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FullSuspension Mountain Bikes
|front suspension tuning
Spring - A system that
offers resistance to force yet allows movement of unsprung mass.
There are several types of springs used on Mountain bike
suspension systems today. The most common types are Coil, Air and
Elastomer. The springs job is to keep the rider and bike
suspended yet allow some movement when the bikes' wheel hits a
Coil Spring - A coil
spring is usually made of ferrous (metallic) material which has
been wound when hot into the shape of a coil. When it cools, it
retains its' shape and will try to return to it's original shape
if compressed or stretched. If it is displaced too far from it's
original shape, it may reach the yield point, the physical
position when the material fails to the point it will no longer
return to it's original shape. This is one of the reasons that
most coil springs operate on the basis of compression (being
squeezed together) as opposed to being stretched apart. Coils
springs usually have two numbers on the side of them. One is the
Spring Rate, a numerical value expressed with a # sign followed
by a number. The second number is the springs travel, usually a
numerical value followed by a " sign.
2.00" X #650
The above example would indicate that the coil spring has up to 2.00 inches of travel at a spring rate of 650 pounds per inch.
Coils springs are preferred
by many mountain bikers, especially downhill racers, due to their
inherit ease in adjusting. Most can be adjusted by simply
spinning the preload adjuster which allows more or less spring
compression in the static position. This adjusts the amount of
force needed to initiate movement of the suspension system.
Air Spring (Gas shock) - an air spring operates on the basis that it takes pressure to compress air. By trapping the air in a closed space, you can use the air to offer resistance to force. Some manufacturers have gone to great lengths to use air spring systems for mountain bikes but there are several inherent problems. One is that you need to seal the air chamber very securely which requires a tight seal. The tight seals often tend to create "stiction" - a condition where the seal requires extra effort to initially allow motion in the suspension system. In recent years, many manufacturers have overcome this problem by using newer materials.
A second problem is spring rate adjustability. You usually need to have a special pump to raise or lower your spring pre load. You also have no visual reference like you do with the coil spring. You cannot "see" how much air pressure you have inside your air shock by looking at it. Also, the spring rate will change during a downhill run. As the shock moves back and forth, the air inside the cylinder will heat up due to the motions of the shock. As air heats up, it will try to expand thus increasing the amount of air pressure inside the shock. Filling a gas shock with an inert substance such as nitrogen gas, will solve this problem. This however, means that you now need more special equipment to change the pressure in your shock.
For Downhill racing, where
shock rate, preload adjustment and consistency all play a major
factor in your success, I personally would not recommend using
and air shock. Still, there are several successful racers who use
Elastomers - Elastomer basically use a special compound of plastics, rubber and other similar materials, produced in the shape of a cylinder, to mimic the attributes of a oil damped coil over shock system. The elastomers are usually in stacks if used on the front suspension. This allows for greater tuning characteristics as they come in various compounds (hard, medium & soft). The elastomers will try to return to their original shape if compressed, thus allowing for a dynamic suspension system. While elastomer systems offer great benefits for some applications, they are not the best system for downhill racing. They offer good weight savings for the cross country crowd. For downhill racing, they are often over damped (see "damping"), especially on the compression side. This means they do not allow fast enough movement when compressing to absorb high frequency hits ( many small hits in rapid succession).
The Elastomer system is also
not as consistent as a coil spring. It is affected by temperature
(colder = harder, hotter = softer). Because elastomers are bound
to the same basic laws of physics that the rest of us are, this
means that if you start a downhill run with your elastomer stack
tuned perfectly, as it starts to move during your run, it will
heat up and become slightly more pliant. Elastomers can also
reach the fail point where they crumble and fall apart. The small
parts can be most annoying to get out of your system. Elastomers
are also used for rear suspension such as on the Proflex bikes.
This works very well for cross country applications because of
the weight savings and inherent damping characteristics of the
Spring Rate - spring
rate is a numerical value often expressed in terms of pounds per
inch OF THE ACTUAL SPRING ITSELF. This is very important to make
note of because many bike manufacturers' are using linkage
systems to give the overall spring rate a "rising" or
"falling" rate. We will discuss these two terms later
when we look at entire suspension systems. A spring rate of 200
means that it should take exactly 200 pounds pressure to compress
the spring exactly one inch. As you may have already guessed,
these numbers are not exact but they are more than close enough
to get a good starting point to tuning your bikes suspension. If
a spring has two inches of travel and is rated at 200 pounds, it
means it will take 200 additional pounds for the second inch as
well, thus, if you put 400 pounds pressure on this spring, it
will compress the full two inches. Spring rates which work in
this manner are referred to a "linear" or "flat"
rates. This means if you drew a graph of the springs' movement vs.
the pressure required, the resulting graph line would be very
Preload - The term preload is used to describe the initial value of resistance a particular spring is set up to offer. For example, if you placed a coil spring on a shock and wound down the retaining ring until it barely touched the actual spring, your preload value would be zero. If the spring had a rating of 650 pounds, and you wound the retainer down one inch, you would have 650 pounds of preload.
The preload setting is very useful in determining the "sag" of a downhill bike. Most manufacturer's recommend that you run approximately 1/3 of your total rear travel in sag. This means when you sit on your bike, the rear suspension compresses 1/3 the distance of your total travel. Sounds easy right? It's waaaaaaaaaaaay more complicated than that. Read on.
Most people set up their
suspension in a static position on flat ground. When your bike is
on flat ground, the wieght bias is about 25%/75% F/R when your
seated. Tilt the bike forward on a slope and that weight bias
totally changes. Additionally, when you stand on the pedals, your
weight bias changes because the center of gravity goes forward to
the bottom bracket. Studies have shown the typical wieght bias to
be about 38%/62% F/R when standing on the average bike, but once
again, this was done on a hardtail bike using cross country
geometry in a static (not moving, on flat ground) position.
Consider your tires! Is the course largely flat with lots of sprinting? If so, use a tire with a low amount of rotating mass. This will increase your acceleration but usually will sacrifice your cornering speed and grip.
Is the course rough? A tire which is light will also usually pinch flat a lot easier.
Is corner speed crucial? If the course has lots of places where corner exit speed is important (i.e.. - a flat section after a high speed corner) a tire which allows for maximum corner speed is important.
Is the course overly rough with lots of small bumps (hits)? Set your tire pressure slightly higher and your suspension very soft for the first 60% of your travel with little or no damping but adjust your shock position for a steep rising rate for the last 40% of travel for maximum progressivity (if this doesn't make sense, read the Suspension Page).
Does the course have several large hits yet is relatively smooth? Set your suspension so that your shock rate is very linear with medium to maximum compression damping and light to medium rebound damping.
Is the course very steep? If so, remember how a bikes weight bias if affected by tilting the center of gravity forward. Set your rear suspension softer that usually and your front slightly harder. I personally will use a harder spring in one side of my front shocks.
If the course is steep with lots of drop offs and steep chutes, rotate your riser handle bars back towards the rear of the bike. This will increase the amount of throw control on your front wheel when going over drop offs. In extreme conditions, such as Mt. St. Anne, I actually use a shorter stem and a bar with at least 2" of rise.
If the course requires a lot of sprinting yet is moderate to light downhill pitch, I rotate my bars forward. This has the effect of providing better arm leverage which results in a faster sprint. Be careful though, your center of gravity is moved forward on the bike and it will go end over end a lot easier.
Raising the bottom bracket on a rough course or using shorter cranks so you can pedal for the whole course.
On courses with slow corners
followed by straights which require fast acceleration, I use 180
mm cranks as they provide more torque out of corners.
Why do you use steel
fork stanchions when most other manufacturers' are aluminium?
The main reasons that we use steel is for strength, fatigue rigidity, corrosion resistance and because a better finish (giving lower stiction) can be produced. Obviously there is a slight weight penalty but we feel that the benefits outweigh the negatives.
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Why can you see all
the machining marks on Pace products?
To remove the machining marks we would have to rumble or polish the parts and by doing this you will lose the definition on most edges and also it can affect the tolerances of the component.
We get a lot of comments that people like the machined look.
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Do the carbon sliders break eventually?
Generally speaking, the fatigue life of carbon can be "tuned" (in layout) t equal other materials, so life expectancy should be no less than other materials. Carbon fibre is very hard wearing and resilient. The only times that we have seen the carbon break is when there has been a car involved in the crash.