During the last year and a half since fitting the LSD unit
in my Ford axle I haven't been happy with the way the rear
end of the car was handling. The initial feeling was good
from the axle, but after approximately 4 months the rear
end of the car felt less inclined to turn into corner. I
have a very stiff front end setup (300lb springs and adjustable
anti-roll bar) and this gives a very quick turn-in. The
rear of the car didn't match this and it felt like a slightly
odd oversteer feeling, the front would dive into the bend
but the back felt like it was on tiptoes and didn't want
to follow it around. The result was that I lost confidence
in the way it went around bends and so set about trying
to find the problem.
The first area that I looked at was the corner weighting.
Towards the end of the first year with the LSD I visited
Steve and we checked over the car. It wasn't very far off
from the previous settings so that wasn't the problem.
Next up was the ride heights. I know from previous experience
that small changes can make a big difference. I tried lowering
the rear of the car by 5mm at a time, the lowering of the
rear tending to settle down oversteer. The car felt a little
better, but when the platforms were all of the way down
to the bottom of their travel the problem was still there.
The car was almost level (no rake at all) so I knew something
must be wrong somewhere else.
I'm not sure what prompted the thought, possibly a chat
with my boss, but however it came about I started to think
about the effect the LSD was having, and how it works. The
LSD locks to rear wheels together by means of clutch plates.
When one wheel starts to slip, the power is transferred
to the non-spinning wheel to maximize grip. The way it knows
that one wheel is spinning is if it is rotating faster than
the other. However, when you corner, the outside wheel will
need to rotate quicker that the inside wheel as it has to
travel further. The way the LSD overcomes this is to allow
a breakaway torque. This is effectively how tightly locked
together the two wheels are and how much torque has to be
applied in order to break the locking of the two. I called
Raceline for some advice as I knew they had built a Ford
axle for a friend. Pete was very helpful and talked me through
how to check the breakaway torque of my axle, and also what
kind of figure it should be set to.
The checking method of the breakaway torque:
||Jack one side of the car up and remove
the wheel, lower the car so it is back to the normal
||Place a chock in front of each front wheel
to stop the car from moving (although it is highly unlikely
that it will).
||Make something to allow you to rotate
the wheel from the canter, to ensure you don't multiply
the torque you apply. I used some 5mm thick steel plate
and drilled two holes for the wheel studs to poke through
and a third hole in the centre. In the middle hole I
fitted an M8 bolt and nut.
||Fit the above plate to the wheel studs
and loosely tighten the wheel nuts to hold the plate
||Using a torque wrench, start with a low
setting, eg. 10 ftlb, and try to rotate the wheel in
the normal direction.
||If the torque wrench clicks then the breakaway
torque of the LSD is higher than the setting.
||Increase the torque of the wrench 5lb
and repeat step 6 until the wheel starts to slip and
the wrench no longer clicks.
||Repeat steps 1 - 7 on the other wheel
as a double check.
When I tested mine in this manner I got results around
the 60 - 65 ftlb area. The recommended values was 25-30
ftlb, so clearly mine was a little on the high side. The
more I thought about it the more I thought this was my handling
problem, the wheels were locked tighter together than they
should have been and the outside wheel less willing to travel
faster than the inside in a corner.
When I was next over at Steve's I tested his car in the
same way, although only one side, to gauge if the recommended
values were in the right area. Steve's car is a De-Dion,
so I expected it to be slightly different. I was slightly
surprised when his wheel started slipping at around 15 ftlb.
The next task was to track down somewhere that could adjust
the LSD settings. This is done by changing or grinding down
some packing shims inside the unit. After asking around
I found a local company called Old Fords in Bracknell, 15
mins away from me. They were happy to take a look so I arranged
to take them the diff/LSD unit.
I decided to take the diff out of the axle to save having
to disconnect the brakes lines, handbrake, etc. It would
also have been a pain removing the axle without taking the
car out of the garage due to the width - it is then fun
getting it back in with no rear wheels!
Removing the diff housing meant that I has to remove the
half shafts. This was a first for me. I looked around for
slide hammer to get them out and thought they were a little
expensive. I decided to make my own. I used my plate from
the testing above and replaced the middle bolt with a length
of M8 studding I had lying around, fixed with a nut either
side of the plate. Onto this I passed a couple of weights
and large sockets and washers (to stop the weights falling
off of the end) and finally a nut. I could then slide the
weights along the studding until it hit the socket and washer
and nut. The four bolts holding the half shaft in place
were removed and the plate fitted to the wheel studs. It
only took a couple of light pulls on the weights to get
the shaft moving - easy. Both sides were removed from the
axle casing and stood up next to the side the came out of.
The propshaft was unbolted next and then the A-frame was
undone so the axle to hang from the dampers and rotate so
the housing was pointing down and easy to remove. Each of
the nuts was undone a little at a time, 1/2 a turn in turn
to avoid distorting the case or the housing. A bowl was
positioned just in time to catch all of the horrible smelly
diff oil that started pouring out of the gaps appearing.
I left this for a night to drain and them dropped the diff
the rest of the way out. It is bloody heavy!
Old Fords had told me they were going to be around on a
Saturday morning so I loaded the diff into a box and took
it to them. It was snowing - so I nearly laughed when they
made appologies that they couldn't do it that week. I decided
to go for a half way setting and asked for 20 ftlb. It was
two weeks later when I got the diff back and fitted it back
into the diff, fitted the half shafts and all the other
bits (prop, drums, etc). When I repeated the above test
I was a little disappointed to get high 20's readings. I
suspect this was down to the extra drag of the brakes and
oil. I would test it when the car was back on the road and
if an improvement was felt, but not enough I could always
do it again - it wasn't very hard to do.
With all my mapping and
misfire issues I almost forgot that I had done anything
to effect the handling when I started driving it again.
The difference was great, it was back to how it was before
the LSD, very neutral turn-in with a rear that just followed
where the front went. As the mapping was improved and the
issue was fixed (all it needed was new leads) the rear end
has started to be much more fun. I softened the front anti-roll
bar as I felt the ride was too stiff (I think the 032's
have too stiff sidewalls) and as a result the front has
more grip, and the rear proportionally less. It made it
quite good fun on roundabouts as you can just apply a bit
more power and prevoke the back to start to slide slightly,
very predictably. The next plan is to change the 032's to
some CR500's which are almost as grippy but with softer
sidewalls which I think will suit the live axle setup as
otherwise you bounce over every bump!