15. "Are dry clutches inherently finicky? If so, why did Ducati put them ontheir bikes?"
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This article is from the Ducatis Motorcycles FAQ, by Duke Robillard duke@io.com with numerous contributions by others.
15. "Are dry clutches inherently finicky? If so, why did Ducati put them ontheir bikes?"
(from a discussion by Godfrey DiGiorgi (ramarren@apple.com), with comments
by Michael Nelson (nelson@seahunt.imat.com), and Tom Dietrich
(txd@mkt.3com.com)
From: Godfrey DiGiorgi
"The 750SS has a wet-clutch -- much less "grabby" and sensitive (especially
in traffic)."
I've heard this same myth over and over again, relating to more than just
Ducatis, and I'd like to set the record straight.
A dry clutch is not more grabby or more sensitive than a dry clutch, nor
can you slip a wet clutch more. The reason that race machines went to dry
clutches was to *improve* disengagement, reduce the amount of clutch
material infiltration into the lube system, and to allow cleaner, smoother
engagement through better cooling and less inter-plate stiction. Clutch
friction material is very abrasive and a major source of bearing wear if it
gets around the filtration in the lube system, a major cause of worry in
older engines without full flow filtration systems.
Wet clutches when slipped expand more because they can't shed heat as
quickly, and the oil stiction masks the chattering of disk/plates as well
as some characteristics of overheating. Just because you can't feel it
doesn't mean it isn't happening. Typically, a wet clutch capable of equal
power transmission at the limit requires more spring pressure to prevent
slippage and longer travel to ensure disengagement.
"Ducati clutches are particularly grabby"
The old bevel driver wet clutches were great, until the 900SS which
produced just enough more torque that there was inadequate spring pressure
to keep them from slipping under hard (ab)usage and inadequate mechanical
advantage to correct that without making them unbearably difficult to
operate for long periods. The solution was to go with a dry clutch for the
lighter spring pressure and cleaner disengagement; this was only done in
racing aftermarket kits and special factory racing fitments, however.
Ducati's clutches since the Pantah have been both wet and dry. The dry
style were typically fitted to the higher performance machines, the wet to
the economy models (it costs more gaskets and housings to isolate the
clutch). It's always been a light clutch for the power output of the
engine, and handicapped by the nearly ideal sporting gearbox (closely
spaced gears with a tall first) and the tall street gearing fitted (for
noise reasons). The detail implementation of the clutches has always been
problematic, both in actuation mechanism and in clutch pack design.
As such, none of them tolerate brutal slippage like an motocrosser well.
Not that kind of design; an motocrosser doesn't have as large a set of
power pulses to deal with, being a two stroke motor anyway. A clutch should
NOT be slipped unnecessarily in any case: just enough to get off the line,
just enough to get gears to mesh without strain or clashing. Slipping the
clutch on any machine is throwing the engine's output away as heat which
will affect some portion of the mechanical bits.
Small factors in the detail assembly of an individual clutch still make a
big difference. The clutch in my bike was actually quite good as delivered,
it would chatter only when oil wet and hot (failure of a small seal
accounting for that) but had a slightly harsh final engagement. It was also
quiet. The repair done to correct the oil seepage and the factory clutch
pack upgrade has now improved the clutch action to perfect, as best as ever
I've had on any bike. It does not chatter, even when abused through nasty
traffic, lever pressure is slightly reduced, engagement is broad but
secure. The downside is it's somewhat noisier. I spent probably 1.5 hours
just examining the clutch plates and assembling the pack into place -- very
small detail differences can make a big difference here. I spent a lot of
time making sure it was right, according to factory spec and my experience
as a mechanic. It works to my complete satisfaction.
Please, let's not propagate myth that dry clutches are finicky and wet
clutches are not. Dry clutches, from an engineering design standpoint, have
several advantages. Detail design implementation and individual assembly
make for large variances in the quality of an individual unit. With modern
materials and techniques available, you can design a clutch of either type
to suit the needs of the application well. How well you implement the
design, how well the assembly is completed will affect greatly the overall
quality of the component.
I opine that a novice rider on a modern Ducati will notice no difference at
all on a properly setup 900SS clutch vs the 750SS unit, it's just not a
valid criteria of differentiation for selection of a new machine. The 750SS
was the price leader in the '92-'93 range (now they have the 600SS in
europe as well, but the US market has dictated the 900SS CR instead). The
wet clutch was fitted as a cost-reducing measure, period: they already had
the design from several previous generations of wet-clutch Pantah motors.
But Michael Nelson disagrees:
Even the Ducati World SuperBike riders such as Polen and Fallappa have
trouble with THEIR Ducati dry clutches. I've read interviews with Polen
where he blamed the dry clutch (which incidentally on HIS bike was a
specially modified billet aluminum jobby.... the BEST one Ducati and Fast
By Ferracci know how to make, and LOTS more expensive than the ones that
come on the production bikes), for his consistently bad starts.
The Ducati dry clutches supplied on recent manufacture rubberband head
motor bikes are NOTORIOUS among the motorcycle press and among a large
percentage of owners as being quite funky in design and execution. While I
agree that a proper multiplate dry clutch CAN be designed (all the 2 stroke
125/250/500cc bikes use them), Ducati has yet to do so. These dry clutches
are especially silly on streetbikes.
Heck, all the Japanese manufacturers seem to be fully capable of designing
and executing wet clutches that are capable of reliably withstanding LOTS
more horsepower on MUCH heavier bikes (ie: the Kawasaki ZX-11, the Honda
CBR900RR, the Suzuki GSX-R1100, ad nauseum). And they still provide smooth,
progressive, easy to use operation with much less lever effort.
The Ducati dry clutch design is difficult to assemble correctly (even
Ducati themselves can't seem to get it right, and they've even revised the
design) , noisy, and it literally beats itself to death with all that
rattling. I had to replace my friction disks at around 5,000 miles, not
because the friction material was worn out but because the tangs on the
edges of the plates were mushroomed and flattened out. As they rattle, they
cause the mushrooming of the edges, that creates more clearance, which
creates more vigorous rattling, which creates more clearance, and so on.
In effect, they beat the crap out of themselves, the steel clutch basket,
and the aluminum clutch hub. You can't have all that rattling and banging
going on without SOMETHING getting damaged in the long run. It's a poor
implementation of a questionable design for a streetbike.
And Tom Dietrich chips in:
... the one point nearest and dearest to the hearts of race bike tuners
everywhere. The beloved and much sought after horsepower. Not having the
clutch spinning in that heavy oil frees up a couple of ponies for duty at
the rear wheel that were previously lost in the oil. Free ponies! It's
enough to make a grin come to the most stone faced tuner/builder.
Michael responds with:
True, and that's probably an advantage worth having on a racebike.
Especially because most roadrace bikes only have to start from a dead stop
once during a race, and they have the $$$ and time to tear the bike down
after each race and correct whatever may be wrong with the clutch. However,
I still maintain it's a silly feature for a streetbike, where reliability
and smoothness are considered more important by most people than a couple
extra ponies.
Finally, Godfrey rebuts:
... I didn't say that *Ducati's* clutch, any of them, was not finicky. I
said that a dry clutch was not in and of itself any more finicky than a wet
clutch, and that a properly installed and adjusted dry clutch was
indistinguishable to a novice from a wet clutch.... By the way, I've had
plenty of experience with blown and improperly assembled Honda, Kawi,
Suzuki and other clutches. I was a parts manager and freelance mechanic on
those machines for a total of about 3 years. They ain't poifect neither...
;) My VFR, Hawk, and CB750F clutches all needed 'clearing' first thing in
the morning, just like the Norton, and I managed to get the Hawk clutch to
slip a couple of times when I was being particularly abusive.
BMW, Guzzi and Morini are three other companies whose dry clutches I have
owned that come to mind immediately. The older BMWs have a somewhat sudden
engagement characteristic, the Guzzi a little less so, due to the high
rotational speed and the mass of the pieces not the fact they are dry. The
Morini I rode for about three weeks before I discovered it had a dry
clutch.
It all depends upon the individual design. The 'finickiness' is not endemic
to the type. With a proper design the action is indistinguishable. On the
Ducati, the clutch design is light and somewhat weak. When properly fitted
and adjusted, no novice can tell the difference. Again, I opine that a
novice rider on a modern Ducati will notice no difference at all on a
properly setup 900SS clutch vs the 750SS unit. It works like a clutch
should work, at least in street use.
The myth is that "dry clutches are finicky and wet clutches are not." The
fact that Pantah and later Ducati clutches, both wet and dry, are
problematic does not contradict this. Nor does the fact that a wet clutch
masks chattering and over-heating problem more effectively from the
operator.
"The beloved and much sought after horsepower."
Tom raises the point that a wet clutch will absorb more horsepower from
sloshing about in the oil. Again, this is a design detail problem, not an
factor inherent in the type. On certain machines where the clutch was
chain-driven from the crankshaft around the clutch basket, it was essential
that the chain dip into the lubricant, and the oil sling would indeed cost
some hp if you overfilled the lubricant. On most designs, this is not much
of a factor (note that very few wet clutches really sit in an oil bath,
they are just made such that oil splash necessary for lubrication to
portions of the drive mechanism can be in with the friction components. A
wet clutch will absorb horsepower but by adding heat to the oil more
likely.
"My dry clutch has lasted practically forever"
Mikhail reports that his modified clutch is hanging in there for 15000
miles so far. I report my 750gt clutch in my original long distance
traveler was in perfect condition condition at 60,000 miles (3mm acceptable
wear limit, less than .5mm measured at that point), and the last I heard
was that this same original 1975 clutch was still running strong at
100,000+ miles when last I heard from the bike's owner somewhere in 1988.
At 13,000 miles, the 907's clutch (all original pieces with the exception
of one new spring plate for the update and one oil seal) is in as new
condition with regard to wear: no warpage, no reduction in plate thickness,
springs at spec, just a little bit of chatter marking on the center driven
hub. I'll report how it fares over the next 13,000 miles. I use the bike
pretty hard but I don't abuse the clutch unnecessarily.
"The Pantah I have has a wet clutch and has hardly been touched for more
than 15K miles, while my 851's needed replacing after just 4K." A 40 hp
engine using a clutch of essentially the same design as a 90 hp engine, and
the 40 hp clutch doesn't wear out as quickly... sounds to me like the
design was probably set up for the 40 hp engine, and the reduced longevity
in the 90 hp case was a compromise result of wanting the same weight and
layout in a similar motor...
Such is as it is. I don't consider these machines to be perfect in any
way... if they were, why would I be looking forward to the next one? I
expect continuous improvements as new designs come around. Hopefully
they'll address the clutch someday soon and give the same perfect action
that mine has now with a bit more longevity and perhaps a little quieter
operation soon. Although I have little complain about, it'll appease those
who do.
Beth W. Dixon sez it best: "I don't care what's happening as long as I like
the feel (tm) of what's happening. A wet clutch may not be any better/worse
than a dry one, but I really didn't want one more thing I wasn't used to
when learning the Duc." I simply purport that if you have a properly setup
clutch on any Duc, Beth would be unable to tell the difference.
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