Any hard data on rotational weight?

[veggieburger]

Originally Posted by benb

Yes but if you look above the discussion is about values less than 10w.

Actually the discussion was about my commute into work. Lots of stops signs, red lights, green lights, hills, slowing down for UPS trucks, etc. You can throw all the science you want at me, but going from 32w Panaracer to 25w Schwalbe is magical.

[Mark McM]

Getting back to the original question, "what's gained/lost between 32mm and 23mm tires"?

Unfortunately, the OP does not specify which specific two tires were being compared. Going to the data charts from the www.bicyclerollingresistance.com website, the median value for the "touring tires", a typical 37mm touring tire has a mass of 600 g and a rolling resistance loss of 30 W @ 18 mph (29 kmh), whereas the typical 25 mm road tire has a mass of 250 g and rolling resistance loss of about 15 W.

Using the example of accelerating from 0 to 30 kmh in 10 seconds, the touring tire would require an extra power of about 2.4 W. But an acceleration of this rate requires a total power of about 360 W (for an average size rider). This kind of acceleration power might be common in races, but probably not on a typical ride - the power to accelerate the extra tire mass at lower accelerations will be proportionally less.

Compare that to the difference in rolling resistance - even under high power accelerations, the extra power to overcome the extra rolling resistance (15 W) is still many times larger than the extra power to overcome the extra inertia. And the power consumed by the extra rolling resistance occurs all the time, not just during accelerations. Maintaining a constant 30 kmh takes about 180 W, so an extra 15 W is about 8% of the total. And that's per tire - for two tires, it could be more than 15% difference in power.

So my initial reply still stands - the performance gains from the lighter tires is primarily the lower rolling resistance.

[Black Dog]

Originally Posted by Mark McM

So, I took a look at the numbers you posted, and at first glance, they just don't add up.

The difference in mass between the wheel alone (797 g) and the wheel plus the Michelin Protek tire (1625 g) is 828 g = 0.828 kg, so this must be the mass of the tire. The tire is at the periphery of the wheel. We know that the effective translational inertia of mass located at the periphy of the wheel (for a wheel that is accelerated without slippling) is twice the actual mass: I(eff.) = 2 x M. Since all of the mass of the tire is inside the maximum periphery of the wheel, we know that it's effective inertia will be slightly less than twice its mass, but 2 x M is a good approximation. So the effective inertia of the tire is 2 x 0.828 kg = 1.656 kg

If the bike were accelerated from zero to 30 Km/sec (= 8.333 m/s), the energy change of the tire would be E = 1/2 I(eff) x V^2 = 1/2 x 1.656 kg x (8.333 m/s)^2 = 57.5 Joules.

If we assume that tire was accelerated with a constant power, than the accelerating power of P = E / T = 57.5 J / 10 sec. = 5.75 Watts.

But according to your data, the difference in power to accelerate the wheel alone (3.1 W) and the power to accelerate the wheel plus the tire (13.4 W) is 10.1 W. So according to your data, it takes 10.1 W to accelerate the tire alone. This is much higher than the known maximum power to accelerate the tire alone. I think there may be an error in either your measurements or your calculations.

I also question the formula in the Wheel Fanatyk web page to calculate moment of inertia - particular the use of the term 4 = gf/N: If the mass is measured in grams, there is no need to convert to or from Newtons of force.

As an aside, the paper referenced for the trifilament pendulum method of measuring moment of inertia was from an engineer at Barry Corporation. I worked for the Barry Corporation in the 1990s, and we did not use a trifilament pendulum for measuring moment of inertia. Instead we used a simpler single wire torsion pendulum. This Velonews article about measuring wheel moment of inertia also used a torsion pendulum.

Mark, thank you for fleshing out the numbers! Reality shows that time and time again that the differences are small, even under exaggerated conditions like the one you used for your calculations. The hype (hysteria) over rotational weight is so very overblown. It is a few watts for a few seconds in a sea of watts and seconds overt he course of a race or ride. This also demonstrates my incredulity over the claims that people can feel that this wheel or that wheel "spins up faster". A few watts during an acceleration effort must surely be well below the detectable threshold for humans. Can people feel the bike accelerating faster when one water bottle is empty versus full? This would have the same effect as the rational inertia of "heavy" versus "light" wheels. No one ever claims that they can sense this yet claims of feeling wheels accelerate faster are all over the place. . I call placebo on these claims, or at the very least folks are conflating (confirmation bias) different levels of feedback from a wheel tire combination with differences in acceleration. I would also like to remind folks that the extra energy they put onto acceleration of these "heavy" wheels is all returned to them though increased inertia. It is not lost into the ether.

[Mark McM]

Originally Posted by Black Dog

Mark, thank you for fleshing out the numbers! Reality shows that time and time again that the differences are small, even under exaggerated conditions like the one you used for your calculations. The hype (hysteria) over rotational weight is so very overblown. It is a few watts for a few seconds in a sea of watts and seconds overt he course of a race or ride. This also demonstrates my incredulity over the claims that people can feel that this wheel or that wheel "spins up faster". A few watts during an acceleration effort must surely be well below the detectable threshold for humans. Can people feel the bike accelerating faster when one water bottle is empty versus full? This would have the same effect as the rational inertia of "heavy" versus "light" wheels. No one ever claims that they can sense this yet claims of feeling wheels accelerate faster are all over the place. . I call placebo on these claims, or at the very least folks are conflating (confirmation bias) different levels of feedback from a wheel tire combination with differences in acceleration.

In regards to the water bottle analogy:

Some time ago, I experimented with using a behind-the-saddle water bottle mount. The one I used could mount two standard water bottle cages.

I compared bike feel with two full bottles in the "standard" cage positions (in the frame triangle, on the seat and downtubes) to the bike feel with the same full bottles positioned just behind the saddle.

When riding in the saddle, I could detect little difference - the bike accelerated and handled roughly the same, regardless of the bottle position. This should hardly be surprising - the total mass and inertia were the same, after all. But when riding out of the saddle and rocking the bike, the bike felt very different - it took more effort to rock the bike laterally when the bottles were mounted higher up, which interfered with my natural pedaling action. I found that I had to re-adjust my pedaling style, and keep the bike more vertical, in order maintain an efficient fluid motion. Once I had re-adjusted my pedaling, I couldn't detect any difference in power or acceleration rates, but the bike definitely still 'felt' heavier - even though the bike obviously weighed the same.

I suspect there is a similar affect with wheels. Heavier wheels will have more gyroscopic precession forces, which will require more force to overcome when rocking the bike, making the bike 'feel' heavier. But gyroscopic precession only affects lateral motions, not forward motion, so difference in 'feel' may be much larger than the actual difference in acceleration.

[numbskull]

Originally Posted by Mark McM

I suspect there is a similar affect with wheels. Heavier wheels will have more gyroscopic precession forces, which will require more force to overcome when rocking the bike, making the bike 'feel' heavier. But gyroscopic precession only affects lateral motions, not forward motion, so difference in 'feel' may be much larger than the actual difference in acceleration.

Interesting insight.

[soulspinner]

Originally Posted by numbskull

Interesting insight.

Is this something like Tom Kellogs explanation on his website?

[Mark McM]

Originally Posted by soulspinner

Is this something like Tom Kellogs explanation on his website?

Could you provide a link?

[SpeedyChix]

Different example but 27.5 vs 29 wheels on trail with repeated acceleration, smaller lighter wheel is less tiring. However, it is more work on open trail as the heavier (or is it due to larger dia.) rolls faster/less pedaling. As I write this I'm now wondering if it's a tire diameter difference, not weight?

This example is based on repeated rides, same routes with riding partner. We've swapped bikes and had similar results.

Is it weight/mass or diameter? Maybe not a good comparison for this discussion.

[soulspinner]

Originally Posted by Mark McM

Could you provide a link?

Checked the website and its all changed, but he says he can feel the diff between a level tt bike and a sloper all other things equal, due to the lower cg.I have one of each and am not sure I can tell....theoretically you could use less energy to stand uphill with the sloper. Your mileage sure could vary on this one...........

[fiamme red]

Originally Posted by soulspinner

Checked the website and its all changed, but he says he can feel the diff between a level tt bike and a sloper all other things equal, due to the lower cg.I have one of each and am not sure I can tell....theoretically you could use less energy to stand uphill with the sloper. Your mileage sure could vary on this one...........

http://www.spectrum-cycles.com/geometry.php

In designing the our first compact prototype back in mid '98, we wanted to discover what if any the real world differences there would be between traditional and compact frame designs. Our first compact frame (still my favorite frame) was an exact replica of my then current titanium frame in materials and geometry save for the sloping top tube. I designed it with a severe (17 degree) slope to ensure that any differences would be as obvious as possible. We had assumed that the new frame would be somewhat stiffer and lighter. It was lighter (about 4 ounces) but it was not appreciatively stiffer. Although we were able to measure a slight increase in stiffness, it was too slight to feel. The big change came when I stood to accelerate or climb. As I stood up, the bike appeared to loose three pounds. The inertia of the bike as I rocked it back and fourth was reduced so much that I felt as though I was on a twelve-pound bike. Interestingly, when seated, a compact frame feels exactly like a traditional design. The compact design has no effect on handling beyond the increases responsiveness during climbing and accelerating.

[Black Dog]

In designing the our first compact prototype back in mid '98, we wanted to discover what if any the real world differences there would be between traditional and compact frame designs. Our first compact frame (still my favorite frame) was an exact replica of my then current titanium frame in materials and geometry save for the sloping top tube. I designed it with a severe (17 degree) slope to ensure that any differences would be as obvious as possible. We had assumed that the new frame would be somewhat stiffer and lighter. It was lighter (about 4 ounces) but it was not appreciatively stiffer. Although we were able to measure a slight increase in stiffness, it was too slight to feel. The big change came when I stood to accelerate or climb. As I stood up, the bike appeared to loose three pounds. The inertia of the bike as I rocked it back and fourth was reduced so much that I felt as though I was on a twelve-pound bike. Interestingly, when seated, a compact frame feels exactly like a traditional design. The compact design has no effect on handling beyond the increases responsiveness during climbing and accelerating.

I have read this and had a hard time not rolling my eyes The longer and heavier (than the tubing) seat post would make up for most of the difference and the effect is not likely as great as he claims. Data would help resolve these types of claims.

[bikinchris]

Ford, Koenigsegg and Porsche have spent a ton of money putting Carbon fiber wheels on their high performance cars.
Lighter wheels seem to make a difference to them.
That being said, Panaracer touring tires ride "dead" compared to Schwalbe.

[Mark McM]

Originally Posted by bikinchris

Ford, Koenigsegg and Porsche have spent a ton of money putting Carbon fiber wheels on their high performance cars.
Lighter wheels seem to make a difference to them.
That being said, Panaracer touring tires ride "dead" compared to Schwalbe.

Automobile wheels are 'unsprung weight' - in other words, it is weight that is not suspended and isolated from road shock. Since the wheels are connected to the chassis by springs, their mass will affect how well the wheels/tires maintain contact with the road. The wheels (rims) are a large part of the unsprung weight, so weight reduction of the wheels will have a significant affect on handling.

On a rigid frame bicycle, the wheels are not isolated from the rest of the bike. The mass of the wheels is just a small percent of the total weight of the bike/rider system. Also, bicycles travel at much lower speed than the racing cars which can benefit from carbon rims, so the rates and magnitudes of the dynamic forces is much less.

[zap]

Originally Posted by Black Dog

I have read this and had a hard time not rolling my eyes The longer and heavier (than the tubing) seat post would make up for most of the difference and the effect is not likely as great as he claims. Data would help resolve these types of claims.

Slap on a full carbon <80g saddle......that makes a small difference.

[soulspinner]

Originally Posted by fiamme red

http://www.spectrum-cycles.com/geometry.php

oh, thanks