"It just can not be emphasized enough that effect of rotational mass of the bicycle does not matter in terms of overall power/energy needed. (Jan Heine claims it effects handling... and it certainly does to an extent, but even there I think that claim is overstated)"
Jan (and others) will also sometimes backtrack to other justifications for their feather light wheels "It's just more fun to throw the bike around in a sprint with light wheels". Others will hand-wavily talk about how every single pedal stroke includes a power phase during which you accelerate a little, plus a rest phase where you decelerate a little. These tens of thousands of "micro-accelerations" all add up and make those light wheels more important. There's no basis for any of that. Buy those $1500 super light wheels because you feel like it, not because speed demands it of you. :) Bill Lindsay El Cerrito, CA On Friday, May 16, 2025 at 2:40:17 PM UTC-7 Ben Miller wrote: > I know y'all are probably sick of me:) But here's a sort of the breakdown > of things I said above: > > 1. Power req'ed to overcome air resistance and maintain 16 mph: 100 W > 2. Power req'ed to overcome rolling resistance and maintain 16 mph: 35 > W > 3. Power req'ed to smoothly accelerate up to 16 mph over 30 sec: 75 W > 4. Power req'ed to smoothly accelerate your wheels to 16 mph over 30 > seconds: 0.05 W > > (Approximate values for a 95 kg human+bike, with C_drag *A of .5 m^2 , > C_rolling of 0.005, and 1.5 kg per each rim/tire combo. C_drag*A and > C_rolling are approximately average values for the majority of > cyclists/bikes per Bicycling Science 3rd Edition. 30 secs to 16 mph is a > pretty reasonably fast acceleration for a recreational cyclist) > > It just can not be emphasized enough that effect of rotational mass of the > bicycle does not matter in terms of overall power/energy needed. (Jan Heine > claims it effects handling... and it certainly does to an extent, but even > there I think that claim is overstated) > > But Ted is still wondering if you want those lightweight wheels to out > accelerate Jasper for the stage win, lets say Jasper Disaster is riding > really heavy 1.5 kg rim/tire as above. And you have super light weight .5 > kg rim/tires. The power savings advantage you'd have is .04 W for getting > the tires up to speed and .8 W for the total kinetic energy. Okay, okay, > but that was for the slow acceleration of 30 s, what about faster > accelerations? The thing is the ratio of those are always the same, so > losing overall weight is going to have a larger impact than the rotational > side of it. You do get the double bonus with rotational weight in that is > overall + rotational, but that power saving in rotational is generally x20 > less even in the extreme case. Maybe that matters in sprinting??? Call me > skeptical. But for us non-racers, no way. > > As for commuting around and lots of stop/starts, it still isn't going to > impact your average power/total req'ed energy much, but what tires > (physical exhaustion, not the rubber kind) humans out isn't so much average > power, but peak power (which in the above example is x1.5 greater; most > everyone's peak sustained power is x1.5 higher than there average; > regardless of overall power output). Consistently hitting peak power is > going to tire you out and make the ride feel much harder than it would be > otherwise. Again, it's really got nothing to do with your wheels. > > And finally, I definitely agree with Bill: the bike hardly matters much in > this at all. It's subtle stated in all of what I said. The main source of > air resistance is you and even on an upright Bosco'ed bar you can crouch > and reduce that. The main source of weight is you and you bike hardly > changes that. The range of coefficients of rolling resistance is very > narrow in general and how you ride probably effects this more than which > bike or at least nearly as much. Performance of a bike lies within such a > narrow range for modern bikes and it really is you and how you ride that > bike determines the "bike's performance." Bike styles/builds is really got > more to do with comfort, safety, and preference. Ride the tires that are > the most comfortable, provide the amount of traction best suited for your > riding style, and a cockpit that is the safest and most comfortable for you > and you're doing just fine. > > Phew, all that said: I need to go out for a ride :) Thanks for indulging > my overly equation-laden posts on the physics of bicycling! I won't be > thinking of any of it while I ride my bike though :) > On Friday, May 16, 2025 at 12:44:46 PM UTC-7 Ben Miller wrote: > >> George, the above equation is looking at only maintaining a constant >> velocity, so by definition the force is equal to 0. Since bicycles can >> accelerate fairly rapidly and the bike/human machine is fairly lightweight >> (relatively speaking), the energy to accelerate the bike over the course of >> the ride is small and can be ignored without changing the average power >> calculation too much. However, it's not nothing! So lets look at it: First, >> when talking about accelerations, it's generally easier to think about >> work/energy as opposed to the forces them themselves. Rotational energy is >> pretty similar to kinetic energy in that it is one half of the moment of >> inertia times the angular velocity: E_rot = 1/2 * I * w^2. For a bicycle >> wheel, this is approximately equal to this: E_rot = 1/(8*pi^2) * m_tire+rim >> * v^2 (note that it does not depend on the radius of the wheel!). So >> 700c/29er tires can easily vary from 200 to 800 g or more, or a factor of >> 4! This seems like a big change in our rotational energy need to get the >> tire up to speed! And since you have 2 of them it is compounded! Indeed, >> the energy needed to get a bikes wheels accelerated from stop to a certain >> velocity can easily change by a factor of eight due to the tires alone. >> Rims also matter, but they just don't vary as much was tire weight (usually >> less than a factor of 2) >> >> But all that rotational energy is very small compared to the kinetic >> energy of the bike. Lets say you have heavy tires like 2.4" MTB (800 g) on >> heavy rims (700g). The total rotational energy of those at 16 mph is ~2 J. >> But if the bike/human weighs 95 kg, the kinetic energy of that is 2000 J or >> one thousand times more! Instinctively we all know this. But a 29er MTB >> with 2.3 tires on a bike stand and spin them up, grabbing the brakes stops >> them nearly instantly. But Get on that MTB and get up to speed: stopping >> now takes considerably longer. >> >> TLDR; although everyone talks about rotational weight of the bicycle >> mattering more, in reality it does not. In particular, I think carbon rims >> entirely unnecessary. >> On Friday, May 16, 2025 at 12:00:18 PM UTC-7 Ted Durant wrote: >> >>> On May 16, 2025, at 1:31 PM, Ben Miller <[email protected]> wrote: >>> >>> This is effected by design of the tire, tread, surface conditions, >>> suspension, "planing" etc. >>> >>> >>> This brings up an interesting and, for me, surprising aspect of the data >>> comparison I made in my original post. The Cheviot is definitely not a >>> “planing” (terrible use of that word, I prefer “swinging”) frame. Stiff as >>> a big, steel I-beam in my experience. The Terraferma is a Jan-approved >>> noodle, skinny tubes, thin walls. I’m light and weak, but I can pretty >>> easily deflect the bottom bracket enough to make the chain hit the front >>> derailer. >>> >>> Without thinking too hard about it, at my commuting speeds, “bike drag” >>> (the whole bike/human rolling resistance bag of stuff) should be more of a >>> factor, relative to wind, than at my recreational riding speeds. So, the >>> difference in position doesn’t make a big difference at the speed, but >>> surely the springier bike would be faster. Thinking more about it, though, >>> Jan has (mostly, not always) been clear that what he calls “planing” is an >>> effect that shows up in high-effort situations, like climbing at high >>> speeds. I’ve seen finite element analysis that confirms that notion - for a >>> given frame, there is a level of effort at which the “swing” kicks in. My >>> commuting level of effort is below that level for both frames. My >>> perception of slowness/stiffness/responsiveness/swing really only happens >>> at initial acceleration from a stop or when giving a little extra effort to >>> climb the tiny hills on my route, i.e. for a tiny percentage of the total >>> time riding. Those perceptions certainly contribute to how I feel about >>> riding each bike, but they don't show up in my data on average speeds. >>> >>> Back to upright vs drop bars … that whole bag of stuff that amounts to >>> bike/human rolling resistance includes the leverage and spring >>> characteristics of handlebars and hand position. >>> >>> And, speaking of that, and Bicycling Science … Gary Boulanger (Riv >>> employee who worked at Waterford during the epoch of Heron) and I went for >>> a ride one day. As we left Thiensville, climbing the 8% bugger of a hill >>> going west, we came upon a somewhat scruffy, not terribly trim guy riding a >>> Columbia 3-speed. He casually matched our speed and chatted breezily with >>> us as we climbed the hill. Not a bit of windedness to him despite the >>> effort. Introduced himself as Jim Papadapoulos, a major contributor to much >>> of what we know about bicycling science. >>> >>> Ted Durant - still using the Second Edition, but those equations haven’t >>> changed :-) >>> Milwaukee WI USA >>> >> -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion visit https://groups.google.com/d/msgid/rbw-owners-bunch/100252af-9380-4c81-9f45-c82c991d88cen%40googlegroups.com.
