Posi-Trac Rear End in Bus ?

[Gary & Peggy Stevens]

Ok, this is something I never heard before, till today, so I thought I would put it out there for validity?

Is it true that only ( One ) set ( side ) of the dual drive tires receives power from the differential? The other opposite set just turns freely? I was sure that in a rig this size they would have a posi-trac differential set up, so you got power from both sets of drive tires?

Is this a stupid question? Did everybody else no the answer to this?

Gary S.

[Kenneth Brewer]

No, it is not true, both sides are driven unless an axle broke or something was mis-assembled. Posi-traction is not something commonly available or normally required because there is generally more than enough weight and traction.

The term posi-traction is or was a trademark and copyrighted name; there are other terms, such as limited slip, for one. In these manifestations, there are clutches incorporated that partially disengage or 'slip' in a 'limited' way when one side tries (has to) to turn at a different rate than the other (note the term 'differential'), as happens when negotiating a turn. However, that said, there are axles that will 'lock' on demand - both sides hard linked to each other. There a many designs and applications.

Newells had/may still have this feature, and of course other vehicles such as emergency vehicles such as certain fire trucks and off road vehicles. As I recall, this was done (Newell) with solenoids (electrically). There are a number of heavy drive axle manufacturers. Eaton, Dana, ZF, and so on. Design may be pneumatic or hydraulic, or mechanical (manual). When this feature is available - and engaged, it is only for poor traction situations -gravel, mud, snow. It may not be used on dry pavement because when one side wants to turn at the same revolutions as the other and can't unless there is forced tire slippage. Steering is/becomes very difficult in this circumstance.

This subject can be easily taken into technical sophistication which I don't think is necessary here, but no. Perhaps we should have a seminar on this at the OKC rally. Or you can ask an avid off-roader/4 wheeler.

[Jon Wehrenberg]

While both sets of drive wheels will be driven on the highway, the purpose of a differential is to allow the wheels on each side to travel at different speeds. When turning in a tight circle for example the wheels to the inside of the turn cover a lesser distance than those on the outside of the turn so in that case the wheels stop turning in sync otherwise one set would be skidding.

When on slippery surfaces our differentials will allow the tires with the least traction to spin. A locked differential or a differential that delivers power to the wheels with traction prevents this. As Ken explains there are all sorts of systems to limit wheel slippage and the cars of today with full time all wheel drive employ many variations of traction control.

Unless you plan on going off-road, or driving a lot in the great frozen north our buses are set up just fine. We drove for many years in snow and the bus did OK.

[merle&louise]

Ken & Gary,

All 3 of my Newells had a button on the dash labled "diff lock". When it is engaged (on) the coach drive axles pull together with no slip. I have used this feature twice when I was stuck in mud. The first time I was able to "walk out" of the muddy ground with the diff lock on and both wheels pulling. It is imperative to remember to turn "off" the diff lock after the coach is freed from the mud. The system worked great!

I became stuck a second time (read dumbass) and the use of the diff lock could not get me out of the mud this time. I had to call a wrecker and with his help I was able to get out. My point is the diff lock is a good thing to have, but it has limitations.

I will never get in that situation again. If I am going to park on grass, first I walk out and inspect the ground. If it is not hard sandy earth, I stay on the concrete or asphalt.

[Joe Cannarozzi]

Does a newel have 2 rear-end gears like wanderlodge or 1 and a tag like Prevo.

[Kenneth Brewer]

Does a newel have 2 rear-end gears like wanderlodge or 1 and a tag like Prevo.

Joe,

If you mean two rear diffs (two rear axles), like many trucks, I am not aware any bus or MH that had/has that, foreign or domestic, particularly Birds. Birds did not, they had a drive and tag like Prevosts (for BB pushers, some of the earlier 90s had a single rear axle). The tag wheel looks exactly like the drive axle, except it is/was just a single, just like the Prevo; but the wheel was directly in line with the outside dual in front of it. And yes, wear was more of a problem because of it, so Birds had the lifting tag axle as well.

Interestingly, for a domestic bus, the Eagle had the tag in front of the drive axle. I do not know why, particularly, but this became problematic when going forward over an obstacle in slippery conditions, such as a curb in snow or ice.

If you mean a (one) 2 speed rear axle, again I am not aware of any BB that had that either, unless it was an optional school bus feature for rural use, or one of their city/highway commercial buses. On those I cannot say.

[Gary & Peggy Stevens]

Thanks guys for clearing this up for me. I am not sure why the guy (Prevo Owner) that I met yesterday thought there was only one side of drive wheels, but he thought that. His bus was a 97 or so Marathon.

I just wanted to get the answer from those that know for sure.

Thanks again.

Gary S.

[Kenneth Brewer]

Thanks guys for clearing this up for me. I am not sure why the guy (Prevo Owner) that I met yesterday thought there was only one side of drive wheels, but he thought that. His bus was a 97 or so Marathon.

I just wanted to get the answer from those that know for sure.

Thanks again.

Gary S.

Many people have this misconception. I think it has to do with seeing one wheel spin in slippery conditions (especially on vehicles that have what is called an 'open' differential, although it happens under certain conditions with "posi's") and incorrectly concluding that only one wheel is ever driven. Wrong.

This would be better explained with a model of a differential. If there is sufficient interest, perhaps I may be able to obtain one and I or someone else can explain/demonstrate the principle of the differential at the OKC rally. If there is sufficient interest, time, and permission to do it.

[Jon Wehrenberg]

Ken,

It would be a good round table type discussion for anyone interested. I know some owners have taken their bus in places they should not thinking they can use the weight of the bus to get traction only to find out that when the surface gets slippery, as in mud or snow they are stuck. I do not know of any coaches with any kind of traction control differentials. But with Prevost in Canada it is likely there are some coaches out there with limited slip differentials.

I won't attend the session however, not because I don't like you, but after far too many years dealing with average annual snowfalls of 240 inches I will not be driving on snow covered roads ever again. At this point I don't care if my bus has all wheel drive or two wheel drive.

[Kenneth Brewer]

Jon,

Actually, I was thinking you would do the session; I don't like me. I think I could get the working model if this demo is wanted.