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Old 09-02-2006, 08:48 AM   #1
Skoolie
 
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Rear axle or 2 speed axle

Hi agian

Anybody know where a good source is for bus rear axles or 2 speed axles I would like to gear up so I can cruse 70 mph, lower my RPM a little, running 3600 rpm at 65 now, I guess a 2 speed would be great for them tall mountains?

Do I need to change the whole axle or can I just change the 3rd member thing?

Thanks in advance.
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Old 09-03-2006, 10:54 PM   #2
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A two speed rear end might have the same limitations as your current rear end. They will surely have two speeds, but if the high gear is the same as the high gear in your current configuration, there really will be no help other than increasing your ability to pull the foundation out from under a house. I know some axles can take a two speed third member, others cannot. I'd arm yourself with all the numbers off the tag on your axle and go contact the nearest medium duty truck place. Ask them what they can find. There usually is no charge for questions. Then you will have something to work with when you go junkyard hunting. I know my local Ford stealership was MORE than happy to print off an entire schematic of the fuel routing and injection system on my 6.6 when I was doing my WVO conversion.
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Old 09-04-2006, 12:16 AM   #3
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Rear axle assemblies are every where to buy

Used rear axles are available every where at very reasonable cost. Choosing a correct and useable rear axle assembly maybe more difficult. Modern rear axles last a very long time. The truck salvage yards have a huge inventory and will sell units quickly and low in cost. The best deal is for a complete assemble as drum to drum. By making a few small changes and a little welding, the opportunity for changes is nearly unlimited. The spring blocks can be relocated on the housing and new ubolts can be bought.
The rear axle ratios available are really good. I'm going to change my Crown bus to an Allison automatic trans. The rear axle will be a 355 or 373
ratio. My goad is to be able to go 75MPH top speed at about 1850RPM. The trans will not mind the shifting and the MPG should be about 12MPG. My usual cruising speed is now about 68MPH with about 7.7MPG at engine RPMs of 2300. My Allison is a MT643. I'm having some difficulties finding a used flexplate to mate a Cummins to an Allison. With a late model truck rear axle, all the late stlye brakes and parts are an upgrade for old buses. My Crown was made in 1968, and that make my rear axle about 38 years. Most complete units can be bought for about $1200-2000. Frank
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Old 09-04-2006, 09:54 AM   #4
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Thanks for the info I guess the first thing I need to do is find the tag and see what ratio I have then locate someone in my area (northern Illinois) that has axles, what should I expect to pay for a axle, I rather just change the 3rd member or the center section if I can.
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Old 09-10-2006, 04:01 AM   #5
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Year: 92
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Hi guys, I'm a newbie here, but I have been enjoying you discussions for a good long time. Just yesterday I bought a 92 International with a dt466 and a 643 auto trans coupled to a 4.44 rear end. This bus runs great , but I don't like the idea of it turning 2250 just to maintaine 60 mph.

Frank_id said somthing about a 3.73 or 3.55 ratio to maintaine 75 mph at 1850 rpm. Unless your running some super tall tires, taller than lp24.5s, I think you will have to go with a 3.08 ratio, if your still using the 643 auto transmission, or you can get an overdrive tranny to fill that bill. The 3.55 axle will get youn 70 mph at 2100 rpm with the 643 trans and 22.5 tires.

For myself, I'm wondering if the 3.21 axle ratio would be a good match for my setup, I'd like to cruise 70 mph at about 1900 rpm. I'm running 295/75r22.5 tires now. Any thoughts, Thanks guys
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Old 09-10-2006, 05:05 AM   #6
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Cruising at 70 mph at such low RPM's is assuming an awful lot....like that your DT466 will be able to push you at such low RPM's and that it won't take a day to get there. Sure Class 8 trucks run lower rpm's than that for those cruise speeds, but they have TREMENDOUS amounts of torque right off idle and some nifty computers in the new trucks that keep everything running as best as it can in the given conditions.

The DT466 is no slug...it is probably the undisputed champion in a quasi-modern school bus in terms of powerplants, but it still can't work miracles. The same gearing that limits your top end also allows you to accelerate faster. I have a feeling your MT643 won't last long with you lugging the engine that low simply because there won't be enough torque available to keep that torque converter locked up. With a lockup torque converter, you've just made it a wear part because of the clutch. I don't claim to be a mechanic, but I'd seen plenty of people toast transmissions in 4wd rigs by trying to turn 38.5 inch tall tires with stock gears. Sure it looks good on paper, but in practice it just won't work and throwing more power at the problem often only makes things worse.

Long story short....before you start throwing lots of money at a set of 3.08 gears, I'd think long and hard about it. Chances are gears that high were never offered in that chassis for a reason.
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Old 09-10-2006, 09:49 AM   #7
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Quote:
Originally Posted by the_experience03
Cruising at 70 mph at such low RPM's is assuming an awful lot....like that your DT466 will be able to push you at such low RPM's and that it won't take a day to get there. Sure Class 8 trucks run lower rpm's than that for those cruise speeds, but they have TREMENDOUS amounts of torque right off idle and some nifty computers in the new trucks that keep everything running as best as it can in the given conditions.

The DT466 is no slug...it is probably the undisputed champion in a quasi-modern school bus in terms of powerplants, but it still can't work miracles. The same gearing that limits your top end also allows you to accelerate faster. I have a feeling your MT643 won't last long with you lugging the engine that low simply because there won't be enough torque available to keep that torque converter locked up. With a lockup torque converter, you've just made it a wear part because of the clutch. I don't claim to be a mechanic, but I'd seen plenty of people toast transmissions in 4wd rigs by trying to turn 38.5 inch tall tires with stock gears. Sure it looks good on paper, but in practice it just won't work and throwing more power at the problem often only makes things worse.

Long story short....before you start throwing lots of money at a set of 3.08 gears, I'd think long and hard about it. Chances are gears that high were never offered in that chassis for a reason.
I agree you need to find the powre band of that engine mainly where the torque is, on my 366 I would be happy at 2500 or 2700 I have no doubt it will push my bus 700 mph at 2000 or less just by the way the engine feels now when run that low of RPM's.

Do you think a 2 speed rear axle would help him with getting up to speed with the higher ratio?
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Old 09-10-2006, 10:56 AM   #8
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Quote:
Originally Posted by kotflb
Hi guys, I'm a newbie here, but I have been enjoying you discussions for a good long time. Just yesterday I bought a 92 International with a dt466 and a 643 auto trans coupled to a 4.44 rear end. This bus runs great , but I don't like the idea of it turning 2250 just to maintaine 60 mph.
Have you thought about turning the governor up on your bus? If you're only turning 2250 RPM's @ 60mph I’d bet that if you turn the governor up a few turns and most likely get 70mph top end. You have almost the same gear ratio as I do (I have a 4.33:1) and after I turned mine up I can go 75mph in most cases.

I don’t have a tach. so I had to use a gear calculator to figure out my RPM's - I called a local diesel shop and was told that my 5.9 Cummins would be fine to run as much as 3,000 RPM's all day long. So when I (actually Jason did it for me) adjusted my governor I made sure I wasn't going to be over 3,000rpm; and I was just under. I know that’s a round-about way of doing it but my bus doesn’t have a tach and I'm too cheap to buy one.

I might add though I rarely cruise at top speed as it seriously affects my fuel mileage. At 55mph I’ve gotten as much as 12mpg at 65mph I get just shy of 10mpg and at top speed (somewhere around 70-77 – depending on wind, hills ect...) I only get about 6.5mpg or so.

Also my speed-o is off in my bus, so you may want to check that out as well. If you know someone with a GPS – use it to get your actual speed.

Here is a link to that gear calculator.

http://www.idavette.net/tech/ratioc.htm
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