Towbar Loads With Bike Carriers (Strada DL3)

[dr_gn]

(also posted in caravans section).

I've got an old Civic fitted with a towbar. Handbook defines a 50kg vertical load limit.

I want to buy a towbar-mounted bike rack (Strada DL3), total weight with bikes almost exactly 50kg.

Simple questions:

1) Even though the vertical load is still 50kg, it'll be located about 400mm behind the vertical axis of the towbar, giving a significant moment (lever effect). Is the max. vertical load figure primarily due to strength limitations of the towbar-body mountings, or suspension compression? If it's the latter, surely the vertical load limit should be de-rated to compensate for the lever effect of something rigidly clamped to the ball?

2) Are tow hitches designed to take this moment loading? Obviously, the're spherical to avoid this kind of load, and provide articulation.

I can't find any information on this on the carrier specs.

Thanks.

[Watchman]

I don't understand what the vertical limit refers to on towbars but you could hang another car off them in practice so a bike rack won't trouble one in the slightest. I have a hefty Thule one which can take 4 bikes, and my tow hook is removable (implying it may not be as tough as a fixed one) but I don't worry about that. When I towed a 2-tonne caravan with it last year, the movement of the van will have exerted massive loads on the tow hook, in all directions, well in excess of 50Kg, even though the stationary rating was so low.

[gazza285]

Don't confuse static nose weight limits with carrying capacity, your bike rack will be fine.

[dr_gn]

gazza285 said:

Don't confuse static nose weight limits with carrying capacity, your bike rack will be fine.

I think it will indeed be fine, but it seems strange that in theory you could weld a scaffolding pole horizontally to to towball like a massive lever, load it on the end (fully compressing the rear suspension), yet still comply with the vertical load and towing capacity limits. All I'm saying is that the loads imparted by a fixed bike carrier are totally different from towing, so do the limits apply?

[AyBee]

But as somebody else has said, a caravan bounces around and exerts much more force on the towball in all directions than the static limit.

[Max5476]

AyBee said:

But as somebody else has said, a caravan bounces around and exerts much more force on the towball in all directions than the static limit.

Unsurprisingly the engineer will have designed considering the dynamic loads, hence giving what appears to be a low static load. Additionally this weight will consider potential changes to braking and stability performance, and is set at a value to keep their pl lawyers happy in case you crash and try and blame them. So unless you are loading it for the life of the car, and drive everywhere flat out you will be fine to exceed it.

With regards to the moment. A degree of moment will always have been assumed. As the cog for a trailer will ideally be between the axle and tow bar, therefore applying a level of moment. I doubt you coud fit 50kg far enough rewards to particularly compress the rear springs, considering the mass of a support to fit it so far rearwards.

[Kawasicki]

Max5476 said:

AyBee said:

But as somebody else has said, a caravan bounces around and exerts much more force on the towball in all directions than the static limit.

Unsurprisingly the engineer will have designed considering the dynamic loads, hence giving what appears to be a low static load. Additionally this weight will consider potential changes to braking and stability performance, and is set at a value to keep their pl lawyers happy in case you crash and try and blame them. So unless you are loading it for the life of the car, and drive everywhere flat out you will be fine to exceed it.

With regards to the moment. A degree of moment will always have been assumed. As the cog for a trailer will ideally be between the axle and tow bar, therefore applying a level of moment. I doubt you coud fit 50kg far enough rewards to particularly compress the rear springs, considering the mass of a support to fit it so far rearwards.

The only moment on a tow ball from a trailer will be due to friction on the ball itself.

I wouldn't worry about your bike rack...engineers will have considered that normal usage.

[dr_gn]

Thanks guys,

There are towing limits and vertical load limits, so I just wondered if there was a cantilever case which I'd missed somewhere. As stated, it puts bending directly into the ball stem which, due to the spherical bearing being locked, would not be the case with a trailer (ignoring frction, again as stated py a previous poster).

I'll be ordering either a DL3 or M3 tomorrow.

[Watchman]

Kawasicki said:

The only moment on a tow ball from a trailer will be due to friction on the ball itself.

I wouldn't worry about your bike rack...engineers will have considered that normal usage.

I understand why you'd think this but if you think of the rearward force a caravan exerts on the hook, it approximates the momentary downward force a bike carrier exerts.

[dr_gn]

Watchman said:

Kawasicki said:

The only moment on a tow ball from a trailer will be due to friction on the ball itself.

I wouldn't worry about your bike rack...engineers will have considered that normal usage.

I understand why you'd think this but if you think of the rearward force a caravan exerts on the hook, it approximates the momentary downward force a bike carrier exerts.

The bike carrier is locked to the ball, so there's a constant moment applied to it (the ball itself), as well as bending loads. If you drew free-body diagrams for the bike carrier and trailer loadcases, they'd be different.

[Chester draws]

Understand where you're coming from having recently got a 4 bike carrier myself.

I was very tentative the first time out, especially over bumps and bends, keeping the speed down and constantly checking them in the rear view mirror!

Over a few runs you will gain confidence in it, and that confidence is worth much more than everyone else telling you "it'll be fine".

It you're really worried, take it out empty to start with, then one bike, then two, then three if you really want to build that confidence in little steps.