Thursday, August 07, 2008

Why Fixed Gear Bikes Are Better On Ice

We're in the midst of what passes for a heatwave here in the Pacific Northwest and somehow I find myself involved in an email discussion about setting up a bike for riding in icy conditions. I'm reminded that I heard once that Jack London wrote one at least some of his famous Yukon stories on a sweltering passage through the tropics and in that spirit I've extracted (with the permission of the other emailers) bits of our discussion of how Nate should build up his winter bike.

Nate wrote:

Fixed or SS?
Any benefit to either in inclement weather?

Michael replies:

You don't get that dérailleur thingy all covered in the omnipresent road grime.

If you go SS you can't join the Sunday morning rides out of River City Bikes.

Real suggestion: get a flip wheel and find out for yourself.

Kent adds:

Fixed definitely has the edge in dicey conditions. You know exactly how much traction you have. I blogged about that phenomena here:

Note, this post predates my days of studded tire ownership. But the ultimate winter commuter would be a fixie with studded tires.

Michael then asks:

That would be because you're never not pedaling on a fixed. You don't think you'd get the same effect by never coasting? ie, is the fixed benefit because it's already there by default.

Nate adds:

I can see benefits to both ss and fixed in the ice. When approaching a known trouble spot, I like being able to approach it in a confident position and coast through it without changing position or momentum on the bike. Just passing silently like a ship through the fog.

But I can see the benefit of fixed for the overall trip. More instant feedback of what you are riding over.

The rear wheel I'll be using is a flip flop, so I'll try it both ways and see how it goes.

I'm taking my frame/fork in for a fresh powder coat this week and will begin building it back up, hopefully finishing before the weather gets cold. Because of clearance concerns, I'll probably go with the Nokian A10 700x32. Even though this is the smallest available 700c studded tire, fender clearance is still going to be tight, so I may be ashioning my first ever set of custom coroplast fenders.

Kent clarifies the fixed advantage thusly:

Michael, here's why the fixed on ice is NOT THE SAME as a single speed. First off let's assume that you actually never do coast your single speed. That makes you absolutely unique on the planet, BTW and leads to the question of why you bother lugging a freewheel mechanism along in the first place. But let's say you do that. Now let's take the case of deceleration, also known as slowing down. You can only slow by applying your rim or disk brakes. In both cases, the mechanism is the same, pads that interface with a rotating surface. Since brakes don't just stop you instantly (you wouldn't want them too!), the brake pads slide along the rotating surface. You increase and decrease pressure to control your velocity.

But (and this is the important thing) you have no way of knowing what slip you are getting comes from the pad/rim interface or the tire/road interface. So you think "hmm, I'm not slowing fast enough, maybe I'll squeeze the brakes more. If the slip is in the pad/rim interface, that will slow you more but if the slip is in the tire/road interface, you worsen your skid.

On the fixed, much of your velocity modulation is via your legs. Even when you use your other brakes, you get the feedback of your legs together with the action of the other brakes. This lets you do the same kind of calculation a modern automobile does when applying its anti-lock brakes, comparing the rotating speed of the wheel with the braking inputs to determine if a wheel is skidding. On a fixed gear bicycle, your brain can do this automatically, in real time. On a coasting bike, you don't have the data to do this calculation.

While slips and skids are most common in deceleration, they can also occur on acceleration. Wouldn't a freewheel and fixed be equal there? Nope. Even a very tightly engaging freewheel mechanism (say a Chris King) will have a bit of slop before it engages. Fixed gear bikes also are never perfect and have a bit of slop but it's almost always less than the slop in a freewheel. And when pulling out from a stop, it's hard to tell if the slip you are getting is from slop in the drivetrain or the tire slipping on the road. Minimizing drivetrain slip makes road slip more noticeable.

Finally, in sub-freezing conditions, freewheels sometimes become sluggish in having their pawls engage. Back in Minnesota every winter I'd see freewheel pawls freeze, making the freewheel spin freely in both directions. Running light lube in the mechanism and keeping water out usually prevents this, as does warming the freewheel/freehub above freezing but fixed gear drive-trains are immune to this particular problem. Various riders on rides like the Ididasport and the Arrowhead 135 have kept a fixed cog in reserve for extremely cold conditions.