Thinking about upgrading my HMMWV half-shafts

actionpaintball said:

I also recommend centering the front differential and being able to carry a single replacement shaft. All four corners of my Humvee use a standard 12k shaft.

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What's involved in centering the front differential? Do I need a lift?

I understand that theoretically both shafts would then be the same length, but which shafts would fit? guessing rear?
Why wouldn't the manufacturer center the differential from the start? must be a good reason?

1993 said:

What's involved in centering the front differential? Do I need a lift?

I understand that theoretically both shafts would then be the same length, but which shafts would fit? guessing rear?
Why wouldn't the manufacturer center the differential from the start? must be a good reason?

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dont waste the time, effort and money to center your diff, you gain nothing IMO. This is a Hummer H1 thing to do more then anything. Yes....the engineers at AM gen designed it this way for a reason.
but im sure some diff centering expert will disagree...

I was wondering if anyone knew why the diff came off center from the factory.

Find out what THIS guy is using:

(HMMWV vs. Porsche): https://youtu.be/Z0eCNOfj9qg

Sparo2 said:

I was wondering if anyone knew why the diff came off center from the factory.

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I think it has something to do with balancing out the whole engine. Isn't it slightly angled on the frame?

Norm, probably the duramax conversion.

1993 said:

What's involved in centering the front differential? Do I need a lift?

I understand that theoretically both shafts would then be the same length, but which shafts would fit? guessing rear?
Why wouldn't the manufacturer center the differential from the start? must be a good reason?

Click to expand...

Not sure why they offset it, but the centering kit lets you use 2 rear shafts up front. i have 4 10k shafts going into mine now that i did the kit. this also allows you to only carry 1 extra shaft, or 2 if you really push it off road.

Just toss a Autozone half shaft in so you can keep driving until you gather the stuff to do the 12K changeover.
I have been beating the crud out of some with my Duramax conversion, just do not drive over 85 or so or the boots will swell enough to catch on something and tear.
Edit, I have no idea why anyone would recommend replacing the 2.56 gears, That is what makes these conversions so bad ass.
BTW the higher gear ratio puts more torque on the half shafts.

I would rather my half shafts break then my transfer case or transmission. I will not be upgrading my axles at any time because the brakes are just fine.

When we use stiffer axles, failure point moves in towards the transfer case. Building a bigger transfer case will then make the transmission fail.

The axles are the cheapest part of that equation. That is why I will not be upgrading my axles. Opinions vary and I understand that.

I respect everyone’s opinion and this is only my own opinion.

EDIT - centering the diffs just screws drive shaft angle. You are treating a non-issue and potentially creating a worse issue.

Just go to autozone like Mogman suggests. Then rebuild the AMG unit and replace.

It’s been proven time and again that diff centering offer no benefit and only benefits the company selling the kit.

Mogman said:

Edit, I have no idea why anyone would recommend replacing the 2.56 gears, That is what makes these conversions so bad ass.
BTW the higher gear ratio puts more torque on the half shafts.

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I'm not sure if I understand what you are saying. Maybe it's a difference of terminology. I assume this is in reference to the mention of 3.08 geared differentials? The 3.08 is a lower gear ratio than the 2.56 gears. Then you say the higher gear ratio puts more torque on the half shafts and that is true but the higher gear ratio is the 2.56 so changing to the 3.08 would lower the torque put to the half shafts. This kind of contradicts the statement of "I have no idea why anyone would recommend replacing the 2.56 gears".

I would keep the higher gear ratio for street driving if that is the primary use. Change out to the lower gear ratio if off road was the primary use.

Just an FYI, the gear ratio in the differentials is not the actual gear ratio because we have geared hubs to consider as well. The geared hubs have a 1.91 gear ratio and if you multiple the differential ratio by the 1.91 that gives the overall gear ratio. So 2.56 x 1.91 is a 4.89:1 ratio and the 3.08 x 1.91 is a 5.88:1 ratio. The nice part about the geared hubs is it reduces the torque on the half shafts by about 1/2 (about 48% actually).

springer1981 said:

I'm not sure if I understand what you are saying. Maybe it's a difference of terminology. I assume this is in reference to the mention of 3.08 geared differentials? The 3.08 is a lower gear ratio than the 2.56 gears. Then you say the higher gear ratio puts more torque on the half shafts and that is true but the higher gear ratio is the 2.56 so changing to the 3.08 would lower the torque put to the half shafts. This kind of contradicts the statement of "I have no idea why anyone would recommend replacing the 2.56 gears".

I would keep the higher gear ratio for street driving if that is the primary use. Change out to the lower gear ratio if off road was the primary use.

Just an FYI, the gear ratio in the differentials is not the actual gear ratio because we have geared hubs to consider as well. The geared hubs have a 1.91 gear ratio and if you multiple the differential ratio by the 1.91 that gives the overall gear ratio. So 2.56 x 1.91 is a 4.89:1 ratio and the 3.08 x 1.91 is a 5.88:1 ratio. The nice part about the geared hubs is it reduces the torque on the half shafts by about 1/2 (about 48% actually).

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If you have a 3.08 advantage rather than a 2.56 advantage you can transfer more torque to the half shafts, simple math and why it would be considered better for off road,.
A 3.08:1 is a higher RATIO than a 2.56:1, just because the majority of people think 308 is lower than 2.65 does not make it correct and just perpetuates and incorrect understanding of what the word RATIO means.
I think everyone understands the geared hub.
EDIT, I think it all started about 100 years ago when someone wrote the first instructions for an automobile, it most likely said "when going up a hill switch to a lower gear" of course they were talking about the gear designation not the ratio, A transmission in first gear has a much higher RATIO at say 5.32:1 than your regular 4 speed with a 1:1 RATIO 4th gear.

Mogman said:

If you have a 3.08 advantage rather than a 2.56 advantage you can transfer more torque to the half shafts, simple math and why it would be considered better for off road,.
A 3.08:1 is a higher RATIO than a 2.56:1, just because the majority of people think 308 is lower than 2.65 does not make it correct and just perpetuates and incorrect understanding of what the word RATIO means.
I think everyone understands the geared hub.
EDIT, I think it all started about 100 years ago when someone wrote the first instructions for an automobile, it most likely said "when going up a hill switch to a lower gear" of course they were talking about the gear designation not the ratio.

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And that is why I started out with it may just be terminology differences. That said, let's forget about terminology and just use numbers and examples.

Does the 2.56 apply more torque to the half shaft or does the 3.08? I'm just trying to understand your previous statement.

"I have no idea why anyone would recommend replacing the 2.56 gears, That is what makes these conversions so bad ass.
BTW the higher gear ratio puts more torque on the half shafts."

All things being equal except the ratio, I think it would be more likely to snap the half shafts with the 2.56 ratio than the 3.08 ratio. The 2.56 ratio requires more power (torque) to the wheel to get it to turn than the 3.08. The 3.08 would be easier to get the wheel to turn and therefor less stress on the half shaft.

OK so the power to the wheel to move a certain weight vehicle a certain distance is the same AT THE WHEEL, how it gets there can be different, so the amount of torque that must be applied to the half shaft is the same unless you change the ratio of the geared hubs.
Now the ability to deliver a given amount of torque to the half shafts changes with the ratio between the engine and the half shafts, it is easy to understand how you can deliver more torque to the half shaft with the transmission in first gear rather that in say 3rd gear because the torque is being multiplied by the higher drive ratio between the engine and the half shafts, (the amount of turns the engine has to turn to then turn the the half shaft 1 turn is higher)
So all the ratios between the engine and the half shafts effects the amount of AVAILABLE torque to the half shaft, so multiplying the engine/transmission/transfer case by 3.08 rather than 2.56 makes more torque available at the half shafts, but of course it has to all equal out so it takes more torque at the driveshaft to deliver X amount of power to the wheel with a 2.56 differential than if you had a 3.08 differential.
For those of you in Rio Linda torque is the amount of rotating force or "twist" and is usually measured in pounds feet in the USA

Edit, so the amount of torque needed at the half shaft to climb over that rock is the same no matter what is between the engine and the half shaft, so in that respect it matters not what the ratio of the differential is, only one has the ABILITY to deliver more torque with a 308 gear rather than a 2.56 gear so it will be somewhat easier to bust a half shaft, the same concept as it is easier to bust a half shaft in lower speed range rather than higher speed range (again when in "low range" there is a higher ratio between the transmission and the differentials allowing more torque available to everything between the transfer case and the wheel.

Guys, let’s make this simple.

We’ve all rode bicycles with gears.

If you put the bicycle in high gear you are able to maintain faster top speed at the cost of more work in the legs/engine. You lose the ability to push the tires as hard from a standing start. There is no mechanical advantage. There’s more stress on the legs/engine and less on the axles as you push the tires.

If you put the bicycle in low gear, (differential gears over three and four), the engine has a mechanical advantage and can twist the tires harder, placing more stress on the axles.