How Much Flex is Too Much Flex?

[Traveling 2gether Journal]

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?

 

[IdaSHO]

3/8" is nothing.

An open C channel on a full size produces considerably more...
But a properly built 3-point pivot handles it no sweat.

 

[quickfarms]

You need to load the truck to the max GVW and then measure to frame flex, this will give you an idea of what the engineers design flex is.

 

[Traveling 2gether Journal]

3/8" is nothing.

An open C channel on a full size produces considerably more...
But a properly built 3-point pivot handles it no sweat.

Glad to see you chime in @IdaSHO
I've read through your build a couple times. I'm planing on doing a steel subframe with a wood/XPS/fiberglass camper.

A 3-point pivot seams like the way to go as far as isolating the camper from the flex. I'm just trying to get a feel for weather or not I really need to be concerned with it in the first place. I've seen a fare number of builds that either welded or bolted subframes straight to their truck frame and reported no issues, but of course I've also seen the people post that this is impossible. Just trying to figure where in the spectrum I fit.

 

[IdaSHO]

As noted above, load it up and re-test.
There is good reason to expect that your flexing will go beyond 3/8" in 8'

But to be honest, I wouldn't sweat the 3-point if it is anything less than 3/4" or even 1" in 8 feet.

Until you find out exactly how much you are dealing with, its all speculation.

 

[Traveling 2gether Journal]

As noted above, load it up and re-test.
There is good reason to expect that your flexing will go beyond 3/8" in 8'

But to be honest, I wouldn't sweat the 3-point if it is anything less than 3/4" or even 1" in 8 feet.

Until you find out exactly how much you are dealing with, its all speculation.

OK. Now I just gotta google how many buckets of water I gotta put in there to equal 1950 lbs ?

 

[quickfarms]

OK. Now I just gotta google how many buckets of water I gotta put in there to equal 1950 lbs

A gallon of water weighs 8.33 lbs

On my truck the flat bed is mounted to the frame with ubolts in the rear and springs in the front

The front of the camper is resting on a gooseneck coupler, makes a really easy pivot for a three point

The rear corners are attached using container fittings that can be unlocked if necessary

The reality is that because the camper only weighs 5,000 lbs there is not enough weight to flex the truck frame enough to worry about

 

[Mickey Bitsko]

Use hot water it weighs less, the truck will never know the difference.

 

[Traveling 2gether Journal]

The reality is that because the camper only weighs 5,000 lbs there is not enough weight to flex the truck frame enough to worry about

That's funny. I'm hoping that my entire truck with the camper (dry) comes out to 5,500 lbs. ?

Unfortunately I can't do anything with my truck where I live, so will have to wait on retesting the frame flex.

 

[socceronly]

The reality is that because the camper only weighs 5,000 lbs there is not enough weight to flex the truck frame enough to worry about

This is the kind of thing us newbs have no intuition for. Only weighs 5000 lbs puts things into some perspective.

So I ... don't have to worry about my sub 1000 lb topper on an F-XXX series truck that isn't trying to go down the Rubicon trail, but just nasty forest service roads.

 

[rruff]

A 3-point pivot seams like the way to go as far as isolating the camper from the flex. I'm just trying to get a feel for weather or not I really need to be concerned with it in the first place.

If you have a fully boxed frame on the T100, then I wouldn't. Your camper and subframe will stiffen it a bit more, but it isn't designed to flex in the first place.

Link to Tern Overland's build?

So I ... don't have to worry about my sub 1000 lb topper on an F-XXX series truck that isn't trying to go down the Rubicon trail, but just nasty forest service roads.

If it's a newer F-xxx with a fully boxed frame then I wouldn't worry. That would be F150s since 2015 and F250s and 350s that came with beds (not chassis-cab) since 2017 I think. Otherwise the open C frames are designed to flex a lot, and you could easily destroy a camper (develop cracks) if it's hard mounted to a subframe that isn't stiff enough to remove that torsional flex. Just going through a ditch at an angle is enough to produce a lot of torsional flex.

 

[Traveling 2gether Journal]

Link to Tern Overland's build?

He posted in a thread about tacoma camper mounting and shared the info.

Attaching DIY camper direct to Tacoma frame.

I nixed the 3-point on my build because I didn't want the extra height and complexity and subframe that a pivoting mount would have. And I don't think I really need it. Going with poly cab mounts. Steel will be heavy. I'm guessing you have metal working experience, but IMO fiberglass, foam, and...

expeditionportal.com

 

[Traveling 2gether Journal]

This is the kind of thing us newbs have no intuition for. Only weighs 5000 lbs puts things into some perspective.

So I ... don't have to worry about my sub 1000 lb topper on an F-XXX series truck that isn't trying to go down the Rubicon trail, but just nasty forest service roads.

The tricky bit is that his camper is going to act a lot different than your topper and I believe his build (I couldn't find a build thread) is on a large military truck of some kind that will act much different than an F-truck. I was hoping to minimize some of the variables by focusing on just the torsional flex measurements.

My goal with this thread is to make a decision on how much flex is acceptable in my subframe. Then I can measure the flex once my frame is built and mounted and if it shows too much flex I can add additional crossmembers or change the bushings until it is with in the acceptable range.

 

[socceronly]

The tricky bit is that his camper is going to act a lot different than your topper and I believe his build (I couldn't find a build thread) is on a large military truck of some kind that will act much different than an F-truck. I was hoping to minimize some of the variables by focusing on just the torsional flex measurements.

My goal with this thread is to make a decision on how much flex is acceptable in my subframe. Then I can measure the flex once my frame is built and mounted and if it shows too much flex I can add additional crossmembers or change the bushings until it is with in the acceptable range.

I think it's great. I learn stuff from almost every post made on this site. It's a LOT of information.

Good luck with the project, I think it is amazing what people take on.

Hopefully someday I'll know enough to help others.

 

[rruff]

He posted in a thread about tacoma camper mounting and shared the info.

Attaching DIY camper direct to Tacoma frame.

I nixed the 3-point on my build because I didn't want the extra height and complexity and subframe that a pivoting mount would have. And I don't think I really need it. Going with poly cab mounts. Steel will be heavy. I'm guessing you have metal working experience, but IMO fiberglass, foam, and...

expeditionportal.com

Thanks! That's about the amount of flex I've gotten out of my Tundra with an 8' bed. The swaybar he has on the Tacoma will increase the frame flex by reducing the suspension articulation.

My goal with this thread is to make a decision on how much flex is acceptable in my subframe. Then I can measure the flex once my frame is built and mounted and if it shows too much flex I can add additional crossmembers or change the bushings until it is with in the acceptable range.

The subframe will stiffen the whole assembly... so will the camper. What you really want to know is if the degree of flex you end up with will overstress any part of the assembly. 99.9% sure you'll be fine on all counts since your frame is already stiff.