Principles discussed in my comment in previous thread apply:
I have got to be away from the house with some time on my hands this spring and summer. I would love to be able to use these chunks of time for my kettlebell practice (currenlty, the pursuit of Simple). What is the consensus about traveling with the kettlebells in the car? Is it an absolute...
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This is what I suggested:
Safest option is a trunk with cage. Then you use a couple of rated load restraint straps to hold it against the cage. The cage is between the occupants and the bell. These cages are in 4 wheel drives and station wagons. Possible some sedans may have something similar.
Another way is the use of a seat belt through the handle of the bell. These are designed to handle a human weight, the standard human weight used to 72kg but I believe that it has been increased a few kilograms due to changes in the average adult weight. I would make sure that any human occupants are out of range of any sideways motion if you use this method. You could also use some load restraint straps to limit the motion of the kettlebell.
A third option is have the bell placed down at floor level with load restraint straps through the handle to secure it in place to some really solid structures. You can use more than one strap and more than one anchor point to achieve a solid connection.
In transporting the bell, if there is a sharp change of speed or direction such as in a loss of car control event or an outright crash then the bell is subjected to massive forces. Essentially it is similar to the large forces you can measure from a kettlebell ballistic like a swing or snatch ie 100s of kilograms or more. You can also see why the anchor points need to be really solid as well.
I've tried sandbags and waterfilled type bags as they can be carried empty and filled at your destination. For travelling on trips I usually go for bodyweight exercises and use a very heavy rubber band for hinge movements (credit to Tom Furman, Armour of War for that idea).
The car I have these days has no cage but I have put a kb in the boot and then a couple of load restraint straps around the seat (these fold down and you can thus get a strap around them).