Originally Posted by matt167
if you put the bars verticle, they are fine, set them horizontal and they will crack. I have put many vehicles on cinder blocks, just not very high nor have I put blocks on more than 1 corner. never had 1 even start to crack. I use standard 12" blocks. my '67 Falcon's front end structure sits up too high for my jack stands to work, so I put the block under, and then set a jack stand center of the block/ frame and lower the bumper jack down just until it touches the frame.
Originally Posted by Kallie49
suit yourself, tell that to the three blokes we had to use airbags to lift the cars off of....well as two of em, they were lucky and lived with minor maiming...the other had his head squashed like a grape...he was actually one of the two that were doing it the way you discribed...but then again experience is the best instructor
There might be some folks wondering how such different extremes, as expressed in the above quotes, can happen. Ask any mason about how block and brick walls stand. Chances are you will get an answer similar to the one I was given, that the parts are like old drunks, each leaning on and depending on the other in order to stand there.
Concrete blocks can withstand compression loads well only when those loads are evenly dissipated over the block. They don't stand up well to impacts, uneven loads or loads concentrated on just part of the block. Note that a block in a wall is resting on a level footer with mortar smoothing out any irregularities and has the same situation above it to spread the load it carries over its entire structure. That load is almost entirely vertical and essentially static.
Placing a 2" wide frame on the top of a 7-1/2" wide by 15-1/2" long concrete block concentrates the load on less than
1/3 of the block, *if* the frame makes full, even contact for the full length of the block. If you turn the block over on its side, with the holes running side to side, it is even more of a death trap because even less concrete is carrying the load. Any rocking of the load will increase the concentration and increase the likelihood of the block failing (shattering) suddenly. You start jerking on a part and the entire weight of the car could suddenly be depending on a very tiny fraction of that block to carry it. That tiny fraction will fracture. So will you.
I have gotten away with using concrete blocks a few times, but have never placed any part of a car directly on the block. I always build a pad of wood on top of the blocks (a length of 2x8 will just cover a concrete block, in the U.S.) and only use the blocks on suitable dirt. The few times I've needed to work under a car when resorting to blocks, I've always used 2 blocks for each 'post' built, as well as the wood padding. (You can see such a rig in one of the early entries in my journal. 4x4's are used to tie the pairs of blocks together and more 4x4's, crosswise of the ties, are used to spread the load across the surface of the blocks). This is still very, very risky and I would not recommend it to anyone.
Just because you might get away with crossing a busy highway without looking first, doesn't mean it's a good way to get from one side to the other.