Flat Towing

[Barrman]

This becomes a hot topic (pun intended) here on the Zone every few years. Rather than pollute Bryant's "I need to move my truck" thread, I figured a new thread about this was in order.

Over the years, I have seen Dave flat tow his M725 800 miles behind his M35 in a single run. Only to have the rear axle walk out of the housing on the way home from the 2005 Texas FE.

I have read about Jon catching his brakes on fire flat towing his M715 behind his D1500 van.

I have seen Bryant flat tow his M715 500 or so miles in a weekend with no trouble for the 2007 Texas FE.

I have also seen Dave almost burn his front bearings and brakes up flat towing the M725 to the 2008 National FE.

Is it luck, chance, a question of truck maintenance or something else that makes a success or flaming failure of a flat tow?

Here is my opinion. While I have seen all of the above. Something I have not seen is any reference in the manuals about flat towing the M715. Maybe Kaiser and the DOD never intended it to be flat towed. After all, the M35 manuals have a section in them about how to flat tow another one and how to be flat towed. Manuals dated 1954 and 1965 have this along with ones published in the 1980's. So it isn't just an oversite for the short production run of our trucks.

But, why did tow bar adaptors get produced? I think the answer to that can be found in the TM for the tow bar itself. The tow bar is designed to be used in conjuction with a wrecker truck. The tow bar provided the spacing and support while the wrecker arm picked up the front of the towed vehicle. Ever wonder why the shackle hooks were made to bolt onto the tow bar adaptors? That is where the wrecker boom attaches to pick up the front. The tow bar TM goes on to say that the tow bar while it can be used to flat tow another vehicle, it really shouldn't for long distances. That of course is in the perfect world of some paper pusher far from the actual use of the equipment.

Practically in our world things aren't so cut and dried. Successful flat tows of M715's I have seen had something in common. The tow bar angled up or was level with the truck pulling. Pulling weight off the front tires when moving. Those that had overheated front brakes and bearings had the tow bar angled down to the towing vehicle. Pulling down on the front tires and adding stressess not designed for.

Take it, argue it, agree with it. I just wanted to write this down before I forgot about it.

[k8icu]

Tim,

When one flat tows a MV be it a M151, M715, M35 or an MB/GPW the tow bar should be as close to horizontal as possible. If the tow bar is pulled down or up it changes the angle and could possible put strain on the drive train and wheels. I've towed my M151 when I had it all over the place. I've towed a M292a2 with a M35a2 500 miles round trip with no issues, but the tow bar was flat horizontal and parralle with the ground. This is the way the bar was designed. Even tow bars that you can buy for civilian vehicles come off of the vehicle level and even with the ground and then angle up to meet the height of the towing vehicle.

That's my $.02 worth.

[kwai]

Actually, the M715 is the one MV that I have seen explicit reference to NOT flat towing for any appreciable distance.

[kwai]

The warning from the medium towbar operators manual is this:

THE M715 OR M725, 1-1/4 TON VEHICLES CANNOT BE TOWED USING THIS TOWBAR. THESE VEHICLES MUST BE LIFT TOWED BY A WRECKER IN ACCORDANCE WITH FM 20-22 (JULY 1970), PARAGRAPH 90B. "VEHICLE RECOVERY OPERATIONS"

[randyscycle]

One thing I can't see is why there would be any difference in towing or driving, regardless of the tow bar angle.

Case in point: All the same components are turning whether being driven or towed. The same lube is being splashed around and the same pudding is being slung about in the knuckles.

Also, if more stress were being added to the front axle by the down force of an angled down tow bar, then certainly anyone with a 4BT, 6BT, Chevy Big Block, etc, would have a lot more weight than a stock 230 over the front axle, creating the same issue and causing brake/bearing overheating and failure.

I suspect that the mechanical condition of the vehicle in question may have a big effect on the end result. Not saying that anyone created their problem, but even a variance of a bit in the wheel bearing adjustment, or shoe to drum adjustment may be the real factor here.

[Barrman]

I agree about the angle of the tow bar. The only people I have encounted having difficulty flat towing the M715 had the tow bar angled down to the pulling truck.

Randy,
I flat towed my M35 home with my M715. The shackle mounts were not on the M35. So, I used the axle wrap mounts on the M35. This put the tow bar angled down to the M35. It pulled great except for going down hill and when stopping. The Gasser would decide to go left, and we went left or right or where ever it decided we should go. The angle was wrong and I was pulling a 14,000 pound truck with one that might have been 7,000 pounds that day.

When I flat towed the gasser to my class back in April, I used the proper shackle mounts on top of the bumper. The tow bar was angled down to the M715. I got stuck in my own driveway almost. The rear of the M715 would pick up and the rear tires would spin. If it hadn't been for my locker, I wouldn't have moved more than a few feet.

I wrote all that to demonstrate that up and down anlges on the tow bar can really throw a M715 around. Now, a 1,200 pound 6BT engine adds a constant load on the front axles in all aspects of driving compared to the 400-500 pound 230. But, the front wheels point where you want them to and tires rolling down the road offer little resistance.

How much of the weight of a 6,000 pound truck is pushed down on the front axles if the tow bar is at a 30 degree down angle? I don't know. Add to that the fact that the tires have to be "self aligning" with the direction of travel instead of driver aligned. Which adds lots of side loads plus the extra weight of the down force. Then add the constant 50-70 mph speed the truck is now going and you are probably past the theoretical 600 pound increase a 6BT adds as far as the bearings are concerned. Factor in maintenance, what kind of grease, ambient temperature and the fact that the stock M715 steering box hates sector shaft inputs which causes the tires to not always be aligned with the direction of travel and I can see problems on the horizon.

Basically, if you can tow dolly it, trailer it or drive it unless you can do as the manual suggest and pick the front end up off the ground for tows longer than a few miles at speed. That is my point.

[brute4c]

Tim, Thanks for posting all this good stuff on the towing. I am so glad you explained it to me at the FE...great stuff.

By the way, the van I tow with is a B250...not a 1500...i wouldnt want to tow far with a half ton with an M715 on the back...

[Barrman]

No problem, I couldn't remember on the van, so I guess the one that would have the lower hitch. Sorry.

[brute4c]

I have a pic of the tow bar angle down to the van...and as you stated, it is LOW! I worried about it and talked to a friend who pointed out that, during braking, this setup would load the rear of the van and plant the rear tires which would help in braking situations...but just driving along, no help at all...heres the pic of what NOT to do for towbar angle:

[airborne]

I have flat towed mine, with a small 'down' angle to the tow bar without any problems. My 3/4t Dodge knows the 715 is back there! I don't like to flat tow, but try and find a 'dolly' or 'wide' trailer to use is impossible.