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Thoughts on adding additional support to a bike rack to overcome the 120 lbs weight limit?

I have read all of the threads discussing the 120 lbs limit for bike carriers and consensus seems to be that it is not a good idea to exceed the manufacturers warning. However, I have not seen any concrete/compelling answers as to why the limit for bikes is so much lower than the tongue weight. The removable Bosal hitch receiver seems to be the weak link as I see no other major difference between the after market Eco Hitch that everyone is installing. It would be helpful to know what the actual problem is before looking for a solution.

That said, I've already gone ahead and ordered a 1up Equip-D rack as I need to make this work one way or another. I was curious to hear your thoughts on adding additional support via straps attached to the car. I've identified a couple of attachment points candidates.

1) You can use something like these Quick Loops for adding lash points to the tailgate or simply attach a strap to the back seat base. My concern with running straps through the tailgate is that it would put stress on the latch.

2) There are two eyelets (see attached image) next to the Bosal Hitch Receiver and part of the hitch frame. Attaching to the hitch frame itself seems like a clean solution but I am not sure if it would solve the problem as I don't actually know where the weak link is.

Any thoughts on the above solutions or has anyone ever contacted Tesla/Bosal to get a concrete answer?
 

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Tongue weight = assumption of a trailer being used. Weight on tongue is supported by trailer wheels.
Hitch rack = cantilevered load.

Hence the note from 1UP:

Note: For 2” Super Duty Double, with addition of two Add-Ons (sold separately), this rack can hold a maximum
of four bikes when all bikes are 50 lb or less. If any bike weighs 51 lb – 75 lb, only one Add-On may be used.
 
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I think I finally understand the issue. Tongue weight of 500 lbs is given at 8" from the hitch receiver. Since a bike rack is a lever arm, this needs to be converted to Torque. 500 lbs at 8" translates to 333 ft/lbs of torque. Given the torque rating, I can now calculate the maximum capacity based on bike weight and weight distribution. I did the rough math with relatively light bikes for two of the racks that 1up offers, see the attached screen shot. Both cases exceed the rated tongue weight even though the weight of the bikes + rack is far less than 500 lbs.

After further reading on Tongue Weight, it sounds like it is more of an issue with handling dynamics than a structural integrity. This would also explain why there is a different tongue weight based on wheel size. Installing a 3rd party hitch would just give you a false sense of security, as the issue is not necessarily a weak hitch receiver, we know that it can handle pulling 5,000 up a hill!

If the tongue is too heavy, the tow vehicle’s steering will be impacted. Traveling ahead of its axis, a too-heavy trailer tongue will also affect the way the trailer moves along the road and the way it stops.

When TW is too heavy, stress is placed on the frame, suspension, tires, drivetrain, or brakes of the vehicle being used to tow. Because of the excessive weight transferred through the hitch ball, the rear tires of the tow vehicle can become overloaded, pushing the rear end of the vehicle around.
What Is Tongue Weight and Why Is It Important?

That said, it sounds to me like with the right choice of rack, bikes and weight distribution, it is possible to carry 4 bikes. The answer is not as strict as the 120 lbs max that Tesla recommends. By my math, every car that has a 500 lbs tongue weight rating would have the same problem.
 

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@akacaj Thanks for doing the calculations....very informative.

If I were carrying 4 bikes, would use a vertical solution....doesn't extend back from the rear of the car as far.


 
I think I finally understand the issue. Tongue weight of 500 lbs is given at 8" from the hitch receiver. Since a bike rack is a lever arm, this needs to be converted to Torque. 500 lbs at 8" translates to 333 ft/lbs of torque. Given the torque rating, I can now calculate the maximum capacity based on bike weight and weight distribution. I did the rough math with relatively light bikes for two of the racks that 1up offers, see the attached screen shot. Both cases exceed the rated tongue weight even though the weight of the bikes + rack is far less than 500 lbs.
Imagine hitting a pothole or something else that bounces and fairly quickly rises and then quickly LOWERs the bikes and rack. Imagine that downward force which is no longer steady dead weight.
 
Same thing can happen with a trailer. Imagine going over a bump with a 5 ton trailer and the amount of force that would put on the car or carrying 5 ton up a steep hill. Again, I don't believe that it has everything to do with the hitch not being able to take the force, it is about vehicle driving dynamics.

The folks who installed an after market hitch are in no better position than the factory hitch. The only difference is that one has a sticker on it warning of carrying too large of an unsupported load, while the other simply shows the tongue weight. The Tesla hitch has the same tongue weight as the Eco Hitch and should work just as well.

Regardless, what I am trying to say here is that you can carry a lot more than 121 lbs as long as the weight is distributed correctly to not exceed the resultant force on the lever arm. It sounds to me like Tesla took the simple approach with the 55 kg / 121 lbs, 2 bike max rather than explaining the physics behind that decision so that people can calculate their own load.
 
I just did a comparison between the Tesla Rack with max weight as per spec VS the 1Up Heavy Duty with the same load. You can see that the Tesla rack imparts twice the amount of torque as the 1Up tray. By this measure you can clearly carry more weight on the tray rack due to the shorter lever arm.

Screen Shot 2022-06-03 at 5.31.59 PM.png
 
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davezforce

IG @davezforce
Jan 18, 2017
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283
NJ
I’ve been using a dirt bike carrier for 20 yrs on my 99’ Pathfinder (500 tongue/5000 towing). The rack is 65lbs, my KX 125 is 196lbs, for a total of 261 + a 5gal jug of fuel attached to the carrier, with no problems. On average it’s a 120 mile round trip on highway plus in trail time to get to the riding spot. So your telling me that this couldn’t be done with the Tesla because of the bosal hitch design or just the design of the whole entire hitch mounting itself? I’ve also towed a 5x8 utility trailer with 2 additional dirt bike with at the same time because the carrier has a 2” ball mount on it as well.
 

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ngng

Active Member
Jul 23, 2018
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I’ve been using a dirt bike carrier for 20 yrs on my 99’ Pathfinder (500 tongue/5000 towing). The rack is 65lbs, my KX 125 is 196lbs, for a total of 261 + a 5gal jug of fuel attached to the carrier, with no problems. On average it’s a 120 mile round trip on highway plus in trail time to get to the riding spot. So your telling me that this couldn’t be done with the Tesla because of the bosal hitch design or just the design of the whole entire hitch mounting itself? I’ve also towed a 5x8 utility trailer with 2 additional dirt bike with at the same time because the carrier has a 2” ball mount on it as well.

Towing is different than a bike rack. I'll tow my race car but I won't overload a bike rack.
 

davezforce

IG @davezforce
Jan 18, 2017
583
283
NJ
Towing is different than a bike rack. I'll tow my race car but I won't overload a bike rack.
The question isn’t towing. The question is why can say a motorack with a dirt bike or a bicycle rack that is well under 500 lb tongue be used on a model X. What is the actual limiting factor. Is it the design of the bosal hitch that come standard with the X or is it the actual way the the hitch mounts on to the body of the X. If you were to install a draw tight hitch would you then be able to use the actual full 500 lb tongue weight? I’m trying to understand what is the failure point of the included hitch that you can’t use more than 120 lbs which seems very poor of a design if so
 
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ngng

Active Member
Jul 23, 2018
1,747
968
Bay Area
The question isn’t towing. The question is why can say a motorack with a dirt bike or a bicycle rack that is well under 500 lb tongue be used on a model X. What is the actual limiting factor. Is it the design of the bosal hitch that come standard with the X or is it the actual way the the hitch mounts on to the body of the X. If you were to install a draw tight hitch would you then be able to use the actual full 500 lb tongue weight? I’m trying to understand what is the failure point of the included hitch that you can’t use more than 120 lbs which seems very poor of a design if so
Yes
 
Below is the question and response that I got from Tesla when asking about clarification on the 120 lbs limit.

My question:
Hi, I am looking for clarification on the max bike rack weight capacity. The manual states that the Model X has a class III hitch with a max tongue weight of 500 lbs, but it also warns that it can only support vertical loads of 120 lbs when it comes to bike/cargo carriers. That seems extremely low for a car that can tow 5,000 lbs!. It is my understanding that a tongue weight of 500 lbs literally means vertical load of 500 lbs at the ball. So which is it, 120 or 500? At what distance to the hitch is the 120 lbs load assumed to be?

Based on the numerous online forums that I have read, there seems to be a lot of confusion among Tesla owners on this topic. It would be great if the user manual can provide greater detail on how the 120 lb limit was calculated.

Their response:
Apologies for the delay, and thanks so much for reaching out. The hitch itself can withstand a 500lb vertical load at the ball of the tow hitch, but that number differs depending on where the load is applied. Through testing, it’s been determined that the tow hitch can support a 120lb load when the hitch is cantilevered rearward, like when carrying bikes on a bike rack.

Therefore, we recommend a 120lb limit, since realistically most users will be carrying accessories on a rack, rather than the ball of the tow hitch itself. We will look into clarifying this content as needed.

To answer your question about distance to the hitch, the maximum distance should not exceed 632mm rearward from the hitch pin center. Therefore, the vehicle can support loads of 120lb within this 632mm distance. Many bike carriers and accessory racks should satisfy this distance requirement.

Their response was helpful, but it left me with more questions. For example 120 lb at 632mm (25 inches) produces less torque than 500 lb would at 8 inches. Even if the center of mass for 120 lb was at 25 inches, it would produce 250 ft/lb of torque which is less than the 333 ft/lb of torque produced by the 500 lb tongue weight at the ball (8 inches from the pin). Based on this I can conclude that the removable Bosal hitch receiver is not the weak link.

It also doesn't explain why the wheel size affects the tongue rating but not the bike rack carrying capacity. To further complicate the matter, Model Y has a lower tongue weight rating of 350 lb but can support a bike rack up to 160 lb!
 

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