What happened to 2-speed differentials?

[ClarkB]

So back in the day having the option of a 2-speed rear differential in a truck was actually there. I haven't seen it on any light-duty truck made in the past 20 years. So why is this?

Many of the 1/2 ton trucks running around with nary an ounce of cargo in the bed would (possibly) benefit from having a numerically lower rear-end. Maybe offer a ~4.10 for towing and a ~3.55 for regular driving. Is this too expensive or too easy a way to eek out a couple MPG?

Clark

 

[Spitty]

I would guess it's because most, if not all, newer trucks have a larger # of gears in the transmission, including an O/D gear.

 

[Black Bart]

They are now building 6 speed automatics.
It is better to build a 6 or more speed transmission than trying to make a transmission out of the differential.

 

[mrsilv04]

Alot of it probably has to do with the fact that I'm driving a half ton pickup with 300 hp stock. I have a 3.42, and can pull a 6000 pound travel trailer just fine. When unloaded, I can get 21 mpg.

15 years ago, the same engine produced 150-170 hp.

When the horsepower doubles with the same displacement in just 15 years, that's a big change.

 

[ekpolk]

Yeah, I agree. Essentially the same capability is included with five and six speed autos, whereas in comparison, 20-plus years ago, you were lucky to get more than three speeds in the autos, and maybe four in the sticks.

 

[Steve S]

Never seen a 2 speed rear end on a pickup?

 

[Black Bart]

Well you young whipper snappers would not have ever experienced driving a 37 Ford with a Columbia 2 speed rear end.
They had a lever and shirted manually.
The three speed tranny with electric overdrive was popular in the fifties.

 

[ClarkB]

Such an easy answer: better transmissions! Can't believe it never occurred to me before.

Now granted, I still think the "cool factor" is much higher having a three speed transmission and a 2 speed rear end rather than a 6 speed transmission. But that's just me.

Clark

 

[dwendt44]

The 'cool' factor wears off quick if you have to shift a lot.
BTDT.

 

[ikeepmychevytoo]

To be informed about its workings one needs to know theory of multiplication. But without it, counting up to ten is ample expertise.

 

[JakeR22]

They definitally would have been a help before the mid to late 80's when OD trannys came along.

 

[MolaKule]

My uncle had a milk truck (5-ton) that had an electric (solenoid-type) two-speed differential.

I think the reaason for the lack of them today in lighter trucks is that with modern Transfer Cases, you can get "granny low" without a two-speed differentials.

 

[Torino]

Hi Black Bart. Being a VERY old timer I remember those Columbia rear ends. The ones I had used a vacuum cylinder at the diff. with a valve located under the dash. (36 Ford three window). The problems were that they shifted slow and would'nt take a lot of torque. The 36 ended up with a bored and stroked flat head (close to 300 cu. in.) and the Columbia would strip the planetary gears. At that time (middle 50's) they were plentiful and one could be picked up at the yards for $10-$15. Best regards. John--Las Vegas.

 

[Black Bart]

Originally Posted By: MolaKule
My uncle had a milk truck (5-ton) that had an electric (solenoid-type) two-speed differential.

I think the reason for the lack of them today in lighter trucks is that with modern Transfer Cases, you can get "granny low" without a two-speed differentials.
The only light trucks that have a transfer case are 4 X 4 the 2 wheel drives have no transfer case.

Electric 2 speed axles were popular in 1 1/2 and 2 ton trucks but with modern high torque engines and Multi speed transmissions the 2 speed rear end has been nearly forgotten.

Back when the 2 speed rear axle was popular the 2 ton truck only offered a 4 or 5 speed transmission.

Like I said before due to the way it worked making a transmission out of the rear axle was not as good as having it all in the transmission.

 

[JHZR2]

Originally Posted By: ClarkB
Such an easy answer: better transmissions! Can't believe it never occurred to me before.

Now granted, I still think the "cool factor" is much higher having a three speed transmission and a 2 speed rear end rather than a 6 speed transmission. But that's just me.

Clark


Thing is, what has happened really? Is the new 6-sp AT really twice as big as an old 3-sp? Has the packing factor improved? Id venture to guess higher packing density, higher power density, and less capability to shed heat.

Complexity is the name of the game these days. So long as a benefit is derived, all is well. We're talking about AT trucks for the most part anyway, there is space, and there is less worry about heft.

Seems to me that if you have a 2-sp diff with a 3-sp AT, you get 6 speeds. Its taken us a few years to get to that point, compared to the fact that it did exist back in the day, according to the posts in this thread.

However, unless my math is wrong, a 2-sp diff with a 3-sp AT yields 12-speeds. The benefit very well may be more great with todays higher-gear ATs. At some point Im sure there are too many gears, but if its an AT, the thing is doing the work for you, its not like youre a tractor trailer driver, working multiple levers (and how many gears do they have? 18? 20?).

In my limited mindset, I see nothing but benefits. It seems like a technological challenge that ought to be easily handled with todays controls and know-how. Put a 2-sp diff on a toyota sequoia, ford expedition, etc., let alone the pickup trucks, maybe put a small diesel in there, and let the torque do its thing... With a super-economy diff ration, plus a "do it all" hauling ratio, Id guess that the automakers could see a nice jump in CAFE from such a thing. Make it settable as a "tow/haul" mode, and EPA ought to allow it to be tested in normal mode.

For people to have their cake and eart it too, i.e. the concept of safety, better road position, carrying capacity, plus lower fuel utilization, something like this seems like a no brainer...

What am I missing? One could effectively have 10-12 gear ratios today...

JMH

 

[Black Bart]

Don't know where you are getting 12 speeds the most with a 3 speed tranny would be 6 but in most transmissions the ratios overlap.
If the 3 speed was a wide ratio then you could split shift and get 6 but the shifting would be slower than shifting a transmission and due to the way a 2 speed rear axle is designed it just is not a good way of changing gears.

A 5 speed with a 2 speed rear axle would have about 8 progressive gears the other two would be so close that they would be useless.

With some combos 4th high axle is faster than 5th low axle so as you shift up you are actually going into a lower gear.
Kinda hard to understand if you have never drove one of these.

The 3 speed with a 2 speed rear axle would be cheaper to manufacture than a 6 speed transmission. Machining the gears is expensive.
The reason you never see a 2 speed axle in a class 8 truck is it can not handle the torque of those turbo diesels in class 8 trucks.

 

[JHZR2]

No, a 2-speed diff with a modern 6-sp AT would yield 12 ratios. The benefit with modern powerful engines is that potentially the diff ratios could be VERY different.

But the overlap isa good point. Then again, with computer control, one might not ever have to think about it...

JMH

 

[Black Bart]

Originally Posted By: JHZR2
No, a 2-speed diff with a modern 6-sp AT would yield 12 ratios. The benefit with modern powerful engines is that potentially the diff ratios could be VERY different.

But the overlap isa good point. Then again, with computer control, one might not ever have to think about it...

JMH
Yes it would be 12 with a 6 speed but that is not what you posted in the above post.
You stated that a 2speed axle with a 3 speed transmission would yield 12 speeds. Now you have corrected yourself.

The typical 6 speed in a car is close ratio if you had a 2 speed rear axle you would have overlapping gears
Say you where in 4th high axle the next gear up would be 5th low but that would be actually lower so you would be down shifting instead of up shifting.
-

 

[JHZR2]

got it... typo... sorry for the confusion.

JMH

 

[bulwnkl]

I drove 60's and 70's American gasoline-powered trucks for ever on the farm. Dodge, Ford, and Chevy 500s, 600s, and 700s before we switched to old, used OTR diesel tractors that we stretched the frames on. Essentially all our trucks were 5-speed manual transmissions with 2-speed rear ends.

Blackbart is right. Split shifting was very useful to make the most of the fairly low power in the big block truck motors, but the 4-5 spread is normally fairly short such that 4-over is a higher overall ratio than 5-under. You can still work out all the splits to progress from lowest gear to highest, but some of the splits are just a waste they're so close together. It also makes for much more complex shifting, having to back off the throttle to up-shift the 2-speed rear end, and having to blip the throttle to down-shift it. I can't even imagine all the inept city cowboys trying to shift that kind of setup.