Hitch Physics Help

[gunafulltime]

I need some physics help.
We have bikes and a rack with a combined weight of 100lbs.
With the hitch extender, the rack is 14" beyond the receiver, and the bikes sit 2ft. beyond the receiver.

I am not a math or science expert, but logic tells me the extension of the weight must weigh more than the base 100lbs. of the bikes and rack, and I would imagine that there is added weight when they bounce in travel.

If this is true, how do I calculate what they actually weigh hanging/suspended 2ft. out from the receiver?

[Wxman]

Of course the weight doesn't actually change, but I believe you are correct there is a lever type action taking place.
100lbs of force exerted on a 2ft lever exerts 200 lb-ft of torque. I believe that would be the 'force' at the receiver if that is what you are wanting to know.

[kirkelli]

Originally Posted by gunafulltime

I need some physics help.
We have bikes and a rack with a combined weight of 100lbs.
With the hitch extender, the rack is 14" beyond the receiver, and the bikes sit 2ft. beyond the receiver.

I am not a math or science expert, but logic tells me the extension of the weight must weigh more than the base 100lbs. of the bikes and rack, and I would imagine that there is added weight when they bounce in travel.

If this is true, how do I calculate what they actually weigh hanging/suspended 2ft. out from the receiver?

Have At...
https://www.vcalc.com/wiki/KurtHeckm...age+Calculator

[Jay2504]

What about the dramatic increase in force that occurs when you hit a big bump? If the OPs static load is 200 pounds because of the 2 foot distance of the 100 pound bikes and rack, that 200 pound number go way up for an instant after hitting a large bump? If so how is that number calculated? Is there a formula to figure this? Jay

[RogerR]

I don't have the math but consider this. A 200 pound man jumps up and down on a bumper. Second, he jumps up and down on a piece of pipe sticking out 2 feet from the bumper. The torque could easily bend the bumper a bit and each time a bit more till failure as you hit bumps down the road.

[kirkelli]

Originally Posted by RogerR

I don't have the math but consider this. A 200 pound man jumps up and down on a bumper. Second, he jumps up and down on a piece of pipe sticking out 2 feet from the bumper. The torque could easily bend the bumper a bit and each time a bit more till failure as you hit bumps down the road.

amazing how many people take Physics for granted and don't realize the best place for weight is it's 'center of gravity'

[craigav]

Keep in mind that the pivot point of the fulcrum is the vehicles rear axle and not the hitch.

[kirkelli]

Originally Posted by craigav

Keep in mind that the pivot point of the fulcrum is the vehicles rear axle and not the hitch.

Yup,, 100%.. The sweet spot

[norty1]

Also consider what the hitch is connected to.

Look at the welds and the thin metal on the rear bumper that is attached to the frame and you don't have to have a physics degree to understand that it is a recipe for disaster.

[craigav]

Originally Posted by norty1

Also consider what the hitch is connected to.

Look at the welds and the thin metal on the rear bumper that is attached to the frame and you don't have to have a physics degree to understand that it is a recipe for disaster.

Absolutely agree, when you move the weight past the hitch, you are subjecting the hitch and frame to a lot of twisting force that wouldn't be there otherwise. When you hit a hard and fast dip, the momentum of that action can easily multiply the torque on the hitch and frame where the hitch is mounted to and certainly that action has damaged a lot of vehicle hitches and frames. It is especially concerning on a motorhome with an extended frame as most all motorhomes have. ~CA

[gunafulltime]

Thanks, Y'all.
For the record, we have the factory 2" receiver that is bolted to the chassis.

[norty1]

Originally Posted by gunafulltime

For the record, we have the factory 2" receiver that is bolted to the chassis.

At least that is comforting. Hope you can find something that will work. Bikes are one thing we hardly ever leave home without.

Our rig came with a bike rack option from the factory. It slides under the frame when not in use. Even it has been known to brake away and dump it's contents on the road. We had ours beefed up at a welding shop and never had a problem with it. Jayco stopped offering it as an option years ago.

[16WhiteColly]

Adding much weight at all to a Jayco square tube bumper is asking for trouble, especially bikes that wobble constantly going down the road. The welds rarely fail, the thin square tube metal, usually rips out.

[gunafulltime]

Originally Posted by 16WhiteColly

Adding much weight at all to a Jayco square tube bumper is asking for trouble, especially bikes that wobble constantly going down the road. The welds rarely fail, the thin square tube metal, usually rips out.

I'll try again. We are not using the bumper. It is a 2" OE hitch receiver that is bolted to the chassis. This has nothing to do with bumper capacity or use.

[dxrobertson]

Originally Posted by gunafulltime

but logic tells me the extension of the weight must weigh more than the base 100lbs. of the bikes and rack, and I would imagine that there is added weight when they bounce in travel.

Correct. But there is no "easy calculation" for your application; too many force vectors. As already mentioned; distance to axle, suspension, bouncing while in motion...
The weight bearing on the rack will most certainly be more than the base 100 lbs, cant easily be calculated how much.

Maybe instead of determining calculated forces, perform some tests. Jump up and down on fully mounted system to see effects. remove the hitch extension and repeat jump test, noting effect difference.

Good you have a frame mounted hitch, no problem there.

Seems the rack rating and weight is the concern?


I have see hitch mounted racks on the road that are overloaded and bending down, it is a valid concern.

[Randel J]

On our last camping trip, I saw a young family drive in to the campground pulling their trailer with several bikes on a rack attached to the rear of their trailer. I don't think the driver was aware that the bikes had bent the rack to more than a 45-degree angle and were leaning precariously, probably due to bouncing during travel.

I don't know if their receiver was bumper mounted or attached to their trailer frame, but in any case, the bouncing had done some major damage to the bike rack and/or the receiver hitch.

[Toad]

Fe de = Fl dl (1)

where

Fe = effort force (N, lb)

Fl = load force (N, lb) (note that weight is a force)

dl = distance from load force to fulcrum (m, ft)

de = distance from effort force to fulcrum (m, ft)

The effort force can be calculated by modifying (1) to

Fe = Fl dl / de

= m ag dl / de (1b)

where

m = mass (kg, slugs)

ag = acceleration of gravity (9.81 m/s2, 32.17 ft/s2)

Originally Posted by gunafulltime

I need some physics help.
We have bikes and a rack with a combined weight of 100lbs.
With the hitch extender, the rack is 14" beyond the receiver, and the bikes sit 2ft. beyond the receiver.

I am not a math or science expert, but logic tells me the extension of the weight must weigh more than the base 100lbs. of the bikes and rack, and I would imagine that there is added weight when they bounce in travel.

If this is true, how do I calculate what they actually weigh hanging/suspended 2ft. out from the receiver?

[Toad]

Fe de = Fl dl (1)

where

Fe = effort force (N, lb)

Fl = load force (N, lb) (note that weight is a force)

dl = distance from load force to fulcrum (m, ft)

de = distance from effort force to fulcrum (m, ft)

The effort force can be calculated by modifying (1) to

Fe = Fl dl / de

= m ag dl / de (1b)

where

m = mass (kg, slugs)

ag = acceleration of gravity (9.81 m/s2, 32.17 ft/s2)


https://www.engineeringtoolbox.com/levers-d_1304.html

Originally Posted by gunafulltime

I need some physics help.
We have bikes and a rack with a combined weight of 100lbs.
With the hitch extender, the rack is 14" beyond the receiver, and the bikes sit 2ft. beyond the receiver.

I am not a math or science expert, but logic tells me the extension of the weight must weigh more than the base 100lbs. of the bikes and rack, and I would imagine that there is added weight when they bounce in travel.

If this is true, how do I calculate what they actually weigh hanging/suspended 2ft. out from the receiver?

[Isaacs]

Originally Posted by RogerR

I don't have the math but consider this. A 200 pound man jumps up and down on a bumper. Second, he jumps up and down on a piece of pipe sticking out 2 feet from the bumper. The torque could easily bend the bumper a bit and each time a bit more till failure as you hit bumps down the road.

Just to visualize this: If the pipe were long and strong enough, that 200 Lb man could lift the front wheels of the truck off the ground!

But having said this, wouldn't a properly adjusted WDH minimize this problem? Would the spring bars be long enough?

[dcdutton]

I agree that it is too complicated to figure accurately. I believe the factory hitch on my Eagle 5th wheel has a max rating of 300 lbs. i am assuming (hoping?) that that rating takes into account the jolting forces of driving down the road. One important aspect regarding bike racks is that you MUST get an RV rated bike rack to ensure that it can handle the extra forces due to the lever effects you’re asking about. That is true of 5th wheels and TT’s that have extended lengths behind the rear wheels. Motorhomes and Class C’s may be different because of their rear suspensions. And finally, this discussion takes on even greater importance with the increasing popularity of e-bikes, which can way 70lbs and upward.