Page 2 - Climbing wheels??????and physics questions...

04-03.-2004, 06:58 AM	#16
Budarz Registered User Join Date: May 2003 Posts: 25	FOr a justification, open any physics text. Look up rotational kinematics or rotational dynamics or rotational kinetic energy. It should be under one of those. Ultimately rotating weight is only "more" when trying to accelerate or slow down the bike such as sprints to the finish. At constant speed it makes no difference. A very brief expanation is that to speed up your bike's non-rotating parts all of your energy goes into moving them faster in a linear fashion. For the rotating parts, they not only need to move faster linearly, but energy is required to rotate them faster as well. Hope this helps. As a test, take an ice cube and "race" it with a ball that will roll (not slide) down an incline. The icecube will win because gravity does work only to produce linear motion on the icecube. Gravity on the ball is split between linear and rotational contributions... same concept as the bike wheel.

04-03.-2004, 07:05 PM

#17

mjw_byrne

Registered User

Join Date: Jan 2004

Location: Oxford, UK

Posts: 525

Quote:

Originally posted by Budarz
FOr a justification, open any physics text. Look up rotational kinematics or rotational dynamics or rotational kinetic energy. It should be under one of those.
Ultimately rotating weight is only "more" when trying to accelerate or slow down the bike such as sprints to the finish. At constant speed it makes no difference.
A very brief expanation is that to speed up your bike's non-rotating parts all of your energy goes into moving them faster in a linear fashion. For the rotating parts, they not only need to move faster linearly, but energy is required to rotate them faster as well.
Hope this helps.
As a test, take an ice cube and "race" it with a ball that will roll (not slide) down an incline. The icecube will win because gravity does work only to produce linear motion on the icecube. Gravity on the ball is split between linear and rotational contributions... same concept as the bike wheel.

Sure, when accelerating, this holds perfectly. But some of the posts in this thread seem to imply that lifting a rotating body somehow takes more effort than lifting a non-rotating body of the same mass, which seems really odd to me.

05-03.-2004, 09:11 AM	#18
Budarz Registered User Join Date: May 2003 Posts: 25	If they claim that, they're incorrect. No difference unless accelerating. As a trackie I never really think of hills relating to bike weight. Reducing weight for us has only to do with improving one's jump. For hill climbs at constant speed, dead weight or rotating weight is the same. T

04-06.-2004, 03:29 PM

#19

rayner

Registered User

Join Date: Nov 2003

Posts: 222

Quote:

Originally posted by vlad
If your tires weigh 100 grams each, and my tires weigh 1100 grams each would I enjoy a benefical flywheel effect?

granted I would exert more energy getting started, but once inmotion would it take more effort for me to maintain a given speed?

would not my heavier tires tend to remain in motion, once they are in motion??

You would exert more energy getting started, but you would also exert more energy throughout your ride. This is because to maintain a constant speed you are constantly (without realising) having to accelerate the wheel due to external factors such as air and rolling resistance. In effect you are accelerating from rest, but on a smaller scale, every time you turn the pedals.

I'm not sure on this one but your tires should stay in motion (on the dead flat) for exactly the same amount of time as the lighter tires. This is because there is an equal weight on both the top and bottom of the wheel so the wheel is accelerating equally as much coming down as it is decellerating going up. For a downhill the heavy tire will obviously go faster and vice versa.

If you like going up hills get a light tire wheel combination and if you like going down 'em get a heay combination.

20-08.-2007, 03:00 PM

#20

wrench23

Registered User

Join Date: Aug 2007

Posts: 1

Re: Climbing wheels??????and physics questions...

Step 1. Draw a free body diagram of the bike. You always have acceleration on level ground and hills even without velocity increase (Force = mass*acceleration). On level ground to maintain a constant velocity you must overcome the force of rolling resistance/ frictional forces and any force due to wind resistance. The forces working against you and your bike (and therefore acceleration associated with them) increase on a hill due to gravity. On level ground gravity is normal to you and your bike, going up a hill it has both a normal component (gravity*sin(angle of the hill)) and a component working against you at the angle of the hill (gravity*cos(angle of the hill)). So in order to keep a constant velocity going up a hill you have to equal the acceleration of the frictional forces/rolling resistance, wind resistance AND the new component of gravity that is working against you.

You know the basic answer you would find in an introductory physics book.

This all boils down to rotating mass (wheel weight) actually does matter more on hills than it does on the flats because you are actually having to put more force ( and since your mass stays the same, this means more acceleration) to stay at a constant velocity on a hill.

Hope this helps someone.

Quote:

Originally Posted by mjw_byrne

I've got serious problems with the idea that rotating weight is somehow "more" than non-rotating weight. Can anyone point me at a document/URL with a physical justification for this?

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