Preload

 

Let's get one thing straight right from the beginning: Preload and spring rate are two separate things. Spring rate is the actual amount the spring compresses when a set load is put on it. Preload is just how much pressure you load on to the spring before the actual load of the bike and rider is applied

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Springs like to work best when they are under a little pressure.And straight rate (constant rate) springs will sag under the load of the machine and rider, taking up travel. And if you hit a bump you will take up further travel. To fix all this we use preload.

When we preload the spring, we already pressurize the spring so we take up less travel to hold the bike and rider up,plus the spring works better because it is already under pressure.The common misconception is that by bumping up the preload you make your spring stiffer, what you really do is move the spring into a different area of operation. the example table follows:

inches of travel without preload using a 125lb/inch spring

1"_____125lbs/inch

2"_____250lbs/inch------>free sag

3"_____375lbs/inch

4"_____500lbs/inch

using 1" of preload

1"_____250lbs/inch------>free sag

2"_____375lbs/inch

3"_____500lbs/inch

4"_____625lbs/inch

so you see that with an inch of preload the spring has an extra inch of travel and is 125lbs stiffer compared to the same distance of travel on the non preloaded unit.

I have found that for sportbike forks, 0.9 to 0.95 kg/mm work best for 190-200 lb riders.For rear shocks, they work through linkages so they require very different springs for different models.

Another thing to keep in mind with springs, the stiffer the spring, the more force gets directed upwards because of the increased resistance, so if the spring is stiffer than required the force will not get entirely soaked up by the springs

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To set sag: sit on the bike in your riding position with all your gear and have a buddy measure from your swingarm to something on your tailsection and from your front fender to your nose fairing. then get off the bike, rock the bike on the kickstand to get rid of stiction, then stand the bike up and remeasure. The difference should be 25-30mm in front and 30-35mm in rear. These are not concrete numbers and like anything to do with setup, these vary from rider to rider.

A tip: put a zip tie on your fork to see how much travel you are using, if you need a bit more travel you can put on more preload, if you need lots more you will need to change springs.

Note the zip tie

Doug Polen came out of roadracing retirement to race in the GSXR contingency cup in 1986.He made more money as a privateer in winnings than the factory riders did with their salaries. The next year Yoshimura paid him enough to sign on with them.Then he went off to race in japan and win more championships for Suzuki.He came back stateside to race with Eraldo Ferrachi on a privateer Ducati, but as a shakedown test of the new bike and rider they went to the world superbike round in mexico, where they promptly won.At that point they decided to go hog wild and go for the '91 world superbike championship.Since Ferrachi's shop is just a tiny dealership, they had to rely entirely on winnings and selling T-shirts at the track to get by. Luckily, Polen dominated like no other,winning nearly every race and finishing second when he didn't win. At the end of the year they still had to pay ducati for the bike that they just won the championship for them with. The next year Ducati made Polen a factory rider, and he won the title again easily.For '93 they sent Polen to win the AMA championship.It was a cakewalk for Polen, so much so that he put a video camera on his bike, would make a lousy start and sell the tapes of the romp through the pack to win, it was just too easy so he went to Honda to ride the RC45. This turned out to be a nightmare and Honda let him go after 1 year. Doug Polen also won the Suzuka 8 hour for Honda in '94 and the world endurance championship for Suzuki in '97