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Originally Posted by cplager;16120983...
A couple important points here: 1) When accelerating a bicycle, you need to not only accelerate the wheels (both linear and rotational sense), but also accelerate the bicycle and the rider. If you compare the mass of the wheels to that of the bike and the rider, you'll see that the wheels are are very small part of a bigger picture. So, yes, it takes more force to accelerate a heavy tire than a light tire, but not (relatively speaking) a lot more force to accelerate a rider, bike, and a [I "heavy"[/I] tire compared to the same rider, the same bike, and a "light" tire.
2) When you are moving at a constant speed, you only need to provide enough power to overcome the frictional forces against you. It will take less energy to keep a heavy but low rolling resistance tire moving than a light but high rolling resistance tire. But going downhill, say coasting down, there are a couple of things going on. We're accelerating at least at first, and the angular momentum will slow that slightly. Very slightly. Then the angular momentum and extra weight will allow a higher speed I think depending on the grade. At the bottom our heavier tires will hold the speed better, again because of the rotating momentum. So tally it up: uphill steady, slowed by just the extra weight. Downhill, helped by extra weight AND angular momentum. Flat after, helped by angular momentum, slowed a tiny amount by extra weight. Going back uphill and slowing, helped by the angular momentum and held back by weight. I'm not so sure that it wouldn't be advantageous to have a heavier tire on some type of rolling course. |
Originally Posted by chaadster
(Post 16121888)
I'm at a loss to understand how you can hinge your whole argument on this 'constant speed' concept and how it means the rider doesn't have to input energy to maintain that speed, then concede 'constant speed' doesn't really exist and that riding is, in fact, a series of constant accelerations while pedaling, yet maintain the position that tire weight doesn't matter. Can you explain what I'm missing here?
It's only when you are accelerating from a stop that most of your force is NOT counteracting drag and in that case, the importance of the wheel weight gets (almost but a little less than) a factor of two.
Originally Posted by chaadster
(Post 16121888)
You also seem to be certain that riders cannot notice, or feel, the weight difference between light and heavy tires, but I don't understand that position, either. If you accept, as you said above, that heavier tires do require more energy to acclerate than lighter ones, why would a rider not be able to sense that difference? We can feel the difference in power it takes to smash a grape under foot, and probably even a tender raspberry, so the idea that we can't feel how much resistance we're facing through the pedals doesn't smack true. I don't know how much force it takes to smash a grape compared to the force it would take to accelerate a 950gm tire versus a 210gm tire (same wheel/gear), so maybe you could address that.
There is one caveat here: A heavier tire has more angular momentum and this could possibly (as in, I can't confirm that it isn't) be noticeable when riding the bike in a slalom course or when "pumping" the bike very hard side to side when sprinting. Neither of these are activities that most people do very often, but it can happen. So, in this case, it's possible that somebody might be able to "feel" the heavier tires (but it wouldn't really slow you down much).
Originally Posted by chaadster
(Post 16121888)
Lastly, if you maintain that tire weight doesn't matter, how do you account for the widespread believe that it does, and the pursuit of lightest possible weights in all matters related to energy efficiency in vehicle development, whether bicycles, human powered vehicles, cars, and motorcycles?
Battle Mountain bicycles are not light bicycles. They are aerodynamic bicycles because if you want to be fast, you need to be aerodynamic. If you are racing a bicycle, then these effects can matter. A 0.5% weight effect may not be noticeable by the rider, but it can make a few seconds difference over 100 miles. If you are a professional racer, then I agree it makes sense to worry about these effects (although, again, rolling resistance is a much bigger issue). Finally, I agree that there is a wide-spread belief in bicycling that wheel weight is important. And, except for racers, it's basically wrong. It's as important as it is, it's very quantifiable, and if weight is an issue, most of us should eat a bit less instead of worrying about the weight of the tires on our bikes. For almost all commuters, having a tire that was heavier, but had lower rolling resistance and better flat protection would almost always be a welcome change. Cheers, Charles |
Originally Posted by cplager
(Post 16122675)
There is one caveat here: A heavier tire has more angular momentum and this could possibly (as in, I can't confirm that it isn't) be noticeable when riding the bike in a slalom course or when "pumping" the bike very hard side to side when sprinting. Neither of these are activities that most people do very often, but it can happen. So, in this case, it's possible that somebody might be able to "feel" the heavier tires (but it wouldn't really slow you down much).
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Originally Posted by cplager
(Post 16122675)
Maybe this is an easier way of thinking about it: When riding at an almost constant speed, almost all of the force you are putting in is being used to counter-act the rolling resistance/aero drag/flex/transmission losses. Only a little bit of it is being used because you are accelerating. In this case, the importance of the tire weight is a factor of (almost, but slightly bigger than) 1.
It's only when you are accelerating from a stop that most of your force is NOT counteracting drag and in that case, the importance of the wheel weight gets (almost but a little less than) a factor of two. In the original case, we were talking about 200 grams extra. Even with the factor of two, that's 400 grams extra (less than one pound). Let's say you have a 20 lbs bike and are 180 lbs. An extra pound is a 0.5% effect. I don't most people here are sensitive enough to notice that. If you weight 100 lbs and have a 10 lbs bike, we're still talking an effect smaller than 1%. There is one caveat here: A heavier tire has more angular momentum and this could possibly (as in, I can't confirm that it isn't) be noticeable when riding the bike in a slalom course or when "pumping" the bike very hard side to side when sprinting. Neither of these are activities that most people do very often, but it can happen. So, in this case, it's possible that somebody might be able to "feel" the heavier tires (but it wouldn't really slow you down much). Cars are different because in city driving, they are spending a lot of their time speeding up and slowing down. And when they try to reduce the weight, they are trying for much bigger percentages than 0.5%. Battle Mountain bicycles are not light bicycles. They are aerodynamic bicycles because if you want to be fast, you need to be aerodynamic. If you are racing a bicycle, then these effects can matter. A 0.5% weight effect may not be noticeable by the rider, but it can make a few seconds difference over 100 miles. If you are a professional racer, then I agree it makes sense to worry about these effects (although, again, rolling resistance is a much bigger issue). Finally, I agree that there is a wide-spread belief in bicycling that wheel weight is important. And, except for racers, it's basically wrong. It's as important as it is, it's very quantifiable, and if weight is an issue, most of us should eat a bit less instead of worrying about the weight of the tires on our bikes. For almost all commuters, having a tire that was heavier, but had lower rolling resistance and better flat protection would almost always be a welcome change. Cheers, Charles Certainly tire weight doesn't happen in a vacuum, and other factors like tire construction, rubber compound, tread pattern, carcass size, and rolling resistance are all part of the tire performance/human performance interface. Usually, heavier tires face other penalties than just weight. Ultimately, I believe that it behooves any cyclist who thinks about their effort and performance to run as light tires as possible (by which I mean, the lightest tire that meets all their demands, such as desired tread pattern, flat resistance level, price, etc.). |
Only when accelerating does the wheel weight matter
but you will get some of that back on your glide Once you are up to speed-only the rolling resistance matters |
Originally Posted by jrickards
(Post 16109781)
Comparing the weights of my Michelin Pilot Sport tires (440gr ea) and my Marathon Greenguard (730gr ea), the difference is 290gr ea which, considering the weight of my 29er, rear rack and somewhat loaded panniers and me, doesn't make much difference (~0.5%). That is the simple consideration of the weight difference. However, given that the tires need to be spun up to speed, is there physics involved that is more complex to suggest that the difference in weight is multiplied because it is spinning and not just simply a static difference like adding more water to your water bottle?
Wheel weight at the axle center has zero velocity in its rotational frame of reference for no additional inertia, and wheel weight where the rubber meets the road is traveling as fast around the axle as the bike is going in a straight line and in its rotating frame of reference accelerating at the same speed as the bike is in its stationary frame of reference so it counts double. IOW, if all that weight was where the rubber meets the road those tires would make a 1% difference which still isn't interesting unless you're a competitive bicycle racer or work in bicycle marketing where claiming that rotating weight counts double might help move product. Heavier carcasses are likely to be less supple which implies higher rolling resistance that can be noticeable (Crr of slick road tires varies by almost a factor of two from slowest to fastest) although that's not a direct result of the weight. |
I know the consensus on this thread is that a few grams of weight amounts to little but, theoretically I find it interesting. Another discussion point regarding this topic is wheel diameter. I'm pretty sure adding 200 grams to a 700c tire would have a slightly different impact than adding the 200 grams to a 26 inch tire. After all, torque (the tendency of a force acting on an object to cause the object to rotate) is dependent on both the radius of the object (or the distance from the applied force to the center of the object) and the force on the object. From what I understand some of the additional wheel/tire mass will act in opposition to the the force supplied by the cyclist. The counter-force due to the additional mass of a tire is at the outside of the wheel whereas the riders force is applied closer to the hub which obviously complicates things, especially with varying gear ratios due to different chain rings etc.
The linear velocity of a rotating wheel is also affected by the diameter of the wheel as it is a function of angular speed and the radius of the wheel. Anyone have some insight on all of this? |
Originally Posted by turky lurkey
(Post 16131121)
Another discussion point regarding this topic is wheel diameter. I'm pretty sure adding 200 grams to a 700c tire would have a slightly different impact than adding the 200 grams to a 26 inch tire.
The linear velocity of a rotating wheel is also affected by the diameter of the wheel as it is a function of angular speed and the radius of the wheel. Anyone have some insight on all of this? |
All physics aside, I can state from empirical evidence that my bikes with larger, heavier tires are slower than the ones with lighter tires. I have tracked my miles and average speeds on all of my bikes for many years and tens of thousands of miles. The bikes with heavier tires are simply slower. My average speed on my touring bike with 35 mm tires is 14.64 mph over 7,277 miles (most of that commuting). My two sport touring bikes, which have 25-28 mm tires, have average speeds of about 15.8 mph over 9,237 miles (mostly commuting). My road bike with the lightest tires (23-25 mm) had an average speed of 17.1 mph over 12,834 miles (all recreational rides and tours).
The bikes with larger tires also feel slower when riding them. They are slower to accelerate and harder to ride up hills. It is hilly where I ride most of the time. If I always rode on flat roads, the differences would not be as large, but heavier tires are still slower to accelerate if you ride in stop-and-go traffic like commuting. |
Originally Posted by tarwheel
(Post 16131211)
All physics aside, I can state from empirical evidence that my bikes with larger, heavier tires are slower than the ones with lighter tires. I have tracked my miles and average speeds on all of my bikes for many years and tens of thousands of miles. The bikes with heavier tires are simply slower. My average speed on my touring bike with 35 mm tires is 14.64 mph over 7,277 miles (most of that commuting). My two sport touring bikes, which have 25-28 mm tires, have average speeds of about 15.8 mph over 9,237 miles (mostly commuting). My road bike with the lightest tires (23-25 mm) had an average speed of 17.1 mph over 12,834 miles (all recreational rides and tours).
Comparison should be made on the same road, with the same goal. I always ride faster when NOT commuting (since when I commute, I try to sweat as little as possible). If you don't take that into account, well - I have empyrical evidence that my tires are pro east - I always ride faster TO work (since I like to sleep as long as possible), than FROM work home. :) |
My commute route is the essentially the same every day, carrying nearly identical loads. That is the primary basis for my comparison. I have commuted on at least 6 different bikes over the years, using a variety of different tires. My speeds are faster when I ride the bikes with lighter tires and less rolling resistance. The biggest change I noticed was when I briefly installed some Panaracer Paselas on one of my bikes and my average speed dropped about 1.5 mph immediately, riding the same route with the same loads. I removed the tires after a month or so, and my speeds went back up.
All of these pronouncements about mathematical formulas remind me of weather forecasts and models. The forecasters speak with near certainty about the upcoming weather based on their mathematical models, yet they are often wrong -- even on the very next day. The problem with using mathematical formulas and models is they usually don't account for all of the factors. Sometimes they turn out to be very accurate and sometimes not, presumably due to factors the modelers are not aware of or taking into account. |
Originally Posted by tarwheel
(Post 16131301)
All of these pronouncements about mathematical formulas remind me of weather forecasts and models. The forecasters speak with near certainty about the upcoming weather based on their mathematical models, yet they are often wrong -- even on the very next day. The problem with using mathematical formulas and models is they usually don't account for all of the factors. Sometimes they turn out to be very accurate and sometimes not, presumably due to factors the modelers are not aware of or taking into account.
The difference here is that, unlike weather forecasting, the classical dynamics involved in understanding bicycle propulsion is very well understood (well, by some, at any rate). Again, it's not that the tires were heavier. It's that they rolled worse. Yes, lighter tires generally have less rolling resistance, but that doesn't have to be the case. Go buy some awful light tires (and put in some light weight tire liners - those do great things to rolling resistance too) and you'll see. It's as if all light tires were black and all heavy tires were red and everybody was proclaiming that "I can state from empirical evidence that my bikes with red tires are slower than the ones with black tires." A lot of people here would do well to remember: Correlation does not imply causation. |
My view is that your formulas for calculating wheel speed are leaving something out. I can't tell you what that is -- just like I can't tell you why weather forecasts are so often wrong -- but my personal observations based on thousands of rides suggest that something is missing from your calculations. On paper, it may be a "proven fact" that wheel/tire weight has little or no effect on average speeds. In practice, many cyclists have observed that their speeds are slower when riding bikes with heavier tires/wheels.
Jan Heine, of the big proponents of fat tires, has conducted a bunch of tests on various tires -- concluding that fatter tires are faster than narrow ones. Curiously, his test involves measuring how far tires roll and how fast going downhill. Hmmm ... Do you think he would have gotten the same results if going uphill? Of course not, but you can't simply roll a bike uphill -- it has to be pedaled. I personally agree with his premise that wider tires are just as fast or faster than narrow ones -- if they are the same weight. However, wider tires are generally heavier than narrow ones, and that's why most cyclists perceive them to be slower. |
Originally Posted by tarwheel
(Post 16131480)
In practice, many cyclists have observed that their speeds are slower when riding bikes with heavier tires/wheels.
The effect of the weight is both very easily quantifiable and accurate. You can notice a huge effect between a good and bad rolling tire. The average rider here would not notice such an effect for different weights. Seriously, why not just start saying that your tires with white reflective letters on the side are faster than those without (probably true, one average since higher end tires are more likely to have it)? |
Originally Posted by cplager
(Post 16131363)
Oy.
Go buy some awful light tires (and put in some light weight tire liners - those do great things to rolling resistance too) and you'll see. |
Originally Posted by tarwheel
(Post 16131480)
However, wider tires are generally heavier than narrow ones, and that's why most cyclists perceive them to be slower.
The fastest tires are made for performance enthusiasts who don't mind 2000 mile lifetimes and have historically run 23mm tires. Their manufacturers have a history of not offering the same construction in wider sizes, even a 25mm tire which won't fit some racing frames. This is slowly changing - breaking with tradition in 2014 Continental will offer its raciest clincher (the GP4000S II) in a 28mm width. Wider tires are usually built for a different markets that value lifetime and flat resistance over rolling resistance with the resulting stiffer carcasses having higher rolling resistance. |
Weight is partly the cause, as cplager noted, the science is clear that heavier tires require more energy to accelerate than lighter ones. Thi
What's in question in this discussion is the extent to which the energy cost of heavier tires is significant; this is why there's the constant qualification, "except for racers" by those who claim it's insignificant cost. I say that, and most evidently at the extremes of the weight range, e.g. between 430gm and 1900gm tire sets, the difference in energy required is noticeable, significant, reduces overall speed (over some distance, but most evidently in longer distance) and increases rider fatigue, irrespective of tire construction, rolling resistance, or any of the other factors that often disadvantage heavier tires. Heavier is always costlier (in terms of energy requirements). Of course, my contention is only true for riders who cycle sufficiently aggressively; if one doesn't ride long or hard enough to tax their power and stamina, then we're literally on a slippery slope to the bottom, where riding it down-- not even around-- the block is the standard, which is clearly absurd. I believe that anyone (well shy of a racer only) who can tire themselves out on a bike can appreciate 1470gm difference in tire weight after rides of equally tiring effort. Granted, I haven't compared the exact same tire in heavy and lightweight versions, so the variables are impossible to rule out, but again, the science says there's a cost, so it's only for us to decide if really an extra 3lbs of tire is reasonable to be indiscernable. No one would argue that they cannot feel if I gave them a 3lb bag of rocks, but some, apparently, would claim they cannot feel it if that 3lbs on their tires, though anyone who has ridden knobby tires in the mud knows that feeling of additional weight when their tires pack up. |
Originally Posted by chaadster
(Post 16133770)
I say that, and most evidently at the extremes of the weight range, e.g. between 430gm and 1900gm tire sets, the difference in energy required is noticeable, significant, reduces overall speed (over some distance, but most evidently in longer distance) and increases rider fatigue, irrespective of tire construction, rolling resistance, or any of the other factors that often disadvantage heavier tires. Heavier is always costlier (in terms of energy requirements).
Yes, the more urban you are, the more time you are accelerating from a stop. But this is still a small fraction.
Originally Posted by Drew Eckhardt
(Post 16133557)
The fastest tires are made for performance enthusiasts who don't mind 2000 mile lifetimes and have historically run 23mm tires. Their manufacturers have a history of not offering the same construction in wider sizes, even a 25mm tire which won't fit some racing frames. This is slowly changing - breaking with tradition in 2014 Continental will offer its raciest clincher (the GP4000S II) in a 28mm width.
Wider tires are usually built for a different markets that value lifetime and flat resistance over rolling resistance with the resulting stiffer carcasses having higher rolling resistance. This is why wide tires get such a bum rap. The difference in rolling resistance between goold and bad rolling tires is HUGE. In my case, I went from an OK rolling tire with a tire liner (100 PSI Maxxis Detonator - which coincidentally has one of the dumbest names for a tire that I've ever heard) to a better tire with no liner (100 PSI Schwalbe Kojak) and my average times have increased 1.5 mph from under 12 mph to 13.5 mph during my commute. |
There are slow riders with light tires, but no fast riders with heavy tires. :)
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Originally Posted by chaadster
(Post 16134428)
There are slow riders with light tires, but no fast riders with heavy tires. :)
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Originally Posted by cplager
(Post 16134769)
That's not true. I've seen guys on 26" knobby tires fly past pace lines. Talk about pissed off roadies! :D
Like I said, there are slow riders on light tires...on roadbikes... in 'pace lines'. I'm sure you can come up with all sorts of anecdotal tales of MTBs crushing roadies, but the fact is, a strong rider on a 26" knobby would be even faster on a light slick, or do you know deny that, too? |
Originally Posted by chaadster
(Post 16134428)
There are slow riders with light tires, but no fast riders with heavy tires. :)
What a funny claim! Not true though, unless your definition of fast is a ridiculously high speed. Think of your fastest cycling friend (or yourself), I bet their are a lot of people in the world who would be faster than that person with tires 2 - 4 times as heavy as the ones that person uses. (Unless your friend is a pro cyclist, but even then my claim might hold true) |
Originally Posted by chaadster
(Post 16120788)
I'm not crispy on the physics, but I don't understand how hill climbing cannot be anything but a series of accelerations, when between each stroke, you not only have rolling resistance slowing you, but also gravity working to pull you back downhill?
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Originally Posted by turky lurkey
(Post 16135211)
What a funny claim!
Not true though, unless your definition of fast is a ridiculously high speed. Think of your fastest cycling friend (or yourself), I bet their are a lot of people in the world who would be faster than that person with tires 2 - 4 times as heavy as the ones that person uses. (Unless your friend is a pro cyclist, but even then my claim might hold true) |
Originally Posted by Pedaleur
(Post 16135216)
The short answer is, it's also a series of deccelerations, where you "get the energy back".
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