Trailer Mod Complete

[Highlander]

The load carrying capacity of the trailer is determined by your tire rating--not springs.

Bob

Very Wrong
Load rating is determined by various things starting with bunks , frame , hitch & suspension they will all have a min- max rating , lastly tires can be determined by usage lightly loaded boat & short distances u can get away with the min required rating, but heavely loaded boat & long distance hiway driving u'd want to go with the max. rated tire !

When the boat is mty " new" & no motor it rides very well no bounce
when the boat is heavily loaded & with a motor it suspension will bottom out badly on small potholes not good
so with 4 independant hubs all rated the same as the original axle u are fine as per the manufacturer
remember reg. tandem suspension transfers the load from one axle to the other when going over bumps etc so is actualy a harder ride
Tounge weight should remain within specs
tandem is safer & with a blowout u;ll be able to take ur time pulling over on a hiway no so with a single axle
tow vehical rear suspension should meet min spec required , min spec will give u sway at higher speeds & less control , & no control if a sudden evaisive action is required this from me who towed his heavily laden boat 2200 km @ 72 mp/h with a ford ranger white knuckle driving for sure

Very nice mod want to do the same J

[mastreb]

All load capacities in all systems are determined by the least capable component. Always. Saying that it would always be any one component is almost certainly incorrect, and that goes without knowing anything about this system in question.

That said, the tires would be the right component to choose to be the least capable in a trailer. But I doubt they always are, especially in the case of the mac.

[RobertB]

OK- All this talk of what is the most critical component of the trailer is starting to make me think - I kind of do not like that this late in the day

I noticed a few things about the trailer "frame"
- If I stand on the fender brackets, the I-beam will deflect angularly (rotate) - not very stiff. I will probably look for some reflectors or plastic bunnies to put on the brakets to discourage people stepping on them. The design placing the fender supports inside helps here.
- Now wondering how the design load analysis showed support of the boat - bow/mid (axle)/stern or just bow/stern. Essentially, is the design load assuming any of the boat weight is supported by the bunk over the axle? If so, a boat loaded up against the bow chock (and thus not resting on the axle bunk as mine is) has a much greater cantilevered load either side of the axle. This could prove to be not such a good thing.

As far as the purpose of adding an axle, I stand by the approach that this is done to add a factor of safety versus increasing the capacity of the trailer.

[Tomfoolery]

I wouldn't worry about the lack of torsional stiffness in those main beams. All such reentrant members, like wide-flange beams, angles, and channels, have very low torsional stiffness. Even a tube with a slit down the length goes from very high torsional stiffness (before slitting) to extremely low stiffness, just by virtue of that disconnect. If torsional stiffness was a necessity, then a tube would have been the order of the day, either round or rectangular. The wide-flange shape of those main beams is, however, very efficient at resisting the bending loads applied as a matter of course for a trailer.

The loads from the boat are transferred to the side beams via cross beams, which impart primarily shear loads directly to the web of the main beams, thereby imparting no such twisting load to the side beams. Even if there was such twisting load, that cross beams would resist it. In other words, the only way the boat loads get to the main beams is via cross beams, which by definition don't put any twist into the beams, and/or they resist such twist in the first place, unless they themselves sag under load, but then only a very small amount of twist would end up in the side beams due to the small change in orientation at the ends of the cross beams.

The axle on those trailers, if I remember what they look like, are bolted to the lower flange of the main beam, and therefore only put a normal (straight up into the flange and on to the web) load into the beams.

Aluminium, by the way, has a Young's modulus (a relative measure of material stiffness) that's about 1/3 that of steel, so even though aluminium can be made as strong or stronger than structural steel (much, much stronger for certain alloys), it will always be much more flexible than a member of the same dimensions but made of steel, which is why aluminium bicycle frames usually use larger diameter tubes than alloy steel - it would be too flexy if the tubes were the same dimension.

I wouldn't sweat it, in other words.

[mastreb]

The Young's Modulus indicates material stiffness. The Old's modulus is the reciprocal property which measures material flaccidity.

[Tomfoolery]

Bada-bum <crash>

[raycarlson]

aluminum trailer has been in use since what 2005, thousands out there in use with never any kind of failure(structurely)that i"ve ever heard of,probably a waste of time worring yourself over its structural integrity, other then one or two rare complaints about some kind of corrosion issue inside of steel tube the hitch actuator mounts to.

[RobertB]

I looked at the trailer with weight resting on the wheels - and lo and behold, contrary to what I stated earlier, the hull rests on the front cradle, rear cradle - and the intermediate one on the axle. It seems that when I jacked the trailer up at the front and very rear to work on it, original intention was to mark axle locations and fit fenders to minimize time at a rented mooring, that the bow (bend - not front) in the I-beams straightened out and thus the hull cleared the middle bunk.

Yes, as recently stated, the trailer aluminium I-beam is fairly elastic. When loaded, it will deflect until the load is spread between the three support points.

[raycarlson]

after fixing the single axle problem,those three tiny lateral supports is what really bothered me,next time your boat is off the trailer look very closely at how really small each contact load bearing point is at those three locations.to me the 2-3 sq inch patches of crushed carpet bunk showed how concentrated the load was at those points.My next project was to convert to longitudinal bunks like a conventional trailer and also added three additional lateral support beams to alum i-beams.

[aya16]

Very nice work you did. Couple things, if part of the reason for the tandem axle was to reduce tongue weight for the tow veh. I think you may have a problem.
Sway from a trailer may come from two diff. areas. the first is tongue weight and the second would be angle of the trailer. Both will cause sway if not right.

Rule of thumb would be a ten percent tongue weight, about 400 pounds for the loaded mac. This can be critical, but your car is , I think you said rated for a 250lb tongue weight. That could be a problem, may need to move up a bit on the tow veh, a truck maybe? another thing you can do is add a weight distribution hitch.
I also tow a starcraft travel trailer, that trailer uses a distribution hitch and has a 4700 pound trailer,(not loaded) the hitch makes the starcraft tow much better than my lighter mac, the star craft also has a 400 pound tongue weight.

having the towed trailer (any) level with the hitch will make the trailer tow straight and true. There wont be any lift while braking as you would have with a trailer that tows at an angle, the braking forces would push straight into the tow ball, as with the braking from the trailer will pull straight back.

The weak link in your set up, looks to be the tow car, if it has a 250lb tongue weight limit, my guess it also has a 2500 pound tow limit, would be ok for short tow's, but any distance not so good, and the ramp may be another problem, traction, and power may not work out for you, although I would guess you have towed and launched with the setup you already have with the tow car.

[RobertB]

I did the mod not only to reduce tow weight but also increase safety factor.

Car is actually speced for only 150 pounds tongue weight - this is really strange - less than 5% of total weight. I suspect the number is based on assuming back seat passengers and cargo loading - both I try to minimize when towing.

The car, a Volvo XC70 with the 4C suspension actually does a great job of towing for a couple of hour trip
(usually less than an hour). That said, after replacing my transmission last month (likely not due to towing, it kind of blew up while commuting to work - cracked the housing at 180k miles, mechanic said fairly good mileage for car), the car will only be towing the boat a few more times - already have my eye on the turbocharged F150. Also, time to get another vehicle with the average miles on our four at over 200k.

Currently the tongue weight is around 330 pounds. Others have done this mod and reported on this board that even with a resultant tongue weight of 250 pounds, the trailer was steady.

[Crikey]

Robert, my guess is that without a transmission cooler, and the load you were towing with that vehicle rating it overheated before finally cracking. Your next vehicle choice should be able to take things in stride.

R

[kevinnem]

as a quick note, to anyone that might be passing by the conversation. Adding a trasmition cooler to my 4 runner was super easy, I hired a guy to help me install it , but if I known how easy it was, I wouldn't have. If you can screw garden hose in to the faucet, you can add a trasmition cooler. They are alos really really cheap in teh big picture. 100$ would get you a really big one, I got a fairly large one for 65.

it mounts on to the existing rad, using these "zip tie" sort of pole things. and then you connect a few hoses, and done! it really is easy to do. I can't see how it is not worth doing.

[Highlander]

read the installation instructions carefully some trans coolers are directional

J

[Catigale]

If you put it in backwards can't I just tow in reverse?