Your stock front springs are likely to be about .7 kg/mm, a surprisingly common (insufficient) value for front springs on street bikes. You want your springs to be more like .85 kg/mm for street use, as much as 1 kg/mm for track use. The best solution is to buy some good progressively wound springs from HyperPro, Bartels, Racetech, or some such. If you don't want to spend $100 on springs, here's a $2 alternative.
This procedure involves some measuring and simple arithmetic. If you skip the measuring and arithmetic, you run the risk of having your forks bottom out due to spring binding instead of running out of travel, meaning you will lose fork travel.
|Spring Wire Diameter:||_______________________||(a)||About 5mm|
|Number of coils:||_______________________||(b)||About 40|
|Spring Free Length:||_______________________||(c)||About 450 mm|
|Spring Binding Length:||_______________________||(d)||a * b|
|Available Spring Travel:||_______________________||(e)||c - d|
|Travel per coil:||_______________________||(f)||measure, or e / b|
|Fork Travel:||_______________________||(g)||About 100mm (street)|
150mm - 200mm (dual sport)
|Excess Spring Travel:||_______________________||(h)||e - g|
|Excess Spring Coils:||_______________________||(i)||h / f|
|Number of widely spaced coils:||_______________________||(j)||About 40|
|10% of coils:||_______________________||(k)||j * .1, About 4|
|Length of (k) coils:||_______________________||(l)||About 35mm|
|Length of stock spacers:||_______________________||(m)||About 80mm|
|Length of new spacers:||_______________________||(n)||l + m + 25|
|Desired static sag:||_______________________||(o)||g / 4|
That's it, you have increased your fork spring rate by 10%. As long as the binding length calculation on your springs says you can make more cuts, you're allowed to make more cuts. Remember, this is like hair: you can't cut it longer.
I get a surprising amount of email on this. If you want to ride a bike that's a bit tall for you, all is not lost. If you ride a Harley, your dealer will sell you a kit. Otherwise, here's what you can do:
If you lower your suspension, try to lower the front forks and the rear shock by the same amount. Otherwise, the handling of the bike will change.
If you choose to lower your suspension, either by changing fork tube level, changing dog bone links, or getting your suspension modified, your bike will sit lower and have less ground clearance. This means your bike will start scraping more stuff in corners, so these are not such great ideas for track days. Also, your kick stand and center stand will now be too long. You can get them cut and welded at any decent machine shop for about $25.