On Bicycles, and.... what else is there?

Last two times, I discussed an article in Bicycle Quarterly which described how low-trail bikes should perform better in cross-winds. I showed that this is true for two effects from the sidewind, one that a bike leaning into a side-wind will tend to steer into the wind if it has positive trail, and the other that a sudden gust will cause a reaction force on the front wheel that will tend to rotate the wheel. Actually, these two effects are really the same, one the steady-state effect, the other a short-term effect. But what the article missed was that the sudden gust is also going to tend to blow the bike over in the direction of the wind, so an effect which causes the bike to suddenly steer away from the wind can compensate for this. But there's another effect. Everyone knows that a deep-dish rim is often less stable than a shallow rim, or even that bladed spokes may be less stable than round ones. This is due to the direct force of the wind on the front wheel, although t...

The description I gave last time was only part of the story described in Bicycle Quarterly and Tony Foale . Suppose I'm riding along when a cross-wind strikes. It will apply a force to the rider, to the frame, to the rear wheel, and to the front wheel. The last time I dealt with the force as a whole, which causes the bike to lean, and the lean induces the front wheel to turn proportionate to the lean. But there's an additional factor. Okay, several, but there's only space to describe two of them in this post. In the last analysis, I assumed a steady wind. But assume instead the wind strikes suddenly from the side. In this case, the bike is starting vertical: not leaned over. But the wind hits and suddenly it's pushing against the rider + bike. Without friction, the bike would accelerate sideways in response to this force, wheels and all. But there is friction: the tires grip the road, the and consistent with Newton's Third, the ground pushes ...

Moving backwards through the four copies of Bicycle Quarterly I recently received, I came across a very interesting article in the Spring 2010 edition. It addressed a topic I'd been puzzled about before: the relationship between trail and cross-wind stability. It's well established that trail contributes to bicycle stability. For example, this reference describes trail as a generally positive thing for stabiltiy: more trail = more stable. So if I'm riding in a cross-wind with deep-dish wheels, I should want a lot of trail, obviously. Yet of my two bikes, a Ritchey Breakaway (somewhat slack 72.5° head tube angle) and my Fuji SL/1 (super-slack 71° head tube angle), the Ritchey seems to do better in cross-winds. What's up with that? Everything I'd read says trail is what makes bikes move in a straight line. That should apply to cross-winds as well as to other conditions, right? Well, not quite. What trail does is cause the bike to steer when it leans. ...