Building a hitch on MCI 7

[mikelutestanski]

Hello.....
   Unless I misunderstand how that hitch in the pix will be mounted:   The 3/8 plate will fail guaranteed.   been there done that bought 2 tee shirts.  I only used 1/4 inch plate for that section. That will fail but take a while longer than mine .   Mine lasted 200 miles from mexico ny to winchester VA.
    And I beg to differ but: it is not stronger than Jacks because there are no structural member on the longitudinal axis ( the same as the rails).
       when I get time I will post my old pix but they should be in the archives somewhere..

       Regards and  happy bussin
       Mike

[poppi]

    Found it Mike........and thanks again.

   http://www.busconversions.com/bbs/index.php?topic=5292.15

   This is page 2 page 3 has a picture just of the hitch.


   Skip

[kyle4501]

How far is the hitch ball from the vertical plates that bolt to the bus?

If 12 inches or more, there is a lot of bending stress on the plates where the tube is welded to it. A 3000 lb tongue load will be very close to the yield strength of the plate. (assuming a 1/2" plate 12" long on each end)

You may not have that much on it in the parking lot, but what about rough roads?

Adding the forward mount all but eliminates the bending of the plate.
It will also reduce the localized hitch load as it is transfered to the bus frame.

Capping the ends of the tube will increase the strength of the tube by preventing it from distorting under heavy load, but I don't think the tube is the weak link in this chain. . . .

It all has to work together.

[Tenor]

OK,
Here are new pictures.  The plates are 10 inches long and the 2x4 is welded on 4 sides.  Here is a shot of the end where you can see the contact of the hitch with the chassis.



Here is a shot of the tongue inserted in the reciever.  It is 10 inches from the centerline of the hitch to the center of the ball.



Glenn

[Tenor]

 Here is a picture of the plate mounted to the chassis from the rear.



And here is a photo showing the thickness and contact area of the plate.



Glenn

[rdbishop]

Good job Glenn,

I think it will do what you want it to do. They said my design was bad also, but it's been there since 1988 lots of miles pulling toad and trailers. So far no cracks.

Richard

[gumpy]

Glenn,

What grade bolts did you use?  They look like Grade 8 in the photo. 

You don't want to use grade 8. You should use Grade 5.

(Or course I could be wrong again  )

craig

[bevans6]

Grade 8 bolts are stronger in every way than Grade 5 bolts, so there is no reason not to use them.  Grade 5 is 120,000 PSI, Grade 8 is 150,000 PSI ultimate tensile strength.  Use 2/3 of that as a rule of thumb for shear applications (SAE bolts are designed for tension applications and are not rated in shear).    In this application, the bolts are in tension.  At first glance, you might think that they would have a shear load vertically, but they do not - the friction created by the clamping load between the bus frame ends and the plates will far exceed the vertical load so the bolts do not come  under vertical load - if the bolts are done up to correct torque.

If you want greater strength than SAE Grade 8, then industrial socket head cap screws ("allen" bolts) and L9 (a manufacturers trade name) are both rated at 180,000 PSI MTS.  Above that, specialty fasteners go up to over 300,000 PSI, but you don't want to know how much they cost...  It's an old wives tale that Grade 8 bolts are "brittle", they aren't, unless they are counterfeit.  If you buy from a reputable supplier, they are a good choice. 

I may be new to buses, but as a race car builder/restorer/driver I've been studying fasteners for 20 years...   

Cheers, Brian

[kyle4501]

There is compromise in every design.

A safety factor of 1 will work, but it doesn't allow any room for error or overload.

I prefer to minimize stress on all the parts & have as large a 'safety' factor as possible. Hence this discussion. I didn't say Glenn's hitch would fail. But, Glenn asked for input & input is what he got.     How he chooses to proceed is, hopefully, his business only. Like most everything else, if he chooses well, he will have success. If he chooses poorly, he will gain experience. 

My '94 Explorer had the towing package & a receiver hitch bolted to the frame . After an unintentional overload was applied, the hitch was fine, but the frame was bent (over the rear axle a couple of feet past where the hitch was bolted). So while the hitch survived, the system failed.
The frame wasn't able to handle the load the hitch was able to transfer. If the hitch had transferred the load differently, the frame wouldn't have seen as much stress, but the hitch would have cost more, hence the compromise. . . .

Quality hardware is a must. So is routine maintenance of checking the bolts - especially at first to establish a base.


Craig, what is the reason for not using gr8 hardware? I'd rather not miss something. . . . but I thought it related to cost. In my designs, I try to design around gr2 hardware to prevent failure caused someone using weaker replacement hardware . . . . You may be surprised at how many don't know the difference.

[mikelutestanski]

Hello;   
      10 years ago  during my hitch tribulations  I consulted with my son in law a mechanical engineer  we discussed stresses moments and stuff that went over my head. The bottom line was he suggested the following:
      On the bottom picture on the right side just inside of the frame rail I added a piece of 3X4 OR 4X4 by 1/2 " thick angle about 8 inches long facing down with the cut ege looking up. we inserted it so the angle fit in the intersection of the plate and the frame rail  running down away from you. In other words the angle was installed to provide a counteraction to the up and down of the hitch.  For the road fix I stopped at a steel place and bought the pieces and had them welded in place..  one piece on each side will be enough to keep the hitch from dropping down Later I replaced the back leg of the angle
     with a plate and put 5 bolts through the side rail. I used half inch grade 8s. THat back part of the  frame rail near the end is pretty meaty.  Actually the later design used a 10 by 10 plate that fit inside the frame rail with the hole cut out for the water pipe and I believe 7 bolts each side.  Never had any trouble with that pulling a 24 foot car hauler loaded  when we moved to florida.
 During my first escapade   I stopped on a flying J to fuel up and when I looked at the saturn the hitch receiver was pointing down. WE unhitched and the following morning I jacked it back up but it only lasted 30 miles so we left the car in winchester Va and headed south.  Christmas 0f 99. We were headed to Savannah Ga to meet the kids for christmas.  Arrived a day late..
     Regards and happy bussin:
     I sure hope your design works better than my initial try....
      Mike

[AIRHOG]

Looking at the photos above I don't think the bolts are the problem, you need to have someone look at the welds.  I have one on my MC-7, don't make it yourself if you have to ask questions like this, think about your fellow bus buddies that may be behind you.  Sorry if I have offended you and if you want pictures of how I did it let me know.

Larry

[PP]

Hi Airhog, Welcome to the board  Your first post and you're already contributing-Way to go
Will & Wife