Fixed gear skidding and pedal force

[[email protected]]

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

 

[[email protected]]

On 19 Nov 2006 11:33:11 -0800, [email protected] 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/gbssci/phys/mmedia/energy/cs.html

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

http://www.stoptech.com/tech_info/The Physics of Braking Systems.pdf

http://www.msgroup.org/forums/mtt/topic.asp?TOPIC_ID=172

Cheers,

Carl Fogel

 

[Werehatrack]

On 19 Nov 2006 11:33:11 -0800, [email protected] 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.
--
Typoes are a feature, not a bug.
Some gardening required to reply via email.
Words processed in a facility that contains nuts.

 

[Johnny Sunset]

Werehatrack wrote:
> On 19 Nov 2006 11:33:11 -0800, [email protected] 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!

 

[[email protected]]

Werehatrack wrote:
> On 19 Nov 2006 11:33:11 -0800, [email protected] 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

 

[[email protected]]

[email protected] wrote:
> On 19 Nov 2006 11:33:11 -0800, [email protected] 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/gbssci/phys/mmedia/energy/cs.html
>
> http://www.physicsforums.com/archive/index.php/t-120420.html
>
> http://www.stoptech.com/tech_info/The Physics of Braking Systems.pdf
>
> http://www.msgroup.org/forums/mtt/topic.asp?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

 

[[email protected]]

Johnny Sunset wrote:
> Werehatrack wrote:
> > On 19 Nov 2006 11:33:11 -0800, [email protected] 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

 

[Mike Causer]

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

 

[Johnny Sunset]

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!

 

[Werehatrack]

On 19 Nov 2006 13:54:07 -0800, [email protected] 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.

 

[David L. Johnson]

On Sun, 19 Nov 2006 11:33:11 -0800, joseph.santaniello 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.

There is no need to whip yourself for using the brake. That is what it is
for. Check what Sheldon Brown wrote about brake-use versus using your
legs to slow down. I do both on my fixed gear, depending on the situation
and my mood.

> 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.

You know, I see kids doing wheelies, or rolling on just the front wheel,
and I can't do that, either. Skidding the rear wheel is the same sort of
circus trick. Not part of real riding. Don't worry about it.

--

David L. Johnson

__o | When you are up to your ass in alligators, it's hard to remember
_`\(,_ | that your initial objective was to drain the swamp. -- LBJ
(_)/ (_) |

 

[David L. Johnson]

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

> 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?

Not close at all, since as you decelerate, you unload the rear wheel,
making it easier to skid but reducing both static and dynamic friction.

--

David L. Johnson

__o | Some people used to claim that, if enough monkeys sat in front
_`\(,_ | of enough typewriters and typed long enough, eventually one of
(_)/ (_) | them would reproduce the collected works of Shakespeare. The
internet has proven this not to be the case.

 

[G.T.]

Johnny Sunset wrote:
> 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".

How dorky. What if you don't race?

Greg

--
"All my time I spent in heaven
Revelries of dance and wine
Waking to the sound of laughter
Up I'd rise and kiss the sky" - The Mekons

 

[[email protected]]

David L. Johnson wrote:
> On Sun, 19 Nov 2006 11:33:11 -0800, joseph.santaniello 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.
>
> There is no need to whip yourself for using the brake. That is what it is
> for. Check what Sheldon Brown wrote about brake-use versus using your
> legs to slow down. I do both on my fixed gear, depending on the situation
> and my mood.

I was just trying to be funny. I too use both depending on mood. It is
just that my mood and situation very infrequently choose the brake. I
ride rural rolling hills with little traffic, so even on my road bike I
usually only use my brake when I get back to my house!

For me, fixed riding is A fun and B good training. Part of that
training comes from having to work downhill as well as up, so I don't
really care that it is inefficient. Perhaps heavy repeated resisting is
"a bad thing" but I don't buy it. I have made a quantum leap in my
performance in the last few months which I attribute to the way my
fixed as forced me to ride lately.


>
> > 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.
>
> You know, I see kids doing wheelies, or rolling on just the front wheel,
> and I can't do that, either. Skidding the rear wheel is the same sort of
> circus trick. Not part of real riding. Don't worry about it.
>

It's just that I can do most of those things (though I don't unless my
7 year old son insists), and on a bike the fixed skid is one of the few
things I cannot do. So I just wondered, is it just because I don't know
how, or because for some reason it is more difficult for me than others
I have seen do it?

I agree, not really something that needs doing. I'd rather wear out my
tires from thousands of kilometers of riding than from tens of meters
of skidding!

Joseph

 

[[email protected]]

Werehatrack wrote:
> On 19 Nov 2006 13:54:07 -0800, [email protected] 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.

I don't doubt that rear only braking requires 2x the distance. If you
can make even a WAG as to how far the skidding stops were, we could
estimate the friction coefficent and I could solve my pointless math
problem! I suppose I could go out on my road bike (after digging the
speedo out of mothballs) and go do some measured skids, but I don't
want to kill my tires.

Joseph

 

[Johnny Sunset]

G.T. wrote:
> Johnny Sunset wrote:
> > 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".
>
> How dorky. What if you don't race?

Since others have brought the subject up:

Are lowracers more dorky than overweight middle-age men wearing replica
UCI team jerseys and riding replica team bikes (especially the
"Lancealikes")?

Example of a "lowrider":
<http://www.sierravistacarclub.org/spring/spring03/bike.jpg>. I would
be hesitant to even try to ride around the block on such a bike.

Example of a "lowracer": <http://www.liegeradinfo.de/li_m5lr.jpg>. Not
for urban commuting, touring or off-road use, but perfectly fine for
long rides on open roads, racing or not.

--
Tom Sherman - Post Free or Die!

 

[David L. Johnson]

On Mon, 20 Nov 2006 00:55:56 -0800, Johnny Sunset wrote:

> Are lowracers more dorky than overweight middle-age men wearing replica
> UCI team jerseys and riding replica team bikes (especially the
> "Lancealikes")?

I think I'm missing a golden opportunity, here. I should call all the pro
teams. I'm sure they'd all pay me to not wear their jerseys.

--

David L. Johnson

__o | Let's not escape into mathematics. Let's stay with reality. --
_`\(,_ | Michael Crichton
(_)/ (_) |

 

[Paul Hobson]

[email protected] wrote:
> David L. Johnson wrote:
>>> 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.
>> You know, I see kids doing wheelies, or rolling on just the front wheel,
>> and I can't do that, either. Skidding the rear wheel is the same sort of
>> circus trick. Not part of real riding. Don't worry about it.
>>
>
> It's just that I can do most of those things (though I don't unless my
> 7 year old son insists), and on a bike the fixed skid is one of the few
> things I cannot do. So I just wondered, is it just because I don't know
> how, or because for some reason it is more difficult for me than others
> I have seen do it?
>
> I agree, not really something that needs doing. I'd rather wear out my
> tires from thousands of kilometers of riding than from tens of meters
> of skidding!

Here's what you should try:
Lean really far forward to unweight the wheel. Once you get the skid
started (which I believe you said you could), quickly move your weight
back to the wheel making sure to pull *up* on your *front* pedal.

My "emergency" braking days are over. Now that I'm riding much further
on slightly treaded 35 mm tires mounted on a heavier bike, I don't even
bother backpedaling unless I'm in a line of moving traffic alternating
between trackstands and creeping along. I ride a bike to get from A to
B as efficiently as possible - using all that leg energy to stop my bike
just seems too wasteful now.


--
Paul M. Hobson
Georgia Institute of Technology
..:change the f to ph to reply:.

 

[[email protected]]

[email protected] wrote:
> David L. Johnson wrote:
> > On Sun, 19 Nov 2006 11:33:11 -0800, joseph.santaniello wrote:

> For me, fixed riding is A fun and B good training. Part of that
> training comes from having to work downhill as well as up, so I don't
> really care that it is inefficient. Perhaps heavy repeated resisting is
> "a bad thing" but I don't buy it. I have made a quantum leap in my
> performance in the last few months which I attribute to the way my
> fixed as forced me to ride lately.

If you ever go back to a freewheel bike you may notice
that you have developed a dead spot in your pedal stroke,
due to the fixed gear's doing some of that work for you,
and you will have to relearn proper pedaling technique.

Robert

 

[David L. Johnson]

On Mon, 20 Nov 2006 08:26:41 -0800, r15757 wrote:

> [email protected] wrote:
>> David L. Johnson wrote:
>> > On Sun, 19 Nov 2006 11:33:11 -0800, joseph.santaniello wrote:
>
>> For me, fixed riding is A fun and B good training. Part of that
>> training comes from having to work downhill as well as up, so I don't
>> really care that it is inefficient. Perhaps heavy repeated resisting is
>> "a bad thing" but I don't buy it. I have made a quantum leap in my
>> performance in the last few months which I attribute to the way my
>> fixed as forced me to ride lately.
>
> If you ever go back to a freewheel bike you may notice
> that you have developed a dead spot in your pedal stroke,
> due to the fixed gear's doing some of that work for you,
> and you will have to relearn proper pedaling technique.
>

Nonsense.

--

David L. Johnson

__o | Do not worry about your difficulties in mathematics, I can
_`\(,_ | assure you that mine are all greater. -- A. Einstein
(_)/ (_) |