The Stovebolt Site Forums The Parking Lot Big Bolts Split shifting

Can someone explain "split shifting" to me with a SM420 4 spd and a 2 spd axle?

I assumed it was 1st gear, axle low then 1st, axle hi
2nd gear low the 2nd hi...

But I got to thinking this can't be correct. The axle split is much larger than this would allow.

I've always had to get under the truck and shift it at the valve, never had the luxury to try it out. Always drove her around in low range on the farm.

I never had that combination. I'm sure someone that has will be jumping in, but I did have 2 trucks with 5 and 2's that had an irregular sequence.

They were,

1 Lo
1 Hi
2 Lo
2 Hi
3 Lo
3 Hi
4 Lo
5 Lo
4 Hi
5 Hi

OK. Running the numbers and it looks like it's as I suspected.

I just kept thinking "4th low" would be lower than "3rd hi".

Axle is 8.1/6.1

Trans:

1st 7.05
2nd 3.58
3rd 1.71
4th 1.0

Rear tires are 900-20 (I think?) Not sure what the OD is.
Engine is a 350ci from a '90 police package Caprice w/Q-jet carb.

I'm not sure what top speed will be. That little sbc likes to rev and has a pretty wide torque band.
I wonder what the mileage will be... 8-10 mpg?

Coyote,

I have driven many rigs equipped with 2 speed rear ends...unless the truck is absolutely overloaded, or you really enjoy shifting, splitting every gear with the rear end isn't necessary.

Save that drill for your brownie! My three cents!

Stuart

Same view here as Atomarc. I put the rear in high and drive it. Without the heavy load, it revs right thru the gears without dragging. Plus it doesn't take long to get to top speed around the city. Stop lights everywhere makes for lots of shifting!

If the mileage matters you may need to make some changes...

Top speed will be better than with a 235, but not much. Those 6.1 gears even with the tall tires mean the motor is spinning pretty fast. Stock the 350 is probably good for 500 to 1000 more RPM than the 235 without strain, but will you want to cruise down the highway at 4,000 RPM? And I'd bet at that speed your mileage will be terrible at best.

I have a car with a 350 and an overdrive. In OD it gets 24 to 26 on the highway running 70 and turning about 2100 RPM. Direct drops the mileage to about 17 and the engine speed to about 3000.

While the car does not directly compare to the truck (effective final drive in OD is 2.0 to 1) I'm using it to illustrate the impact on fuel mileage turning the motor faster at the same speed.

Turning the motor 4000 or so to drive a 5000 pound big box through the air at speed is going to use a lot of fuel! You might want to measure how tall the tires are and then use one of the online RPM calculators to determine how fast it will need to turn to achive your desired speed.

Then consider your options to reduce the speed of the motor once you have achived that speed. It will never get great mileage by today's standards, but I'm sure it can be improved.

I drove a Cornbinder flatbed for a building supply company this summer that had a 5 speed trans and a 2 spd axle, with an inline 6 diesel without a turbo. The guy that taught me to drive it said to do this pattern unless it is absolutely piled high:

1-low
2-low
3-low
4-low
5-low
5-high

Staying in low range helped out driving around in developments, subdivisions, and in the city where the speed limit is 20-35 mph. Once it was out on the open highway, it would go about 55-60 at about 1500 rpm. That keeps you from having to change so many gears.

I ran the numbers real quick, assuming you have 32" OD, you'd get about 60MPH @ 4000 RPM, or 70 @ 4500 RPM. It's definately a more drivable combo than mine which tops out at 56MPH at redline. :p
Get an actual diameter and what RPM range you think it'll handle well and I can give you an estimated speed.

As for shifting, depends on the gearboxes, but the numbers come up with an even 1-lo, 1-hi, 2-lo, 2hi progression. If you have a weak motor and a heavy load it's necessary, but really you could stay in low through 3-low, then shift the rear to be in 3-hi and be OK from there.
Keep in mind too that the read end may have a lot of slop on it and not be real shift freindly. Mine is pretty bad, still solid as a rock, but I'd hate to have to use it regularly. in the meantime it drives just fine in high range.

I agree with 66stepside. My '65 GMC 4000, with 1,000 gallons of water on board (that and the fire body puts us close to Max Gross), does fine with that pattern. So did my '73 Loadstar 1800 dump.

Pulling a steep hill yesterday I had to drop down and split both 3rd and 4th gears, though.

What's the trick to downshifting the axle wihout grinding/clashing?

John

i know with my engine if i just lug fourth high or just before lugging it and shift the axle down to low and just tap the clutch and get off and then back on the gas (at the same time) it shifts smooth. this works well in any gear for me even down to 1st

if im going from 4L to 3H i just shift the axle to high then do a normal downshift and its smooth (up with the one and down with the other) if i do it going either on way up from second and try to hurry it into third then i just catch a short grind because the tranny isnt ready yet(shifting too fast)and visa versus.

my dump works the same way even though its different parts between the gmc and chevy but same principal. because of where i live its a must to split shift on both trucks.

i had to practice to get it down.

John,

Just to add a little extra, hit the button down just before you need it. I've done it a little too fast sometimes and didn't allow it enough time to shift. There is a very small window to both let it shift and eliminating the "grinding" sound. Once it is perfected you will be shifting it with just a little "click".

Also, we have a similar subject going under "Making my 69 Chevy Dump go faster" for some more interesting reading.

Bill

Tire measured about 39" OD so, It looks like it should be turning about 3200rpm at 60mph.
If I take her out to pick up a load or two of rock for the drive at the ranch I'd like to be able to keep up with traffic.

Don't want to get pulled over for goin' to slow.

OK, with a 39" tire I came out with these approximate speeds and RPM's:
Gear RPM Lo Hi
1st @ 4000 8 11
2nd @ 4000 16 21
3rd @ 4000 33 44
4th @ 4000 57 76, or 66 @ 3500

Definately a split shift required when running it's max load capacity and a weaker motor, but thats with an older motor, the one you have is runing at least twice the ponies. With a more modern 350 you should be able to pull a respectable load up through 4 lo then shift it over and ease up to speed.


As for shifting the rear, I have the instructions that were on my visor (just fabricated a new visor and instruction sheet).
Dunno if it will help, I'm still figguring out my truck, but again it's pretty sloppy back there.

TWO SPEED AXLE SHIFT
To shift axle only:
Into Low-
With accelerator down, move lever to low. Ease up accelerator, then press down, at the same time quickly disengage and engage clutch.
Into High-
With accelerator down, move lever to high, remove foot from accelerator, disengage clutch-pause-engage clutch and press down on accelerator.

To shift transmission and axle:
Into high gear and low range-
Ease up on accelerator, shift transmission, using double clutch method, and just before engaging clutch, move lever to low and press down on accelerator.
Into low gear and high range-
With accelerator down, move lever to high, then shift transmission, using double clutch method. Axle shift will take place while transmission is shifted.

1. Always keep accelerator down when shift lever is moved, except when shifting into higher gear and low range.
2. Always park in low range and start in low range (this was printed large and bold).
3. See instructions in owners manual.

Might help, or might confuse the heck out of ya, whatever works!