Heavy rims

[Minion]

 

 

Is one area of the wheel not constantly decelerating' date=' while the other is accelerating?

[/quote']

When adding or reducing weight on a bicycle, it does not matter if you do it on the wheels or on any other place on the bike. The effect is the same on acceleration and deceleration.

This is not true. Although parts of the wheel are undergoing different accelerations in opposite directions, the forces are internal and cancel each other out. The force in the spokes causing the rim to speed up as it nears the top is balanced by the opposite force slowing it down as it reaches the bottom.

 

The wheels do act like flywheels by storing additional kinetic energy in their rotation.

 

 

Look at it like this:

 

Adding 1kg to the frame, moving at 36km/h (10m/s), stores an extra 50J of KE (1/2 x 1 x 10^2).

 

Or,

 

Add 1kg extra weight to a wheel as follows:

200g added at the hub (doesn't rotate)

4x200g added at 4 points around the rim.

 

If you look at the KE of each mass you see that:

- the hub mass is moving at 36km/h and has a KE of 10J

- the bottom mass is not moving and has no KE, but will soon start moving and gain KE.

- the top mass is moving at 72km/h and has a KE of 40J. It will start slowing down and lose KE.

- the front mass is moving at 50km/h (combine down and forward) and has a KE of 20J. It is slowing down and losing energy.

- the back mass is moving at 50km/h (up and forward) and has a KE of 20J. It is speeding up and gaining energy.

- since the masses are rigidly connected to each other by the spokes, the KE lost by the masses that are slowing down is gained by the ones speeding up.

 

The overall KE stored by the masses in the wheel is 90J. You gain an extra 40J of energy storage by putting the masses on the wheel, rather than the frame.

 

[Cassie]

All this is making me think of those toy super cycles we played with as kids...had a solid back wheel and a pull device to give it momentum - Wow those jobbies could go!!!  In a straight line only, though...

[parabola]

 

 

 

 

 

Is one area of the wheel not constantly decelerating' date=' while the other is accelerating?

[/quote']

 

When adding or reducing weight on a bicycle, it does not matter if you do it on the wheels or on any other place on the bike. The effect is the same on acceleration and deceleration.

 

 

You are forgetting about rotational inertia. Rotating mass stores energy as well as the foward momentum of bike and rider.

 

Edman, you are correct but I suspect you have not correctly calculated the rotational energy, which will increase the deficit between adding non-rotating weight and weight to the wheels. Adding weight to the hub only adds linear momentum correct. Check here : http://en.wikipedia.org/wiki/Flywheel

 

Lets stick to the extreme example of a very lightweight frame, with very lightweight hubs, and two sets of rims, one proportionaly heavy, one very light. The rims will help conserve momentum by storing the rotational energy. But you will lose time in accelerating.

 

Over a long, perfectly flat course, where an average rider can make up the time lost from the initial acceleration, heavier rims will win as they will smooth the power delivery from the average riders uneven pedal stroke.

 

Over a shorter distance, the riders benefit from the smooth power delivery will be lost due to his increased initial acceleration time, so the lighter rims will win.

 

For a professional rider with a trained even power delivery, lighter rims will always win, no question.

 

If you start looking at an uneven course, the time depends so heavily on the extra energy required for climbing that it will most likely outweigh the energy saved from the smoothed power delivery by a margin dependant on the ratio of climbing/descending/flats.

 

So its a compound problem heavily dependant on the riding conditions and the conditioning of the rider. However its late and I needed to dig out my dynamics textbook to check this, so there may be more to it.

parabola2009-11-27 07:57:43

[Woofie]

Very interesting guys.

Nice one with the formulas as well.

Is Joules=Watts?

 

I had another thought last night.

 

Heavier rims may carry you up a hill quicker because of the stored up momentum, but going down on the other side it will take longer to accelerate the rims to a high speed.

Obviously at the bottom of the hill the rims will be up to full speed again but the 2 seconds gained on the climb may be lost on the decent again.

 

 

[Johan Bornman]

You have to perform one joule of energy per second to do one watt of work.

 

Put differently, 1 watt is 1 Joule/s

 

 

A Joule is the energy required to lift 1KG to a height of 1m. It has no time component.

 

[Minion]

 

Edman' date=' you are correct but I suspect you have not correctly calculated the rotational energy, which will increase the deficit between adding non-rotating weight and weight to the wheels. Adding weight to the hub only adds linear momentum correct. Check here : http://en.wikipedia.org/wiki/Flywheel[/quote']

The calculation is correct. It shows an additional 40J of energy over and above the translational energy (that is 50J). If you calculate the KE posessed by the 4 rims weights using the rotational formula (1/2 Iω^2) about a point that is stationary relative to the hub you also get 40J.

 

The calculation I used works because it accounts for the fact that each mass has two velocity components. The first component has a constant magnitude and direction (the translation). The second has the same magnitude, but it's direction is always tangent to the rim, so it changes relative to the stationary point on the ground (the rotation).

 

At the top, the components add linearly, at the bottom they cancel. On the front and back they are perpendicular, giving a vector sum for the magnitude. At the hub, the rotation component is zero.

 

[Woofie]

 

Edman' date=' you are correct but I suspect you have not correctly calculated the rotational energy, which will increase the deficit between adding non-rotating weight and weight to the wheels. Adding weight to the hub only adds linear momentum correct. Check here : http://en.wikipedia.org/wiki/Flywheel[/quote']

The calculation is correct. It shows an additional 40J of energy over and above the translational energy (that is 50J). If you calculate the KE posessed by the 4 rims weights using the rotational formula (1/2 Iω^2) about a point that is stationary relative to the hub you also get 40J.

 

The calculation I used works because it accounts for the fact that each mass has two velocity components. The first component has a constant magnitude and direction (the translation). The second has the same magnitude, but it's direction is always tangent to the rim, so it changes relative to the stationary point on the ground (the rotation).

 

At the top, the components add linearly, at the bottom they cancel. On the front and back they are perpendicular, giving a vector sum for the magnitude. At the hub, the rotation component is zero.

 

You have so lost me there.

I can understand the first sentance and then everything slowly starts to become a blur.

 

 

[Gumpole]

this is all completely pointless....

the extra KE in the wheel/bike/system comes from extra energy put in by the rider!!

this would only help you if the TT consisted in a race to see how far you could  freewheel for after getting up to a maximum speed.

 

[techguy]

Its a compound problem. It depends on the riders performance as well as the course. Flywheels in a car even out the power delivery and are selected based on both motor performance as well as the course. So it would depend on how smoothly the rider can pedal. A heavier wheel may actually help a rider with uneven pedalling force  for example.

If the course was entirely flat I dont think the obvious answer would be 'as light as possible' but rather there would be a wheel weight/design which allows for peak performance.

I'm just speculating based on the physics' date=' I've never even ridden a road bike for that matter, so take this with a pinch of salt.
[/quote']
I think you are onto something here.
I have heard that some "suuped up" cars get lighter flywheels which allows them to accelerate faster, but then as soon as they hit a hill they end up slowing down faster than normal.

 

Race cars have their entire engine and related components lightened  and balanced for better acceleration and they adjust the gear ratios, just as on a bike, to suit the track so they don't lose momentum...

This is where we need mythbusters to do the tests for us.

[bikemonster]

Race car stuff...now we're into my other obsession.  Pull up a chair!

 

Lightening and balancing of engine components is done to improve efficiency and allow for higher max rev limits.

 

Lighter flywheels make for better acceleration, for exactly the same reason as lighter bike wheels make for better acceleration.  In a car though, a heavier flywheel makes the car more tractable and easier to drive, particularly at low revs, such as when pulling away from a start.

 

The basics of car racing are not that different from bike racing, and the obsession with weight is about the same.  And the same basic rule holds: lighter is always better than heavier.  Any advantage that comes from weight comes with a penalty which is heavier (see what I did there?) than the advantage.

 

Finally, back there somewhere  ^^^ somebody talked about the "advantage" of heavier wheels in "helping" to compensate for a cyclist with an uneven pedal stroke.  Oh yeah?  And how smooth do you think your pedal stroke is, given that you are putting in heaps of torque at 3 o'clock/9 o'clock and almost zero torque at 6 o'clock/12 o'clock?

 

James

 

 

[mountain_lion]

OK, Edman has convinced me.

 

But as Gumpole said, the stored rotational energy will mostly come from the rider (except if there is a very steep downhill on the course).

 

Ignoring Woofie's original example and looking at the more usual case of reducing the weight of a bike to increases performance, I am still not sure if there will be a noticeable difference in reducing weight on the wheels compared to reducing weight elsewhere given that we talking about real world components (not concrete wheels etc).

 

 

[Iwan Kemp]

The heavier rims would indeed act as a flywheel.

 

Time to accelerate not a prob?? We both ride a 40km TT at an a steady max speed of 40 km/h.? (As if I could!)With my super-light' date=' wispy wheels I accelerate to 40km/h in 90 seconds.With your hefty, slower accelerating wheels, you accelerate to 40km in 120 seconds.

 

I win!

 

While there is some advantage to sacrificing lightness for aero, there is never any advantage to adding weight for its own sake.

 

James[/quote']

 

 

 

Yip. Plus the effort of keeping it at 40k's will be a lot more if the course is not FLAT or slightly downhill.

 

 

 

Sacrificing weight for aero makes sense as there will be a trade-off and "balance point". Just as sacrificing weight for stifness

[Iwan Kemp]

If you want more mass for more momentum' date=' you can add it anywhere on the bike.

 

[/quote']

 

 

 

Not a scientist (or even particularly clever for that matter!), but I believe this will only be true when going downhill.

 

 

 

A 10kg bike with 1kg wheels will accelerate faster than a 9kg with 2kg wheels even though they will both weigh in at 11kg.

 

 

 

On a downhill when freewheeling this will probably be evened out as the laws of gravity will apply.

 

 

 

But as long as you have to pedal to get it going rotational weight will be the one where you'd wanna save as much as possible. That and smooth, fast hubs with quick engagement.

[parabola]

 

The calculation I used works because it accounts for the fact that each mass has two velocity components. The first component has a constant magnitude and direction (the translation). The second has the same magnitude' date=' but it's direction is always tangent to the rim, so it changes relative to the stationary point on the ground (the rotation).

[/quote']

OK I see what you did now, you just did it by adding up the components rather than using the formula. Same answer, same thing

 

[Iwan Kemp]

I am still not sure if there will be a noticeable difference in reducing weight on the wheels compared to reducing weight elsewhere given that we talking about real world components (not concrete wheels etc).

 

 

 

 

Sorry, I'm joining a very interesting topic late...

 

 

 

In my experience you'll feel 150g in the wheels immediately but not necessarily elsewhere. Experimented with that on my Mbuzi. The Kobra adjustable post weighs a ton so hacking 350g off it was fairly easy. Didn't feel much of a dif (complete bike weighs in at 15k's so it's a small % compared to RB weights).

 

Then I tried new tires. Dropped 319g and could immediately feel it. And the dif felt BIG. Yeah you have to factor in grip and things like that that also has an influence...

 

 

 

Educate away.

[parabola]

One place I really do feel the weight difference is in my saddle. Its like a big pendulum up there. A lighter saddle makes the bike feel so much more stable under out of saddle climbing.