Quote:
Originally Posted by Riggodeaux
(Post 997236)
JCL, you confuse me here. I don't have the spec in front of me, but the tongue weight is, as I recall, 10% of the gross towed weight, so maybe 500+ lbs. Are you saying that 2-300 lbs on a hitch mounted cargo rack, or 4 mountain bikes on a rack, is too much, simply because of the added torque on the very robust OEM hitch from extending out more than the 8" from lock pin to center of ball spec? I'm just a simple caveman X5 jockey, and don't understand that higher-math engineering mumbo jumbo, but this seems a little severe :)
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Correct. Unless you can get the load in close enough to the hitch pin, and usually that isn't possible.
BMW lists 600 lbs as maximum tongue weight for a braked trailer, when driving on smooth roads, and when the load is positioned no more than 8" from the pin. Let's stay with smooth roads for a moment, and use the 600 lb figure. The failure mode seen early on in the X5's life was bending of the hitch receiver downwards. It happened with aftermarket hitches. BMW cares enough about it to specify a maximum 8" distance, and to instruct their dealers to affix that decal to every hitch they install. They also specify a maximum drop, because extra drop also applies additional bending torque.
If you apply the load at 16", the equivalent bending torque is achieved with 300 lbs load. If you apply the load at 24", the limit would be 200 lbs. If you apply the load at 48", the limit would be 100 lbs.
We don't know the value of the maximum bending torque that the hitch was designed for, but we know that the maximum bending torque according to BMW specs is as laid out above.
A cargo carrier with 8" of offset plus 24" to the centre of gravity of the load, ie 32", would reduce the tongue load by 4 times, so 150 lbs.
It isn't really high math, just think of a torque wrench. You are loosening a tough bolt, and apply 100 lbs force to the wrench. It isn't enough. So you put an extension pipe on the wrench to get twice the leverage. People who focus on the tongue weight without considering where it is applied are essentially saying that it doesn't matter how long a wrench is, just use the approved force and extend the wrench as much as you like to get it tight.
Now, since we are in this deep, look at the limits BMW applies for bouncing loads. A trailer ball 8" out with 600 lbs tongue weight rarely rattles the hitch ball, it is a varying load but always applied downwards. If you have a carrier, the load is by definition bouncing, there is no trailer axle and trailer momentum to keep it in place. For offroad use, where you can have a bouncing load even with a trailer, BMW specifies a much reduced maximum tongue weight, something like 330 lbs IIRC. It is in the instructions for the hitch. That should be the starting point for a carrier, even on road, IMO. We can perhaps decide individually that BMW is too conservative and ignore it, using the 600 lb figure, but they went to some trouble to calculate the maximum bouncing load.
My personal approach is to use the 600 lb limit, pay attention to where it is applied, and drive such that any load doesn't bounce, whatever the road surface.
There is a parallel to other BMW published specifications here, things like oil change intervals. I happen to have had good luck with 24,000 km or 15,000 miles. BMW recommended that, and it works well for me. Putting a 600 pound tongue weight 16 inches out (double the spec) is just like running 30,000 miles between oil changes, on the basis that there is lots of safety factor and the legal guys wrote the rule, not the engineers. OK. Some might get away with it. I wouldn't do it.
Jeff
