Conclusions and Recommendations Based on Two Seasons of Drag Racing With the IRS on 10" ET Streets (Originally Posted on BON and Corral 12/01)
I have to date accumulated 173 runs on my 99 Cobra with 10" ET Streets and 4.10/4.30/4.56 gears and a variety of halfshafts. I probably had another 50 launches on street tires with 3.27 or 4.10 gears before going to the ET Streets.
I received the first aftermarket halfshafts from Raxle and GKN in March of 2000 and ran them until the GKN Stage 3 parts were installed in June of this year.
I run on two local drag strips, both of which usually have poor surfaces unless they lay down a lot of VHT. Normally, my 60' times will only be in the 1.80-1.85 second range. This is at 3000 feet altitude in a heavy convertible with a stock engine.
I broke an outboard stub shaft at run 87. This failure is the only one that really concerns me and it should be taken seriously by anyone with an IRS who runs their car with sticky tires. This failure was at the shoulder of the outboard stub shaft and resulted after accumulating a lot of runs on the car. The part that failed is the same part whether you have a Raxle halfshaft or a GKN, unless it is a Stage 3 GKN part.
This failure of the outboard stub shaft is a safety issue and is very expensive to repair. The wheel hub comes out of the knuckle and the wheel is only held under the car by the brake caliper. The caliper mounts bend on the aluminum knuckle and the knuckle has to be replaced at your expense, which will also include the new outboard CV joint, the tow, and the labor.
If the stub shaft fails at speed, you could have a wreck because the wheel is jammed and locks up under the car. Mine failed at launch at the line, but this failure is preventable as described later.
I have had two CV joint failures, at runs 165 and 167. Both occured in December, immediately after the VHT was put down at my local strips. These were failures of the spider in the inboard CV joint itself. These parts are not changed on the Stage 3 GKN joints. I was lucky, the only real damage was to my pocketbook. With the Locker, I was able to drive the car back to the pits and up on the trailer.
I have not had an intermediate shaft failure of the axle between the two CV joints. These parts are tough as they can be, which is why other parts fail now.
What is unique about my car? There are two main differences: 4.56 gears and a Detroit Locker. The load on the halfshafts is proportional to the gear ratio. A 4.56 gear will apply 4.56/4.10 more load on the halfshafts than a 4.10 will.
As the Locker locks and unlocks, it adds to the fatigue loading on the halfshafts. I didn't even consider this as a contributing factor on the halfshaft loading until I talked to the axle suppliers, who were very knowledgeable about the C-Locker's effect on their standard axles.
I have also kept in contact with other IRS owners who have either very quick normally aspirated cars (high 11s, low 12s) or supercharged cars. They have all had some failures, usually in the spiders, and one input shaft in the differential. No one else has had a outboard stub shaft failure, fortunately.
Here are my conclusions, after three seasons with the IRS, and two seasons with really good parts...
1. If you have a power adder, don't even attempt to run with the IRS. You
don't know
when it is going to break, but it will. Go to a 31-spline live axle minimum.
2. A normally aspirated near stock engine car with 4.56 or lower gears
and a Locker and
slicks is going to be hard on IRS parts. Carry a spare halfshaft. On a
surface good
enough for 1.7x 60', you are going to break something eventually.
3. Normally aspirated cars with 4.10 gears and a clutch type differential
and slicks will
be fine, unless you are running sub 1.80 second 60' times all the time; then
you can
expect to have a problem or two, but certainly not very often.
4. Normally aspirated cars with 4.10 gears, Traction-Loc and drag radials
are going
to be fine with aftermarket halfshafts for a long time, in my opinion, unless
you run
on a surface good enough for 1.7x 60' times all the time. Then, if you
have a light
coupe, you might still be OK. A heavy 'vert may or may not last as long.
5. This is the most important thing I can pass on to everyone. You MUST
inspect the
outboard stub shafts every 50 runs and replace the outboard CVJoints if
you see cracks
at the radius between the shoulder and the splines!! If you don't, you could
have the
same failure I had. I don't want that to happen to anyone.
Here is a picture of what this looks like. This picture is of
the stub shaft from
the OTHER side of the car from the one that failed completely. This was
at run 91 for
this joint. I replaced it before it failed. You should do the same....
The Stage 3 parts have been modified to increase the life of the outboard
shafts, but we
haven't accumulated enough runs on the parts yet to see how effective the
modifications have been.
I hope it will extend the life to around a 100 runs or so before cracks
start to show up.
We need to accumulate everyone's experience here to see.
The outboard stub shaft size is set by the wheel hub diameter; that is,
by Ford, not by
GKN. The inboard stub shaft diameter is set by the differential used, so
keep those
limitations in mind before you blast GKN.
I am working with GKN to see what can be done to increase the strength
of the CVJ
spiders but I am not sure it is practical. There are some physical constraints
as well as
manufacturing problems in making any changes.
Why this dissertation now from the guy who said he would enjoy the IRS
as long as
possible? I believe several folks will accumulate enough runs on their
cars in 2002 to
duplicate some of my experiences, especially those running heavy cars at
good drag
strips with more power than I have at this altitude. 2002 is going to be
hard on IRS
parts. I just wanted to make sure you knew what the limitations are with
the current
parts. I also wanted to make SURE that no one breaks an OUTBOARD STUB SHAFT!!
Inspect yours!!
(Added by Joe Lynch on New Year's Day, 2002...)
The Stub Shaft checking procedure can be found here.
To put this in perspective: Folks with 28 spline live axles have problems
with the same
gears, tires, and power as I have. You have to spend some money on a live
axle to
make it live under the stress. I am building one now. And, yes, I will
do a page on the
swap for my site.
Joe Lynch, aka El Presidente
*CCL 5/6/04*