Effect of changing fork springs and oil to original specs?

I just got a 93 BMW 1100RS. It seems to ride pretty stiff. the guy who
sold it to me said he changed the fork springs and oil to heavier. I am
considering returning the forks to original factory spec. What effect
should I expect in handling the bike. I am a very gentle rider and
enjoy cruising mountain roads. I do not ride fast or aggressively. Thanks.

 

What is your weight compared to his? Did he usually ride two up? Do you?
Luggage?

I don't remember the 1100RS being particularly softly sprung. If it were
me, I'd go back. I weight about 180 and ride alone.

 

Is that a K-1100RS? Ask the guy exactly what brand of spring he put
into the forks, in terms of spring rate, preload settings, oil weight,
and oil level before you start experimenting. He just may know
something about what he's doing if he tells you that he's installed a
straight wound spring and may know nothing at all if he says he
installed Progressive springs...

I looked at the www.racetech.com site and plugged in 175 pounds in
their spring calculator and come up with a recommended spring rate of
0.961 kg/mm...

That's a 53.6 pound per inch spring...

Race Tech offers four springs for the K-1100RS
29.0mm diameter x377mm long $110.00
..80kg (44.7 lb)
..85kg (47.9 lb)
..90kg (50.92 lb)
..95kg (53 lb)

The 94 K1100RS came with a stock fork spring rate (measured)of only
0.460 kg/mm. That's only 25.7 lbs per inch!

What are the gentlemen engineers at Race Tech trying to do, by
recommending that you install springs with twice the rate per inch as
the stock springs?
Are they *crazy*?

No, they aren't crazy. They are trying to set up the forks to use only
half their available travel so the machine will have more predictable
steering...

There's the old story about Giacomo Agostini, and how the MV Agusta
factory offered him a factory ride on their GP bike, so they sent him
out for a test ride on what they had available, which was a 600cc shaft
drive inline 4...

The machine was set up as a touring bike, it had long travel suspension
like BMW's have. It had about 6 inches of travel in the front, and 4
inches in the rear. Agostini came back from his test ride and told MV
that he would race for them if they would reduce the front fork travel
to about 3 inches and the rear travel to about 2 inches (or less, I
disremember the exact numbers). The MV representatives were horrified,
and refused to modify their carefully engineered suspension system to
meet the demands of this racer. Who did he think he was? Then they
reconsidered and hired him and he won about 15 world championships in
various racing classes...

But, obviously, BMW expects their machine to be ridden straight up for
comfortable long distance touring, they don't expect the rider to be
doing any dangerous maneuvers, like turning the machine at high lean
angles. Or racing in MotoGP...

A motorbike wallows in the corners with a long travel soft spring. I
could even feel my touring machine wallowing slightly on the freeway,
in long corners with half a mile radius. Preloading the rear shocks a
bit by turning the adjuster made the wallowing go away, and gave mne
more confidence in the handling...

Now, what if you went back to stock springs and left the rear springs
preloaded all the way stiff? Your motorbike could turn into a very
scary
Frankenstein machine, as the rear tire bounced across sharp bumps and
tried to transfer weight to the front wheel. Your motorbike would feel
absolutely dangerous under those conditions...

However, some experienced BMW riders do get frisky. They do track days.
When you lean a motorbike over to a 45 degree lean angle so you can
corner at 1 g lateral acceleration on a race track, the effective
static weight of the chassis on the springs is 1.414 times the static
weight of the machine when riding in a vertical attitude...

A motorbike with really soft stock springs will compress its
suspension, and I suppose a BMW with a heavy rider and stock springs
would be compressing all the travel out of the springs at high lean
angles...

If the suspension travel is all used up by static loads, there's
nothing left for sudden dynamic loads imposed by sharp bumps except the
compression damping of the fork due to the movement of fork oil through
whatever valving
the fork has...

If a fork has a compression blow off valve and really soft springs and
the rider hits sharp bumps while leaned way over, the motorbike will
effectively have a rigid chassis, and the tires will be the only point
of suspension compliance...

Like most other manufacturers trying to cater to the sport touring
crowd, BMW has worked their way into a corner with that tiny little
120/70 X 17 front tire. It has to have really rigid sidewalls to
support the excess avoirdupois of a porky rider on a heavy motorbike...

If the front forks bottom out due to soft springs while you're trying
to corner, that little front tire will skate away from you and you'll
find yourself in a huge speed weave that will try to spit you off. If
you find both legs on one side of the motorbike, it's best to *let go
of the handlebars before the motorbike flips over and launches you into
the air*...

You may not race or want to ride really fast, but the point I'm trying
to make with this tale is that performance riders don't like long
suspension travel and soft springs on their motorbikes. It makes the
steering too unpredictable. That's what you'd get if you went back to
stock on your BMW. The forks would move up and down more than you're
used to and the rake and trail would change a lot more than you've
gotten accustomed to and you'd find the handling scary when you
approached a sharp turn and tried to take it at the speed you're used
to.

Springs have a natural frequency as they compress and rebound.
Automotive engineers work with those frequencies to get a ride that
appeals to whatever group of customers they are trying to attract. Your
father's Oldsmobile's springs had a suspension frequency of about 1
cycle per second. The ride could be described as "boat-like". It gave
kids motion sickness. They had to take Dramamine...

Your BMW with the original springs would fall into the "boat-like"
category...

Sportscar spring frequencies are about 3 cycles per second. The car
handles great, but the driver wants to get out of the car and rest
after several hours behind the wheel...

Sportbikes and racing cars can have suspension frequencies of 3 to 5
cycles per second. They feel taut and responsive on smooth race
courses, but a sportbike with a 5 cycle per second frequency will
jiggle your eyeballs so hard on a concrete freeway, you can't focus
your eyes...