Flutterby™! : Measuring forces 2004-07-05 20:49:37.512513+00

Measuring forces

2004-07-05 20:49:37.512513+00 by Dan Lyke 4 comments

Yesterday Charlene and I loaded down the tandem and pedaled up to Petaluma for a party at a friend's house. The plan was to camp overnight and pedal back today. It wasn't a a long trek by my solo standards, 29 miles on the odometer, but on the tandem with a lot of gear, and with stopping regularly to massage out some of Charlene's muscles, it was a long pedal. Especially that 8 mile stretch on the shoulder of highway 101 between Novato and Petaluma. Yuck (although we did keep a respectable pace on the flat, and it was another chance to clear 35mph on the downhills... Much more stable now!). And the house was well up a hill, so while toodling through Petaluma proper was fairly flat, by the time we got to the last mile or so I totally bonked. We got off and walked for a bit, then I got back on the bike and was able to pedal it up the hill, so it was clear that Charlene had bonked earlier than me.

This morning Charlene hurt in places she doesn't normally, so she decided it'd be easier to catch a ride south (with the gear), and I pedaled the tandem home (and I'm about to go out skating, 'cause I haven't had this good a workout in a long time and I wanna feel the burn).

But we got to thinking... It'd be cool if we could measure the force applied by each rider, both for historical information, but also so that I could know when I'm bearing the load and when I'm not, so that I know earlier than "oh damn that hurts" that we're not going to make a hill, for instance.

The two easiest places to measure that seem to be by seeing if there's any measurable crank deflection or deformation, or by measuring tension along the top chains. Crank deflection would require some sort of wireless transmission, and I doubt there's much of it going on so it'd be difficult to distinguish from noise (same with, say, measuring seat weight). But we started brainstorming about a chain link that had something measurable that changed under tension, and suddenly it hit me: Measure the temperature differential between the chain on the top (under tension) and the chain on the bottom, and see what could be derived from that. Anyone know if this sounds reasonable to pursue? Yes, there's a lot of noise, but the two places would provide a reasonable baseline, and I know the chain's gotta get hotter under tension but I'm not sure I ever got to that place in physics.

Anyone got suggestions?

How hard would it be to add an idler pulley that deforms the top chain, and then measure the tension on that pulley? You're going to be trading off friction and slack for accuracy (analogous to the loss you get when you try to measure a large voltage or current). The pulley doesn't have to be in the middle, it can be close to the back gear cluster where there might be more things to weld it to. Mount it on a strain gauge and smooth the results over two or three pedal cycles.

The problem with using temperature is that you've only got a few seconds to measure it before the top link becomes the bottom link, and my hunch is that it will take longer than that for the tension to translate into heat enough to be measured. Unless you do something wacky like point a pair of those remote-read thermometers (the kind with the laser pointer and some kind of radiatively-coupled sensor) at the chain just before it hits the gears.

The only other idea that came to mind was to fabricate two round plates that sit between the front crankshaft and the gear cluster, one bolted to each, joined with perpendicular flanges with strain gauges between them. At a guess, you'd need at least half an inch of space to work with, and the linkage between the crank and the gears might not lend itself to wedging anything in between them.

Measuring temperature at the resolution you need would be expensive. Adding the idler pulley is a more reasonable approach. You could even use a potentiometer instead of a strain gauge for this.

Another way you could discover the information you are looking for is for you to slack off for a moment just before you hit the hill and see how much complaining you start hearing.

This link lists some of the various devices on the market for measuring power output (input?) on a bike.

One monitor by Polar ( http://www.polar.fi/polar/channels/eng/segments/products/S625X/ accessories/Power_Output_Set.html ) uses measurements of chain tension, speed, and vibration to calculate power. Much easier than using a special hub and whatnot. Unfortunately, I'm not so sure that it would work on the stoker chain of a tandem. You could certainly put it in back and measure total power output though. Seems like the crank based units would be your best bet (although they're terribly expensive and you probably wanted to homebrew something).

I discounted the idler pulley idea because I thought the additional mechanical drag was more than I wanted to deal with.

Thanks for those links, Berticus, although I'm having trouble telling if those devices have real-time output. And, yes, in hoping for something that wouldn't require a machine shop to fabricate a lot of those prices seem like more than this information is worth to me.

I need to brush up on the mechanics and figure out what sort of crank deformation is happening, because either that chain vibration trick or measuring the crank seem like the easiest (and cheapest) ways to go about this.