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Platform bike rack
- Thread starter Poncherelly
- Start date
LOL my rational is 1k for a bike rack, then 1k minimum per bike target one (4) that is light enough to carry back there ... or ... just buy a trailer and pull the crazy bikes I already have. LOL.For all you heavy weights I guess you will all have to switch to lightweight road bikes
Model Y manual says that while tongue weight capacity is allowed to be 350lb, the vertical weight capacity for accessory carriers like bike rack to be 160lb. I am little obsessed about this phenomenon that the two are different and have been doing some calculations why that might be in if I can safely carry four bikes with my Kuat NV, which in my case totaling 210 lb.
Before I go further I should be explicit that I am not recommending anyone to put a bike rack more than 160lb. Given my calculations, I intend to put 210 lb but that is my decision and I take the risk and I am not recommending anyone to do the same. Plus your configuration might be different than mine and even with my methodology it may not be safe to carry more than 160 lb in your configuration (weight and how far back the center of gravity (CG) of your bike rack + bikes is from the tow bar screws are the determining factors)
There are 6 bolts that holds the tow hitch to the body of the car. 4 being at the bottom and 2 at the top. Bolts have two capacities, one for shear load, which is load carrying capacity perpendicular to the axis of the bolt, and the second is tensile load which is along the axis of the load.
We can eliminate shear loads to be the limiting factor, as vertical load and tongue weight generates the exact same shear load for the 6 bolts, so the difference should arise from the tensile load.
When pulling a trailer, in addition to the tongue weight, there is also draw bar pull force (DP).
DP = (T x R / r ) - RR where T= torque of the car in inch pounds, R = Gear Reduction, r= tire radius in inches and RR is Rolling resistance in lb.
And RR = GVW x R / 1000, where GVW is for the trailer in lb, R is Rolling resistance of the surface.
Let’s first calculate RR for model y with 3500lb trailer limit from the manual and R of 15 which is for concrete (there are various tables that you can locate R online for different surfaces).
RR = 3500 x 15 / 1000 = 53 lb
To calculate DP, we also need T, R and r
T = 471 lb-ft x 12 inch/ft = 5,652 lb-inch
R=9:1 (just google Tesla Model Y gear ratio)
r=19 inch (which is what I have)
DP = ( 5,652 x 9 /19) - 53 = 2,677lb
If you assume hitch ball is along the same height as the bottom bolts (You can see the location of bolts on the body from the eco hitch aftermarket installation video from i1Tesla. Depending on what type of hitch ball you use, this is a fairly likely scenario), then bottom bolts would be taking the whole Draw Pull, while upper bolts would be taking the tongue weight load, which is minor. So 2,677lb is pulled by four bolts on the bottom under this scenario.
If the front bolt and the bottom bolt designed to carry the same load (in the video they mention the bolt is 12 mm for all 6 bolts), then it is safe to assume that top bolts can carry half the load of bottom bolts, because there are 2 bolts on the top vs 4 on the bottom. Which gives us top bolt tensile capacity of 1,338lb for two bolts. We'll use this number in below to see if it is safe to carry 210lb bike rack + bike. Btw, this is well below the capacity of an M12 bolt that is 5,000lb+ per bolt.
Now let’s move on to the scenario of carrying an accessory like a bike carrier.
In this scenario, there is no DP, only a vertical load. Depending on how far back the CG of the bike rack is, there will be more tensile load on the top bolts due to cantilever effect. In fact, when carrying a bike carrier, the bottom bolts is not working as the whole bike rack is being pulled by gravity which only pulls the top bolts while applies negative pressure to the bottom bolts. There is a clockwise rotational force in the system, when looked from the side.
Because the system needs to be in equilibrium: T x V = W x D, where
T=tensile load on the top bolts
V= vertical distance between top and bottom bolt
W= weight of bike rack
D= distance between horizontal CG location of bike rack and the bolts - in other words how far back bike rack CG is from the bolts
I have a Kuat nv with 4 bike racks. With 4 of my bikes the total weight comes to 210lb. I also measured roughly the horizontal CG from tow bar mounting bolts, which is ~36 inches looked from the side of the car.
Again from the eco hitch installation video, the distance between top and bottom bolts is somewhere between 8-10 inches. Let’s take 8 inches to be conservative.
T x 8 = 210 x 36
T = 945 lb. This is lower than the 1,338lb top bolts capacity we calculated from the trailer pull scenario, therefore it should be safe for me to put 4 bikes.
Now what could be wrong with this calculation:
Before I go further I should be explicit that I am not recommending anyone to put a bike rack more than 160lb. Given my calculations, I intend to put 210 lb but that is my decision and I take the risk and I am not recommending anyone to do the same. Plus your configuration might be different than mine and even with my methodology it may not be safe to carry more than 160 lb in your configuration (weight and how far back the center of gravity (CG) of your bike rack + bikes is from the tow bar screws are the determining factors)
There are 6 bolts that holds the tow hitch to the body of the car. 4 being at the bottom and 2 at the top. Bolts have two capacities, one for shear load, which is load carrying capacity perpendicular to the axis of the bolt, and the second is tensile load which is along the axis of the load.
We can eliminate shear loads to be the limiting factor, as vertical load and tongue weight generates the exact same shear load for the 6 bolts, so the difference should arise from the tensile load.
When pulling a trailer, in addition to the tongue weight, there is also draw bar pull force (DP).
DP = (T x R / r ) - RR where T= torque of the car in inch pounds, R = Gear Reduction, r= tire radius in inches and RR is Rolling resistance in lb.
And RR = GVW x R / 1000, where GVW is for the trailer in lb, R is Rolling resistance of the surface.
Let’s first calculate RR for model y with 3500lb trailer limit from the manual and R of 15 which is for concrete (there are various tables that you can locate R online for different surfaces).
RR = 3500 x 15 / 1000 = 53 lb
To calculate DP, we also need T, R and r
T = 471 lb-ft x 12 inch/ft = 5,652 lb-inch
R=9:1 (just google Tesla Model Y gear ratio)
r=19 inch (which is what I have)
DP = ( 5,652 x 9 /19) - 53 = 2,677lb
If you assume hitch ball is along the same height as the bottom bolts (You can see the location of bolts on the body from the eco hitch aftermarket installation video from i1Tesla. Depending on what type of hitch ball you use, this is a fairly likely scenario), then bottom bolts would be taking the whole Draw Pull, while upper bolts would be taking the tongue weight load, which is minor. So 2,677lb is pulled by four bolts on the bottom under this scenario.
If the front bolt and the bottom bolt designed to carry the same load (in the video they mention the bolt is 12 mm for all 6 bolts), then it is safe to assume that top bolts can carry half the load of bottom bolts, because there are 2 bolts on the top vs 4 on the bottom. Which gives us top bolt tensile capacity of 1,338lb for two bolts. We'll use this number in below to see if it is safe to carry 210lb bike rack + bike. Btw, this is well below the capacity of an M12 bolt that is 5,000lb+ per bolt.
Now let’s move on to the scenario of carrying an accessory like a bike carrier.
In this scenario, there is no DP, only a vertical load. Depending on how far back the CG of the bike rack is, there will be more tensile load on the top bolts due to cantilever effect. In fact, when carrying a bike carrier, the bottom bolts is not working as the whole bike rack is being pulled by gravity which only pulls the top bolts while applies negative pressure to the bottom bolts. There is a clockwise rotational force in the system, when looked from the side.
Because the system needs to be in equilibrium: T x V = W x D, where
T=tensile load on the top bolts
V= vertical distance between top and bottom bolt
W= weight of bike rack
D= distance between horizontal CG location of bike rack and the bolts - in other words how far back bike rack CG is from the bolts
I have a Kuat nv with 4 bike racks. With 4 of my bikes the total weight comes to 210lb. I also measured roughly the horizontal CG from tow bar mounting bolts, which is ~36 inches looked from the side of the car.
Again from the eco hitch installation video, the distance between top and bottom bolts is somewhere between 8-10 inches. Let’s take 8 inches to be conservative.
T x 8 = 210 x 36
T = 945 lb. This is lower than the 1,338lb top bolts capacity we calculated from the trailer pull scenario, therefore it should be safe for me to put 4 bikes.
Now what could be wrong with this calculation:
- First, because the bolt sizes are the same for top and bottom, I assumed they can handle the same load. However, before bolt breaks, where bolt attaches to the body could break and it is possible the body design have different strengths on the top bolt support area and bottom bolt support area. This would be unlikely but plausible.
- Second my 8-10 inch measurement between top and bottom bolts is based on me comparing hand sizes with bolt distances from the video. This measurement could be wrong as well but I am quite certain it is no less than 8 inches which I used in calculations
- Maybe bolts are not the limiting factor but the welding of tow bar is the weaker part under the rotational forces. This is also unlikely as even the cheap tow bars can carry far more than 350lb for other cars and can't imagine Tesla making the welding just strong enough to carry the rotational force under 160lb limit.
- If you are not bored yet and read until here and you see other reasons why my calculations could be wrong, please let me know.
Last edited:
While reading, I felt like this
Google Image Result for https://media3.giphy.com/media/pKEufUXBqsLi8/giphy.gif
But great job. I personally won't take the risk, but I can see your justification.
steve841
Active Member
Yes I was wondering about the restrictions myself.. my thought was that having so much weight on a long lever puts a lot of strain on the frame the hitch is attached to (not the hitch itself).. my concern would be that it could bend the car’s frame.. it’s hard to know where the weakest link is. Also, going over bumps could further stress things.
That being said - keep us posted... I just bought a Thule xt pro dual carrier that’s 1.5 inch so hopefully I can find a way to jig it to work with the 2 inch hitch. Ideally, I’d like to get the extension to carry 4 bikes..
That being said - keep us posted... I just bought a Thule xt pro dual carrier that’s 1.5 inch so hopefully I can find a way to jig it to work with the 2 inch hitch. Ideally, I’d like to get the extension to carry 4 bikes..
I agree for a few lbs. However in my case I have 120lbs of bikes. Then I you add a 90lbs carrier, I'm 50lbs over and although probably fine, I'm not willing to take the risk yet.
Of and some additional good news, the Thule roof box I've been holding on to since I sold my last Audi doesn't fit the TESLA. The roof rack is spaced too far apart. So now that's going up for sale.
Catbiscuits
Member
OMG. How wide is the spacing and which box is it? Do you have a solution for a new box?
The only solution is to sell the box and maybe buy a replacement ... assuming a replacement is capable to crossing the gap.
This is the closest image I can find. Free Shipping To Canada And Usa For Sportrack Sr7018 - 18 Cubic Foot Rear Opening Cargo Box | Tdot Performance | Tdot Performance
The problem is the Thule box has a knob inside that turns to close the clamps around the roof rails. They move to adjust to fit most roof systems but the one on my Y apparently is too spaced out to accommodate.
One more reason I need to buy a trailer I guess.
This is a really interesting discussion. I also have only ever looked at "tongue weight" capacity and have never considered "vertical weight" capacity.
Like Dcd7041 had stated earlier it's probably not the hitch which is going to fail first -- it's probably something else (bolts, frame, etc.). If that's true that means that the people who have installed the EcoHitch on a Model Y also have that same 160lb limitation, right?
Like Dcd7041 had stated earlier it's probably not the hitch which is going to fail first -- it's probably something else (bolts, frame, etc.). If that's true that means that the people who have installed the EcoHitch on a Model Y also have that same 160lb limitation, right?
Catbiscuits
Member
Sure, will need to do that for you tomorrow though when we're back from camping. No tape measure with me
Catbiscuits
Member
Well even if my Kuat will only ever carry two bikes, here it is on the Y on a recent trip.
Lo
Ok good. And I think at 2 bikes you're safe with a little room to spare. At 4 bikes and the needed extension though ... you go over.Well even if my Kuat will only ever carry two bikes, here it is on the Y on a recent trip.
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