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  #12441  
Old 07-07-2020, 11:20 AM
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What did you do to / for your E53 today??



Option two: These are 16' 2x6. They were wet and super heavy. My crossbars are only 3/4" tall not designed to hold much load but with only 16' I can shift toward the back and support up from my tail rack.

I borrowed wife's tow hook in case I needed to do a bipod but the mono pod was perfect

My 7 yr old at the time son was with me and loved how it makes the car feel like a tank
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  #12442  
Old 07-07-2020, 11:47 AM
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Option 2 is another good idea. I'll probably go that route in the future since I've got a cargo carrier also. I've determined the maximum roof load with the two factory crossbars is 5 sheets of 3/4 inch walnut plywood - 335 pounds. The crossbars are fine because the load is spread evenly across them, but the front tips of the side rails lifted up off the roof 1/4 inch . Fortunately they sprung back in place when the load came off. I wish there was a center crossbar to even out the load on the side rails better. Two just aren't enough.



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  #12443  
Old 07-07-2020, 12:29 PM
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Quote:
Originally Posted by srmmmm View Post
Did somebody say trellis? This one was 14 Ft x 8 Ft. Notice the roof crossbars lost their inherent upwards bow.
That looks like a stylish trellis! Thanks for sharing!
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  #12444  
Old 07-07-2020, 12:44 PM
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I made my own crossbars so they are flush with the top of the stock roof rails, sheet goods lay flat agains the whole rail front to back following the curve.

I try to stay close to the 100kg load limit per BMW but I’m sure I’ve gone over a few times. Those 16’ wet 2x6 was one of those times.
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  #12445  
Old 07-07-2020, 12:49 PM
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Quote:
Originally Posted by andrewwynn View Post

Option two: These are 16' 2x6. They were wet and super heavy. My crossbars are only 3/4" tall not designed to hold much load but with only 16' I can shift toward the back and support up from my tail rack.
As shown in the photo there, the support from your tail rack is adding stability (and is surely helping a lot in that regard), but cannot be taking much load at all off the roof racks.

To understand why, without drawing a free body diagram, imagine if you yourself were holding those beams with your hands, in the middle of course. Then your 7-year-old tank commander went and tried to help by lifting at the very end of the beam. They could very easily move that end up and down, without taking hardly any of the load. You'd still be carrying the same weight.

With the beams strapped securely to the two cross bars, it may be possible to exert some force on the end. But that will not be taking any of the net load off the bars, the force will be counteracted by the torque between the two bars. The net force on the bars remains the same, with the extra torque superimposed - so it's actually worse in terms of having a peak force causing structural failure.

The added stability and reduction in bouncing will definitely be helpful though.
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Last edited by oldskewel; 07-07-2020 at 03:17 PM.
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  #12446  
Old 07-07-2020, 07:00 PM
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What did you do to / for your E53 today??

Quote:
Originally Posted by oldskewel View Post
As shown in the photo there, the support from your tail rack is adding stability (and is surely helping a lot in that regard), but cannot be taking much load at all off the roof racks.



To understand why, without drawing a free body diagram, imagine if you yourself were holding those beams with your hands, in the middle of course. Then your 7-year-old tank commander went and tried to help by lifting at the very end of the beam. They could very easily move that end up and down, without taking hardly any of the load. You'd still be carrying the same weight.



With the beams strapped securely to the two cross bars, it may be possible to exert some force on the end. But that will not be taking any of the net load off the bars, the force will be counteracted by the torque between the two bars. The net force on the bars remains the same, with the extra torque superimposed - so it's actually worse in terms of having a peak force causing structural failure.



The added stability and reduction in bouncing will definitely be helpful though.

Incorrect. The things you may be overlooking is that I preloaded the back post. Not enough lift to transfer much weight from the back to front crossbar but enough that the back was probably holding 20-30% of the weight with maybe 40% on the front. (40-30-30).

The thing is: those 2x6 are flexible so even though they were secured at the two crossbars, the first 30-50# of force up on the back maybe half of that is unloading the back cross bar the other half is removing flex from the beam and loading the back point.

I did similar when transporting the ladder. But since it weighs so little I actually set it up 40-0-60 or so. Actually more like 60, -40, 80. I set it up with the ladder floating off the front crossbar and then ratchet straps pulled it down tensioning the ladder so it wouldn't have any wobble at all.

The only wobble was side to side when one side of the car hit a bump.

It is a delicate balance to lift the back without just transferring the load to the front bit it's not difficult when paying attention as it's definitely better to spread the load.
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  #12447  
Old 07-07-2020, 07:06 PM
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After 3 error codes for Thermostat over 8 months, I decided today was the day to change it. Job done.
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  #12448  
Old 07-07-2020, 09:27 PM
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Quote:
Originally Posted by andrewwynn View Post
Incorrect. The things you may be overlooking is that I preloaded the back post. Not enough lift to transfer much weight from the back to front crossbar but enough that the back was probably holding 20-30% of the weight with maybe 40% on the front. (40-30-30).

The thing is: those 2x6 are flexible so even though they were secured at the two crossbars, the first 30-50# of force up on the back maybe half of that is unloading the back cross bar the other half is removing flex from the beam and loading the back point.

I did similar when transporting the ladder. But since it weighs so little I actually set it up 40-0-60 or so. Actually more like 60, -40, 80. I set it up with the ladder floating off the front crossbar and then ratchet straps pulled it down tensioning the ladder so it wouldn't have any wobble at all.

The only wobble was side to side when one side of the car hit a bump.

It is a delicate balance to lift the back without just transferring the load to the front bit it's not difficult when paying attention as it's definitely better to spread the load.
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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  #12449  
Old 07-07-2020, 09:50 PM
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After battling with a sticking fuel gauge (stuck at about 12 gallons) for a 2400mile round trip, pulling 4,000# of U-Haul trailer and load, I finally changed out the left side fuel level sending unit.

I had thought the float was sticking because when I tapped on the top of it with a long extension and BFH, the gauge would go to the proper level.

The float was not sticking, it slid effortlessly on the wire wound resistors embedded in the unit. The resistance of the old unit was about 18 ohms at empty, 250 ohms at half travel and 500 ohms at full up.



The new unit has moved down from 12 gallons to 10 which was never happening with the old unit, which would hover around 11 and 12 gallons forever.

On my trip I used the miles traveled and MPG shown in the iBus app to estimate my fuel consumed. In all but one fuel stop this proved to be workable. One fuel stop I took 23.75 gallons for a presumably 24 gallon tank



This was a much nicer fuel stop (SAM'S CLUB Joplin, MO) on that trip!



PS: I also installed the bigger O-ring that this forum highlighted for the jet pump connection.

Hopefully all well for now.

Mike
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  #12450  
Old 07-07-2020, 09:56 PM
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Missing a CRITICAL Part of the equation.

The load is almost .centered on the REAR crossbar so when lifting up from the back it puts more load on the front but the goal to end up split the load closer to equal

The beams were almost teetering on the back crossbar before I put the back support.

If you started with a centered load the brace in back or front just limits additional forces from the turret torque which is very helpful but when the forces can actually be held by the front and back that's a whole different story. The tow eye could easily hold more than the rating of the entire roof hence my desire to get it into the mix.

The next time I move some 16' lumber I might favor the front if I can get to also reach the back. Just like I did with the ladder, it could have negative load on one of the roof crossbars putting most of the load on the back and front bumpers
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