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Old 02-16-2007, 07:44 PM   #1
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Driveline Gear box/transfer case?

Hey guys. I know many of you have been really helpful before and may know I am looking for better gear ratio solution.

Three years ago I was pointed toward an overdrive transfer case in which the drive shaft would have to be shortened. I was told about $2000 for the job.
Of course at the time I wasn't ready to spend 2k on a transfer case and then have a tranny randomly go out on me that I could possibly have replaced with a 5 or 6 spd. Anyway. My AT545 is in great shape, shifts like a cadillac(or any other model you thinks shifts great
I just received some info that I have yet to follow up on directly, but I heard that I could have the drive shaft shortened and get the gear box and install it for less than I would spend on getting gears changed. The information comes from an good friend who is a retired railroad engineer and former professional sprint car driver. he knows his stuff but I haven't talked to him personally about this yet.

Anyway My tranny is good. Tools for the installation are no problem. I think it would be ideal because I have low end gears and an overdrive on open highway. I guess I am just asking for info and opinions from anyone who has done it or checked into it themselves etc.. Is having a bus size drive shaft shortened expensive? Do you have any idea who makes these units and where to look for them?

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Old 02-16-2007, 08:39 PM   #2
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Auxiliary transmissions, both over and underdrives, are pretty common from Jeeps all the way up to to big rigs. As to whether or not it's going to be cheaper for you...well....that depends on how expensive the unit itself is for you. I'm thinking you would be getting a junkyard unit. I really don't know if a divorced or married type woulbe be cheaper to be perfectly honest. A married unit would simply require that you shorten your existing driveshaft, but you would have to mate it to the tailshaft of the transmission which can sometimes mean a bit of work. It would also likely need another crossmember. A divorced unit wouldn't require transmission surgery, but would require that your existing driveshaft be shortened and a new one made up for between the transmission and the overdrive unit itself. You would also need a crossmember for it. Do you have more information on the specific unit you're looking at?
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Old 02-18-2007, 08:35 AM   #3
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i've also contemplated adding an auxillary trans/overdrive unit.

i think another consideration is how fast you spin the drive shaft. ie: you woulnd't want to double the top end speed of the bus, but a small increase should be fine. Spinning teh drive shaft faster also spins the gears inside the differential faster.

I think the aux overdrive unit is a great idea. One idea i toyed around with is finding an aux trans out of an old logging truck. These transmissions were divorced and were used to lower the gear ratio of the trucks, and many had 4 speeds. The input/output shafts are the same size which allows the unit to be turned around and used to increase the gear ratio ie: make the truck go faster. You could even set up a small air cylinder to move between gears. (I don't think it would be easy to move to all 4 gears that way, but you could choose between 2 gears that are straight up/down from each other) I also thought that a small trap door in the floor would be a good idea so you could manually move the gear selector in case the air shifter didn't work.
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Old 02-18-2007, 06:03 PM   #4
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married? divorced?

I am still single so I don't fully get the metaphor I don't have a specific unit in mind. I have a high mechanical aptitude with a medium amount of experience. Based on hat i've heard I visualized what I think is a married unit but didn't anticipate any surgery to my tranny. Is married one that connects directly to the tranny on the front end and the other side it connects to the shortened drive shaft? I guess I imagine that a short shaft could be used between unit and tranny. ???

I am not really concerned about high speeds. I just want to reduce RPMs(and engine noise/fuel consumption) at 55mph without permenantly giving up my low gear power. Also it would be great to go 65 steady for a few hours when the interstate is the only option.
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Old 02-18-2007, 09:16 PM   #5
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You've got it on the married versus divorced set up, Joe. Married attaches to the tailshaft of the transmission and usually requires a different output shaft/housing. A divorced unit mounts aft of the transmission and is connected with a small "spud shaft."
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Old 02-19-2007, 06:21 PM   #6
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Thanks

Thanks guys. Yeah, I am going to look for a used unit, but I want to find out who makes them new etc..
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Old 03-01-2007, 07:18 PM   #7
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google gear vendors, i think they make the product you are looking for
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Old 04-24-2011, 07:04 PM   #8
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Re: Driveline Gear box/transfer case?

just found this site, they have a good Q&A section, they say that they have a married gear splitter that can match up to almost any tranny to give under / overdrive, just thought id pass it along
www.gearvendors.com
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Old 05-24-2021, 11:51 AM   #9
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I'm considering the same thing - a divorced gearbox, two-speed, to lower crusing RPMs at highway speed. Has anyone done this?
What are some good models candidates for a auxiliary transmission / gearbox, two-speed? I've also read that you might be able to use a transfer case installed backwards. Thoughts?
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Old 05-24-2021, 07:38 PM   #10
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Originally Posted by wanderingturtl View Post
I'm considering the same thing - a divorced gearbox, two-speed, to lower crusing RPMs at highway speed. Has anyone done this?
What are some good models candidates for a auxiliary transmission / gearbox, two-speed? I've also read that you might be able to use a transfer case installed backwards. Thoughts?

A spicer 5831 has a 15% overdrive in high gear. In the lower hp bus engines less then 200hp it would be good. Higher horsepower and look for the 6000 series auxiliary. I did this in my Dodge one ton truck, and it worked great for many years but did break a few teeth off eventually. These auxiliary trans do need to be double clutched. If you have driven a twin stick Mack you are good to go.



Just out of curiosity why not regear the rear end? Do you need more gears?


I used the auxiliary to give me split shifts for heavy loads, and because the 5831 is a three speed, it gives me a nice low range for off road.


For my bus I have a two speed rear and prefer this over the auxiliary. Much easyer to shift, even when split shifting.
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Old 05-24-2021, 11:15 PM   #11
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i would have to change the entire rear end to have a two speed rear. I thought an auxillary transmission might be cheaper.
I don't want to just regear the rear because i still want a low gear for take off and pulling.
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Old 05-25-2021, 07:46 AM   #12
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Lower rear end and wide ratio transmission (3500SP). So far, so good. I like to keep things simple, adding another element on the drive line (another gearbox) is adding a source of failure.
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Old 05-25-2021, 09:49 AM   #13
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Originally Posted by wanderingturtl View Post
i would have to change the entire rear end to have a two speed rear. I thought an auxillary transmission might be cheaper.
I don't want to just regear the rear because i still want a low gear for take off and pulling.



Do you have a automatic or manual trans? The auxiliary would likely have to be put in the range you want then left there and not shifted while under way if you have an automatic.


My bus did have an automatic in it when I got it. Did not drive it that much till changing to a manual trans. When it was automatic I did not shift the rear while driving.



For a two speed rear, normally to go from low to high you let off the gas, and reapply a bit of throttle and you feel it engage. To go from high to low, keep foot on the gas, put the switch to low, then kick the clutch pedal momentarily. Not sure what would happen with an automatic , it might try to downshift also.
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Old 05-25-2021, 10:27 AM   #14
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I owned a spicer 7041, a 4 speed with OD auxiliary box. I was going to install it in my bus for lower highway rpm like you're wanting to do. Mine did not come with the shifter, so I drew up an elaborate mechanism of air cylinders and solenoids to shift the manual box, only to find out that spicer made air shifting boxes, so I sold my manual box and went on the hunt for an air shift model.

Then I realized that I just don't drive my bus enough, and that cruising 55 is a-okay the little bit I do drive it.

Like Ronnie said, my plan was to pick a gear in the auxiliary before starting out. So if you're on the highway, choose OD in the auxiliary and let the automatic shift normally. If you're crawling through stuff and wanting low range, shift it into low range, and let the automatic do the work.

There's a guy on youtube that installed an auxiliary transmission in his bus, and shows how he shifts it. A lot of popping into neutral and rev matching, which still isn't ideal IMO. I think I'd prefer it if you preselected your gear before moving.

2 speed rear axles can be shifted while moving, but a lot of old timers told me not to. There was a school bus accident in the mountains years ago where a driver shifted the 2 speed rear axle while moving, breaking it in the process, and crashed because he overheated the brakes.
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Old 05-25-2021, 06:35 PM   #15
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I must say I grew up driving Dad's old trucks shifting the two speeds rears, then I had my own dump truck with one, and now the bus. So for me it is a natural to shift while driving, but I do it with a manual, and have plenty of experience.


I think the old timers think the "younguns" can not do anything so they tell you not to.


I will say if you miss a gear with an auxiliary or a two speed rear you are out of luck. Stop and start all over. Reason is that if you miss a shift you will quickly get out of the speed range to be able to rev match. So make sure you are in the gear you want before going down a mountain....of course if you follow the rule of using the same gear as you came up the mountain to go back down then all is well anyway.
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Old 05-31-2021, 06:17 AM   #16
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I have a Spicer 7231 I’m installing on my 1995 AmTran. I currently have 4.78’s in the rear and with a .86.1 overdrive I end up with 4.11’s and still able to retain my stock gearing when needed. If that still doesn’t give me the result I want, I’ll change out to 4.44’s and get 3.81’s in overdrive. Time will tell what I end up with. I’ll probably not use the 2:1 underdrive as I can’t see a practical use for it. I cannot use the standard shifter so I’m shifting using a heavy duty push/pull cable coming in from the side and running to the cab. I don’t plan on shifting on the fly but rather put it in the range I need for the terrain I’m traveling in.
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Old 05-31-2021, 07:00 AM   #17
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We had a bunch of busses in Florida a but ago and a number of them got stuck in soft sand. This is where the low range is nice, and for any other off road adventures. As long as you know how to get low range, you could always crawl under and manually move the shift rails to get it.
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Old 05-31-2021, 07:10 AM   #18
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buddy of mine built a twin stick.. spicer 5 speed as the main trans and a divorced 4 speed Aux trans that has overdrive.. that bus can pull a house in low gear and haul ass on the highway.. fun as hell to drive too
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Old 06-01-2021, 10:45 AM   #19
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Originally Posted by sojourner View Post
I have a Spicer 7231 Iím installing on my 1995 AmTran. I currently have 4.78ís in the rear and with a .86.1 overdrive I end up with 4.11ís and still able to retain my stock gearing when needed. If that still doesnít give me the result I want, Iíll change out to 4.44ís and get 3.81ís in overdrive. Time will tell what I end up with. Iíll probably not use the 2:1 underdrive as I canít see a practical use for it. I cannot use the standard shifter so Iím shifting using a heavy duty push/pull cable coming in from the side and running to the cab. I donít plan on shifting on the fly but rather put it in the range I need for the terrain Iím traveling in.
That seems like it might work. Got a link to the cable?
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