Center of Mass questions

[bicycle_disciple]

Some questions bothering me. I may be wrong here, please correct me.

1. Position of CG of a combined system seems to be dictated more by
the system with higher mass. That makes sense. Hence, won't a change
in the rider himself (weight, height etc) contribute more to a change
(lowering) in CG than say, something on the bike that weights
considerably less than him, like BB drop, sloping top tube etc?

2. A side question to the above is, how do we know what changes or
affects CG more. What makes people think BB drop will aid in lowering
CG? I haven't seen any literature that says, ok, so many cms of BB
drop will lead to so much % decrease in CG.

-Ron
http://cozybeehive.blogspot.com

[Tom Sherman]

bicycle_disciple wrote:
> Some questions bothering me. I may be wrong here, please correct me.
>
> 1. Position of CG of a combined system seems to be dictated more by
> the system with higher mass. That makes sense. Hence, won't a change
> in the rider himself (weight, height etc) contribute more to a change
> (lowering) in CG than say, something on the bike that weights
> considerably less than him, like BB drop, sloping top tube etc?...

Er, no. The design of the bicycle is most important in lowering the
combined center of mass. See
<http://www.lmb.org/mhpva/newsletter/2006-12/images/02-DonSmithsVKNoCom.jpg>.

--
Tom Sherman - Holstein-Friesland Bovinia
POST FREE OR DIE!

[Michael Press]

In article
<5f564c9e-545a-4e55-8ec2-c22371d25714@n20g2000hsh.googl
egroups.com>,
bicycle_disciple <1.crazyboy.only@gmail.com> wrote:

> Some questions bothering me. I may be wrong here, please correct me.
>
> 1. Position of CG of a combined system seems to be dictated more by
> the system with higher mass. That makes sense. Hence, won't a change
> in the rider himself (weight, height etc) contribute more to a change
> (lowering) in CG than say, something on the bike that weights
> considerably less than him, like BB drop, sloping top tube etc?
>
> 2. A side question to the above is, how do we know what changes or
> affects CG more. What makes people think BB drop will aid in lowering
> CG? I haven't seen any literature that says, ok, so many cms of BB
> drop will lead to so much % decrease in CG.

Center of mass is relatively unimportant.
Lowering the bottom bracket lowers the entire
mass of the rider, but what is a couple centimeters?

The formula for center of mass is distance weighted by mass.

5 kg 7 kg

---^-------------------^----------
1 m 4 m


COM is

(1 m).(5 kg) + (4 m).(7 kg)
---------------------------- = 33/12 m
5 kg + 7 kg

from the left margin.

--
Michael Press

[joseph.santaniello@gmail.com]

On Dec 28, 2:01 am, bicycle_disciple <1.crazyboy.o...@gmail.com>
wrote:
> Some questions bothering me. I may be wrong here, please correct me.
>
> 1. Position of CG of a combined system seems to be dictated more by
> the system with higher mass. That makes sense. Hence, won't a change
> in the rider himself (weight, height etc) contribute more to a change
> (lowering) in CG than say, something on the bike that weights
> considerably less than him, like BB drop, sloping top tube etc?
>
> 2. A side question to the above is, how do we know what changes or
> affects CG more. What makes people think BB drop will aid in lowering
> CG? I haven't seen any literature that says, ok, so many cms of BB
> drop will lead to so much % decrease in CG.
>
> -Ronhttp://cozybeehive.blogspot.com

Unless you are talking about an amazingly heavy bike, the rider's
position will have a hugely greater influence on where the CG is than
anything else on the bike.

Why would lowering the CG be of a concern?

Joseph

[Ron George]

On Dec 28, 1:27 pm, "joseph.santanie...@gmail.com"
<joseph.santanie...@gmail.com> wrote:
> On Dec 28, 2:01 am, bicycle_disciple <1.crazyboy.o...@gmail.com>
> wrote:
>
> > Some questions bothering me. I may be wrong here, please correct me.
>
> > 1. Position of CG of a combined system seems to be dictated more by
> > the system with higher mass. That makes sense. Hence, won't a change
> > in the rider himself (weight, height etc) contribute more to a change
> > (lowering) in CG than say, something on the bike that weights
> > considerably less than him, like BB drop, sloping top tube etc?
>
> > 2. A side question to the above is, how do we know what changes or
> > affects CG more. What makes people think BB drop will aid in lowering
> > CG? I haven't seen any literature that says, ok, so many cms of BB
> > drop will lead to so much % decrease in CG.
>
> > -Ronhttp://cozybeehive.blogspot.com
>
> Unless you are talking about an amazingly heavy bike, the rider's
> position will have a hugely greater influence on where the CG is than
> anything else on the bike.
>
> Why would lowering the CG be of a concern?
>
> Joseph

Joseph,

I started thinking about this when a someone said he's making a cross
bike with a lower BB (and hence lower CG) to negotiate tight turns at
the natl's cross race. He says he found he can make such a bike with
adequate pedal to ground clearance. I was wondering if a cm or two of
BB lowering would actually help him do these turns in a much more
significant fashion than before.

[Ron George]

On Dec 28, 5:56 am, Tom Sherman <sunsetss0...@REMOVETHISyahoo.com>
wrote:
> bicycle_disciple wrote:
> > Some questions bothering me. I may be wrong here, please correct me.
>
> > 1. Position of CG of a combined system seems to be dictated more by
> > the system with higher mass. That makes sense. Hence, won't a change
> > in the rider himself (weight, height etc) contribute more to a change
> > (lowering) in CG than say, something on the bike that weights
> > considerably less than him, like BB drop, sloping top tube etc?...
>
> Er, no. The design of the bicycle is most important in lowering the
> combined center of mass. See
> <http://www.lmb.org/mhpva/newsletter/2006-12/images/02-DonSmithsVKNoCo...>.
>
> --
> Tom Sherman - Holstein-Friesland Bovinia
> POST FREE OR DIE!

Tom,

That bike looks awesome. I was thinking more on the lines of road and
cross bikes.

Ron

[joseph.santaniello@gmail.com]

On Dec 28, 2:53 pm, Ron George <ron.r.geo...@gmail.com> wrote:
> On Dec 28, 1:27 pm, "joseph.santanie...@gmail.com"
>
>
>
> <joseph.santanie...@gmail.com> wrote:
> > On Dec 28, 2:01 am, bicycle_disciple <1.crazyboy.o...@gmail.com>
> > wrote:
>
> > > Some questions bothering me. I may be wrong here, please correct me.
>
> > > 1. Position of CG of a combined system seems to be dictated more by
> > > the system with higher mass. That makes sense. Hence, won't a change
> > > in the rider himself (weight, height etc) contribute more to a change
> > > (lowering) in CG than say, something on the bike that weights
> > > considerably less than him, like BB drop, sloping top tube etc?
>
> > > 2. A side question to the above is, how do we know what changes or
> > > affects CG more. What makes people think BB drop will aid in lowering
> > > CG? I haven't seen any literature that says, ok, so many cms of BB
> > > drop will lead to so much % decrease in CG.
>
> > > -Ronhttp://cozybeehive.blogspot.com
>
> > Unless you are talking about an amazingly heavy bike, the rider's
> > position will have a hugely greater influence on where the CG is than
> > anything else on the bike.
>
> > Why would lowering the CG be of a concern?
>
> > Joseph
>
> Joseph,
>
> I started thinking about this when a someone said he's making a cross
> bike with a lower BB (and hence lower CG) to negotiate tight turns at
> the natl's cross race. He says he found he can make such a bike with
> adequate pedal to ground clearance. I was wondering if a cm or two of
> BB lowering would actually help him do these turns in a much more
> significant fashion than before.

If he thinks it will make him faster, it probably will no matter what
the physics say.

The height of the CG makes no difference in how far a bike has to lean
in a given corner. Having a lower CG would mean the CG had a shorter
distance to move side to side in S bends, so theoretically a bike with
alower CG could negotiate two sequential turns faster. But only if is
was how fast the rider was able to change direction was the limiting
factor. But it is traction that is the limiting factor, even more so
in 'cross.

Joseph

[meb]

On Dec 28, 2:53*pm, Ron George <ron.r.geo...@gmail.com> wrote:
> On Dec 28, 1:27 pm, "joseph.santanie...@gmail.com"
>
>
>
> <joseph.santanie...@gmail.com> wrote:
> > On Dec 28, 2:01 am, bicycle_disciple <1.crazyboy.o...@gmail.com>
> > wrote:
>
> > > Some questions bothering me. I may be wrong here, please correct me.
>
> > > 1. Position of CG of a combined system seems to be dictated more by
> > > the system with higher mass. That makes sense. Hence, won't a change
> > > in the rider himself (weight, height etc) contribute more to a change
> > > (lowering) in CG than say, something on the bike that weights
> > > considerably less than him, like BB drop, sloping top tube etc?
>
> > > 2. A side question to the above is, how do we know what changes or
> > > affects CG more. What makes people think BB drop will aid in lowering
> > > CG? I haven't seen any literature that says, ok, so many cms of BB
> > > drop will lead to so much % decrease in CG.
>
> > > -Ronhttp://cozybeehive.blogspot.com
>
> > Unless you are talking about an amazingly heavy bike, the rider's
> > position will have a hugely greater influence on where the CG is than
> > anything else on the bike.
>
> > Why would lowering the CG be of a concern?
>
> > Joseph
>
> Joseph,
>
> I started thinking about this when a someone said he's making a cross
> bike with a lower BB (and hence lower CG) to negotiate tight turns at
> the natl's cross race. He says he found he can make such a bike with
> adequate pedal to ground clearance. I was wondering if a cm or two of
> BB lowering would actually help him do these turns in a much more
> significant fashion than before.

If he thinks it will make him faster, it probably will no matter what
the physics say.

The height of the CG makes no difference in how far a bike has to lean
in a given corner. Having a lower CG would mean the CG had a shorter
distance to move side to side in S bends, so theoretically a bike with
alower CG could negotiate two sequential turns faster. But only if is
was how fast the rider was able to change direction was the limiting
factor. But it is traction that is the limiting factor, even more so
in 'cross.

Joseph

Likewise a shorter Length to lean the center of mass will make emergency cuts quicker.

Lowering CG (as well as rearward relocating the CG) will also improve braking, as lowering the CG will:

1) increase the speed at which a bike would endo if the bike has a high enough cg relative the distance behind the front contact patch point;

or

2) on bikes having a cg already low enough relative the front contact patch: more evenly distribute the front/rear weight between the tires maximizing traction.

[David L. Johnson]

bicycle_disciple wrote:
> Some questions bothering me. I may be wrong here, please correct me.
>
> 1. Position of CG of a combined system seems to be dictated more by
> the system with higher mass. That makes sense. Hence, won't a change
> in the rider himself (weight, height etc) contribute more to a change
> (lowering) in CG than say, something on the bike that weights
> considerably less than him, like BB drop, sloping top tube etc?
>
> 2. A side question to the above is, how do we know what changes or
> affects CG more. What makes people think BB drop will aid in lowering
> CG? I haven't seen any literature that says, ok, so many cms of BB
> drop will lead to so much % decrease in CG.

This is self-contradictory. Since, from paragraph #1, the rider is the
largest part of the system, then lowering the rider will indeed lower
the center of mass. Also, lowering the bb height lowers the rider. So,
the conclusion is...

--

David L. Johnson

I believe that the motion picture is destined to revolutionize our
educational system and that in a few years it will supplant largely,
if not entirely, the use of textbooks
-- Thomas Edison, 1922

[unforgiven99@juno.com]

On Dec 28, 8:53 am, Ron George <ron.r.geo...@gmail.com> wrote:
> On Dec 28, 1:27 pm, "joseph.santanie...@gmail.com"
>
>
>
> <joseph.santanie...@gmail.com> wrote:
> > On Dec 28, 2:01 am, bicycle_disciple <1.crazyboy.o...@gmail.com>
> > wrote:
>
> > > Some questions bothering me. I may be wrong here, please correct me.
>
> > > 1. Position of CG of a combined system seems to be dictated more by
> > > the system with higher mass. That makes sense. Hence, won't a change
> > > in the rider himself (weight, height etc) contribute more to a change
> > > (lowering) in CG than say, something on the bike that weights
> > > considerably less than him, like BB drop, sloping top tube etc?
>
> > > 2. A side question to the above is, how do we know what changes or
> > > affects CG more. What makes people think BB drop will aid in lowering
> > > CG? I haven't seen any literature that says, ok, so many cms of BB
> > > drop will lead to so much % decrease in CG.
>
> > > -Ronhttp://cozybeehive.blogspot.com
>
> > Unless you are talking about an amazingly heavy bike, the rider's
> > position will have a hugely greater influence on where the CG is than
> > anything else on the bike.
>
> > Why would lowering the CG be of a concern?
>
> > Joseph
>
> Joseph,
>
> I started thinking about this when a someone said he's making a cross
> bike with a lower BB (and hence lower CG) to negotiate tight turns at
> the natl's cross race. He says he found he can make such a bike with
> adequate pedal to ground clearance. I was wondering if a cm or two of
> BB lowering would actually help him do these turns in a much more
> significant fashion than before.

BB height doesn't affect tight turn performance nearly as much as
front end geometry. The only advantage to lowering the BB on a cross
bike is making it easier to jump onto. On the other hand, in order to
keep the same saddle to bar drop, the head tube needs to be shortened
by as much as the BB is dropped. If your friend is short enough to
already be at the low end of head tube length, he could be trading
real straightaway speed for perceived cornering improvement. There
have also been claims that a higher BB lets you get through wide turns
faster because the tires are tracing a much bigger arc than your cg,
allowing you to carry more speed without the front tire sliding out.
Personally I find this almost as shaky as the claim that a high BB is
more aerodynamically efficient because it puts your center of area
over into a smaller and thus lower velocity arc through the corner.

[joseph.santaniello@gmail.com]

On Dec 29, 5:31 pm, unforgive...@juno.com wrote:
> On Dec 28, 8:53 am, Ron George <ron.r.geo...@gmail.com> wrote:
>
>
>
> > On Dec 28, 1:27 pm, "joseph.santanie...@gmail.com"
>
> > <joseph.santanie...@gmail.com> wrote:
> > > On Dec 28, 2:01 am, bicycle_disciple <1.crazyboy.o...@gmail.com>
> > > wrote:
>
> > > > Some questions bothering me. I may be wrong here, please correct me.
>
> > > > 1. Position of CG of a combined system seems to be dictated more by
> > > > the system with higher mass. That makes sense. Hence, won't a change
> > > > in the rider himself (weight, height etc) contribute more to a change
> > > > (lowering) in CG than say, something on the bike that weights
> > > > considerably less than him, like BB drop, sloping top tube etc?
>
> > > > 2. A side question to the above is, how do we know what changes or
> > > > affects CG more. What makes people think BB drop will aid in lowering
> > > > CG? I haven't seen any literature that says, ok, so many cms of BB
> > > > drop will lead to so much % decrease in CG.
>
> > > > -Ronhttp://cozybeehive.blogspot.com
>
> > > Unless you are talking about an amazingly heavy bike, the rider's
> > > position will have a hugely greater influence on where the CG is than
> > > anything else on the bike.
>
> > > Why would lowering the CG be of a concern?
>
> > > Joseph
>
> > Joseph,
>
> > I started thinking about this when a someone said he's making a cross
> > bike with a lower BB (and hence lower CG) to negotiate tight turns at
> > the natl's cross race. He says he found he can make such a bike with
> > adequate pedal to ground clearance. I was wondering if a cm or two of
> > BB lowering would actually help him do these turns in a much more
> > significant fashion than before.
>
> BB height doesn't affect tight turn performance nearly as much as
> front end geometry.  The only advantage to lowering the BB on a cross
> bike is making it easier to jump onto.  On the other hand, in order to
> keep the same saddle to bar drop, the head tube needs to be shortened
> by as much as the BB is dropped.  If your friend is short enough to
> already be at the low end of head tube length, he could be trading
> real straightaway speed for perceived cornering improvement.  There
> have also been claims that a higher BB lets you get through wide turns
> faster because the tires are tracing a much bigger arc than your cg,
> allowing you to carry more speed without the front tire sliding out.
> Personally I find this almost as shaky as the claim that a high BB is
> more aerodynamically efficient because it puts your center of area
> over into a smaller and thus lower velocity arc through the corner.

The idea that a higher CG allows greater speed before washout due to
the tires describing a wider arc is rubbish. Tire-patch to CG lean
angle is determined by speed and turn radius, which also determines
the turn acceleration. Traction is determined by the acceleration and
the lean angle. Distance to CG has nothing to do with it.

The lesser aero drag in corners has been "discussed" ad-nauseum over
in rec.bicycles.racing. There the focus was on straight ahead vs in a
turn on a Velodrome.

The differences between two turns, one with a center of pressure a few
cm's higher would be microscopic indeed.

Joseph