One of my students is working at Upper Echelon Fitness, which shares space with Portland Bike Studio, an excellent cycling boutique on Portland's East Side. UE Fitness, run by Russell Cree, is a coaching company paired with a fit studio, and my student is mostly assisting with bike fitting. The cool thing about going there today is that Molly Cameron, badass cyclocrossista and vegan, was running her side of the business, the Bike Studio bit. Molly manages to dominate the Northwestern Cyclocross scene while running two bike shops, the aforementioned PBC and the excellent Veloshop on the other side of town. She rides a Ridley and stocks Ridleys in the store, and I saw a chance to ask her something that's been niggling at me for a while.
"What do you think about the lower bottom brackets showing up on 'Cross bikes everywhere these days?" I said.
"Hate 'em," she replied.
"Can I quote you on that?"
"Please do!" she answered.
Here's the deal. 'Cross bikes have been appearing with lower bottom bracket drops. "Bottom bracket drop" is the distance from a horizontal line drawn through your front and rear axles (you're looking at the bike from the side, OK?) to the line, parallel to the ground, that passes through your bottom bracket. Big name builders such as Sacha White and Richard Sachs like bikes with bigger drops (White's Speedvagen lists a BB drop of 70 mm, and Sachs only builds bikes with 80 mm drops—that's lower than most road bikes, mind you). Bottom bracket drop is important in many directions. Here are the rules of thumb: lower the bottom bracket, and you lower the center of gravity, which makes the bike more stable while turning. Raise the bottom bracket, and you get more clearance underneath the bike, which can be a downright safety concern off-road, in criteriums, or on the track (Captain Dondo used to run a high bottom bracket and 165 mm cranks so he could pedal through the corners at crits back in the 70s—he won a lot of races that way). So you've got that consideration. Here's another one, though. Raise the bottom bracket, and the chainstays shorten a little bit, so the bike climbs better. Lower it and the wheelbase stretches a bit, giving you (again) a more stable/smooth ride.
The 'Cross community seems to headed in a low direction (the Focus Mares sports a BB drop of 70 mm; the Habanero Cyclocross goes 8 farther, with a drop of 78 mm!). But the best bikes in the cyclocross world, Ridley Bikes, still boast a very small BB drop—57-61mm throughout the whole range of bikes. Another great (European) bike maker, Stevens, sports drops of 62-70mm throughout its range, which is just low of center for 'cross bikes (a good median is 65 mm, I believe). I'd been thinking that, perhaps, Ridley had fallen behind the times.
"I used to ride low bottom brackets," Molly continued, back at the studio, "and I was always hitting pedals and crashing. I crashed right in front of Sacha at Providence in '06 and was like, 'See!' But he wouldn't change the bottom bracket drop. I thought I just couldn't handle the bike, but then I rode one of these things (she pointed up at one of several Ridleys on the wall) and thought 'Whoa, I can sprint out of any corner on this bike.'"
Molly detailed a whole bunch of good reasons to keep a high bottom bracket, mostly having to do with the type of cornering one does in 'Cross racing. Bikes with low bottom brackets sweep through long turns, like long shallow descents out of the mountains. But you almost never make that kind of turn while racing a 'Cross bike. Most of the turns are hairpin, requiring the rider to almost stop, navigate the turn, and then sprint out of it—that's why your heart rate stays so high in 'Cross. Since you're moving slowly you can't lean the bike over dramatically, which is where your low bottom bracket would come in handy. But with a high bottom bracket you can turn and keep pedaling, which will help you stay upright (you know how motocross riders correct the back end of their bikes? They give the bike some gas, which is the same thing as pedaling). Then, with the turn almost completed, you can sprint out of the corner. With a low bottom bracket you'll probably spend a lot of time whacking your pedals on the ground. That won't kill you, but it'll cost you, and a few half-second a lap, over 6-12 laps, can mean around 30 seconds. 30 seconds is an eternity in 'Cross racing.
Keep those bottom brackets up! I'll be visiting Molly to price a Ridley in the coming months.
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