ST1300 Sonic Fork Springs

Update 8/9/2012: 30mm - 35 mm should be more like 1-3/16" to 1-3/8". Guess I need to tweak my spacers....

Update 9/2/2007: With the help of Steve Wendelboe, I was able to check my front end sag
while we were in Redlodge for BearSTOC. We measured the front sag to be 3/4" which
is what I was shooting for. I wasn't wearing my gear, nor did I have the bike
loaded with gear, but I was shooting for 3/4 to 1". So I should be right where I wanted it!

Remove the front wheel. It's easier to pull the left fork first, then the right. This
is due to the brake assembly that bridges between the forks.

If you have bar risers like I do, you can't get the cap removal tool onto the cap
unless you lower the fork first as shown here.

The sonic springs are longer than the stock springs. So the spacers will be cut shorter.

130mm gives you a stack that's the same height. But you'll lose 5mm in the cap as shown below.

You lose about 5mm as shown here. The washers supplied by Sonic interfere with the cap retaining bolt and can't be used.
They supplied 4 washers, I only used 2 of the 4, one in each fork between the springs and the spacers.

With the stock spring, washer and spacer you get 3.2mm (1/8") of preload. Add this to the cap preload shown below to get the
total preload.

You get an additional 15mm (19/32") of preload from the cap when using the stock spacer. Total stock preload is a little
over 18mm (23/32" nearly 3/4").

So, I cut my spacers to 140mm. I get 15mm (.59") extending beyond the fork tube as shown. Add the 15mm (.59") from
the cap, less the 5mm (.2") of spacer that rides up into the cap, and you get 25mm (.98") of total preload.

Here's the fork with the cap tightened just enough to contact the riser. The gap between the top of the fork tube and the
portion of the cap that contacts the tube is 25mm (.98"), or the preload.

I used a turkey baster with a zip tie at 125mm to set the fluid level in the fork. Just overfill it and slip the baster into
the fork and suck out the excess fluid. I opted for Bel-Ray 7wt fork oil. One quart is not quite enough.

I used a piece of 1/2" lumber to hold the spacer down so I could tighten the retaining bolt.

Setting Sag on Street Bikes

Setting the amount of sag your suspension has is an important step in optimizing both the handling and the comfort of your bike. Unfortunately, there's a lot of confusion on the subject, so we'll try to keep things as simple and straightforward as possible. First off, what is sag, and why does it matter? Sag is the amount of suspension travel that is used up with the bike sitting still, with the rider and any luggage or passenger on the bike also. It matters because it governs how much of the available travel is used for bump absorption and how much extension is there to "fill in" as the bike travels over dips or pot holes. Having the right balance between these two is critical to performance, safety and comfort.

To measure sag you'll need at least one assistant, preferably two. First, measure the exposed section of the fork tube with the suspension fully extended. For standard forks this is the distance between the top of the wiper or dust seal and the bottom of the lower triple clamp. For inverted forks, measure from the seal to a convenient point on the tube. Write this number down. Next take the bike off the stands (This is important!! Do not measure sag with the bike on the stands!) and get on it. If you have a second assistant have him hold the bike upright while you assume your normal riding position. If not, hold the bike as vertical as possible with one foot, keeping as much weight as you can on the bike. Have the first assistant push down on the front end a little and slooooowly let the bike rise until it stops. Measure the exposed fork tube length and write it down. Now have him extend the front a bit and let it settle back down slooooowly. Measure the exposed fork tube length and write it down. Now average last two numbers and subtract them from the first, this is your total sag. An example:

First measurement: 5 ˝" Second measurement: 4" Third measurement: 4 ˝"

Average of second and third is 4 Ľ", subtracting that from 5 ˝" gives 1 Ľ", or approximately 32mm of total sag.

This is right in our recommended range of 30-35mm for street bikes.

If you have too much sag, add some pre-load. If too little, take some pre-load out.