|I worked so damn hard on this post that I'm repeating it.||bill|
May 23, 2001 8:12 AM
|With respect to the wheels/rim/mass/weight/speed problem, I think that almost everyone on this thread had something right, some of the people had it all right but haven't explained it all, and a fair number of people had something wrong with their assumptions in that they either confused forces or properties or ignored 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 the rotational 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. Assume that the road surface friction is a constant and let's play with the other variables.
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 (you can spin your wheel in the stand, but the bike isn't moving forward; moving the bike forward requires additional force). 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 source 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.
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, which I think is fair enough). 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 or at least equal kinetic energy to the bowling ball, since we aren't doing it with greater mass, we have to do it with greater speed (the same force applied to move the smaller mass to have the same kinetic energy will give the bullet much greater speed -- speed and force are NOT the same; it all depends on the mass). So, at a certain point, you won't care very much whether you're creamed by the bowling bowl or by the bullet. O
|I don't understand a friggin thing you said...but that's ok!||Bob in Indiana|
May 23, 2001 8:38 AM
|This kind of intense scientific/technical discussion requires WAY too much brain power to process, and is precisely why I skipped finals that year in College (to get a head start on the Ft. Lauderdale sprind break trip I can't remember a thing about) to allow myself to be unceremoniously "dismissed" from school...Like I will tell my daughters...do what dad SAYS, not what dad DID (will that fly?)
So can I just pretend to understand the physics involved here, and remain impressed with the level of knowledge (and thought/time) you guys sometimes put into posts!!!
|Believe it or not, there was more to the post (you know,||bill|
May 23, 2001 9:12 AM
|the part where it all becomes clear) that was cut off. |
Basically, the point is that intuition in physics can be right (and usually is), but only if you ask the right question. To ask the right question, you have to think about the parts to the question and your assumptions. Once you've got that stuff lined up, then intuition usually can pick up where it left off.
|So...guns don't kill people, bowling balls do? (nm)||MeDotOrg|
May 23, 2001 9:02 AM
|Overcoming inertia||Tom C|
May 23, 2001 9:48 AM
|Interesting what you're saying, as I just read that Moser's bike in which he broke Merckxs' hour record record in 1984 weighed 23.8 lbs. with much of the weight in the "moon disk" wheels. One reason why as a midwesterner (flatlands) I don't worry about weight weeniedom.|
|re: I worked so damn hard on this post that I'm repeating it.||mon t|
May 23, 2001 10:04 AM
|well so what? are you trying to say that light wheels are good?|
|kinetic energy||Duane Gran|
May 23, 2001 10:22 AM
|Provided that we raced our bikes straight downward, weight really wouldn't matter. ;) This is an interesting topic, but I believe someone else put the finishing touch on the matter. I believe the person stated that someone in the upper echelon of the sport would be riding a lead wheel if it really was very helpful, but when you look around they all have very light wheels. With all the time spent in the wind tunnel and with SRM units I'm sure the people in the lab coats have it pretty well figured. I know I ride better with lighter equipment.|
|re: I worked so damn hard on this post that I'm repeating it.||dug|
May 23, 2001 10:24 AM
|Thanks for the brief... I actually followed every word you typed, and have absolutley NO idea why you typed it. Just ride your damn bike.|
|re: I worked so damn hard on this post that I'm repeating it.||Steevo|
May 23, 2001 10:29 AM
|bill gives a good, simple, non-technical discusson about the physics involved. I am a graduate mechanical engineer and there is nothing in his post that is technically wrong.|
|re: I worked so damn hard on this post that I'm repeating it.||mon t|
May 23, 2001 10:51 AM
|this is why we all like to make jokes about you nerds. what in the hell does any of this treatise mean? there is a world out there, nerds, relate yourselves to it!|
|re: I worked so damn hard on this post that I'm repeating it.||Tom C|
May 23, 2001 10:56 AM
|Nerd=Inventor=Bicycle. Try and relate to that.|
|re: I worked so damn hard on this post that I'm repeating it.||mon t|
May 23, 2001 12:23 PM
|good one. how 'bout this? nerd-inventor-personal hovercraft. you still haven't made any sort of real world relevancy statement, or summary, so all you nerds are doing is mental mast....... we're waiting.|
|ah, look, you're obviously happier not thinking about this||bill|
May 23, 2001 12:36 PM
|stuff, so . . . |
When it comes to design of bicycles and the choice of materials and components, there is nothing more "real-worldly" than Newtonian physics. We all know that you need legs and lungs. After that, it's all physics.
|ah, look, you're obviously happier not thinking about this||mon t|
May 23, 2001 1:52 PM
|ok, ok. let me make this simple for you gniuses. WOULD SOMEBODY PLEASE SUMMARIZE WHATEVER THE ORIGINAL POST MEANT? thanks.|
|All things are not equal....||128|
May 23, 2001 2:18 PM
|How'd I do prof?
Squeeze breaks harder to stop a heavier bike
Heavier wheels don't mean slower bike (because, alas, all things are not equal; friction, weight, stopping distance, speed, direction, what you ate for breakfeast etc...)
|it means that a heavier wheel is harder to get to speed, but||bill|
May 23, 2001 2:27 PM
|when you do, it tends to stay at speed "better" than a lighter wheel. Same with a heavier bike. If you are on the flats, and steady speed is your issue (and how fast you get to speed is not so important) you're probably losing nothing with a heavier wheel vs. a lighter wheel. Or a heavier bike. Where the wheel carries the mass matters, too -- lower rotating mass, easier to get to turn. Between two wheels of the same weight, the wheel with the lighter rim weight will accelerate faster. Their forward momentum will be the same, although the heavier rim will "want" to turn at a steady speed more than the lighter rim. If you had fifty pound rims and managed through a burst of incredible strength to get them to speed, you could maintain your speed on the flats like a machine. If you are comparing light wheel rims, heavy bike frame vs. heavier wheel rims, lighter bike frame (same weight overall), you probably will be able to accelerate the lighter rims faster, resulting in being able to accelerate the whole bike faster because of the momentum issue in the second frame of reference (the rotational mass issue, as opposed to the moving against friction mass issue, which is the same for both bikes). And so on. If you think about what you want to improve (do I need to climb faster, do I need to accelerate faster, can I afford to keep some weight on the bike because all I really want to do is maintain my speed on the flats), you can make more intelligent choices about the features of the various stuff out there. In truth, the margin for error with most of this stuff is probably greater than the differences, and the componentry ALL represents compromise to some extent. There still, however, are some relative constants in terms of what we discuss as "better" (including that lighter is generally "better," because we all have to get to speed and climb some and whatever we lose in downhill momentum we can probably make up with a little power), but at least we may be more thoughtful than slavish in pursuing light and we would understand what it means when comparing things such as rim weights and overall weights for wheels and frames.|
|re: I worked so damn hard on this post that I'm repeating it.||speedchump|
May 24, 2001 11:07 AM
|nerd - computer - cycling forum.
what does your presence in this forum have to do with your "real world" ?
talking about a C-40 doesn't have any "real world relevancy" for me because I can't afford one.
talking about physics does because I can comprehend it.
online forums are the ultimate in "mental mast.....", but you're here, aren't you?
|re: I worked so damn hard on this post that I'm repeating it.||nn23|
May 23, 2001 11:28 AM
|I'll skip talking of inertia and moment of intertia here and stick to layman terms....
Now since you spent less energy to reach your steady speed, it's only fair that its requires less energy (by friction, wind etc) to stop you (just like it took less energy to stop the falling bullet).
Moral of the story: Lighter wheels means a more responsive bike - gaining speed as well as losing speed. Also you spend less energy from the start to the finish line and require less power/force/muscle to reach/sustain your max speed.
|masses set in motion tend to remain in motion unless acted ....||Tom C|
May 23, 2001 11:42 AM
|Since much of this is really coming from ballistics, following your observation a light bullet should shed velocity slower than a heavier one when in fact the opposite is the case. I think this is why we are cautious crossing railroad tracks. Hm?|
|no, tom, i actually said the opposite. i think. a heavier||bill|
May 23, 2001 12:16 PM
|object starting at the same speed as a lighter object will lose velocity slower than the lighter object.|
May 23, 2001 12:28 PM
|no, tom, i actually said the opposite. i think. a heavier||Tom C|
May 23, 2001 6:47 PM
|I agreed with you Bill my remark was directed to nm23.|
|Getting Hit by a Train is Slow||grz mnky|
May 23, 2001 1:07 PM
|You're mixing and matching concepts. Ballistics is really the study of how something travels, given an intial velocity, is acted upon by gravity and air resistance. A bullet path comes close to parabolic and would be perfectly parabolic without air resistance. Bikes really aren't ballistic, he was just using it as an example to illustrate a point, not make his case. There are always some interesting results when you manipulate some or all of the variables. |
There usually are special situations where doing something counter intuitive can actually have a positive result. This is why you need all sorts of qualifications when you talk about things in general terms. Lighter wheels are usually better than heavier. Lower rotational moments of inertia are usually better than higher. Less bearing friction is better than more. Better aerodynamics (spokes, rims, hubs, & tires) is better than worse, etc. What you can't do is focus completely on one single aspect while ignoring all others and then blindly declare that design/concept the hands down winner. This is why there are so many choices when it comes to wheels and why someone might use different wheels in different events.
You are right about the real world intuitive point about heavier vs. lighter bullet_all_other_things_being_equal (usually they're not). You have to be careful when you start talking real world with air resistance and the fact that most bullets are made of lead and are solid (except for special things like hollow points, and rubber bullets) and the fact that the muzzle velocities are not always the same.
Take two identical rounds, one made of lead and the other of rubber, fire them from the same gun and at the _same_ velocity. The lead bullet has a higher KE (and momentum = mv) and will take longer for the effects of air resistance to disapate all of the energy. The heavier one will travel further or mess you up more from the same distance. Now recognize that the intial amount of energy required to make these rounds travel at the same speed is significantly different. NFL Principle (No Free Lunch) also says that if you give two rounds the same initial energy the one with the smaller mass will be going faster (using either F=ma or an energy ballance). Since one is smaller it has lower air resistance, but mass is a function of volume (three dimensions) and most of the resistance is related to the frontal area (two dimensions) so things are not linear. Start talking about how bullets behave aerodynamically when they start out supersonic (Mach # > 1) and end up subsonic (M<1) and you have a whole new level of complexity.
It all comes down to one thing - getting hit by a bullet while on your bike has gotta suck no matter how fast you think your wheels are. ;-)
|Getting Hit by a Train is Slow||Tom C|
May 23, 2001 7:23 PM
|My comment Grz, earlier in this discussion was in realization that Francesco Moser deliberately used a heavy bike (23.8 lbs )in his attempt to better Merckx. Much of the weight was on the wheels in an attempt to "create" momentum. Moser had the HP to provide the initial energy to set the aggregate mass in motion. Moser's engineers knew pretty much what engineers know now that in this specific instance, the combined aerodynamic design of Moser's bike and specifically the moon disk wheels gave Moser a better chance of bettering Merckx.As I recall Merckxs' Colnago built Windsor was a mere 15 or so lbs. but was, as a track bike, conventional in every way i.e. built with really no aerodynamic consideration. Using your observation that I made a casual reference to bullet flight to make a point and didn't equalize that is make things linear lets make them linear between the 2 bikes by imagining both bikes with both riders having the same frontal area, both riders being capable of the same power output but finally Mosers bike weighing 23.8 lbs and Merckxs' all of 15. Which goes faster?|
|Initial Conditions||grz mnky|
May 24, 2001 11:34 AM
|I understand what you're saying and think that we agree. A lot depends upon the initial conditions and the duration. I'm not familiar with the specifics of your example - it would be best for me to become familiar with this. Once the more aero wheels with higher angular momentum have been accelerated they have a distinct advantage. |
Initially the heavier bike requires more energy input to get to speed. On a flat level course with identical aerodynamics (assuming they differ only at the wheels) the only input required is to overcome the drag so both bikes should be equal. Now if we recognize the fact that one set of wheels is heavier, but has aerodynamic advantages and there is no change in height then the heavier more aerodynamic setup has a distinct advantage assuming that the race is long enough to overcome the disadvantage of the intial acceleration to speed. In a short sprint it would be reversed. With a "running start" the aerodynamic wheels would have the advantage.
I guess the time/distance thing on a track is one reality, while road riding is another. This would explain why lighter wheels are favored over heavier more aerodynamic wheels by climbers.
|another way of looking at it is that, assuming your pedaling||bill|
May 23, 2001 12:22 PM
|force is constant, the force of the pedaling will give the mass the same kinetic energy, but, because the mass is less, the same force will give more speed to the mass. Of course, you have to go downhill, too. |
Got that, mon t?
|That's OK!||grz mnky|
May 23, 2001 11:48 AM
|You assume that every one wants a full blown explanation. I've found that in actuality they just want the short and simple answer (light is better than heavy, aero is better than not, etc.). The only people who really want all the gory details are us engineers, who all pretty much mastered this stuff in freshman physics and those of us who just couldn't get enough went into mechanical engineering. The only real problem is that the non-techies tend to focus on one single aspect to the exclusion of the others and don't really have the means to ballance out the trade offs. |
But, that's OK. You did a good job explaining it.
May 23, 2001 12:11 PM
|Do the heavier wheels also create more friction against the road all other things being equal?|
|I'm glad we've got guys like you||mike mcmahon|
May 23, 2001 12:26 PM
|to figure all this stuff out for us because all the technical details bore the bejeezus out of me. Just keep designing the lighter, faster, more aero stuff. And thanks on behalf of all the "non-techies" who are looking for the simple answers.|
May 23, 2001 12:25 PM
|Mass requires force to accellerate it. Less mass requires less force to accellerate; the same force applied to less mass will accellerate faster. Air has mass. Therefore, if I expel all the air from my lungs during a sprint, I'll accellerate faster.
This is perfectly logical, but true?
|the one that, I swear, made me think a few seconds through||bill|
May 23, 2001 12:40 PM
|the haze of an endorphin rush on the rollers was picking out the precise fallacy in the hypothesis that, if I turn the fan on me while I'm on the rollers, then it should be harder to pedal.|
|Order of Magnitude||grz mnky|
May 23, 2001 1:12 PM
|The real art in engineering is realizing that some things have a larger effect than others, sometimes much larger, and then optimizing a design to take advantage of this. Now if your lungs were full of water...... ;-)|
May 23, 2001 1:33 PM
|The old 'lighter but less aero' wheel argument precisely turns upon the 'larger effect' issue. Also, 'all else equal, lighter is better' -- problem is, all else is never equal.
Something to be said for empirical evidence, and to hell with the theories and general principles.
|I learned a lot about life when the ol' physics teacher said,||bill|
May 23, 2001 1:36 PM
|"Now, this variable is approaching zero, so, let's make it zero and kick it out of the equation." I didn't know that you could do that.|
|..is this gonna be on the test...?||128|
May 23, 2001 1:50 PM
|Just kidding, interesting stuff.
All this and biking is still fun too!
But! Don't think and ride...
|heres the bottom line!!!!!!!!!!!!||techie|
May 23, 2001 2:46 PM
|The reason lighter wheels are better is because we are constantly (even while riding along at what we think is a steady speed) slowing down due to friction whether it be uphill or flat. After we slow down (maybe only .001 mph) at the top of the stroke we have to pedal again to maintain that speed, and with a lighter wheel we can do this easier. I think its as easy as that.
By the way, did you guys know that a 700x23 tire pumped up to 120 lbs weighs 12 grams more than a non-inflated tire??? So the air in both tires almost weighs a whole ounce!!
|I have wondered why lighter rims help you get down the hill, too||bill|
May 23, 2001 3:11 PM
|because I think that they do. |
If you look at the kinetic energy discussion, you might conclude that the lighter object would "fall" no faster down the hill than the heavier object. And, because the heavier object possesses greater kinetic energy as compared to hub and ground friction (which we have assumed are constant), then we might think that the heavier object actually would accelerate faster down the hill. But.
While the lower mass wheel's reaction to the force of gravity is the same as the reaction of the heavier rim to the force of gravity (Galileo, remember?) in terms of speed down the hill, because the rim's rotational mass is less, the lower mass rim requires less force to accelerate it in the rotational perspective. When the same force is applied (the force of gravity on bike and rider), gravity's effect on the entire system results in a greater acceleration and greater speed for the lower rotational mass rim.
It's the same effect as with the pedaling force, but it required a little more thought.
|Oh my..... :(||JohnG|
May 23, 2001 7:30 PM
|That's a pretty fundamentally confused post! I'm not even sure what your point was. |
Try again please. Be more concise the next time though.
good rides to you
May 23, 2001 8:53 PM
|First Law: The more it costs the better it performs. Expenseive wheels are better that cheap wheels, up hill, on the flats, down hill...they better be better, we paid more for them!
Hap (We didn't have to take Physics in B-school)
Enjoyed the thread!