Took the towing (i.e. withidl) plunge...

[Bayerische E53]

After hundreds of hours of research and after speaking at length with several experts, I finally decided on my lash-up.

This is a very lightly covered topic on this board and so I figured I'd document my journey for everyone else's information.

As it turns out, towing is a highly misunderstood undertaking and the concept of weight distribution is ever more misunderstood. To begin with, the X5 is rated to tow up to 6,000lbs. Normally, hitches give two maximum towing capacities: un-weight distributed and weight distributed. The BMW (actually, Westfalia) only provide an un-weight distributed capacity. Generally speaking, there is a 2:1 relationship between a vehicle's un-weight distributed and weight distributed towing capacities. In other words, the X5 chassis can, in theory, handle up to 12,000lbs of weight (go ahead, cry bloody murder). Several other variables go into calculating towing stability as well. For example, the distance from the ball to the center of the rear axle, the tow vehicle's tire side-wall flex, suspension geometry, wheel base, whether you exceed the gross axle weights, braking power, tongue weight, etc. It really is a science.

Indeed, from the research I've done, I've actually found that the Ford F-150 is actually not nearly as stable when towing as X5s and, depending on how it is equipped, has about the same towing capacity as a Porsche Cayenne (7,900lbs Ford vs 7,700lbs Porsche). Two of the main reasons why the F-150 is more unstable than an X5 or Cayenne is because the tires flex more and because the ball is further away from the rear axle than on an SUV. Further, it turns out that independent suspension may actually be superior to a live axle for towing. Whoodah thunk, eh? (Again, go on, cry bloody murder).

Note: Interestingly, the F-150's tow rating increases with the engine size and gear ratio - it goes from a minimum of 5,500lbs with a V6 and 3.55 gears up to 8,300lbs with a V8 and 3.55 gears. Go on the Ford website and you'll see. The 7,900lbs figure above is for an F-150 equipped with a V8 and 3.31 gears. These figures are all with the 126" wheel base. When you upgrade to a longer wheel base (145"), the F-150 is actually rated to tow up to 10,000lbs! It's also interesting that most guys who drive bigger trucks say they would never tow more than 6,000lbs with an F-150. Many people say that you cannot tow a 6,000lbs enclosed trailer with anything less than a 1/2 ton - the real hardcore guys jump to a 3/4 ton. What gives? Well, the main complaint is stability. But how can an X5 possibly feel more stable towing 6,000lbs than an F-150 towing the same load? Is it even possible? How about all those crazies towing 20'+ 6,000lbs+ Airstreams with DODGE MAGNUMS? The answer really all depends on the numbers you get from all the variables I mentioned above. Again, towing is very misunderstood.

Looking at the Cayenne, I honestly cannot see how the chassis has a 1,700lbs higher towing capacity than the X5. The only reason I can come up with is that the transmission in the Porsche is stronger than on the X5. Indeed, the X5 actually has a longer wheelbase than the Porsche - a plus for towing. My X5 has a manual transmission as well, so it's at least stronger than the X5 automatic.

Anyways, after seeing Withidl's Airstream lash-up, doing tons of research, and speaking to several experts, I've come up with a lash-up that should weigh a maximum of 7,700lbs. I just ordered a 24', all-aluminum enclosed trailer which weighs in at 4,300lbs unloaded (it is furnished on the inside, hence the increased unloaded weight; it is 8'6" wide and has an overall height of 9'6"). The manufacturer will be installing a Hensley Arrow on it. The Hensley unit weighs about 100lbs. The trailer will then be loaded with the track car and tools (figure about 2,800lbs for the car and 400lbs of spares/tools) and I'm at 7,600lbs. Additionally, I am using the Hensley TrueControl brake controller - the best currently available. Of course, I'm using the factory BMW hitch.

In short, based on the actual dimensions of the X5 and the variables outlined above (statistics indicating that the X5 and Cayenne are at least as capable at towing, within reason, than an F150) the conclusion is that the X5 can certainly safely tow at least 7,700lbs WITH a weight distribution hitch (and, indeed, I'll be below that limit).

Of course, because I'm making this thread, I'm open to comments/questions/scepticism.

I will post pictures up of the lash-up when the trailer is ready/when I pick it up in about 8 weeks.