CBR Metalworks M920 hauler 'Mad Max'

[patracy]

I had gone down this path when I had a M55A2 and a MK17 unit. I was to the point where I had all the numbers worked out to make it work with the sprag unit but I still was afraid to try it. But with a M939 series truck the t-case still worries me (weakness).

That said, as I recall, the numbers worked out that a 16.00 tire up front and 395/85R20's on the rear would make everything happy.

The MK axles are worth keeping. They have air lockers.

The other side of the coin would be to swap the front axle in the M939 series chassis to a front axle from a MK48 or a HEMTT. But that'd be a lot of fab work to make it happen. I'd love to take my SEMTT and swap in a MK48 front end and MK1* tandem set for the air lockers. Most likely I'll just find one more detroit locker axle and swap out the rears.

 

[CBR Metalworks]

I won't take apart MY M936, it is too nice of a truck (was refurbished in 2009).

TadB, that was the truck I was thinking about, I should have bought it 5 months ago, but other stuff kept coming up. Was just always hoping it would still be there when the time was right.

 

[CBR Metalworks]

Well, I have gotten the MK15 home, it has a few missing items, but nothing too major. Did not have a bit of the bii in it, every box was empty...

After studying it over some, I have come to the conclusion that it will be easiest to go with my original plan and just chop the M920 frame and splice this onto it, that leaves me without 6x6, but I have never even used 6x6 on my M936 wrecker, so I can live without it at least until I am able to source a MK48 front axle, or a new ring/pinion for the M920 (this is what I would prefer as it would be less fabrication, and therefore 'safer')

The plate on the front of the MK15 that bolts to the MK48 front unit actually bolts in, leaving bare framerails. IF the frame width were slightly wider on the MK15, the M920 frame would actually slide inside the MK15 perfectly, sadly, they are the exact same width. My plan is to cut off the OUTER frame on the M920, leaving as much inner frame as possible (hopefully at least 4-5 feet), slide the two together, add lots of bolts and welds, connect some hoses, and call it good. I'll remove the MK15 'bed' to make the swap easier, I really want to put an ISO bed on it so I can have the container locks, I may do that as I know where one is with damage that I can shorten by a couple feet.

SO, if anyone knows where to get gears for an M920 fron axle that are close to 5.43:1, or where to get a complete MK48 front axle, let me know.

 

[73m819]

AS far as the front and rear axle ratios go, you really do not have to worry about the difference because the front axle is selectable, with normal driving will be with the rears, off the hard road is where the front is needed, so the bit of slip due to the different ratios will be absorbed by the loser footing. With the WEIGHT of the truck, if it can not pull something, locking in front wheel drive will more then likely break something even if all the ratios were the same..

 

[phil2968]

Pctrans did this a few years ago. http://www.steelsoldiers.com/showth...eas-peppered-with-too-much-testosterone/page2
Another, bragging at this point. http://www.steelsoldiers.com/showthread.php?70123-Soldier-quot-B-quot-never-gets-tired&highlight=

 

[patracy]

AS far as the front and rear axle ratios go, you really do not have to worry about the difference because the front axle is selectable, with normal driving will be with the rears, off the hard road is where the front is needed, so the bit of slip due to the different ratios will be absorbed by the loser footing. With the WEIGHT of the truck, if it can not pull something, locking in front wheel drive will more then likely break something even if all the ratios were the same..

6.17 vs 5.43 is a great deal of difference on a 16.00 tire. Like 50rpm difference at just 10mph. Think about how much damage happened with your wrecker between 11.00's and 12.00's on the road. Yes, I know it's an apples to oranges comparison since you were on hard pack. But with a slower front end tire speed, you're going to be fighting on the steering. If it was the other way around, it'd make steering a little easier actually. Lots of mud racers use a taller front axle ratio. But we're talking a lightweight vehicle compared to a 30K lbs one.

By the math, you could get ratios closer by running a 16.00 up front and a 14.00 on the rear. With new tires, that's only about 50rpm difference at 55mph speeds from the pinion shafts. A little worn 16.00 and new 14.00's would just about be perfect.

 

[CBR Metalworks]

Unless I can get the correct ratio for the front axle, I will likely just not have 6x6. I know on very loose terrain it would possibly be ok and would try it if it were a lighter vehicle, but with something that weighs this much it is not worth the chance of breaking something. If it won't go with both rear axles locked, I am somewhere I shouldnt have been in the first place. I would LIKE to eventually get the front to match, but that will not be high on the priority list.

I also need to source a small engine/pump to run the hydraulics, as I really want to avoid running the hydraulics off the PTO.

 

[CBR Metalworks]

Anyone have any idea the hydraulic requirements needed to run everything on the MK15? I am looking for a hydraulic power pack to run it all, either engine driven or electric motor (to be driven by a generator).

 

[Artisan]

Michael what is the name and model number of the crane?

 

[hailstone]

Do you have pictures of the RV part?

 

[CBR Metalworks]

I finally sold the box off the M920 (was a expandable box off a M934). I did some measuring today and am guessing the wheelbase will be stretched by about 3-5' depending on configuration. It will definitely be a LONG truck! This not the actual picture of my S280 that I plan to use, but it is an S141 which is basically the same size. If someone is good with photoshop maybe you can splice all this together? I am horrible with MS Paint

The problems I found today with a tape measure:

1: The top of the frame on the M920 is about 52", top of the frame on the MK15 was ~44" (measured in front of front tire, tires slightly low, frame sitting low in front) Hopefully it won't be as far off as it measures. If it is I guess I could run 14r20's on the front and 16's on the rear.

2: Unless I leave the 'box' above the middle axle and put the S280 in front of it, I won't be able to sit the 280 directly on the top of the frame because the tires would rub it. If I sit the S280 on top of the 'bed' (or the same height) I think it would be getting close to being over height, especially with the A/C unit on top (I can remove the A/C unit if need be).

3: If I leave the 2' wide box in front of the hoist, it has nice storage spots for the ground spade legs, a medium tow bar, and plenty of other stuff. BUT if I leave it that will make the truck need a ~5' wheelbase stretch. How long is too long? My M928 is 34' long and doesn't seem bad, maybe I need to compare it's wheelbase to what the wheelbase on this would end up being.

 

[R Racing]

opcorn: I can't wait to see this 1 come together. Are you planning to reuse the m920 rear axles ?

 

[CBR Metalworks]

opcorn: I can't wait to see this 1 come together. Are you planning to reuse the m920 rear axles ?

Planning on cutting the M920 frame right behind the middle axle. Then cutting the OUTSIDE of the outer frame rail off the 920 frame (leaving the top and bottom in place). Based on measurements the Mk15 frame should slide right over the 920 frame after that, with about 7.5' of 'splice'. Should be plenty strong enough I'd imagine.

This will give a higher gear ration which will be great for highway driving, but it will make me lose the option of 6wd.

 

[R Racing]

Sounds like it will be a neat set up. It's to bad your loosing the 6x6. But understand wanting to use the mk15 rears for higher road speed.

 

[patracy]

I wonder if anyone is scraping out any MK48's? With how simple the perches are on these trucks. I would think it'd only be a matter of working out a steering arm to swap in a MK48 front axle.

 

[CBR Metalworks]

Yea I figured I would keep my eyes open for a mk48 front axle. I have never had my m936 anywhere it needed 6wd, with a truck that weighs that much, if you NEED 6wd, you are probably already stuck.

 

[patracy]

M548?

There's a HEMTT front axle on a popular website right now. VERY steep asking price, but has a make an offer option.

 

[CBR Metalworks]

Well, my m928 is 34' long bumper to bumper. I just went out and walked off the 920, it appears to be about 24-25 ft long. I guess even if I stretch the wheelbase by 5 feet it still won't be as long as the 928, and would probably have a similar wheelbase as the 928.

 

[CBR Metalworks]

All done! What does everyone think?

 

[R Racing]

What's in the big box ??