How Much Flex is Too Much Flex?

[broncobowsher]

Having been around service trucks and service beds and spending some time with the manufacturers I have learned a few things over the decades. The service boxes are solidly attached to the frame at the rear bumper. The front part floats on the frame typically on hardwood sleepers. Normally U-bolts keeping it attached tot he frame but really flexy stuff has spring loaded clamps that will allow the box to lift off the frame. Never seen a pivot at the rear.

During one of the chat sessions they were talking about when GM needed a few boxes to crash test trucks. They tried attaching by the cab and float the back. The box flipped forward and squished the cab. Attaching the box at the rear bumper was much safer in a crash. That is what everyone does and has done for decades now. At least the professionally installed service bodies. And if attached at front and rear solidly, frame cracks. The front still flexes, actually takes all the flex now, That is why the sleepers and U-bolts to allow things to move just a little.

That pivot at the rear of the bed looks neat. But would be better if it were at the front of the bed. Harder to show off, but more functional.
By functional, that rear pivot will allow some wicked frame harmonics. Attaching the box to the back of the frame is a huge mass that will tame the rear frame rails.

 

[rruff]

That pivot at the rear of the bed looks neat. But would be better if it were at the front of the bed. Harder to show off, but more functional.
By functional, that rear pivot will allow some wicked frame harmonics. Attaching the box to the back of the frame is a huge mass that will tame the rear frame rails.

I've seen just as many complaints about fixing the rear as fixing the front. You may get some odd movements either way. And if your frame is stiff and you don't have a suspension that articulates really well, you won't be able to keep your wheels on the ground. Plenty of compromises however it's done.

Not sure what you mean by "harmonics". With a solid mount on the rear, that means the rear of the frame can't twist to follow the terrain without the camper tilting with it. If it's forward mounted (really mounted about half way between the axles) the tilt of the camper will be in a middle zone between the front of the truck and the back. Seems like rear mounting would create the most extreme camper motions.

 

[Chuck1]

They tried attaching by the cab and float the back. The box flipped forward and squished the cab. Attaching the box at the rear bumper was much safer in a crash.

Thats a very good point on crash safely, im going to remember that one.

 

[rruff]

Thats a very good point on crash safely, im going to remember that one.

Only a problem if your box mounts are inadequate.

 

[Chuck1]

yeah if you have one in the back, overbuild using grade 8 bolts and make sure it wont pull out of the shell, that will take engineering because of tension when its normally under compression, g forces in a crash can be very high, a 4000lb shell at 10 g's would be 40,000 lb's of force.

 

[ITTOG]

I've read quite a few threads about torsional flex in truck frames and various mounting systems to help isolate campers from this flex. So my question is how much torsional flex in the camper subframe is OK and how much is too much?

The best way to answer this question in my opinion would be to hear from people who have successfully built and used their campers and shared their measurements.

I've only seen one post where someone shared this info. Tern Overland has a 3 point mounted flat bed on the back of a 2nd gen tacoma. He measured 3 inches of flex in the truck frame and 1/4 inch of flex in the flatbed.

I jacked up the driver side rear tire on my Toyota T100 until the passenger side rear tire started coming off the ground. (Trying to mimic Tern Overland's test) I measured 3/8 inch of flex when comparing the truck frame behind the cab to the frame at the back of the 8 foot bed. This seams like a small amount of flex to me, but I don't have any experience to compare it to.

Any of you measure your truck frame flex? or the flex in your sub frame or flat bed?

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[billiebob]

this might be the limit, Canadian style

 

[Mickey Bitsko]

Wow.. For anyone who's ever pulled a loaded set of doubles like these getting of the line like that takes talent.

 

[billiebob]

yep, jake shifting terrifies me