Cracked Frame

[Bill]

I think it is fair to give TM a little time to evaluate the problem, and formulate a course of action. I'm sure most of us do. No one is trying to cover up anything - that would be silly, since the info is already out there in front of 1000 board members, a dozen TM dealers, and half a dozen welders across the country. Not to mention a bunch of emails between the affected owners and the factory. Remember, emails never disappear - just ask Ken Lay (oops, make that Martha Stewart).

So why don't we all just ease off a bit and watch what TM does, all by itself, without our prodding and moralizing. TM has never failed us before, as a review of the posts on this board for at least 5 years will easily demonstrate.

(Mr Hulsey, I've just stuck my neck out for you here ...!)

Bill

[commodor47]

I would like to thank everyone for posting their information on the cracked frame. We have spent the last two winters traveling from Western MA to the southwest for a total distance of about 14,000 miles. Like most travelers, I do an inspection of the trailer and the TV before setting out on a days travel. However, a close inspection for cracks is not something I routinely look for -but will now.

I was wondering if the affected trailers have the lift kit? Wether or not the lift kit puts more stress on that part of the frame?

Originally Posted by commodor47

I was wondering if the affected trailers have the lift kit? Wether or not the lift kit puts more stress on that part of the frame?

fwiw, I have:

2005 TM 2720
factory lift kit
factory 40 gallon water tank, always full when leaving home
no WD hitch
factory swing away tongue
do not use dirt or other rough roads (other than the dirt loops in the campground at destination at slow speeds)

I don't think anything else is a parameter for the problem.

Originally Posted by commodor47

I was wondering if the affected trailers have the lift kit? Wether or not the lift kit puts more stress on that part of the frame?

Interesting thought. I have two factory lift kits.

On another tangent, when I used the word "Coverup" I was not at all suggesting that TM is trying to cover something up - it was in response to some earlier suggestions that we should not be posting this problem in public view.

I have full confidence that the TM factory will evaluate this and take action, including notification of all affected owners.

[RockyMtnRay]

Originally Posted by Texas_Camper

I was wondering what the parameters could be:
Lift Kit
WD Hitch
Swing Tongue
Heavier Axles
Larger Tires
Water Tanks Filled
And,,, date of mfg of the trailers involved...
Might be interesting when all the facts are in...if they ever are......

One thing that Michael Hulsey and I (as well as several others who've contributed to this thread) can definitely agree on is that the cause of these fractures is repetitive and large dynamic loads concentrated in a small area (i.e. the attach points of the swing tongue hinge).

That means we can immediately discard any factor that does not effect the static or dynamic tongue loading, or in other words, factors like lift kits, heavier axles, larger tires, etc. are unlikely to have any effect.

What does contribute is anything that increases static and especially dynamic loading on the tongue. And more importantly, any factor that concentrates all that loading on one side of the tongue...which is pretty much what the swing tongue does since there is enough slop in the hinge that a substantial amount of torque has to occur before the other side makes contact. Therefore, this is definitely only going to be an issue with swing tongue TMs. And it is likely going to be more of a factor where manufacturing variances resulted in a slightly larger gap between the two parts of the tongue when it is locked in the closed/towing position.

To clarify things, static loads are those that exist when the trailer is not moving. Dynamic loads are those that come from movement of the trailer....when it is subject to road shocks and pulling forces from the tow vehicle. Most static loads become dynamic loads...a 650 lb static tongue weight can easily become a transient 1200 lb dynamic tongue weight if the rear of the tow vehicle encounters a pothole or bump or at speed and that road imperfection causes the hitch ball to sharply/rapidly rise or drop (2G), even if the vertical movement of the hitch ball is only 1/2 inch. However, some static loads do not become dynamic loads....for instance if the swing tongue is rotated to the side with the trailer resting on it, you have a static load (of maybe 600 lbs) that will not become a dynamic load. And some dynamic loads have no static equivalent...for instance, in mountain towing on sharply curving roads there are large lateral (side to side) dynamic forces on the tongue caused simply by the tow vehicle forcing the trailer to suddenly change directions during cornering.

Focusing on static loads....these will include front loaded weight within the trailer (e.g. full water tanks on the models that have the tanks under a front couch); stowing of heavy items in the front of the trailer; and most importantly the use of a WDH. For instance, the use of a WDH with 750# spring bars with those bars at 80% deflection when the trailer is not moving means about 300 lbs of downforce are being applied on each side of the tongue and, more importantly, that downforce is being applied only an inch or so aft of the hinge ear attachment welds. Given that without a WDH, each side of the tongue is probably carrying 250-350# of weight, the use of a WDH has likely doubled the static load on each side of the tongue to around 600 lbs, probably even more on the hinge pin side. And then if you hit a bump ...or do anything that causes an acute angle between trailer and tow vehicle (e.g. crossing a gutter)...the loading of a hitch with a WDH can easily go from 600 lbs per side to around 2000 lbs per side!

Every time the dynamic loading goes into these high ranges, the frame tube flexes a tiny bit. After many thousands of flex cycles, the metal crystallizes and eventually develops tiny cracks. Those tiny cracks gradually coalesce into a full crack.

And finally, contributing to the problem is the fact that TM fabricated the hinge pin ears using thick (roughly 3/8) steel and welded them to the much thinner (~1/8 inch thick) walls of the tongue frame tubes using a heavy welding bead. That thick bead needed a lot of heat...and that heat in and of itself helped crystallize (or embrittle) the metal of the frame tube...making it more prone to crack when subjected to the thousands of flex cycles discussed above.

Soooo, the bottom line is that factors which increase dynamic tongue loading...and concentrate the tongue loading on one side...will be the causal factors of this cracking.

That means that TMs without swing tongues should not have any problems. It also means that tire size, lift kits, heavy axles, etc...anything that does not strongly affect tongue loading...will not have any significant effect.

However, things that do affect dynamic tongue loading will be culprits: use of a WDH; towing long distances over rough roads that produce substantial and frequent shock loads on the tongue; towing frequently on sharply curving mountain roads; and heavy front loads inside the trailer (e.g. 40 gallons of water).

In my case, I strongly suspect that my regular mountain towing is a significant factor, particularly in combination with use of a WDH and a fairly front loaded trailer (my tongue weight is around 600 lbs).

Hope this clarifies things.

Ray,

Thanks for the fine description of the stresses involved. Boy, that sure brings back fond memories of my courses in statics and dynamics, before I switched to computer science from engineering.

My TM has the 40 gallon water tank under the dinette, on the street side, just behind the point of failure. Quite likely a significant contributor to the failure. I always leave home with a full tank of water.

The welder that repaired my TM has removed all slop in the pin on the curb side. The original factory design has the pin going through a hole at the top and bottom and sliding behind the flat plate. This design allows about an eighth inch of slop. As part of the repair, my TM has been modified so that the pin goes through relocated holes and also through a pair of holes in the box tube. There is now absolutely no slop. This likely shifts some of the load from the street side to the curbside, improving the situation.

There is also the additional strap that has been welded onto the bottom of the box tube, on the street side only.

Though I have weighed my TV and TM once, I did not weigh the tongue. In general, all discussions of tongue weight imply that more is better. Between TMO and RV.net forums, I don't recall anyone ever discussing tongue overload. Underload of tongue weight is quite common as it seems to be a significant cause of sway. Maybe I should weigh the tongue this year.

When I started this thread I was mainly looking to hear if anyone else has had the problem. Having never really thought about it, I suspect my goal wast to also:

get an appropriate repair under waranty
inform others that there may be some risk, so keep on eye on things
perusade the factory that they should make an enginering change

I believe that all three have been accomplished.

I'm thinking that use of a WDH would produce compressive forces at the bottom of the frame tube, and so may be the better case, since any cracks forming on the bottom half would tend to be kept closed. The few data points we have would seem to bear this out, with utahsue and popbeavers problems occuring within a year (no WDH), while RMR's (with WDH) TM went several years before the problem arose. Pity, not enough info to add TMs of Russ-Betts and the recent NYSINYD case.

Just a thought.

-Paul

[RockyMtnRay]

Originally Posted by pbuck1

I'm thinking that use of a WDH would produce compressive forces at the bottom of the frame tube, and so may be the better case, since any cracks forming on the bottom half would tend to be kept closed. The few data points we have would seem to bear this out, with utahsue and popbeavers problems occuring within a year (no WDH), while RMR's (with WDH) TM went several years before the problem arose. Pity, not enough info to add TMs of Russ-Betts and the recent NYSINYD case.

Just a thought.

-Paul

The other way around...the lift bracket for the WDH rests on the top of the frame tube. When the WDH chain is snapped up, it puts a compressive downforce on the top of the frame. Resisting that downforce are the hitch ball a foot or so in front of the bracket and the trailer suspension many feet behind the bracket. The net is that the downforce from the WDH lift bracket will very slightly compress the top of the tube downward which causes application of tension across the bottom of the tube (which is being "stretched" by this bending). This tension compounds the normal bending tension from the normal weight on the tongue. Michael Hulsey and I were in full agreement that a WDH does compound the loading on the bottom of the frame and increase its proclivity to crack.

And you're right...3 data points are not enough to draw many conclusions. Though I suspect there are several more TMs out there in TM Land with incipient cracks that haven't yet been noticed.

As I said above, I suspect that my cracking is at least as much a function of repetitive mountain road cornering as it is use of a WDH...and FWIW, the last trip I took involved 2 crossings of Berthoud Pass, a 20 mile stretch of highway with many, many sharp corners. The pattern of paint shedding along the rear weld beads of both the upper and lower hinge ears on my trailer would tend to point to repeated flexing from lateral (not vertical loading); also the fact the crack on the bottom of the tube had only migrated part way across also indicates at least some lateral as well as vertical flexing. And finally, I load my TM quite heavily...the last time I had it weighed, the total weight was around 3800 lbs...not much below the GVWR of 4050 lbs for this model (2720SL)...and if anything, I'm carrying even more now (likely closer to 3900 lbs total weight).

[RockyMtnRay]

Originally Posted by Texas_Camper

Ray
That was one of my points, ie, the heavy axle would tend to encourage us to carry more "stuff", hence the more hitch weight.

Sorry, I was thinking purely in terms of the physics of the situation where a heavy duty axle will not, per se, have any effect on the tongue loading. But you are correct...a heavier duty axle could have the psychological effect of causing owners to think they can safely carry more "stuff", hence heavier tongue weights and more proclivity for cracking.

I wonder if it would be a good preventive measure to go ahead and have a strap welded on the bottom of the frame? If so, just exactly where would it be? On the bottom edge, or on the side near the bottom? I am waiting for more photos of "after" before I do anything.
Thanks

Yes, I think that a strap (technically a "gusset") welded to the bottom of the frame would be a good preventative measure. However this gusset should be 1/4 to 3/16 inch thick (but definitely not thicker), about two inches wide, and at least 4 inches long with it's front edge as far forward as possible. Very important note: the gusset should only be welded along the sides. It should NOT be welded across the bottom of the frame tube on either end. The reason is that welding across the tube will embrittle the tube and make it much more likely to crack crosswise.

Waiting for photos would be a good idea.

[RockyMtnRay]

Originally Posted by PopBeavers

Ray,

Thanks for the fine description of the stresses involved. Boy, that sure brings back fond memories of my courses in statics and dynamics, before I switched to computer science from engineering.

Thanks. My academic background is Physics/Chemistry (have a masters in Physical Chemistry) with a minor in CompSci. My knowledge of stresses (static and dynamics) resulted from many years of piloting light aircraft. I did a lot of reading so that I could better avoid accidents resulting from losing assorted aircraft appendages (wings, tails, etc. ) due to stress fractures. Nowadays I build data driven business software.

My TM has the 40 gallon water tank under the dinette, on the street side, just behind the point of failure. Quite likely a significant contributor to the failure. I always leave home with a full tank of water.

Yep, water is troublesome stuff. It's quite heavy and it sloshes. So the dynamic loads from carrying water can be both high and varying.

The welder that repaired my TM has removed all slop in the pin on the curb side. The original factory design has the pin going through a hole at the top and bottom and sliding behind the flat plate. This design allows about an eighth inch of slop. As part of the repair, my TM has been modified so that the pin goes through relocated holes and also through a pair of holes in the box tube. There is now absolutely no slop. This likely shifts some of the load from the street side to the curbside, improving the situation.

Clearly a welder who understood metallurgy and stresses..and the effects of loosely connected parts. I'm hoping the guy who will do mine this week has similar ideas.

Taking your descriptions of which side the failure was on and where the fixes were applied at face value, it's seems that sometime between '02 and '05 TM switched the sides of the hinge pin and locking pin. On my '02, the hinge is on the curb side and the locking pin (where the slop is) is on the street side. Exactly the opposite of yours.