Trailer hitch moments

[trainguy]

Hi guys.

Structural engineer here, specializing in railcars and in a former life, buildings.

I'm looking to attach a 4-bike bike rack to a Honda CR-V trailer hitch. Total weight at the hitch will be around 250 lb (Rack + 4 bikes). However, there are 4 bikes and a substantial rack that are essentially cantilevered off the hitch, some distance from it.

Trailer hitch sites list towing capacity and tongue capacity, none of these refer to moment capacity, or something which addresses this moment. Any limitations on maximum CG distance of the assembly? Are there ways to reduce this moment by attaching to a set of roof rails?

I'd hate to be doing 60, hit a bump and than have a plastic hinge occur...

tg

 

[MikeHalloran]

The part of the hitch that's attached permanently to the car is called a receiver.

If the receiver's socket is 2" square inside, that's a Class 3 receiver, and you don't have to worry.

If the receiver's socket is 1-1/4" square, that's a Class 2 receiver, and you might need to worry a little about your bikes bouncing a bit.

There are standards defining the design loads for receivers, which you may be able to find somewhere at sae.org.
... it may not be easy; their publications index is too big for a CD.

Or you can call the outfit that manufactures pretty much every brand of hitch and receiver sold in the US: Cequent Group:

http://www.cequentgroup.com/Brand/All/

Mike Halloran
Pembroke Pines, FL, USA

 

[trainguy]

Mike, or others,

I am planning on using a class 3 hitch, so 2 inches square.

Let's say I hit a large bump, what's the chance that the car will be damaged (instead of the hitch or bike rack)?

Does the hitch receiver and frame spread the loads out sufficiently well to the frame rails?

Some of the Thule racks have a 1.25" horizontal member that has a 2 inch expandable end that jams itself into a 2 inch receiver. Is this a good way to ensure the rack part fails before the car frame? Or should I just avoid all things 1.25" wide?

tg

 

[GregLocock]

The structure from the receiver back to the car is designed to resist translational forces, not moments.

Bracing the rack against the roof is exactly how my very flimsy looking 3 bike frame gets around any necessity for additional structure at /any/ of its contact points.

http://www.cargogear.com/imagelibrary/images/srbrider.jpg

If it is a factory fit towbar then I wouldn't worry too much, but if it is aftermarket then I doubt it has been through durability

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[ornerynorsk]

250 lbs for 4 bikes and the rack seems like an over-estimation, in my opinion.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.