Need help determining magnet size/strength for a car project

So for the Raider project, I'm building a second swing-out, which will hold the spare tire (about 60lbs) as well as a couple water/fuel tanks (maybe another 30-40lbs). The swing-out is the usual style, with a spindle-style pivot on one end and a heavy latch on the other, at the bottom crossbar. The bottom is very secure.

Then there's a twin-tube (square tubing) vertical and the tire hanger, which holds the tire to the rear. Like so:

So with the tire and tanks on, it has the expected issue of some vibration fore-and-aft as the lower crossbar and/or the verticals are torqued. I want to minimize that vibration, and additional steel bracing won't do it. I also want to not destroy the spindle with constant vibration.

So Jeep guys have a "bumper" for the swing-out that basically when the swing-out is latched, it compresses the bumper higher up on the tailgate, reducing vibration at the top. I want to do similar on mine. For a first test, I basically made a rubber bumper out of some old engine mounts:

This reduced the vibration somewhat, but only stops movement in one direction. I had thought of putting a latch back there somehow, but it would be hard to access with the tire on. So then I got thinking about "what about a magnet?"

I woudn't have to be strong enough to hold the swing-out closed per se (the latch does that, as well as a secondary spring pin), but if I could put a magnet where I have those bumpers in the previoius pic, I'd just need it strong enough to stop any for-and-aft movement at the top of the swing-out. And of course weak enough that I can actually pull it open with a good tug and not use all my body weight lol. 

Ideally it would attache to the truck's door (there are reinforced bolt holes there) and use its magtetism against the steel of the vertical bar, so method of attachment is also a concern, of course. 

So, any thoughts on what size magnet to get? There are plenty out there, in a lot of styles....150lbs, 50lbs, whatever. I had thought about maybe just getting making an electromagnet that would be strong when "on" and non-magnetic when off, but that seems like unnecessary complexity. 

So, what size magnet would YOU get for this as the "first try?" I'm not against dropping some cash and then it being too strong/too weak and having to find a different one. Just want an idea of where to start.

Thanks in advance!

This company just does Magnets,

https://www.kjmagnetics.com/

You need to invest in a fish scale first and pull the assembly with enough force to see a change in the gap where the rubber bumpers are located. That will get you a good starting point for the strength of the magnet required. Also note that magnets pull force is usually stated at full face to face contact and the pull strength drops off very significantly with a small increases in separation. You may be able to find the values or calculate the magnetic force vs air gap. I think it is material/grade specific.

If you can't get the information here, it may be time to call in some experts-

stafford1500 said:

You need to invest in a fish scale first and pull the assembly with enough force to see a change in the gap where the rubber bumpers are located. That will get you a good starting point for the strength of the magnet required. Also note that magnets pull force is usually stated at full face to face contact and the pull strength drops off very significantly with a small increases in separation. You may be able to find the values or calculate the magnetic force vs air gap. I think it is material/grade specific.

not a bad idea. back to Amazon!

side note: not concerned with the air gap - I don't need them to pull together from any distance, as they'll be in contact directly (and if the right strength, will not separate until pulled with some force by me opening it. 

californiamilleghia said:

This company just does Magnets,

https://www.kjmagnetics.com/

Just for posterity - 

I touched base with KJ and one of their reps read this thread and recommended a 60# neodymium magnet (about 11 bucks). It's installed, and is pretty much perfect for this application. 

That's it there on the crossbar (circular):

Just be aware that neodymium magnets are very strong magnetically, but very brittle physically.  They don't like impact or shock loads so you should be gentle with closing that rig.

stukndapast said:

Just be aware that neodymium magnets are very strong magnetically, but very brittle physically.  They don't like impact or shock loads so you should be gentle with closing that rig.

noted. he suggested the rubber-coated ones specifically for that reason.

Even if I slam it, the magnet shouldn't take any significant impact, since the lower urethane bumper hits first and stops it. It's set up to just exactly touch the magnet once I clamp down the main latch at the bottom. So hopefully won't be an issue. I did order a couple spares just in case (and/or for other future projects that might need a magnet). 

jmho, but you might want to look at how a dial indicator magnetic base works and consider doing something like that.  Then you can have very strong clamping force without having to yank it super hard to release it.  

JBinMD said:

jmho, but you might want to look at how a dial indicator magnetic base works and consider doing something like that.  Then you can have very strong clamping force without having to yank it super hard to release it.  

I don't have to yank this very hard at all, just a little tug. It's just enough to hold the vibration movement, but certainly wouldn't hold the swing-out closed by itself if the main lower latch wasn't latched. 

I had also thought for a second about having an electromagnet that only had power when the truck was running, which would be a strong hold. But didn't feel like adding any more electrical crap than necessary :)