From:  Eric Christensen [SMTP:edc@remedy.com]
Sent:   Monday, March 17, 1997 11:39 AM
To:     'Bruce Kapraszewski'
Subject:        RE: Clutching phase of race car setup.

>O.K. all of you F5 race hardened veterans out there, we've heard about
>engine selection and tire pressures relative to car setup. Would any of
>you care to enlighten us newcomer's and interested fans as to the art of
>selecting the proper clutch springs for a race setup?

Oh boy Bruce, now you've opened a can of worms! :-)

Clutching is a gray art form. First remove your clutches and rebuild
them, replacing all worn parts. Then draw a pentagram on your
workbench with the blood of a freshly killed chicken..... :-)


Seriously, Olav Aaen's Clutch Tuning Handbook is the bible of clutch
set-up. If you don't already have a copy, get one from Aaen
Performance before you start messing with anything. Even though the
clutches appear to be pretty simple devices, they are actually quite
complex and have enough tunable components to leave you hopelessly
lost if you don't *really* understand how they work.

Before I get into this, I'm going to put the BIG disclaimer that this
is highly subjective information. I'm sure there are several on the
list that will disagree with something or another I say here. Please,
if you have other ideas, opinions or techniques, feel free to
contradict me. I am NOT an expert clutch tuner. I happen to have had
the good luck to work with a couple of bona-fide experts and have
learned a little bit from them along the way. 

As always, your mileage may vary and this advice is probably worth
about as much as you paid for it. :-)

The basic idea is to set the spring and flyweights in your primary
(engine) clutch bring the engine to a specific RPM (hopefully right
at peak HP). Then adjust the spring tension and ramp angles on the
secondary (driven) clutch to get the shift characteristic you want.
That shift characteristic is largely dependent on your personal
preference and driving style. The theoretical ideal is a slight RPM
peak at engagement, then flat at the peak HP out all the way though
the shift-out phase with the RPM slowly climbing above the peak HP
once you're shifted all the way out into overdrive. Again, that's
theoretical... you may find that you never get there. If you do, you
may find that it doesn't work all that great for your track and/or
driving style.

There are thousands to combinations of springs, weights, ramps, shims
etc. to choose from. And in fact, there are at least 3 different
primary (Polaris, Comet and Ski-Doo) and 2 different secondary
(Polaris and Arctic-Cat) that are commonly used on F500s. I'm sure
there are a few more that I don't know of that people have used as
well. And there are several different versions of many of these
clutches. 

The point of this being that generalizing clutch settings is
virtually impossible. Even if I told you exactly what I run in my KBS
Mk-8, it's almost guaranteed not to work for you. Besides, I don't
think too many F500 racers would give away their "trick" clutch
setups. Clutch tuning is the one area in an F500 where you can gain a
competitive advantage over the other guy. 

So, where to start? Well, using standard Polaris primary and
secondary on a AMW with the stock pipe, here's what I'd put on it as
a starting point:

Polaris Primary:

Aaen orange spring or Polaris blue spring. I personally prefer the
Aaen orange. Flyweights depend on your engine and pipe combination.
For the AMW with the stock pipe I would start with something around
51 grams and adjust from there. You're shooting for about 7600 -
7700  RPM with the stock pipe on the AMW. 

Polaris Secondary:

34 degree ramp (helix), Polaris blue spring, 11-13 pounds of
pre-tension (about the 3rd hole in the stock Polaris helix).

This gives you a nice, drivable set-up that should be reasonably
competitive. 

Now, how to tune from there....


For the starting point, you first want to focus on getting your
engine RPM right. That means right at peak HP and steady through
acceleration on level ground. You'll mainly play with flyweights to
achieve this. Lighter weights raises the RPM, heavier weights lowers
it. There are also several different curvatures on flyweights. Make
sure you're using the same curvature on your weights when you're
changing them or you'll end up chasing your tail. Don't use "notched"
weights that have a flat spot cut in the heel. These are for drag
racing sleds and are intended to increase the engagement RPM for hard
acceleration off the line. In our cars we want nice smooth engagement
around 4000-5000 RPM. Too high of an engagement speed induces wheel
spin. 

Do NOT change the secondary to adjust engine RPM. I've seen too many
people increase the pre-tension on their secondary to bring up their
engine RPM. You are shooting yourself in the foot doing that. It will
work (for awhile), but you're decreasing the efficiency of the
transmission and adversely affecting the ability of the secondary to
keep the shift curve flat. 

A trick for the Polaris (yes, I'll give away *some* tricks) is to
remove shims from under the spider to minimize the belt clearance in
the sheaves. In stock form the Polaris primary has about .2" of belt
clearance. By removing shims to tighten up this clearance you'll get
smoother engagement. You'll need a grunt tool to pull the spider in
order to remove the shims. 

I'm not even going to try to go into spring tuning in the primary
(mainly 'cause I don't understand it enough myself). Just suffice it
to say that the Aaen orange and Polaris blue springs work well. Get
Aaen's Clutch Tuning Handbook if you want to get into spring tuning.


Once you have your RPM where you want it, you next job is to get it
to remain flat. That's the job of the secondary. This again, gets
into more gory detail than I really want to (or feel confident about)
get into. The combination of spring and helix (ramps) is what really
determines this. There are now multi angle helixes, Teflon coated
helixes, and roller bearing kits that replace the standard plastic
buttons that the helix slide on. These all change the shift curve of
the clutches. 

As a general rule, if you use the Polaris Blue spring with a straight
34 degree helix and 11 pounds of pre-tension, you'll be pretty close
to flat. Close enough to run in the front pack at the track anyway.


The main thing we play with in the secondary is the pre-tension on
the spring. This directly equates to side pressure on the belt. The
idea is that the more side pressure you have on the belt, the more HP
gets eaten up with belt drag and the less efficient the transmission
is. What I try to do is get to the minimum side pressure that I can
drive. Too little belt pressure and the clutch will backshift too
fast and put you in too low of a ratio coming out of the corners
(wheel spin, engine screaming, going nowhere). Too much and it won't
backshift fast enough and you'll loose RPM climbing hills. 

Ok, this is getting way, way too long already. Again, take this as my
personal thoughts which may or may not work well for you. They work
for me. That's the only claim that I'm willing to make when it comes
to clutch tuning. 

Now, I've opened the floodgates (and probably confused everyone)....
there must be enough clutch tuning knowledge out there between all of
us that we can collectively put together a useful summary of how to
set up the clutches on an F500. Throw your thoughts in there.