Whatever you choose to use to pre-load the springs wether metal or plastic is fine, Many modern sport bikes use plastic spacers in the forks. On my CB400F I can adjust the pre-load from the top of the forks just like a modern bike. I'll get round to making some for the 750 one of these days.
The question is....Do we know for what reason and the effects it will have to do this???
For me tuning my suspension to the best it can be(given the limitations in the design and adjustability) is as important as tuning the motor for optimum performance. Setting up suspension can and is a confusing task that not many riders understand. I raced a RVF400 for a few years and what I have learned is from trial and error and talking to more knowledgable racers than myself.
Go to a track day and I suspect 9 out of 10 riders of modern bikes do not have a clue how to set up there bikes suspension.
This is what I have learned and I am most definitely no expert. You can find many articles on the web so check them out.
We should be setting the suspension up to be able to use the full travel available without bottoming out. How do we do this?
Well the first setting to address is the suspension pre-load. This is for the front forks. It is best to have a second person to help and supply the beers..!!
Step 1. Stand bike up level with no rider on board, stand at front of bike and pull up on the bars forks fully extended. Measure from say the top of the dust seal to the underside of the lower tree and record the measurement.
Step 2. Stand bike up level with the weight of the bike only on the suspension. (pump the forks a bit between measurements) Now take a measurement from the same place as is step one.
Step 3. Sit on the bike with both wheels on the ground and feet on the pegs, make sure your helper is holding the bike upright from the back of the seat/fender, if he/she is drinking more beer than helping..!! then well yes you will fall over....!!!! Actually this one needs a third person to measure from the same place as in steps 1 and 2, record the measurement.
Ok what do we have here? we have measurements for static sag and rider sag.
Static sag = measurement from step 1 minus measurement from step 2 = ideally should be around 25-30mm.
Rider sag = measurement from step 1 minus measurement from step 3 = ideally should be around 35-45mm.
If you are a heavy rider and need to incease the pre-load to maintain the rider sag figure, this may remove the static sag figures. If this is the case then you need harder springs.
If you are a lightweight and need to decrease the pre-load for rider sag figure, this might end up with too much static sag. Then this will require lighter springs.
Another quick check is to put a tie cable around the fork tubes and set it at the bottom not too tight,(look at modern race bikes they all have them) go and ride the bike normally, cornering, braking,pot holes etc. this will show you how much of the travel you are using and will be a starting point for which direction to proceed with setting up pre-load.
These steps 1-3 should be performed for the rear suspension as well.
Static sag Rear = 5-10mm
Rider sag Rear = 30-40mm
Now with our machines we do not have the option for adjusting the compresion or rebound damping.
Our only thing we can do is experiment with the fork oil weights or by changing the hole sizes in the damping tubes.
With my 400 racer and my road 400's I use a fork oil of 10weight. with an air gap of 100mm, most sport bikes use air gaps of 90-100mm.
To set your air gap (ignore the cc volume in the manual) with springs removed and the forks compressed, fill with fork oil until the level is 100mm from the top of the fork tube.
Sorry about the long post but I was on a roll and thought this might help get a better understanding and get our machines to perform as best as they can.
Kevin.
