OEM Trailer Hitch

[TerminatorX5]

here is the physics 101 from the NASA website... in our case only the example 1 is applicable:

A force F is a vector quantity, which means that it has both a magnitude and a direction associated with it. The direction of the force is important because the resulting motion of the object is in the same direction as the force. The product of the force and the perpendicular distance to the center of gravity for an unconfined object, or to the pivot for a confined object, is^M called the torque or the moment. A torque is also a vector quantity and produces a rotation in the same way that a force produces a translation. Namely, an object at rest, or rotating at a constant angular velocity, will continue to do so until it is subject to an external torque. A torque produces a change in angular velocity which is called an angular acceleration.
The distance L used to determine the torque T is the distance from the pivot p to the force, but measured perpendicular to the direction of the force. On the figure, we show four examples of torques to illustrate the basic principles governing torques. In each example a blue weight W is acting on a red bar, which is called an arm.
In Example 1, the force (weight) is applied perpendicular to the arm. In this case, the perpendicular distance is the length of the bar and the torque is equal to the product of the length and the force.
T = F * L

In our case, the Force is stationary for the sake of the argument and is
F = m x a (mass multiplied by acceleration).
the mass is known, 240 pounds (108.5 kg),
the acceleration is 9.8 m/s*s (earth gravity),
so our force is 1063 kg*m/s*s,
and the length of towbar is 18 inches (0.4572 m) = 486 kg*m*m/s*s = 486 N*m (not to confuse with 486 joules, which is the energy resulted from the same calculations)...

naturally, the longer the towbar, the greater the "lever" effect...

1 Nm = 0.7375621 lbs*foot
486 Nm = 358.46 lbs * foot (or, more common term, "foot-pound)

so, in the stationary condition, we are applying a torque of roughly 350 pound foot onto the hitch bar and the hitch assembly.

Once we introduce the additional forces of driving forward and bouncing the load vertically, the numbers will decrease on "up" and increase on "down" move, also, as the vehicle is moving on the decline, the torque will decrease, as if the vehicle is moving on the incline, up the hill, the center of bike's gravity will move further away from the 18" mark and the torque will increase...

I am just too lazy to venture into these calculations...

The main point is, will the mounting points sustain the load, and more importantly, will the places where they are mounted to, sustain the load... those items can be calculated, based on the type of metal used, the thickness of the metal, the type of the attachment of different materials, the ability of the metal to take the repeating bending stress without losing the resistance property and the metal fatigue that will eventually settle in the material before anything would break.

These are the calculations taken into the account by the engineers who design these things...

Remember the Fast 5 movie - when Dominique Torreto and Co. ripped the "unbreakable" safe from the enclosure, as the enclosure was not reenforced enough to protect the vault... so, it did not matter that the vault was good, the surroundings weere not up to par...

oh, and by the way, I am not here to rain on your parade - I am in engineering myself, and can only appreciate a well executed mechanical (or electrical/electronic) masterpiece... And I think, the OE tow hitch is an engineering masterpiece...

[JCL]

Originally Posted by TerminatorX5

here is the physics 101 from the NASA website... in our case only the example 1 is applicable:....<snip>

It is perhaps easier to approach it by agreeing that the bending moment that BMW designed for can be easily compared, without worrying about the absolute value of that bending moment.

600 (or 330) lbs x 8 inches. Relate everything to that. If you have half the weight, at three times the distance, that is 50% beyond the design stress.

And all this is for a static load. The real issue is fatigue, as you inferred.

Anyway, I don't want to rain on anyone's parade. Just pointing out that there are more things to consider than the grade of attachment bolt. I fully agree with your comments on it not being about the receiver itself, but rather the integration of it into the vehicle, and the strength of the surrounding structure.

[xsx450]

I wish my 6-speed transmission was an engineering masterpiece!!!

That said it does shift very nice and i do enjoy everything about the way it works...

[Porschesolutions]

I installed the factory hitch/wiring as-recommended by this forum: very heavy-duty, no worries from now on. Wish my ball/carrier didn't bounce around so much inside the 2" square opening with light trailer loads on a light trailer.

[JCL]

Originally Posted by Porschesolutions

I installed the factory hitch/wiring as-recommended by this forum: very heavy-duty, no worries from now on. Wish my ball/carrier didn't bounce around so much inside the 2" square opening with light trailer loads on a light trailer.

In my experience the 2" square piece you insert in the receiver can vary in size depending on the supplier. You can also shim it. However, the first thing to check is whether you have sufficient tongue weight.

[xsx450]

They make stabilizers, look them up.

I get what you guys are saying, but you are just SAYING. I am doing. They are not bolted to sheet metal, or the floor pan. This is not one of those mounts that screws through your spare tire floor in the trunk.

All of our equations mean jack shit because for one they aren't accurate, and two I have loaded up over 900 lbs on this thing over 24" out of the hitch, jumping up and down. No stress fractures, no tearing subframe. Nothing.

The thing works perfectly, it is sturdy as shit for what i am using it for. and expressions like "Will fail much sooner" Sound retarded because its not going to fail. I am not an idiot that is going to try to pull a stump out of the ground with this thing and I am not an idiot that likes to spend $300 on nothing.

You guys on these forms make me laugh sometime (like right now). I think some among us need to spend money on anti anxiety medicine not throwing money down the drain to ease their conscious....

[TerminatorX5]

Originally Posted by xsx450

They make stabilizers, look them up.

I get what you guys are saying, but you are just SAYING. I am doing. They are not bolted to sheet metal, or the floor pan. This is not one of those mounts that screws through your spare tire floor in the trunk.

All of our equations mean jack shit because for one they aren't accurate, and two I have loaded up over 900 lbs on this thing over 24" out of the hitch, jumping up and down. No stress fractures, no tearing subframe. Nothing.

The thing works perfectly, it is sturdy as shit for what i am using it for. and expressions like "Will fail much sooner" Sound retarded because its not going to fail. I am not an idiot that is going to try to pull a stump out of the ground with this thing and I am not an idiot that likes to spend $300 on nothing.

You guys on these forms make me laugh sometime (like right now). I think some among us need to spend money on anti anxiety medicine not throwing money down the drain to ease their conscious....

where is the bottom arm bolted to? It is not clear from the pictures posted earlier on this thread.

[xsx450]

Originally Posted by TerminatorX5

where is the bottom arm bolted to? It is not clear from the pictures posted earlier on this thread.

It is bolted to the subframe of the x5 There is an extremely heavy duty box frame that runs right behind the rear differential which you drill a hole into and then use a special bolt which expands when you torque it the first time like a pop rivet and then you retorque it with a 1/2" Bolt.

Any downward force would be transfered very evenly with the mount. I believe the shear strength of the bolts does matter, unless you think the rear frame is going to rip off?

[TerminatorX5]

Originally Posted by xsx450

It is bolted to the subframe of the x5 There is an extremely heavy duty box frame that runs right behind the rear differential which you drill a hole into and then use a special bolt which expands when you torque it the first time like a pop rivet and then you retorque it with a 1/2" Bolt.

Any downward force would be transfered very evenly with the mount. I believe the shear strength of the bolts does matter, unless you think the rear frame is going to rip off?

sounds like a "Jesus" bolt... Interesting, a frame that is behind rear diff... i wonder, what the purpose of it there... i need to pull out some of the blueprints, see what it does...

[JCL]

Originally Posted by xsx450

I get what you guys are saying, but you are just SAYING. I am doing. They are not bolted to sheet metal, or the floor pan. This is not one of those mounts that screws through your spare tire floor in the trunk.

It appears that the Reese cross bar bolts on to item 14, in the parts book drawing attached. That is a lateral tie piece that holds the two axle supports (2 and 3) from spreading apart. It is not strong in the fore/aft directions, unless you through-bolt to item 1 (not sure if your hitch does that, or not, as some do). It shows as being braced to item 1 in any case. Item 1 is the trunk floor. Item 14 is braced to the trunk floor because it isn't very strong by itself, it is very thin. It is the piece that BMW discards in their OE hitch installation. So, it appears that despite your claim, this is one of those mounts that is attached, either directly or indirectly, to the spare tire floor in the trunk.

All of our equations mean jack because for one they aren't accurate, and two I have loaded up over 900 lbs on this thing over 24" out of the hitch, jumping up and down. No stress fractures, no tearing subframe. Nothing.

This equation is totally accurate. The math is pretty simple. The design load is published as 600 lbs vertical at 8", if you use the reinforced OE hitch. You have applied 1.5 times the rated load, at 3 times the rated distance. That is 4.5 times the design load. Glad that worked out for you. Some of us just believe that some calculations do mean something. You don't have to agree, you are certainly entitled to your own opinion. You are saying that because it hasn't broken yet, it is strong enough to last forever, that fatigue isn't a factor. OK.

As an aside, the strength of the front bolt doesn't matter. It is strictly a locating point. All the force from the hitch stabilizer bar at that end is upwards, not downwards, so the bolt doesn't see any significant stress.

The failures that we did see back in the early days of the X5 were in fact to the rear pan, item 1. Yes, it was ripped off. Repairs were expensive. That was why the aftermarket hitch manufacturers added that longitudinal brace. They also include a line in their hitch warranty that states that they do not guarantee that the hitch is fit for use. Their 6000 lb rating is likely based on bench tests of their hitch in a jig, not on having it mounted on a vehicle. We all accept that it has a 6000 lb rating, just not that their way of rating the hitch has any relevance.