Shocking isn’t it
In the years of
dealing with Bugattis I have seen just about every type of shock absorber used,
from the early friction dampers on 8 valves through Bugatti drums, Hartford,
Houdailles, Armstrongs and telescopics. With all these types some things are
evident. If you can use telescopics,
thank your lucky stars and talk to Koni.
If you can get away with hydraulic lever-arms, make sure they are big
enough to stay in the working temperature range. If you are blessed with
friction dampers, the Bugatti drum has some serious advantages over the
Hartfords but only if you’re prepared to learn how maintain them. This may cause some raised eyebrows, but
then you may not have heard of “Coulomb” friction. (Charles Augustin Coulomb, 1736-1806, also contributed his name
to the lexicon of electric power). So here is how do we set our Bugatti shock
absorbers.
Firstly, shock
absorbers do not absorb shocks; that is the job of the springs. They are more correctly termed “dampers” as
they damp the resultant motion of the sprung mass of the car and its suspension which results from driver
inputs (turns and braking) or from hitting bumps. A pure friction damper (Hartford) obeys the equation.
Mf
La Cos. Oa
Where Fd is the
damping force at the end of the lever arm.
Mf is a frictional moment giving
the damping force, La is the length of the lever arm and Oa is the arm
angle. Further, the friction moment
depends on the number of frictional surfaces, their effective coefficient of
limiting friction, the…….. you get the idea by now. As more variables are included, the equations quickly become very
complex. How do they relate to our
pride and joy bowling down the road or tearing up the racetrack? With the hindsight gained in the 80-odd years
that Bugattis have had dampers, we can apply these first principles in
combination with data logging, chassis dynos and computer modeling to find that
what appears to be a very crude damping system is actually quite good.
As our understanding
of the complexity of the damping picture has improved, so has the complexity of
our modern dampers. The height of
technology fifteen years ago, the double adjustable Koni, is now replaced by
four-, five-, and even six-way adjustments.
Rebound settings are generally softer than bump, as bump controls the
un-sprung mass (wheels, axles etc.)
whereas rebound controls the sprung mass (the rest of the car). This may seem academic for the leaf sprung
Bugatti, but it is important to understand that a car used primarily for
circuit competition needs to be stiffly damped, using its dampers to control
chassis attitude. Cars that need to be
good over bumps, such as road cars, will be less stiffly damped, and a trial
car will be the softest damped of all.
The Bugatti damper
(which first appeared on the Type 13 at Brescia) is as good as any prewar
friction damper, and when properly assembled from original parts or correctly
made replacements, it damping characteristics are superior to the widely used
Hartford damper. This is basically due to the very small differences between
static (Couloub) friction and sliding friction in the properly maintained
Bugatti system. Picture this: As our competition car enters a corner, a
load builds on the outside suspension which is resisted by the spring and its
damper. At a given load, the forces
overcome the static friction of the damper, and the suspension starts to
move. If the relatively constant sliding-friction
resistance to motion afforded by the damper is now significantly less than the
starting static friction, the car effectively falls over onto the outside of
the corner.
As the corner ends,
the suspension is slow to return until the forces build up again to overcome
“stiction”, after which it suddenly jerks back up. This is typical of a conventional friction damper, and a similar
chain of events occurs under braking.
The Bugatti damper works by shearing of events occurs under
braking. The Bugatti damper works by
shearing grease between the does and the outer drum, mimicking a hydraulic
rotary vane damper, giving a progressive, almost hydraulic, action. This effect did not go unnoticed in the
contemporary marker: Late Hartfords are
encased and run submerged in oil to counter the jerking action.
Now to get to the
specifics of making the Bugatti damper work.
The most important part is the shoe.
In order to evenly distribute the loads across the friction face, the
shoe is tapered from the spring end, thickening all the way to the lever
mount. It is the same principle as the
longbow: thick in the middle, thin at the ends. Some replica shoes are made parallel; they won’t work correctly
as the springs will apply pressure unevenly and create an uneven layer of the
all-important grease. All the internal
components must be free from buss and smooth.
Then we coat each working face liberally with Linklyfe, a waxy
high-pressure graphited-moly grease used as a motorcycle chain lube. (Linklyfe is a available in the UK but
not exported to the USA. It is made by
Weldtite Products, Ltd., which does send other products to these shoes. Ed.)
The each spring must be fitted. We find
the best way is to hold the spring compressed in a vice with about a third of
it above the jaws. Hold the assembled
damper body tightly against the jaws (use a jig!) and gently tap the spring up
into the body. Be careful! There is a lot of stored energy here and if
the spring comes out, it can fly across the workshop at quite a speed, damaging
anything in its path.
Once the damper is
assembled, you can check the damping rates, static and sliding, with a spring
balance. What rates do we use? A Type 35B racing car front is set to around
80 pounds pull at the attachment bolt with a maximum static to sliding
variation of 5 pounds. The rears are
set to 40 pounds, obviously in matched pairs.
Whichever car we are setting for, the proportions stay around 2:1. For alight car like a Brescia we would
probably have settings of 50/25 pounds.
These are guide figures. We use
BOC-supplied springs. They are usually
a bit too long and too strong. To
shorten, put them carefully into a three-jaw chuck in a face grinder and being
sure to use lots of coolant to avoid heating and subsequent loss of temper,
remove no more than 0.010 inch per pass.
0.010 inch equates to roughly 5 to 8 pounds in the pull test.
Now we get into
related areas such as the leaf springs and maintenance. Leaf springs by their nature are self
damping with inter-leaf friction doing the job. To counter this, we strip the springs, deburr and grease between
the leaves with the same Linklyfe used in the dampers. The guide rates assume properly prepared
springs. If you disconnect the dampers,
the car should bounce fairly easily.
Finally, check the integrity of the linkages and mountings for looseness
and proper travel. There must be no
play in any of the linkages/leathers.
This is undamped motion and, along with play in the steering ball
joints, is a leading cause of axle tramp under braking. To function correctly, the dampers must be
correctly greased. We have found that
this needs to be done after every tow race meetings. The grease nipples fitted as standard simply can’t work, so we
modify the shoes and liners to carry an annular groove that lines up with an
extra nipple on the drum. This allows
us to simply re-grease with a grease gun and strip and clean the dampers only
once a season.
After the steering
links, the dampers play the biggest role in making a Bugatti a delight to
handle. Don’t underestimate their
importance of ignore them. Equally,
don’t write off the Bugatti drum shock absorber; if you get it all working
right, there is no better non-hydraulic system.