Tow Hitch

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I got a shop to install the Torklift hitch (x7373) and the Tekonsha Modulite ZCI (119250) trailer wiring package and the setup worked fine in towing a small UHaul trailer for 300 kilometers but power consumption was quite large as I averaged around 270 wh/km for the trip. I’ve got a longer trip coming up and I know the standard Supercharger planning in the car won’t be accurate as the car doesn’t know it is towing a trailer. Anybody have a suggestion for an app to help with trip planning?
 
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I'm going to look into coming up with a lighter weight option than the torklift offering, as 55 lbs seems like a huge amount of weight to add to a performance vehicle. Will start by finding out if the crash beam has enough strength on it's own. On my subaru, I reinforced the crash beam by replacing the spot welds with full seam welds, and found it to be much stronger in torsion and bending than the 2" square tube normally used for tow bars.
 
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I got a shop to install the Torklift hitch (x7373) and the Tekonsha Modulite ZCI (119250) trailer wiring package and the setup worked fine in towing a small UHaul trailer for 300 kilometers but power consumption was quite large as I averaged around 270 wh/km for the trip. I’ve got a longer trip coming up and I know the standard Supercharger planning in the car won’t be accurate as the car doesn’t know it is towing a trailer. Anybody have a suggestion for an app to help with trip planning?
How big was the trailer and how fast were you going?
 

android04

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I'm going to look into coming up with a lighter weight option than the torklift offering, as 55 lbs seems like a huge amount of weight to add to a performance vehicle. Will start by finding out if the crash beam has enough strength on it's own. On my subaru, I reinforced the crash beam by replacing the spot welds with full seam welds, and found it to be much stronger in torsion and bending than the 2" square tube normally used for tow bars.
In case you don't know, the crash bar in Model 3 is aluminum. And it clamps onto the crash supports with 3 nuts on each side, which tighten only into the inside surface of the crash bar. I would not modify the crash bar to pull trailers, but it might be ok for a bike rack.
 
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In case you don't know, the crash bar in Model 3 is aluminum. And it clamps onto the crash supports with 3 nuts on each side, which tighten only into the inside surface of the crash bar. I would not modify the crash bar to pull trailers, but it might be ok for a bike rack.
Of course, I'd first make a 3D model of the beam and run a static simulation. I'd then verify by clamping a cantilevered arm to the approximate distance of a trailer hitch and applying a test load. In this case, 200 lbs tongue weight, X2 for bumps or pot holes, X2.5 for factor of safety. Should result in something on the order of 1000 ft-lbs of torque. Regardless, I'll practice due diligence.

Worth noting for the uninitiated, resistance to bending increases with the cube of the thickness, and resistance to torsion increases to the forth power of thickness, i.e. if a beam or shaft is twice as thick, it can handle 8X bending load and 16X the torsion. Assuming the crash bar is made from a common structural aluminum alloy, it might be comparable in strength to the relatively low grade of steel used in common aftermarket automotive accessories. This is a good thing as it must provide enough strength to accelerate a 4000 lb car from 0-5mph in only a few inches in order to meet federal standards. An aftermarket parts seller probably wouldn't be willing to use it as a tow bar for liability reasons, but it's very likely that it is more than strong enough to handle the load.
 
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Quick update:

1559066142178-png.26276


With 1000 lbf applied vertically to the ball, the hitch is barely breaking a sweat. Aside from some data artifacts due to the over simplified way in which I modeled the bolted connections to the car frame and the receiver tube itself (assumedly those have been properly designed already), we don't see anything over 12 kpsi (83 MPa), with the yield strength of 6061-T6 aluminum being 40 kpsi (276 MPa). I'll do a bit more modeling of other load cases to verify, but this should be the most severe.

I used only a single piece of square tube with some holes drilled in it, available on McMaster for $32. This will work with a 90º receiver available on amazon Best of all, this adds only 3 lbs. (1.5 kg) of weight permanently attached to the car.

1559066438787-png.26277
 
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Note one minor issue: This design leaves only a small gap between the square tube and the rear bulkhead of the car. It is likely that the rear bulkhead could be dented by a rear end collision because of this. The bulkhead is hidden from view both from inside and outside of the car and does not appear to serve a structural purpose, so I'm not concerned for the sake of my own vehicle.
 
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Small update:

I located the SAE spec. for tow hitches and found that the official requirement is that the hitch widthstand half of the full gross weight of the trailer plus 480 lbs. applied in the forward direction and the same load applied simultaneously in the vertical direction "without becoming detatched". Note that they don't require the hitch to survive this load intact, so this is a what I'd refer to as a "crash load". Looking at the commercially available tow bars and their ratings, it certainly seems that most of them would be pretty mangled, but still "attached" after such a load. Nonetheless, I set out to design a system which will not even bend with the crash load. Given such loading, my original configuration would cause fastener pull through and/or buckling where the down tube attaches to the lower half of the crash beam, so I reinforced it with a tension strap. Shown below is my model of the worst case loading with 2,000 lb trailer, which consists of 1480 lbs of weight applied vertically and 1480 lbs applied to the ball in the forward direction simultaneously for a total force of 2092 lbs. (per SAE J684 spec):

1559851153771-png.26617


This slight modification halves the peak load in the critical region. I might be pessimistic here, because my modeling now shows that the receiver I bought on amazon is likely to fail before my assembly attached to the car does, and it is rated to 5,000 lbs tow capacity. I've thereby decided to self-rate the assembly at 2500 lbs towing capacity and 250 lbs. tongue weight. One concern going forwards is that the vertical tube will suffer accelerated wear being made from aluminum, but this will be used infrequently and I'd rather have problems with the tow assembly than suffer galvanic corrosion of my crash beam. Hard coat anodizing could help, but the company I work for doesn't normally spec. that coating and it would be $250 to pay for my own run.

I've already machined all of these features into the square tube. Annoyingly, the 2" hitch adapter is actually more like 2.02" square, whereas the aluminum tube is a smidge under 2". To remedy this situation, I used our company's milling machine with some fairly long end mills to carefully open up the lower end of the tube to 2.05" square.
 
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Feathermerchant

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That looks pretty good. My application will only be a bike rack. Probably never more than 100 lb.
At that weight the rack could be Al as well. I have a steel one I designed and fabricated but it is pretty heavy. 21 lb 4 oz. I just need to carry 2 bikes.
 

garsh

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... so this is a what I'd refer to as a "crash load".
...I set out to design a system which will not even bend with the crash load.
If there's a crash, you mighty prefer that the hitch bend rather than transferring all of that energy to the vehicle and causing part of the frame to bend.
 
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If there's a crash, you mighty prefer that the hitch bend rather than transferring all of that energy to the vehicle and causing part of the frame to bend.
I should make a distinction here; when I said "without bending" I meant "without permanently bending". The beam bends plenty under crash loads, 1/2 inch at the ball! In terms of a standard rear-end collision (without hitch installed), this single strap shouldn't add significantly to the overall strength of the crash beam; what it does do is make the beam better able to handle the torsion imposed by applying an offset force at the end of the trailer hitch.

Once I install this stuff on the car and test it out, I'll post up some drawings with precise measurements.
 
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If someone want's to copy what I'm doing, I can't stop them. There are liability reasons that I wouldn't do this for someone else, however. In fact, I don't even recommend or certify the suitability of what I've done for any particular purpose. This thread is strictly to share my experience and thought process.
 
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How big was the trailer and how fast were you going?
It was the smallest U-haul covered trailer, 4 x 8 i think(see pic attached). On the trip from Kingston Ontario to Bolton Ontario I was traveling at around 100 Km/hr (62 mph). We just completed the longer trip on June 3, from Bolton to Baltimore, about 780 ish Km's or about 460 miles. I used the suggested BRP and made 5 charger stops on the way down during the 12 hour trip, wh/km during the trip was about 275 just from monitoring during the ride. (aside: how does one turn on the odometer/trip meter functions in the car? I can't find it in the manual ), with excursions to above 300 wh/km and down to 250 depending on the terrain. It was a long slow trip due to the multiple charging stops and reduced speeds. On the way back the next day it took us 2 charging stops (1 of which was done just for peace of mind) and 10 hours at about 110 kmh average since we weren't always on interstates and I was trying to run efficiently to avoid additional SC stops. I don't think I will be doing such a long trip with a trailer again as it was quite annoying feeling the drag and bumps of the trailer on the hitch. I will use it for my hitch mounted bike rack regularly though and perhaps for much shorter trips with a trailer.
 

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Dr. J

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I got a shop to install the Torklift hitch (x7373) and the Tekonsha Modulite ZCI (119250) trailer wiring package and the setup worked fine in towing a small UHaul trailer for 300 kilometers but power consumption was quite large as I averaged around 270 wh/km for the trip. I’ve got a longer trip coming up and I know the standard Supercharger planning in the car won’t be accurate as the car doesn’t know it is towing a trailer. Anybody have a suggestion for an app to help with trip planning?
abetterrouteplanner.com can handle that. It has a setting for Extra Weight under Show More Settings specifically for "Extra vehicle weight, like a trailer or heavy cargo."
 
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It was the smallest U-haul covered trailer, 4 x 8 i think(see pic attached). On the trip from Kingston Ontario to Bolton Ontario I was traveling at around 100 Km/hr (62 mph). We just completed the longer trip on June 3, from Bolton to Baltimore, about 780 ish Km's or about 460 miles. I used the suggested BRP and made 5 charger stops on the way down during the 12 hour trip, wh/km during the trip was about 275 just from monitoring during the ride. (aside: how does one turn on the odometer/trip meter functions in the car? I can't find it in the manual ), with excursions to above 300 wh/km and down to 250 depending on the terrain. It was a long slow trip due to the multiple charging stops and reduced speeds. On the way back the next day it took us 2 charging stops (1 of which was done just for peace of mind) and 10 hours at about 110 kmh average since we weren't always on interstates and I was trying to run efficiently to avoid additional SC stops. I don't think I will be doing such a long trip with a trailer again as it was quite annoying feeling the drag and bumps of the trailer on the hitch. I will use it for my hitch mounted bike rack regularly though and perhaps for much shorter trips with a trailer.
Unsurprising. Fortunately I have no plans on towing road trips with this car. I'd mostly use it for towing a 5'x8' garden/utility trailer around town, or maybe borrowing someone's popup camper to visit one of the local camping sites within 20 or 30 miles. A Tesla should do just fine for this.