caravan axle

I too would go with the broken centrebolt.
Often happens when U bolts stretch or become loose for whatever reason.
The wheel assembly moves back on the spring causing "toe out" the usual cause of inside edge wear on the tyre.
mike

That would mean that the whole axle has moved, therefore both tyres would scrub out.

You would think so Steve.
But I had the left hand side wheel suffer from this problem and it only affected the wheel where the dislodged centre bolt was.

If the axle "relocates" on one side, all you SHOULD get is crabbing To my mind, as long as the axle remains straight (and the bearings aren't sloppy) the wheels will remain parallel so there should not be asymmetrical wear as reported. I'm always open to learning - even at my age - so I'd welcome attempts to explain otherwise.

If you do a little sketch on paper with ruler and pencil it is easy to see how the "intact" centrebolt acts as a fulcrum for the axle to pivot around.
The difference is the ratio of the intact centrebolt to the centre line of each wheel.
mike

Good explanation Mike

I've seen more beam-axle trailers than I would like to count. Assuming the axle itself is straight and the bearings are half-reasonable, the wheels themselves remain in parallel planes. If you line up that axle assembly square under the trailer body, every body seems happy that it will tow straight with no odd tyre wear pattern.

From the viewpoint of the axle/wheel combo, it is oblivious if you put the trailer on top of it cockeyed. The ONLY result is crabbing. In extreme cases, that will produce asymmetric wear patterns - inside on one wheel and outside on the other, from scuffing.

juliew,
If you are in Sydney, give Brett a call at Narellan Truck Align. They also specialise in caravan and 4WD alignments.
Address: 2/3 Campbell St, Narellan NSW 2567
Phone:(02) 4647 1811
Keir