# Has anybody ever inflated there bicycle tires with water and done a power study

Originally Posted by howardjd .

Yeah I realize all of that. The point is that because gravity is a conservative field if you finish where you start you've done no work in the Gravitational field, you've only expended energy to overcome frictional resistance. Speaking along the lines of the Work-Energy theorem Work is only done in conservative fields(path independent in calculus terms). The frictional losses in cycling in work-energy physics jargin are not technically considered work, they are considered simply frictional losses.

That'd be great if you were not moving around in a fluid. To eke out the most efficiency on a decent, you have to coast - or else you are loosing that potential energy gained on the climb to overcome increased aero drag of moving faster than your average speed over the course.

howardjd said:
Yeah I realize all of that. The point is that because gravity is a conservative field if you finish where you start you've done no work in the Gravitational field, you've only expended energy to overcome frictional resistance. Speaking along the lines of the Work-Energy theorem Work is only done in conservative fields(path independent in calculus terms). The frictional losses in cycling in Â work-energy physics jargin are not technically considered work, they are considered simply frictional losses.Â
So what? We have to deal with frictional and other heat losses because we cycle in the real world. Is there a point you're tying to make? Feel free to choose whatever terminology you feel most comfortable with, but the physics is still the same: climbing up a hill on a bike and descending are not path independent processes in a world in which energy losses always exist.

The point is that by finishing where you started you eliminate the variable of work done on or by the gravitational field making the test a more controlled study.

Having the same start and stop sounds like a good general setup for an experiment, but it really depends on what you are testing.

If anyone is going to take your study seriously, it will require several controlled trials. You need to minimize variation in everything but the variables you are intentionally introducing. One person doing a TT course using different setups days apart will not cut it - too many variables in the mix.

Talk to a professor about you plans, they always love to help out with some research and can help you design an experiment. As a bonus they get to take all the credit as well - but you may be able to get a scholarship or some pay from the work.

Originally Posted by alienator .

That would be a great waste of good beer.
Beer should only be used to inflate stories, egos and heads not tires.

howardjd said:
The point is that by finishing where you started you eliminate the variable of work done on or by the gravitational field making the test a more controlled study.Â
Well that point could have been made just by saying, "Make sure to do an out and back test." Note that starting where you finish does not guarantee anything. Doing a 5 mile, 3,000 ft climb that starts at point A that is followed by a 15 mile descent that ends at point A does not negate work done by gravity. Again, there is no path independence in the real world on a bicycle. I also don't see how this clarifies any test that's been done.

Originally Posted by alienator .

The best fluid for inflation will be the fluid that has the least friction with the inner tube and the least internal friction. That means that a gas is the best fluid, and as such, liquids are out. As for which gas, it really doesn't matter. Helium wouldn't be a good choice as helium is a difficult gas to contain. That pretty much leaves price as the decider, and in that case air wins since it's freely available via a pump.
Speaking of friction, I've been wondering what the water inside an inner tube really does when the wheel starts to turn in the vertical plane. Do you think it moves as a column stuck inside the tube, or does it require some time to spin up, so that the acceleration of the water lags that of the tube? At a steady-state, would the water column rotate at the same speed as the wheel, or would there be a small amount of slip (and resulting power loss)?

the trials weren't done days apart. They were done about 15min apart.

If you road a steady velocity because of friction the water would eventually approach the same rotational speed of the bike wheel. Accelerating and deccelerating the water would lag the accelerations of the bike wheel. If the frictional coefficient of the water and bike tube were known it would be a straight forward but still difficult calculation because of the relative motion of the bike wheel to ground.

work done by gravity does not include frictional losses. Really we're just talking semantics at this point.

I speak to my professors regularly about this. I've taken many lab courses and know how difficult it is to control a experiment and get worthwhile data, i've spent many weekends redoing and redoing labs to get good data. One problem I have is I do not have friends whom ride and could help me perform trials to get more data. I'm also injured and don't know how long it will be before I can ride enough to at least test. I'm pretty sure I am at least injured to the point competitive cycling is out of my future. I've been working on trying to develop a 3dimensional model predicting perpendicular motions of the bike as a result of applying force at the pedals which is off center line, and from producing forces across the seat and handle bars. The calculations would be pretty difficult and require tensors and Lagrange or Hamaltonian mechanics, right now I'm stuck on finding my equations of restraint. This model should help do determine how much the extra angular momentum of heavier bike wheels would reduce the small but visible side to side rocking motions that occur during riding. A high speed slow motion camera would be very helpful but I certainly don't have the money for that.

dhk2 said:
Speaking of friction, I've been wondering what the water inside an inner tube really does when the wheel starts to turn in the vertical plane.Â  Do you think it moves as a column stuck inside the tube, or does it require some time to spin up, so that the acceleration of the water lags that of the tube?Â  At a steady-state, would the water column rotateÂ at the same speed as the wheel, or would there be a small amount ofÂ slip (and resulting power loss)?Â Â
If we simplify things a bit, we can say that the water in contact with the tube will tend to travel faster than water in the center of the tube volume because of skin friction betwixt the water and the tube. However, that skin friction is acted upon by gravity, surface tension in the water, and shear in the water (as the result of the water velocity increasing as you move from the center of the tube volume to a tube surface). All that results in turbulent flow. So the answer would be that there will be slip, and it will be chaotic. Of course it has to be remembered that turbulence is generated from energy put into the system by the rider (and the road surface).

Another ****ing double post.

howardjd said:
If you road a steady velocity because of friction the water would eventually approach the same rotational speed of the bike wheel. Accelerating and deccelerating Â the water would lag the accelerations of the bike wheel. If the frictional coefficient of the water and bike tube were known it would be a straight forward but still difficult calculation because of the relative motion of the bike wheel to ground.Â
So you think that modeling the turbulence in the water is a straight forward calculation? Well, first you have to decide which turbulence model you want to use, then you need to the computing power (there is no simple hand calculation for determining losses from turbulence), then you need to crunch everything, and finally you'll need to repeat everything for every different water volume and inner tube. Turbulence modeling isn't trivial.

howardjd said:
work done by gravity does not Â include frictional losses. Really we're just talking semantics at this point.Â
And I said something different....where? The point I made is that if you're going to do real world tests to come up with your theory, you need to take into account the fact that path independence is hard to find in the real world, and as a result, you'll have to factor that into your error budget. It's not at all a matter of semantics but rather a matter of considering everything that influences testing.

howardjd said:
I speak to my professors regularly about this. I've taken many lab courses and know how difficult it is to control a experiment and get worthwhile data, i've spent many weekends redoing and redoing labs to get good data. One problem I have is I do not have friends whom ride and could help me perform trials to get more data. I'm also injured and don't know how long it will be before I can ride enough to at least test. I'm pretty sure I am at least injured to the point competitive cycling is out of my future. I've been working on trying to develop a 3dimensional model predicting perpendicular motions of the bike as a result of applying force at the pedals which is off center line, and from producing forces across the seat and handle bars. The calculations would be pretty difficult and require tensors and Lagrange or Hamaltonian mechanics, right now I'm stuck on finding my equations of restraint. This model should help do determine how much the extra angular momentum of heavier bike wheels would reduce the small but visible side to side rocking motions that occur during riding. A high speed slow motion camera would be very helpful but I certainly don't have the money for that.Â
Frankly, I think you should first look to theory to find the appropriate equations and the relevant parameters. The theory will take you a long way. For instance, theory can show you in 3-space how a 3D wheel will respond to a given force. In four space, you can see how the wheel responds over time. You can then take those predicted responses and equations and see how they match up--via experimentation--with the real world. Unless physics gets turned on its head you'll find they should match up well assuming all independent variables have been accounted for. With respect to a wheel's angular momentum, MOI, and the rocking from side to side of a bike, you're going to have some parameters that are going to be real fuzzy and difficult to state explicitly, like how an individual's pedaling style influences the dynamics of interest, how a given rider's body influences said dynamics....... The human influence on the system will introduce all manner of uncertainty, hurdles, and headaches into your effort. IMHO, I'd break the problem down into manageable parts that can be tested with accuracy and precision and modeled in a straightforward way. Let's look at this in terms of a Hamiltonian: if you want to construct a Hamiltonian, the first thing you need to do is figure out the number of degrees of freedom of the system. You can try it head on and just attempt a single overarching Hamiltonian, but that might actually result in a serious cerebrovascular accident since there are, using a scientific term, shitloads of DOF. It's much easier to break things down into manageable segments: rear wheel dynamics; crank dynamics, front wheel dynamics...or however you need or want to break it down. There's a reason that particle physicists didn't start with writing an all encompassing Hamiltonian for the Standard Model of particle physics but instead assembled it out of parts. Have you see the thing?

what if you were riding on the moon? and you did not have to deal with aerodynamics?

Yeah its tough stuff but its still interesting.

Making the approximation for steady velocity riding that the water column will began to flow in unison over time yes it would be a straight forward calculations. Otherwise the calculations would be very difficult your are right again.

howardjd said:
what if you were riding on the moon? and you did not have to deal with aerodynamics?
You'd still have to deal with friction in all of its forms.

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