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/cdf8da87-dfea-41f2-83e0-e14943d62b58n%40googlegroups.com.
