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Path: utzoo!watmath!clyde!cbosgd!ihnp4!ihlpa!animal
From: animal@ihlpa.UUCP (D. Starr)
Newsgroups: net.cycle
Subject: Re: front suspension (aircaps)
Message-ID: <867@ihlpa.UUCP>
Date: Thu, 7-Nov-85 16:00:56 EST
Article-I.D.: ihlpa.867
Posted: Thu Nov  7 16:00:56 1985
Date-Received: Fri, 8-Nov-85 22:05:50 EST
References: <120@copper.UUCP>
Organization: AT&T Bell Laboratories
Lines: 128

* there are about two pages worth of previous opinions below this point
	(aren't you glad you have 9600 baud?)

> > I own a '84 Yamaha FJ600 that I use mainly for fast rides on
> > back roads. I have been experimenting with the front suspension,
> > specifically air-caps. The '85 FJ600 comes with air-caps, the
> > '84 doesn't. 
> >
> > Recently an amateur racer, who also rides an '84 FJ600, 
> > suggested I forget the air-caps and install progressive springs 
> > in the forks and bump up the fork oil from 10w to 15w. He claims 
> > that tuning with air-caps is impossible since the air expands and 
> > contracts due to temperature. Also, air tends to leak somewhat. 
> > 
> > My question is: Is he correct?  Why then do most sport bikes 
> > now come with air-caps? For hard, fast riding, which is more 
> > likely to improve handling, the air-caps or progrssive springs?
> > Would some combination of the two be even better?
> > 
> > Thanks,
> > Jay
> 
> I don't know anything about aircaps.  Never heard of them.  I also don't know 
> anything about racing.  So I may make some inccorrect assumptions.  But if
> I do, I *know* that those on the net will correct me.  :-)
> 
> Let's
> assume your friend is absolutely correct.  Then aircaps would still be fine for
> reacing but useless to you.  Why?  Because a race is a one shot deal held
> under fairly constant conditions of temperature and pressure.  It's not a big
> deal to make sure they are set correctly before every race and in the pit stops
> if it's a long race.
> 
> When the typical casual biker rides however, he doesn't want to have to tune
> his suspension everytime he gets on the bike (depending on how hot it happens
> to be at a particular time of day.)
> 
> A racer only wants his bike to perform for  a limited time under controlled
> conditions.  A rider wants his bike to be comfortable, manueverable, and
> safe under a wide range of conditions and road surfaces.
> 
> *************
> OPINION
> *************
> When I see someone riding a racing bike, I have to assume
> 1) He races on the track and puts up with a racing bike on the street,
> 2) or he's a goofy kid who wants to look cool at the cost of practicality.
> 
> 
>            
>                                               Todd Vierheller
> 
> 
> There are old bikers,
> And there are bold bikers,
> But there are no old bold bikers.

* a relatively brief tutorial on air forks follows *

A lot of the above discussion is based on incorrect assumptions, or that
"little bit of knowledge" that so frequently turns out to be a dangerous
thing.  Air-assisted suspension components *are* useful, both for racing
and for regular day-to-day riding, and they may be indispensable for the
seemingly self-contradictory concept of "sport touring."

Air caps on forks provide two useful features not found on the stock
spring forks:  adjustable preload, and progressive resistance.  The first
is easy to understand--the only way to change the preload on a spring
is to compress it.  This is relatively easy to do on the rear (watch carefully
what the cam-type adjuster back there does when you twist it), but rather
difficult with telescopic forks.  Further, the adjustment range with a
spring tends to be rather limited.  Air, on the other hand, can be added
or removed easily (there's usually plenty of it about), and because of
the way it compresses it provides a much greater range of adjustability.

The progressive-resistance aspect of air forks comes from a fundamental
difference between the physics of spring compression and air compression.
With a spring, the resistance force rises linearly with compression--that
is, for each inch you compress the spring, the force required increases
by a fixed amount (e. g., 100 lb for one inch, 200 lb for 2 inches, 300 lb
for 3 inches, 500 lb for 5 inches...) until the coils touch.  Air obeys
a different rule:  pressure times volume remains constant.  This means that
each inch of compression produces more force than the preceding inch
(example--for a fork with an eight-inch air space and 10 pounds of pressure
at rest, compressing four inches  produces 20 lb of force, compressing
6 inches produces 40 lb and compressing 7 inches produces 80 lb.)  The
result is that the air produces little effect when the forks are near full
extension (for soaking up those little ripples) but push back *hard* when
compressed most of the way (like on a Chicago-style pothole). 

Air by itself is generally insufficient to support a bike at rest, and
at the low end air tends to build up pressure to slowly, so the optimum
setup is an air-spring combination.  The spring takes care of the little
jounces, then the air kicks in on the big bumps and under sudden
deceleration.

As for the effects of temperature on air forks, they should be minimal.
Pressure in the fork will rise proportional to temperature--absolute
(Kelvin) temperature!  This means that, on this planet, you'd have
to heat the air in the forks to about 590 degrees F to double the
pressure in the forks.  Not very likely.

What about progressive springs and heavy oil?  Progressive springs are
rather expensive and aren't adjustable.  Heavy oil affects damping, not
springing, and is probably a good idea with air caps also.

Do air forks lose air?  Of course they do, over a period of several days.
If your forks don't hold air for at least a week without leakage, you've
probably blown your fork seals.  

If you're going to fit air caps, I offer the following two recommendations:
install a setup with a tube connecting the two fillers, so that you can
get the same pressure in both sides (something that's very difficult to do
otherwise), and buy a syringe pump with attached guage for filling them.
The syringe pump (marketed under the "Quik" name, among others) allows you
to fill forks easily and gives an accurate readout of the pressure (checking
with a tire guage is guaranteed to lose at least one psi each time you do
it).  And never, NEVER fill an air fork from a gas-station hose; you're 
almost certain to blow the seals.

...and one last benefit of air caps:  If you have them, you can change your
fork oil without pulling the fork caps and watching your springs fly into
the next county.  Just pull the crossover tube and inject the new oil
through the air fitting with a turkey baster (found at your local supermarket;
don't go looking for one at the motorcycle shop!).  On bikes with fairings
this can save a lot of work!

			Dan Starr