New OEM Hitch Specs R a "Gotcha"!

[LVP]

I got the OE ball from the dealer. 72-11-0-009-120. 2" ball with a 1" diameter threaded shaft. Torque specs were different for different shaft diameters. 3/4" was 160 lb-ft and 1" was 250 lb-ft.

[Riggodeaux]

Thanks, LVP. So, as I understand it, with the BMW hitch shaft, fitting the Westphalia specs [8" long, 4" minimum drop] and matching our 6k max load capacity, we need a ball with a 1" diameter shaft. The BMW hitch from ECS Tuning comes with a 2" ball. So, if one of your trailers uses a 1 7/8ths ball, common with light trailers used to haul lawn tractors, etc., you need to pick up a ball with a 1" shaft - and be prepared with breaker bars to open that highly torqued mount if you use the same hitch shaft! Of course, with that light a trailer/load, conforming to the strict westphalia length/drop criteria is probably less important, and I can use the cheap northern tools 4" drop hitch I already have with a 1 7/8ths ball and not worry about swapping/torqing a ball into the "OEM" shaft ......

[upallnight]

Originally Posted by JCL

The hitch was designed for that particular limitation of drop and extension. Anything more than that would apply a higher bending moment that the unibody attachment points are designed for. You can take that chance if you want, but I wouldn't recommend it. The limitation hasn't changed with the new hitch manufacturer, those are the same dimensions in my 2003 installation instructions, and the same label that was supplied by BMW with my hitch with instructions to attach it to the vehicle.

I bought my ball mount from my dealer when I bought my hitch. It was within the specs shown on the schematic. There are threads about posters who have purchased non-OEM ball mounts and had rattles. You can always shim them, but I figured it was easier to just buy the ($$$) BMW part as it fit snugly.

I don't agree that you necessarily want a Class IV mount. Most will be labelled Class III at 6000 lbs/600 lbs tongue weight. Class III isn't a precise specification, there are 5000 lb hitches and 6000 lb hitches, all labelled Class III. Some Class IV hitches are labelled 6000 lbs, others 7500 lbs, and others 10,000 lbs. If a Class III ball mount is labelled 6000 lbs, that would be sufficient.

It is a side point, but the spec doesn't cover bike racks. The hitch is designed for the load to be applied at the hitch ball. If you mount a bike rack, you need to figure out the new CoG, and weight. Then figure out if you are applying a greater bending moment than the hitch was designed for. Most bike racks are very light and it won't be an issue, but it is worth checking.

Good luck.

Jeff

Bikes shouldn't be a problem I carry my Felt and Cervelo on a Saris bike carrier attach to my OEM hitch, not unless your ideal of a bike is a HD Electra glide.

[LVP]

The hole in the hitch was good for the 1" diameter. Not sure how a smaller shaft ball would fit (i.e. it wouldn't centre nicely). If you got an exact dimensioned hitch but with the 3/4" hole and mounted the 1-7/8" ball in it, that is the route I would go. Ideal would be the 1-7/8" ball with a 1" shaft, but not sure if that's common or not. I just went with the dealer one, because they had a 2" (what I needed) and I was there. Sometimes lazy prevails…

[Riggodeaux]

I think my cheap northern tool 4" drop hitch has a 1.25" shaft. I have 2" and 1.75" balls for it. It is too long for the OEM designated 8" link. If/when I start pulling 5k+ of aluminum and horseflesh, I'll be sure to have the OEM compliant hitch and ball.

[admranger]

Originally Posted by Riggodeaux

I think my cheap northern tool 4" drop hitch has a 1.25" shaft. I have 2" and 1.75" balls for it. It is too long for the OEM designated 8" link. If/when I start pulling 5k+ of aluminum and horseflesh, I'll be sure to have the OEM compliant hitch and ball.

Moving loads are so fun to tow. Shifting weight in the trailer as you go down the road makes things more interesting.

I had a live load once when my race car came unstrapped after a particularily large bump on a two lane road.

I was the only car on the grid that had a diamond plate design embedded into the black plastic rub strip of the rear bumper!

[Riggodeaux]

Hauling horses encourages you to drive very gently. If you haven't made your horses neurotic, once the get used to it, they tend to ride steady and not move around much at all, just leaning into the dividers and looking out the windows. Strangely, ours seem to enjoy riding in the trailer [if they can go together - herd animals], will step right in. They do seem to think of trailers as outhouses, though . Most of my experience has been hauling in a gooseneck trailer behind Ford superduty diesels - the best combo for this task, but more truck and trailer than we now need. I'm looking forward to trying a two horse bumper pull behind the X5.

[JCL]

Originally Posted by upallnight

Bikes shouldn't be a problem I carry my Felt and Cervelo on a Saris bike carrier attach to my OEM hitch, not unless your ideal of a bike is a HD Electra glide.

Four-bike holders loaded with full suspension mountain bikes are typically over the limit due to the bending moment resulting from the rear offset over and above the 8" spec. Cargo trays are nearly always over the limit.

I put my Cervelo R5 and my wife's R3 on the roof in fork mounts, with two wheel holders. Heading out this weekend.

[Riggodeaux]

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

[JCL]

Originally Posted by Riggodeaux

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

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