Originally Posted by oldskewel
(Post 1187301)
If you want to really know then, a free body diagram will show it clearly, but ...
Anything you're doing with "flex" is primarily creating a torque which is offset by the pair of crossbars which must create a counteracting couple. The 40-30-30 ratios you're talking about may be true, but what matters is the net actual forces on the bars, not the ratios.
Here's one way to think about it:
1. consider the case where there is no rear support, just that of the crossbars. Assuming the load is centered, it's pretty easy to see that the weight will be split equally and carried by each crossbar.
2. then consider the case where the beams weigh nothing at all, and you jack up that rear support, creating internal stress in the beam, countered by forces on the crossbars (since the beams are strapped down). In this case, the front bar will push upward on the beam, and the rear bar will pull it downward, with the couple providing a torque exactly offsetting that from the rear support. The amplification factor will be greater based on ratio of overall beam length vs. the spacing of the bars.
3. now consider the case you actually have, where there is a rear support, and the beam is not massless - a combination of 1 and 2 above. Since all this analysis is linear, the answer in this case is a sum (linear superposition) of those two other cases. So whatever you had in case 1, you can just add the answer from case 2 to it.
4. OK? If so, since the answer from 2 showed that the front bar had an upward force on the beam, then this means that the force on the beam applied by the front bar (so the force on the front crossbar is this amount, downwards) will be greater than it was with no rear support. So although the pre-loading from the rear beam may be decreasing the stress on the rear crossbar (if the forces and distances are in the right range), it is guaranteed to increase the loading on the front crossbar. And of course that is not good. You just need things to fail in one spot to cause problems.
Another way to possibly help understand it - consider if you had a single crossbar supporting things at the center of the beams, and a pin-joint trying to push up on the rear end. Think that all the way through. Then consider two crossbars closely spaced, etc. - that will change things slightly, but not materially. In this extreme case (very closely spaced bars), the forces on the bars needed to create the counter-acting torque will vastly exceed the net upward force due to the rear support.
As long as the bar spacing is a lot shorter than the overall beam length, the force increase due to the counter-acting torque will exceed any reduction in force due to the rear support.
Main thing is to not think you have any extra rack load capacity due to something you're doing just on the back end like that, with something that is basically a pin-joint. That could lead to problems.
If still not convinced, it would be a lot easier to just do the calcs vs. trying to explain it intuitively, so feel free to provide the dimensions, weight, and rear support load (estimates of course are perfectly useful) and I'll get hard numbers.
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