Fixed gear skidding and pedal force

Hi All,

I have been riding my fixed gear bike exclusively for the past few
months. I have a brake which is reserved for emergencies. I have not
had any emergencies yet, but out of laziness I have used the brake a
few times. Otherwise I just slow down by resisting the pedals. I don't
live somewhere where I need to make short stops, so I don't "skip" or
skid my wheel. Just for laughs I have tried, but I can't really do it.
Even back in the days riding a track bike around in NYC, I never really
could do it.

So I am wondering about how my weight and gearing affect my (in)ability
to skid. I can of course effect a skid by leaning all the way forward,
but I am more concerend about stopping than seeing how far I can skid.

Traction is a function of downward force and friction. Assuming normal
road surface, with a weight of 100kg, a speed of 25km/h and a 48x18
gear on 165mm cranks, how much force is required at the pedals to stop
the wheel? What difference would it be with say 70kg, 175mm, and 42x18?

Tips on how to figure this out appreciated. I don't really care about
the answer, I am just interested in the physics of it.

Joseph

On 19 Nov 2006 11:33:11 -0800, wrote:

Hi All,

I have been riding my fixed gear bike exclusively for the past few
months. I have a brake which is reserved for emergencies. I have not
had any emergencies yet, but out of laziness I have used the brake a
few times. Otherwise I just slow down by resisting the pedals. I don't
live somewhere where I need to make short stops, so I don't "skip" or
skid my wheel. Just for laughs I have tried, but I can't really do it.
Even back in the days riding a track bike around in NYC, I never really
could do it.

So I am wondering about how my weight and gearing affect my (in)ability
to skid. I can of course effect a skid by leaning all the way forward,
but I am more concerend about stopping than seeing how far I can skid.

Traction is a function of downward force and friction. Assuming normal
road surface, with a weight of 100kg, a speed of 25km/h and a 48x18
gear on 165mm cranks, how much force is required at the pedals to stop
the wheel? What difference would it be with say 70kg, 175mm, and 42x18?

Tips on how to figure this out appreciated. I don't really care about
the answer, I am just interested in the physics of it.

Joseph

Dear Joseph,

These might help you get started:

http://www.glenbrook.k12.il.us/gbssc...energy/cs.html

http://www.physicsforums.com/archive.../t-120420.html

http://www.stoptech.com/tech_info/Th...%20Systems.pdf

http://www.msgroup.org/forums/mtt/to...p?TOPIC_ID=172

Cheers,

Carl Fogel

On 19 Nov 2006 11:33:11 -0800, wrote:

Tips on how to figure this out appreciated. I don't really care about
the answer, I am just interested in the physics of it.

Before wasting your time trying to figure it out, consider that for a
conventional bicyle as well as most other wheeled vehicles, when
braking force is applied and deceleration begins to occur, the
downward vector shifts forward; there is "weight transfer" to the
front wheel. If enough braking effort is applied to the rear wheel,
there will be a point reached at which sufficient forward weight
transfer will occur to reduce the traction of the rear wheel to the
level at which it goes into a skid...and at that point, the weight
transfer is partially reversed *but the braking effectiveness is still
lost because the wheel is in a skid*. Thus, the place that braking
needs to be able to occur for safety is in the front wheel, *no matter
how effective the rear brake may be*.

Rear braking capability is not the key to stopping safely in an
emergency. Front braking capability rules the day in this.
--
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Werehatrack wrote:
On 19 Nov 2006 11:33:11 -0800, wrote:

Tips on how to figure this out appreciated. I don't really care about
the answer, I am just interested in the physics of it.

Before wasting your time trying to figure it out, consider that for a
conventional bicyle as well as most other wheeled vehicles, when
braking force is applied and deceleration begins to occur, the
downward vector shifts forward; there is "weight transfer" to the
front wheel. If enough braking effort is applied to the rear wheel,
there will be a point reached at which sufficient forward weight
transfer will occur to reduce the traction of the rear wheel to the
level at which it goes into a skid...and at that point, the weight
transfer is partially reversed *but the braking effectiveness is still
lost because the wheel is in a skid*. Thus, the place that braking
needs to be able to occur for safety is in the front wheel, *no matter
how effective the rear brake may be*.

Rear braking capability is not the key to stopping safely in an
emergency. Front braking capability rules the day in this.

While the static coefficient of friction between the tire and the
pavement is greater than the dynamic, the latter is not zero. Braking
force is not decreased that much at the rear wheel due to skidding. The
real issues are controlling the bike directionally and maintaining
balance once the rear wheel starts to skid.

--
Tom Sherman - Post Free or Die!

Werehatrack wrote:
On 19 Nov 2006 11:33:11 -0800, wrote:

Tips on how to figure this out appreciated. I don't really care about
the answer, I am just interested in the physics of it.

Before wasting your time trying to figure it out, consider that for a
conventional bicyle as well as most other wheeled vehicles, when
braking force is applied and deceleration begins to occur, the
downward vector shifts forward; there is "weight transfer" to the
front wheel. If enough braking effort is applied to the rear wheel,
there will be a point reached at which sufficient forward weight
transfer will occur to reduce the traction of the rear wheel to the
level at which it goes into a skid...and at that point, the weight
transfer is partially reversed *but the braking effectiveness is still
lost because the wheel is in a skid*. Thus, the place that braking
needs to be able to occur for safety is in the front wheel, *no matter
how effective the rear brake may be*.

Rear braking capability is not the key to stopping safely in an
emergency. Front braking capability rules the day in this.

I am quite aware of this weight transfer. In a thread called "Heat
resistant tubular glue?" a while back I figured that riders have easily
over 90% of their weight tranferred under heavy braking.

But I don't know anything about the friction of the surface, and what
sort of limitations this imposes. Front-wheel braking is more or less
limited to about .5G due to weight transfer and the size and
configuration of regular bikes. Much more than half a G and weight
transfer goes over 100% and you flip over the bars. Rear braking is
limited by traction. I wonder how close to .5G one can get with a back
brake?

This is all theoretical for me. I have a front brake on my bike. These
are just things I ponder as I ride around with nothing more exciting
than the occasional cow in the road to distract me.

Joseph

wrote:
On 19 Nov 2006 11:33:11 -0800, wrote:

Hi All,

I have been riding my fixed gear bike exclusively for the past few
months. I have a brake which is reserved for emergencies. I have not
had any emergencies yet, but out of laziness I have used the brake a
few times. Otherwise I just slow down by resisting the pedals. I don't
live somewhere where I need to make short stops, so I don't "skip" or
skid my wheel. Just for laughs I have tried, but I can't really do it.
Even back in the days riding a track bike around in NYC, I never really
could do it.

So I am wondering about how my weight and gearing affect my (in)ability
to skid. I can of course effect a skid by leaning all the way forward,
but I am more concerend about stopping than seeing how far I can skid.

Traction is a function of downward force and friction. Assuming normal
road surface, with a weight of 100kg, a speed of 25km/h and a 48x18
gear on 165mm cranks, how much force is required at the pedals to stop
the wheel? What difference would it be with say 70kg, 175mm, and 42x18?

Tips on how to figure this out appreciated. I don't really care about
the answer, I am just interested in the physics of it.

Joseph

Dear Joseph,

These might help you get started:

http://www.glenbrook.k12.il.us/gbssc...energy/cs.html

http://www.physicsforums.com/archive.../t-120420.html

http://www.stoptech.com/tech_info/Th...%20Systems.pdf

http://www.msgroup.org/forums/mtt/to...p?TOPIC_ID=172

Cheers,

Carl Fogel

Ok, so traction (friction) is linear. This means my 100kg stick to the
road twice as much as a 50kg flyweight. As is evident by said flyweight
dropping me in the hills, I cannot produce 2x the power and by
extension 2x the force of said flyweight. A lighter person has therefor
a greater chance of being able to lock a fixed rear wheel than a heavy
person. Does that sound right?

I think I could figure it out, but I don't know a friction coefficeint
to use as a starting point.

I assume under rear-wheel only locked braking, an equilibrium is
reached where weight transfer balances friction.

Does this sound right?

Joseph

Johnny Sunset wrote:
Werehatrack wrote:
On 19 Nov 2006 11:33:11 -0800, wrote:

Tips on how to figure this out appreciated. I don't really care about
the answer, I am just interested in the physics of it.

Before wasting your time trying to figure it out, consider that for a
conventional bicyle as well as most other wheeled vehicles, when
braking force is applied and deceleration begins to occur, the
downward vector shifts forward; there is "weight transfer" to the
front wheel. If enough braking effort is applied to the rear wheel,
there will be a point reached at which sufficient forward weight
transfer will occur to reduce the traction of the rear wheel to the
level at which it goes into a skid...and at that point, the weight
transfer is partially reversed *but the braking effectiveness is still
lost because the wheel is in a skid*. Thus, the place that braking
needs to be able to occur for safety is in the front wheel, *no matter
how effective the rear brake may be*.

Rear braking capability is not the key to stopping safely in an
emergency. Front braking capability rules the day in this.

While the static coefficient of friction between the tire and the
pavement is greater than the dynamic, the latter is not zero. Braking
force is not decreased that much at the rear wheel due to skidding. The
real issues are controlling the bike directionally and maintaining
balance once the rear wheel starts to skid.

How much is it decreased, and how much was it to begin with?

Joseph

On Sun, 19 Nov 2006 13:54:07 -0800, joseph.santaniello wrote:

Rear braking is limited by traction. I wonder
how close to .5G one can get with a back brake?

How high is your CoG? If you're not in lowrider recumbent territory
then forget it.


Mike

Mike Causer wrote:
On Sun, 19 Nov 2006 13:54:07 -0800, joseph.santaniello wrote:

Rear braking is limited by traction. I wonder
how close to .5G one can get with a back brake?

How high is your CoG? If you're not in lowrider recumbent territory
then forget it.

The correct term is "lowracer". Lowracers are for riding, "lowriders"
are for show.

--
Tom Sherman - Post Free or Die!

On 19 Nov 2006 13:54:07 -0800, wrote:

...I wonder how close to .5G one can get with a back
brake?

Not having the burning desire to invest in an accelerometer, all I can
relate is this: Long ago, I got curious about the relative
effectiveness of the front and rear brakes on my bike, and I tried a
very simple experiment. I chalked a line on the street in front of my
house, and then made a dozen runs toward it, each at 15mph. (This was
back in the days of cable-driven mechanical speedos, by the way.) As
I hit the line, I jammed on either the front brake, the rear brake, or
both; four runs were made for each. Where the front wheel stopped, I
made a chalk mark. I ended up with three fairly closely spaced groups
of marks and a couple of outliers. I couldn't tell you what the exact
measurements were, but I recall that the average rear-only stopping
distance was well over twice the average front-only stopping distance,
and a bit over three times the front-and-rear stopping distance. Most
of the rear-only stops were skids, but the shortest was not...and it
was shorter than the other three by a significant margin. That
street's paving was aged but sound asphalt, by the way.

I was a lot younger then, and I had expected the outcome to be much
different. I was actually trying to find a reason to simply get rid
of the front brake on my bike; adjusting it had turned into a
continual pain in the neck, as it wouldn't even stay centered
overnight. I was really fed up with the way it would just start
dragging at random moments for no apparent reason, and I figured that
if I could prove that the front brake didn't really make any
difference (and I typically didn't use it much, because I thought it
it wasn't all that good) then I could pull it off and not have to put
up with the misbehavior anymore.

Needless to say, I ended up spending a little time getting the front
brake to work better and more reliably, and I used it a lot more after
that. Once I had it set up right, in dry weather I could stop in
pretty much the same distance with just the front brake that I could
with both front and rear. In the wet, the rear seemed to fare better,
but I couldn't tell you why it felt that way.
--
Typoes are a feature, not a bug.
Some gardening required to reply via email.
Words processed in a facility that contains nuts.