Wheel chocks 1

[andrewjmorin]

I'm working on a system to immobilize cars on flat-beds during transportation. The design envelope is very tight (chock must fit under the 'shadow' of a 24" outer-diameter tire; web-strap on tire circumference IS allowed; system must survive up to a 2-Gee impact repeatedly; connect to the floor directly beneath the tire). The practical problem has shown itself to be keeping the chock from pulling the floorboards up.

Any thoughts, suggestions or prior-art discussions will be most appreciated. I'm stuck inside 'the box' at the moment.

andy

 

[Fabrico]

It sounds as if the strap is attached to the chock and the chock attacthed to the floor. The preferred arrangement would have the straps attached to the floor.

 

[andrewjmorin]

O.K. Why would that be preferred?

 

[evelrod]

Because it separates the horizontal and vertical loads.

Besides, why reinvent the wheel. Just about any trailer that I've seen set up to haul an automobile, the wheel chocks (if used) do not act as tie downs. There must be a reason!

Alternate: Beef up the floor.

Rod

 

[andrewjmorin]

I'd have no job if reinventing the wheel was outlawed. <grins>

Perversely, I'm restricted from *actually* re-inventing anything: I'm required to develop a restraint that uses the existing flimsy floor. It was designed for chocks only, not straps, from which I deduce that it was 'intended' to resist the horizontal load only. Sometimes a car skips over the chocks under the right/wrong conditions, and so straps are now the order of the day.

What makes the floor inadequate is that it is actually a wire mesh designed to hinge up to allow cleanout. There is a steel sub-floor that normally carries the weight of the cars.

I've got a few dozen different concepts, each with their own flaws. My first, best hope was to use the chock as a lever to convert the ~vertical over-tire-strap-tension into a horzontal load. Am I way off-base here?

 

[MikeHalloran]

Ah. Tying the strap to the chocks can work if the chocks for a given wheel are tied to each other, e.g. by chains inside and out, or by an outer bar that can resist bendng loads like the back of a c-clamp.





Mike Halloran
Pembroke Pines, FL, USA

 

[andrewjmorin]

Ding! I never thought of that, Mike!!

<starts new free-body sketch>

 

[IceStationZebra]

Mike beat me to the punch.

Two questions
1. Do your straps tie into the mesh or the sub-floor?
2. Could you have the wheel drive onto a steel plate that would help to redistribute the loads? I know this falls outside the shadow of the wheel and requires more effort, but it would have virtually no height. Projecting the plate forward would give you a longer lever arm to resist the vertical forces.

ISZ

 

[andrewjmorin]

ISZ,
The pre-existing system hooks/locks a pair of chocks into the mesh, and the mesh is comfortably over-designed for that.

I'm pretty free, within the envelope described and with allowances made for the system being light-weight and unlikely to damage the cargo.

Two prototypes have been tested:
-one took the straps directly to the mesh, and simply peeled it up 6" to 8" from the sub-floor, which could obviously contact the underbody of the chocked vehicles;
-the second was intended to provide the strap load with a lever arm, using the tire-to-chock contact-point as a fulcrum, and convert the mesh 'lift' to a 'stretch'... when the test load was applied, displacements & deflections (compression of the tire, strain in the strap, movement of the mesh, etc) were hardly any different from those in the first test. (much to my surprised dismay)

As to 'driving onto' a plate, I think that would make the product a tough sell. However, I am going to brainstorm some ways of implementing Mike's (and your) suggestion of rigidly connecting the two chocks, and creating as large a footprint over the mesh as possible. One new-ish problem I anticipate with this approach is making it adjustable to the range of tire diameters. I'm self-inflicting the critera that 'possible' tire sizes range from 24" dia up to the low 40's.

 

[MikeHalloran]

Easy. Weld the strongback to one chock, and make it slide through a sleeve welded to the other chock.

You could put a rollpin through the end of the strongback to keep the paired chocks together ... or you could connect them with a chain. The strongback lets you place both chock with a single motion. The chain allows you to cinch the chocks against the wheel with one hand.




Mike Halloran
Pembroke Pines, FL, USA