"Weak" axles

OK some discussion has taken place regarding the axles in these things, and their survivability behind more powerful engines. Let's take a look.

The front axle is rated for 935 lb-ft of torque.
The rear axle is rated for 1,450 lb-ft.

Divide these ratings by the combined low gears of the transmission and transfer case, to get a worst-case value. Transmission 1st [6.398] X transfer case low range [1.96] gives us a multiplication factor of 12.54. This 12.54 x axle gear ration =
crawl ratio, or 73.6098:1, but that's not important in this discussion.

Then, the stock engine is rated at 191 net lb-ft of torque. So in high range (Transfer case) and 4th gear, the axles would see a total of 191 lb-ft of torque. Child's play. Now, multiply the 191 by the factor above [12.54], and we see that the axles then receive 2,395.14 lb-ft. WOW! If we suspect that the front axle will do 39% of the work, and the rear does 61%, we see that the front receives 934 lb-ft (rated at 935) and the rear receives 1,461 lb-ft (rated at 1,450), and we clearly see that the axles are right up against their design limits in low-low, with the original engine (when operated at the most extreme end of the vehicle's capabilities)! Then you get into the situations of shock loading where spinning tires suddenly find traction? Ugliness may soon follow.

Naturally, there are other factors to consider. The degree of "go-pedal" used, state of tune of the engine, and perhaps more importantly, does one ever stress the truck to the point of stalling the engine in low-low at WOT? I did in a truck pull, may not again after seeing these numbers!

Is it any wonder that these axles CAN fail when the truck is repowered with an engine that produces two to three times the torque of the 1bbl Tornado?

Now, I'm a big fan of these trucks. I applaud those who wheel them, resore them and modify them. I'm NOT slamming these trucks. At all. This is just a posting of the results of my study of the manuals here on-line.

Makes a bit of sense now to keep the itty-bity u-joints, huh? Easier and cheaper to change than an axle!

Food for thought.

Bob

Bob, Has it been slow lately.
I don't think that I could think that deep.
redneck1

your scholarship is noteworthy. Thank you for the tutorial. I always wondered what the torque at the axle was on the stockers.
The above illustrates why I'm a fan of the 14bolt as well. Also why I'm spending most of my money on the axles and not the motor.
I'd bust out the Nomex, the stock guys are going to fry you for that one LOL....

Fry ME for reporting what is in the freakin' manual? I think not. I'd think they'll bask in the glow of their wisdom for keeping the Tornado.

Hrmph. Optimist. Could you be so kind as to post the formula to calculate torque at the axle? I know it's in your other post , but I mean in a straight out form. I hate math.

That is a really great post Bump !! Add to that the wheel spin that the open rears allow transfering ALL the power to the wheels with the least amount of traction. Now We put in lockers and a real set of traction tires . Now what follows is even uglier !!! The wheels with the MOST traction MUST turn. BANG !!! Twisted off axle shafts. I've been there and done all that. I've broke NOS shafts in less then 5 minutes. It's no fun being broke . M1028

I'm trying to decide what to make the weak link: the easiest part or the cheapest LOL. I suppose if I make it the easiest it will cost me. If I make it the cheapest (front u-joints I guess) then it will be a pain in the ass. Have seen some very interesting ways to secure a vehicle out here.. and none of them were very safe lol..
Maybe the front driveshaft u-joint, as it is both easy AND cheapish.

anyway post that formula, I want to compare apples. And I feel sort of lazy today too....flu bug.

Engine torqueXtransmission gear ratioXTransfercase ratio=torque applied to axles. pick any gear/transfer case range you please.

This does not take into account parasitic losses. Nor does it specify front-to-rear bias - I just noted the 39%:61% based on torque specs listed in the manual. [front spec/(front spec+rear spec)] and [rear spec/(front spec+rear spec)]

Tacoma, We argue the weak link point alot . IT seems that the only time that something breaks is when you really need it. Two feet deep in mud and a front driveshaft joint lets go, taking a piece of the yoke with it. Or worse yet putting a hole in the oil or tranny pan. That is not my idea of a fun time or a well built truck. M1028

Agreed. Conventional wisdom states that a $12.00 u-joint should be the weak link. I agree with that as long as you have a drive shaft hoop to retain it!

Warn makes front hub fuses, but they tend to 'splode differentials when the let go. Not exacly my idea of a safety device, no?

I look at it as an endless cycle. More tire , more gear, more motor. You should not have any driveline failures. You should always need one of the things in the endless cycle. If you have enough motor and gear to turn the tires , Then you need more tire. If you can't turn the tires , then you need more gear or motor. And it just keeps going and going. I use the three strikes rule. A part that breaks on me three times is getting upgraded, No Exceptions !!! Guys in our Mudbog club that are kind of on a budgit use the tires as a weak link. They run a big tire that is about 75% wore out . That gives them some traction, alot of wheel speed , but very few driveline failures. M1028

That's an interesting take on things. I think I work off a modified or stopped version of that: build for the biggest thing. Like, 60's might be overkill for 31's. But I'm building around 40" sticky pig tires. That requires a few things. Rock sticks, shock loads parts, so I'll need some good strong axles to handle it. Then I'll need some motor to move all that stuff... suspension to get power to the ground... etc. I think out here I'll start on the suspension first, as it can make up for the tire and motor to some extent. After that tire, to keep me off the rocks. Then finally motor to break something so I can start over! LOL..

Tires as a weak link... interesting concept. I think I'll try the driveshaft thing, after weld some 1/4" plate to the oil pan hahaha.

Interesting way to look at it.

Spicer, what have you done to upgrade your axles so that they live behind your 427T?

I know that he has purchased an amount of one (1) complete set of NOS shafts as replacements. Also recall that he is planning on Rockwells. hehehehe. It's all true though.

What is a 14 bolt rated at? What about a open knuckle dana 60? Can I swap in 35 spine axleshafts into my truck?

the 14bolt is I think just a bare touch above the d70. Plus, it has a pinion support bearing. The newer open knuckle 60's have much bigger shafts than the stockers.

So that is for the total axle, not just a per axle shaft #?

Good stuff Robert.

So can I upgrade to stronger shafts on the stock axles? Can I replace the carrier and hubs to get 35 spline axle shafts? Or am I stuck with stock?

Spicer - correct. The ultimate shaft strength is unknown to me, but based on M1028's history, they DO appear to be the weak links, torque-wise. Thanks for the compliment.

Kali - you MAY be able to upgrade carrier, but getting custom shafts made to mate to out oddball hubs could be quite high. Even if our hubs were "standard", getting custom long-side/short-side would be necessary. Prolly cheaper in the long run to swap axles. You'll have to weigh your options and desired results. If the corp 14-bolt is only marginally stronger (I don't know the numbers) you may find it more desireable to jsut keep a spare set of axle shafts with you.

That's what's so crazy about modifying these things, there's no right way or wrong way to do it, just gotta go with your own thoughts. Well, no wrong way except for swapping in 1/2 ton axles, like someone we know!

Newer Dana 70 vs 14 Corp. 14 bolt . They are almost identical in strength . The 70 will give you about 3/4" more clearance under the pumpkin, and there are more gear ratio's available. The 14 bolts are easier to find and alot cheaper. The newer Dana 70's are alot stronger than the original ones that were put in the M715. The old 70's have coarse spline (23) axles , The new ones are 35 spline. The alloy's that the shafts are made from are stronger in in the new ones too. M1028

also worth noting... while researching axle shaft costs (mostly for the little Jeep), I noticed a few sites have comparo's between say, Dana 60 and the 14bolt-- be very careful when reading these comparisons!! A lot of them for some reason compare the D60 to a SEMI-floating, piece of 14bolt. I dont' even consider them 14bolts, but apparently some people confuse the 2.

as Barry said, 14bolts are cheap and easy, and they can be re-configured w/a hub switch. Plus disc swaps are cheapish.