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Old 26th September 2006, 03:59 AM   #11
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Quote:
Originally posted by kelticwizard
It might be that rare earth magnets might be the way to go here.
Don't forget to put one arm out the window to help hold the stuff down...

I_F
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Old 26th September 2006, 04:33 AM   #12
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Old 26th September 2006, 05:55 AM   #13
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There are some *seriously* strong magnets around..
I'd bet some could hold an elephant on to the roof of your car! Really!

First,Be warned: you *MUST* be careful when handling them,you can easily remove fingers if you get "pinched" with them
They can come together with tremendous force.
Be very careful when installing/removing your rack!

The guys that make DIY wind-power and such use big strong magnets alot in thier generators,so if you know where to look,you can find quite strong magnets.
Here's some..
http://www.forcefieldmagnets.com/cat...hp?cPath=23_37

Or,Maybe find a few of those magnetic handles used for carrying big metal plates,etc. Those puppies are pretty strong too. Kinda large though.
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Old 26th September 2006, 12:29 PM   #14
Schaef is offline Schaef  United States
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There are a couple of things to consider with your idea KelticWizard. All of them point to don't.

First, with a lot of magnets, their force is in holding them together, which means pulling them apart is diificult, but you can slide them apart. Now, in your application, what direction are the main forces going to be in? Perpendicular to the magnet, meaning, while it will hold the roof rack down, it won't hold it in place. Particularly if you put felt in there to protect your roof.

Second, in addition to the strength of the magnet, you also need to consider the roof itself. There may not be enough metal in the roof to be able to hold the magnet well. I found out about this by watching a Mythbusters episode where they had magnets that worked great on 1/4" steel but on 1/32" sheetmetal ducting, they couldn't hold.

Third, if you're going to use the very powerful magnets others have pointed to, have you thought about what its going to do to the contents INSIDE the car?

Finally, the magnets you talked about have nowhere enough strength to do the job.

Oh, and just as a quick real life example. I did a theatrical show where the director wanted a bulletin board that could be attached and removed from a wall without any visible means of support. My final solution was to use two old hard drive magnets behind the flat and two mending plates glued to the back of the board. It took both magnets to hold the 2 pound board on the wall, otherwise it would slide. (Hence my comments on #1) Oh, and these magnets were out of full height 5 1/4" hard drives and could hold each other through my hand, so they're strong.

So, short answer, I agree with I_Forgot, let me know your plate # so I can avoid you if you do this.
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Old 26th September 2006, 12:47 PM   #15
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Schaef:

Thank you for your well thought out comments. However, I find myself getting committed, (you can interpret that last sentence any way you want ).

Digital Junkie made a good post pointing me to those lifting magnets. Available inexpensively, they are ranked at 25 lb, 50 lb and 100 lbs lifting weight. Each crossmember will have two, so you double that.

Of course, the roof is slightly curved, so the I imagine the lifting power is reduced somewhat since it won't be contacting a completely flat surface. And there will be thin coating of something, (rubber, fabric) to weaken it slightly more. Still, I think there will be more than sufficient force here.

By the way, if someone has a suggestion of a nice sticky coating to put on the bottom of these magnets so they won't scrach the paint, by all means please jump in.
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Old 26th September 2006, 12:55 PM   #16
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Just a quick point, in the UK roof racks have to be approved for use, so if something goes wrong you are insured. If your idea fails and damage or injury occurs, then you might well find your insurance doesn't cover you...
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Old 26th September 2006, 01:01 PM   #17
Wizard of Kelts
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On the issue Schaef raised, the matter of the the rack sliding across the top of car. Let me relate something that actually happened a couple of years ago, when I was using the car which the hooks of the roof rack had something to hook onto-thereby making a completely secure system.

Once I accidentally put an 8 ft stepladder on top of the racks without lashing it down. It was just resting on the racks entirely unsecured.

I drove off. For about half a mile, there was a gentle downward slope. I stopped at the stop sign. Made a right turn, drove off at around 35 or 40 mph.

It was only about a mile down that road that the unsecured ladder fell off the racks. I picked up the ladder, secured it and drove off none the worse for wear.

Granted, there was no wind. Granted, this was not highway speeds. But the fact that the ladder stayed on the roof, unsecured for a mile and half of normal driving indicates to me that if the racks are being held to roof by two lifting magnets of 50 lbs lift each, sliding across the roof is not likely to be a problem.

Of course, once I do this, I have every intention of going onto an empty road and driving this around, slamming on my brakes, etc, to make sure this thing works before I get onto any other roads with traffic on them.
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Old 26th September 2006, 04:24 PM   #18
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Quote:
I have every intention of going onto an empty road and driving around, slamming on my brakes
I've always wondered what folks do in “The NUTmeg State” for fun.
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Old 26th September 2006, 04:44 PM   #19
wwood is offline wwood  United States
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As mentioned previously, the holding force of the magnets is perpendicular to the face of the magnet. Friction is what keeps it from moving laterally. If your roof and the magnets have a large coefficient of friction then it might work. I have some neodymium - iron boron magnets (about as strong as they get) and they slide over finished steel quite easily. Stopping or turning sounds like a big risk, plus damage to the finish of your vehicle.

If you must do this, make sure the magnets have a large surface area, the larger the better, then roughen the mating surfaces to further maximize friction.

What did you say your plate number is?

Bill
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Old 26th September 2006, 05:04 PM   #20
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Hallo,
Magnetic ski racks are not new. They do have approval by the German authorities (TÜV - not easy to deal with).
I give you one link. It is in German but a translation programme will give you the essence:
http://www.stiftung-warentest.de/onl...6/1224616.html

Regards
Wilfried

One more link to an Italian manufacturer:
http://www.fabbrisrl.com/catalog/gb/auto/index.htm
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