GenVibe - Community for Pontiac Vibe Enthusiasts

Ok, I bought a bike computer and I have to attach one part of the sensor to the front fork of the bike and the magnet to one of the spokes. The sensor and the magnet have to align when the wheel turns and also be no further than 2 mm apart. The problem is, I can't. My forks are too wide to do this. The only place that might be possible is near the axle, and the picture shows it to be attached near the tread.My question is does the wheel turn at the same RPM near the axle as it does at the outer edge. Because in order for the computer to properly register speed and distance it needs an accurate measurement of how fast the wheel is spinning. My brain can't recall the formula to figure this out after 8 years of non use. Thanks

well, the entire wheel turns at the same rate. think of a spoke spinning at the tire rotates. is it possible for one end of the spoke to rotate at a different speed?

All parts of the wheel makes the same revolutions per minute (RPM). Your device depends on the RPM, so the magnetic sensor could technically be placed anywhere on the spoke.Where you might get confused is that the flyby speed of the sensor does vary depending on the location on the spoke. Obviously, the magnetic sensor speed as it flys by the fork detector is slower near the axle, and faster near the tire. But it still registers the same RPM.

The rpms will be the same at any point on the rim. The difference is the closer to the center will travel at a slower speed since there is less distance for it to trvel one revolution and the closer to the tire will travel at a faster speed since there is more distance for it to travel one revolution. But the time it takes for one revolution at any point on the spoke is the same.

Cool thanks. That is what I was thinking, but years of television has destroyed my mental capacity for all things educational.

as others have said, all parts of the wheel rotate at the same RPM, but the parts of the wheel closer to the axle have lower linear speed than parts closer to the tire.This may or may not be a problem. A changing magnetic field will generate an electrical current in a conductor. The amount of current depends on the size and rate of magnetic field change. What this means is that if you mount the pickup close to the axle, it is possible that the magnet will be moving past the pickup too slow for the pickup to notice. Probably not, but it is possible. There is probably a minimum speed the thing will work at. The practical impact may be that miminum speed will be higher the closer you mount the sensor to the axle. But that is physics, you were asking for math and bicycle info Assuming you have to calibrate the computer by telling it your wheel size, then as long as the pickup senses the magnet going by, the speed and distance calibrations should be as accurate as they would be if the sensor was mounted close to the tire.

I even got a B in college physics. Too many formulas to remember. I would think that just as long as the magnet creates the electric field in the sensor it will register. The duration shouldn't have any effect on it. Guess I'll have to test it out and see. If I am cruising at 3 MPH or 70 MPH then I will know it doesn't work. And if at all else, I will just have to take another computer back to the store minus the zip ties.

Quote, originally posted by redlava » I would think that just as long as the magnet creates the electric field in the sensor it will register. The duration shouldn't have any effect on it.The sensor is just a reed switch that closes when the magnet gets close enough - the same type of switch is used in burglar alarm systems to detect when doors/windows are opened. If you listen closely, you can frequently hear a faint click when you place the magnet near the sensor and cause the switch to close.The bike computer sees the switch closures and determines your speed by measuring the time between successive closures. As you wrote, the duration of the closure has no effect on the indicated speed and distance. Placing the sensor near the hub will work fine at all speeds.

Quote, originally posted by prathman »The sensor is just a reed switch that closes when the magnet gets close enough - the same type of switch is used in burglar alarm systems to detect when doors/windows are opened. If you listen closely, you can frequently hear a faint click when you place the magnet near the sensor and cause the switch to close.cool, then distance from the axle shouldn't matter