My first Twin!

bollock chops said:

Mate don't go by the markings on the swing arm for alligning they are ****!! Use a long straight edge like a 6ft level from rear wheel to front with even distances each side

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gregorious77 said:

ah yes ~i know what you mean now.
Ideally you should have spacers on the front wheel which are exactly the right size (difference between front width and rear width / 2)

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millemille
Superstock racer


Here's how I measure and set sag, it may seem a bit over the top but it's worth doing properly and once you've got the method down pat it doesn't take long at all....

Wind all of the damping adjusters to fully off so that the damping doesn't affect the springs. Support the bike so that the front and rear suspension hangs down and measure from the centre of the wheel spindles to a datum point on the forks/frame that is easily found accurately and repeat-ably and that is on the wheel's suspension path above the wheel.

Record the measurement front and rear. That's the suspension's extended length.

Next, measure the unladen (free) sag.

Take the bike off the stand and support it so that the bike is upright and the weight of the bike only is taken by the suspension. In turn on the front and then the rear, gently push down on the suspension, compressing it a couple of inches and hold it down and then release it and let the suspension find its' rest position. Measure the distance from the centre of the wheel spindle to the datum point and record it.

Now gently lift the suspension a couple of inches, hold it up and then release and let the suspension find its' position and measure the distance form wheel spindle to datum point and record it.

Work out the average of the two readings for the front and rear and this tells you your unladen sag taking into account the drag from the fork and shock seals and any friction in the rear suspension linkage.

Ideally you are looking for unladen sag of between 15 and 20mm on the front and 5 to 12mm on the rear (with an Ohlins shock err towards the 12mm as they lose free sag as they heat up). So if you have too much sag increase the preload and if you have too little decrease the preload. When change the preload and remeasure the sag don't just measure the end that you've altered. Because adding or subtracting preload alters the weight distribution fore and aft you need to check the other ends' sag as well.

Once you've got the unladen (free) sag dialled in you need to do the laden sag.

Put all of your riding gear on and sit on the bike. With the bike supported upright repeat the compress/release/measure and raise/release/measure and average of the two to establish your laden sag.

On a track bike you are looking for somewhere between 20 and 25mm of laden sag front and rear and on a road bike between 25 and 30mm front and rear.

If your laden sag isn't correct add or subtract preload until it the laden sag is in the right range BUT you constantly need to be checking the unladen sag.

If in achieving the correct laden sag your unladen sag goes outside of the acceptable range then your springs are the wrong rate and need changing.

If you have to wind in loads of preload to achieve the correct laden sag and as a result your unladen sag is now too little then you need stiffer springs and if you have to take off loads of preload to achieve the correct laden sag and as a result you have to much unladen sag then you need softer springs.

Once you've got the correct rate springs and correct unladen and laden sag return the damping adjusters to their original position and then I fit a tie wrap around one fork slider (making sure that on full compression the tie wrap can't damage the fork seal) and push the rear shock bump stop up the shock shaft and go out and ride the bike, ideally finding a nice twisty and bumpy piece of road (if you live in the south east the A264 from Langton Green to East Grinstead is where I did my all of my suspension testing) and building up the speed until you are at your maximum pace. When you come back have a look at how much suspension travel you have used. If the damping feels ok and you've used all of the travel then think about adding a bit more preload to give a little suspension travel in reserve or even going for a slightly stiffer spring and vice versa if you are not using 95% of the suspension travel. If the damping needs adjusting make these adjustments before looking any more at preload/spring rates as any change to preload and spring rates can be fairly easily accommodated by adjusting the damping.

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