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

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.

:wavey:

Very interesting! Thanks for sharing!!

There is not a 2:1 relationship between weight-distributing hitch capacity and non-weight-distributing hitch capacity. There may be such a relationship for a specific model of hitch drawbar, but that doesn't change the vehicle's towing capacity. If it did you could put a class 4 hitch on an X5 and tow 10,000 lbs, which you can't do. You can't extrapolate and say that the chassis can handle 12,000 lbs, that would be extreme for a unibody.

There is a chassis limit, a braking limit, a transmission limit, a cooling system limit, and so on. Whichever one you hit first defines the vehicle towing capacity Early 3.0 X5 models had a 5000 lb published towing limit with the automatic, until 2004. European models had either a 7700 or 7900 lb towing capacity with a special factory towing code option, as I recall.

I think you are brave to tow 7700 lbs with a significant aero drag, with a 3.0. Perhaps you don't have any hills. I wouldn't be as worried about stability, with your Hensley hitch, but I would worry about power, clutch life, and the smaller 3.0 brakes.

Perhaps I wasn't clear about the 2:1 relationship. What I meant to say is that GENERALLY speaking, that is the relationship. Of course, towing 12,000lbs with an X5 would be utter stupidity.

You make a good point as to the aero drag. However, I think we can say that aero drag would be an inconvenience rather than a safety concern if it's not affecting stability, correct? At least that's how I see it - if it was stability, I could see it being a safety issue, but I don't see a lack of sheer power being a safety deal. We're also talking about 65mph, not 80. But, I digress. If you have any counter opinion I'd be interested to hear it (I'm serious - not trying to be snide).

I've considered the brake issue. Brakes are upgraded with Carbotech track pads and Motul fluid so I'm not worried about overheating or anything. If the towing goes well, I will upgrade to a large brake kit for extra peace of mind.

Your European numbers are actually quite interesting. Where did you hear about that? If that's true, I may be better off than I thought. Of course, the drag issue still exists, but as far as stability and safety, there should be no question if the 7,700/7,900lbs figures are correct.

As far as aero goes, time will tell. I'll be sure to report as soon as I experience the first tow.

Wow super cool. Yea I have to tell you, I had to hitch up a couple of trailers, tools and material for long hauls. I am not impressed with my F150 towing capabilities. I have the long ass bad boy quad-cab. Here is where the difference comes in. While towing the trailer, I had close to 800lbs worth of sheet goods in the bed. I also don't need a trailer to transport building supplies. I just put them in the back. It is just a matter of different uses for both cars. I would try towing that with my X, but once the trailer is unhitched, the X would become useless as far as for what I need to do at work.

Fair point. OK, so it sounds like what I've been told and what I've read about the F-150 isn't hearsay.

I don't know of anyone who bought an F150 to tow. That is what F250s, F350s, and F450s are for.

Aero drag isn't safety related on terms of power of the 3.0. Crosswinds will be safety related but all models will be the same on that respect.

Tow limits are listed on the Euro BMW sites. Back in the day, when the E53 was current, there was an option called Increased Tow Capacity in Europe. It was a no charge option as I recall. I suspect it was a safety label calculated differently. I think it had a maximum grade percentage. The current E70 still has a similar option in Europe.

Do you have extensive towing experience? The driver will end up making most of the difference between safe and less safe.

One more issue is the insurance liability relating to exceeding manufacturer's ratings. Some worry about insurance being void. YMMV.

Good luck.

I also have to mention that the trips home, with empty trailer and empty truck are way more wierd. It is like the truck need the weight in the back to feel more stable. And yes, I did not get the F150 to tow. I got it to be able to pack it full of crap and people and go on a job, and be able to park it in and around NYC. I am going for a shorter beast next time.

I see; thanks for the info. I would never hold myself out as a tractor trailer driver or even someone with years and hundreds of thousands of tow miles under my belt. That said, I am relatively new at towing, but I do have over 10,000 miles under my belt and I have towed with 5 different trucks and 3 different types of trailers. My maximum towing weight has been just under 6,000lbs - all without weight distribution. I've also driver several thousand miles in box trucks through cross-winds (monsoon conditions in the southeast, actually) for a fair bit of the time. Like I said, I'll never say I'm an expert, but I also wouldn't think that I'm merely an amateur, much less a newbie.

I have also always taken more precautions than the average person towing (In my opinion). I keep logs of wheel torques, tire pressures, miles travelled, fuel economy, trailer weight vs. tongue weight, etc. I think it's fair to assume that most people are not that anal every single time they trailer. But who am I to say...

Perhaps those who have significant towing experience could chime in and express their opinions. I'd be interested to hear their opinions. Of course, I am interested in your opinion as well, JCL, as I've seen that you've towed a fair bit albeit never exceeding about 5,000lbs (is that correct?).

Very good points. I see your need for an F-150 is quite specific.

I'm not surprised that the truck towed a bit weird with an unloaded trailer. Trailers typically tow better the closer you load them to their maximum axle rating - the tongue weight proportioned to that total weight is also important. What happens is that the trailer tends to bounce around more and the tongue doesn't put enough pressure, let alone constant pressure on the hitch ball. Indeed, the pressure on the hitch ball tends to be more volatile because of the unloaded trailer's tendency to want to buck and up and down more than a loaded trailer. Simple physics, really. Great observation.

There you have it - it's merely a transmission duct for extra air cooling. That says everything - it's the transmission heat that limits the X5's towing capacity. Otherwise, it's good to 7,700lbs as JCL said. Just like the Porsche Cayenne. That seems to explain how I couldn't see how the X5's unibody could be that much weaker than the Cayenne's.

With the manual transmission, I'm sure I won't have to worry as there won't be a tendency for it to hunt for gears.

The other two things I found were that the increased towing capacity vehicles have different rear differentials - all the parts and ratio are the same, however, so I'm guessing it's maybe got a finned cooling cover on it. I also found that those vehicles also come with a 220A instead of 180A alternator. Thing is, the 220A comes on vehicles with 4-zone climate control, which my car has. So I'm good there too.

As for the diff, a little more heat just means replacing the fluid a bit more often. Big deal.

The E70 also seems to have a small additional engine radiator but I couldn't find this for the E53. Even still, on the highway, I don't see this being an issue.

I'll install the cooling duct for the trans anyways for safe measure. So it looks like I'm actually NOT overweight! That's very promising! Thanks for that!