| Author | Message | ||
| JAMES R. RINK (Fakeguy)
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hello. I have a 1962 PD4106 with the 8v71 engine mounted cross wise (not inline). i need to attach a trailer hitch to pull my 11 ft long open landscaping trailer with my golf cart. i pull it now with my 4 cyl ford ranger so it cannot be much. however i have heard that one has to be careful of structural damage to bus when installing hitch. at some point, will be towing heavier trailer, but looking for tips/sketch of best way to fabricate. thanks, fakeguy | ||
| Brian (Bigbusguy)
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There is a lot of differnt ways to put one on a GMC . I would just make sure that it extends up to some point where the axle is connected to . The toung weight I would not go to heavy . I would try to keep it under 600 lbs. You can probley go heaver I know of some the have gone up to 1000 lbs toung weight with out ever having any problems . No one has ever pulled the motor down from the roof on any GMC . Some older GMC's have had cracked bulkheads but not from towing. http://pg.photos.yahoo.com/ph/bigbusguy/detail?.dir=c933&.dnm=c15d.jpg&.src=ph http://pg.photos.yahoo.com/ph/bigbusguy/detail?.dir=c933&.dnm=401b.jpg&.src=ph Brian 4905 | ||
| Jim (Jim_in_california)
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Yeah, apparantly you don't want "constant pulling" on the engine cradle. It wasn't meant for that. Remember your bus has a LOT of aluminum framework, it's got enough strength for what it was made to do but not a hell of a lot extra. The engine is suspended from basically an aluminum cradle top and forward of the motor/tranny set. Not a good thing to yank on hard. Pull from the most solid axle mounts you can find by having the hitch reach "forward" under the engine and to the axle mounts. THEN don't load the tongue "downwards" much. If you're "flat towing" a car/truck "toad" or a modest motorcycle trailer your vertical hitch load will be very low, as little as 300lb or less which is no sweat. It's the big 20ft+ cargo hauler or full car carrier trailers (where none of the car's wheels roll) that you have to watch for, the ones with the axles in the middle. Tongue weights go up significantly and...I suspect they're a bad idea in a classic aluminum-frame GMC, esp. if you're towing that a lot. | ||
| Mark & Michele (Busnut_pd4106)
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How much does it cost? Where can I buy one of these hitch? And how much is involved in stalling one? | ||
| RJ Long (Rjlong)
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You cannot walk/drive in to your local U-Haul or Camping World and buy one of these hitches "off-the-shelf" - they're custom-made. Might check around locally. Where I live, there's lots of ag work, which translates into lots of custom fabrication shops manned by guys who know how to build something that won't fall apart the minute you swipe your credit card. Daris Boutillier has a hitch similar to Brian's, here's the link: http://www.freewebs.com/darisb/ You'll find the hitch in the "Exterior" section. Ideally, you should have the hitch tie into the radius rod mounts, as well as the cradle. Keep your tongue weights in the 300 lb. range for best longevity. If you decide to haul a "monster" trailer, then make sure it's equipped with a Tuff-Tow unit on the front. www.tufftow.com HTH, RJ PD4106-2784 Fresno CA | ||
| Don/TX
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Usually trailer hitches are wild overkill. Although I don't necessarily recommend it,I had a friend with a 4104 who towed an exceptionally heavy trailer, SERIOUS toungue weight, with nothing more than a 6 inch angle iron bolted onto his rear bumper with four bolts. I thought it would never hold, so kept checking for cracks etc every time I came home, never did have a problem.. I did not trust the bumper that much, so I tied mine into the cradle arms never had a problem with thousands of miles of towing. I think if you ever pull an engine cradle, you will agree that it would be awfully hard to overstress that connection, as the pull is NOT on the bolts as most think. | ||
| Jim (Jim_in_california)
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A 4104 six cylinder has less engine weight than a 4106 V8. If you then swap to an auto tranny as a lot of people do, that's more weight yet. One of our guys on this board just lost his hitch and had his Datsun toad pass him up and wreck hard. Fortunately nobody was hurt and other property damage was minimal but it's hard proof this stuff SHOULD be taken seriously! If what RJ and I are saying isn't clear yet: working with a combination of suspension mounting points and the engine cradle, your *horizontal* pull forces available will be very high...enough to (briefly) pull another bus if you had to. The *vertical* loadings down onto the hitch should be watched carefully, or if necessary managed with a Tufftow or similar hitch-weight-support device. | ||
| Brian (Bigbusguy)
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Mark and Michele, You will have to make it your self of have one made. If you have one made for you make shure you show them the pic's of the ones made. I have about 8 hrs and about $60 in steel and grade 8 bolts. On My 4905 the plate the bumber is bolted to and the part of the engine cradle is steel A very high strenght type. If you bus has the same. You could probley get a way with bolting a hitch to that. What ever you build try to keep it up high .I have mine tied into the radious rods mounting on the motor side of the bulkhead that does the same thing as going to the rods you will just have to look under you bus to see what Im talking about. I all ready had it come close to the hitting the ground and mine is no lower then the factory cradle. I will try to put more pic's in the links I posted earler. If your going to go to the rally in Oregon Im still thinking I may attend if I dont get in I will be parked out side of the fairgrounds and stay for a day or 2 you can see it then. Brian 4905 | ||
| doctor al Rating: N/A |
I just had Fred Hobe work the hitch on my 4905. Now, we're towing a 20' enclosed trailer. No problems so far. I'll let you know how well it works. | ||
| linda alexander (Noopdoggy)
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where is two dogs to set this straight! Please don't try to scare people Jim, do you even have a bus yet?The way he explained it to me, seems the flipper lever on the hitch lost its security bolt, and it came loose from the ball!nothing to do with the hitch mounting to the bus..also he has an eagle not a gmc!anyone wanting to get a wonderful custom built hitch for a gmc or mci, just has to visit Fred Hobe in northern florida. My hitch is a work of art.well worth $300 and much more!!!Gosh I hate when non owners spout out nonsense!!Lin | ||
| TWODOGS (Twodogs)
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Linda....you are correct...the lever flipped up& it unhooked...nothing wrong with the hitch...keep tounge weight under 500 pounds by shifting the load forward & aft on the trailer.. | ||
| Jim (Jim_in_california)
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Ah. OK. Maybe that's too conservative  . | ||
| Jerry Liebler (Jerry_liebler)
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All, I keep seeing these cautions about toungue weight and I feel, based on stress analysis of the rear engine cradle support of my 4107 that toungue weights of 2000+ pounds are safe. Can anyone prove this wrong? I'm assuming 30,000 PSI yield steel and 8g vertical acceleration (this would require the suspension bottomed and solid tires). As Brian pointed out no one has ever pulled the engine down on a GMC. I know I've exeeded 1000 pounds for 2500+ miles without problems of any kind. Regards Jerry 4107 1120 | ||
| David Hartley (Drdave)
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Metal fatigues, Bolts break or shear, Welds crack and unfortold things happen. Hitch balls break,shear and bend, Recievers even the class IV are not rated for that much weight. I know that on a GMC RTS that the whole cradle assembly is held in the bus with 4 (count them 4) 3/4 inch Grade 8 bolts. Two lower front mounts that see shearing stress, And two that hold the whole cradle up in the rear that see shear stresses and downward stretching stress along with side to side stress. Any attachment is only as good as it's weakest part. | ||
| Brian (Bigbusguy)
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Grade 8 , 0.7500 16 Tension Capability (lb) 53408 Shear Capability (lb) 40200 I think the bolts will hold up over 50000(lb) toungue weight. Brian 4905 | ||
| Don/TX
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Unless an RTS is different than earlier GMC buses, there is NO shear action on the front cradle connection. It is a tounge in groove sort of thing, the bolt simply keeps the tounge in the groove. Even on the rearmost bolts, there is a friction factor as well as shear strength, and as Brian pointed out, those two will hold up an 80,000 lb tounge weight. Far beyone the engineering, too many GMC's have hauled too heavy of trailers for too long for someone to say it won't work. I have never heard of a cradle bolt shearing under trailer loads. I think the real worry should be on the effects of heavy tounge weight on the roof. If you follow those vertical supports up a ways, you find they are essentially hooked into the roof, which would probably be the first failure point from too heavy of loads. | ||
| Jerry Liebler (Jerry_liebler)
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Brian, Thanks for proving the weakest link in the hitch system isn't the cradle bolts. My suspicion is that the toungue weight paranoia is kind of an urban legend and based on TLAR engineering. TLAR= That Looks About Right. I believe your hitch system, or mine, have their weakest link not in the bus but in the ball mount or reciever tube itself. I can almost guarantee that the trailer's toungue itself has greater stress than any part of the bus, it's a beam as opposed to the pure tension and compression GM designed into these marvelous machines. Regards Jerry 4107 1120 | ||
| Jerry Liebler (Jerry_liebler)
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Don, I agree, sort of, I believe the weakest link in the cradle support is the riveted joint (30 each 3/16" steel rivets) to the roof. This joint is good for at least 24,000 pounds. The roof itself has a cross section of over 6 square inches so it's good for well over 60,000 pounds in tension (my roof is aluminum). Regards Jerry 4107 1120 | ||
| Jim (Jim_in_california)
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Except...we've had numerous reports of stress-cracking of engine cradles. We've got a recent one just cropped up, guy is trying to figure out how to weld it. At least one guy around here reported the entire engine cradle on a classic GMC "dropping" while on the road - and that's *without* towing, just from the long-term stress of the engine bouncing up and down. Paranoia? Or... | ||
| JJ
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Greetings, Not an engineer here, but do have some common sense...Here's my understanding. First, anyone hazard a guess as to how many g's are involved when the bus and trailer go over a bump or dip in the road and the tongue moves up or down and then reverses direction suddenly? Gotta multiply your tongue weight by this amount, right? Then, we don't really expect immediate failure, rather eventual failure due to repeated stress...kinda like repeatedly bending a wire. First bend shows no effect, but after a while, failure. The bus parts move and stretch. By the way, I don't think anyone has argued that the bolts cannot take the forces. Folks have had cracks in bulkheads and structural members. I had an MCI with cracked engine cradle. The cradle had been used to tow. Who's to say what caused the failure. JJ | ||
| Jerry Liebler (Jerry_liebler)
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Jim Jim, The SOB bus with the cracked stainless tubing, due to weld induced stress? Since much of the significant structure of the classic GMC is steel rust is a likely explanation. JJ, For a while I had aircraft accelerometer bolted, 'c' clamped actually, to one of the steel tubes inside the rear cap. Drove about 7,500 miles with it there and the peak recorded was -0.5,+ 2.5g. Not very exciting though some of the roads under construction sure felt much worse than 2.5g. Regards Jerry 4107 1120 | ||
| Kyle Brandt (Kyle4501)
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If you were to place the accelerometer on the hitch insert, you would see higher g forces. The main structure will have to endure localized 'impact' forces which can be very destructive. The cracks most people find on the structure are from fatigue. Fatigue damage is usually cummulative and is a difficult enemy to battle in these compilcated structures since it is impossible to know the stress history of the part in question. At best you look for cracks at the conclusion of every trip and monitor the problem areas. Simply beefing up the area does not guarantee a solution. This is the reason most will urge you to use caution when adding a trailer hitch. If it were mine, I would add the best hitch I could and go out and enjoy my time spent bussin'. I would also add to my pre & post trip an inspection to monitor the structure for cracks. (Get others to look too as they may see things you don't.) Enjoy, kyle4501 |