Exceeding maximum weight on towbar when using cycle rack

[Peteski]

I realise that the maximum towbar vertical (nose) weight is only 120 lbs (54 kg) when using a cycle rack. I've just bought a very nice 4-bike rack that weighs 44 lbs (20 kg) and I've just added up the weight of our 2 adult carbon mountain bikes and 2 small lightweight kids' bikes and the grand total (including the rack itself) surprisingly comes out at 137 lbs (62 kg). So I'm 17 lbs over the limit and obviously a little concerned. Now I see lots of photos of people carrying similar loads or quite possibly much heavier loads in some cases, so should I really be worried about 17 lbs? I'm not expecting Tesla to approve as they clearly state a maximum nose weight, but just wondered what sort of loads people are actually carrying on the X towbar in practice without any issues? I wouldn't be slamming it violently along bumpy roads or offroad, just regular smooth roads and motorways.

Many thanks for any info.

 

[SabrToothSqrl]

**Chuckles, as he hooks up a 12k trailer to a truck rated for 7k** 17lbs? that's breakfast.

 

[Peteski]

**Chuckles, as he hooks up a 12k trailer to a truck rated for 7k** 17lbs? that's breakfast.

Well that's great, but totally irrelevant to my issue. But it is why I'm asking. Is the Tesla nose weight ridiculously conservative or close to the real limit? I don't want to put £9Ks worth of premium mountain bikes on the back to find that connector fails.

 

[SabrToothSqrl]

Must be nice. Where are these $9k in bikes located?

 

[NeilsMX]

I'm in the same situation, Thule XT hitch rack with extension weighing 70-80lbs for rack alone, plus four bikes weighing about 60lbs total (family bikes - kid sized, and adult ones are light as well) puts us at close to 140lbs. We're hesitant despite only being ~20lbs over because the tongue weight has to support additional torsional forces due to not having wheels supporting from below (ie. you're correct in noting a wheeled trailer is not the same type of load).

I haven't seen any posts of catastrophic failures, if anyone has lost bikes - and I'm assuming Tesla owners in general will have not-inexpensive gear - we would have seen at least one "caution" post based on the value of the stuff that got destroyed.

It would be safe to assume, if the route was generally smooth and your weren't hitting switchbacks over and over again for an extended amount of time, that the hitch wouldn't hit the manufacturer calculated limits for the tongue weight, and 17lbs wouldn't be too risky. For us, a trip to the beach would probably be ok, but a trip to a national park might push the load limit more often than would be advisable.

 

[SSonnentag]

Due to the long extension of the bike rack hanging out the back, the torque on the hitch mount is MUCH more than the 137 lbs. I wouldn't be surprised if it were double that. This is my main beef with bike racks on cars. Trucks have 500+ lb tongue weight ratings, so it isn't an issue.

 

[Peteski]

Due to the long extension of the bike rack hanging out the back, the torque on the hitch mount is MUCH more than the 137 lbs. I wouldn't be surprised if it were double that. This is my main beef with bike racks on cars. Trucks have 500+ lb tongue weight ratings, so it isn't an issue.

Being a mechanical engineer by profession I'm totally aware of torque vs pure vertical load. The trailer tongue weight on the Model X is actually 500 lbs, but only 120 lbs for bike/ski racks. They clearly take into account the torque applied by such racks in their much lower nose weight rating. My rack is actually pretty compact for a 4 bike carrier, so not the worst case scenario from that point of view. I was just surprised how easily I could go over the limit with a couple of fairly lightweight adult mountain bikes plus a couple of kids bikes. 4 full sized adult mountain bikes would be way over the limit.

 

[Peteski]

I haven't seen any posts of catastrophic failures, if anyone has lost bikes - and I'm assuming Tesla owners in general will have not-inexpensive gear - we would have seen at least one "caution" post based on the value of the stuff that got destroyed.

That is a good point. I haven't heard of any failures either and people do certainly carry 3 or more bikes on the X towbar. I'm not really expecting a catastrophic failure, but don't want to damage the towbar frame etc over time. I'll be very careful when going over bumps and undulating roads etc. I'm also thinking of when the kids get bigger and their bikes get heavier, then I'll be another 20 lb over again!

 

[SSonnentag]

The trailer tongue weight on the Model X is actually 500 lbs, but only 120 lbs for bike/ski racks.

That's cool. I don't recall seeing dual ratings on hitches/vehicles before. It's a great idea though. I've never looked at the X ratings as I have an S.

 

[SSonnentag]

What we need is a spring-loaded swivel-wheel underneath the bike rack to help with some of the load. It could double as a wheelie bar.

 

[flar]

It would be nice if they would list the maximum torque and let an owner calculate their own load. The 120lbs limit is a rule of thumb and makes assumptions about the length of the rack and distance between mount points to keep owners from applying too much torque. A single 120lb. bike on a single carriers applies less torque than 4 30 lb. bikes spaced out along a 4-bike carrier (ignoring the rack weight for sake of the point I'm making).

If you learn to fly (at least for gliders), you learn to compute moment arms for Center of Gravity calculations to make sure the plane is flying within spec. There are no "total weight" rules of thumb, you actually calculate the specific point where your CG is so that you can make sure the plane is still stable. (Actually, there is still a total weight limit, but that isn't the end of determining if you are safe, you have to compute CG as well.) The level of detail on the calculation, though, is limited to the location of the seat (longitudinally along the length of the plane) and the weight of the passenger (or similar for cargo), but still they provide that information and you are expected to compute it to know if the flight will be within specs.

While it makes sense to have a rule of thumb weight limit on the hitch for owners who won't be bothered to run the equations for their car, for those who notice they are over the limit, an actual torque rating would let them see if they are still OK even if they are over the conservative spec. The calculation would likely involve the torque limit, the length of the receiver, similar specs for the distance to the rack's CG, and distance to the mount points for each bike.

 

[flar]

Also, if someone loads their 120 lbs worth of bikes and rack with the heaviest bike on the far end of the rack, then they will likely be applying more than the torque you have with 137 lbs if you keep the heavier bikes closer to the car.

 

[Peteski]

Also, if someone loads their 120 lbs worth of bikes and rack with the heaviest bike on the far end of the rack, then they will likely be applying more than the torque you have with 137 lbs if you keep the heavier bikes closer to the car.

Yes a maximum torque figure would be nice, but most people are not capable of working out the CofG position of their fully loaded rack and then calculating the resulting torque on the tow bar. Obviously I will load the heavier bikes on the front of the rack to minimise the torque and I expect it will be okay if I go easy on any bumpy roads.

The other unknown is the safety factor they built into the stated weight limit. That would also be useful to know. I expect it’s a very conservative limit, but I can only guess!

 

[Barklikeadog]

What if I shoot a deer that weighs 185lbs dressed, but the weight is spread out on the luggage rack? Would I then be under 120lbs in this case?

 

[Peteski]

What if I shoot a deer that weighs 185lbs dressed, but the weight is spread out on the luggage rack? Would I then be under 120lbs in this case?

Well no, it’s still 185 lbs of load. The torque applied by that 185 lbs would depend how far its CofG was from the towbar nose (torque = force x distance). But we don’t know what the maximum torque allowed actually is. The best you could do is position the deer as close as possible to the back of the car on the rack.

 

[Yinn]

Well no, it’s still 185 lbs of load. The torque applied by that 185 lbs would depend how far its CofG was from the towbar nose (torque = force x distance). But we don’t know what the maximum torque allowed actually is. The best you could do is position the deer as close as possible to the back of the car on the rack.

Although, I think a bike rack or a large cargo carrier goes upwards with the mounting point typically higher towards the spoiler. This amplifies the amount of torque placed on the tow bar, especially on bumps.

With a flat carrier or a typical deer rack; it's usually flat/level with the tow bar. This means the amount of torque is much closer to whatever the designed limits are and should be a closer match to typical tongue weight.

 

[Peteski]

Although, I think a bike rack or a large cargo carrier goes upwards with the mounting point typically higher towards the spoiler. This amplifies the amount of torque placed on the tow bar, especially on bumps.

With a flat carrier or a typical deer rack; it's usually flat/level with the tow bar. This means the amount of torque is much closer to whatever the designed limits are and should be a closer match to typical tongue weight.

Actually that's not correct. It's the perpendicular offset distance of the load's CofG from the towbar nose that defines the torque, not its height. Remember the load is acting vertically downward with gravity, so only the distance to the towbar nose perpendicular to that load matters. The height of the rack is irrelevant to the torque calculation. A 10m tall bike rack would put the same torque on the towbar as a 1m tall rack with the same rear overhang.

As it happens my rack is the flat tray type anyway, so it's more or less in line with the towbar.

 

[Yinn]

Actually that's not correct. It's the perpendicular offset distance of the load's CofG from the towbar nose that defines the torque, not its height. Remember the load is acting vertically downward with gravity, so only the distance to the towbar nose perpendicular to that load matters. The height of the rack is irrelevant to the torque calculation. A 10m tall bike rack would put the same torque on the towbar as a 1m tall rack with the same rear overhang.

As it happens my rack is the flat tray type anyway, so it's more or less in line with the towbar.

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By wouldn’t that be the same issue with a trailer and tongue weight? Sure there’s wheels in the back, but tongue weight is the actual weight at the hitch point.

 

[Peteski]

By wouldn’t that be the same issue with a trailer and tongue weight? Sure there’s wheels in the back, but tongue weight is the actual weight at the hitch point.

No, it's not the same thing at all. That's why the trailer tongue rating is 500 lbs, while the bike/ski/luggage rack rating is only 120 lbs. The latter applies a direct torque to the towbar because the load is overhanging, while the former has its weight supported on its wheels. So there is only a pure vertical load with no significant torque applied to the towbar.

 

[Yinn]

No, it's not the same thing at all. That's why the trailer tongue rating is 500 lbs, while the bike/ski/luggage rack rating is only 120 lbs. The latter applies a direct torque to the towbar because the load is overhanging, while the former has its weight supported on its wheels. So there is only a pure vertical load with no significant torque applied to the towbar.

Only some of the weight though. And depending on where your wheels are set in relation to the overall trailer, that will change the amount of weight - and resulting impact/torque on the tow bar.