Compression and springsÉ sounds related anywayÉ
Welcome back to another issue. The first response from our readers to this column was a fellow planning on racing one of the brand new 05Õ 600Õs in amateur class. He had a concern of a very high advertised compression ratio, in the neighbour hood of 13.5:1. He also mentioned the promotional article he read contained something about a top out spring in the fork and wondered what the heck was that?
To address the first part of his question, he wanted to know if I
thought he needed to run race gas with a compression ratio (CR) that high. The short answer to this question is
no, although 13.5:1 is very high, he should be able to run this thing on premium
pump gas. It is just within the
last 4-5 years that I have noticed OEMÕs starting to request Premium fuel in a
few of their higher end performance models. Even more recently a couple of OEMÕs have suggested Race gas
in their motorcross models. They
state that these bikes are sold as race bikes with little or no warranty. I have yet to have seen this request on
a ÔroadÕ bike, other than of course YamahaÕs TZ and HondaÕs RS series of pure
race Grand Prix bikes.
The requirement of Race Fuel is based on many things, although CR is
among the leaders, engine load and rpm expectations and bore size also come
into play. The smaller bore size
of a 600 can get away with more compression before detonating than can say a
1000. You will see 1000Õs probably
top out at a more modest 12 or 12.5:1 as a peak pump gas CR. Anywhere above these numbers and you
need to seriously consider Race Gas.
Race fuels are used by most professional road race teams for a number of
reasons. The primary reason is of
course protection, the factory built motors have been optimized in the area of
piston to cylinder head clearance, overall compression ratio and ignition
timing, all these things invite detonation under heavy load, and Race gas
protects against detonation.
Secondly, is performance.
Race fuels are designed to protect against detonation, not add
power. They allow the engine
builder to assemble a more radical motor that would otherwise not survive on
pump gas. HoweverÉ having said
that, a few race fuels out there will add horsepower, even to a stock
motor. I have seen Nutec and Elf
for example add 2-4 horsepower across the entire rpm range to a stock engine
just from pouring it in. If you
have ever priced engine modifications, the price of race gas isnÕt so bad a
deal if you are just using it at the track. The final reason teams will settle on a race gas for the
season is consistency. In this
current age of fuel injection, most of the production based street bikes being
campaigned in the National Superbike series across this country are using some
method of fuel injection mapping software. The bikes are built and tuned in the race shop for their
peak performance, and then mapped very carefully to the race fuel. The team can then go all the way to
Calgary at the foot of the mountains, or out to Nova Scotia just above sea
level and not have to worry about local fuel quality or availability and they
know their mapping will remain consistent. This is currently very important as the Canadian National
Road Racing series is enforced by the Dynojet rear wheel Dyno. So, having made a short answer long, I
would rather suggest to this amateur that he put his limited budget toward
tires. The bike he is purchasing,
if geared properly, will be one of the fastest on the grid in stock trim, but
if he has the extra cash and wants a decent power gain across the board, he can
definitely benefit from a few drums of ELF.
The second part of his question involved top out springs (TOS). All modern OEM style forks offered from
KYB (Kayaba) and Showa, the two main suppliers to most of the big volume
manufacturers, use top out springs.
These are small springs that reside inside the forks to resist the very
top part of the forks stroke. They
can be as short as 75mm or as long as 200-300mm (OAL, not stroke), depending on
their required effect. The main
function of these springs is to provide a cushion or deceleration to the very
top of the forks travel. Just
imagine how distracting, not to mention damaging it would be if every time you
opened the throttle in the lower gears the forks made a big mechanical clunk
just as you wheelied. The TOS also
helps the fork track better over the bumps while hard on the throttle. Most of you are probably aware as to
how adjusting the preload affects the static and dynamic sag of the
motorcycle. If you add preload
front or rear the bike will sit higher in itÕs suspension stroke. In much the same way the OEMÕs have
chosen the top out spring for your bike to help the front end sit statically in
a certain spot. This may help them
(with a longer TOS design) to maintain an initial desired rake and trail,
depending on the intended purpose of the motorcycle. If there were no TOS, it would just be you and the bikes
weight against the fork spring and the fork would be fairly stiff all through
its travel. Now, with a properly
selected TOS, the fork will act softer in the upper part of its travel,
allowing you to comfortably run a stiffer spring that will perform better at
higher speeds down deeper in the forks travel. This is even more important as stock spring rates have
climbed fairly high in the last few years. Another consideration is that Top out springs make a
straight rate spring act more like a progressive rate spring. A straight rate spring in this instance
is a typical wound coil spring with uniform coil thickness and spacing from top
to bottom, (other than where the last ÔdeadÕ coil is ground flat to provide an
even surface for the spacer or retaining system). Progressive rate springs can have two or more rates
incorporated into one spring, these springs can be recognized visually by the
change in spacing of the coils near one end. This changing spring rate will allow a soft supple action in
the first part of the springÕs compression, ramping up to a more aggressive
rate deeper in the springs travel (sound familiar?). This gives smooth, softer compliance over the bumps, leaving
the rider and chassis less distracted, yet the stiffer coils control the big
chassis dive caused by higher cornering speed or during heavy braking. Although progressive springs are still
used extensively in some applications, typically the TOS combined with a
stiffer straight rate spring seems to be the direction of the future.