Axles

Expense is number one, the heavier spec axles cost more so if your freight doesn't need heavy axles then a fleet won't spec them. Ride comfort and stability are also affected, heavier axles have stiffer suspension. Maintenance expense is another factor, heavier axles have more expensive parts. On an individual basis it may not sound like much but multiply the cost by three or four thousand trucks and it adds up. Even fuel economy is affected by axle spec, not just the gear ratio but the weight. Some fleets make it a goal to have the lightest weight truck possible, not for more payload but just to save fuel, again scale of economy when looking at several thousand trucks.

 

The newer rigs are often heavier up front and you see more fleet trucks spec’d with heavier steer axles than in years past. My rig has a 14,600 front axle which is standard for an auto hauler. I often run that weight up front. Fleets don’t want to spend a dollar more when they buy thousands of rigs, but an owner operator buying one would be more likely to spec it heavier just in case they need it on occasion.

 

With air ride rear suspension, ride quality shouldn't be affected so much by going to heavier drive axles. Steer axles are still leaf springs for the most part so a heavier steer axle usually means stiffer ride.

Then you get into weight. Once you hit a certain weight rating on the steer axle you have to have a secondary assist of some sort (usually a 2nd power steering box on the passenger side). Heavy duty drive axles have beefier suspension. All this adds weight. They also make axles called Super 40's which are like a hybrid of (I believe) 40k lb rated housings with 46k lb rated gear assemblies.

Not sure where you're seeing 30k rated drives though. I've never heard of them anyways and legal US tandem weights (for the most part) is 34k lbs. Can't see why any manufacturer would build a truck that can't even do the bare minimum. There'd be next to no resale market for it either.

 

Mack offers 46K, 58K and 65K tandems on the drives. If you've got a liftable 3rd axle, you want enough axle weight capacity so that when you get to a jobsite or logging road and need the extra traction on your drives, you can raise that 3rd axle and put the entire weight that was on the 3 axles onto the 2 drive axles without exceeding ratings. Same for those tandem dumps with multiple lifts...want axle ratings high enough to carry the entire load on 3 axles (steer + 2 drives) at the job site, even if you need 3 or 4 lift axles lowered to meet bridge law for axle weights.

 

I pull 140,000 lbs and go off road with it too in to farm yards, fields and gravel roads.
I use to have a highway truck that only had 40,000 lbs rears. When pulling these kinds of weights, the rear end temperatures would go over 300 degrees climbing large hills and if I was on a gravel road long enough. After two years of this kind of work, the bearings gave out inside the differential housing.
The last two trucks I have 46,000 lbs rears. The temperatures never go over 175 degrees, usually under 150. And they have lasted over six years without failure.