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Old 04-07-2016, 04:57 PM   #29
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Originally Posted by sdagro View Post
I would like to extend this topic to include the "QUICK LINKS" used on the car end of the safety cables on the Roadmaster "All Terrain Falcon II". I was told by a very reputable machine shop that the QUICK LINK is the WEAKEST LINK!! Steve
How can a safety cable be the weakest link? It is the backup when the primary towing system fails. Maybe you need to start a separate thread on this topic

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Old 04-07-2016, 05:34 PM   #30
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Bolt Vs Hitch Pin

OK....here is my take on it. You have to do some research and I did it quickly....so PLEASE take this as a SUGGESTION....and do your own due Diligence.

First, from the following link.


Scroll down to page 19. These are the values for a SINGLE Shear load. Note that the value is about 13,600 Lbs for a typical, 5/8" Draw Bar pin.

NOW, go to the following link.


If the link breaks, then google Shear Strength 5/8" grade 5 bolt.

Scroll down and there is a good explanation of Single versus Double Shear. SO, in the above, we used Single....so lets use SINGLE to keep it Apples to Apples. The Single Shear for a Grade 5 bolt is 22,100 Lbs. If you look to the left, a standard Grade 2 bolt is 13,600. HMMM....we've seen that before.

Therefore, a piece of 5/8 bar stock (probably 1038) whether used in a Bolt or as a Drawbar pin is the same. NOW, if you upgrade the metallurgical properties of the bolt to grade 5, it goes up almost 90%. That is because the core hardness of the bolt is harder. Obviously it has to be properly case hardened and also stress relieved to achieve the correct values.

Bottom line, a Grade 5 bolt, properly torqued will obviously be stronger. NOW, comes the practical side of it. On MY Roadmaster XL, I have TWO 5/8" pins connection the baseplate arms to the hitch receiver. They are the equivalent of a Grade 2...but there are two of them...so you have, in theory, 13,600 Lbs per side. However, in actual use, the load varies from near zero to near 100%.

NOW (hang in here with me....there IS a conclusion and a recommendation), you have TWO Grade 2 pins holding the hitch spreader bar to the tow bar. They are 3/4", or 19,600 Lbs. The Hitch has two pivot bolts, which are (probably) Grade 5 or 31,800 Lbs. Therefore, if you have the hitch in a position where ONE side (one Baseplate) is carrying the load, you would have ONLY 13,600 Lbs or the rated load of a SINGLE drawbar (5/8") pin.

Therefore the weakest link in the whole assembly is the drawbar pin in the MH Receiver and the drawbar pin in the baseplate arm. They are EQUAL....

Therefore, if you increase the shear strength of ONE, you have to kick up the shear strength of the OTHER. In my case, I would have to use a Grade 5 bolt on BOTH of the arms and also on the MH Receiver.

Assuming THAT I did that....you would need to properly torque the bolt(s). You should use a FLAT washer, not a SPLIT one. You should use a Grade 5 nut and torque it to 128 Ft. Lbs. See the link below. This assumes you use zinc plated fasteners and NO loctite. The K factor is 0.17.


128 Ft. Lbs. is a LOT of Torque. Once you torqued the hitch bolt with a nut, you really SHOULD add a "vibration resistance" nut. So you use a Nylock Nut to "Lock" the first nut in place. Now you have to have both an open end wrench as well as your socketed torque wrench. You PROBABLY have room to do that on the MH.

BUT, I would NOT, I think have any WAY of getting the torque (to the proper MEASURED value) on the XL Baseplate arms. There is NO room for a torque wrench (with a socket). YES, you could use a Crow's foot attachment, but you would have to use a higher value.

Therefore, unless you intend to replace the Drawbar pins in a typical Roadmaster mount, why do it? I pull my tow bar off (it is locking) and do not leave the MH. It would be quite a task to bolt and unbolt it each time.

NOW, if you wanted an additional 25% increase in shear, some of the Tractor manufacturers have a "high grade" 5/8" Hitch Pin. New Holland has one that is PN SPC71021WCDS. It is much longer than your standard Draw Bar as the usable length is 4 1/4" versus 3 3/4". It also has a handle, so it will take MORE clearance. IF you used that for the MH Drawbar, you SHOULD drill out the hitch pin clip hole. You need to drill them to 5/16" to accomodate most locks. That is what I did with mine as I lock ALL the hitchpins.

I doubt, based on clearance, that I can get the longer ones, with the handles, in where the bent ones go to attach the arms to the baseplates.

BOTTOM LINE....you probably have ENOUGH safety factor in the 5/8" pins. Swapping the MH pin for a hardened bolt and then properly torquing it and adding a properly torqued Nylock nut gives you MORE shear strength in attaching the Tow Bar to the MH. BUT, the load on each individual draw bar pin (assuming you have the Roadmaster XL series) can approach 100% and you gain nothing. However, it will take a LOT of effort to remove or sabotage your MH attachment.

BTW, if you look at the classes of 2" receiver hitches, the max is 14,000 Lbs for a Class IV. If you upgrade to a Class V, it is a 2 1/2" receiver. BUT, all the 5/8" hitch pins are rated for Class I to V. Therefore, unless you find a specially hardened or higher strength draw bar pin and also BEEF UP the Base Plate arm attaching system, you gain nothing.....

Blue Ox.....Have NO idea how they rate the attaching arms or whatever.

There WILL be a pop quiz....so read this again....

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Old 04-07-2016, 05:44 PM   #31
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Originally Posted by Algoma View Post
How can a safety cable be the weakest link? It is the backup when the primary towing system fails. Maybe you need to start a separate thread on this topic
Something has to be the weakest link. Doesn't mean they won't do the job they were intended for.
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Old 04-07-2016, 05:50 PM   #32
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Quick Link IS the WEAKEST point in Tow Bar Assembly

Typically, if you look at the Quick Links, they are "around" 3/8" diameter. If you google the specs, you will find that they are rated at 2200 Lbs. The SS ones are rated at 2100. I have a BRAND new Sterling Tow Bar in the garage. I checked the diameter of the link and plainly stamped on it is 2200 Lbs.

I did call Roadmaster a few years ago and debated this with a tech support person. Their position was that the Falcon 2 was rated at 6000 pounds. My retort was that their cables were probably much stronger (memory fails me....I think they were 6,000 EACH). SO, you have a cable on each side that is rated at the capacity of the tow bar. BUT, each is attached with a 2200 Lb. Quick Link. BINGO, that is a max of 4400 Lbs.

They are aware of this....but the logic that they use or the Failure Effect Mode Analysis (FEMA) is that you are trying to "restrain" the TOAD from detaching and that you are required, BY LAW, to have a break-away system that will lock down the TOAD.

Great point....and the machine shop is 100% correct.

I did NOT get into a lengthly discussion on the mounting points of the Safety Cables and the "shorties" that eventually go to TOAD".

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Old 04-07-2016, 06:55 PM   #33
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Angry Base plate problems

I recently had my Blue Ox base plate on my Cadillac SRX come unbolted. Only one bolt held it on when I finished a three hundred mile run. It dropped down an inch or so damaging the front fender and paint. Lucky I caught it before it came off completely. It was installed by a hitch shop in Hesperia, CA. The repair shop in Texas said there was no Locktite (as spec'd by Blue Ox)on the threads so the bolts backed out. $375.00 repair bill. Make sure you know your installer is qualified!!!!!
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Old 04-08-2016, 06:35 PM   #34
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not quite

Originally Posted by twinboat View Post
Cold*rolled*has a shear strength of around 40,000 psi and your area under load for the pin is about 0.3 square inches for a 5/8" pin so in single shear you are good to around 12,000 lbs. The pin is in double shear so you might expect 24,000lbs...but it is not just the shear strength of the pin it is the compressive strength of the wall of the tube. You have 0.188" by .625" across the diameter of the hole which gives an area of about 0.12 square inches so each side of the tube can take 4700 lbs for a total of just over 9,000 lbs if the tube is standard hollow structural with about the same yield strength as*cold*rolled. Obviously the pin is not really the limiting factor it is the*tube*thickness.
twinboat, you used shear strength to calculate what is a bearing stress on the tube. The bearing strength is stronger than shear strength. You may still be right about the tube being weaker than the pin but its stronger than you thought. Cheers!

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