Weight calculations, any engineer types out there?

[Matt400]

So I want to hang a motorcycle off a 5th wheel, I know its not ideal but I have been working the numbers and trailer specs to get some sort of safety margin.

What I have found is a 5th wheel with a load capacity of 4088 lbs along with an optional upgrade to "G" 3410 lb rated tires.



The carrier is 225 lbs and the motorcycle is 475 so we are talking 700 lbs back there.

I realise 700 lbs over the axles is different then 700 hanging 11. 5 feet out past the axle so found this calculator:

http://www.blueox.us/Carriers/carriercalculation.pdf

It shows me that 700 is really adding 3402 lbs to the axle. I am guessing this is because of the distance that 700 lbs is from the rear axle in addition to unloading some of the trailers pin weight off the truck and placing that trailer weight on the axles where it would have been on the truck.



Filling in the blanks I have:

• 1 = 105"
  
• 2 = 36"
  
• 3 = 475 lbs
  

Then there is:

• X = 225 lbs
  
• Y = 34"
  

So correct me if I am wrong but. .

425 lbs + 225 lbs = 700 lbs x 36" + 105" + 34" and then divided by 36" = 3402 lbs



Am I really adding 3402 lbs to the rear axle on the 5th wheel? Seems like allot and while within the 4088 available it leaves me with only 686 lbs of supplies I can add before being over loaded.

 

[Hoefler]

Simply no. You can not use this example for you scenario. You will be adding at most 1,400 lbs to the suspension. Just think of it as a scale or teeter totter. As you add weight to the rear of the trailer, you will be taking weight off of the tongue or pin. In order to calculate how much you would loose off the tongue or pin, we need to know the length of your trailer, distance from center of suspension to the pin or tongue and center of suspension to the rear most part of the carrier.

 

[Grizzly]

The worst part is you will be adding all the extra weight to the rear axle and taking weight away from the pin. What I would do is put the truck on a scale, then hook up the 5er and put the truck only on the scale with the 5er off the scale to determine the pin weight. Then put the truck and the front axle on the scale. Then put the truck and both axles on the scale. Now you know your pin weight and each axle weight as well as the weight of your truck. Now if you can rig up a way to hang the motorcycle and the carrier to the back of the 5er and go through same procedure on the scale. If you can't hang the motorcycle and carrier to the back of the 5er maybe you can hang the equivalent amount of weight, 700lbs, of something, like RR rails. This can be calculated, but like Hoefler said we need more information and I would have to dust off my college calculus books.

 

[Matt400]

Just think of it as a scale or teeter totter. As you add weight to the rear of the trailer, you will be taking weight off of the tongue or pin.

That makes better sence then the M/H calculator I was using.

In order to calculate how much you would loose off the tongue or pin, we need to know the length of your trailer, distance from center of suspension to the pin or tongue and center of suspension to the rear most part of the carrier.

• Overall lenght = 31' 6"
  
• Center of suspension to pin = 256"
  
• Center of suspension to rear of carrier = 162"
  

Besides knowing what the reduction in pin weight is, shouldn't I also be concerned with the pounds added to the trailers rear axle?

This particular model has these specs:

• Dry weight = 9292
  
• Load capacity = 4088
  
• GVW = 13,380
  
• Pin weight = 1995
  
• Axles rated to 6,000
  

I would have thought the GVW would call for 7K axles but they say its not needed due to the pin weight.

Thanks for the help / advise on this.

 

[WestTN]

The additional weight will not be only on the rear axle. That triangular connector you see between the trailer axales is called a walking beam. If the trailer sits level on a level surface when hooked up, then the weight is shared by both axles. If your trailer nose is a lot higher than the rear in the same conditions, then suspension travel over bumps will overload the rear axle when it bottoms out.

 

[Boondocker]

You can get a trailer for the 5th wheel that has 2 mounting points so it tracks with the trailer in forward and reverse. You can't jackknife it. They put only modest hitch weight on the trailer. They're expensive though.
RV Motorcycle Trailers

 

[Matt400]

You can get a trailer for the 5th wheel that has 2 mounting points so it tracks with the trailer in forward and reverse. You can't jackknife it. They put only modest hitch weight on the trailer. They're expensive though.

RV Motorcycle Trailers

Actually they are less than the carrier, about 1/2 price which is enticing. When you add the platforms hitch weight to the bike it ends up saving me just 260 lbs. Not a bad way to go but it worries me what would happen if that wheel got a flat? Back there I doubt I would know it before something is really messed up.

I saw that unit here:

SWIVELWHEEL-46



Would be kinda cool if its possible to add a 5 tire TPMS to the truck for the trailers wheels and the little swivel. Maybe it could be set up as a spare tire monitor?? I found one of those here:

RV Tire Pressure Monitoring Systems by Doran

 

[Hoefler]

If my math is correct, you lose 259 lbs off the pin and transferred to the axles in addition to the 700 lbs of the rack and bike.



As far as the axle rating, it is expected that you will carry 10-15% of your load on the truck, thus giving you your GVW.

 

[Matt400]

Thanks for taking time on this for me,

The carrier would be more easy to load, unload & give some piece of mind with no added flat tire to worry about.



The swivelwheel platform would take 260 of that 959 lbs off the back + double as a utility set up behind our truck when not pulling the 5th wheel.



Gonna have to give this some more thought.

 

[PToombs]

Matt, people I know had the pressure monitor on the car they towed behind their RV. It worked great. If the pressure dropped it would sound the alarm, and show which wheel on the display. (small pic of the car showing tires) He showed me one day, I watched the monitor and he cracked the cap loose. All heck broke loose, flashing lights, alarms.

Just some input for ya.

 

[jelag]

I own a 30ft 5er I've done this too... .

Before we had the rack installed and the extra weight we found that the springs and tires were not up to the wet weight of the 5er before the extra weight... .

We lifted the 5er up and added a 2x2 tube from the front axle mount to the rear of the trailer on both sides for 2 reasons... one was to allow for extra clearance because the trailer wasn't high enough for a dodge 4x4 and the other was to add strength for the extra weight... .

we upgraded the tires and springs to handle the wet weight of the 5er... we than added the receiver to the end of the frame on both sides... . we often camp with a gen set on the end of the frame, or our 450 lb Honda Rincon 4 wheel atv... .

Our rack, which we built plugs into the receiver just like a hitch...

We found that we have easily 250 lbs of extra stuff in the front of the trailer... . extra battery, tools, junk... . so we didn't do much to worry about the way it works... my son, a mechanical engineer helped and said there was so little change that he wouldn't even take the time to run the numbers... .

It tows well.....

 

[Matt400]

I own a 30ft 5er I've done this too... .

We found that we have easily 250 lbs of extra stuff in the front of the trailer... . extra battery, tools, junk... . so we didn't do much to worry about the way it works... my son, a mechanical engineer helped and said there was so little change that he wouldn't even take the time to run the numbers... .



It tows well.....

Thanks jelag for sharing,



I found a different carrier that significantly reduces the weight to a total of 560 lbs for carrier & bike combined. Basically a 20% reduction in weight so based on Hoefler's numbers I am guessing that 20% would reflect across the board and come out to a 207 lb pin weight reduction along with an increase of 767 lbs to the rear axles.

Based on the specs of dry trailer & hitch weight this would put hitch weight at just over 19% of the trailers weight which sounds good to me. Add in generator, tools, camping gear & such to the forward compartment should even things out and the "G" rated goodyears add piece of mind.