My Own Trainer Project

Alex Simmons

Oh yes - the chain!! Funny - it looks worse in the photos than in real life - but it's a shocker!!. I've considered some form of chainguard to keep the sweat away.

My model will have the adjustment bolts in better places etc. We could use round tubing as well.

I've ordered two large (46x30x10 mm) powerful rare earth magnets for the grand total of A$16 and we are going to rig up a device on the prototype to see what effect they have.

I've bought an SRM for the trainer.

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mikesbytes

Assume you are using track chain

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Training Log

Alex Simmons

The chains will suit the chainrings/cogs chosen. They'll cope. It's not like I'll be putting out any more power or torque than I normally do on the road.

I spoke with the guy from the hysteresis brake company today - most helpful - so he is following through with engineering in the US on the model that would best suit. It's probably a ~ A$1,500 solution though. Still, I'm investigating anyway.

In the meantime, we'll play with the eddy current effect of the rare earth magnets.

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Alex Simmons

Just another quick update.

The uber strong rare earth magnets arrived for the eddy current resistance experiment.

I got two of them. They are not big - 40x30x10 mm rectangular prisms but boy do they stick!! @ A$8 it was an experiment that wouldn't break the bank.

I have to tap them with a rubber mallet to remove them from anything they are so damn strong. Impossible by hand to remove them from magnetically attracted materials. No pacemakers or hearing aids near these suckers!

Anyway, we rigged up a bodgy device which enabled me with a wooden lever to move the magnets in and away from the flywheel on the prototype trainer. No more than basically close or not close - no fine tuning possible.

It definitely caused quite a deal of resistance - John said it felt like going from 1st gear (in the 7-speed the hub gear) to to 4th gear. And all I did was bring them in a bit closer. I could sense some mechanical pulsing through the wooden level but John couldn't detect any through the cranks. Given the flywheel is "spoked" (refer earlier pictures) I suspect the effect will be better on a solid flywheel. I calculated how much to reduce the radius of the 44kg flywheel by, so mine will be a solid disk unlike John's prototype.

So John is taking the magnets to our machining friend to come up with an easy way to have the magnets held close to the flywheel and moved up/down and/or in/out. I figured installing some kind of small screw lift device that could lift the magnets up and down with reasonable precision would be ideal.

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Steve_B

I'm visualizing something like a vise where the magnets are on both jaws of the vise, the wheel is between the two jaws and the screw gives you infinite adjustability to resistance.

Alex Simmons

Nice thinking, and that's the sort of action I meant, although only one jaw moves in a vice, so for perpendicular control that wouldn't work for a magnet rither side of the flywheel.

I was thinking of the magnets moving parallel to the plane of the flywheel, more than perpendicular. My gut tells me that the resistance will vary depending on the radial position of the magnets relative to the flywheel axle. I figure that moving the magnets a mm parallel to the flywheel axle will be a much finer change in resitance than a mm perpendicular. But I could be oh so wrong!

But being able to easily control both directions would be nice.

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WattsAMatta

I think that you're right about that.
These are the basic assumptions that I'm working with on my project (although in this instance, "working with" means "thinking about after I've had a few beers")...
* Resistance increases with strength of the magnetic field.
* Resistance increases with speed of the conductor through the magnetic field
* Resistance decreases with the SQUARE of the distance between the magnet and the moving conductor.
This implies that if you increase the perpendicular distance by a mm, say 10 to 11 mm, then you'll decrease the resistance by about 21%. (1.0^2 compared to 1.1^2) But if you decrease the radial distance by a mm, say 200 to 199 then the speed of the conductor will only decrease by about 1% . (200^2 compared to 199^2)
I've got ten of the strong magnets to experiment with on my 300PT, and my current 'best idea' is to use two or four or six or eight of them at a time at a pre-set position to set the rough range of resistance, and use the last pair moving radially to fine tune it.
-- Bryan

vladav

Disected my RavX trainer a while back. It uses eddy current braking too. The way they did the cable actuation is kinda neat. The Braking occurs in a flat aluminum ring with 6 magnets on the inside diameter, and 6 on the outside. The inside 6 are stationary while the outside 6 rotate concentric with the spinning aluminum ring. When all 12 magnets are aligned as close as possible (There's a gap to allow the ring to spin between them) the resistance is highest. When the 6 outer magnets are rotated away (staggered) from the inner 6 the resistance is lowest.

So on your flywheel, I'm thinking one magnent could be stationary, while the other one simply moves in-plane (not sure if the xy axis would matter) with the disk.

If it helps I can draw a diagram...

Dave

Alex Simmons

Yes please

It need to be a simple mechanism, easy to craft up from something and keeping the cost down.

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swampy1970

Ugggh, me likey simple.

http://render-2.snapfish.com/render...C/of=50,554,443

AutoCAD it ain't.

vladav

This isn't simple, but I believe it could be greatly simplified given all the room your working with. Keep in mind that this mechanism had to be compact at only 5 or so inches in diameter to be marketable...

The left side shows highest resistance, right is low resistance. The red band is driven off the barrel which in turn is driven by the tire.

I am seeing a possible(?) problem with trying to do the eddy current braking in a solid disk - compared to a limited conductive path (ie Wire, thin Band, Ring etc.) there's plenty of room for the electrons to simply avoid the traffic jam created by the eddys. Hopefully I'm wrong

But if it proves true, then maybe a thin aluminum ring insulated/spaced off the disk with magnets on either side? (I doubt you need them evenly spaced around the circumference, just spaced enough to get the modulating effect.)

Anyways, hope this helps!

Attached Thumbnails

 

vladav

Probably left abunch of ambiguity - let me know where I should expound...

john979

My goal is to turn my PT300 Pro into a cheap Velotron. Actually, I am hoping that Saris sees the possibilities of the same. I really think they could substantially increase market share with an optional electromagnetic brake.

From a conceptual point, I like this: http://www-ph.postech.ac.kr/Edulab/phy-esp1/web/eddy/eddy.htm.

Commercial electromagnetic calipers are available: http://www.entecnational.com/

But I wonder if Velotron would sell just their electromagnetic braking unit?

john979

Does anyone know the diameter of the Velotron 1) flywheel and 2) the copper conductors?


Thanks.