Magnets To Attach Ski Rack To Car Top?

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This sure doesn't have anything to do with audio, but it does everything to do with magnetics, so I thought I would post it here.

I have a second car, well used, on which I usually keep a simple ski rack-actually just two simple bars with pads on them. Crude drawing of one below. The pads are 2.5" by 4.5" with rubbery plastic on the bottom. The racks attach to the car by hooks which hook under the top of the windows.

The ski racks are useful in case I have to transport anything like plywood or ladders. The ladders I usually use are a simple 10 foot step ladder and the occasional-very occasional-20 foot extension ladder.

The problem is that I got rid of my old second car, for which the ski racks fit beautifully. My new second car does not have anything for the ski racks to hook onto in the window area.

I really, really don't want to glue anything or drill anything to the car rooftop. I've looked around for some comparable ski rack which might work, but I haven't found one.

I was thinking of taking flexible magnet material as anchors and gluing the bottom pads onto that. If that is not strong enough, I was thinking of several small solid magnets to be glued onto the pads.

A) Does anybody think the magnet material will be strong enough? I imagine a flexible magnet material piece of perhaps 4" square per pad.

B) How about some hobby small magnets, say one inch flat rounds Would several of them work glued to the bottom of the pads??

C) I would like to attach some thin material to the bottom of the solid magnets if I use them. Felt would be nice except I don't think it holds up to rain and snow well. Anybody have an idea for a fabric material to glue to the bottom of the magnets to prevent the roof from getting scratched?


I realize I am not giving you much in the way of specific figures here, but I think a lot of people on this forum have worked with magnets and can guess what flexible magnet material and hobby type solid magnets can and cannot do.

Any advice would be appreciated.
 

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I seem to have pointed to the Lee Valley site a number of times in various posts - perhaps it's time to issue a disclaimer: I have nothing to do with this company except as a long-time satisfied customer.

Anyway, here is a link to magnet information, including some non-skid material intended to be glued on to the magnets which may fit your application.
 
Please post your license number and car description so I know not to ride behind you when you go down the freeway with a 20 foot extension ladder held on the top of your car by a few magnets.

Have you seriously considered the safety aspects of what you are intending to do?

I_F
 
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I don't think I'll be going down the freeway with plywood or ladders on top of the car. Just the local streets.

Yes, I have considered the safety aspects of this and intend to test it after buying the magnets. If I don't have confidence in it, if I don't honestly think this thing is being held down really securely, then I'll reluctantly drill the holes for ski rack.

I was hoping that someone who works with magnets might be able to give some advice, even if that advice is "Forget it".
 
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Variac said:
How will you get them off?

Thank you for asking. :)

My basic plan is as follows.

Take piece of plywood a foot long by six inches wide, with a small piece of canvas underneath it, and place a few inches ahead and to the side of one of the pads.

Take tire iron or perhaps longer piece of steel pipe, slip between ski rack crosspiece and car top, and put one end down on plywood. The ski rack cross member should bisect the lever much closer to the end contacting the plywood than the end you will be lifting.

Lift the crossmember, which will cause the pad to separate from the car top.

With the lever still holding the crossmember in the air, slip a piece of two by four with a towel wrapped around it under the pad.

The first pad should now be separated from the car top by at least a couple of inches, well outside point where the magnets make much difference.

Repeat for other three pads.

I figure it might take five minutes or so to get the rack off, but I don't put it on or take it off the car that often, so it shouldn't be a bother.
 
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 :att'n:
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/catalog/index.php?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.
 
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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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 :D ).

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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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.
 
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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