shave grams & grave smash
To make my bike lighter, it would be wise for me to cut back on the breads and occasional gorging of sweets. But, that’s the wise solution. Sometimes I prefer the long way around a problem; oftentimes the view is more enlightening, actually.
The goal was to lighten a wheelset by 25 grams. Yes: 0.055 pounds. The problem: those pesky magnets that are included with chronometers, so that, with each revolution of the wheels, the magnet will pass by the fork-mounted unit for the calculations. In addition to scarring the beautiful appearance of bladed spoked with this gaudy piece of magnetic-molded-into-plastic, the unnecessary size kills aerodynamics and creates an obvious imbalance for rotation.
The solution: find a small, powerful magnet capable of triggering the unit while minimizing aerodynamic drag and aesthetic losses. I posed my problem to a few of my engineering friends and one of them mentioned I should search for “rare earth magnets”. And I did. And I found a very small magnet, indeed, to further this experiment of mine. And, truth be told, I found it at a magic shop online.
I’ll be honest, when I realized that magnets were sold through magic shops, my respect for the field was seriously destroyed. But, for my purposes, the magnets will serve a mighty purpose, far less clouded in mystery than their expected use. Also, neodymium magnets are remarkably strong. This little thing is able to pick up my keys, which is a respectable feat for my pockets let alone a miniscule magnet.
Doing the calculations to approximate the gain in efficiency is pretty neat. The two factors that change are the moment of inertia of the wheel and the drag. However, finding the new drag coefficient is practically impossible without windtunnel testing. So, just from the weight loss, I am looking at a gain in performance of about 0.002%. Nice.
Mission Accomplished.
December 19th, 2007 at 10:09 pm
Are you interested at working to solve some of my challenges with similar determination?? If you are interested, please let me know.
A friend introduced me to neodymium magnets several months ago. He had a few, each about the size of a pencil eraser. They were incredibly strong. He could stand several feet from an old PC monitor (CRT) and create distortion on the screen. One day he had them in his pocket as he walked past a metal desk and the magnets stuck to his desk and just about ripped the pocket from his pants.
His experience raised my curiosity in these rare earth magnets. Have you looked at the large super neodymium magnets that are available online? I am talking about those that are about 2 inches tall and 2 inches in diameter. Get 2 of these things near each other and you can permanently crush your hand. In fact, you may not be able to pull them apart. The flashing warning messages at the purchase sites were a dead giveaway that these magnets were lawsuit material. I would love to have a pair of smaller ones for home play use but I am concerned what potential damage could occur to magnetic media and electrical equipment in the same room.
Last thing, I was again talking to my friend this week about these magnets. We were wondering about the influence of these magnets on FedEx/UPS delivery trucks and drivers. Picture this… one of these super magnets can have greater than 200 lbs magnetic force. If they shipped these magnets in small boxes and the box landed on the floor of the truck, the driver would not be able to lift the box from the truck (assuming the box would stay intact). I suspect they ship these magnets in large boxes to prevent near-contact with a metal surface.
Another last thing (Forgive me, I am on a roll here). We concluded having one of these large super magnets would be interesting to have in an apartment if your neighbor on the other side of the wall ever cranked up his massive home theater system next to his large picture tube TV. Just roll that large super magnet on the shared wall and his TV would have a changing warped image and the magnets in his speakers could not stand up to the dominating super magnet force and would start disintegrating. All along, he would think something is wrong with his equipment. Once the sound volume resided to a desired level, you could remove the magnet from the wall. It might take a time or two, but I bet you could train your neighbor (without his knowing) to keep the volume down. DISCLAIMER: This above situation was intended for mental entertainment value only. Do not, I say, DO NOT go off and try this curious feat of magnetic dominance. Personal injury and financial ruin can occur. I did not even mention the potential flying debris injuries that could occur in the neighbor’s room as loose metal objects start flying towards the wall. I am dead serious. If you do not believe me, you need to read those flashing warning messages of the online suppliers’ websites a couple times and think about the lawyers just waiting for a case of magnetic abuse.
December 19th, 2007 at 11:57 pm
during the phoebe’s field stuff, we had talked about having a room with an electromagnet that kids could point at a 3-dimensional grid of small bar magnets. the electromagnet would cause the bar magnets to all rotate to wherever the electromagnet was positioned at the time. it made for a neat concept and sketch collection.
then it occurred to us how many small metal things we typically have on our bodies, even what kids might have. something like pants rivets are subject to magnetic forces. if we had a room like this, we would be legally required to make sure that the kids & their guardians signed assumption of risk forms prior to entering the museum. which, in hindsight, is probably a terrible way to start off a learning adventure for a middle school field trip.
plus, an electromagnet that size would require a power supply on the order of 5000 W which means currents around 40 amps are running nearby. in effect, we’re talking about bringing kids into an texan execution chamber and hoping they walk out having learned some physics.
December 20th, 2007 at 4:50 pm
I used rare earth magnets to attach my Darwin fishy to my car. The Darwin fish had a sticky tape back and I didn’t want that to touch my paint job. I bought two tiny magnets for 50 cents and they have kept my profession of evolution attached to my car even through an automated car wash.