|
DRIVETRAIN ISSUES: A Treatise On Rear Axle
Problems and
Selection
By Sandy
Mandel
If your
rear axle seems to be making many unpleasant noises, it has a lot of
mileage on it and probably needs work. What are your options and what
should you be concerned with?
Before
doing anything, you should make sure that the noises are really coming
from the axle. Noises in the drive line are very tricky and hairy to solve
as all the metal parts transmit noise all up and down its length. What
sounds like a driveline noise, may, in fact, be coming from the brakes,
transmission, U joints, etc. If all is well in these areas, then these
are the usual options:
Some Common
Types of Differentials:
Posi:
Positive Traction Differential. Typically a clutch and spring differential
which resists an extreme difference in wheel rotation speed, but, allows
enough slippage to provide differentiation around turns smoothly. Good
for both street and track applications.
Limited
Slip: See Posi.
PosiTraction: See Limited Slip and Posi
Limited
Slip Torsen: Provides continuously variable torque control between both
wheel and allows up to 4 time rotational differential speeds between the
wheels while going around turns. As soon as the car finishes rounding a
turn, torque from the inside wheel is smoothly transferred to the outer
wheel. The Torsen uses ‘invex’ worm type gears to provide limited slip
rather than clutches. Especially useful on wet roads. Great for street,
somewhat fragile for track use.
Spool
Differential: A type of ‘Locking’ differential. For straight line only.
Detroit
Locker or Locker: A series of dog type ratchet system which when going
straight, physically locks the rear wheels together more so than a clutch
and spring system which winds up having almost a straight through axle. As
soon as the car rounds a turn, the force of differentiation, breaks the
ratchets free then relocks after round the turn. Great for track use,
uncomfortable for street use.
1. Get a
used axle with same ratio and posi. The problem is, there are very few
axles available with less than 75Kmiles for under $500.00 and they are
mostly 2.73:1 ratios. This is also a very ‘high wear’ part. These things
are usually abused by kids doing 4,000 rpm dropped shifts. These are also
NOT high strength Dana or Ford 9 inchers or even a GM 8.5 incher that
could take the guff. This axle was a ‘point price’ unit that the bean
counters - Sorry Jeff G. ;-) - helped with the engineering. We all know
what that means.... Also, the axle was made for the cars’ space
limitations as the axle has a lot of vertical travel to absorb bumps.
Unlike a more space saving independent system, a live axle requires a lot
of room and can’t be too large.
2. Have the
axle rebuilt by someone who knows what they are doing. Around Connecticut,
there is: Bill Mitchell=Special Vehicle Development in Cheshire CT. He’s
an ex GM Engineer who also races and he’d be happy to rebuild it to a
3.73:1, but, it was going to cost me about $2,250.00 with taxes. The new
parts are real pricey and the labor was also expensive, but, it’d be done
perfectly. There is also Reeves Calloway, but, I’ve not personally dealt
with him. In PA, there is Art Houser's Rear End Service
www.rearman.com
.
3. Get a
4rth gen axle. Unfortunately, all the mounting brackets are different and
the whole unit is 4” wider than the 3rd gen unit. A proportioning valve
also needs to be installed in line with the master cylinder to make the
rear brakes function properly. This is now looking like complex and
expensive surgery if you want the car to have a stock track width. The
axle may only be a few hundred, but, the time and expense to get it in
correctly may not be worth while.
4. Have the
dealer rebuild the axle. This is expensive and not all dealerships have
people well enough versed in axle rebuilding as most cars today are FWD.
The entire transaxle is usually replaced as a unit. Also, the ring and
pinion gears aren’t normally on a ‘bi-level’ relationship to each other.
This is done with rear axles to keep the driveshaft low to reduce
driveshaft tunnel height. This makes it very tricky to align the gears
properly. It’s almost a lost art, and very few mechanics these days have
the experience or the patience to do this kind of work.
5. Get a
rear axle as a complete assembly with all new parts from some dedicated
service. This would include a completely refurbished axle housing and all
new, stronger than OEM innards for a really good price. All Timken
bearings, Federal Mogul seals, 7 5/8” OEM ring & pinion - good for up to
about 350 HP - and Moser Eng. high strength race quality axle and hubs.
Total cost, $1370.00 plus differential choice and shipping for a ‘bolt in’
turn key assembly. (See again
www.rearman.com .)
Should I
change my ratio from stock?
If you have
2.73s, most definitely, yes. What to go to? If you want tremendous
acceleration but still be streetable and have a manual transmission with
overdrive, 3.73:1 should be the maximum, whether for a 305 or 350 engine.
Just make sure you have the 5,500 RPM camshaft because you’ll be using all
of your available rpms. 1rst will be a creeper, and all other gears will
be moved down about 1 gear range. Like, 5th will be where 4rth was, 4rth
will be where 3rd was, et. The engine will run about 600 rpm higher in all
gear ranges. 5,000 rpm will still get you to 150 MPH in 5th. Gas mileage
may suffer or stay the same depending on driving stile. Starting up from a
dead stop in 2nd will be a breeze. Especially with a 5.7L TPI.
If you have
an automatic 4 speed, go with a max of 3.42:1 as the torque converter in
the auto multiplies torque and you’ll still be able to keep some semblance
of gas mileage.
What kind
of differential should I go with?
This
depends on your application. If you’re drag racing all the time, you
probably want the Auburn posi. They go for about $280-$400 depending on
application and where you buy it. If you buy from the dealership, a
differential will already be bolted to the ring gear. This pinion, ring
gear and attached differential carrier is called a ‘chuck.’ This is the
only way these parts are purchased from the dealer. This parts are not
sold individually. The differential carrier can be either be a posi or an
open carrier. The dealership can supply any ratio chuck you desire. But,
you’ll pay through the nose. Make sure you ask for your bar of soap and
your kiss before hand, as they always seem to forget that after they take
the money.
If you want
maximum traction, especially in the wet, and want the best traction device
possible, the Zexel Torsen is superb. It has all the best of both worlds.
It has the smoothness of an ‘open carrier’ with more ‘locking’ ability
than a standard posi, with completely variable torque division between the
back wheels. Instead of over coming ‘stiction’ of the clutches, which are
not linear in torque transfer between the rear wheels, the Torsen provides
smooth, variable torque to the wheel that still has traction. It works
with the ‘worm gear’ principal, in that, you can turn the worm gear itself
but the worm gear can not be driven because the gear pitch is too
perpendicular. This is the same action that goes on inside this type of
diff. The torque is instantly transferred because the wheel that’s loose
is now working against the worm connected to the wheel with traction;
frictional resistance of pitch. Bias transfer ratio is adjustable by
varying the amount of gear tooth pitch of the worm gears. It’s a very
complicated mathematical issue. Lots of Cos ß’s and Cos <theta>‘s. There
is a link provided on the site to explain the Torsen operation. It’s quite
unique.
The Torsen
is used in such cars as BMW, Lexus, Nissan, HummVee, Subaru, Audi Quatro,
et. The Torsen is used in many of these cars. In 4 wheel drive
applications, it’s mounted between the front and rear axles. It’s also
used as a front wheel drive posi traction device.
Its main
failing is that the Torsen is torque limited and care must be made when
selecting for high HP applications - 400 HP and above. When a Torsen
breaks, in simply turns into an ‘open’ differential. At least if you have
to replace it, its pretty simple for a competent mechanic to do, as the
ring an pinion gear lashes don’t have to be reset.
If I change
my ratio, will it affect my speedometer and odometer? What can I do about
it?
Yes, and
your speedo can be re calibrated. It’s expensive to do a direct conversion
because the speedo drive gear is mounted on the output shaft of the
transmission. This requires taking apart the tranny to change the ‘drive’
gear. There is a much easier solution. A company in California makes a
mechanical ratio re calibration box that mounts in line with the speedo
cable. It’s mountable at either the transmission or on the back of the
speedo.
You have to
remember that if you go from a 2.73:1 to a 3.73:1, that is a 36.6% change.
This will make your speedo read about 10mph faster than actual road speed.
The best and easiest way to determine this is to go out onto the highway,
look for the mileage markers on the side of the road. Note your odo
reading as the first marker passes by the front of your hood. Then note it
again as the following marker passes your hood. Deduct one reading from
the other. In the case of this kind of ratio change, one mile will be
about 1.15 miles on the difference. This is about a 22-23% change. You’ll
need to do this with your car to get an accurate re calibration. The name
of the company is:
http://www.speedometershop.com
The cost of
the unit is a reasonable $61.00 + shipping, and they take major credit
cards. Ask for Carl.
This is
pretty important if your speedo is an Carter Admin. 85 mph unit. It’ll
loose about 10 mph of usefulness without re calibration.
Also, if
your car is equipped with cruise control, it probably makes the most sense
to connect the ratio converter to the transmission side of the speedo
cable, as there may be two cables, one going to the cruise control
controller and a cable running from the cruise controller to the
speedometer gage head.
If I do go
with a whole axle assembly, where can I get one?
There is
really only one place; Art Houser’s Rear End Services, or AHRES for short,
or simply, The Rearman. One problem is getting a good rear axle housing
that’s been fully inspected, as GM probably doesn’t stock these anymore,
and if they do, they probably cost a small fortune.
Advantages:
• The Axle
Housing:
You can get a fully inspected, degreased, sandblasted axle housing that
is painted with rust resistant primer and a black epoxy gloss type paint.
It is magnifluxed for cracks and reinforced welded in known weak areas.
The connection mounting brackets and plates appear to be from a type of
carbon steel, rather than cast.
• The
innards:
All the best. For standard high performance street applications under 400
HP: OEM ring and pinion, Auburn, Torsen, et. differential, Timken
bearings, Federal Mogul seals and Moser Engineering high strength steel
axles and Hub spools. Also included is a quality lubricant, all the wheel
lug studs - high strength - and a high strength driveshaft collar yoke.
AHRES has
been lashing and patterning rear axles since 1973 and does about 4,000 per
year. If you do that many, how can you not be good at it?
• Pricing:
A typical 3rd gen axle with the above specifications will run about
$1770.00 plus shipping to your area. It will take about 3-5 hours to
install at your local garage if you don’t do it your self. It does weigh
in at about 200-250 lbs, so its’ major surgery. Your brake callipers will
need to be transferred. So, total cost should be in the $2020.00 area.
Again, a lot of money, but, if you can install it your self, you’ll save a
lot.
• Available
Kinds:
Ford 9”, GM 8.5”, Dana, GM 75/8”, all kinds and custom. Just ask. They
are exceptionally friendly and Arts helper, Yvonne, can help set you up.
Just see their link on the link’s portion of the site.
•
Disadvantages:
Cost. Working with this major part of your car ain’t cheap and can be
more than buying a new transmission when all is said and done; if not
abused, it should last the life of the car.
• Art is
fairly settled in the new building and both are legitimate addresses.
Mail goes to Topton and service/shipping is done at Mertztown (only two
miles away). AHRES also has contracts with local police, fire and
ambulance services. If their vehicles break, Art has to fix them first, so
your project might have to be temporarily shunted to the side. Also,
summer time is car show time and the racing season. So, a delay might
occur due to racers getting their cars ready for the season. They do a lot
of that, and they can get really loaded in the summer. Recommend that the
winter might be the best time as they might not be quite so jammed.
• Payment:
AHRES does do layaways and they accept checks but no credit cards.
Fortunately, they are honest and provide a very high quality product. They
are very well known in the industry.
What kind
of unexpected things can happen when changing the rear axle in this way?
Some of the
brake caliper holes might not have threads. Your local shop can easily
helicoil the holes as it’s not that big a deal. Your brake lines might be
corroded and have to be replaced. Your calipers or drum brakes might not
be in the best shape and need R/R. It’s also a good idea to check out your
universals and motor/tranny mounts, just in case. It’s always a good idea
to catch problems before they occur down the road. Also, there may be the
usual damaged rubber mounts, bushings, et.
What if I
have a problem?
One of the
biggest problems is shipping damage. Be sure to inspect the axle or have
your mechanic inspect the axle thoroughly for shipping damage. Damage can
occur internally or externally. Any damage should be photographed and the
shipper needs to be informed immediately.
In some
ways, you’re kind of on your own, unless you can get the axle back to it’s
builder. If it’s something like a leaky seal, you can either bring/send
the axle back to AHRES or they will send you new parts under a 1 year
warranty. If you can do the work yourself, that’s fine. But, a lot of us
would have to have this professionally installed. Art will gladly send the
new part gratis, but, if the shop you take it to messes up, its now your
problem. Unfortunately, I’m in the process of getting a leaky pinion seal
resolved. The original seal was leaking and Art and Yvonne were kind
enough to not only send me a new seal, but also pay for the first labor to
have it installed. This is an area of the car I seem to have problems
with. AHRES is sending me another new seal gratis, and I will have to see
if the shop who installed the last seal will reinstall the new one under
their warranty. If not, I will probably just go to the dealer and have
them install one of their seals so I can at least get warranty on it.
Bringing in someone else’s part to a dealership allows them to finger any
blame for repair induced problems elsewhere. Unfortunately, this is one of
the risks of buying a large, complex to assemble subsystem which carries
fluids. At some point in the future, I hope to have this problem resolved.
Seal
Addendum: After placing a bead of RTV around the inside bore to seal edge,
the leak eventually turned into a seepage, then, with the cold weather,
has stopped. I’m hoping this will be the case once warm weather returns
and the car us used more. But, the resultant seepage after RTV application
was almost insignificant, as the car is parked over a slab that shows up
everything.
If you
cannot find your receipt, Art takes will still take care of you. It is
rare to have a problem, but some just have Art look to see if it needs
more oil. They don't want to void the warranty, so they bring it over. (A
little extreme, but it's great to have someone refrain from screwing up
the work.)
What if I
numerically increase my gearing (shorter gears) and I get a ‘thrumming or
drumming’ vibration coming up from the floor boards at highway speeds
(55-80 mph) like a tire is going over a ‘highway wakeup rub strip’?
Most
F-bodies come with a steel driveshaft, which for gearing at or below
3.42:1 are fine. Aluminum driveshafts were a special order item available
from the factory. The differences are that a steel shaft is stronger and
heavier while a aluminum shaft is about 5-6 lbs lighter but not as impact
resistant. If you’re trying to get after that extra 1/100th of a second
time in bracket or road/track racing, going to a aluminum shaft for the
street really makes no sense. As to balance, a steel shaft can be balanced
as well as a aluminum shaft. Any good driveshaft specialty shop can not
only balance your shaft but check it for: twist, bow, fractures, etc. The
strongest most economical shaft is one made of seamless DOM steel tubing.
Sound deadening material is added to limit tube ringing and resonance. It
can be high speed balanced and is suitable for drag racing.
For axle
ratios of 3.73 and higher. At this point, you may experience some
vibrations at the transmission slip yoke area where both the transmission
and driveshaft can vibrate regardless of how well balanced a driveshaft
is. To limit these left over vibrations, a special steel only shaft is
available from the factory which incorporates a harmonics balancer on the
slip yoke side. Otherwise, typically 50% of all F-bodies have drive line
vibrations using 3.73:1 or higher gears.
In terms of
cost to power ratio for ‘street’ applications, aluminum driveshafts are a
terrible value. Your better off buying a set of headers or a exhaust cat
back.
Additional
information for those wanting 12 bolt rear axles:
From
Yvonne of AHRES: The first attempts by aftermarket manufacturers to create
a viable 12-bolt rear for these rears failed. We never sold or made
those, but we surely fixed quite a few... Improvements have been made,
and we offer a solid 12-bolt rear option, which allows owners to keep ABS
brakes. For more information, email
houser@rearman.com and/or see
www.rearman.com .
Art
Houser offers 10-bolt rears, 12-bolt rears, and Ford 9-inch conversion
rears. For those of you with 9-bolt rears, parts and service are
available.
|
