Physics Help Forum physics of rear-mounted bicycle carriers

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 Jul 3rd 2019, 11:15 PM #1 Junior Member   Join Date: Jul 2019 Posts: 5 physics of rear-mounted bicycle carriers I have what is probably a simple question to most of you as I have had no luck getting this information from the manufacturer or elsewhere on the web... I want to buy a particular bike carrier that sits on a tow bar/hitch and projects rearward from there. The carrier/rack can support 4 bikes (roughly evenly spaced) with a total weight of 68 kg/150 lbs. The carrier/rack itself weighs 18.1 kg/40 lbs. My current tow bar/hitch is rated to only support 95 kg/210 lbs. The rack projects at least approximately 41 cm (16 inches) back from the tow bar of the car. Is there a formula I can use to determine the "effective weight" that will be placed back on the tow bar/hitch if the carrier/rack is holding the maximum 68kg of bikes? I need to determine the minimum hitch weight rating I need to safely transport bikes on this particular carrier/rack. Thanks in advance
 Jul 4th 2019, 04:57 AM #2 Senior Member     Join Date: Jun 2016 Location: England Posts: 1,000 Just a Moment I think what you are after is the bending stress you will be applying to your fixing point. The limits you have been given only apply to total load and you are safely within that. However you have a longer lever arm, so the bending moment will be higher. If you work out the centre of load (the point where there is as much load closer to the car than this point as there is further away from the car than this point) Then multiply this distance by the total load, you will get the bending moment at the hitch attachment (pounds.feet) However I don't know how you can find out what the safety limit might be. Perhaps if you are able to quote this value to the manufactures, they will understand what you are querying and may be able to provide reassurance. __________________ ~\o/~
 Jul 4th 2019, 05:00 AM #3 Senior Member   Join Date: Apr 2017 Posts: 506 This is what a tow bar looks like .. to think that this could only support 95Kg beggars belief ... it would probably fail when supporting 1 tonne on the flat beam part .. the dome part (with green label ) would probably fail with 500Kg pushing directly down ... Manufacturers are notoriously conservative with such figures to prevent lawsuits ...and to allow for very uneven roads which put extra stress The further the mass extends behind the vehicle the more leverage is exerted putting stress on two uppermost bolts .... Realistically you can carry a dozen bikes unless your vehicle is very old and rusted. The fact that neither the manufacturer or the web has the answer you need suggest nobody has asked such a question , most people just load on whatever they like, and manufacturers have to take this into consideration when designing such things and put in a massive safety margin .... You are worrying too much! Last edited by oz93666; Jul 4th 2019 at 05:19 AM.
 Jul 4th 2019, 05:14 AM #4 Senior Member     Join Date: Jun 2016 Location: England Posts: 1,000 My own feeling would be the same as Oz. But don't quote me in the lawsuit when your bikes fall off on the free-way... __________________ ~\o/~
 Jul 4th 2019, 06:06 AM #5 Senior Member     Join Date: Jun 2016 Location: England Posts: 1,000 Safe Limits This looks to me like a sales-mans or possibly lawyers limit, rather than an engineers limit. An engineer will calculate what it will take before it breaks, then quote half that to leave a nice safety margin. A Sales Man will ask how much is needed to do its (normal) job? How much can we reasonably expect it to have to withstand in "ordinary" use? At what point can we claim that it was being subjected to unreasonable treatment? 95kg is equivalent to a moderately hefty person standing on it... __________________ ~\o/~
 Jul 4th 2019, 04:05 PM #6 Junior Member   Join Date: Jul 2019 Posts: 5 Thanks. Just to clarify...so for arguments sake, if we just say that half way along the 16 inch arm the bikes sit on was the centre of load (i.e 8 inches or 2/3 of a foot) would I multiply the 190 lb total weight (rack and bikes) by 1 and 2/3 to get the bending moment (317 lbs). Is that correct? If so, this is just over 50% above the quoted limit which sounds significant to me!
 Jul 4th 2019, 04:13 PM #7 Junior Member   Join Date: Jul 2019 Posts: 5 Thanks. Until now (for many years) I have been just piling bikes on to my current three bike rack without so much as a thought but I just got to thinking...remembering that these tow bars are meant to be used for trailers that have wheels supporting the load rearward. It is easy enough to measure the 'tongue weight" imposed by a trailer using a scale made for the purpose. I just wonder if these quoted ratings for tow bars/hitches make allowance for design specs on these carriers/racks.
Jul 4th 2019, 08:39 PM   #8
Senior Member

Join Date: Apr 2017
Posts: 506
You don't say weather you are also towing a caravan/trailer at the same time . The manufacturers figures certainly allow you to do this as well as tie on bikes ... Bumpy retain is a big consideration , again the manufactures figures will allow for the worst pot holes imaginable.

 Originally Posted by mmelinz Thanks. Just to clarify...so for arguments sake, if we just say that half way along the 16 inch arm the bikes sit on was the centre of load (i.e 8 inches or 2/3 of a foot) would I multiply the 190 lb total weight (rack and bikes) by 1 and 2/3 to get the bending moment (317 lbs). Is that correct? If so, this is just over 50% above the quoted limit which sounds significant to me!
It's hard to give a precise answer , as the load extends backwards leaverage will cause different parts of the tow bar to experience more stress , but the overall downward force will not exceed the weight of the bikes ...as pointed out particularly the two upper bolts feel the leverage effect , these may be designed extra strong to cover this eventuality ....

 Jul 5th 2019, 08:38 AM #9 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,352 Regarding the 210 pound limit - it is derived from the towing capacity of the hitch. The smallest standard tow hitch one can buy is a Class 1 hitch, which is rated for pulling a loaded trailer up to 2000 pounds. These hitches have the smaller 1-1/4" square receiver. Ideally you try to balance the load on a trailer so that 10% of the trailer's weight is supported by the hitch. Ten percent of 2000 pounds is 200 pounds, so consequently Class 1 hitches are rated to carry a bit over 200 pounds of vertical load. IMO the actual vertical weight limit that the hitch can withstand is higher than that - the manufacturer says 210 pounds so that you don't exceed the 2000 pound towing weight limit of a Class 1 hitch. In your case you're not towing anything. I'd say go with it, don't worry about it. Last edited by ChipB; Jul 5th 2019 at 12:52 PM.
 Jul 7th 2019, 03:21 AM #10 Junior Member   Join Date: Jul 2019 Posts: 5 For the record I am only transporting bikes, not towing a trailer/caravan as well bikes. Thank you for your responses.

 Tags bicycle, carriers, physics, rearmounted

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