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Suspension - It used to be easy

16:48 23rd February 2000 by Bikemagic
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IT
USED TO BE EASY




 
Jump
straight to it…

What
should it feel like?
Springs and things
Virtual Pivot Points
Bob – what is it?
Pro Setup Tips
Oval Chainrings -how?

 

Years ago, full suspension,
indexed gears and V brakes didn’t exist and bike set-up was a doddle. Buy
a bike and ride it. The only problem was that in those early days, bikes
didn’t actually work very well, and magazines were obsessed with things
like ‘handlebar buzz’, subtle nuances in tyre compound and weld type minutely
altering ride quality.
Then suspension arrived. Rockshox, Offroad and Manitou suspended the front
of bikes, and everything went mental. Cannondale added new bottom brackets
halfway up the seat-tube and used them to mount a swingarm on. They flexed
so much the front mech would rub on the rear tyre. Suspension on bikes these
days has such an air of simplicity that it’s puzzling why it’s taken us
so long to get there. Over-simplicity in early bikes, with corners cut on
bearing and shock materials, made bikes that stank like the dogs they were.
These days we’re used to coil-over shocks, metallic rolling-element bearings
and the like. Everything’s getting much better. One reason things have moved
on so much is set-up. Early bikes used stiff springs and binary damping
circuitry, and it was all just guesswork really. Early Rockshox forks remained
locked out unless you hit a big rock. Compare that to the modern breed of
low-stiction fork that’ll sag under the weight of the unloaded bike. As
softer spring rates and damping settings have come into play suspension
has been adopted by more people. Suspension requires a learning process
to get the most out of it though. No more sprinting up hills, or nose-diving
off rocks. Now fast riding is all about smooth power and wheelying off drop-offs.

 

JARGON
BUSTER

Preload
Adding
a loading onto the spring in its static position to create a force
against suspension movement. Doesn’t increase the spring ‘weight’
but does increase the load needed to start it moving. Normally done
by winding collar (rear coil shocks) or top cap adjuster (forks)
down onto spring.

Negative
spring

A spring that pushes the shock back past its static position when
unloaded. Creates a plusher feel and allows wheel to extend quickly
into depressions for better traction.

Spring
weight

The force needed to compress a given spring one inch.

Spring
rate

The change in effective weight of the spring as leverage within
the suspension system changes under compression. A rising rate system
will increase its ‘weight’ as it compresses – often described as
‘ramping up’ or ‘progressive’. A straight rate will remain constant,
and a falling rate will decrease.
Coil springs can be wound specifically to a rising, straight or
falling rate, while air shocks will always be rising rate.

Leverage
The ratio between the travel of the rear wheel and the movement
of the shock.

Sag
The amount of travel used up when the rider sits on the bike in
a static position. Again allows the suspension to extend into depressions,
increasing traction and creating a far more fluid, plush ride. We
recommend around 20-25 per cent sag for XC work and up to 50 per
cent for downhill traction

Damping
A resistance applied to the shock’s movement to prevent pogo stick
lunacy. Can be tuned at manufacture in elastomer springs or provided
by parts of the shock moving through oil.
Compression damping is the resistance against the compression (impact)
of the spring to prevent it diving too fast.
Rebound damping controls the return of the spring to stop it throwing
you from the bike.

Linkage
An
extra link in the suspension
system used to take structural stress away from the shock. A four-bar
linkage can be used to alter the path of the rear wheel under compression.
Also allows the shock to be repositioned around the frame, and the
leverage and rate of the shock to be modified to designers’ needs.

Air/oil
Air spring damped with oil.

Coil/oil
Coil
spring damped with oil.

Elastomer
Chewy,
springy rubber available in different weights.

Piggy
back

A shock where the damping reservoir is mounted on a mount above
the shock.

Remote
reservoir

The
damping oil or air spring chamber is linked to another frame-mounted
chamber to increase the volume and decrease progressiveness.

 

What
should your suspension feel like?

The best suspension bikes gobble up the trail without worrying the pilot.
Any obstacles smaller than the amount of travel your bike has can be ridden
over without a thought. Anything larger than the available travel can be
ridden over with far less weight shift than you might expect. All this is
a bit surprising at first. It’s often only when you have an unplanned crash,
or take a line you didn’t expect to, that you find the upper limits of your
suspension. Hitting foot-high rock steps square on is possible on some of
the longer travel forks – but you’d never believe it until you’ve seen it.
We’re not saying you should go out and crash your bike to determine whether
your suspension is working properly. A full suspension bike might not necessarily
be more comfortable, but you’ll end up riding harder trails faster until
the bumps you’re getting through the bars feel the same. To achieve this
level of performance, a bike needs to be set up properly. Some bikes can
be set up ‘more properly’ than others. And herein lies a problem.

Compromising positions

The process of transferring power from your legs to get the back wheel
turning is difficult enough without having to move the back wheel up and
down as you ride over over bumps. Simply having enough gears and getting
those gears to work in muddy conditions is remarkable enough – look at the
trouble Shimano are having with their nine-speed system – but having it
all happen with up to eight inches of travel is quite a feat, though it’s
achievable. Many manufacturers claim that their suspension systems isolate
rider pedalling action from the suspension movement, but it’s a hopeless
cause. All powered vehicles react differently whether being driven or decelerated.
Putting a spring at each end affects everything. And, besides, who says
it’s bad? By placing your pivots cleverly, you can affect the way the suspension
moves when the power is applied. If the pivot point of the bike is above
the chainline, then the suspension will be extended under power. If the
chainline is above the pivot, then the suspension will be compressed. Some
bikes with pivots below the chainline use four-bar linkages – other pivots
within the rear triangle – to modify the path that the wheel axle travels
through, thereby modifying where the ‘effective’ swingarm ‘effectively’
pivots from. These instantaneous centres of rotation follow the same rules
as real pivot points, it’s just harder to spot where they are. But it’s
possible – see page 58. Then there are two different designs: conventional
and Unified Rear Triangle (URT). URT bikes have the bottom bracket (and
cranks, chainrings and other gubbins) all mounted on the swingarm. The pivot
point can be close to the bottom bracket, around the bottom bracket, or
a long way from the bottom bracket, depending on the model of bike. All
these things have an effect too. Does your head hurt yet?

But why should that
matter?

The location of the swingarm pivots has an effect on the suspension
set-up. How the swingarm pivots and how the shock compresses have an effect.
You don’t need to be an expert to set your bike up right for you, but it
helps to have a number of things clear in your mind. As we said before,
a bike with a pivot point below the chainline compresses the shock when
the power goes down. Add to this, the other force that’s being put into
the suspension – your legs flapping up and down – and the bike will bounce
down the trail. A bike with a pivot point above the chainline can, in the
best case, eliminate bob by countering the force that your legs moving downwards
exert on the suspension with an upward force generated by chain tension.
Match these properly and you’ll have a rear end that doesn’t move under
pedalling pressure, within a limited range of gearing. The problem is that
suspension designs can’t yet detect what gear you’re in, and so though a
gear combination of middle ring/large sprocket might give the same gearing
as a small chainring/other sprocket, the way that the suspension reacts
to that combination is very different. Because the chainline is lower, the
resultant force on the shock will alter. This increases the further away
the chain is from the pivot point. To get the bike to work properly for
you, the amount of spring rate, type of spring and amount of damping all
need to be tailored to your weight, riding style and the type of riding.

But don’t bikes come set up like that?

You may imagine that suspension manufacturers spend huge amounts of
cash and resources in tuning suspension for different people. After all,
only boy racers shell out for new springs and shocks on their cars – normal
men in the street don’t have to set their cars up for their weight, do they?
The difference is that a bike weighs a fraction of what the rider does,
and it’s one of the lightest suspended vehicles. The result is that a small
change in rider weight requires a significant change in suspension set-up,
and a similarly small change in rider weight positioning can also affect
suspension performance. So not only do you have to take into account how
much you weigh, but also how you ride and how your bike is set up – small
wonder manufacturers don’t offer pre-tuned bikes for sale.

Springs and springing

Everyone knows what a spring is, and thankfully, the bike industry is,
to date, free from the complexities of torsion bars and leaf springs. We
use ‘simple’ coil springs – actually wound-up torsion bars, but let’s not
worry about that – and air springs. Coil springs can be wound to give a
continuous rate – add a force and they compress a given amount, double that
force and they double their compression. Or they can be progressively wound
– double the force and they compress 75 per cent of the initial amount,
for example. Air springs are progressive too. Setting the spring rate is
the most important aspect of suspension performance. Without the correct
spring rate, the dampers will have to support the machine’s weight, leading
to a lumbering ride with frequent bottom-outs and the possibility of blown
seals.

When it goes wrong
If you’re the owner of a bike with a steel-sprung shock, you might be
feeling pretty smug. You’ve heard that air shocks aren’t as plush as coil-over
ones, and you’re happy that you’re getting a better ride. But what happens
when the spring in your coil-over shock isn’t right? Air shock users simply
add more air whereas you’ve got to buy a whole new spring. If your spring
doesn’t give you the sag you require it’s time for a new spring. But how
much stiffer do you need the spring to be? Spring rates are dependent on
many things – rider weight, on-bike position, the shock length, the leverage
ratio of the bike, the type of riding you’ll be doing… all complex stuff.
To this end, we’ve calculated various different things through and graphs
accompanying this article (see left) will give you an excellent starting
point to set up your suspension with the correct spring rate. You’ll already
know the travel of your shock, from the above sums, but you’ll also need
to measure the travel at the rear wheel. This can be calculated reasonably
accurately by measuring from the centre of the rear wheel to a point on
the back of the saddle at full extension and full compression. From there,
it’s a matter of looking at the graphs (left). Because spring rates are
shock-dependent, look at the graph that gives you the length of shock you
have and compare it to the amount of travel your bike has. Read off, along
the bottom, your weight (we’ve allowed 10lb for clothing) and see where
the lines intersect. This gives you the spring rate you need, which you’ll
have to take in increments of 50lb to your local shop who’ll order you up
a spring to suit. Of course, if you’ve an air spring, you’re laughing. Simply
pump more air in and it’ll be sorted. All you’ve got to do is fiddle with
the pressure and get the sag right. Simple!

But that’s only the beginning
Damping next: the damping has to be as light as possible. As we’ve already
mentioned, the fact that the bike is so light when compared to the rider
means that any additional force slowing down the movement of the suspension
will hinder the operation of the suspension. Damping circuitry is designed
to control the movement of the shocks, but should be kept a minimum. Compression
damping is hardly needed by light riders, and only needs to be increased
with heavier riders. There’s no hard and fast rules for applying damping
– if your shock will let you change it anyhow. But if you’re still bottoming
the bike out despite the correct spring rate, then you’re going to need
more damping. Either dial it in on the shock, or send it away for a service.
Rebound damping governs how fast the shock returns. It too should be set
as light as possible. Too much rebound damping causes the back end to pack
down over repeated bumps, but more often makes the bike feel sluggish at
the back. Wind off rebound damping until the bike’s kicking up at the back
over big hits and then wind it on a little more.

Want to
find the spot your linkage bike is actually pivoting about?

Firstly, check you’ve got a linkage bike in the true sense of the word.
Some models use a linkage to drive the shock via a swingarm. Others mount
the rear wheel on part of a true four-bar linkage. You can spot this by
looking at the bit of the swingarm that connects the wheel to the frame.
Is there a link between the rear wheel axle and the main pivot? If yes,
then you’re looking at a four-bar linkage design where the rear wheel doesn’t
pivot about a simple arc around the main pivot – it follows a ‘modified
path’, modified by the addition of the extra linkage. Think Spirograph,
but with straight lines rather than circles… Find the centre point of
the modified arc, then project lines through the top and bottom arms of
the four-bar link. Where they cross is the instantaneous pivot point. This
point will move, as the suspension compresses. Where it is relative to the
chainline will provide the same outcome as if it was a real pivot point.
But why a moving pivot point? It allows precise control of the axle path
of the rear wheel to keep chain tension exactly as they want. However, most
virtual pivot points provide an axle path very close to that which most
single-pivot bikes are settling for. Strange.

There
are things that can be done to that help you tweak and tune your suspension
easily. Things that you can d
o
to make your bike work better if it’s like one we’re describing. Keep
them as your armoury of shock-setting knowledge, and use them as you
see fit.

URTs
Because of the way a URT works, the spring rate you need when you’re
standing up is different to that when you’re sat down. When you’re
sat, most of your weight is going through your saddle, into the main
frame. When you’re stood up, all your weight is going through the
pedals onto the swingarm. When you stand up on a URT bike, the shock
extends, as there’s less weight bearing on it from the main frame.
Setting the spring rate so that there’s the right amount of sag when
you’re stood just off the saddle means that you get a great dual-purpose
ride. When you’re sat down, meandering along, you’ll be using lots
of the travel. Stand up, the bike stiffens and you’ll be able to take
those big hits too. The bike should just bottom out when you slam
your weight down on the saddle as you’re going along. URTs are different
beasts, and work well when set up in this softer manner, as opposed
to setting up ‘normally’ and then becoming harder when you stand.

“You’ve
got it in the wrong hole”

Lots
of bikes out there have different holes to mount the shock in. It
allows you to change the rate of the shock and the amount of travel.
Generally, dropping the back of the shock increases travel and dropping
the front of the shock changes the rate. A rate change means that
the shock will tend to compress more easily in the early part of the
stroke, stiffening up at the end. This can make the bike feel a little
wooden – particularly with an air-shocked bike as an air spring is
itself a rising rate system. By dropping the shock down (or raising
the front up) you can set the shock up to tend to be a falling rate
system, counteracting the rising rate of the shock, giving a more
comfortable straight rate bike. We’ve used this trick to good effect
on a few SID-shocked bikes and got a plusher feel.

Faster
rebound

If
your shock has no rebound damping adjustment and your spring is a
little soft, then wind on some preload and get something approaching
the ride you need. It’s not perfect, it’s not the answer to all your
problems, but it will give you a better ride.

Harder,
yet softer

This
last trick – winding on preload to speed up rebound – is one way that
a stiffer spring can make a bike feel softer. If the shock is returning
faster, with a stiffer spring or one under more compression, then
the shock will be able to react faster. If you’re not getting the
ride you want then add preload and see where it gets you.

Spotting
correct rebound

When
a suspension system is correctly damped, you’ll feel ‘one and a half
overshoots’. Hitting a simple bump, like a kerb (either up or down)
you’ll feel the suspension compress, then extend, then compress to
its median level. If you feel one thud then it’s wrong, you should
feel down-up-down. If you can actually feel, or see yourself bouncing
multiple times after hitting an obstacle, then you’ve not got enough
rebound damping wound on.

Setting
correct rebound – 2

Of
course, as we’ve mentioned at the top of this article, because the
chain tension changes with pedalling load – or in the case of URTs,
the weight distribution changes – the bike will bob. Low pivots do
it, all URTs do it. What this means is you’ll have to crank on more
rebound damping than you need for correct suspension action. Either
that or pop the sea-sickness tablets.

Oval
chainrings

Though
no one uses them for the reason they were invented any more – to smooth
out the pedal stroke – we’ve used oval chainrings to tame bounce on
a couple of suspension bikes. They work by changing the chain tension
and chainline throughout the pedal stroke, and can totally eliminate
bob and actually increase traction throughout the stroke. We’re not
pursuing it because of the chop and change of test bikes and the nice
way Shimano chainrings shift. Anyone care to take up the cause and
cut us in on a tidy share of the profit?