one time the front tyre deflated for no particular reason and the bike sunk in the cradles, it was still in place at the destination but I thought if it weren't on road and for much longer it could have escaped out the top tube foamy hook, so I try to remember to run a safety strap nowadays.
You’ve raised an important point Shred – I hope one that will be of interest to others.
When a tyre goes flat – and they do, a bike needs to climb up out of the wheel support cradles in order to be ejected from the wheel support cradle – from both ends. The only way that a bike can be ejected is if the ramp angle on the top tube is steeper than the curve described by the tyre rolling up and out of the cradle at the frame support contact point. In other words if your bike has a very steep top tube and you’ve clamped at that steep part, then the bicycle will be ejected. Easy.
Now, the way to fix the cause – rather than to address the symptom is to move the frame top tube contact point towards an area of the top tube that isn’t steep – which is closer to the seat tube. Much like this example below:
If the Yeti shown above was clamped at the steep part of the top tube, it wouldn’t stay on the carrier. But by moving the frame support contact point further back towards the seat tube, the frame is now clamped on a flat part of the top tube. This ensures that the bike will not be ejected.
In the case of the iSi product, there are two vertical and offset support posts. Whilst that allows for each bicycle to be clamped at the ideal position, it also allows for both bicycles to be pulled together for very narrow load. Much like the animation below:
Shred, I do welcome being contacted with any of that sort of stuff - and take advantage of our experience in travelling rough terrain with potential for all sorts of chaos. Chances are we’ve experienced it before and can at least offer some advice. If something is worth worrying about, we’ll tell you.
Hope that helps.