Converting an IC FE300 from a single rear axle to a tandem axle setup.

[John W. Miller]

In search of information on converting my 2004 IC FE300 from a single rear axle to a tandem rear axle setup. What IC tandem axle assembly would work the best?

My current rear axle gear ratio is 6:17, but would like to get better top end speed. What would be the better gear ratio to accomplish this project?

The engine is a DT-466, Allison trans, standard school bus tires.

[rossvtaylor]

Are you trying to make a twin-screw tandem, with both axles driven? Or are you looking for a tag axle, for additional weight distribution? What is your goal here with the tandem configuration idea?

[John W. Miller]

Twin screw would work best, this conversion is for going off road camping. Traction is more important than weight distribution.

[Booyah45828]

Search "axle cutoff assembly"

Then you can narrow it down by brakes, hubs, wheels, axle ratios, etc.

Any rear axle that is on a class 8 semi will typically be heavier then your school bus axle, so weight rating isn't something that I would necessarily worry about.

I'm not sure a tandem axle would improve traction if off road. Maybe if you were in sand or something that you needed more surface area, but I don't think this is the route I would go for off road. I'd got with a single axle with a locker, then a powered front axle, over going with a twin screw tandem.

[Sehnsucht]

Better than twin-screw tandems for offroading would be 4WD with power sent to the front axle as well as the rear drive axle. Dual drive axles paired close together don't actually increase off-road performance, and in fact can become worse in off-camber situations because they cannot flex very far out of alignment with one another. With tandem axles, when one side's tire goes up, it'll also lift the frame until the second axle's tire on the same side will be stretched to it's suspension limit until that tire is actually being lifted off the ground. Now you have a free spinning tire in the air and no forward momentum. It would take a highly exotic suspension that would isolate each axle independently and adapt to off-camber by allowing one axle to carry all the weight periodically. Semi trucks adjust their air bags to compensate for their load and road conditions but it only takes almost non-existent ruts to get hung up because there's no way to dump one airbag without the rest. Leaf spring suspension would be even less forgiving.

There's also the issue of gear ratio. Semi truck drive axles have an additional reduction gear in the differential. If you ever look closely, you'll notice the drive shaft doesn't go into the first axle in the center of the differential but is closer to the top and often offset. This provides a gearset which further reduces rotational speed for increased torque. So without further gearing changes elsewhere you'd likely find your top end speed significantly reduced! Big rigs have 10- to 18-speed transmissions which make up for this gearing but without changing out your transmission as well you're mismatching transmission and drive axles.

Large expedition rigs favor 4WD over dual rear drive axles as long as the axles are sufficient for the carrying capacity. There's some pretty cool top-loading axles configurations and couple that with some coil spring suspension for lifted rigs but then you're into purpose built vehicles and undoubtedly far outside to scope of a school bus turned off road RV.

[John W. Miller]

My idea of offroad camping is not extreme 4 wheeling.
I mean poor dirt roads, open fields or campgrounds.
If I was going hardcore terrain, I would just buy a 6x6 military drivetrain. Thanks for your advise.

[rossvtaylor]

I would respectfully suggest that your bus, with the existing dual wheels on a single axle, will get you on any dirt or Forest Service road that you can navigate...given the long wheelbase, the clearance, and the rear end overhang. I'm not poo-pooing your idea, just offering my observations from having driven all of our buses on such roads here in Arizona. The shorter buses, like the 5-7 window IH 3800s will take steep hills and rutted roads like a mule...and the low first gear makes it easy. But I expect that your limiting factors will be the maneuverability and clearance more than traction.

[John W. Miller]

Thank you.

[Sehnsucht]

I agree with RossVTaylor, for what it sounds like you're planning you may be surprised how far a stock bus will go! Of course driving skill factors into the equation and preparedness in case you overestimate your obstacle. Jeep off-roaders (smart and experienced ones at least) know they need to be equipped to rescue themselves but then also know not to wheel alone. You may find yourself alone anyways simply because of the nature of your rig and therefore preparedness is even more important. Roadside service isn't likely to get out to where you are and they'd likely get stuck also.

As for 6x6 military grade, you are correct they are very hardcore but they also have locking differentials and interaxle locks to force power to all wheels uniformly. This would compensate for airborne tires and those I've seen have even less actual suspension travel than what you have now simply because they're so overbuilt. The drawback? They can only go straight forward in this configuration. So I'm thinking your best potential powertrain upgrade would be a rear axle with a locking differential, so that if you do get into a soft soil or off camber situation, at least you can put power to both tires long enough to overcome the obstacle.

[Drew Bru]

Our experience driving for a year and a half on forest service roads and rural dirt roads makes me think you'll be fine with the bus as it is...aside from the top end speed issue.
Just for frame of reference, we've got an 11 window bus and when we put new tires on it we went with a more aggressive tread for the rears (nothing outrageous though). The only time we ever needed to be towed out of a stuck situation was in the storage lot, before we set off on our adventure, and we were in post-winter wet grass. We just spun the tires and dug ourselves in.
We almost exclusively boondocked in Forest Service land or BLM land. Our strategy was usually to find a spot on Campendium or Free Campsites .net and find a good spot to camp for a night or 2 (but sometimes just a couple hours). Then we'd scout around with the truck to find a nice private/remote spot for an extended stay. We drove on some pretty crazy roads into some pretty remote places and more than once someone would ask us "how the hell did you get your bus up here?!!"
Maybe it was a bit of foolhardiness but I think learning the bus' capabilities and scouting ahead gave us the confidence to go where we needed to go and a healthy wariness that kept us out of the places we shouldn't attempt.

[John W. Miller]

Thank you for your opinion.

[John W. Miller]

Thank all of you for your opinions/advice, it is greatly appreciated.
But, I would like to hear from someone that has actually performed this type of conversion.
My bus body/cab will be shortened by approximately 50% in length, this will allow me to haul my H2 over top of the tandem axles setup. This would also allow me to pull a gooseneck trailer with my 32 foot aluminum express cruiser when necessary.

The final product will look like a flat nosed semi tractor with a oversized sleeper. I can go long highway miles, yet be able to handle light off pavement driving also.
The conversion will get done, not sure yet of what type of drive capacity (single or dual drive).

Is there anyone here that has performed this conversion?

[DeMac]

Originally Posted by John W. Miller

Thank all of you for your opinions/advice, it is greatly appreciated.
But, I would like to hear from someone that has actually performed this type of conversion.
My bus body/cab will be shortened by approximately 50% in length, this will allow me to haul my H2 over top of the tandem axles setup. This would also allow me to pull a gooseneck trailer with my 32 foot aluminum express cruiser when necessary.

The final product will look like a flat nosed semi tractor with a oversized sleeper. I can go long highway miles, yet be able to handle light off pavement driving also.
The conversion will get done, not sure yet of what type of drive capacity (single or dual drive).

Is there anyone here that has performed this conversion?

-----------------------
Welcome to the Insane Asylum, John.


This fantasy is not common, here. The variety of opinions, expressed above, originate from skilled fabricators, using constructive theory & hypothesis methods. After reading the analysis offered by the aformentioned replies, I'm inclined to believe that a capable person would not select a front engine flat-nose to perform the desired functions listed in the original post.

Occam's Razor: "the simplest solution is almost always the best.”

Purchace the desired platform.
4x4 Dognose ✅
Tandem RE ✅
Tandem ME ✅
Tandem FE maybe


Drivetrain conversion is task of capability. If we have to ask, we're not. Dreaming is still fun, though, right. ☺

Start a build thread & show us. Anything is possible. I'll subscribe. This is good stuff.

[John W. Miller]

Never said I was a professional, if you cannot have Dreams, why continue along life's pathway....
Thank you for your opinion, but still looking for that person that followed their dreams and have completed this conversion..... I saw a flat nose bus with this vary conversion in real life, it was magnificent. Since then I have seen photos in several other media platforms. I am still researching all other groups online.

Thank you all again for so much valuable advise.

[Sehnsucht]

I can Photoshop it but I'll be honest I don't think anyone here has actually done what you're asking. I've never seen it in real life but I've given it much thought myself and I usually always land at the same conclusion DeMac suggested... Buy the right base vehicle in the first place.

[rossvtaylor]

Hi John, I know you want to hear from someone who's done what you envision and it may seem like everyone here is poo-pooing your idea. We're not a bunch of naysayers and all the advice is meant in positive spirits, really. This group is, by far, the most experienced and helpful bunch of folks with actual bus expertise. It doesn't mean there aren't some really creative builds and mods done and shown here... But almost everything I see here is tried and tested. And if something fails to work well, I've seen admissions of that (unlike the FB groups, where people post all kinds of things without any followup). We look forward to seeing your pics and experience as you do this, but I think you're in untested waters here. Nothing wrong with forging a new path, though!

[John W. Miller]

Thanks for the support, have put out some feelers to photos I have found online that show this conversion, not the ones produced by the manufacturers.
Thx again

[sportyrick]

Start educating yourself on just grabbing a tandem cut off from a road tractor, that's the easiest way. Cut your frame where you see fit (frame height should match) and weld the twin setup in place, even easier if you get the body out of the way. Use a professional welder to do that work but no reason you can't set it up yourself. Go visit some big rig junkyards and look at what is available, tell them your intentions. My rearend came from a twin screw set, I grabbed the rear axle with a 4:33 ratio, it is the same or beefier than your single axle. I had trouble getting the pinion angle right but afterward it has very worked well (there are shims to work out that angle). I am with the rest above, you will find that a sprung twin screw has very limited suspension travel so some high centering will happen BUT an air ride will give you quite a bit extra travel. There are only two things holding you back from you dream, fear of failure and ignorance of the situation, both will be overcome with education, go for it. (your bus can't have juice brakes, that won't work with a twin screw)

[John W. Miller]

THANK YOU SO MUCH.... I have air ride suspension already and air brakes. I will need to talk gear ratios with you later. The other topic in my research is DT-466 engine mods for adjusting HP and fuel mapping to control speed and torque.
Thank you again, cannot fail if you do not try.....

[sportyrick]

Some of the other guys will have to pipe in because you have a DT466E and it's out of my realm of knowledge, mine is a manual pump. For reference I did install a larger turbo, removed the pump and had it turned up and pulled the timing up a little, runs much better. I guess you will have to turn yours up electronically, probably install larger injectors and turbo and I don't know about timing. There is plenty of knowledge here about gear ratio's. DT466, 3060 Allison, 4:33 rear ratio, low pro 22.5 tires gives me 70mph @ 1550 rpm, I would go with a 4:50 or higher ratio if I re-geared again, 1550 is a little on the low side but a 466 does have the torque coming in at 1360rpms so I am not out of the torque range. It does drop down to 5th when I throttle it up to pull a hill which works well.
You will be able to sell your air ride because you should be cutting it off to install a twin screw setup. Education! Ask questions to people in the know.