Thread: Front and rear recovery points - bolt sizes

I'd definitely go for bigger bolts, I'd also align them if possible more in the direction of the pull.

IMO 2xM12's are the minimum, for loading like yours you want 3 or 4 M12 bolts. Are those M8 or M10?

The other issue is clamp load. Bolts shouldn't take shear themselves, they should be done up tight enough that friction between the two surfaces they are clamping together takes the load.
To acheive this across a chassis section requires crush tubes to be welded in. Crush tubes suitable to support a fully torqued M12 grade 8.8 bolt need to be at least 22mm diameter since they have a 13mm hole through them.
Bigger if you're using higher strength bolts torqued up tighter.

To get a friction grip connection on a chassis is a bit af a bummer.
Fro a true friction grip connection the surfaces should be bare metal.
There are already crush tube at these holes and made for M10 bolts.
The STD front bumper mounting bolts.
The rear is already being altered but the front stay a problem.

Originally Posted by Rudolf

To get a friction grip connection on a chassis is a bit af a bummer.
Fro a true friction grip connection the surfaces should be bare metal.
There are already crush tube at these holes and made for M10 bolts.
The STD front bumper mounting bolts.
The rear is already being altered but the front stay a problem.

That swaybar location gives you a lot fewer options than on a D1/RRC etc. I have a hook on my RRC with the bolts drilled up through. No crush tubes yet because I need to take the front of the body apart to do it properly. I plan to add a flat plate with a shackle eye to the other side when I finally do it properly.
In the mean time i risk ovalising the holes if I pull too hard.

Oval holes is another warning I got from our resident engineer.
The chassis tubes wall thickness is not enough and could deform with small diameter bolts.
Justifies goin the friction grip route with a minimum of m12 bolts.
For design pusposes I would still design it for shear only.

Here is a recovery point gone wrong.
Four bolts, looks like M12.
Chassis frame had no stiffeners or doubler plates.
Oh, by the way. This is from a Toyota Hilux.
The lug did not come off but damadged to chassis and body work.

That's ugly.
I'm currently doing some head scratching to fit a decent recovery point to a mates bighorn (jackaroo). the chassis rail on that is completely boxed in, maybe 2.5mm wall and has M12 captured nuts inside.
The nuts will just pull through the 2.5mm rail if anything serious happens but to get something decent in there will require cutting open the rail and welding some decent anchors in.

I think a lot of bodywork needs moved to even be able to do that. But interestingly enough, if you just bolt a 4500kg rated hook with M12 grade 8.8 bolts then most 4wd clubs will deem it okay.

Another theory for bolts sizing.
Recovery points are mostly loaded at a angle.
This does no good for a strict friction grip connection.
Rather design the connection for a load combination.
The bolts should be able to handle both shear and tension.
For the friction grip design 8.8 bolts should not be used.
Rather use 8.8S or 10.9S bolts and torque them to spec.
These bolts should never be reused once torqued.
Again for friction grip the chassis rails are too thin and would not stay flat.
A doubler plate of min. 6mm should be welded on for a better flat surface.

Some reading I found at a local site about recovery point design and a local 4x4 forum.

Vehicle Recovery Points And some technical aspects, by Mike Lauterbach
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