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Old 10-06-2013, 01:19 PM   #178 (permalink)
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Join Date: Mar 2012
Location: Salt Lake City
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00C - '00 Toyota Corolla
90 day: 43.54 mpg (US)
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Originally Posted by Xist View Post
I really like the latest progress!

What did you put down under the magnets? Does that prevent scratching?
I believe what's shown is 0.4mm (1/64") neoprene rubber, but that stuff 'feels' a little too thin, I'm worried it would wear through or involve excessive lateral twisting. I think the final version is going to be 40A durometer 0.8mm (1/32") EPDM rubber. The idea is to prevent scratching. Rubber has some capacity to absorb vibration, and so in principle it might spare the paint (or at least extend its life). BTW, EPDM and neoprene have very similar weather, UV, and ozone resistance, and temperature range, from what I've read, but automotive gaskets use EPDM for some reason, so I'll probably be favoring EPDM in the future. (Neither one is very oil/solvent resistent, but I think neoprene has the edge there.)

NdFeB magnets also are subject to rust, which can destroy their magnetic capacity. They are nickel plated for that reason. I'll be using a cold-galvanize spray on them (and the steel plate and screws touching them are also zinc plated). I've had good luck with the cold-galvanize on the hitch-box frame so far. Also I will form the EPDM into a boot to keep water out (and to streamline the leading edge). I might even pot inside the boot with caulk, though there are reasons not to do that. Maybe I'll just pot the tail end of the boot.

Originally Posted by Thenorm View Post
nice construction. I guess you don't know yet if the magnets will hold?
I think on paper each anchor (pair) might be good for around 40lbs pull at 0.8mm separation (could be a lot lower than that in real-life, due to thin-ness of car's steel panel, as well as its mild curvature). Rubber-on-metal presumably having a coefficient of friction around 1, they should have about the same strength against lateral forces. The Achiles-heel of a magnet anchor is torque, and they can be pried up by hand, but since the back of the Kamm will be strapped down, and given the location of the hinge, I don't see a lot of torque on these anchors.

I can grab hold of the frame and give the Kamm a good stout shake and the magnets hold tight, so I believe it's good to go.

They're also strong enough to pinch fingers, quite painfully.

These ones, if you're curious (if I recall):
K&J Magnetics - Products
(Except that somehow I managed to order them in N, S pairs, which is best, by far, for maximum pull.)

Last edited by christofoo; 10-06-2013 at 01:26 PM..
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