How is a Tow Bar supposed to work?

[bamaboy473]

Originally Posted by AKIQPilot

I mentioned in your other thread that what you have built isn't properly designed or engineered. There are many things wrong with this design including but not limited to: bolts in single shear, safety chains connected to towbar hitch points.

You bent 1/2" plate steel on a very short tow. The vertical difference between the Toad and Tow Vehicle isn't what caused your problems today. Among many other things, a properly designed baseplate has a cross member connecting the left and right hitch points only a couple of inches back from the hitch pin. This crossmember is critical. It helps transfer the load equally between the two hitch points on the toad.

Your current configurations isn't safe to operate like it is and it will take a complete redesign before it is even close to safe.

Thanks for the critique, and it may come to that point where we don't use the vehicle; life is too short.

I have a Blue Ox baseplate that features a 2" box crossmember, with protrusions for the tabs, and the BMW has a similar set-up, except that instead of a box, the 1/2" bars are gusseted. Our thinking was that loads were mostly of a Push/Pull on a horizontal plane, and that is no longer accurate, IMO.

The cross-member is there, and attached to the front ends of the frame, and how the bar bent as it did baffles me, but our setup is a 1/2"x 2" bar instead of a box section. I'm going to do another test in the morning and see if the tab fails again. If so, then Houston has a problem.

If the test passes after I've lowered the tow bar to a height similar to the base plate eyes, then I'll want to take it to the shop in order to enhance the tabs, which includes replacing that bar that's suffered two bends.

[AKIQPilot]

Originally Posted by bamaboy473

Thanks for the critique, and it may come to that point where we don't use the vehicle; life is too short.

I have a Blue Ox baseplate that features a 2" box crossmember, with protrusions for the tabs, and the BMW has a similar set-up, except that instead of a box, the 1/2" bars are gusseted. Our thinking was that loads were mostly of a Push/Pull on a horizontal plane, and that is no longer accurate, IMO.

The cross-member is there, and attached to the front ends of the frame, and how the bar bent as it did baffles me, but our setup is a 1/2"x 2" bar instead of a box section. I'm going to do another test in the morning and see if the tab fails again. If so, then Houston has a problem.

If the test passes after I've lowered the tow bar to a height similar to the base plate eyes, then I'll want to take it to the shop in order to enhance the tabs, which includes replacing that bar that's suffered two bends.

And you need to address those bolts in single shear and the safety cable chains attached to the hitch point bolts and not directly to the frame.

[Healeyman]

Rick,

One of the most stable and strongest "figures" in Geometry is the equilateral triangle.

In the equilateral triangle all 3 legs (the distance between the corners) is the same (a 1:1 ratio) and the included corner angles are the same.

What tow bars and baseplates usually give us ins an Isosceles triangle where 2 of the sides are of the same length and one side is shorter or longer than the other two.

The greater the length disparity between the two equal sides and the third unequal side, the more unstable and weaker the triangular figure becomes.

On MY Roadmaster Tow Bar and Roadmaster Base Plate, the towing arms of the tow bar are 48" from the pivot point on the back of the RV and the Attachment points to the Base Plate. The distance between the Drivers side and Passenger side Base Plate attachment points is 28 inches.

A 1.7:1 length ratio between the tow bar arms and the baseplate attachment points.

On your BMW, the distance between the baseplate attachment points looks to be slightly more than the width of the license plate (12")

Those numbers give you about a 4:1 ratio or "DISASTER" as you have found out.

Your baseplate would be MUCH better and stronger if you moved the attachment points outward so that they are AT LEAST 24 inches apart.

At that 2:1 length ratio you could probably get way with the 1/2" steel plate that was used, although I'd like to see it stacked double and the layers welded together. Contributing to that is how much lateral force it takes to turn your BMW to follow the RV.

Comments were made above about the vertical forces (in addition to the lateral forces) on your baseplate. REMEMBER ALSO that because your RV hitch is higher than your baseplate attachment points, when you accelerate, an UP vertical force is applied to the attachment point on the baseplate.

Conversely, when you brake the RV, the back end of the RV goes UP, INCREASING the vertical distance between the RV hitch and the Baseplate attachment points. Because the RV rear is UP, the BMW may have a tendency to try to drive UNDER the RV, again increasing the vertical DOWN force on the baseplate attachment points.

Like you, I designed and fabricated my first stiff-arm Tow Bar and Baseplate.

I am an engineer (electrical, not mechanical, but Engineering 101 is still Engineering 101) and I am a MIL Certified welder. I pulled that tow bar and baseplate over 35,000 miles.

There is a LOT of engineering and risk abatement that goes into successfully designing a strong and safe Tow Bar and Baseplate, not to mention the product liability. That is why the Roadmaster and Blue Ox equipment is expensive.

I hope that I have helped you. Please PM me if I can add more.

Stay Safe.

Tim

[bamaboy473]

Thanks for the considered education, Tim

From pivot point to the ears is 34", and there are 20" between the ears, so oddly enough, it happens to be a 1.7 ratio as well. We chose the 20" distance by taking two factors into consideration; that the large bolts and flanges that hold the front bumper backer to the frame are 20" apart and we wanted to be as close to the frame as we could be, and
I looked at several toads that were in an RV park and measurements were as little as 16" wide, so it appeared that there wasn't a standard for these things.

I'm busy trying to hammer the drivers side tab down far enough to insert the first bolt, and danged if it isn't hard to try to bend that steel. Makes it even more baffling that there was enough force placed on the RH tab just by exiting the driveway. As I think about it, the 5" height disparity was increased by the slight grade as I entered the street, so we'll see...

These photos show the original tab, along with a Before and After of the bracket being flipped.

The safety chain attaching point can't be changed, IMO, because the the way the car is built. 1/2" Grade 8 shouldered bolts will have to suffice.

More photos to come.

[georgelesley]

There are a lot of other cars that can be safely towed because professionally designed and tested baseplates are available. Life is too short to risk your and other peoples safety on a home made job.

[bamaboy473]

Test drive was successful. I went 12 miles and made about 20 turns, and checked the setup about 6 times. All seemed to be working as planned, so you all should pat yourselves on the back for getting this fabrication much closer to being on the right track

The hitch on the Sequoia is about 1.5" lower than the ears, but the coach might be even closer, so I won't change anything right now. During turns the pivot worked properly (see photo) and, yes, all parts of the tow bar have been greased.

Look at the photo taken from the top and note that the aluminum insert is canted over to one side during a hard turn. I have some washers in there to act as shims, but do I want to add more washers to almost eliminate any side-wobble?

[Alan_Hepburn]

Has that Blue Ox tow bar been modified? The ones I've seen, and the one I've owned for 10 years has 3 prongs to connect to the car - the center one that yours has, and two that go on the outside of the connector. Here's a copy of the manual for the Aventa LX towbar...

[bamaboy473]

Originally Posted by Alan_Hepburn

Has that Blue Ox tow bar been modified? The ones I've seen, and the one I've owned for 10 years has 3 prongs to connect to the car - the center one that yours has, and two that go on the outside of the connector. Here's a copy of the manual for the Aventa LX towbar...

The model is BX _345, and could be as much as 20 years old. The triple-prong system would sure be stronger, so I'd guess it was a design improvement.

[bamaboy473]

Originally Posted by georgelesley

There are a lot of other cars that can be safely towed because professionally designed and tested baseplates are available. Life is too short to risk your and other peoples safety on a home made job.

The Wright brothers did a home-made job, too, as did most NASCAR drivers at the start. A factory produces what can sell, and how many base plates could Blue Ox sell for BMWs?

Secondly, I rarely use the coach, so can't see any logic in buying a 5th car that will sit most of the year. The BMW is old, paid for, looks good, drives great, has soul and personality (it's a 5-speed) and I'm in the camp of making things work if I can. Thanks to all those that have guided me with this project.

Safety is important to all of us, so no worries about whether I'll be safe. However, it doesn't take much time to find threads about Blue Ox base plates cracking and failing, so there's that. Engineers try hard, but their work can sometimes fail, too.

[Old Scout]

By now you know this is a tough audience...most of us appreciate the DIY approach but not if you are modifying safety stds:
1- Safety cables are usually designed to attach to towed cross-frame in the event the "baseplate" fails.
2- Std. Blue Ox arm connectors are an off-set design to help redistribute lateral forces.
3- Regardless of use, a 20-year old tow-bar is well beyond its life cycle.
4-BO tow-bar looks like the lighter weight aluminum model--how heavy is your BMW?
5-Most std. transmission cars can't be pulled 4-down without modification--no lube to rear of tranny.

[bamaboy473]

Thanks for the caveats. Designing something for the first time pretty much always begs the questions of, "...why didn't it get done this way, or that way?", and if I had a welding shop I would do things differently. I've paid more money to fabricators than I would to Blue Ox IF they made one, so money isn't the object I'm interested in. It needed to be done, that's all.

We also don't use the coach 5,000 miles/year, so this first trip later this month will be the test. If it works, great. If not, find a Jeep.

Originally Posted by Old Scout

By now you know this is a tough audience...most of us appreciate the DIY approach but not if you are modifying safety stds:
1- Safety cables are usually designed to attach to towed cross-frame in the event the "base plate" fails. Can't change that because there is not cross-frame that's accessible. Given a tow-bar, safety cables, and a break-away system, redundancy is in place and if all those fail, chances are the toad and motor home are off the bridge somewhere...
For better or worse, Blue Ox uses 1/4" box steel and we used 1/2" plate bars that are welded to the ends of the frame itself. Maybe not perfect, but not lightweight, either
2- Std. Blue Ox arm connectors are an off-set design to help redistribute lateral forces. Please explain off-set design. I have a Blue Ox base plate that I can follow along with your reasoning
3- Regardless of use, a 20-year old tow-bar is well beyond its life cycle. Age doesn't bother me because the unit got minimal use for most of it's life, and no use for the past 5 years. All components show some wear, but nothing severe.
4-BO tow-bar looks like the lighter weight aluminum model--how heavy is your BMW? 3200 pounds
5-Most std. transmission cars can't be pulled 4-down without modification--no lube to rear of tranny. Nice thing about older cars is that manual trannies are simple. This is a ZF unit, works just fine and no pump needed.

[powerboatr]

you are obviously working out the kinks
but using a mount bolt for the tabs as the point to connect the chain for the safety cables hook is at best slim.

i know you stated getting safety cable or harnessed cable to the frame is not easy.


i would at least change that bolt out to one with a link as part of the head.

[bamaboy473]

Originally Posted by powerboatr

you are obviously working out the kinks
but using a mount bolt for the tabs as the point to connect the chain for the safety cables hook is at best slim.

i know you stated getting safety cable or harnessed cable to the frame is not easy.


i would at least change that bolt out to one with a link as part of the head.

Are you saying to weld a link to the head of the bolt? Seems to me that tearing a welded link off the head would take less force than ripping a link apart that is around a Grade bolt. Did I read the statement correctly?

And, no, there isn't a way to attach chain or cable to the frame unless we weld two lengths of chain to the cross bar and dangle them through the bumper. That would look goofy driving down the street when it wasn't being towed, wouldn't it?

[Triple H]

Originally Posted by bamaboy473

I've paid more money to fabricators than I would to Blue Ox IF they made one, so money isn't the object I'm interested in. It needed to be done, that's all.

If money isn’t in your decision, then why mess with this setup? It just looks and sounds like a problem waiting to happen. And anytime someone brings up safety you basically shrug it off with “this can’t be changed” or “if it doesn’t work I’ll buy a Jeep”.

If it doesn’t work, chances are someone’s life, either yours your family or some poor soul who happens to be next to this contraption when it “doesn’t work” might end up paying the ultimate price for your lack of responsibility for safety. You act like that because you only use the MH 5k miles a year, (which is actually quite a bit) nothing can happen. But hey your GCWR will only be around 26-27k lbs, what could possibly go wrong. My advice ABORT, ABORT.