Electronic Design

What's All This Clinometer Stuff, Anyhow?

When I decided to go on a bicycle trek in Nepal a few years ago, I planned to instrument the tough hills with a clinometer to measure the slopes I'd be ascending and descending. Well, designing a "clinometer" shouldn't be Rocket Science. A weight dangling from a pivot can tell you the slope of a hill.

So I set out to engineer one per these needs: A clinometer must be enclosed to read correctly despite winds. It must have good damping to reject vibration. If I'm gonna carry it to 18k ft, it must be light. It also has to be waterproof and durable to keep working despite knocks and bumps.

So how to plan this? I mounted the pointer inside a slice of a clear plastic cottage-cheese bowl and bolted it to a light frame made of copper-clad with spacers. I cut holes in the copper-clad frame to cut weight, and I covered those holes with thin mylar to keep the wind off the pointer. The copper-clad frame had arms that I duct-taped to the frame of my bike (Specialized Rock-Hopper). Very stable. The pivot was a piece of brass tubing from an old ball-point pen soldered to the copper, with paper-clip wire inside the tubing, weights hanging below, and a pointer above. Calibration was easy—mark the dial versus a measured slope.

But when I started riding, I realized that I had not solved a basic problem—the damping. Even with heavy oil inside the "pivot"—inside the brass tubing—there was not enough damping. I tried adding a few thin mylar vanes to damp the pendulum's motion, versus the air inside the dish. This had minimal effect: The Q of the damping fell from 5 to ~4. H'mmm. I rinsed out the oil and put in grease. Now the damping was much too heavy. The pointer would never settle to the true value. Okay, now what?

I thought: Where's the compromise between too little damping and too much? I thought about it and found the solution. While the tubing was still full of grease (SAE 90), I pushed out a little of the grease and added a little oil (SAE 10-30). The oil was along the paper clip, inside the grease. There was some mingling. The resulting gradient of grease and oil worked well! The grease and oil did not contaminate each other, and the damping stayed good. It did not freeze up when cold or get too soft when hot. What did I learn from this meter?

I could ride uphill okay, but as I pedaled, the meter did not indicate well because the bike was surging. That's okay. When I came to a steep hill, I'd ride as hard as I could, then stall. Then I could stop and read the slope.

But when riding downhill at a smooth and constant speed, with brakes on smoothly, I could peek down, and the meter would indicate the true slope.

In Nepal, the uphills were sometimes steep or moderate. But the trails were mostly rocky, and not much riding uphill could be done after the first day, except for the fifth, sixth, and eighth days. So the meter wasn't useful to indicate any "average" uphill slope.

On high slopes, many trails were more than 28%—too steep to ride. But I biked many slopes less than 14%.

On the downhill at Phedi (below the 17,771-ft pass, Thorong La, north of Annapurna), there was an "improved" trail, rough and stony, as steep as −50%. I'd let my bike down a few feet, then clamp on all brakes, then walk down a couple steps, and repeat this. Tough trail!

So I didn't learn much from this meter. But what a great trek—see at www.national.com/rap/trekking/bek. I didn't need a meter to tell me that. And I learned that a suitable gradient of oil and grease could provide good damping.

Comments invited!
[email protected] —or:
Mail Stop D2597A, National Semiconductor P.O. Box 58090, Santa Clara, CA 95052-8090

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