Down sides to extending vehicle wheel base?

[Checksix]

Good morning, I am currently in the process of building a 1979 Ford f250 4x4 crew cab into a flat bed long distance family camp truck. My previous trucks have been about the same size and have all suffered from long departure angles. I am considering moving the rear wheels back about 5 – 6 inches to improve the departure clearance.

I am rebuilding the suspension hangers and installing a longer drive train (auxiliary transmission) so the additional work is fairly minimal. This change will also allow me to increase on board fuel storage up to about 55 gallons.

Now what are the negative effects? I assume the turn radius will suffer and the frame stress may increase. How serious are these issues? Are there other negative effects?

Thanks for your time.

 

[R_Lefebvre]

Breakover angles will also get worse.

 

[KG6BWS]

So will your turning radius.

 

[Wonderland]

n/m

 

[chasespeed]

If you are interested in reducing your departure angle... I would consider bobbing the back end a little as well, before increasing the wheel base. I have a R/C L/B, and wouldnt increase the W/B any more than needed. I will be trimming the bed, and making some adjustments to other things, to reduce the depature angle though.

Some functional trimming of the bed sides behind the rear wheels, and tucking the bumper up, and in, would go a great distance to helping with your problem.

Chase

 

[ntsqd]

If you're going to be at or near max GVW then the added span between the suspension mounting points may mean that the frame will actually be overloaded in the middle even though technically the truck isn't overloaded. I wouldn't predict immediate failure, more like a shortening of the frame's lifespan as what you'd be doing is creeping into the frame's designed-in Factor of Safety.

What I would suggest trying to do is to find and measure the frame of an F-350 or F-450 Cab & Chassis truck of roughly the same vintage and near your proposed new wheelbase. Measure the frame thickness and the height at the middle of the frame. If those two numbers are the same or really close to yours then I'd say proceed w/o concerns.
If they're not the same then I'd consider a frame transplant from a heavier spec truck.

I'm basing this on the assumption that you're building this for the long haul. If it is a short life expectancy vehicle then I'd say go for it as-is.

 

[bat]

I think making the wheel base longer is going to kill your turning radius and breakover angle.

 

[cascadeoffroad]

x2 on bobbing the bed. This is a much better way for getting a better departure angle. Stretches are normally only for vehicles who's stock wheeling is under/around 100. If a compramise for room vs departure angle becomes an issue you will have to decide what you will be using your truck for and what's more important.

 

[max adams]

Lengthening the wheel base will allow the truck to twist more, the f250 is reinforced in critical areas by doubling or reinforcing the frame and you would be moving those stress points to new locations. I would bet it would be hard on cab mounts and cross members, speaking of cross members you would need to move them as well.

 

[Checksix]

Frame thoughts...

OK, it sounds like this may not be the easy win I was looking for. I actually can’t bob the bed as I need to maintain the full size flatbed for farm work. Also the larger gas tanks will not fit with a shorter frame.

I want to keep this truck for a long time so need it to be functional and safe. I think I can somewhat work around break over angle issues by installing flat plate under the driveline, transmission and transfer case. But the frame strength and turning radius are a bit harder to overcome.

Is there any method to calculate the loss in turning radius?

Will boxing the frame and reinforcing the body mounts mitigate the risk of overstressing the frame?

Thanks for the advice. I think I’ll go work on the front axle and think this over a bit longer.

 

[ntsqd]

Boxing the frame is simple. Boxing it so that the twisting stresses don't kill it is another matter entirely. Such a frame is designed to twist from the beginning, it is impossible to make a ladder frame of any length rigid enough to not twist. Where and how the cross-members are attached the rails is a critical issue to the twisting not breaking welds or cracking the rails. So any boxing has to allow the cross-members to continue to function as designed.

 

[chasespeed]

You dont exactly have to "bob" the bed. BUT, as stated, trimming the bed sides, and tucking the bumper up and in, will increase the departure angle. You can trim so things are are a couple inches below the bed floor, and work from there.......keeping your bed functional, and allowing a functional departure angle............

Chase

 

[Checksix]

You dont exactly have to "bob" the bed. BUT, as stated, trimming the bed sides, and tucking the bumper up and in, will increase the departure angle. You can trim so things are are a couple inches below the bed floor, and work from there.......keeping your bed functional, and allowing a functional departure angle............

Chase

Thanks. The truck is flatbedded already ther are no sides to trim as the bed rail sits above the frame. I tend to hit the rear of the frame, bed and "bumper".

Another friend is suggesting larger tires as an alternative.