You can determine whether your rear spring is the right rate for you or not by adjusting the race, or rider sag
, then comparing it with the free, or static sag.
Race sag is the amount the the rear suspension compresses under the weight of the rider. The rider should have as much of his weight as possible on or directly over the foot pegs for this measurement. First, with the bike on a stand and the rear wheel
hanging free, measure from the rear axle to some repeatable reference point. Then, with the bike on the ground, and the rider on board as described, take another measurement at the same two points. The rear suspension should settle a total of about 100mm (3 15/16"), with 97 or so (3 13/16") being preferable.
Free sag is the amount the bike settles from full extension when sitting on the ground under its own weight. Once the race sag is set, get off the bike, lift the rear fender a little, and let the bike settle under its own weight. If the rate of the rear spring is correct, the free sag will be about 25mm, or 1".
Now you can check the spring rate. If the spring is too soft, you will have to preload it too much to get 100mm of race sag, and the result will be that you have too little free sag, or none at all. If it's too stiff, you'll have too unwind it too far, and end up with more than an inch of free sag as a result.
At your weight, I would suggest that you first approach this test backward. Check the free sag first and set it to 25mm, then measure the race sag. If the race sag is pretty close, you can go through and do the process the right way around and double check. If, on the other hand, you find the race sag is way too much, you need heavier springs. I suggest this because there is a minimum length the spring should be preloaded to in order to avoid it coil binding, or bottoming against itself at full compression, and it's important not to exceed that limit. I don't have that info handy
, and it's hard to measure it with the shock on the bike anyway.
Springs are almost always replaced as a set when people do it, so if the rear is wrong, the fronts most likely are, too. Springs are the single most important element of adjusting suspension for weight variances, and even at 280#, the right springs will probably put the damping within the range of the adjustment clickers, so by all means, start with that. I have among my riding pals a father and son duo who are both big guys at around 240-265, and when I resprung their bikes for them, they couldn't believe the difference it made.
Springs are available through any suspension shop, Smart Performance, Eibach
, Cannon Racecraft, MX-Tech, Race Tech
, etc. For your weight, Race Tech
calls for 0.52kg/mm front springs and a 6.3 kg/mm rear.Setting Race Sag
Checking Spring Rates