Cracked Rear Cross Frame 17 Footer

[Thane]

While working under my 2006 17B I noticed two cracks in the small cross beam between the fresh water tank and the rear bumper. The passenger's side crack was very obvious but the driver's side crack was hard to see until I wire brushed the surface.

The rear cross frame has a 3 inch crack on the passenger side and a 2 inch crack on the driver side. The crack centers are 7 1/2 to 8 inches from the side frames that run from the rear bumper forward. I find it odd that both sides of the rear frame are cracked in the same area.

Escape suggests entrapped water froze and cracked the frame. On my trailer the only water path is a bolt I added for a different stinky slinky holder. The only water that could access the bolt would be residual water on the stinky slinky when I put it away, and the occasional rinse of the holder. Rain water cannot access the bolt. The holder has a lot of water drains. To check for water entrapment, I put a hose right next to the only bolt and ran water for at least an hour. I figure this represents years of water exposure. I could not detect any water in the cross frame when I put a strip of paper towel inside via the bigger crack; so perhaps it is not a frozen water issue.

The lower and front frame surfaces are bulged slightly in the area of the cracks.

The first picture shows the cracked rear cross frame from the driver's side looking aft. This is after wire brushing the frame and the cracks are circled.

The second and third pictures show the cracks before I wire brushed them.

The fourth and fifth pictures show the cracks after I wire brushed them.

I am going to have angle doublers welded in place and talk to the welder about the best way to remove the bulges adjacent to the cracks. I don't know if a good whacking with a hammer or a good heating then a beating is best.

I suggest you take a look at your rear frames and let us know what you find.

[PGDriver]

It does look like freezing damage as it appears as if it's pushed out and split. If it was just rusted through you would be able to crush the area around it easily.

Cheers

Doug

[paulw]

Interesting....Any large loads carried on the rear receiver?

[Ron in BC]

Originally Posted by Thane

I find it odd that both sides of the rear frame are cracked in the same area.

Me too. I don't think water is the culprit, it looks more like fatigue cracking to me.

Have bike racks or storage boxes etc. been carried on the rear receiver over sustained rough roads?

Ron

[Brian B-P]

Although those are cracks, they also certainly look to be locations of road debris impact (gravel, etc, kicked up off the road). The gravel takes the paint off and even scrapes the steel, the moisture which is routinely around the underside gets at it, and corrosion has a good head start. This is particularly apparent in the second photo, which is of the passenger side, before wire brushing. I suspect that this crossmember is the most exposed to this sort of road debris damage; the others are higher and/or not behind the tires.

I don't see how the entire square tube could get so entirely filled with water that bursting due to freezing would be a concern, and if that happened wouldn't the flat sides of the tube bulge? They don't look bulged to me, but even though the photos are good it can be hard to tell from just photos. Bulging at the crack locations could be distortion after the crack formed, due to bending stress on the tube, rather than anything which happened to cause the crack; the relatively high stress in this location when the bike rack is in use would also exacerbate the crack. In any case, a couple of drain holes would prevent a recurrence, if water is a concern.

I assume that "angle doublers" would be "angle iron" (actually steel) over the cracks, "doubling" just the lower front corner of the tube. This makes sense, but careful coating before welding (presumably with a weld-through coating in the weld seam areas) and coating (protective paint) after welding would be appropriate to avoid having a more extensive version of the same problem later. Some rolled angle won't fit against the tubing properly (the legs are tapered, so the inside faces are not quite at right angles; also, the corner radius may be too large compared to the tubing corner), so I assume that you would check any angle for fit and consider using a formed angle (if you can find it with a sharp enough bend radius).

In this illustration, the "hot mill product" (typical structural steel) is the type that might not fit, and the "cold finished product" is the cold-formed type that might be better:

From Foll Formed Angle Strength - Samson Roll Formed Products Company - Samson Roll Formed Products Company

[Thane]

Thanks for the feedback. The bottom and rear side of the beam are bulged but I can't tell if it is due to freezing or applied load. We sometimes carry 4 bikes, none fancy light ones, on a rack on the rear receiver hitch but not over unusually bumpy roads.

it'll be interesting to see if anyone else has the same problem.

I am going to add drain holes after the repair and seal the one bolt in the beam.

[Dave Walter]

My opinion based only on the OP's original post was over-loading on the rear receiver. It is possible that with normal highway type roads, even four bikes on a rear rack bouncing over occasional rough spots may have overloaded the cross member. I would beef up the cross member and consider lightening the loads on the rear receiver.

[Brian B-P]

If the cracks were just cracks, and located elsewhere, I would suspect simple overloading... but they are at exactly the places thinned and exposed by road debris. I wouldn't be surprised if these points had rusted though without any stress at all, although of course stress has presumably made it worse. It seems likely that they show as long cracks, rather than just holes, at least in part because when you grind away at a corner of a box, the resulting hole is a long slot along that corner.

[Thane]

I think they are cracks though weakening of the frame due to abrasion is a good possibility, thanks!

[Devil Dog]

Yes , I agree as well . Looks like ice or frost damage . No drain holes I think?

[Thane]

Turns out it is due to water intrusion as I found rust inside when drilling stop drill holes.

Must have filled up from a bolt hole I added and cracked during a hard freeze. I am going to plug the upper hole and weld the frame. I 'll leave the lower hole as a drain.

Oh well...

[Brian B-P]

Originally Posted by Thane

Turns out it is due to water intrusion as I found rust inside when drilling stop drill holes.

Must have filled up from a bolt hole I added and cracked during a hard freeze...

Perhaps, but I wouldn't assume that. It has had two open slots on the lower leading corner for some time, so there will have been water inside due to those openings. Since the tubing is not coated on the inside, it will have rusted inside.

[azjack]

Years ago with another brand of trailer I welded a hitch receiver on the back across two cross members, put two bicycles on a rack plugged into the receiver and took off for a 1000 mile trip. When arriving I discovered a crack on each side of the receiver through the aft cross piece which was the bumper. The bikes spent rest of the trip inside. I believe the bikes probably did a lot of shaking and caused the cracks. I needed a upper mount attachment to stabilize the bikes.
Jack

[Dave Walter]

Originally Posted by azjack

Years ago with another brand of trailer I welded a hitch receiver on the back across two cross members, put two bicycles on a rack plugged into the receiver and took off for a 1000 mile trip. When arriving I discovered a crack on each side of the receiver through the aft cross piece which was the bumper. The bikes spent rest of the trip inside. I believe the bikes probably did a lot of shaking and caused the cracks. I needed a upper mount attachment to stabilize the bikes.
Jack

Sounds plausible to me.

[Ron in BC]

Originally Posted by ice-breaker

Sounds plausible to me.

Yup, 3 theories, water, abrasion or torque from the load on the bike rack.

I'm still going with the torque from the bike rack. I guess we'll never know for sure.

Ron

[Zardoz]

For this to be the result of freezing water, wouldn't the entire frame have to be filled to capacity and then frozen (causing the expansion that would crack the frame)? I find that hard to imagine. If the frame was even half full of water, there'd still likely be plenty of space to accommodate the expansion within the frame and not cause cracking. Hummm ....

[Brian B-P]

Originally Posted by Zardoz

For this to be the result of freezing water, wouldn't the entire frame have to be filled to capacity and then frozen (causing the expansion that would crack the frame)? I find that hard to imagine.

I don't think freezing is the likely explanation either, but for it to happen only this crossmember would need to be full of water. It is welded to the main frame rail tubes, but there is no opening in the side of the frame tubes so this crossmember is an isolated volume - only this crossmember would fill up if you made a hole in the top and pumped in water.

Also, it might not need to be absolutely full. If you leave water in a bucket with vertical sides, as the ice forms it still pushes against the sides even with the top open, since the ice structure resists flowing to expand upward. I don't know how deep is deep enough.

[Thane]

I am confident that my cracks were due to water expansion. The one bolt I added was very corroded, the bulges looked reasonable for such an event, we didn't use the bike rack that much, and no one else has had a similar problem. I also added another bolt to the side frame and it is still in great shape. I screwed up and didn't seal the leaking bolt and 7 years later I got burned. It gets down in the teens every so often.

Lesson learned is seal all bolts. I am going to remove other bolts and seal them.

Thanks for all the opinions and help. Lots of valid insight. Post if you ever find a crack back there.

[Klem]

Food for thought....

Glance at an ice cube tray. The water will expand up and not crack the bottom of the cube forming shape .... ever. IF it froze can you imagine it full of water to create the pressure?

Do you have mud flaps? Is the cracked places 'in line' with where debris would fly up from the road?

To me it looks like it could be a combination of factors ... including lower graded, with impurities .. but sold as comparable imported steel.

I see a battery box, is that a pair of 6's or just 1 twelve?

[Thane]

I just have one 12 V battery.