extended cab frame flex?

[sawdustnsteel]

looking through the threads I've failed to find much on this. With the frame flex on existing stock m931 ( found one, just finishing up bargaining) I'm wondering if anyone stiffens the frame with X bracing or boxing ( which would require some additional welding attention since the frame is tempered) to keep the extended cab from ripping free from the frame.

one builder used an air ride bag on the back of the cab ( the up armored build) most I've contacted used bushings and bolts. None thus far Ive spoken with or read of had any issues with bolts pulling through. ( I'm not sure if its a non issue or just no one has really done some hard climbing in an extended cab m939 series.)

the Truck I've found has had a rear bumper welded on ( which will be cut free and and a new one bolted up. Not anything against the weld job, I just doubt there was preheating or specific weld wire used)

Im leaning toward X bracing and bolting just to make the frame more rigid, adding the body extension on and having a unibody construction basically just to ensure there is minimal issue between the frame and the body. ( I got a gut feeling ever **** pebble in the road will feel like a 3' hole) my other grand thought was long bolts and bushings but how much flex I may get from the motor mounts to the rear bumper might be much more than I'm bargaining for.
thoughts?
( can't seem to find anything on specs for frame flex even)

 

[wheelspinner]

They can twist up pretty good, but that's by design. Take a look at how the 923 beds are mounted with spring loaded bolts. Maybe that would be an idea for the back of an extended cab?

 

[tobyS]

I've wondered about this also.

I notice the mount at the back of the cab has a lot of flexibility. Maybe it can be moved back when the cab is lengthened.

At least with a 931, you are starting with the short frame.

 

[74M35A2]

I drove my 925A2 over a car, and when the front wheel went atop the car roof, and I looked back and saw all 4 rear wheels still on the ground providing traction and stability, it made sense to me to be a fascinating design feature.

The rear of a conventional M939 cab is supported in the center only atop a frame crossmember. It will accomodate the majority of the frame flex. Just move this rearward.

I'm not so certain these frames are tempered, vs those of a conventional civilian road tractor.

 

[sawdustnsteel]

I drove my 925A2 over a car, and when the front wheel went atop the car roof, and I looked back and saw all 4 rear wheels still on the ground providing traction and stability, it made sense to me to be a fascinating design feature.

The rear of a conventional M939 cab is supported in the center only atop a frame crossmember. It will accomodate the majority of the frame flex. Just move this rearward.

I'm not so certain these frames are tempered, vs those of a conventional civilian road tractor.

my concern is moving that cross member further back might either cause stress cracks/ load on that portion as well. IE ripping steel since its mostly a side to side sheer pressure.

how much did your frame flex when rolling over the car? ( rough guess is still better than no info at all)

 

[Jericho]

Its a quick fab job to mimic the rear cab support on the present cab to match the rear of the new cab, if your lucky you can get a spare one and mod it it, Ive Rockwell tested my 109 frame after reading the blurp on the "special treatment the frame recievied during manufacturing, I have read a lot on the ups and downs of welding on the frame and must say I used a 109 frame to construct a trailer, bogie assy installed, welded the new tounge and frame to the frame with no problems, I used 7018 rod and got great penetration, the fuse was solid and I have hauled 18 k on it many times with no problems, It still twists nicely when needed, I tested the welds on the edge of the welds and at the fuse points and found no appreciable difference, I never had any cracking or "rip'seperation" the welds are solid! I think the concern over welding is over rated, I suspect it is driven more toward the possible flex encountered repeatedly over rough country in that solid welded joints would have little flex, The "LOADED" stress would have little chance of release thru frame or joint flex other than to concentrate at joints of convergence and BEND to shed the energy. It aint rocket science, Well yes some of it is. Where as riveted or bolted joints would by there very design allow flex and disipate stressors. considering the small amount real hard core repeated loaded rough country use I SUSPECT THE CONCERN FOR THE No WELD embargo is over zealous. Ill post an up date in 5 years on my trailer welds

 

[sawdustnsteel]

Its a quick fab job to mimic the rear cab support on the present cab to match the rear of the new cab, if your lucky you can get a spare one and mod it it, Ive Rockwell tested my 109 frame after reading the blurp on the "special treatment the frame recievied during manufacturing, I have read a lot on the ups and downs of welding on the frame and must say I used a 109 frame to construct a trailer, bogie assy installed, welded the new tounge and frame to the frame with no problems, I used 7018 rod and got great penetration, the fuse was solid and I have hauled 18 k on it many times with no problems, It still twists nicely when needed, I tested the welds on the edge of the welds and at the fuse points and found no appreciable difference, I never had any cracking or "rip'seperation" the welds are solid! I think the concern over welding is over rated, I suspect it is driven more toward the possible flex encountered repeatedly over rough country in that solid welded joints would have little flex, The "LOADED" stress would have little chance of release thru frame or joint flex other than to concentrate at joints of convergence and BEND to shed the energy. It aint rocket science, Well yes some of it is. Where as riveted or bolted joints would by there very design allow flex and disipate stressors. considering the small amount real hard core repeated loaded rough country use I SUSPECT THE CONCERN FOR THE No WELD embargo is over zealous. Ill post an up date in 5 years on my trailer welds

Thanks Jericho. That alleviates some of my concern. Is there any warping in the bed?

I'm rather worried hard pointing a full length can on a 931 will warp the door jams eventually and my handles will become bungee cords part of what I'm looking at doing is putting an matv hood on, boxing the cab more square, slanting the front windshield, and ending the cab about 4' from the rear. Getting a matv look on a 931.

 

[Jericho]

Not at all, in fact I installed two cross pieces in the tongue area I scrounged of another frame, I kept the spacing roughly the same , BUT, I welded them in with continuous full joint welds, rater than bolting. I did thought build my mount tabs for the deck similar to the factory and I welded the tabs, ( HD angle iron, ) to the frame . I added rubber( stiff) between the tabs , I ran 8 Tabs on each side . I routinely haul my goats (M561) on it, longest run I have made is 5 hours at highway speed, But I live three miles up a very rough road, I would run a two point air bag mount at the rear of the add on cab, will fab it from box steel , bolt to frame, they airbag to cab . I am in the process of doing a four door to my 109.

 

[Jericho]

just an aside, but I did see a photo of the rear frame area of an M 923 XLB a while ago, it had X bracing (bolted and riveted ) at the rear area

 

[74M35A2]

Spring bolt mount the cab-frame crossmember to the frame like the bed is to the frame, and it should be able to handle nearly anything.

 

[prholley]

Or use a 927/934 chassis. Reinforced and better proportions for crewcab anyways.

 

[Csm Davis]

Or use a 927/934 chassis. Reinforced and better proportions for crewcab anyways.

This is going backwards the shorter frame will flex less. The 931 frame is braced way more at the back 5 feet than any other 5 ton. The front half will move more than the rear of a tractor. If you use the factory mounts you should be fine as it is basically a 3 point mount for the cab, two front on each frame rail and the rear is two body pucks side by side in the middle so it can handle the twist of the longest wheelbase trucks. I have twisted up tractors and wreckers pretty hard and the hood vs cab gap was much larger on the wrecker even though the wrecker is much more reinforced than the tractor the longer frame equals more torque to twist.

Sent from my SM-G900V using Tapatalk

 

[prholley]

He's talking about crew cabs though. I haven't seen one done on a 931, but I would imagine it would look pretty goofy. Between the other two chassis I can assure you the 934 flexes the least.

 

[sawdustnsteel]

PRholley,

ill likely cab the entire top, after bobbing it. right now I'm weighing options on how to pull off what i want to do.
thus far there are two options since I'm not going to ever load the truck with pallets of stuff.
1. mod the crap out the body find an MATV hood and make the entire thing close to the MATV in shape. leaving a small 4" bed thats got support brackets boxed around it.
2. do a full length cab with 4 doors and two back center opening doors with a tommy gate on the back that tucks under the frame.

I'm looking at about 70% on road 30% light off road, mostly riding through the public land to go out shooting, Ill have firearms and passengers, both of which won't enjoy being in the dust at 100 degrees. so building a comfortable cab with a serious AC unit, decent sound proofing, and a relatively comfortable ride to go out and ride to far reaching places is mostly what I'm after. I intend to put Mrap Fuel tanks on both sides. a 12v converter,ac, stereo, storage area for rifle boxes and ammo, POSSIBLY a ladder and platform on top. when not being used to go shoot it will be driven to get groceries, trips to lowes, local events and such because going through valet in a 5 ton sounds like a **** fine idea to me. I live in vegas, my entire city was created for excess.

why do i ant to do this? because i can

 

[grendel]

Air ride the entire cab. I've done it on a different project.

Build a sub frame for the cab.



it uses a hinge in the front and this one has 4 air bags with a pan hard bar in the rear. Cab was built on top of it, with a 13.5' sleeper behind it.


The frame needs to flex.

 

[74M35A2]

Then you can tie your stereo to your air bag solenoid and make it dance.

 

[Flyingvan911]

As said above, I'd make the cab able to move with the flex like the bed does. The frame flexes as it should. Strong enough to bend.