Tuesday, March 29, 2005

Engineering meets Reality

One early summer day in 2000, the shop got a call from an engineer working on a project he told us was secret. All he needed from us was some simple mechanical work.

"I want you to take apart the front wheel of a mountain bike and replace the hub with one I'll send you. The spokes will need to be about 7 inches long."

"How did you derive that number?" I asked.

"Well, that's just the 26-inch rim diameter, minus the diameter of my hub," he said. "That'll do it, right?"

"Not quite," I said. "Got a pencil?"

The bike spoke length formula is a longer chunk of math than most of us readily deal with, involving cosines and square roots and stuff. Personally, because I had difficulty with math class, I would only tackle it with a calculator. I reeled it off to the engineer.

"Whoa," he said. "I thought bikes were simple."

"Maybe, but not that simple. You're looking for a tangent running from one circumference out to another. Your spoke has to be long enough to reach with ample thread, but short enough not to stick into the area where the inner tube goes.

"It will be cheaper to build you wheels than to sub in your hubs, too."

We ended the conversation and each ran the calculation using his hub dimensions and the effective rim diameter of the rim I suggested.

Over the course of several days and numerous phone calls and emails, we determined the wheel would have to be spoked radially, because the dimensions he had already specified meant that we couldn't adjust anything to make room for tangential, crossed spokes.

"You shoulda come to see me sooner," I said.

We ordered the parts and extra tooling we would need to produce three front wheels for the three prototype bikes he finally told us he was developing. Then we waited for the hubs.

The engineer did not know a lot about the specific details of bike construction when he started with us, but he knew them by the time we finished. He'd gone into this not knowing the standard diameter of a front axle or the over-locknut width of a front hub, yet he hoped to use stock forks. So he had parts being machined before he knew exactly how to make them fit existing pieces of his planned structure.

A friend of mine describes an engineer as a person with all the common sense taught out of them.

He had enthusiasm and a willingness to persevere. Good thing, too, because our shop put in a lot of hours. I would ride this thing through to a billable result, come what may.

Despite his assurance that the product would make a big splash when it came out, and his promise that he would send us any press items about it, the wheels went out and his payment came in, and we heard no more about it.

Then, last summer, a local family brought in their spiffy new electric bikes. The wheel configuration looked oddly familiar. The motor was no longer in the front hub, but the front hub was still a large drum to accommodate the battery.

Ralph and I rode the bikes after we peformed the minor repairs they needed. They're fast, with two power settings, a low-power cruise mode that supposedly yields a 20-mile range, and a "turbo" setting, for very snappy acceleration.

Like all electric bikes, they weigh a lot, around 50 pounds, so no one is likely to pedal them much. Like all electric bikes, they're really just a clean-running, rule-beating small motorcycle. Shhh. Don't tell the government. You'll have to register them and stay off the bike path. But they're the best example I've seen so far, much sportier than the EBikes another customer brought in.

The models we saw use regular rim brakes. These are definitely inadequate for a vehicle of that mass. At the very least, pad wear will be rapid. The arms flex alarmingly under hard stopping.

Of course you may not notice the brake arms flexing if you're distracted by the way the standard mountain bike suspension fork bends under its load.

Bike stuff is light because the human motor is weak, so the vehicle itself has to be built lightly. Motor vehicle stuff is heavy because fuel or batteries, and motors, engines and drive trains are chunky, to contain and direct the forces generated with that extra horsepower. It is hard to merge the two. Any motor, even a woefully inadequate one, will make a bike weigh much more than a rider of any ability will want to push around by muscle power. The better it is as a motor vehicle, the more irrelevant its pedals become.

That being said, I'd rather share the road with electric motorcycles than gas-guzzling barges.

It was a fun project. We all learned a lot and shared what we knew.

So what will this summer bring?

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