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Test Drive
Driveline Vibration Rears Its Ugly Head
With the Jeep sitting on all fours, ready to drive out of
the garage, I felt myself bursting with excitement. A trip
to the bathroom diminished the feeling but I was excited
nonetheless. I carefully drove it onto the driveway and
went backwards and forwards a few times to ensure that the
brakes were in good working order. After one last
inspection I joined our small procession of 4x4s to meet up
with our friends on Blue Mtn. Almost immediately after
getting onto a main street, I noticed a serious driveline
vibration which occurred between 40 to 60 km/h. It was bad
but not bad enough to warrant turning around and heading
back to the shop.
The ride on the street was noticably better and the 6 inches
of total lift didn't seem to make the Jeep handle any worse
than when I had my 2.5" lift. Best of all, the lack of a
front track bar didn't not significantly affect the
handling. I noticed some mild bumpsteer when I braked hard
but it was easily controllable and not much worse than with
my 2.5" lift.
Offroad, it was even better. With the tires aired down to
20 lbs and the front sway bar disconnected, the ride was
absolutely beautiful. The articulation was dramatically
improved over my old 2.5" lift which, in itself, was no
slouch, either. Trail sections which used to cause me to
lift a tire were negotiated with all four tires on the
ground.
Addressing the Driveline Vibration Problem
The next day, after work, I set about to find the source of
the driveline vibration. The first thing I did was to
measure the transfer case output angle and the axle pinion
angle. Sure enough, there was a difference of just over 5
degrees. Certainly enough to cause the vibration.
My transfer case skidplate has taken a lot of damage over the
past three years so I decided I should lower it another inch
which should bring it in line with a "stock" transfer case.
I also obtained and installed a set of rear 5-leaf spring
packs (now I'm running 5-leafs in the front and back). That
improved things so now the rear axle is only pointing 2-3
degrees higher than parallel with the transfer case output.
The driveshaft angle is approx. 20 degrees (I have an MIT
transfer kit so the transfer case is effectively 4 inches
shorter than stock). The vibration is still there but it
has diminished significantly and only occurs around 40 km/h.
Basically, it's livable, although I'll probably go with a CV
joint driveshaft when I swap in a D44 rear axle later this
year.
Bonuses
I noticed a few positive side effects of having the stock
spring packs on my Jeep again:
- With a lift kit, the added arch of the lift springs
usually causes the rear wheels to move backwards when
they are pushed up into the wheel wells because the
springs straighten out under comrpession. This backwards
motion results in contact with the rear, bottom portion
of the fenders, which is why many YJ owners with 33" or
greater tires must trim this area. With the flat, stock
springs, the rear wheels actually move a little bit
forward when they move up into the wheel wells so the
available fender clearance is maximized.
- Similarly, because the springs are at their maximum
length (as measured between the spring eyes) when the
vehicle is on level ground, the shackles will not move
any farther outwards when the suspension cycles. The
result is that you don't have to worry about the rear
shackles' movement being hindered by the frame
crossmember because they won't be going any farther back
when the suspension is being maxed out.
- Removing and re-installing the sway bar links at the
beginning and end of trail runs used to be a hassle
because it was often difficult to level the front axle so
that the bar could be re-attached. Now, because the
springs are so soft, all it takes is one person to
lightly jump up and down on my front bumper in order for
me to align the link with the sway bar ends.
- Thanks to the added articulation at the rear, it's not
as frequently necessary to disconnect the sway bar as before.
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