, not on the drops. I
think that's more reasonable. In a panic stop situation, riders are
unlikely to shift their hand position.

In any case, I got 0.66g deceleration for the normal position, and 0.79g
for the belly-to-the-saddle position, a 20% increase in deceleration.
It's an improvement, for sure.

One question, though, is how much of that is attainable during a panic
stop. As I said, I think few riders would shift their hand position to
the drops at the start of a true panic stop; they'd just squeeze the
levers from the hoods (especially since cantilevers and dual-pivots give
very hard braking from that position.) Similarly, I think very few
riders will get their belly to the saddle as I did. It was difficult
for me to do stationary in the basement, and probably much more
difficult in a "right now!!" emergency situation.



--
- Frank Krygowski

On 25/09/14 01:10, Frank Krygowski wrote:
On 9/24/2014 3:03 AM, Rolf Mantel wrote:
Am 24.09.2014 07:08, schrieb James:
On 24/09/14 12:28, Frank Krygowski wrote:

No matter what you have for brakes on your bike, your maximum
deceleration is limited by pitchover. That limit is determined by the
angle above horizontal of the line between the bike+rider center of
gravity and the front tire's contact patch.

I very much doubt that anyone on an upright bike can achieve a lower
angle than that of a recumbent, especially a long wheelbase one.

I would have thought there must come a point at which the long recumbent
doesn't transfer enough weight to the front wheel to prevent it from
locking up.

In that case, the recumbent (or tandem) can effectively use the rear
brake.

Generically, as soon as the angle pitchover angle as described above is
below 45 degrees (cars, motor bikes, tandems, recumbent bikes with lots
of luggage), deceleration is limited by traction (typically 1g
deceleration for the typical traction coefficient between tire and road
around 1). When the angle is above 45 degrees, deceleration is limited
by geometry.

That makes sense to me.

IIRC, Jobst Brandt and/or Sheldon Brown pointed out that it's even
trickier for a bike rider. They claimed most over-the-bars incidents
also involved the rider inadvertently cocking the front wheel to one
side, which must be easy to do when one must attempt to hold one's body
back by pushing forward on the handlebars.

Close to the point of pitchover, what's needed is a quick balancing act
on the front wheel. Nose wheelie experts can do it when they're
prepared, but I think few riders can do it spontaneously in an
emergency. IOW, it's tough to achieve even the theoretical maximum
deceleration.

That being said, I seldom need to brake very hard. My riding style is
pretty careful, and I devote a lot of attention to working out road
situations ahead of time - as in, "Don't ride near that guy; he's too
squirrely" or "Move further left so that driver notices me."


I must be a natural. I do a lot of city riding at night, and with other
riders. Sometimes there's a need to hit the brakes pretty hard. Like
when a pedestrian steps out, or a motorist turns unexpectedly.

Of course, we ride slower and take less risks in wet weather, because
brakes obviously don't work as well, but in the dry, it's happened a few
times that I've been braking on the front wheel, and notice it's
floating when it bumps back to earth. It happens fast. There's not
time to prepare.

It's like master Yoda said, "No. Try not. Do... or do not. There is no try."

--
JS

On 25/09/14 01:54, Frank Krygowski wrote:
On 9/24/2014 10:26 AM, Joerg wrote:
James wrote:
On 24/09/14 10:55, Joerg wrote:
Phil W Lee wrote:


Fitting tyres that are appropriate for the road instead of off-road
will help as well.
Something which is "very grippy" on mixed trails is almost by
definition far below optimal on the roads.


Well, my impression is that I can out-brake pretty much anything with
rim brakes. Also, my MTB tires have a much larger road contact area
than
my 25mm Conti Gatorskins. Hugely larger. So braking is much better but
of course that comes at the price of less efficient travel on asphalt.
However, I try to avoid asphalt anyhow because that wears out the tires
very quickly.

My impression is that traction is not the limiting factor when braking
in a straight line on the road on a bicycle, more the maximum braking
effort that can be applied before the rear wheel lifts off the ground
and follows and arc over the riders head.


Not if you slide behind the rear seat, if necessary hovering inches
above the rear wheel. That puts the center of gravity almost over the
rear axle und greatly decreases the angle at which the load pushes
towards the front axle. In fact so much that you have to watch out not
to skid the front tire, at least not too much.

The downside is that your belly slams into the back of the seat and if
you don't have strong abs that can hurt. But that's still better than
going over the handlebar or crashing into something. This way you can
perform an incredibly fast stop on a mountain bike.


You can't aquaplane a bicycle tyre in any circumstances short of fully
streamlined downhill record setting, so a slick tyre is always better.
All the gaps in the tread achieve is to allow it to squirm around and
reduce traction.

Of course, getting your weight back and down will help with ultimate
stopping power as well, so good luck outbraking a recumbent,
particularly if it's a lowrider design.


If I slide behind the seat I could unless the guy has disc brakes.
Today
I was behind a recumbent for the first time. Couldn't pass because his
bike was so wide but he rode at a pretty good clip.


Disc brakes may provide better modulation, but cannot alone prevent the
rear wheel from passing the front wheel.


That's why mountain bikers instinctively slide past the rear seat and
then downward. An instinct often developed in lots of steep downhill "Oh
s..t!" situations.

For those interested in quantifying the benefit:

Once semester before I retired from teaching, I decided to assign some
class exercises to bicycle problems. Some of them involved bike braking
systems. To that end, the students had to calculate the center of
gravity position of a bike+rider. Here's the method we used:

After measuring various bike dimensions, we used two bathroom scales on
the level lab floor to measure the tire loads. That allowed us to get
the X coordinate of the center of gravity. To get the Y coordinate, we
repeated but with the front wheel's scale elevated a bit, something like
a foot or so. I donated all my class notes to those who took over my
job, so I can't double check, but IIRC the COG turned out to be quite
close to the nose of the saddle. (BTW, that puts the COG to front tire
contact patch pretty close to 60 degrees above horizontal.)

The next part of the exercise was to do the belly-to-saddle position
Jeorg mentions, to see how much difference it made. As I recall, the
difference was not dramatic, and the improvement in maximum possible
deceleration was pretty slight.

I don't have a roomful of students to help me now - instead, just one
wife - so I didn't repeat the entire exercise. But my touring bike has
a 42" wheelbase and its saddle nose is about 38" high.

I did just now get wheel loads in standard riding position and in
belly-to-saddle position. Standard was 85 pounds front, 124 pounds
rear. Belly-to-saddle was 60 pounds front, 146 pounds rear. Yes, the
totals are off by 3 pounds, probably because it's difficult to keep
balanced against a radial arm saw table without inadvertently exerting
some vertical force. And the digital scale used in the rear gave only a
few seconds reading time before blinking out to save the battery.

I'm being called outside to enjoy the weather now. Someone where it's
cold and raining might want to assume a COG height of 38" (it will be
close) and see what the effect of the rearward shift will be on maximum
deceleration.

I wasn't going to bother. Us "roadies" don't know about mounting biker
stuff.

--
JS

Frank Krygowski writes:

On 9/24/2014 5:41 PM, Joe Riel wrote:
Frank Krygowski writes:

On 9/24/2014 12:37 PM, Joerg wrote:
Frank Krygowski wrote:
On 9/24/2014 10:26 AM, Joerg wrote:
James wrote:
On 24/09/14 10:55, Joerg wrote:
Phil W Lee wrote:

[...]

You can't aquaplane a bicycle tyre in any circumstances short of fully
streamlined downhill record setting, so a slick tyre is always better.
All the gaps in the tread achieve is to allow it to squirm around and
reduce traction.

Of course, getting your weight back and down will help with ultimate
stopping power as well, so good luck outbraking a recumbent,
particularly if it's a lowrider design.


If I slide behind the seat I could unless the guy has disc brakes.
Today
I was behind a recumbent for the first time. Couldn't pass because his
bike was so wide but he rode at a pretty good clip.


Disc brakes may provide better modulation, but cannot alone prevent the
rear wheel from passing the front wheel.


That's why mountain bikers instinctively slide past the rear seat and
then downward. An instinct often developed in lots of steep downhill "Oh
s..t!" situations.

For those interested in quantifying the benefit:

Once semester before I retired from teaching, I decided to assign some
class exercises to bicycle problems. Some of them involved bike braking
systems. To that end, the students had to calculate the center of
gravity position of a bike+rider. Here's the method we used:

After measuring various bike dimensions, we used two bathroom scales on
the level lab floor to measure the tire loads. That allowed us to get
the X coordinate of the center of gravity. To get the Y coordinate, we
repeated but with the front wheel's scale elevated a bit, something like
a foot or so. I donated all my class notes to those who took over my
job, so I can't double check, but IIRC the COG turned out to be quite
close to the nose of the saddle. (BTW, that puts the COG to front tire
contact patch pretty close to 60 degrees above horizontal.)

The next part of the exercise was to do the belly-to-saddle position
Jeorg mentions, to see how much difference it made. As I recall, the
difference was not dramatic, and the improvement in maximum possible
deceleration was pretty slight.


Any experienced mountain biker will attest to the contrary. Have you
ever ridden a mountain bike hard? By hard I mean with emergency stops,
wildlife jutting out of the bush right in front of you, a few "Oh dang!"
moments after misjudging a diwnhill section steepness, and so on.

Even experienced road bikers slide behind the seat in an emergency
situation:

http://janheine.wordpress.com/2013/0...-on-a-bicycle/

Yes, I've ridden a mountain bike hard, and I've used the
technique. I've also taught cycling classes, which included practice
at moving back over the seat during hard braking. What I'm saying is
that one can quantify the benefits; and they're not as large as some
people seem to believe.


The only way to find out is actually doing it, attempting maximum
braking with and without sliding behind the seat. IME the effect is
stunning.

What's "moderate" for some may be "stunning" for others, I suppose.
But "The only way to find something out is by actually doing it"
sounds like a recipe for a trial-and-error approach to everything in
life. There are ways to apply more intelligence.

I'll work out some numbers later.

About a decade ago I carefully measured my center of mass
both on the saddle and behind the saddle, on a road bike,
then used this to compute the effect.

I see someone archived it on the web:

http://www.industrializedcyclist.com...-pitchover.pdf

The computed max declerations on level ground
are 0.63*g and 0.83*g. The difference is not insignificant.

Right, that's 32% better acceleration.

I didn't use your equations (which I don't remember seeing before) but
I worked it out for the numbers I gave earlier, with the assumption
that the center of mass is at 38" or 96.5 cm. That's higher than
yours, and admittedly it's based only on what I remember from the
class exercise. However, my measurements are for hands on hoods, not
on the drops. I think that's more reasonable. In a panic stop
situation, riders are unlikely to shift their hand position.

In any case, I got 0.66g deceleration for the normal position, and
0.79g for the belly-to-the-saddle position, a 20% increase in
deceleration. It's an improvement, for sure.

One question, though, is how much of that is attainable during a panic
stop. As I said, I think few riders would shift their hand position
to the drops at the start of a true panic stop; they'd just squeeze
the levers from the hoods (especially since cantilevers and
dual-pivots give very hard braking from that position.) Similarly, I
think very few riders will get their belly to the saddle as I did. It
was difficult for me to do stationary in the basement, and probably
much more difficult in a "right now!!" emergency situation.

At 20 mph = 30 ft/sec, the stopping distance is 23.4 ft at 0.6g
and 17.6 ft at 0.8g. The difference is 5.8 ft. So you'd need
to make the switch in 5.8/30 = 0.2 seconds to save distance.
Not going to happen. For a real emergency situation, with
no time to prepare, the time required to move off the saddle,
unless you can do it while also braking, is counterproductive.


--
Joe Riel

On Wednesday, September 24, 2014 7:15:14 PM UTC-4, James wrote:
On 25/09/14 01:54, Frank Krygowski wrote:

On 9/24/2014 10:26 AM, Joerg wrote:

James wrote:

On 24/09/14 10:55, Joerg wrote:

Phil W Lee wrote:





Fitting tyres that are appropriate for the road instead of off-road

will help as well.

Something which is "very grippy" on mixed trails is almost by

definition far below optimal on the roads.





Well, my impression is that I can out-brake pretty much anything with

rim brakes. Also, my MTB tires have a much larger road contact area

than

my 25mm Conti Gatorskins. Hugely larger. So braking is much better but

of course that comes at the price of less efficient travel on asphalt.

However, I try to avoid asphalt anyhow because that wears out the tires

very quickly.



My impression is that traction is not the limiting factor when braking

in a straight line on the road on a bicycle, more the maximum braking

effort that can be applied before the rear wheel lifts off the ground

and follows and arc over the riders head.





Not if you slide behind the rear seat, if necessary hovering inches

above the rear wheel. That puts the center of gravity almost over the

rear axle und greatly decreases the angle at which the load pushes

towards the front axle. In fact so much that you have to watch out not

to skid the front tire, at least not too much.



The downside is that your belly slams into the back of the seat and if

you don't have strong abs that can hurt. But that's still better than

going over the handlebar or crashing into something. This way you can

perform an incredibly fast stop on a mountain bike.





You can't aquaplane a bicycle tyre in any circumstances short of fully

streamlined downhill record setting, so a slick tyre is always better.

All the gaps in the tread achieve is to allow it to squirm around and

reduce traction.



Of course, getting your weight back and down will help with ultimate

stopping power as well, so good luck outbraking a recumbent,

particularly if it's a lowrider design.





If I slide behind the seat I could unless the guy has disc brakes.

Today

I was behind a recumbent for the first time. Couldn't pass because his

bike was so wide but he rode at a pretty good clip.





Disc brakes may provide better modulation, but cannot alone prevent the

rear wheel from passing the front wheel.





That's why mountain bikers instinctively slide past the rear seat and

then downward. An instinct often developed in lots of steep downhill "Oh

s..t!" situations.



For those interested in quantifying the benefit:



Once semester before I retired from teaching, I decided to assign some

class exercises to bicycle problems. Some of them involved bike braking

systems. To that end, the students had to calculate the center of

gravity position of a bike+rider. Here's the method we used:



After measuring various bike dimensions, we used two bathroom scales on

the level lab floor to measure the tire loads. That allowed us to get

the X coordinate of the center of gravity. To get the Y coordinate, we

repeated but with the front wheel's scale elevated a bit, something like

a foot or so. I donated all my class notes to those who took over my

job, so I can't double check, but IIRC the COG turned out to be quite

close to the nose of the saddle. (BTW, that puts the COG to front tire

contact patch pretty close to 60 degrees above horizontal.)



The next part of the exercise was to do the belly-to-saddle position

Jeorg mentions, to see how much difference it made. As I recall, the

difference was not dramatic, and the improvement in maximum possible

deceleration was pretty slight.



I don't have a roomful of students to help me now - instead, just one

wife - so I didn't repeat the entire exercise. But my touring bike has

a 42" wheelbase and its saddle nose is about 38" high.



I did just now get wheel loads in standard riding position and in

belly-to-saddle position. Standard was 85 pounds front, 124 pounds

rear. Belly-to-saddle was 60 pounds front, 146 pounds rear. Yes, the

totals are off by 3 pounds, probably because it's difficult to keep

balanced against a radial arm saw table without inadvertently exerting

some vertical force. And the digital scale used in the rear gave only a

few seconds reading time before blinking out to save the battery.



I'm being called outside to enjoy the weather now. Someone where it's

cold and raining might want to assume a COG height of 38" (it will be

close) and see what the effect of the rearward shift will be on maximum

deceleration.



I wasn't going to bother. Us "roadies" don't know about mounting biker

stuff.



--

JS

Experience shows that moving weight rearward and/or down lets you brake a lot harder at speed on a steep downward grade.

Simple test. Hit a certain downward grade at a certain speed and hit tthe brakes hard whilst sitting on the saddle. Repeat with same grade and speed but shift rearward and/or downward. Notice difference in stopping distance and/or ease of stopping.

Cheers

On 25/09/14 07:41, Joe Riel wrote:
Frank Krygowski writes:

On 9/24/2014 12:37 PM, Joerg wrote:
Frank Krygowski wrote:
On 9/24/2014 10:26 AM, Joerg wrote:
James wrote:
On 24/09/14 10:55, Joerg wrote:
Phil W Lee wrote:

[...]

You can't aquaplane a bicycle tyre in any circumstances short of fully
streamlined downhill record setting, so a slick tyre is always better.
All the gaps in the tread achieve is to allow it to squirm around and
reduce traction.

Of course, getting your weight back and down will help with ultimate
stopping power as well, so good luck outbraking a recumbent,
particularly if it's a lowrider design.


If I slide behind the seat I could unless the guy has disc brakes.
Today
I was behind a recumbent for the first time. Couldn't pass because his
bike was so wide but he rode at a pretty good clip.


Disc brakes may provide better modulation, but cannot alone prevent the
rear wheel from passing the front wheel.


That's why mountain bikers instinctively slide past the rear seat and
then downward. An instinct often developed in lots of steep downhill "Oh
s..t!" situations.

For those interested in quantifying the benefit:

Once semester before I retired from teaching, I decided to assign some
class exercises to bicycle problems. Some of them involved bike braking
systems. To that end, the students had to calculate the center of
gravity position of a bike+rider. Here's the method we used:

After measuring various bike dimensions, we used two bathroom scales on
the level lab floor to measure the tire loads. That allowed us to get
the X coordinate of the center of gravity. To get the Y coordinate, we
repeated but with the front wheel's scale elevated a bit, something like
a foot or so. I donated all my class notes to those who took over my
job, so I can't double check, but IIRC the COG turned out to be quite
close to the nose of the saddle. (BTW, that puts the COG to front tire
contact patch pretty close to 60 degrees above horizontal.)

The next part of the exercise was to do the belly-to-saddle position
Jeorg mentions, to see how much difference it made. As I recall, the
difference was not dramatic, and the improvement in maximum possible
deceleration was pretty slight.


Any experienced mountain biker will attest to the contrary. Have you
ever ridden a mountain bike hard? By hard I mean with emergency stops,
wildlife jutting out of the bush right in front of you, a few "Oh dang!"
moments after misjudging a diwnhill section steepness, and so on.

Even experienced road bikers slide behind the seat in an emergency
situation:

http://janheine.wordpress.com/2013/0...-on-a-bicycle/

Yes, I've ridden a mountain bike hard, and I've used the
technique. I've also taught cycling classes, which included practice
at moving back over the seat during hard braking. What I'm saying is
that one can quantify the benefits; and they're not as large as some
people seem to believe.


The only way to find out is actually doing it, attempting maximum
braking with and without sliding behind the seat. IME the effect is
stunning.

What's "moderate" for some may be "stunning" for others, I suppose.
But "The only way to find something out is by actually doing it"
sounds like a recipe for a trial-and-error approach to everything in
life. There are ways to apply more intelligence.

I'll work out some numbers later.

About a decade ago I carefully measured my center of mass
both on the saddle and behind the saddle, on a road bike,
then used this to compute the effect.

I see someone archived it on the web:

http://www.industrializedcyclist.com...-pitchover.pdf

The computed max declerations on level ground
are 0.63*g and 0.83*g. The difference is not insignificant.


You might also find that on a MTB the riding position is slightly
different. Some also slam the seat down prior to a descent, so lower
the CG and make pitching over less likely.

--
JS