Break in difference

I have always wondered about rigs that leave the factory at nearly a full load, like buses and motorhomes. Most of these are driven from the factory to the dealer or customer and many of the motorhomes have a toad behind them.

As mentioned, its for rear axle gear break-in.

Skipping break-in, which heavy trucks have to do, just means the gears make a slight amount of noise under light load. You probably can't hear in in a semi-tractor, and drivers of straight trucks just get used to it.

You can follow the manufacturer's recommendation for service intervals, weight ratings, break in period, tire pressure... or not.
Your call.

My sample set for my claim though is only 30-40 trucks in the last 25-30 years though.
Some didn't see high miles under my watch, 10-15 of them went over 100k before I dumped them, and a couple over 200k.

However trucks that I was responsible for, working for a large construction company are well in the hundreds or possibly over 1000. Have seen precious few repair invoices for differential problems or failures, save for pinion and axle seals.

^ Becasue you can't load a set of diff gears for breakin like engines are.
Never bought a semi so can't comment on them. But, to your point, vehicle will likely be fine if you tow with it when new.
I've had countless new company trucks. Never paid attention to breakin and several of them have gotten hooked to a trailer well under 500 or 1000miles and driven cross country like a rental. Never any gear issues in those or ANY of the other newer trucks I've had since I started driving.
There was one defective truck that had rear diff problems but it was grossly defective.

Bottom line, differentials are one of the more bulletproof components on vehicles and issues are rare, regardless of use.

ACZL wrote:
Why are rear ends not ready from the get go to haul? If motor and tranny are, rear end should be as well. Just don't understand why the rear has to be "broken in" while the rest not. Should be no different in pick-ups or semis. Both are HD in their own right, but seems from the replies, the back ends aren't wholly robust right away. Can understand taking it easy towing 1st time or so, BUT put a driver in a new semi and his/her foot is right to the floor.

Rear gears need to wear in to match each other. That takes time which you can’t practically do at the factory. The result of not doing it is a noisier (howling) rear end. I doubt a semi operator would know or care about that, however your average pickup truck owner would be back at the dealership complaining.
I see Ben posted similar info as I was writing this.

Have to have an understanding of gear sets and how they work...

They 'marry' themselves to each other during breaking

Design science and manufacturing science has come a long ways and will continue to get better and better...but...they still have to marry themselves to each other

But...there are many factors here...

Like in a perfect world, gear faces do NOT touch each other, metal to metal...the spec'd lube is of high enough film strength and viscosity to keep a film of lube between them and not allow them to touch during all phases of their job...including dynamic loading & shock loads (expected)...in a perfect world

They do touch metal to metal at the extremes of whatever poop situation happens

And the pressures exerted on the lube film does exert the same force on the metal

Risk of not breaking in properly is gear whine...not an absolute that it will happen...just the possibility. Am old and experience enough times trying to help folks who did NOT break in their diff gear sets and their complaints of that whine... Nothing short of changing out the gear set solved it totally....addition of molyb powder knocked one guys down a bit, but it still had some whine to it (regular open diff...molyb is too low in friction to be used in any LS or clutch based diff)

Why are rear ends not ready from the get go to haul? If motor and tranny are, rear end should be as well. Just don't understand why the rear has to be "broken in" while the rest not. Should be no different in pick-ups or semis. Both are HD in their own right, but seems from the replies, the back ends aren't wholly robust right away. Can understand taking it easy towing 1st time or so, BUT put a driver in a new semi and his/her foot is right to the floor.

Most everyone says incl GM, Ford and Ram to run a new diesel at least 500-1,000 miles to break it in before towing.

The Ram/Cummins owners manual provides for full towing capacity for the engine from mile zero. It does prescribe 500 miles break in before towing for the rear end. In my three Ram/Cummins trucks I have found that the engine seems to 'break- in' some around 30,000 miles, a little better fuel mileage and the engine seems to be a little less tight. Then again at around 80,000 miles it seems to 'break- in' some more.

Picked up my new '03 Dodge/Cummins and made a 270 mile trip at 75-80 mph. The rear axle whined after that. Part of the problem was Dodge was shipping trucks with the AAM 11.5" axle with gear lube 1" below the plug hole along with my not reading breakin procedures before leaving the dealership. At 2500 miles I added close to a quart to bring the fluid to the bottom of the hole.
Dodge dealer was going to swap out the rear axle but Star says no. Finally in the 15k miles range (4 months) and towing heavy, the whine went away.
Pretty common issue at that time for those that didn't follow break in recommendations.