TheDirtBiker wrote:
Hey Guys newbie here, hope I found the right forum for my question.
I've just put down a deposit on a 2018 Tracer 24DBS
http://primetimerv.com/product-details.aspx?LineID=475&Image=24433&ModelID=3690#Main
I want to add a receiver to the rear of the trailer so that I can use a "hitch hauler" to haul two dirt bikes (less than 500lbs, more than 400)
Now common sense tells me that this frame is a pretty beefy piece of I Beam steel. I have a buddy that's a fabricator (and mechanical engineer) and we were planning to either use one of the standard recievers below that appear to support up to 500lbs tongue weight or fab something up ourselves.
Hi DirtBiker,
I will try to help you see some of the issues. Some are easier to work through then others.
First is the TT weight and balance. If I found your camper correctly, Yes/No? See here
Tracer Specs The 24BDS has a dry (empty) VUW of 4940# with a empty tongue weight of of 505#.
Your camper is starting at 10.2% dry tongue weight (TW). That is the bottom end of TW for stable towing. You need to load the camper to get it up ideally into the 13% or higher range. And with that floor plan and the amount of storage upfront it will allow you to get there. So now the camper is balanced well with just your camping gear.
When you add weight on the back you will start losing TW from what you just gained by adding the camping gear. The camper will have to be balanced out again and not go over the tires/axles limit or GVWR of the camper. Have your engineer friend can do static moment calculations about the camper axles to determine how much more weight you need to add up front to offset the rear overhung load of the hitch platform on the back to get back into that 12 to 13% TW range.
If you add 500# in the back say 8 feet behind the center of the axles, you have to add 500 more pounds 8 feet forward up front from the axles to offset the overhung load on the hitch platform. That is 1,000# more to the camper just to get it back to the needed balance before you added the bikes, frame etc. Part of the issue is, that floor plan and where they placed the axles the camper starts with a very low dry TW. Coming up with 500# more gear forward of the axle will take some work.
You will need to keep track of just how high in weight the loaded cargo is to not go over the GVWR of the camper and the axles/tires.
Next is the rear axle loads. When you add the 500# that X feet behind the rear axle hanger, the rear axle will take on more weight than just the 500#. Again your engineer friend can calculate the moments above the rear axle and see just how far you are from your rear axle rating. With that much weight, that far back you might overload the rear axle. It is something to check. You probably only have 3,500# axles.
Next is the main frame rails. Your camper is a 7,505# GVWR rated camper frame. The RV industry sizes these frames to the bare minimum. The word "beefy" is an oxymoron when it comes to RV frames. You stated it was an "I beam". It is more of a I shape and is very thin with ultra week flanges. Odds are high a frame sized for that rating is maybe only 6" tall and 1/8" thick center web and lower flanges. The problem is the concentrated load overhung behind the rear axle hanger. The area behind the rear axle hanger is the weakest spot for the overhung load.
When you hit a pothole doing 50mph the impact loads into the trailer frame is massive and that overhung load is not your friend at this point. Does your camper have shock absorbers or a rubber equalizer? Most do not, but yours might. It is much worse without the shocks. Have your engineer friend calculate the moment of inertia of the frame section. You will have to figure out if your frame is made from 36ksi yield steel or 50 ksi. Hopefully it is at least 50ksi yield strength steel. They do make them from high strength low carbon steel (HSLC) just I do not know if your is.
He can then back into what is the maximum bending moment at the rear hanger along with the maximum stress of running over a pothole at speed. That will then tell if the stress are high enough to blow out the lower flange on the frame behind the rear axle hanger as that is where the failure will most likely fail first. Have him look up inelastic flange local buckling. Basically it means the lower flange will permanently deform from the bending loads and the frame rail will no longer be straight or can handle the normal towing loads without added deflection.
These camper frames are not made like an industrial flat bed trailer that has a lot of steel in them with tall thick frames and lots of frame reinforcing.
Potholes and bumps are an unfortunately realty to towing a camper. I suspect your engineer friend will find that there is very little to no safety factor left in the frame with 500# of bikes and bike rack attached to the back of the camper. If that amount of weight can be over the axles, then no problem. Yes I'm sure they can create a mount to hold the bike platform to the trailer frame, but that is not the weak point. The whole frame rail is.
From what I have learned, I would not attempt a loading like that unless the main frame rails had a minimum area moment of inertia of 51 in.^4 with added lower flange reinforcement to prevent the buckling and made from 50ksi steel or higher. That and shocks and a rubber equalizer is a must. Odds are high your frame rail is not 1/2 of the size needed.
Hope this helps
John
2005 Ford F350 Super Duty, 4x4; 6.8L V10 with 4.10 RA, 21,000 GCWR, 11,000 GVWR, upgraded 2 1/2" Towbeast Receiver. Hitched with a 1,700# Reese HP WD, HP Dual Cam to a 2004 Sunline Solaris T310R travel trailer.
