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From: AMuzi <am@yellowjersey.org>
Newsgroups: rec.bicycles.tech
Subject: Re: Bicycle physics question
Date: Thu, 20 Jun 2024 08:07:05 -0500
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On 6/19/2024 9:52 PM, Jeff Liebermann wrote:
> On Wed, 19 Jun 2024 22:39:07 -0000 (UTC), <bp@www.zefox.net> wrote:
> 
>> Jeff Liebermann <jeffl@cruzio.com> wrote:
>>> On Tue, 18 Jun 2024 00:16:01 -0000 (UTC), <bp@www.zefox.net> wrote:
>>>
>>>> While out for a motorcycle ride this morning a question
>>>> applicable to both bicycles and motorcycles came to mind:
>>>>
>>>> When a bike/cycle is leaned into a turn, its center of gravity
>>>> is lowered.
>>>
>>> Gravity doesn't move.  However, your center of mass does move and is
>>> lowered.
>>> <https://en.wikipedia.org/wiki/Center_of_mass>
>>>
>>>> That would seem to remove some potential energy.
>>>
>>> True, but it's a tiny amount of energy.
>>>
>>> Potential_energy = mass * gravity * height
>>> or
>>> joules = kg * 9.8 meters/sec^2 * meters
>>>
>>> Notice that it's the same change in potential energy whether you're
>>> moving of standing still.  You could be riding furiously or at a
>>> traffic light, and the change in potential energy would be the same.
>>> Your forward motion is also not involved in the potential energy
>>> calculation, because it is perpendicular to force vector (gravity).
>>>
>>> If you were to lean the bicycle over 1/2 meter and you and your
>>> bicycle weigh 80 kg (176 lbs), the change in potential energy would
>>> be:
>>> Potential_Energy(change) = 80 * 9.8 * 0.5 = 392 joules or 392
>>> watt-seconds
>>>
>>> <https://www.omnicalculator.com/physics/potential-energy>
>>> I like calculators that allow me to mix metric and imperialist units.
>>>
>>>> To undo the lean, the wheels have to be steered back under
>>>> the CG, which requires pedal effort on the bicycle and extra
>>>> throttle on the motorcycle.
>>>
>>> Correct.  Assuming 100% efficiency (most of which is lost in
>>> compressing the tires), in the above example, you will need to supply
>>> 392 joules of energy to return to an upright position.  Note that the
>>> energy is supplied only in the upright direction (perpendicular to the
>>> ground) and does not involve anything in the forward direction.
>>>
>>> There are some interesting comments in this discussion:
>>> <https://www.bikeforums.net/advocacy-safety/288303-what-makes-bike-turn.html>
>>>
>>>> But, leaning a bike/motorcycle doesn't seem to make it go
>>>> perceptibly faster, so if it takes work to stand it back up,
>>>> where did the energy of leaning over go?
>>>
>>> It didn't go anywhere.  It's all POTENTIAL energy, not kinetic energy.
>>> You can use potential energy to do work.  Only kinetic energy can do work.
>>        ^
>>      can't <-typo?
> 
> Oops.  It should be "can't".  Unfortunately, it's also 1/2 wrong.  One
> CAN use potential energy to do work, but the work isn't done until
> after things start to move.  This article sorta fumbles through the
> concept:
> "Is potential energy and "work done" the same thing?"
> <https://physics.stackexchange.com/questions/94077/is-potential-energy-and-work-done-the-same-thing>
> 
>> Potential energy can certainly do work, think of a trebuchet.
> 
> Notice the word "can".  Yes, potential energy can do work, but no work
> is done until the trebuchet starts to move.  For an exercise in
> frustration, try calculating the amount of work done by an object that
> isn't moving.  Work = force * distance_traveled.  If the distance
> traveled is zero, no work is done.
> 
>> Potential energy is lost in leaning.
> 
> True.  None of that potential energy goes into moving the bicycle
> forward (or backwards).  Easy enough to test:
> Dismount from your bicycle.  Jam the wheels against something that
> keeps the bicycle from moving sideways.  Grab the top tube.  When you
> lower or raise the top tube, do you feel any force the might move the
> bicycle forward or backwards?  You shouldn't.  Therefore don't expect
> leaning the bicycle over to gainfully contribute to forward or
> backward movement.
> 
>> Tom thinks it's going into tire friction,
> 
> "Tom thinks" is an oxymoron.
> 
>> we all seem to agree the amount is smallish compared to the KE of
>> the bike and dissipation caused by air drag making it hard to detect..
> 
> Before the discovery of quantum theory, it was axiomatic that if it
> can't be detected or observed, it doesn't exist.  Fortunately, nobody
> has invented a quantum bicycle.
> 
>> Maybe Tom's right. Front tires on motorcycle certainly wear faster
>> on twisty roads, even at low (35 mph) speeds.
> 
> Tom is certainly politically to the right.
> 
> When you turn the motorcycle handlebars on a twisty road, you feel
> quite a bit of resistance to turning.  The motorcycle wants to
> continue going forward in the direction of travel and your turning the
> front wheel tries to convince it to go not in a different direction.
> The resistance to the turning force causes the tires move slightly
> sideways.  Going sideways is highly abrasive and causes tire wear
> through friction.  A side effect of the tire wear is heat or energy
> loss.
> 
>>>> "The epitome of futility is the analysis of velocipedes with z
>>>> wheels, where z is a complex number..."
>>>
>>> That's true only on a rough road, where the velocipede can move in the
>>> Z direction (up and down) going over the bumps.  The z axis is also
>>> involved in making a turn, where z component of the centripetal force
>>> keeps the rider and bicycle from falling over.
>>
>> In the context of the original joke z=x+iy, where i is the
>> square root of minus one. Apologies for the obscurity...
> 
> Apology accepted.  I've been trying to keep the math at a very simple
> level or totally avoid the math by using gedankenexperiment (thought
> experiments) and analogies.  When I start using anything more complex
> than basic arithmetic, I instantly lose half my audience.
> 
>> Thanks for writing,
> 
> You're welcome.
> 
>> bob prohaska


" Work = force * distance_traveled.  If the distance
traveled is zero, no work is done."

Yes, that's right.

Losses from tire scrubbing are extant in two wheelers but 
significant in four wheel vehicles.
-- 
Andrew Muzi
am@yellowjersey.org
Open every day since 1 April, 1971