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The specification for car hitches routinely state the tongue weight, but do not specify how far that weight can/should be applied. What am I missing? Shouldn't the maximum bending moment be the one to be specified?

It matters because I identified one hitch manufacturer who states

This tow hitch is rated at 1588 kg (3,500 pounds) towing and 238 kg (525 pounds) tongue weight.

That is terrific, because every other hitch builder for this vehicle only handles 91 kg (200 pounds) (the so-called class I).

Nominally, a 238 kg (525 lb) hitch is amply sufficient to handle the load from a 2-bike rack (24 kg; 52 lb), a 2-bike add-on (20 kg; 44 lb), and four mountain bikes (13.5 kg; 30 lb; each), for a total of 98 kg (216 lb).

But the third and fourth bikes, as well as the add-on rack, are so far from the hitch I am concerned the torque would be excessive.

How do I relate tongue weight to weight x distance?

hitch tongue weight

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  • $\begingroup$ I suggest that the normal ball distance is about 4 inches from the end of the hitch. I would compare my loads to 525 lbs at 4 inches. This is not a documented area, but there is clearly some slop in the ratings for longer "drop" balls. But you are thinking correctly that the longer moment arm could be a problem. $\endgroup$
    – Tiger Guy
    Nov 30, 2023 at 5:45
  • $\begingroup$ Is there a good reason you don't use metric units? $\endgroup$ Nov 30, 2023 at 13:30
  • $\begingroup$ @12431234123412341234123 I followed the unit that most of the specs used. Good point. Fixed. $\endgroup$
    – Sam7919
    Nov 30, 2023 at 14:10

1 Answer 1

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hitch

you need to jack up the car and check the structural layout of the hitch. Usually, there are two arms welded to the sides of the hitch receptor reaching to the left and right side of the mainframe of the car bolted to the frame of the car through two rows of bolts on each arm.

You measure the moment of 4 bicycles times their distance from the hitch, say

$$M = 200 lbs. * 1ft= 200lbs.ft$$

Half of this goes to each side bracket and induces tension and shear on front bolts and compression and shear on rear bolts, in addition to 500/4= 125lbs tension per bolt. $$T= C =100/d$$

My estimate is a hitch that is rated 500lbs. tongue load can handle this because it has already been designed to take the moment of 500 * D, the distance of the receiver to the brackets. That distance is many times bigger than d.

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  • $\begingroup$ That makes sense, and it ought indeed to be this simple. So if the bolts are horizontal, we'd compare with the maximum shear strength for each bolt (do we estimate it or do we ask the vendor?), and if the bolts are vertical, we'd compare with their tensile strength (ditto re max value). $\endgroup$
    – Sam7919
    Dec 1, 2023 at 1:12
  • $\begingroup$ exactly. but if you can measure the distance D and compare it to distance d and it is more the 4 times, you need not worry. $\endgroup$
    – kamran
    Dec 1, 2023 at 6:24

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