How
to choose a clutch
The selection of the correct clutch for the intended application is
critical to good operation, including the characteristics of (but not
limited to:
- clutch feel
- operating temperature range
- wear characteristics / durability
- clamping force
- break-in period
Quite often, the first impulse is to get "too much" clutch. This is
often a very big mistake, as there will be compromises in some
or all of the operational features listed above.
The first step in identifying what clutch to get is dependent on the
characteristics of the car. Ask these questions:
- How much power does the car make?
- How is it used? Street driving or track use? If for
racing, what kind of racing?
As we are dealing with BMW applications here, let's use an
widely-understood example - the 1992-2004 6-cylinder 3-series. Over
this 12 year period comprising two different chassis, we are still
dealing with virtually the same fundamental engine and transmission
designs. Power ranges from around 190hp to 333hp. Typical
bolt-on modifications bring many of the early models up to the
215hp-260hp range and later models around 350hp. Extensive
modifications including supercharging or turbocharging bring power to
the 350hp-450hp range, with some examples in the 500hp-600hp
range. The typical BMW enthusiast uses the car primarily for
sporty street driving, and the occasional auto-x or track day. The
3-series is also popular as a heavily-tracked car and dedicated
race car. Some see time spent drag racing, whether occasionally
or as dedicated drag racers.
So it is apparent that we have a broad range of power and use to
contend with.
Presented in increasing "aggressiveness", here is a brief overview of
clutch compounds, their power handling, and other characteristics:
(note that hp figures are generalized, pressure plate clamping force
and torque curves are additional factors)
| organic |
| CHARACTERISTICS |
Metal-fiber woven into "organic"
(actually CF aramid with other materials), original-equipment
style. Known for smooth engagement, long life, broad operating
temperature, minimal-to-no break in period. Will take hard use,
somewhat intolerant of repeated abuse (will overheat). Will
return to almost full operational condition if overheated. Material is
dark brown or black with visible metal fibers. |
| USE |
Street-driven cars up to 400hp,
auto-x and track use. |
|
| kevlar |
| CHARACTERISTICS |
A high-durability material more
resistant to hard use. Engagement is similar to organic, but may
glaze slightly in stop-and-go traffic, resulting in slippage until worn
clean when used hard again. Higher temp range in general, but can
be ruined from overheating - will not return to original
characteristics if "cooked". Has a break-in period of 500-1000
miles during which slippage may occur. Care must be taken during
this period not to overheat from excessive slipping. Material is
uniform yellow/green and may look slightly fuzzy when new. |
| USE |
Street-driven track cars up to
500hp, auto-x and heavy track use. Will take hard use, intolerant
of abuse (will overheat and not recover completely). Due to the
unforgiving nature of Kevlar, it is not recommended for street cars,
especially those that see frequent stop-and-go traffic which will cause
surface glazing of the clutch. |
|
| segmented
kevlar |
| CHARACTERISTICS |
Same
material and characteristics as solid kevlar above, but segmented
(blocks or sections missing) for better heat dissipation. New
generation of kevlar offered by UUC is resistant to glazing and is an
excellent choice for smooth operation in high-powered cars or those
equipped with SMG transmissions. |
| USE |
Street-driven track cars up to
650hp, auto-x, and heavy track use. |
|
| hybrid
carbon/ceramic/organic |
| CHARACTERISTICS |
Organic material on one side and a
segmented carbon or ceramic material on the other. The idea is
that the organic side will help smooth the engagement, reducing the
shuddering from the segmented side. Engagement is same as
organic, but still with shuddering. Temperature and hp range is
identical to organic. Carbon/ceramic side will wear flywheel or
pressure plate surface faster and will wear out faster in traffic
situations. Material is organic on one side (described above) and
segmented or completely separate pucks (described below) on the other. |
| USE |
Can be used in same situations as
standard organic. The "hybrid" design appears to be more of a
marketing gimmick rather than an actual performance advantage. Some
brands are poorly designed and wear unevenly due to flexation of
the clutch disk. For
examples of hybrid
failure, click here. |
|
| carbon/ceramic/cerametallic |
| CHARACTERISTICS |
Very high temperature materials,
usually only found on multi-puck disks. Will accommodate 500hp+
Engagement is more abrupt. Will wear flywheel surface faster,
especially in traffic situations. Carbon is slightly more durable
and flywheel-friendly, ceramic has a higher temp range. Multi-puck
design may result in slight shuddering or "stepped"
engagement when used in traffic situations, although many users report
completely acceptable operation. There may be a sound during engagement as is expected of this material. Color is any of several light
hues - gray, pink, brown. |
| USE |
Street/strip applications for
drag-racing and heavy track use cars up to 500hp. Will take
very hard use, suitable for extreme-clamping applications. |
|
| sintered
iron |
| CHARACTERISTICS |
Extremely high temperature
material. Will accommodate 700hp+ Engagement is on or
off. Requires special flywheel surface. Material is
metallic gray. |
| USE |
Strictly for high-horsepower
endurance racing. With correct pressure plate, capable of
extremely high clamping force. Engagement is like a switch,
either on or off. Does not work well when cold. High-durability
flywheel surface required, standard flywheels will be
destroyed quickly. |
Back to our original question, how to choose? The
answer depends on the answers to the two basic questions asked earlier
regarding intended use of the car and power output. Based on the
answers, this gauge will help the decision:
To repeat the important point, do not buy more clutch
than you need. A simple organic disk will handle a wide variety
of use - including
street use, auto-x, and even true racing. In fact, SCCA ITS
racing rules require a standard OE-spec organic disk. UUC has
tested organic disks to reliably handle up to 475hp in long-term street
use. M3s regularly run through several auto-x seasons without
problems.
A kevlar disk is a good choice for a heavily-tracked or road-raced
cars, especially with forced induction.
Carbon/ceramic should be left to high-power cars that see lots of drag
racing, or are dedicated track/drag cars.
Sintered iron clutch disks are strictly for endurance
racing.
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