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  1. #1
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    small wheels (myths and facts)

    Many of us around the folding bike forum are engineers and, as engineers, we cringe at the seemingly endless supply of fallacies perpetuated about small wheels (by both proponents and opponents alike).

    Below is a short summary of what I've found to be the most common myths and the facts which dispell them. Theoretical and experimental proofs of the facts have been posted over the years in this forum and elsewhere and are left as an exercise to the reader.

    Myth: Small wheels are slower because you have to pedal more.
    Fact: No. The invention of the mechanical geartrain eliminated wheel size from the consideration of how fast one needs to pedal at a given speed. If this were not the case then even 700c or 26" wheels would not be able to go faster than a walking pace.

    Myth: Since small wheels have less inertia they accelerate faster.
    Fact: No. Since a properly designed geartrain will make small wheels spin faster (so you don't have to pedal more) they will not have less inertia and they will not accelerate any faster unless they are lighter. Physically speaking this is really the same myth as the "pedal more" myth, but, paradoxically, proponents of small wheels usually cite the former while opponents usually cite the latter.

    Myth: Since bigger wheels have more inertia they are faster.
    Fact: As above, bigger wheels do not have more inertia unless they are heavier. Anyone that's ever ridden a bicycle knows that a heavier bike is not faster. Although some "flying start" world speed record bikes have purposely added weight to their (small) wheels, "flying starts" are an exception because the extra weight allows one to essentially cheat by generating and storing energy before the clock starts ticking.

    Myth: Small wheels are slower because they generate more friction by spinning faster.
    Fact: While this may be true in theory, the effect is likely far too small to make any measureable difference. Most world speed records are set on small wheels and to the best of my knowledge the aforementioned friction is not even a consideration for these bikes which are literally designed to be the fastest bikes on earth.

    Myth: Small wheels are generally slower because they are more difficult to manufacture and/or profit from the manufacture of quality wheels.
    Fact: This may actually be true.

    Myth: Small wheels are weaker/underbuilt.
    Fact: No. Just as stretching pizza dough into a bigger pie makes it thinner and more likely to tear, so also bigger wheels are weaker. Most small wheels are seriously overbuilt to the same specifications as their larger weaker counterparts.

    Myth: Small wheels are unstable and/or have poor handling at high speeds.
    Fact: No. At high speeds the most important parameter influencing handling/stability is trail which can be arbitrarily set with a properly designed frame for every wheel size. The source of this myth probably stems from the fact that a small wheel fit to a frame designed for a large wheel will result in less trail and, thus, less stability just as we might expect any number of strange problems to occur from using incompatible parts. Small wheels are also clearly more nimble at low speeds; So small wheels can have the best of both worlds.

    Myth: Small diameter wheels have difficulty on rough surfaces and bumps.
    Fact: While this may be true if tire width is not increased to compensate, since the tires used by smaller wheels require less material to go around the circumference of the wheel, the weight penalty for increasing tire width is much much lower. One could argue that larger diameter wheels actually have more difficulty on rough surfaces and bumps because they can't accommodate wide tires without a significant weight penalty.

    Myth: But wide tires are slow.
    Fact: No. Wide tires are actually faster because they have lower rolling resistance and, once again, small wheels are superior in this regard because they can accommodate the wider faster tires without a significant weight penalty. While increasing wheel diameter can also lower rolling resistance for skinny tires, comfort is inherently compromised unlike with wide tires on small wheels which can regain comfort by simply having their pressure lowered.

    Myth: Small diameter wheels are less comfortable.
    Fact: Not if wider, lower pressure tires are used, at which point they are more comfortable than large diameter wheels with skinny tires without incurring a significant weight penalty.

    Myth: But low pressure tires are slow.
    Fact: Yes, low pressure tires have greater rolling resistance on smooth roads, but lower pressure tires may actually be faster on rough roads. However comfort is not as much of an issue on smooth roads anyway. So small wheels again offer the best of both worlds with the least weight penalty: wide high pressure tires with low rolling resistance on smooth roads and wide low pressure tires for comfort and low rolling resistance on rough roads.

    Myth: Small wheels are less able to climb curbs, logs, offroad obstacles, etc.
    Fact: True.

    Myth: Small wheels are dangerous because they are more likely to fall into potholes and stop short, sending the rider flying.
    Fact: No. Wheels do not fall into potholes at speed, they mostly sail over them (falling mere centimeters for every meter traveled forward at even moderate cycling speeds). Moreover, naturally occurring potholes do not have tall steep edges like curbs, so there is no reason to expect a wheel that has fallen in to stop as opposed to roll out as if going over a miniature ramp. The actual danger of a pothole is that the impact may cause one to lose control of the bike as the sideways forces on the wheel surprise or overpower the rider who may then be unable to correct the steering before crashing. If this were not the case it would be near impossible for a pothole to cause a crash with a 26" wheel (or even a 20" wheel) and, yet, we know that hitting a pothole is not an uncommon crash cause. So small wheels are actually safer because they give the rider more leverage to hold the handlebars steady and allow the rider to correct the steering more quickly. Wheel trail can not help in these situations because the self correcting property of trail depends on the interaction between the rolling wheel and an even road, which is literally undermined in the case of a pothole.

    Feel free to post more myths and facts if you have found a convincing scientific proof.
    Last edited by makeinu; 10-07-08 at 11:07 AM.

  2. #2
    Part-time epistemologist invisiblehand's Avatar
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    Interesting. As you know, I am neither engineer nor physicist. Just from rejurgitating common lore, I have been telling people that small wheels accelerate faster. So where the mass is located has an effect on acceleration; but whatever advantage a small wheel has is offset by the amount the small wheel has to accelerate to keep up with its larger bretheren.

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    Quote Originally Posted by makeinu View Post
    Myth: Small wheels are dangerous because they are more likely to fall into potholes and stop short, sending the rider flying.
    Fact: No. Wheels do not fall into potholes at speed, they mostly sail over them (falling mere centimeters for every meter traveled forward at even moderate cycling speeds). Moreover, naturally occurring potholes do not have tall steep edges like curbs, so there is no reason to expect a wheel that has fallen in to stop as opposed to roll out as if going over a miniature ramp. The actual danger of a pothole is that the impact may cause one to lose control of the bike as the sideways forces on the wheel surprise or overpower the rider who may then be unable to correct the steering before crashing. If this were not the case it would be near impossible for a pothole to cause a crash with a 26" wheel (or even a 20" wheel) and, yet, we know that hitting a pothole is not an uncommon crash cause. So small wheels are actually safer because they give the rider more leverage to hold the handlebars steady and allow the rider to correct the steering more quickly. Wheel trail can not help in these situations because the self correcting property of trail depends on the interaction between the rolling wheel and an even road, which is literally undermined in the case of a pothole.
    A Dahon Cadenza, a Swift, and a CarryMe roll into a pub. As they approach the bar, they hit a pothole, 50mm deep and 250mm across. Luckily, the edge of the hole slopes at about a 45° angle.

    -- "You OK, buddy?" the bar-keep says to the CarryMe. "You looked fine going in, but you should have seen the look on your face coming out!"

    To scale:
    Attached Images Attached Images
    Last edited by timo888; 10-07-08 at 11:49 AM.

  4. #4
    Raleigh20 PugFixie, Merc LittlePixel's Avatar
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    Quote Originally Posted by timo888 View Post
    A Dahon Cadenza, a Swift, and a CarryMe roll into a pub. As they approach the bar, they hit a pothole, 50mm deep and 250mm across. Luckily, the edge of the hole slopes at about a 45° angle.

    -- "You OK, buddy?" the bar-keep says to the CarryMe. "You looked fine going in, but you should have seen the look on your face coming out!"

    To scale:
    Heh I concur - this is the only one I disagree with. Great list otherwise - you spent a lot of time on it I feel...


  5. #5
    Senior Member Speedo's Avatar
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    Quote Originally Posted by makeinu View Post
    Myth: Small wheels are dangerous because they are more likely to fall into potholes and stop short, sending the rider flying.
    Fact: No. Wheels do not fall into potholes at speed, they mostly sail over them (falling mere centimeters for every meter traveled forward at even moderate cycling speeds). Moreover, naturally occurring potholes do not have tall steep edges like curbs, so there is no reason to expect a wheel that has fallen in to stop as opposed to roll out as if going over a miniature ramp. ...
    Count me as another who disagrees with this. This time of year (fall), the leaves have turned a beautiful orange, and the potholes have all been fixed. Life is good. By March, freeze and thaw cycles will have caused cracks to open into wheel-eating craters. The worst will be six or more inches deep and have very steep sides. I definitely need to be more cautious in the spring with the small wheeled bike. It's not enough to keep me off the bike, but it is a real issue.

    Speedo

  6. #6
    Drops small screws noteon's Avatar
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    Quote Originally Posted by makeinu View Post
    Many of us around the folding bike forum are engineers and, as engineers, we cringe at the seemingly endless supply of fallacies perpetuated about small wheels (by both proponents and opponents alike).
    Sorry, it won't work. I've known too many engineers.

    Myth: Small diameter wheels have difficulty on rough surfaces and bumps.
    Fact: While this may be true if tire width is not increased to compensate, since the tires used by smaller wheels require less material to go around the circumference of the wheel, the weight penalty for increasing tire width is much much lower. One could argue that larger diameter wheels actually have more difficulty on rough surfaces and bumps because they can't accommodate wide tires without a significant weight penalty.
    One could argue whatever one likes, but if one were to ride one's small- and large-wheel bikes on the same cobblestone street in lower Manhattan, as this one has, one would be likely to expand one's argument as one's rattling fillings provided new data.

    Myth: Small wheels are dangerous because they are more likely to fall into potholes and stop short, sending the rider flying.
    Fact: No. Wheels do not fall into potholes at speed, they mostly sail over them (falling mere centimeters for every meter traveled forward at even moderate cycling speeds). Moreover, naturally occurring potholes do not have tall steep edges like curbs, so there is no reason to expect a wheel that has fallen in to stop as opposed to roll out as if going over a miniature ramp. The actual danger of a pothole is that the impact may cause one to lose control of the bike as the sideways forces on the wheel surprise or overpower the rider who may then be unable to correct the steering before crashing. If this were not the case it would be near impossible for a pothole to cause a crash with a 26" wheel (or even a 20" wheel) and, yet, we know that hitting a pothole is not an uncommon crash cause. So small wheels are actually safer because they give the rider more leverage to hold the handlebars steady and allow the rider to correct the steering more quickly. Wheel trail can not help in these situations because the self correcting property of trail depends on the interaction between the rolling wheel and an even road, which is literally undermined in the case of a pothole.
    There's nothing even vaguely "scientific" (your word) in this. Wheels do fall into potholes. Smaller ones fall in more. "At speed" means nothing. "Naturally occurring" is not a useful distinction when discussing potholes. The leverage you're saying gives the rider an advantage also means the weight of your hands can make the bike fall over faster.

    I've also noticed that my folder is much more prone to pitching up its back wheel in a panic stop than my road bike. There are reasons besides wheel size for this, but if you're on a small-wheeled bike, chances are good you're on a folder; and things like trail, wheelbase, and position of center of gravity relative to front axle are a little dicier than on your 700cm non-folder.

    So maybe I can't blame the wheel size, but I can blame the nature of a small-wheeled bike.

    You know the joke about the engineer and the guillotine, right?
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  7. #7
    Senior Member Sir Lunch-a-lot's Avatar
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    Um... I'm not exactly sure how you figure that wider tires have a lower rolling resistance, since there is more tire in actually in contact with the road. More tire, greater resistance. If fatter tires made you go faster, I'm pretty sure that road bikes would have fat tires.

    Great list otherwise
    Pythagorean Theorum: 24 words. Lord's Prayer: 66 words. 10 Commandments: 179 words. Gettysburg Address: 286 words. Declaration of Independence: 1,300 words. U.S. Government Regulations on the Sale of Cabbage: 26,911 words.

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    ...poet... timo888's Avatar
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    And even though, as makeinu asserts, the wheel travels in a parabolic arc downwards as a result of its forward momentum while falling, if the wheel should happen to encounter the ascent of the opposite wall on its parabolic way downwards, the angle of impact could be nearly head-on, almost 90°, with a very small wheel such as the CarryMe's wheel.
    Attached Images Attached Images
    Last edited by timo888; 10-07-08 at 02:40 PM.

  9. #9
    eight spokes somnatash's Avatar
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    Quote Originally Posted by Sir Lunch-a-lot View Post
    Um... I'm not exactly sure how you figure that wider tires have a lower rolling resistance, since there is more tire in actually in contact with the road. More tire, greater resistance. If fatter tires made you go faster, I'm pretty sure that road bikes would have fat tires...
    I also would say that in general fatter tires are not faster than slim ones. Rolling resistance and "air resistance" are important for speed. The faster the speed, the more air resistance gets important and the less important is rolling resistance. In road bikes (which are fast) the air resistance the the more important factor and of course slim tires have less air resistance than fat tires.

    plus, the probably more important factor is, that fatter tires are (all else the same) heavier than slim tires. Weight slows you down. So these factors are reason for slim tires in road racing bikes.
    Last edited by somnatash; 10-07-08 at 01:15 PM.

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    i remember seeing a diagram for 16" bike where you change with 48T crank and when you change to certain speed, it is equivalent to peddling a 26" full size bike...does any1 here know what i'm talking about and display that diagram again?
    Last edited by vincentnyc; 10-07-08 at 02:33 PM.

  11. #11
    Part-time epistemologist invisiblehand's Avatar
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    Quote Originally Posted by Sir Lunch-a-lot View Post
    Um... I'm not exactly sure how you figure that wider tires have a lower rolling resistance, since there is more tire in actually in contact with the road. More tire, greater resistance. If fatter tires made you go faster, I'm pretty sure that road bikes would have fat tires.

    Great list otherwise
    Here is the best description I have seen on the web.

    http://www.schwalbetires.com/tech_in...ing_resistance

    Tire pressure determines the size of the contact patch. The width of the tires changes the _shape_ of the contact patch. What determines rolling resistance appears to be the length of the contact patch which is longer with narrow tires.

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    Ah, makeinu, how dare you challenge the accepted consensus of the unknowing with reasoned facts!

    BTW for those who want to read further about the difference in rolling resistance between fat and skinny tyres, there's a good explanation in layman's language on the Schwalbe site.

    I'd sooner hit a pothole at speed with a small wheel with a fat tyre than with a large wheel with a skinny tyre. Anyway as makeinu points out, at road speeds the wheel does not drop into the bottom of the hole.

    As far as the aerodynamics of fat v thin tyres, I think it's irrelevant to us because few of us are averaging 25mph and over.

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    Part-time epistemologist invisiblehand's Avatar
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    Quote Originally Posted by somnatash View Post
    I also would say that in general fatter tires are not faster than slim ones. Rolling resistance and "air resistance" are important for speed. The faster the speed, the more air resistance gets important and the less important is rolling resistance. In road bikes (which are fast) the air resistance the the more important factor and of course slim tires have less air resistance than fat tires.

    plus, the probably more important factor is, that fatter tires are (all else the same) heavier than slim tires. Weight slows you down. So these factors are reason for slim tires in road racing bikes.
    What often changes with tire widths is tire construction and materials. Well ... regarding the second part of that sentence ... higher-end racer tires often have fancier names for the materials. I will assume that there is actually a difference.

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    ...poet... timo888's Avatar
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    Quote Originally Posted by datako View Post
    I'd sooner hit a pothole at speed with a small wheel with a fat tyre than with a large wheel with a skinny tyre.
    Happily, they're not mutually exclusive options. You can hit your pothole with a larger wheel and a fatter tire.

    Regards
    T

    P.S. Here in the States, where so much of our tax dollar goes towards creating very big potholes elsewhere in the world rather than fixing them here at home, we are becoming veritable pothole experts. Not to make light of the matter...just a reminder as November nears.

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    I'm not going to argue every point here, but I hope this picture clarifies this business about about parabolic trajectories and a wheel not really going into a hole as those with "vivid imaginations" might think it would


    Quote Originally Posted by invisiblehand View Post
    Interesting. As you know, I am neither engineer nor physicist. Just from rejurgitating common lore, I have been telling people that small wheels accelerate faster. So where the mass is located has an effect on acceleration; but whatever advantage a small wheel has is offset by the amount the small wheel has to accelerate to keep up with its larger bretheren.
    They may be accelerate faster if they are lighter. A pair of 700c Marathon Racers are 280 grams heavier than a pair of 406 Marathon Racers, which for the purposes of acceleration are equivalent to losing about 1-1/4 pounds off the frame.

    Quote Originally Posted by somnatash View Post
    I also would say that in general fatter tires are not faster than slim ones. Rolling resistance and "air resistance" are important for speed. The faster the speed, the more air resistance gets important and the less important is rolling resistance. In road bikes (which are fast) the air resistance the the more important factor and of course slim tires have less air resistance than fat tires.

    plus, the probably more important factor is, that fatter tires are (all else the same) heavier than slim tires. Weight slows you down. So these factors are reason for slim tires in road racing bikes.
    Yeah, but all the same could be said in favor of smaller diameter tires too (lower air resistance, lower weight, etc).

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    Professional Fuss-Budget Bacciagalupe's Avatar
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    Erm.... OK, time to bust the myths of the myth-buster. I agree with some of the assertions, but:

    1) When comparing equivalent tire and wheel types, small wheels are undoubtedly harsher (and faster) than larger wheels. Even wider tires (e.g. 1.75" 406's) tend to be harsher than larger sizes at the same PSI's.

    BTW put in another vote for doing a test on potholes. They're hard on an unsuspended 700c bike but unbearable on 20", as I found out in Belgium....

    2) Anything you can think of to make a small tire / wheel more comfortable can also be done with 26" and 700c, with an equivalent effect on performance. As a result, I suggest you compare apples to apples rather than modify various parameters.

    3) Bike weight, especially non-rotating weight, has almost no effect on performance. Aerodynamics, rider position, and drive train efficiency are more important. E.g. TT bikes and recumbents are both heavier and faster than standard bikes. Similarly aero, trispoke and disc wheels are heavier than traditional wheel designs but, in many circumstances, are significantly faster.

    4) I have never heard anyone suggest that smaller wheels are weaker. It's pretty well known and understood that smaller wheels are stronger.

    5) Smaller wheels undoubtedly have more responsive handling than larger wheels. In most cases, viewing this as a pro or con is a matter of personal taste; however I've found it extremely difficult to do hand signals on fast club rides with 20" wheeled bikes while maintaining good control. It'd be irrelevant for slow or solo rides.

    By the way, if it was possible to set up a 20" bike with the same handling characteristics as a 700c, someone would have done so by now and trumpeted it in their marketing materials.

    6) Wider and lower-pressure tires definitely have higher rolling resistance; wider tires also have more drag. Ergo wider + lower psi = worse performance. There are some exceptions based on designs and casings, notably the Big Apples.

    7) There is absolutely no question that wider tires are available for 26" and 700c bikes; it's only racing-style road bikes that tend to limit tire size. Typical hybrids will run 32c's, my cross can take up to 40c, Surly even makes a 26" wheel that's 65mm (!) wide for its Pugsley.

    To suggest that wide tires are somehow unavailable in 700c is incorrect. There are far, far more tire options in 700c than for 451, 406 or other small sizes.

  17. #17
    pooh bear joose's Avatar
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    Just one thing makeinu

    I ride really slow so I'm going all the way in that hole whatever sized wheel bike I'm on.. lol

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    Quote Originally Posted by Bacciagalupe View Post
    Bike weight, especially non-rotating weight, has almost no effect on performance. Aerodynamics, rider position, and drive train efficiency are more important. E.g. TT bikes and recumbents are both heavier and faster than standard bikes. Similarly aero, trispoke and disc wheels are heavier than traditional wheel designs but, in many circumstances, are significantly faster.
    This is the crux of our disagreement. So let me ask you what evidence is there for this assertion? Bear in mind that:
    1. Theoretical arguments based on the pseudo-physics of mechanical energy "conservation" are entirely unconvincing if not completely misguided.
    2. The vast majority of riders out there, including myself, obviously think that weight has a very important effect on performance.
    3. I even see many recumbent riders arguing that the biggest roadblock preventing recumbent bikes from beating the pants off uprights in the real world (as aerodynamic considerations suggest they should) is that they're mostly too heavy.
    4. The only controlled real world experiments I've seen demonstrating an insensitivity to bike weight apply only to the obviously very limited context of a flat course with only a single stop at the end (the absolute best case scenario for weight insensitivity).

    Am I missing something? Because it seems to me that in all practical riding weight is extremely important.
    Last edited by makeinu; 10-07-08 at 05:28 PM.

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    jur
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    I think bike weight (and rotating weight) has a bigger than proportional effect on performance. I perform better on my Swift than on my heavy R20, and can't be absolutely sure about this but perhaps the effect of the extra weight that I have to handle is to put a subtle psychological block such that I don't feel like working as hard. Like a placebo effect? On the Swift I feel fast and strong and like to go like stink, on the R20 I feel like I'm lugging the world on a chain behind me, so I can't muster much effort. I think.
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    Senior Member Speedo's Avatar
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    Quote Originally Posted by datako View Post
    I'd sooner hit a pothole at speed with a small wheel with a fat tyre than with a large wheel with a skinny tyre. Anyway as makeinu points out, at road speeds the wheel does not drop into the bottom of the hole.
    For me the fat tire/skinny tire issue has been a wash. I use skinnyish small tires. The volume of air in my small wheel tires is about the same as the volume of air in my large wheel tires, and I pump both up to 120 psi.

    Since you are riding with tyres I wonder if where you are from has something to do with your opinion about potholes. While I agree that for a small pothole, the wheel does not drop into the bottom, the potholes that concern me are not that puny. Where I live, by the time we get around to March, roads that have significant cracks at the start of winter will have crater-like potholes. The town crews can't keep up with them. Worse, they are often filled with water, so in poor light you can't see how deep they are. They're a problem with small wheels. Definitely more of a jar than with the bigger wheels. By summer they have mostly been patched, and the worst roads get repaved.

    This is the second time that I can recall that there seems to be a disconnect on the effect of potholes. I really think it has to do with the experience of where you live. Last spring on South Road in Bedford Massachusetts a pothole 2 meters long and 0.5 meters wide and 0.2 meters deep opened up over the course of 24 hours. Day 1 it wasn't there. Day 2 it was there, and filled with water. I should take some pictures this winter for the edification of those who don't get to experience the joys of potholes!

    Speedo

  21. #21
    ...poet... timo888's Avatar
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    Quote Originally Posted by makeinu View Post
    I... I hope this picture clarifies this business about about parabolic trajectories and a wheel not really going into a hole as those with "vivid imaginations" might think it would

    Are you sure this graph supports your point?

    The graph users meters as the unit of measure and shows the trajectory of a traveling body falling off a cliff -- macro-view. So any micro-adjustments that the realities of a pothole might require (e.g. crumbling edge) won't appear in the graph. The graph says that the body falls 30mm for every 12mm it moves forward when traveling at a speed of 5 meters per second (18 kph). Doesn't that indicate that the wheel would not "sail" over the pothole if it's 250mm wide, but fall into it and hit bottom (on a typical pothole) and if the pothole is not very broad, the wheel could smash head on into the other side of the pothole?

    The Endo.

    Regards
    T
    Last edited by timo888; 10-07-08 at 06:08 PM.

  22. #22
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    Quote Originally Posted by Speedo View Post
    ...a pothole 2 meters long and 0.5 meters wide and 0.2 meters deep opened up over the course of 24 hours...
    I usually notice something that size and use avoidance techniques. The sort of potholes I hit tend to be smaller.

    I agree that potholes (as opposed to your chasms) would be less of an issue if I was using a larger wheel, but at any reasonable speed I don't fall into them even with the small wheel. The other point is that the rider is not a fixed weight on the bike (or shouldn't be!) so I move my weight right back before hitting such road perils and then forward before the back wheel hits, so that tends to minimise the impact anyway.
    Last edited by datako; 10-07-08 at 06:16 PM.

  23. #23
    Drops small screws noteon's Avatar
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    Quote Originally Posted by datako View Post
    I usually notice something that size and use avoidance techniques. The sort of potholes I hit tend to be smaller.

    I agree that potholes (as opposed to your chasms) would be less of an issue if I was using a larger wheel, but at any reasonable speed I don't fall into them even with the small wheel.
    I do think speedo's on to something, though. We may not mean the same things when we say "pothole."

    In the context of this conversation, my mind goes immediately to the kind my 20" wheels would be more likely to have trouble with. (And as any heavy traffic commuter knows: Sometimes you just have to eat the pothole.)
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  24. #24
    Drops small screws noteon's Avatar
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    And while we're at it, we mean different things by "falling in," too. It doesn't have to bottom out to count as "falling in." All that needs to happen is for there to be significant banging, whether on the bottom, against the far side, or in some other way that hasn't occurred to me yet.

    The fact that "banging" isn't a neatly quantifiable term in this context (in others, it's universally understood) does not hinder clear communication among anyone but the aforementioned engineers.
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    Quote Originally Posted by makeinu View Post
    Myth: Since small wheels have less inertia they accelerate faster.
    Fact: No. Since a properly designed geartrain will make small wheels spin faster (so you don't have to pedal more) they will not have less inertia and they will not accelerate any faster unless they are lighter. Physically speaking this is really the same myth as the "pedal more" myth, but, paradoxically, proponents of small wheels usually cite the former while opponents usually cite the latter.
    Your words "unless they are lighter" are critical on this issue because small wheels are lighter than comparable large wheels.

    Assuming that we are talking about comparable wheels with similar width and style of tyres, similar rim cross sections similar hubs and similar spoke types:

    Small wheels have both less linear inertia and less effective rotational inertia.

    Linear inertia

    Small wheels have less linear inertia because they are lighter. They are lighter because the smaller circumference means that the tyres tubes and rims weigh less and the smaller radius means that the spokes weigh less.

    Effect of Rotational Inertia

    Your argument correctly identifies that the rpm of a small wheel is correspondingly greater than a large wheel. At any given road speed, the tangential velocity is independent of the size of the wheel.

    Since its the mass at the rim that matters for rotational inertia and it has to be accelerated to the same velocity regardless of wheel diameter there would be no significant difference in effective rotational inertia if the mass at the rim were the same.

    As the mass at the rim (rim, tube and tire) is actually much less, then so is the effect of rotational inertia on your ability to accelerate the bike.

    David

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