Why huge differentials on trucks?

[4570Man]

I can understand the torque bit, especially for large equipment. However, how do they keep the ratio down to around 4.0 in regular pickups with such huge ring gears? Ratio on the tiny Miata differential is 4.1. Think the ratio in my Tacoma is about 3.6. It revs MUCH lower at speed than the Miata.

Most trucks get those granny gears via the gearbox and/or transfer case, not in the differential.

Ralph

The car is geared a lot higher overall. Compare the trans ratio. A truck with a 4:11 rear end and a 1-1:25 overdrive gear will probably be at 3,000 rpm to maintain 70 mph. I’d be fairly surprised if the car is really got 4-1 gears in the rear end. With such little tires you’d have an effective gear reduction of a lot more than a pickup. It doesn’t make any sense to put 2 or 3 overdrive gears in the trans to make up for a rear end gear that’s too low.

 

[Egon]

Force applied and material strength required to oppose force.

 

[ovrszd]

Good thread. I was reading about Mercedes Benz EV trucks with no transmission or differential because the motor output shaft is at the wheel. I grew up in an area where they used "third members" instead of differentials. Maybe I should have put that in the You Might be Old thread.

Yep, as well as "rear end" and "pumpkin".

 

[oosik]

The first time I peaked under the Taco Wagon( Ram Power Wagon ) I noticed how enormous both ends were. The front & rear are both - Dana Spicer 80 Track Lok. I have the electrically activated locking knob on the lower dash in the cab.

 

[ruffdog]

Trucks have pumpkins....Miatas have small gourds.

 

[ovrszd]

The first time I peaked under the Taco Wagon( Ram Power Wagon ) I noticed how enormous both ends were. The front & rear are both - Dana Spicer 80 Track Lok. I have the electrically activated locking knob on the lower dash in the cab.

Dana 80s are very strong systems. The e-lock system demands strong systems. The rear axle is also equipped with a limit slip for normal driving. A full understanding of the locking system should be applied when using this system. Very easy to damage the powertrain when both axles are locked. The manufacturer added every precaution they could to protect the vehicle from the inexperienced operator. Lockers will only engage when T-case is in 4Lo. Front locker will not engage unless rear locker is engaged.

Very capable system. To quote a saying my Dad used a lot. "What this requires is a high level of maturity".

 

[the old grind]

Miata should have a fraction of the maturity that we who post here represent.

Planetary final drives on axles reduce size & torque demand on pinion and increase what's then delivered to axles by their ratio. Works for tractors, HumVees, and .. mine trucks. Hummers gain ground clearance and can use lighter, faster turning drive components by spur reduction at hubs, IIRC at 2:1 or so.

 

[Jstpssng]

Trucks have pumpkins....Miatas have small gourds.

I thought that you were discussing vehicles, not women.


Oh, wait a minute...

 

[Industrial Toys]

Up until the 90s, I thought all difs were of cast construction. Working in an automotive stamping/welding plant, I saw how they crafted a Dif out of stampings for HONDA and other vehicles.

I always feel rather guilty seeing my 3/4 ton Dif on the hoist at my Mechanics. That thing only transports little old me, 99.999% of the time!

 

[ruffdog]

I thought that you were discussing vehicles, not women.


Oh, wait a minute...

Uh oh.......

 

[LouNY]

I believe that most people would be amazed at how small flimsy looking so many gears are
in transmissions and other locations.
I know the first time I ever saw the cover off a tractor (Farmall 560) I could not believe how small the gears were.
Thin and not very robust looking for the transmission and differential, then you got to the pinion gear that was much more as to what would be expected in a tractor, then the bull gear, that was a gear.
And then to see the design that went into those older drivetrains, simple seeming 5 speed trans with a single reverse, then a mechanical disc type brake mounted on the pinion shaft so that the braking force was multiplied by the pinion / bull gar ratio just as the power was.

 

[Industrial Toys]

Maybe the Gears in my Childhood Meccano set would dwarf those in a HONDA. lol

 

[oosik]

Ahhh.....OVRSZD, that is exactly what my Owners Manual says. I really don't think there will be many times that I'm in 4LO and need to lock both front & rear differentials. At that point a person had better be looking for a friend with a large vehicle or a good dead man. Either to act as a winch point.

From over twenty years of off-roading in AK - perspective is superior to charging forward. In a tough situation, my Taco Wagon @ 7540# could be a tough pull. Lower gears & higher HP are little comfort when the mud is coming in over the window ledge on the drivers door.

Besides - when the vehicle is that deep in the mud - it is NO FUN diving down to get the hook on the end of the winch cable.

 

[RalphVa]

I figured it out. It's the size of the gears, and those Subaru, Benz and Miata differentials aren't so small. First, with more power/torque input, you need to have bigger teeth or to go to higher strength steel. Bigger teeth = bigger ring gears. The Subaru, Benz and Miata differentials appear small because you only see the lower ends of them. The ring gear is there, with smaller teeth than on a truck or pickup, but is mostly above the shaft input from the transmission.

Ralph

 

[masscity]

In the mid 90's we were pouring a basement wall. A rear dumper, a Mack I think was heading to the back of the job in granny gear in peanut butter mud.

There's a big boom and oil spilling out of the transfer case. Dead in the water, I had to bucket the 11 yards so they could tow it back to their yard.

Watching Graveyard Cars, all the Hemi's and maybe the 440 six pack got a Dana 60 in them.