General Axle Discussion (Jeep Axles 101)
*Note : This e-mail will probably make more sense to you knowing that
Brent's and Todd's Jeeps are Yjs ('91 and '89, respectively) and Kerry's
is a 97 TJ.
I'm copying Brent and Todd on this e-mail in case they were curious too...
I was busy with family and axles over x-mas vacation or I would've called.
I'll probably be up again the weekend after next. I'll call then, I
promise ;-)
You might wanna print this thing out and read it later--it's really long.
Kerry,
I'm 90% sure that you've got a Dana 35-C rear end (You can get the 4.0
litres with a Dana 44 now, but I don't think the D44 was an option in the
TJs until '98 or '99) and I know for sure you have a standard-rotation
Dana 30 in front.
Brent's got a Dana 35-C basically identical to yours in the rear end.
Todd and I have regular Dana 35 rear ends and Brent, Todd, and I all have
identical Dana 30 reverse-rotation front ends.
The rear ends in all of our Jeeps are all about the same strength
(marginal with 33s, open diff and stock gears) but Todd's and my Dana 35s
are considered slightly more desirable to the Jeeping crowd because they
don't use C-clip axles. The ONLY advantage that the regular D35 has over
the D35-C is that IF an axle ever breaks, you can still throw it in 4HI
and limp home. When a C-clip axle breaks, the wheel, tire, drum and
whatever's left of the axle are absolutely free to part company with the
Jeep (quite spectacularly if this happens at freeway speed). Not good.
Your front end is slightly stronger than the ones in our Jeeps (probably
fine with 33s and stock gears and an open diff unless you really abuse the
thing rock crawling) because it has beefier 297-X Spicer U-joints in the
axles (same size as the U-joints in a 1/2-ton D44 like the one I'm
rebuilding). Our front ends use the smaller (weaker) 260-X Spicer
U-joints. However, the ring-and-pinion assemblies in our front ends are
slightly stronger than yours when going in a forward gear because the
pinion teeth are pushing on the strong side of the ring teeth while going
forward. However, this advantage means nothing when you're going in
reverse because our reverse-rotation diffs are pushing on the weak side of
the ring teeth while your diff is pushing on the stronger side... Another
advantage is that, on the reverse rotation diffs on our Jeeps, the pinion
contacts the ring at a higher point on the diff housing. This helps keep
the U-joints on the front driveshaft at happier operating angles, as well
as keeping the driveshaft itself tucked up closer to the frame rails and
out of harms way.
This is a doubly important consideration for your particular vehicle,
because it has no axle disconnect like our front axles do. The upshot of
this is that your front driveshaft is ALWAYS rotating, regardless of what
gear your transfer case is in. This a good thing because it does away
with the spooky, vacuum actuated disconnects on previous Dana 30s (like
ours). But it's also a bad thing because it costs TJs a bit of gas mileage
(but not much--the TJ axle offsets this by employing extremely
low-friction bearings in it's front end). It also exacts a price in the
form of extra wear and tear on your front axle assemblies and
differential. But most of all it's a bad thing because you have to be
VERY careful about front driveshaft angles when lifting a TJ. If angles
are off, (even a little) it'll shake like a wayward whore on a Saturday
night when you get up to freeway speeds. YJs (like ours) are much more
tolerant, because the highest speed the driveshaft will ever see is only
about 40 or 50 MPH because it only rotates when the transfer case is in
4wd (and technically, we shouldn't be going any faster than that in 4HI
;-)
another thing to keep in mind is that when you re-gear, you're driveshafts
will be spinning faster. So, in a Jeep equipped with 4.10 gears,
driveshaft vibrations that were previously noticeable only at speeds above
100 MPH are now noticeable at 75 MPH because the driveshaft (both
driveshafts in your TJ's case) will be spinning 25% faster. (More on this
later.)
Jeep actually used a reverse-rotation Dana 30 with the larger 297-X
U-joints for only one year--1995 and these axle assemblies had the best of
both worlds: reverse-rotation diffs and big U-joints. Brent and Todd
could buy 1995 axles and duplicate this setup for a little extra beef if
they wanted, but you already have the bigger U-joints, so that doesn't do
you any good.
With that said, you pays your money and your takes your chances. In your
case, dumping money into gears for your current axles could go down the
drain if you keep snapping axles or strip a ring-and-pinion assembly.
Keep in mind that this (probably) wouldn't happen in normal every day
driving, it'd happen when you're stuck in a foot of gumbo mud or climbing
a hill or otherwise beating on your Jeep. Only you can decide whether to
spend money now or possibly have to spend it again down the road.
What you have to realize is that shorter gears (higher, numerically) will
multiply the stress on your entire axle assembly by whatever percentage
your ratio increases. For example going from your stock 3.07 to the 4.10
ratio I mentioned earlier (the maximum I'd recommend for such small axles)
is approximately a 25% increase of the torque seen by your axle shafts and
your R&Ps as long as you still have open diffs. This 25% increase may not
seem like much, but in a marginal strength situation (like with the Dana
35 rear and 33-inch tires) this can mean the difference between an axle
shaft twisting harmlessly and an axle shaft snapping like a twig.
A locker compounds this further by additionally DOUBLING the maximum
possible torque seen by your axle shafts. So by adding lockers to the
example mentioned above (a Dana so and 35 with 4.10s 33-inch tires) your
axles would be seeing 250% more torque than a stock rig. Driven
carefully, with a light right foot or a low-torque 4-banger you can get
away with this setup, but I wouldn't recommended it on any of our Jeeps.
Think of it this way: The engine in your Jeep maxes out at approximately
220 lbs.-ft of torque. Take that figure and multiply it by 3.93 (first
gear ratio), 2.72 (transfer case LO ratio) and 3.07 (stock R&P ratio) and
then divide it by two (an open differential splits the torque seen by
either individual axle so it will never be higher than 50%--both tires on
dry pavement). This gives you a maximum of 3609 lbs.-ft. It sounds like
a lot, but in the real world your axles will probably never see anything
near this.
Now take the same engine (220 lbs.-ft) same first gear and T-C ratios
(3.93 and 2.72) and then multiply by the new R&P ratio (4.10) and then
DON'T divide it in two because a locker will ensure that each axle sees
the maximum available torque flowing into your diffs and you end up with
9462 lbs.-ft! Almost three times the amount that the axles see in the
stock setup. Once again, they'll never see this kinda torque unless you
park against a brick wall on dry pavement, throw her in 4LO first gear,
rev the engine up to four grand and pop the clutch, but you get my point.
The axles (or something else--the wall perhaps;-) has to break.
Of course, these figures are also heavily affected by the following
mitigating factors: The possible amount of torque seen by your axles is
actually going to be much less than the maximum possible amount in 4LO
because the engine is driving both the front and rear axles, which absorbs
a lot of power. On the other hand, larger, heavier tires will increase
the amount of torque seen by the axles substantially because of leverage
and their much greater weight.
Well, now you're probably sorry you asked. I guess I got a little carried
away. I must have diarrhea of the word processor today...
I should really get to work now.
Later,
Jake
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