Suspension with Terry Hay

This entry is part 12 of 22 in the series Adventure Rider Issue #2

Adventure riding often needs bikes to carry big loads, and those loads have a major impact on the bike’s suspension and behaviour. Even weight distribution is the key.

Fully loaded man

Adventure riding is taking man and machine deeper and deeper into the Australian wilderness. As we get further afield we generally get further from assistance, creating a need for us to carry more and more gear with us. It’s great to be prepared, but all that extra gear means a lot of extra weight on the bike. How you arrange that weight can be extremely critical to bike performance.

Balance

Even weight distribution is the key to getting the best from your bike.

When the manufacturer produces a motorcycle it’s done with the intention of having an even weight bias between the front and the rear of the machine. The centre of gravity is also very important. We can easily upset the balance when we load our bikes up by positioning our load poorly.

Our bikes are subject to several forces that generate varying reactions within them. Acceleration, braking, cornering and steering all serve to upset the attitude of the motorcycle and take away from its stability. These need to be controlled to minimise their unwanted influences.

Acceleration and braking will make the bike pitch fore and aft (diving under brakes and squatting under acceleration).

As we lean the bike in a corner we experience the ‘roll’ effect as the bike is tipped left or right. Steering will bring about a displacement of the wheels away from the centre line of the vehicle known as a ‘yaw’ effect.

So essentially our bike rotates around three separate axes: pitch, roll and yaw.

The axes are easy to imagine if you think of a spear pushed through the motorcycle in three different points.

The pitch axis would see the spear pushed through the side of the bike somewhere around the airbox region.

The roll axis would see the spear pushed through the motorcycle from front to rear, roughly level with the cylinder head.

The yaw axis would see the spear pushed vertically down through the bike just in front of the footpegs.

Mass hysteria

In an ideal situation these points would all intersect creating a single point that would be our “mass centre” of the bike. This would create the most balanced motorcycle possible, providing maximum potential for the best possible handling.

Unfortunately we don’t live in an ideal world and sometimes have to put up with “good enough”.

But knowing this we can imagine how an imbalanced load will effect the handling of the bike.

• Weight too far back will result in understeer and excessive squatting under acceleration (by far the most common scenario due to the ease of storage by means of racks or strapping load to the rear of seat)
• Weight too far forward will result in oversteer and excessive diving under brakes (typical with oversized tanks with no other gear on the bike)
• Weight too high will see the bike drop into corners too aggressively and be difficult on corner exit (as with heavy tank bags)
• Weight too low will make the bike hard to turn, but see the bike exit corners easily (as with additional fuel cells or saddlebags/panniers).

Obviously the further the weight gets away from the mass centre, the more influence it will have in upsetting the stability of the bike, while weight placed at the mass centre will have very little effect.

Centre point

Naturally we can’t place weight in the mass centre. It’s generally where the rider will spend most of his or her time, so we seek alternatives.

Knowing this we can now start to see that weight needs to be countered with other weight to maintain control and stability. A larger tank can be countered easily with saddlebags or panniers. Gear placed on the rear seat or racks can be countered with gear mounted down near the front of the motor. This is a great spot for small heavy (durable) items such as tools, tent pegs etc. Front fender bags don’t provide massive amounts of storage but they do provide a valuable medium for forward weight placement. Tank bags should be considered a must in my opinion.

Naturally all this weight can be placed on the bike as carefully as we like and it will still have an effect on the bike’s handling. As we load the bike the weight will naturally compress the springs in our suspension, lowering the centre of gravity. This can be helpful in cars but not so in motorcycles. It will make them harder to manoeuvre. Not only that, it will also reduce our ground clearance, severely limiting our off-road options.

The trick here is to account for the weight you will be carrying and upgrade the suspension springs to suit. Companies such as RaceTech or Eibach produce a range of springs to suit most popular off-road bikes and their load carrying requirements.

Overall

Consider the total combined weight of bike, rider and equipment and consult with your local suspension supplier as to the appropriate springs for your needs. This will allow you to maintain original ride height and geometry specs, providing the best handling outcome.

Most shock absorbers or forks will have adjustable damping to allow you to control the movement of the suspension. This will need to be slowed down when the bike is loaded compared to its unloaded state. If your bike doesn’t have external adjusters you can still influence the suspension movement with internal changes such as oil weight or system modifications… once again, consult your local suspension tuner.

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