Charity Drive and Saronni’s Colnago


Our charity drive for the Museo del Ghisallo was a great success. We sold 80 calendars and have donated the entire proceeds – more than $ 1200 (900 Euros) – to the Museo. Not only will the donations help re-open the museum, but they also show the interest and passion for this unique place. Hopefully this will help secure the public funding that any museum needs.

Above is one of the amazing bikes I saw at the Museo when I visited: a beautiful Colnago with 650C wheels. It is said to have been built for Guiseppe Saronni in 1982, the year he won the World Championships.

Saronni, as a sprinter, should have benefited from smaller wheels, which weigh less and thus “spin up” faster. It doesn’t seem to have worked out that way, since race photos show him on a 700C bike in 1982. Why didn’t Saronni use the advantage of the smaller wheels?

If you have a flat on 650C wheels, you’d have to wait for your team car, since no “neutral support” would carry the correct wheels. As a secondary concern, the handling of a small-wheeled bike inherently is less stable. (Our 650B bikes are stable because they use much larger and heavier tires than a 21 mm-wide tubular.) Despite these disadvantages, if the smaller wheels provided a significant benefit in a sprint, then a strong sprinter like Saronni would gladly accept the disadvantages. He might have to be a bit more careful in corners, but then he’d win every race in which he doesn’t have a flat!

The fact that Saronni – and many others who experimented with smaller wheels – didn’t race them indicates that the rotational inertia does not make a big difference in how fast a bike accelerates. Physics tells us that even world-class sprinters don’t accelerate all that quickly – a sprint begins at 65 km/h and about 100 meters later, racers may reach 85 km/h.

While we are at it, did you notice something else that is unusual about the Colnago? The cranks have six arms to hold the chainring. Knowing that the three arms of a René Herse crank are plenty to transmit the torque of even a strong tandem team, I wonder why they came up with that idea.

Those who contributed to the charity drive will enjoy this Colnago on their wall come May next year. For those who missed out on the charity drive, you can still donate to the Museo here.

And if you still don’t have the 2014 calendar, limited quantities still are available here.

About Jan Heine, Editor, Bicycle Quarterly

I love cycling and bicycles, especially those that take us off the beaten path. I edit Bicycle Quarterly magazine, and occasionally write for other publications. One of our companies, Bicycle Quarterly Press publishes cycling books, while Compass Bicycles Ltd. makes and distributes high-quality bicycle components for real-world riders.
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13 Responses to Charity Drive and Saronni’s Colnago

  1. Jan,

    I’ve always assumed that the increased RPM of the smaller wheel cancelled out the lower mass, so there would be no advantage in a smaller wheel to a sprinter. But I only got a “B” in high school physics. ;-)

    • The rotational inertia increases with the square of the radius, but as you note, the speed goes up, too. The speed increase is only directly proportional to the radius, so the smaller wheel still has an advantage. Going from a 622 mm (700C) wheel to a 571 mm (650C) wheel would give you an 8% reduction in rotational inertia – which is like reducing the wheel weight by 8%.

  2. Jan: Great post! You make a great point about acceleration being over-rated in a sprint, although with a well-executed leadout I think you even over-estimate how much acceleration there is. On the track kieren, a super-explosive event, riders even use heavy disk wheels, because wheel rotational inertial simply isn’t that important, while aerodynamics is huge.

    But I believe rotational kinetic energy is ½ 𝞈² I, where I is the rotational inertia and 𝞈 is the rotation rate. So even though 𝞈 is 1/radius, 𝞈² is 1/radius², so the radius² in I is canceled. The result is wheels of the same rim mass have the same rotational kinetic energy at the same translational speed. The advantage may be the smaller wheel is lighter. Angular momentum is 𝞈 I, which will be proportional to radius, so the larger wheel will still be more stable.

    • I looked it up, and you (and Peter) are right, and I was wrong: The radius does cancel. (We’ve been thinking mostly in terms of handling, where the radius does not cancel, as you point out.) I corrected the post to reflect that the smaller wheel’s accelerative advantage is due to the lighter weight alone.

      How much lighter is the smaller wheel? The reduction in radius – and thus in circumference – between 622 mm (700C) and 571 mm (650C) is 8%. If you use the same rim extrusion and the same tire construction, both the rim and the tire will be about 8% lighter, if you discount the valve on the tire. However, since the spoke bracing angle is better on the smaller wheel, you probably can get away with a lighter rim extrusion and fewer spokes, which adds to the weight savings. Even that isn’t enough to make the smaller wheel worth while.

      One place where smaller wheels do feel radically different is when sprinting out of the saddle. The rocking of the bike from side to side of course has much larger accelerations than the forward acceleration of the sprint. When I raced, the difference between my training wheels (480 g rim and 450 g tire/tube) and my racing wheels with 330 g rims and 240 g tubular tires was very noticeable. I believe that this effect often is erroneously interpreted as quicker forward acceleration.

      • Ted kelly says:

        As you observed, for the same extrusion rim mass will scale with circumference which scales with radius, and that puts another r in the numerator by way of the m in the expression for I. So I think that makes the rotational kinetic energy of the smaller wheel smaller. That is on top of the linear kinetic energy which is of course also smaller for the smaller lighter wheel.

      • I just compared the Velocity Atlas rim specs, 700c versus 650c, and the fractional mass difference is 3.2% while the difference in bead seat diameter (BSD) is 8.2%. So rim mass in practice doesn’t scale with radius. One reason could be the spoke holes don’t scale with radius, typically.

      • Do the spoke holes add or remove weight? With single or no eyelets, I would assume that they remove weight, although it’s negligible. I once contemplated drilling holes in the rim bed, where they would be covered by the rim tape. I calculated that the savings were about a gram, and not worth the trouble.

        I suspect the specs aren’t all that accurate. Perhaps they weighed the 700C rim when the extrusion was new, and the rim walls thinner as a result, and the 650C rim later in the life of the die, when it was worn and the walls were thicker.

  3. David Pearce says:

    “While we are at it, did you notice something else that is unusual about the Colnago? The cranks have six arms to hold the chainring. Knowing that the three arms of a René Herse crank are plenty to transmit the torque of even a strong tandem team, I wonder why they came up with that idea.”

    –My intuition tells me that the triple circular design of the Herse cranks is very strong, from the bottom bracket to the bolt-circle, and even though it only has a three bolts, the bolts are in the middle, radially. I really don’t know what I’m talking about, and after Christmas my math is very bad (except the subtraction from my bank account), but it looks very narrow to me where the six chainring arms on the Colnago center into the bottom bracket–almost as if one good sprint could just shear the cranks clear free from the chainrings.

    • The highest torque occurs when starting from a stop, not in a sprint. That is why on bikes with horizontal dropouts, the rear wheel tends to pull out of alignment when starting from a stop, but never in an all-out sprint. The Herse cranks are strong enough for a strong tandem team starting a hillclimb from a standing start. In fact, I wrote a blog entry about that.

  4. ted kelly says:

    On the topic of experimentations with smaller wheels that never caught on. In “The Custom Bicycle” by Kolin and de la Rosa, in the chapter on Cinelli the authors quote Cino, “the bicycles that are produced now are good bicycles but they are the wrong bicycles for the good roads we have today”. Starting on pg 127 they relate his interest in steeper angles, smaller wheels etc, and say that “one of Cino’s first prototypes was ridden by Ole Ritter in Mexico when he broke the hour record in 1968″.

    • I was thinking of Cinelli, among others, when I wrote about the “others who experimented with smaller wheels.” When I raced in the 1990s, local builder David Levy of Ti Cycles raced a 650C bike, too. And then there was Vélocio’s famous “Trotinette” with its very small wheels in the 1920s, which looked a bit like a forerunner of the original Moulton.

      • ted kelly says:

        What such experimenters have (or had) to say in retrospect would be interesting. It seems more is written about why some person thinks something is a good idea than why they feel it didn’t exactly work out.

  5. Larry T. says:

    BRAVO! Since we already have the wonderful calendar we made our donation directly via the Museo webpage. We’ll be there in July 2014, when we hope the Museo will be open and thriving. One thing to note about this is the wonderful Museo di Campionissimi in Novi Ligure has very restricted hours due to budget cuts. But nothing has been removed or sold off to my knowledge and when we visit in June of each year, a phone call is all it takes to have it opened and the lights turned on. If this happens at Ghisallo, it wouldn’t really be too bad in my opinion.

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