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Old 02-03-2013, 12:46 AM   #1
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Upgrading rear axle

Going to put 373 in, replacing all bearings and such....what all is needed to upgrade to 31 spline? I know the axles, anything else need to be changed?
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Old 02-03-2013, 06:49 AM   #2
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You would have to change the rear diff too
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Old 02-03-2013, 06:34 PM   #3
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The side gears, though generally people just swap out the carrier assembly...
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Old 02-03-2013, 07:06 PM   #4
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Originally Posted by cliffyk View Post
The side gears, though generally people just swap out the carrier assembly...
So I can do it without swapping the whole carrier? Is it cost effective to swap the gears instead of the whole carrier? Just wondering, I wasn't planning on this as of yet but I don't want to pay labor more than once to rip the rear end apart
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Old 02-03-2013, 07:33 PM   #5
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Originally Posted by 2002VertGT View Post
So I can do it without swapping the whole carrier? Is it cost effective to swap the gears instead of the whole carrier? Just wondering, I wasn't planning on this as of yet but I don't want to pay labor more than once to rip the rear end apart
I believe the actual carrier is the same on all 8.8" axles and it is just side gears that are needed--but verifiy that before proceeding.

I have to aks why you want to "upgrade" to 31-spline axles? They (the Ford axles) are only 10% stronger than the 28-spline axles.

COT: Continuous output torque rating
MOT: Maximum output torque rating

Ford 8.8 28-spline COT: 1250 lb-ft MOT: 4600 lb-ft
Ford 8.8 31-spline COT: 1360 lb-ft MOT: 5100 lb-ft

31-spline COT = +8.8%
31-spline MOT = +10.8%


The aftermarket axle makers claim the splines on their 28-spline axles are stronger than the stock axles, one maker claims 35% stronger.

That said it is foolish to ever believe anything anyone trying to sell you something says.

Are you planning on a super-build in the future? The 28-spline axles are solid to 380 rwHP¹ or so--and that is only ceiling if you can hook up the the road VERY well...

--------------------------------------------
¹ - Axles and other power transmission components are rated in torque capacity, not HP, so some assumptions have to be made (I selected 2nd gear because it is unlikely one would get a 100% solid hook up in 1st):

If we assume 4.10:1 gears then 4600 lb-ft out would be 4600/4.10 = 1122 lb-ft in to the rear end. Now if we assume 2nd gear (2.0:1) with perfect traction that's 1122/2.0 = 561 lb-ft into the tranny. If we further stipulate that the engine's peak torque happens around 5252 rpm then that would be 561 fwHP, or 476 rwHP with a 15% powertrain loss.

Now that would be pushing the axles right up to their limit, so if we include a 0.8 design buffer that becomes 476 * 0.8 = 382 rwHP--or just about what most people here would consider to be the limit for the stock 28-spline axles.
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Old 02-03-2013, 07:53 PM   #6
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I'm not planning a "super build" persay, but I am getting a super charger. Eventually I will rework the motor so it can handle more but others who have a supercharger on their stock motor with full bolt ons make around 400-430 on a conservative tune. This is my thing. If I'm going to need it, I want to get it done when I put my 373's in there so I don't have to pay someone a second time later to rip the axle apart.

---------- Post added at 08:53 PM ---------- Previous post was at 08:51 PM ----------

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Originally Posted by 2002VertGT View Post
I'm not planning a "super build" persay, but I am getting a super charger. Eventually I will rework the motor so it can handle more but others who have a supercharger on their stock motor with full bolt ons make around 400-430 on a conservative tune. This is my thing. If I'm going to need it, I want to get it done when I put my 373's in there so I don't have to pay someone a second time later to rip the axle apart.
Eventually when I rebuild I will have forged internals so I can turn up the boost, trick flow heads, stage 2 cams...ect..
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Old 02-08-2013, 02:45 AM   #7
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Ok this thread kinda died down....let me restart this... At what HP range is it best to have 31 spline rear end?
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Old 02-08-2013, 05:15 AM   #8
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Originally Posted by cliffyk View Post

Now that would be pushing the axles right up to their limit, so if we include a 0.8 design buffer that becomes 476 * 0.8 = 382 rwHP--or just about what most people here would consider to be the limit for the stock 28-spline axles.
According to this (all other math removed for length purposes) 382 is the limit for stock 28 spline axles. A supercharger would put me between 380 and 430 depending on tune and other supporting mods. Sooooo should I upgrade the axles or not?
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Old 02-08-2013, 05:39 AM   #9
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Just build it now. I snapped a axel and shattered the rings teeth and a few spider gears at the track last summer. All that is needed is a 31 spline diff and axels. That's it. It's simple to do since changing gears you'll already be 99% into doing it anyways. The biggest weak point in the 31 rear end I've seen is the diffs themselves, but they will hold to just about anything you'll give it. I'd suggest just sticking with the 31 trac lok frpp diff. It's cheap as hell, and rebuildable and from what I've read, they can take a beating. I'm doing the exact same thing your doing with prepping for big HP and that's exactly what I did. No need for a new carrier or tubes. Just that internal parts themselves. Obviously new bearings and such.
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Old 02-08-2013, 05:59 AM   #10
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Cliffyk mentioned 28 spline are generally good for up to 380 HP but that torque was the actual measurement to judge by.
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Old 02-08-2013, 06:04 AM   #11
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Lol I wouldn't press your luck. I'm NOWHERE near that number and it looked like a grenade went off in the carrier.
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Old 02-08-2013, 06:12 AM   #12
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I put in 31 spline axles and differential with 4.10's in my mustang and have launched it at 6k with slicks like 100 times and I haven't broke anything. I know people with stock 28 that have broken there axles so many times and they don't have slicks. Get at least the moser 31 spline axles and any Posi differential I got mine from a ford f150 and it fit perfectly. I know people without slicks that have broken so many 28 spline axles at the drag strip I didn't want to worry about it. You should get 33 spline moser axles which have a lifetime warranty if your axles ever break you get new free ones.
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Old 02-08-2013, 06:20 AM   #13
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I was gonna do the 33 spline but the options on diffs are so limited. Especially in the price region. You'd never worry about breaking the axles though. Guess if you really wanted, spools are available everywhere and cheap as dirt.

---------- Post added at 12:20 PM ---------- Previous post was at 12:19 PM ----------

Did you do any work to the tranny or still kickin the stock 3650
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Old 02-08-2013, 07:30 AM   #14
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Just build it now. I snapped a axel and shattered the rings teeth and a few spider gears at the track last summer. All that is needed is a 31 spline diff and axels. That's it. It's simple to do since changing gears you'll already be 99% into doing it anyways. The biggest weak point in the 31 rear end I've seen is the diffs themselves, but they will hold to just about anything you'll give it. I'd suggest just sticking with the 31 trac lok frpp diff. It's cheap as hell, and rebuildable and from what I've read, they can take a beating. I'm doing the exact same thing your doing with prepping for big HP and that's exactly what I did. No need for a new carrier or tubes. Just that internal parts themselves. Obviously new bearings and such.
Your post about that is what got me thinking..
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Old 02-08-2013, 07:50 AM   #15
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I was gonna do the 33 spline but the options on diffs are so limited. Especially in the price region. You'd never worry about breaking the axles though. Guess if you really wanted, spools are available everywhere and cheap as dirt.

---------- Post added at 12:20 PM ---------- Previous post was at 12:19 PM ----------

Did you do any work to the tranny or still kickin the stock 3650
I did fix my synchros cause they were grinding really bad but other than that I got a Valero clutch.
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Old 02-08-2013, 09:20 AM   #16
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Originally Posted by t0t4lbl4ck0ut View Post
Just build it now. I snapped a axel and shattered the rings teeth and a few spider gears at the track last summer. All that is needed is a 31 spline diff and axels. That's it. It's simple to do since changing gears you'll already be 99% into doing it anyways. The biggest weak point in the 31 rear end I've seen is the diffs themselves, but they will hold to just about anything you'll give it. I'd suggest just sticking with the 31 trac lok frpp diff. It's cheap as hell, and rebuildable and from what I've read, they can take a beating. I'm doing the exact same thing your doing with prepping for big HP and that's exactly what I did. No need for a new carrier or tubes. Just that internal parts themselves. Obviously new bearings and such.
I am curious as to how many hard launches before it broke? What gear ratio? I'd like to run some numbers and see what sort of output torque is was carrying when it let go.

Do you have HP and torque number for your engine?
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Old 02-08-2013, 11:50 AM   #17
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Originally Posted by cliffyk View Post

I am curious as to how many hard launches before it broke? What gear ratio? I'd like to run some numbers and see what sort of output torque is was carrying when it let go.

Do you have HP and torque number for your engine?
Cliffy, I just wanna say I applaud your math ability..
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Old 02-08-2013, 12:55 PM   #18
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Just go 31 spline. You can pick up a cobra diff for under $200 now. Pair of strange axles and be done with it. One less thing to worry about.
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Old 02-08-2013, 01:17 PM   #19
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Just go 31 spline. You can pick up a cobra diff for under $200 now. Pair of strange axles and be done with it. One less thing to worry about.
Didn't know cobra diffs were 31 spline....that's what I was thinking when I started it. SC will put me at 400ish and I knew totalblackout had blew up his axle so I figured just do it while its open
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Old 02-08-2013, 01:42 PM   #20
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No I sure don't. It's just a full bolt on, cammed, clutch car. I had 3.90 gears, spec stage 3+ clutch, and 315 555R radials. That was the first time it had been to the track and I launched it at 5k and BOOM.

---------- Post added at 07:39 PM ---------- Previous post was at 07:36 PM ----------

Yea you might as well jut do it now lol spend the cash and get good axles too.

---------- Post added at 07:42 PM ---------- Previous post was at 07:39 PM ----------

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I did fix my synchros cause they were grinding really bad but other than that I got a Valero clutch.
Where did you get your synchros. I didn't wanna spend 800+ on the D&D upgrade kit when it ultimately won't hold the final power I wanna put out.
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Old 02-08-2013, 01:56 PM   #21
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No I sure don't. It's just a full bolt on, cammed, clutch car. I had 3.90 gears, spec stage 3+ clutch, and 315 555R radials. That was the first time it had been to the track and I launched it at 5k and BOOM.

---------- Post added at 07:39 PM ---------- Previous post was at 07:36 PM ----------

Yea you might as well jut do it now lol spend the cash and get good axles too.

---------- Post added at 07:42 PM ---------- Previous post was at 07:39 PM ----------



Where did you get your synchros. I didn't wanna spend 800+ on the D&D upgrade kit when it ultimately won't hold the final power I wanna put out.
Ok so, moser 31 spline axles and frpp diff...what else do I need? I have 373 gears
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Old 02-08-2013, 05:48 PM   #22
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Quote:
Originally Posted by 2002VertGT

Ok so, moser 31 spline axles and frpp diff...what else do I need? I have 373 gears
Depends on how far you wanna go. Would be perfect time for diff cover with a girdle
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Old 02-08-2013, 05:52 PM   #23
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Quote:
Originally Posted by t0t4lbl4ck0ut View Post
No I sure don't. It's just a full bolt on, cammed, clutch car. I had 3.90 gears, spec stage 3+ clutch, and 315 555R radials. That was the first time it had been to the track and I launched it at 5k and BOOM.

---------- Post added at 07:39 PM ---------- Previous post was at 07:36 PM ----------

Yea you might as well jut do it now lol spend the cash and get good axles too.

---------- Post added at 07:42 PM ---------- Previous post was at 07:39 PM ----------



Where did you get your synchros. I didn't wanna spend 800+ on the D&D upgrade kit when it ultimately won't hold the final power I wanna put out.
I got my synchro's here

http://www.pro-forceperformance.com/tr3650.htm
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Old 02-08-2013, 09:56 PM   #24
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Depends on how far you wanna go. Would be perfect time for diff cover with a girdle
Ok what does a diff cover with a girdle do?
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Old 02-09-2013, 03:40 AM   #25
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--------------------------------------------
If we assume 4.10:1 gears then 4600 lb-ft out would be 4600/4.10 = 1122 lb-ft in to the rear end. Now if we assume 2nd gear (2.0:1) with perfect traction that's 1122/2.0 = 561 lb-ft into the tranny. If we further stipulate that the engine's peak torque happens around 5252 rpm then that would be 561 fwHP, or 476 rwHP with a 15% powertrain loss.

Now that would be pushing the axles right up to their limit, so if we include a 0.8 design buffer that becomes 476 * 0.8 = 382 rwHP--or just about what most people here would consider to be the limit for the stock 28-spline axles.
I was lying in bed just now pondering worldly issues and it occurred to me that the numbers I presented above are "subject to misinterpretation and potentially misleading"² because I elected to use 2nd gear (2.0:1) in the example, so as to make the math easier to follow. After 50+ years of working with this stuff it comes so naturally to me that often I do not express it well. So allow me to present a better (I hope) explanation.

===========================================

First let me repeat that power transmission (PT) devices are rated by torque, not horsepower. This is because with PT equipment horsepower is torque (work) applied over time, in most instances revolutions per minute, and without specifying some rpm a gearbox cannot have a "HP rating"¹.

For example let's take a differential rated for 1000 lb-ft maximum input torque and turn that input at 100 rpm. The maximum horsepower (HPmax) rating is therefore 1000 * 100 / 5252 = 19.0 HPmax.

Now lets spin it at 1000 rpm: 1000 * 1000 / 5252 = 190.4 HPmax;

At 5252 rpm it becomes 1000 * 5252 / 5252 = 1000 HPmax;

Same differential, however as you can see it can be rated from 20 to 1000 HPmax depending on how we use it.

So getting back to the numbers I presented above, let's now use 1st gear (3.38:1 for the 3650), the same 4.10:1 final drive ratio, and the same 4600 lb-ft MOT for the 28-spline axle:

4600 / 4.10 = 1122 lb-ft as before, however the lower ratio of 1st gear means that getting 1122 lb-ft into the rear-end will only require 1122 / 3.38 = 332.0 lb-ft from the engine into the transmission. At 5252 engine rpm that is only 332.0 * 5252 /5252, or 332.0 fwHP. Again assuming 15% powertrain loss that is just 282 rwHP.

This explains why t0t4lbl4ck0ut, with what must have been excellent hook up, was able to "grenade" his rear end.

The most used (in Mustangs) T-56 has a 1st gear ratio of 2.66:1, meaning that the HPmax for the 28-spline axle would be 1122 / 2.66 = 422.0 fwHP, or 358 at the wheels. It's all about gear ratios and rpm.

I hope this better explain how horsepower ratings of transmissions, final drives, etc. must be determined?

--------------------------------------------
¹ - In practice it is not unusual for a PT device to have a maximum input rpm rating in addition to a maximum torque rating, which of course does imply a maximum HP rating. A gear box rated for 1000 lb-ft, with a 2400 rpm maximum input speed would have a 1000 * 2400 / 5252 = 457.0 HPmax (rpm-limited) rating.

² - From Bill Clinton's characterization of his prior testimony during the "Lewinsky" impeachment hearings.
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Old 02-09-2013, 09:01 AM   #26
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Quote:
Originally Posted by 2002VertGT

Ok what does a diff cover with a girdle do?
Helps keep the diff cover from shattering on hard launch. I'm pulling my 8.8 again to weld the tubes, add front bracing and one of these covers. I launch on the extreme side of the spectrum and your input shaft or clutch will go before a 31 spline rear.
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Old 02-09-2013, 10:29 AM   #27
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Helps keep the diff cover from shattering on hard launch. I'm pulling my 8.8 again to weld the tubes, add front bracing and one of these covers. I launch on the extreme side of the spectrum and your input shaft or clutch will go before a 31 spline rear.
Ok Looks like I may not be getting the SC...I did find a 94-98 8.8 rear with 373 installed would be as cheap as a gear install and would give me a second axle. Is there any difference between that and the 02 rear ends?
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Old 02-09-2013, 10:37 AM   #28
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It's narrower than yours. It bolts up but I believe it's 1/2 inch shorter on each side.
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Old 02-09-2013, 10:51 PM   #29
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A lot of great knowledge on this thread. Are 4.10 gears discouraged from a car with 28 spline axle and Trac LOC diff? Car will never see a track. Daily driven with a little aggressive attitude from time to time.
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Old 02-09-2013, 10:58 PM   #30
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A lot of great knowledge on this thread. Are 4.10 gears discouraged from a car with 28 spline axle and Trac LOC diff? Car will never see a track. Daily driven with a little aggressive attitude from time to time.
The gear ratio has nothing to do with the strength of the rear end Ur rear end will hold up to about 400-450 rwhp I think
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Old 02-10-2013, 12:24 AM   #31
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The gear ratio has nothing to do with the strength of the rear end Ur rear end will hold up to about 400-450 rwhp I think
Reason I'm going 373 and not 410 is because of the supercharger.

---------- Post added at 01:24 AM ---------- Previous post was at 01:23 AM ----------

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The gear ratio has nothing to do with the strength of the rear end Ur rear end will hold up to about 400-450 rwhp I think
Like tot4lbl4ckout said, his car was just full bolt on and he grenades his rear..
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Old 02-10-2013, 05:28 AM   #32
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I got the 4.10 with the Bama tuner that AM sells. No major modifications to my car. Borla mufflers, K&N CAI, and Ford Racing Traction LOK for the 28 spline. Everything else is cosmetic.
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Old 02-10-2013, 08:43 AM   #33
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The gear ratio has nothing to do with the strength of the rear end Ur rear end will hold up to about 400-450 rwhp I think
I knew I did not explain it well enough. While gear ratio cannot of course affect the strength of the side gears and axles, it does greatly affect the torque that those component will be expected to handle.

Assume an engine that has a peak torque of 300 lb-ft at the flywheel, a 3650 transmission with the 3:38:1 first gear ratio, and 3.73 gears in the rear-end; and ignoring drivetrain loss to make things simple.

This means the side gears and axles¹ will be expected to carry:

300 * 3.38 * 3.73 = 3782 lb-ft

Now let's install 4.10:1 gears, the side gears and axles will now see:

300 * 3.38 * 4.10 = 4157 lb-ft, 10% more torque.

Which not coincidentally is the same percent change as that of the final drive gear ratios 4.10 / 3.73 = 1.10 = 10% more.

------------------------------------------------
¹ - Note that here and in my posts above, I am considering only one side of the differential output, when in theory the output torque is split between the two axles. This is to force a "weak-link" service factor into the equations. In practical use the side gears/axles would only share the load 50/50 if if it were a locked or "spool" differential, and road traction was the same for both tires.

Once one tire breaks loose the other axle will be carrying 90+ percent of the load.
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Old 02-10-2013, 08:55 AM   #34
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Cliffy, I have a question, the math formula you are giving is for stock axels correct? How would I be able to determine how would much torque moser 28 spline forged axels will hold?
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Old 02-10-2013, 10:17 AM   #35
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Cliffy, I have a question, the math formula you are giving is for stock axels correct? How would I be able to determine how would much torque moser 28 spline forged axels will hold?
Depends on how much stronger than stock they claim them to be. Just multiply the 4600 lb-ft MOT (Maximum Output Torque) by that percentage.

I.e. if they claim 25% stronger then 4600 * 1.28 = 5888 lb-ft MOT...
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