Towing Bike Rack and Tongue Weight

[tknick]

I am interested in taking 4 bikes with me when we camp.  I have seen towing bike racks that go between the the trailer and the TV.  The bike rack goes in the receiver on the TV and the trailer connects to the bike rack.  The problem with the ones I have seen in the past, is they were only rated to 300-350lbs tongue weight and 3000-3500lbs tow weight.  I have a 2002 Coleman Bayside and that was not high enough.

I was reading the thread about the pro rack that goes on the tongue of the PU and the rear tires rest on the roof of the PU.  The $412 price seems a little steep.  I found a Towing rack for $209 rated at 600lbs/6000lbs.  That is more than my class III hitch without weight distribution (which I do not use).
Towing Bike Rack

When using a rack like this, does it add to the tongue weight from my PU possibly exceeding my 500lb limit (300ish from my PU and 200ish from the rack and bikes) on my receiver?  Or, does the receiver on my TV have a 500lb limit (which the bike rack wouldn't exceed) and the bike rack have a 600lb limit (which my PU wouldn't exceed)?

BTW, I would not be exceeding the 5000lb limit on the hitch even with the rack.

Any help would be appreciated.

[abbear]

I found a Towing rack for $209 rated at 600lbs/6000lbs.  That is more than my class III hitch without weight distribution (which I do not use).
Towing Bike Rack

When using a rack like this, does it add to the tongue weight from my PU possibly exceeding my 500lb limit (300ish from my PU and 200ish from the rack and bikes) on my receiver?  Or, does the receiver on my TV have a 500lb limit (which the bike rack wouldn't exceed) and the bike rack have a 600lb limit (which my PU wouldn't exceed)?

Yes, I believe it would add to your tongue weight.  While technically it is mounted on the vehicle side of the hitch, the weight is still levering at the receiver and I would count it as tongue weight because of that.

[wynot]

I am interested in taking 4 bikes with me when we camp. I have seen towing bike racks that go between the the trailer and the TV. The bike rack goes in the receiver on the TV and the trailer connects to the bike rack. The problem with the ones I have seen in the past, is they were only rated to 300-350lbs tongue weight and 3000-3500lbs tow weight. I have a 2002 Coleman Bayside and that was not high enough.
 
I was reading the thread about the pro rack that goes on the tongue of the PU and the rear tires rest on the roof of the PU. The $412 price seems a little steep. I found a Towing rack for $209 rated at 600lbs/6000lbs. That is more than my class III hitch without weight distribution (which I do not use).
Towing Bike Rack
 
When using a rack like this, does it add to the tongue weight from my PU possibly exceeding my 500lb limit (300ish from my PU and 200ish from the rack and bikes) on my receiver? Or, does the receiver on my TV have a 500lb limit (which the bike rack wouldn't exceed) and the bike rack have a 600lb limit (which my PU wouldn't exceed)?
 
BTW, I would not be exceeding the 5000lb limit on the hitch even with the rack.
 
Any help would be appreciated.

Put a front receiver on your Expedition, like I have on my Tahoe.  Then, you don't have to worry about the tongue weight, etc.

[zamboni]

Also, importantly, when using a hitch extension (to put the ball farther from the TV to allow for a bike rack)...

I don't remember the formula, but you MUST reduce the max tongue weight your TV can handle.

If your TV can handle 500 lbs tongue weight, that is assuming the weight is at a point, say, 8 inches from the mounting brackets to your frame.  If you increase that to 16 inches, you've DOUBLED the force on the mounting bolts that hold your hitch to the frame (of course, most hitches have several mount points, so it is not really a doubling - but a definite increase).

[mike4947]

Nope, it's the old distance from the mounting points/axle centerline compared to the new distance from the mounting points/axle centerline. Depending on whether your figuring dtress on the mounting ot stress on the TV.

[edhawk]

I found this posting recently and found it to be helpful (although it stirred quite the debate in my head.... store bought vs. home made... argggghhhh)


http://home.insightbb.com/~tkmcgrath/bikerack.htm

[zamboni]

old distance from the mounting points/axle centerline compared to the new distance from the mounting points/axle centerline.

The mounting points are never all on the centerline or middle of the axle of a TV.  Aside from that, most under-body hitches are a "U" shape.  The bottom of the "U"is near the hitch.  Under theoretically-ideal circumstances, the camper-hitch weight (say, 500 lbs) is acting right at the 2 bolts at the bottom of the "U".  It exerts 500 lbs down on those bolts, which in turn pushes 500 lbs up on the "top" end of the U-hitch.

The weight of your camper creates a moment-of-force centered around the closest bolts.

I assume you have used a shovel -- this nothing more than a simple lever, just like a trailer hitch.  The further you are from the weight, the easier it is to "move" it with the lever.

Let's assume your hitch extension is 10 feet long.  Putting the same 500 lb camper-hitch-weight on this new extension would theoretically just push the TV down 500 lbs... but NOT.  Since the weight is now cantilevered out 10 feet from the mount points, and we assume the "top" of the U-hitch is 1 foot in from the main mount points, your moment of rotation around the main mount points from the 500 lbs of hitch weight is now 5,000 lbs

Newton's Moment Of Force, says that Moment = Force(kg*10) * distance (in meters).

The average hitch ball is maybe 1 foot (30 cm) from the mount point.  Manufacturers would have considered this.  Thus, the Moment of Force of a 500 kg trailer on the frame (bolts) is (500 * 10) * 0.30 = 1500 Nm.

Now, extend that same hitch 1 foot futher with an extension, and you have:
(500*10) * .60 = 3000 Nm.  This is twice the force exterted on the mount points.  This is the same thing as a 1,000 kilogram (for this, interchangeable with pound) hitch weight.

Result:
Your frame was NOT meant to carry that much force.  Adding an extension to your hitch MUST result in decreasing your "maximum" hitch weight, not to save your GVWR, but to protect your frame.

[mike4947]

But the pivot point on the TV is the rear axle, not the mounting points. Weight behind the rear axle will #1 compress the rear suspension and #2 try and lift the front of the vehicle.

On the average a 14-16" extension lowers the weight carrying limit 20%. Most are rated for 400/4000 pounds when used with a class III 500/5000 pound rated receiver.