| Racing
Clutches – What did they ever do for us ? (part1)
One of our two
“Driver of the Day” awards has, this year, been sponsored
by Super Clutch. As a former hillclimber, I've known Super Clutch's directors,
Tony and Shirley Tewson for over 25 years. So it seemed natural to ask
Tony to tell us all about racing clutches.
I must freely admit that until I spoke to Tony, my idea of a clutch was
that as long as it didn't slip when engaged and cleared when the pedal
was pressed, the job was done. It turns out there is rather more to it
than that, and with more knowledge of why racing clutches exist (as distinct
from uprated road car clutches) we can all make a more informed selection
clutch specifications, not to mention helping lap times.
But to begin with some background, Tony and Shirley started Super Clutch
14 years ago when Tony was a clutch research and development engineer
and Shirley was in an engineering parts supply organisation. Both were
highly successful hillclimbers and realised from their viewpoint on both
sides of the fence that a small specialist racing clutch company could
potentially do a better job for their customers than the established big
names. This meant having the technical knowledge, direct experience of
motor sport, and offering clutches from stock.
Tony told me Super Clutch equipment is now used throughout motor sport
in diverse applications from International to Clubman Race & Rally,
Historic F1 to super-tuned Track & GT. “We don't do brakes and
we don't do road car or carbon clutches, but we do specialise in metallic
friction material racing clutches of the type used in Monoposto probably
better than anyone else. I handle the sales so almost invariably it's
me that anybody ringing in talks to, not a salesman. We're not as well
known as some of the big names because so much of our business is direct
to engine builders and car preparation specialists. If they would only
fit glass bellhousings you would have seen our name more! We've grown
by word of mouth recommendation.”
The Super Clutch approach is to help match the product to a customer's
requirements. This may seem obvious, but Tony says “How many times
have we all been frustrated by non-specialist suppliers of racing parts
telling us “you want one of these mate, they all use these”
with no justification of why or attempt to establish actual requirements.
If you already know what you need then you are in a good position, but
it is often worth asking what other options are available in case there
are new developments or options that better match your style of driving
and the aspects of performance you require.”
So what would you need from a typical racing clutch, compared with an
uprated original equipment style clutch? Tony answers “better fitness
for purpose, and the opportunity to downsize the clutch for performance
benefits and reliability”.
“The performance improvement comes from reducing the rotating mass
of the engine, and the “moment of inertia” – the flywheel
effect to you and me. In practice this means not just the overall weight
of the clutch, but how much of that weight is concentrated in the centre.
When the inertia is reduced, the engine revs more easily, which can, for
example, help acceleration out of tight corners, and give an advantage
when overtaking. It makes matching revs easier during gear changes, and
as a bonus helps reduce drivetrain wear especially to gear dogs or synchros.”
“There are three main ways to reduce the moment of inertia. The
first and most obvious is a lighter clutch cover. Machining from solid
aluminium billet rather than using a casting or a steel pressing can save
up to a kilo on a specialised racing clutch, and even more compared with
a road-based clutch, and still be a stiffer and stronger component.”
“The second way is more radical – to reduce the diameter of
the clutch. By way of comparison, a typical road car clutch is 8.5”/215mm,
whereas racing clutches are normally 7.25”(184mm), 5.5”(140mm)
or 4.5”(114mm) diameter. The size denotes the diameter of the clutch
plates.”
“The third way, usually done in conjunction with the clutch, is
to reduce the diameter of the flywheel or design it so there is as little
material as possible at the outside where it counts the most for reducing
inertia.”
“When the clutch diameter is reduced, usually at the same time as
other mods to increase engine power/torque, it will be necessary to restore
clutch torque capacity by having a greater number of plates and/or increase
the clamping load - for example as in the ubiquitous twin-plate race or
rally clutch.
Tony continued “The huge advantage of reducing inertia is
improved engine response and reduced lap times. Every branch of motor
sport has recognised this fact, even rallying where it was traditionally
thought that low inertia would stop the driver keeping his rhythm or momentum.
Overall there is a good performance advantage, and the only significant
downside is that starts require more control from the driver. With a 7.25”(184mm)
clutch this is not normally an issue.”
“However with a 5.5”(140mm) clutch with matching miniature
flywheel as per Formula three spec Dallara the starts become more sensitive.
I know this from personal experience on the hills where the start is critical,
and have yet to find a circuit racer who thinks the start is any less
crucial. I’m talking about that window of opportunity on the start
between stalling and using too may revs. Even so, a 5.5”/140mm clutch
has an overall performance advantage compared with 7.25”/184mm even
on a 1000 yard hillclimb. We have made huge improvements in clutch spring
design and friction material, so a good compromise may be to go from a
twin-plate 7.25”/184mm clutch to a single plate clutch of the same
size This is a viable compromise more often than not” - of which
more later.
"We have developed new friction materials and other improvements to help
open out that start-line window again. Of course launch control could
be the ultimate solution, but until closed loop control clutch operation
is perfected and generally available then the best “manual”
launch will generally be better than an assisted launch”.
Tony said “The often repeated on/off light switch clutch control
syndrome associated with racing clutches is the paddock equivalent of
urban myth. When investigated it is always due to abused or just plain
knackered, over-heated and worn out clutches, mismatched clutch operating
systems, poor engine tune, over-tall first gear etc. A racing clutch is
never going to be like a road car clutch, but properly spec’d and
installed, it should be second nature to use, not difficult or unpleasant”.
Any clutch is effectively doing the same job as a brake. It uses friction
to stop the relative motion between two surfaces. During take-up of the
clutch at the start-line the working surfaces of the clutch are rotating
at different speeds and must be matched by the driver controlling the
clutch until full drive lock-up is attained. During this period the clutch
must be able to deal with the driver induced slippage by absorbing energy
in the form of heat and dissipating it. Tony observed that club racers
paying for equipment from their own back pocket seem better able to control
this procedure and prolong the life of the clutch, than professional racers
using sponsors' money !
Of course, it may be that the clutch is spec'd to deal with a mechanically
unsympathetic driver (see Top
Gear, any week - ed), but the general ideal in a circuit
race environment such as Monoposto is that smaller and lighter is quicker.
To illustrate this consider the ubiquitous Vauxhall two litre red top
engine. In Pick-up racing using a single rather than twin plate clutch
may gain as much as 250 rpm down the straights –a small but critical
advantage when slipstreaming and overtaking. Yet the same two-litre Vauxhall
in a Rally car will usually require a much heavier duty twin plate rally
clutch with plenty of heat-sink capability due to the nature of the sport.
Coming soon - Tony Tewson discusses clutch materials
Tony Tewson was talking to Tony Cotton
|
Shirley Tewson in the Pilbeam MP40 with a V8 4500cc Rover, here at
Shelsley Walsh.
5.5”/140MM Billet Aluminium Clutch
An assortment of components
3 Sizes of Clutch Plate
Organic and Metallic Friction Material (getting ready for part 2)
|
