Question regarding Bosal 2" Receiver Adapter

[romad]

The Bosal removable hitch on the Tesla X is similar to the Bosal hitch VW used in Europe except that the ball is 50mm (2") on a swan neck.

I was able to source one of the 2" Adapters for the Tesla X and will use it with my Bosal tow bar on my VW Jetta, but I have a question regarding the ball mount. On the receiver it says the distance from the hitch pin to the ball should not exceed 8". What brand ball mounts have been found with this 8" distance and what were their drop/lift dimensions?

 

[ShawnA]

Hi Romad,

Try a Curt 45030

Shawn

 

[mknox]

In addition to the length restriction, Tesla does not allow any drop in height and only a 3/4" max rise.

I bought this.

 

[romad]

Thanks for the suggestions, I'll have to look at what Curt & QuickProducts carry. It looks like for me the distance between the hitch pin and the back end of the receiver is the most important dimension, then I'll have to see what the rise is on my swan neck ball. IIRC the top of the ball is about 175mm (7") above the bottom. This puts the ball at the middle of my Jetta's bumper cover, and about 450mm (17.75") from the ground. I also have to see exactly how high the coupler of a U-Haul 4' x 8' cargo trailer is when the trailer is level; all the UHaul website says the ball can't be higher than 25"

 

[mknox]

This is probably more of a Bosal issue than a Tesla one, but they have some pretty specific limits and restrictions printed right on the receiver. It is likely to do with the weird way it attaches to the tow bar on the car. The one that blows me away is that it's okay to put a 500 lb. trailer on the hitch, but your bike and rack better not weigh more than 121 lbs.

 

[mongo]

This is probably more of a Bosal issue than a Tesla one, but they have some pretty specific limits and restrictions printed right on the receiver. It is likely to do with the weird way it attaches to the tow bar on the car. The one that blows me away is that it's okay to put a 500 lb. trailer on the hitch, but your bike and rack better not weigh more than 121 lbs.

View attachment 370673

That is due to the longer level arm of a bike rack versus the 8 inch hitch ball. A 32" rise in the bike rack creates 4 times the torque: 500/4 = 125 lbs. So at one G, you are producing as much force as the trailer does.

 

[romad]

This is probably more of a Bosal issue than a Tesla one, but they have some pretty specific limits and restrictions printed right on the receiver. It is likely to do with the weird way it attaches to the tow bar on the car. The one that blows me away is that it's okay to put a 500 lb. trailer on the hitch, but your bike and rack better not weigh more than 121 lbs.

View attachment 370673

Interesting. My install instructions for the Bosal tow bar on my VW Jetta Wagon gives a max tongue weight of 75 kg or 165 lbs and a trailer weight of 1600 kg or 3520 lbs for the same type of connection to the tow bar. Of course the weight of the swan neck is probably less than the combined receiver adapter, ball mount, and ball

 

[romad]

That is due to the longer level arm of a bike rack versus the 8 inch hitch ball. A 32" rise in the bike rack creates 4 times the torque: 500/4 = 125 lbs. So at one G, you are producing as much force as the trailer does.

Could you clarify the physics for me, please? The 8" dimension is a horizontal one while I believe the 32" dimension is vertical.

 

[ShawnA]

Hi Romad,

Some quick geometry - a right triangle with one side 8" the other side 32" will have a hypotenuse of approximately
33". Any weight or force acting on the top of the bike rack applies this force to the connection point through
the 33" lever arm....
100 lbs at 8" = 800 in/lbs or 66.6 ft/lbs.
100 lbs at 33" = 3300 in/lbs or 275 ft/lbs

Still roughly a 4 times multiple...

If the bike rack was an impractical 8" high the forces would be nearly identical...
I think at that height you would be scraping your bike tires on the road.

Shawn

 

[romad]

Hi Romad,

Some quick geometry - a right triangle with one side 8" the other side 32" will have a hypotenuse of approximately
33". Any weight or force acting on the top of the bike rack applies this force to the connection point through
the 33" lever arm....
100 lbs at 8" = 800 in/lbs or 66.6 ft/lbs.
100 lbs at 33" = 3300 in/lbs or 275 ft/lbs

Still roughly a 4 times multiple...

If the bike rack was an impractical 8" high the forces would be nearly identical...
I think at that height you would be scraping your bike tires on the road.

Shawn

Thanks, Shawn, that makes it more clear.

I'm actually considering one of those hitch cargo carrier platforms so I should be good using the 350 lbs tongue weight rating.

 

[mongo]

Thanks, Shawn, that makes it more clear.

I'm actually considering one of those hitch cargo carrier platforms so I should be good using the 350 lbs tongue weight rating.

Careful, you can overload the hitch that way also. First approximation is 500*8inches =your load * distance from bosal attachment to load center of gravity. if it is a 24 inch platform that starts at 8 inches, rough center is at 20 inches, so max load is 200 lbs. However, inertia is the square of distance, so in a dynamic situation (hit bump in the road), max allowable load is even less.

 

[romad]

Careful, you can overload the hitch that way also. First approximation is 500*8inches =your load * distance from bosal attachment to load center of gravity. if it is a 24 inch platform that starts at 8 inches, rough center is at 20 inches, so max load is 200 lbs. However, inertia is the square of distance, so in a dynamic situation (hit bump in the road), max allowable load is even less.

Thank you, that information will be of great help. I'll look at aluminum carriers to keep the load down.

 

[smorgasbord]

That is due to the longer level arm of a bike rack versus the 8 inch hitch ball. A 32" rise in the bike rack creates 4 times the torque: 500/4 = 125 lbs. So at one G, you are producing as much force as the trailer does.

Well, just because the Tesla vertical arm is 32" long doesn't mean that the center of mass of the combined bike rack and bikes is anywhere near that distance. I believe the center of mass is what should be used in the torque calculation. For instance, using a 40 lbs platform bike rack that holds the bikes in the same relative position as the Tesla hanging rack should apply the exact same forces, modulo any dynamic swinging effect of the bikes on the hanging rack.

To make matters worse, Tesla's own Towing page says::

When carrying bicycles or other items on the Model X hitch, always check to ensure that the maximum weight is not exceeded. When calculating weight, remember to include the weight of the accessory carrier. For example, assuming the carrier weighs 40 lbs, the weight threshold is sufficient for carrying two bicycles weighing approximately 40 lbs each, or four bicycles weighing approximately 20 lbs each.

Which clearly disregards the additional torque of 4 bikes versus 2 bikes mounted on the same carrier. I understand the desire to dumb the specification down to a simple weight limit, but it would seem the physics involved are more complicated. Is Tesla's towing page equating 2-bikes to 4-bikes of the same weight wrong, perhaps created by some Technical Writer who didn't understand the actual physics and not reviewed by Bosal/Tesla engineers?

Then there's that the Tesla rack extends out horizontally quite a bit from the hitch. My 1-Up Super-Duty is much closer - from the pictures I'd guess that my 1-UP's first bike slot is an easy 8"-10" closer to the vehicle than Tesla's rack's first bike slot, and a bit lower as well. Here's a side by side, although my photo wasn't taken from the exact same position as Tesla's, which distorts things a bit:

 

[mongo]

Well, just because the Tesla vertical arm is 32" long doesn't mean that the center of mass of the combined bike rack and bikes is anywhere near that distance. I believe the center of mass is what should be used in the torque calculation. For instance, using a 40 lbs platform bike rack that holds the bikes in the same relative position as the Tesla hanging rack should apply the exact same forces, modulo any dynamic swinging effect of the bikes on the hanging rack.

Agree, distance to the CG is the critical factor.

To make matters worse, Tesla's own Towing page says::

When carrying bicycles or other items on the Model X hitch, always check to ensure that the maximum weight is not exceeded. When calculating weight, remember to include the weight of the accessory carrier. For example, assuming the carrier weighs 40 lbs, the weight threshold is sufficient for carrying two bicycles weighing approximately 40 lbs each, or four bicycles weighing approximately 20 lbs each.

Which clearly disregards the additional torque of 4 bikes versus 2 bikes mounted on the same carrier. I understand the desire to dumb the specification down to a simple weight limit, but it would seem the physics involved are more complicated. Is Tesla's towing page equating 2-bikes to 4-bikes of the same weight wrong, perhaps created by some Technical Writer who didn't understand the actual physics and not reviewed by Bosal/Tesla engineers?

Does it? 4 bikes at 20lbs each is 80 pounds, as is 2 bikes at 40 pounds. Or are you speaking of the incremental offset? In which case, it covers the 2 at 40 in the worst position and the four at 20 is theoretically less load.

Then there's that the Tesla rack extends out horizontally quite a bit from the hitch. My 1-Up Super-Duty is much closer - from the pictures I'd guess that my 1-UP's first bike slot is an easy 8"-10" closer to the vehicle than Tesla's rack's first bike slot, and a bit lower as well. Here's a side by side, although my photo wasn't taken from the exact same position as Tesla's, which distorts things a bit:

With the 120 pound limit, the torque is the same for a 500 hitch weight at 8 inches from mount as 120 pounds is at 33 inches. Of course, closer results in less force.

 

[smorgasbord]

Does it? 4 bikes at 20lbs each is 80 pounds, as is 2 bikes at 40 pounds. Or are you speaking of the incremental offset? In which case, it covers the 2 at 40 in the worst position and the four at 20 is theoretically less load.

No, 4 bikes is the worst since that moves the center of mass further away from the car, assuming when you load 2 bikes you load them at the closest to the car positions, which most people would.

 

[mongo]

No, 4 bikes is the worst since that moves the center of mass further away from the car, assuming when you load 2 bikes you load them at the closest to the car positions, which most people would.

Or...

In which case, it covers the 2 at 40 in the worst position and the four at 20 is theoretically less load.

Depends if they were being conservative or not.

 

[smorgasbord]

OK, I misunderstood what you meant by "worst position." But that's because that's not how Bosal or anyone specs these things. The sticker says: "Max vertical load for max 2 bikes" so that would seem to preclude using the worst positions in a 4 bike rack. There are 5 bike racks out there, too - are they handling that?

I think we can agree Bosal were being conservative, but not to the point of mis-using a 4-bike rack (all instructions say to use the closest positions for the heaviest bikes first). It does appear that Tesla accessories either didn't read the Bosal spec or knows something more.

It is interesting that Bosal sells a bike rack with a carrying capacity of 132 lbs (60kg): Bicycle carriers | Driving innovation in mobility

 

[mongo]

OK, I misunderstood what you meant by "worst position." But that's because that's not how Bosal or anyone specs these things. The sticker says: "Max vertical load for max 2 bikes" so that would seem to preclude using the worst positions in a 4 bike rack. There are 5 bike racks out there, too - are they handling that?

I think we can agree Bosal were being conservative, but not to the point of mis-using a 4-bike rack (all instructions say to use the closest positions for the heaviest bikes first). It does appear that Tesla accessories either didn't read the Bosal spec or knows something more.

It is interesting that Bosal sells a bike rack with a carrying capacity of 132 lbs (60kg): Bicycle carriers | Driving innovation in mobility

Yeah, the guide is not for engineers. I would suppose that 120 is conservative.

FWIW, that bike carrier page calls out "conventional towbar" at the bottom which I think is differentiated from the detachable versions like Tesla has.

 

[kchau]

Hi Romad,

Try a Curt 45030

Shawn

I have looked everywhere, and this is the only one that meets the <8" requirement. flip it upside down and it gives you 3/4" rise as well. This is the one to get.

also very cheap at amazon right now: https://www.amazon.com/gp/product/B000P3WW12/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1