Motorcycle Chassis and Suspension Specialists
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  ProMecha + Graham Byrnes PhD   motorcycle chassis and suspension specialists
     < Myths and Misconceptions >     

Is it true that a "Non-rebuildable Shock" is not able to be re-built?

  Over the years we have not yet found a shock that we can't rebuild. The biggest factor is usually the cost to rebuild such things. Most modern shocks (let's say from late 80's) are able to be converted into a re-buildable and fully serviceable shock for a minimal cost increase, usually a regas valve. Some require machining to have them opened and then we install clips and regas valve to complete the job. The most expensive rebuild is when we have to return the shock to original condition i.e. chrome and overall appearance for a restoration.

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I can 'borrow' my mate's suspension settings and have them work for me?

  Each rider has a different riding style and favourite way of turning a motorcycle into a corner. The adjusters on motorcycles are for you to trim or set-up the way the motorcycle behaves to suit your riding style. So the best thing you can do is work them out for yourself, as you have your own unique way of doing things. (See road/race set-up and tuning; offroad set-up and tuning)

More Preload makes the spring stiffer?

  Preload makes the bike sit higher, or lower - it does NOT make the spring stiffer.  
  "So if someone tells you that you should reduce your preload to make the bike feel less harsh, they probably donít have a clue" (GB). A spring's job is to be able to compress almost fully and then return to it's free length without any changes to length or rate. When you "preload" a spring it simply means you compress the spring with a load or adjuster before any vehicle/bike load is put on the spring. So if you have a spring that has a rate of 1Nm per mm and when you assemble the forks you compress the spring (preload) 10mm with the adjuster backed right off, then that is "fitted preload". The usual preload adjuster has a further 15mm of preload range, this means the total force you have stored in the spring is 25Nm. To make the fork move you have to exceed this load, and then the rate increases by 1Nm because that's the spring rate. What makes it feel more stiff is that instead of starting at 10Nm it starts at 25Nm because the one force is higher than the other. This is what gives you the feeling of a stiffer spring.

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I don't use my adjusters as one setting should cover all types of riding I do?

  (See road/race set-up and tuning; offroad set-up and tuning)  
  The tests are to check spring rates and damping effects. And adjusters are there for adjustments to be made to suit different applications e.g street versus competition. A setting for solo street riding which works perfectly, gripping in the wet etc, will not suffice when used in a race scenario where late, hard, heavy braking is called for and quick acceleration out of corners. Much firmer settings are required for race track use; it has to be when you are using the suspension much more vigorously. A touring bike has to compromise its settings for a variety of road surface irregularities but always on a sealed road. To get a broad range of damping control we individualise the set-up to the person, bike, and type of riding planned. The broader range means that the overall set-up will be at a slightly less high level and the adjusters must always be used if you want to achieve the very best compromise for a particular type of riding.
  For example, PMA race equipment allows for a large, wide range of adjustment whereas standard equipment is concerned with much lower wheel travel rates and more bump control.

      Springs and parts


Are Aftermarket Shocks the solution to poor suspension action?

  You have to establish what level of performance you want to gain by modifying the suspension. For the rear shock, a new spring and a revalve will take a lot of the harsh feel out of the shock, and will improve plushness and help control squat. These mods will give you a superior shock when compared to the standard unit. The aftermarket shock will have an advantage, mostly in the way the adjusters work, the piston design and the slippery surface finishes.

My suspension appears to handle better two-up than solo?

  People often say a bike that is underdamped and undersprung works better with more load. It actually doesn't but will move around less. The suspension action will still be harsh and to get the best you would need to re-spring and at least revalve or consider PMA options. The harshness will be masked by the extra load and give the impression of a better ride. Basicly you need to decide what application/ riding style is most important to you (e.g. ride-day, sports, touring) and from there you can narrow down the equipment necessary to do the job.

Suspension mods review

Triumph Bonneville T100
Bloody brilliant! The first thing you notice is of course that the bike doesn't slump on its rear springs like a cringing dog the moment you get on board.

Rob Smith


Steering is too light and shakes - I need a steering damper?

  Most headshake problems, especially with an application of sports/touring come from the rear shock. It unloads the front as the back squats too far, and this causes headshake in most situations.
  A steering damper's job on a roadbike is to control side-to-side oscillations of the front wheel (headshake). As an extreme example - consider a Race Bike where the front wheel will very likely come off the track. If it hits a bump when it comes down it can cause headshake; the job of a steering damper is to stop these oscillations. They may be of a particular amplitude to not self-cancel. The steering damper does this for you. When the suspension is correctly set-up with springs and damping to suit the rider and the application, for most road and not-too-serious track conditions, the steering damper is not needed.

Nitride treatment to forks makes a substantial difference?

  No, while the surface is a lot more slippery, it doesn't produce massive real-world differences. It also means you can't repair stone damage - it just will not polish up like chrome, it's too hard. The surface treatment demands closer tolerances, better alignment or the advantages will be mostly lost in a sloppy front end. So if you want the best in low friction and you put a high end coating on, then the rest of the fork must be brought up to a similar high end level of engineering.

Can I use brake fluid in my front forks?

Dedicated suspension oil has added chemical agents which control viscocity over a range of temperatures. It is important that these specially designed damper oils should be used with the desired viscocity, viscocity index, appropriate lubrication properties, compatible with all the materials used in forks (e.g. bearing & sealing materials) and resistance to foaming. Please refer to  Damping and Oils  for the reasons why you can't just put any oil in your suspension and expect it to work correctly.

Suspension mods review

Kawasaki ZRX1200R
I've commented in previous tests.. that a suspension fiddle was on the must-do list. Well, it's now mission accomplished - and the transformation is impressive.

Ken Wootton


Does the rear actually RISE under acceleration?

  "Does the rear actually RISE under acceleration? If the swingarm were not connected to the bike via a rear shock, the force of the drivechain would obviously pull the swingarm away from the tail unit and it would spin around the swingarm pivot towards the front wheel. If the bike is moving, will the momentum of the rider force the rear to squat? or will the force of the drivechain still make the rear end rise?"  (Question posed by Jack Anstice 10 Jan 2007)
  Yes, but it depends. It's not just the chain, but the fact that the tyre is pushing back against the road and so the road is pushing the bottom of the tyre forward, equal and opposite reaction, Newton's 3rd law. Which is after all what causes the bike to accelerate.
  Now if you look at the swingarm, it slopes down to the rear axle. So when the tyre is pushing the axle forward, it tries to move forward underneath the swingarm pivot, which lifts the back of the bike. This is easy to check; squeeze the front brake and start feeding out the clutch as though to do a burn-out and the back will rise. This is called "anti-squat" geometry. If you use the rear brake, it will pull down because the force is being applied to the brake caliper rather than the ground.

Suspension mods review

Honda CB1300
On the surface, the range of adjustability appears to allow individual owners to get a setting just right for them so why did we consider additional tuning? And here's the answer...


  However, there are 2 complicating factors:  

# 1.  As you accelerate, the bike tries to tip backward, requiring more "holding up" force from the back suspension. So this tends to compress the back (squat), in opposition to the lifting effect (anti-squat) from the swingarm angle. Depending on how high the centre of gravity is and how steep the swingarm is angled, one of these will win out and the bike will either rise or fall. That's one reason racers sometimes adjust the swingarm pivot up or down, to get just a small rise. Drag racers go the other way... if the bike squats, the CoG is lower and the bike is less likely to wheelie (but more likely to wheelspin).


# 2.   If you're in the middle of a corner or riding up a hill, the rear suspension will be more compressed to start with, so there will be less swingarm angle. Hence if you open the throttle mid-corner, the rear is less likely to rise and more likely to squat than when accelerating in a straight line. That's a reason race bikes are often given much stiffer rear springs, to reduce the change in attitude between straights and corners.


Rear Spring is too stiff as it doesn't use all the travel?

  One little myth that needs to be de-bunked: some people conclude that the rear spring is too stiff if they don't "use all the travel". Generally, you'll use less travel at the back, because braking loads the forks so much. Accelerating doesnít do the same to the shock, because the geometry of the swingarm and chain help hold the rear of the bike up: that's why some race bikes have adjustable swingarm pivots, to modify the degree of anti-squat. Yamaha's set up instructions for the TZ250 suggest leaving 20mm of travel unused at the rear, before hitting the bump stop. At the front, they suggest 5mm.

KTM 450EXC mods

KTM 450EXC 2006 suspension modifications
Tony has recently purchased the 450 and found the gearing too tall, not enough grip in the tyres and suspension springs and damping not looking after the control side on the bike.

So we'll fix all of these and get his 115kg into a more comfortable and controlled ride.
  There are two reasons for this: the need to match front and rear rates. Also, if you bottom the forks braking into a corner, you might lock the front for a second and run wide. If you bottom the rear on a bump while accelerating out of a corner, thereís a very good chance you'll high-side.
  In reality, we need to maintain traction for handling while braking, cornering and accelerating over bumps. So any choice of spring rates is a compromise, even on the race track. The other important issue is that front and rear springs should be related: since both ends of the bike are loaded during cornering, if one end is much softer it will drop much more than the other. That will change the geometry. Typically if the front is soft relative to the back, the front will try to "tuck" or oversteer. On race bikes it's more common that the front has been stiffened to deal with heavy braking and the back drops more. In that case the bike will tend to run wide.
Myths and Misconceptions
© ProMechA + Graham Byrnes