Monday, March 18, 2013

randonneuring tire mass versus size

Last week I rushed to my bike to catch my morning train and the tire was flat. There wasn't nearly enough time to swap the tube, but there was an old Mavic Open Pro wheel laying there with an old Continental Supersonic 20 mm tire. I grabbed the wheel, inflated it to 100 psi (it held!), put in on my bike and caught the train.

I couldn't believe we used to race, let alone train, on these things. Every bump felt like it threatened to cause a pinch flat. Even the speed bumps in the parking lot at work seemed like they'd launch me into ballistic trajectory.

James Huang, CyclingNews
Ellis Randonneuring bike, 2011 North American Handbuilt Bike Show (James Huang, CyclingNews)

The improvement in ride quality going from 20 mm to 23 mm tires is profound, and it's even better making the jump from 23 to 25 mm or 26 mm. I love my Michelin Pro 25's, and my Grand Bois Cerf 26's. They're even good for a little fun on the dirt, riding fire roads and smooth hiking trails. But even 26 mm tires have their limits: once last year I was riding to work in an SF2G paceline when I aced a pot-hole not far from home and dented both of my rims (the tires stayed inflated, however). Something fatter with more air volume and there's a good chance I would have been able to finish the ride rather than walking home in my cycling shoes to catch a late train.

It's the advantage of fat tires which I view as one of the primary attractions of randonneuring bikes. Yet despite the rapid growth in 29'er mountain bikes designed for 700C wheels and "gravel racers", there's a shortage of high-quality wide tires. Traditionally the space above 25 mm has been dedicated to either heavily-treaded cyclocross tires or to super-heavy touring tires. Touring tires are designed for maximum endurance and minimal flat probability, but the rolling resistance associated with their thick casings is appalling. These tires have done a disservice to width: it's not that wide tires are slow by nature, but rather they tend to be slow by design.

Jan Heine and Bicycle Quarterly have gone a long way towards promoting the advantages of supple, light wide tires. He even advocates the use of huge 650B/42 mm tires for his dream-bike, a custom-built Rene Herse (built in Colorado, not Paris). The wider the tire, the bigger the bumps it can absorb. And when riding on gravel roads or on typically poorly maintained American roads useful for bike commuting, that's a good thing.

The obvious issue with fatter tires is mass: fatter is heavier. So I decided to look into what's available. I made a plot of some popular randonneuring tires, plotting claimed mass versus tire width. Here's the result:

mass vs width

For a range of sizes (Paris-Moto available in only 650B/38 mm), you can see that the Grand Bois Extra Léger tires are by far the lightest of the bunch. Next come the standard Grand Bois tires. Jan Heine likes these tires and sells them through Compass Cycles, his on-line bike component shop. Curiously, wheel size plays a relatively small factor. The Grand Bois 650B and 700C tires fall on roughly the same curve. The Extra Léger tires are 80% - 87% the mass of the standard Grand Bois tire of the same size, although fewer sizes are available so far. I extrapolate the results to predict the mass of future Grand Bois Extra-Léger using a multiplication factor of 80% for 700C tires, and 87% for 650B tires, based on comparing the sizes which are already sold. I get Grand Bois tires from Box Dog Bikes in San Francisco.

In addition to the Grand Bois, Compass sells a tire under its own brand in the 26-inch size, which Jan advocates for widths greater than 42 mm. This tire falls on a bit of a heavier trend line than the Grand Bois tires.

The Panaracer Pasela is a popular randonneuring tire, and is available in a broad range of sizes. Yet it's much heavier: the 700C/32 mm Pasela is 70 grams heavier per tire than the corresponding Grand Bois standard, and 128 grams heavier than the corresponding Grand Bois Extra-Léger. Deforming all of that extra mass surely comes with a huge increase in rolling resistance losses. Panaracer sells another tire, the T-SERV PT, which is quite similar. In both cases I plot the tire with the synthetic bead, rather than the wire bead: the wire bead is substantially heavier still.

Then there's the Vittora Randonneur. With a brand with the racing tradition of Vittoria (their tubular road tires are excellent) you'd expect high performance. But these tires are real anchors, coming in even heavier than the Paselas. Maybe the goal is to maximize the effort to build strength for racing.

With all of the tires, the mass increase is relatively modest up to approximately 28 mm, but beyond this and the mass increases more rapidly. 32 mm tires are clearly heavier than the tires up to 28 mm, but going above 32 mm you pay an even stiffer penalty. For me this suggests 32 mm is close to a sweet spot. But maybe if I had experience riding fatties I'd feel differently. Certainly if I want to go fatter than 32 mm I'll want to hope that Grand Bois extends its Extra Léger range.

added: For 650B tires, the Panaracer Paris-Moto tires are notable in that plot for their lightness. There's two: the black (lighter, higher thread count) and the tan (heavier, lower thread count). Of course I would prefer the black. I don't understand the retro sidewall aesthetic.

3 comments:

jefaisduvelo said...

The Vittoria Randonneurs are a horrible tire. I was hoping to opt out of buying Grand Bois Cypres tires and bought some Randonneurs. They, as the French would say, sucked.

My only solution has been to purchase Grand Bois in large enough quantities to get a discount.

djconnel said...

Thanks! I'll avoid the Vittorias...

I really think it's a misperception of what the market wants. They think anyone riding wide tires doesn't care about efficiency or speed unless they're racing cyclocross or riding a 29er MTB. Grand Bois has realized this isn't the case. I suspect there will be more competition in the future as the trend has been towards wider rims and wider tires. 19 mm rims will be extinct.

Jan Heine pointed out elsewhere my correlation of mass with rolling resistance doesn't apply to the bead. True: a wire bead is heavier but shouldn't increase rolling resistance. For this reason I tried to select tires with synthetic beads.

As to the price of Grand Bois: tires are an important part of ride quality so I don't mind spending money on them. Some people try to reduce costs at tires but I'd rather save money buying slightly heavier components. Of course if you're riding SRAM Apex, for example, you're already doing that. But it hardly makes sense, for example, to spend $$$$ on carbon handlebars or on a fancy carbon fiber frame because they improve ride quality then get cheap on tires.

jefaisduvelo said...

Certainly find a tire which works for you. I experimented in 2011, and the Cypres was the only thing which worked for me. Unfortunately, they are expensive but I will shell out the cash for them. I put my Cypres tires through 2 1200k's, and I had to rotate them. At roughly 2000 miles and 3 total flats between the set, they were shot. I did just put some Pasela w/ TG on my commuter (26x1.25) and I have liked them so far.

Mass is only part of the equation. "Suppleness" (not to be confused with friction) plays a big part as well and is independent of mass (wire vs kevlar beads is just one example - a low mass tire could be made of carbon fiber, yet it wouldn't be very supple).

Good luck, roll safe.