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Light wheels-does this make sense???(25 posts)

Light wheels-does this make sense???Badger
May 21, 2001 4:16 PM
I did a long training ride yesterday with three other riding buddies. I used my lightest wheelset, my Spinergy Spox wheels.

What I found was that I felt like I had to "accelerate" the entire time I was riding. The more I thought about it and physics, the more I thought that even though the wheels are light, there is no momentum with them. Think of a flywheel. Once you get that going, its easier to keep it going.

I also ride Spinergy Rev-x and Rolf Vector Pro wheels. These both seem to maintain speed and momentum a lot easier. The lighter wheels feel like they "loose it" and you have to stay "on top of it" to keep them up at speed.

Has anyone else noticed this with light wheels and does this make sense? I just felt more tired than I usually do and I felt like the wheels spin up quick, but it was a chore keeping them there for four hours.
re: Light wheels-does this make sense???MrCelloBoy
May 21, 2001 4:22 PM
I'm not a physics expert but I believe what you say is basically accurate. Lighter "rotating weight" is easier to accelerate, but doesn't have as much momentum once it's turning. Same to be said for bikes with 26" wheels Vs 700c wheels.
re: Light wheels-does this make sense???Skip
May 21, 2001 4:40 PM
Sorry, but the more mass that is rotating, the more momentum you have, not less. Ever try to stop a freight train that's only going 5 MPH, by standing in front of it - NO, it has alot of momentum.
re: Light wheels-does this make sense???mike mcmahon
May 21, 2001 4:27 PM
I'm no physics expert, but I can tell you that long climbs have been a joy on my new Xaeros when I compare them to my old 32x3 Open Pros. Regardless of what the laws of physics might say, I'm getting up hills faster on the light wheels.
re: Light wheels-does this make sense???MrCelloBoy
May 21, 2001 4:36 PM
Yup,
The momentum we're talking about is best utilized "cruising at speed".
Getting to speed and climbing will benefit from the lighter rotating weight. Heavier wheels will keep you going once you reach that speed.
So you've got to evaluate what the ride has in store and plan appropriately.
re: Light wheels-does this make sense???mesa
May 21, 2001 4:35 PM
Momentum = mass x velocity (even about a radius) The math is simple.
Practical application is much more subjective: tire pressure, wear, road surface, cornering, hills, etc.
re: Light wheels-does this make sense???LC
May 21, 2001 5:01 PM
The Spox also seem to have a little more aerodynamic drag than most wheels. I can feel it more in a head wind, but not sure why because they are supposed to have aero rims. I have both Open Pros and Spox and can feel the difference too on the flats. Maybe the fatter spokes or that monster hub?
Bingo!grz mnky
May 21, 2001 6:31 PM
Well I studied physics and more imporantly mechanical engineering and you get an "A" for the day - go put a gold star on your forehead! ;-)

All joking aside this is really the crux of the whole matter. To "feel like you need to constantly accelerate" means you are either constantly climbing a hill or there is some constant type of loss. Friction (i.e. wind resistance) is a nasty thing. So is riding with a dragging brake, but hopefully that's another story. Those Spox with the phatty spokes and the mongo hubs are an aerodynamic nightmare. If you want to see a direct comparison hook up a set of Mavic Ksyriums (or some other popular svelt spoked hoop) with the same tires and do a little comparison riding with your buds or use some power instrumentation if you have it. I immeidiately noticed that I had better "glide" when I jumped on my Ksyriums a year ago. I could glide with and sometimes past my regular buds, much better than with the pair of 36h training wheels. Some of them even developed a bad case of wheel lust.

The worst thing about the drag is that it is non-linear velocity dependant and a spoke at TDC is travelling twice as fast as the hub. It's a real whammy.

YNSH Knerd
An "A" from grz; nice workmike mcmahon
May 21, 2001 7:01 PM
Those Spox hubs and spokes do look a bit like they belong on a Peterbilt. So far at least, I feel like Spinergy got it right with the Xaeros: nice Edco hubs with thinner spokes. They climb like champs, but I've never had the feeling that I was fighting the wheels on the flats. I'd be interested to hear a comparison between Ksyriums and Xaeros from someone who has spent considerable time on each under varying conditions. Any volunteers?
Nucleon vs XaeroMGS
May 21, 2001 8:09 PM
I rode Nucleons for two seasons and just switched to the Xaero. By my digital scale, the Xaero weighs in about 200 grams heavier.....but....

On my standard 36 mile route, which I time as my training ride, I averaged my fastest every speed, by .5mph, using the Xaero. Maybe it was the psych of the new wheels, but it's the beginning of the season, and the ride is a combination of steep hills and flats.

The wheels feel faster than the Nucleons, and on my bike, they are.
Nucleon vs Xaeromike mcmahon
May 22, 2001 3:44 PM
Thanks for the comparison. I'm glad to know that, for you at least, the Xaeros are faster. How would you compare the ride quality of the Xaero and Nucleon?
Nucleon vs XaeroMGS
May 22, 2001 6:47 PM
This is entirely subjective, but I believe the ride of the Xaero is and easier ride. There appears to be less transmission of vibrations from a rough rode surface.

Their claim of an enhanced ride secondary to ride quality and smoothness, does appear to be true.

The nucleons are great wheels, true and light, they just don't ride as fast or feel as comfortable, despite their weight advantage.
Grz, nice evasion of the question at hand.speed lover
May 22, 2001 10:42 AM
Sure the fat spokes are less aero, but do light wheels slow down quicker than heavy ones????
What's the Question?grz mnky
May 22, 2001 11:50 AM
Well, yes they do! assuming by light you mean lower moment of rotational inertia, which is all that really matters when discussing the "weight" of a rotating mass. It works like this if two objects are rotating at the same angular velocity (i.e. rpm) the one with the higher moment of inertia will have greater rotational energy. If the sum of all the drag forces (bearing friction, wind resistance, etc.) are the same then it will take longer to disapate the wheel with the higher rotational energy. it is entirely possible to have a wheel that is lighter than another, but to still have a higher moment of inertia. It's how the wieght is distributed that counts.

I understand your point, but I wasn't trying to evade the question - I was pointing out that the mechanism at play was different than what the original poster thought. To feel like one is constatnly having to "accelerate" while on the flats and travelling at constant speed is directly tied to the loss mechanisms. The lighter wheel will feel like it loses "speed" quicker _all_other_things_being_equal. Which they are not.
re: Light wheels-does this make sense???likesbikes
May 21, 2001 7:04 PM
I dont' know much about this topic, all I can say is this. I was on a fifty mile ride this weekend and was in a pack with 4 other strong riders. My regular ride was sick with the bottom bracket flu, so I took out my commuter...24lbs. At any rate, though I was slower in acceleration from a stop sign, and pushed hard on the hills, I was able to coast faster than the rest of the group on downhills and while in draft had to be on my brakes most of the time to keep the proper distance with the rest of my group. And yes, I am lighter by about 10lbs than three of my group and I found myself coasting a whole lot more than the other guys riding ultra light rigs. So, was it the weight of my bike, (heavy wheels) that aided in my ability to keep up with the rest of the pack, or just some phenomenon that felt sorry for the litte guy with the heavy bike?

Just my .02 worth
Randy
Few things to tryixiz
May 22, 2001 6:03 AM
Simple experiment to try at home
1. set a fan up pointing to a fix point on your wheel(prefer top of wheel), best if you have an old fork on a vise or a bike upside down with your computer visible. do this in your garage so less errors
2. dont move the fork and the fan (fix comparison)
3. let the wheel spin record the time it took to get up to speed at 10 sec increments and what is the max speed.
4. plot a graph and see which wheel is best

This test by no means compares to biking but gives you aero drag
the faster the wheel on this test is the one that has the most drag but may have the best bearings......
re: Light wheels-does this make sense???Ian
May 21, 2001 8:27 PM
I am not an engineer, but no it does not make sense. Grz Mky had a good answer concerning aerodynamics. And someone else mentioned a freight train. What takes more power, a train or a car? And not from a stop, but cruising at 50 mph. A train will, there is just more mass to move, regardless of whether it is already moving, the motor has to keep it moving. Or a better example might be a car vs. a SUV. Which gets better gas mileage?

If heavier wheels were better, everyone in the mid-west and Florida would be on 3000 gram wheelsets, not 1500 to 2000 gram wheelsets.
if you want heavy wheelsixiz
May 22, 2001 6:05 AM
fill the tube with water and go for a ride and see if heavier wheels are better. i think i know hte answer
energy transmission and friction (incl aerodynamics)ixiz
May 22, 2001 6:11 AM
those are the major things that makes a fast wheel
Weight does play a big difference if its not constant speed and we all know its never constant speed. So light weight wheels makes the biggest difference on hill climbs but not so much on time trials and thats why time trials heavy disk wheels dont matter much but helps on the aero side of the equation.

Energy transmission and sometimes adrenalin is not accounted for when we test these wheels. Bearing friction toooo.
inertiammaggi
May 22, 2001 8:23 AM
When Francesco Moser set the hour record back in 1984 in Mexico City, he used heavier hubs on an aerodynamic wheelset than what other riders had previously used.

Why? Because after you finish accelerating and you maintain a particular speed, the heavier hubs create more inertia and require less of an effort from a cyclist in order to maintain that speed as opposed to using a lighter wheelset.

Well... at least that's how Moser explained it.
errrr you mean momentumixiz
May 22, 2001 9:41 AM
Angular Momentum is a function of mass and radius
I have the answerIsaac Newton
May 22, 2001 3:37 PM
I don't think you guys are seeing clearly to the crux of the issue. The basic equation that governs the speed you go on a bike is Newton's 2nd law: acceleration=(net force)/mass. If there is a net positive force then the rider speeds up, if there is a net negative force he slows down - if there is no net force then he maintains constant speed. In this case the speed is determined by the resistance force, and the amount of power the rider is putting out: speed x resistance force = power output of rider. As someone said, the resistance force has contributions from friction (wind resistance, and friction in the bearings), and also from gravity if you are climbing a hill.

Suppose you have two identical people riding similar bikes along a flat road, with wheels that are identical in every way except one is made of a denser material so that they weigh more whilst still having the same profile. The answer is simple: if the riders pedal equally hard they will go equally fast. Period. Provided the resistance force is the same, the wheel weight does not matter, and feelings of "having to accelerate the wheels" are just figments of your imagination. Maintaining speed on the flat is independent of wheel weight.

If you are climbing hills though, the lighter wheels are better because as we all know gravity does care about weight. Conversely, going downhill, heavier wheels will make the rider faster, because gravity is then better able to overcome the wind resistance.

I think what you are feeling is the fact that light weight wheels accelerate and deccelerate more quickly than heavy wheels. If you are riding along and the wind picks up a bit it will deccelerate you more quickly if you are on light wheels (from the earlier equation), so that might explain your experience. But your new "equilibrium speed" with the stronger wind will be the same with either wheelset. You will just reach it more quickly with the lighter wheels. I guess this does mean, though, that in gusty conditions heavier wheels will be faster (since most of the time on a bike winds are headwinds).

As I say, this assumes that all other factors are equal (tire pressures, bearing friction, aero profile etc). Moments of inertia, and angular momentum in the wheels only matter when you are accelerating or steering. In dealing with climbing hills, if one could climb at constant rate then only the mass would matter, but the fact is we all tend to acclerate during the pedal stroke (when standing climbing) so the wheels with the lowest moment of inertia are best. Same for sprinting - unless you run out of steam in which case the higher moment of inertia wheels will mean you slow down less quickly. The lower the moment of inertia, the more nimbly your wheels will steer.
I have a hard time buying this.Ian
May 22, 2001 9:52 PM
So what you are saying is that once two objects are in motion, they will require the same energy to move forward, regardless of the weight? Wouldn't this be in the vacuum of space, but not on Earth where gravity will come into play?

What if I take two Ford Explorers. One has only a driver. The other has a driver and is loaded with ballast. Gross weight, for arguements sake, would be 4000 and 8000 lbs. They set off on the highway, cruise control at 55 mph. You are telling me that they will run out of gas at the same time?

And I am not trying to be difficult. I like to think of myself as having a brain and maybe I'm showing that I don't, but I just don't get this.
I think that almost everyone on this thread has somethingbill
May 23, 2001 7:51 AM
right, some of the people have it all right but haven't explained it all, and a fair number of people have something wrong with their assumptions in that they either mix forces or properties or ignore them.
The only way to analyze this problem is to break down the forces and properties at work. The wheels have mass. Even with two wheels of the same overall mass, the mass may be nearer the hub or further from the hub, which can matter in terms of whether momentum or inertia (as I remember the terms from physics, they address the same property of matter even though the colloquial distinction lives on -- mass in motion tends to remain in motion, in a straight line, thank you very much, and mass at rest tends to remain at rest). The wheels have an aerodynamic profile in two frames of reference, with respect to the friction as the wheel spins through air and with respect to the friction as the wheel moves forward into the air. The hub provides another source of friction, as does, obviously, the road surface.
Only after you separate these concepts and put them back together, figuring your variables in the process, can you discuss the problem intelligently.
The forces include the force required to rotate the wheel from stop to speed and the force required to move the mass forward from start to speed. Once the wheel is at a constant rotational speed and the mass is moving forward at a steady speed, the only forces required to maintain speed are against the friction created by the air and by the road and by the hub. It gets a little confusing, because the force used against these forces to keep the bike in motion tend to come all from one place in this system, because of the relationships among the mechanical parts, but the resistances remain separate forces.
The more mass that is moving forward, the more kinetic energy the mass possesses. This, as above, occurs in two frames of reference. The more massive rim requires more energy to get it to speed, but, because it then possesses more kinetic energy, it is harder to slow down. And vice versa. And so on with the mass of the wheel (or with the bike with the heavier rider moving forward -- took more for him to get up the hill; he now possesses greater kinetic energy, but NOT GREATER SPEED until his greater kinetic energy is hampered less by friction, as follows). So, if two masses are moving at the same speed with equal friction, the one with the greater kinetic energy (with more mass, given these variables and assumptions) will be easier to maintain at speed, because the friction would be a lesser force relative to the force created by the kinetic energy of the mass.
Got that?
A bullet and a bowling ball will fall through the air at about the same speed (assuming that air friction would matter little to either's progress). But, you'd much rather get hit by a falling bullet than a falling bowling ball, right? The bowling ball, in this scenario, has a MUCH greater kinetic energy. It's only fair; it took more energy to carry the bowling ball up the stairs. Now, I don't want to confuse you, but if we change the variables to create a scenario where the bullet has greater kinetic energy, since we aren't doing it with greater mass, we have to do it with greater speed (the force applied to move the smaller mass has to be so much greater to create a greater kinetic energy). So, you'd much rather get hit by a bowling ball thrown at your legs by, say, a wimp like me, than a bullet shot out of a gun, right? The kinetic energy imparted by the gunpowder explosion created a greater speed, and, at speeds that much greater, the bullet has MUCH more kinetic energy to impart to you.
I have a hard time buying this.DonB
May 23, 2001 7:52 AM
That is exactly correct -- provided the road is level and the weight difference does not affect wind resistance or tire rolling resistance. In reality, the heavier vehicle will sit slightly lower and have a larger tire contact patch, but these effects will be extremely small. Wind resistance is by far the dominant force for this example and for a bicycle on level ground. All the differences among bicycle wheels can be explained by aerodynamics.

Hills, of course, are where weight matters.