Why huge differentials on trucks?

[RalphVa]

Why do pickup trucks have such huge differentials?

The ones on Subarus are small. The one on our Miata is small, and its final drive ratio is 4.1. Same for our 1983 240D Benz. It was basically the same drive with independent rear axles as the Miata and Subaru have.

Why are pickup trucks (and the big truck's) differentials so large?

Ralph

 

[RandyT]

A larger diameter ring gear requires less torque input to rotate than an smaller diameter gear.

 

[Rustyiron]

Ralph my first guess (in a heavy vehicle) would be the horsepower and torque sent through it along with the tens of thousands of pounds resisting these forces are not present in a light car where the " circuit breaker " is the tire simply breaking traction.
Then there's the less stress with a mechanical (think leverage) advantage.

 

[k0ua]

I will guarantee that differentials are engineered to be no larger than they need to be to perform in the worst case scenario the truck is likely to be called to perform. The bean counters will make sure the engineers don't run wild. As pointed out by others trucks often have to move large loads, have "taller" tires and sometimes have dual wheels or even tandem axles. All of this equates to more torque needed to be applied to the propeller shaft. The differential must be able to "hang together" under these circumstances. The Engineers make certain that it will. The bean counters make certain it "just" does that and no more. This applies to every other component of the drive train as well. Enough, and that's all. So to sum up, truck differential are not huge.

 

[the old grind]

How do tire diameters and tow ratings compare between the Miata, the Benz, and our HD or Super Duty pickups, even half ton models?

Like James says, everything proportional and darned little is 'overbuilt' these days ..

 

[ruffdog]

The contact patch between the teeth of the ring gear and pinion increases with the diameter of the pinion and the depth of the gears. Gear oil has its limits on keeping those gear teeth apart so they don't wear down from friction. The weight of the vehicle, tow rating, engine torque, all dictate how large the differential gear contact patch must be so the oil can succeed. If the gears are bigger, the case is bigger too. With a larger load bearing on the gears, there also must be bigger bearings to support the load. All of this makes heat so a large case to hold more cooling oil is needed too. Hypoid gears (off center pinion) means the gears do a bunch of rubbing as they rotate. This is why the anti-friction additives are so important. Hypoids are smooth and quiet, and that is why they use them.

 

[ruffdog]

So why don't they over-build the diffs on all vehicles? Because big diffs cost MPG, especially in cold weather. Big gears and big bearings swimming in thick oil takes some HP to turn. So they come out with better lubes which allows them to go a bit smaller on the diff design. If you try towing 25k up a mountain with 900 ftlbs of diesel torque, all through a Miata diff, the gears will be toast. Why? The oil can't keep 2 tiny gears apart and it will self destruct.

 

[Agvg]

On trucks and larger equipment you usually have this solution, and then you can reduce the size of the differential.

 

[ovrszd]

I dug around in my shop and could only come up with two ring/pinion sets. Sorry. This would have been a great thread to display 3 or 4 different sizes.

First is from a Dana 44 differential. It's a common size in 1/2T equipment and actually some autos. This set is 4.56 ratio. The larger the number, the slower the gear ratio. The larger the number, the smaller (less teeth) the pinion gear has and more teeth the ring gear has.

This set failed. They came out of one of my offroad buggies. I'll let you guys see if you can spot the failure.

I laid the T square in the pic to give an idea of size.

Second is a Dana 30 set. It's commonly found in the front axle of Jeeps and other compact trucks. This set is 3.55 ratio. Notice larger pinion count compared to ring count.

This set was replaced to get a lower ratio, slower vehicle speed, more pulling power.

Here's a comparable pic with both sets. Notice how much larger the 44 is over the 30.

My Son's run Dana 60 axles. Their ring/pinions sets are gigantic compared to these sets. Dana 60 size is what's common in 3/4T or 1T trucks.

Over the road trucks have ring/pinion sets that would make these look tiny.

Hope this helps explain the differences in strength that larger axles have. Wish I had gears from one of the vehicles you listed to show how tiny they are.

 

[k0ua]

I see some "pieces parts" (teeth) chipped on the ring gear of that Dana 44