Bob driveshaft length?

[Jeep Nasty]

I am almost done bobbing my 5 ton crew cab. I got the leave springs attached to the axle yesterday and took my measurements for my driveshaft.

I got a measurement of 76 inches from mounting surface to mounting surface. Is there a magic number with these driveshafts to the Length you can go before you need to start thinking about a carrier bearing. If anyone else who has done a Bob could chime in with their respective Drive shaft lengths. This is a measuring contest I know bigger isn't always better.

 

[Jeep Nasty]

How Do I calculate the driveshaft rpm's? My crew cab. Has the Cummings 8.3 & Allison five speed.

 

[porkysplace]

With the motor rpm's , transmission gear ratio and transfer gear ratio .

 

[Jeep Nasty]

With that being said I can't open the TM's on my phone where I'm at and I barely have cell service never mind data, does anyone know offhand the final gear ratio for fifth gear in the Allison 654 five speed and what the gear ratio is in high range for the transfer case?

I should've been more clear I know how to get the calculations I was wondering if someone knew the number to multiply by my engine RPMs are by to get an estimation of my maximum driveshaft RPMs

 

[VPed]

Data I have indicates 1.00:1 final drive ratio, 0.732:1 t-case hi range, and 6.44:1 final drive ratios.

You can also use tire rollout (circumference), final drive ratio, and of course, your target speed.

 

[Ford Mechanic]

You can also use tire rollout (circumference), final drive ratio, and of course, your target speed.

I was also going to suggest the figuring from the tire side of the equation.

Measure the circumference of your tire.

# of feet in a mile ÷ tire circumference = Tire rotations per mile

Tire rotations per mile x 60 (mph) = Y

then Y x 6.75 (gear ratio, I think)= rev per mile at 60mph

 

[Floridianson]

I thought it was around 60/62 inches. This truck has a wheel base of 164 and drive line from mounting flange to flange is 59". I don't like a short bob and Pete gave me the drive shaft so I placed the rear to match it. Just for smits and giggles I fill up the fuel tank put the battery box's on and weighted her today. 15,960. She was a M818.

 

[4XDesign]

I built a super shout one recently so that's of no use. But a respectable driveline shop should be the best one to answer that question. They will base it on tube size and wall thickness. When I made this tractor my shop used thicker tubing and said that if it was just a bit longer id need a mid shaft bearing.

 

[dilligaf13]

I just re-used the original drive shaft to my front tandem, from the M813 that was being bobbed. Can't argue with that...

 

[Floridianson]

I just re-used the original drive shaft to my front tandem, from the M813 that was being bobbed. Can't argue with that...
View attachment 574204

I will never do such a long wheel base again but yours looks a little to short and after you put on the 53's think it might be a bit tight.

 

[dilligaf13]

28L-26s cleared so I think I could fit some 53's on there no problem

 

[Floridianson]

28L-26s cleared so I think I could fit some 53's on there no problem

That's a hacked up mud truck and where is the fuel tank. I rather built a street / mud truck than something that spends it's life on a trailer.

It's all what you like I guess but if a truck is too short and you ask me I will tell you I think it looks to short.
The truck I just did was long and as said with a 164 wheel base and no tool box/tire carrier between the bed and the cab I had 14" to fill and with 53's it put my tire 28" away from the smaller fuel tank that is on the tractors.
Now to keep the 105 bed in line with wheel wells no tool box and 1" gap from the cab there would be around a 150 wheel base and 14" between 53's and the small fuel tank is what I found. As said your truck all good but I just don't like to see any tire inches from the fuel tank. Trying to make 5 ton with a smaller wheel base than a 4 door jeep to me just does not make for a balanced looking truck.

 

[dilligaf13]

Fuel tank's in the bed. My F-350 on 38's is the street / mud truck. The 5 ton is for getting to places the F-350 can't go...

 

[Floridianson]

Very good and if the 5 ton was ever on the road and God forbid you have something go wrong and you spill fuel into a creek and the EPA says you owe 50 k for clean up the insurance can say the tank was moved by someone and they don't have to pay for clean up.

 

[dilligaf13]

Anything is possible but I haven't seen the EPA set up shop in any of the swamp buggy guy's a$$es around here yet, when they crash their contraptions while their being trailered, so I think I'll be ok. Besides, if the feds want to screw with you they'll find a way and there's not a lot you can do about it, so I think I'll choose to live my life rather than fearing the what ifs of big brother....

 

[Jeep Nasty]

Well thanks for the advice I was planning on going to a good driveline shop for their advice but I wanted to make sure I armed myself with good information before I walked in.
I find that some folks are so eager for your business they say yeah I can do that and the last thing I want to happen is a catastrophe on the highway when that thing spinning like a son of a gun. I went to Gilberts drive wine in Manchester New Hampshire the guy Bruce was very knowledgeable and we came to the conclusion that we have to be set up to 4 1/2 inch tubing to accommodate the max RPMs.

It's definitely a lot longer than I was thinking when I planed to Bob. He assured me this driveline would be safe without having to look at doing a two piece shaft with the carrier bearing. Essentially built an all new driveshaft except for the mounting flange. Was definitely a cost I did not factor in to bobbing this