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Old 17th September 2007
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drsctang - Thanks for the info and the links. I read through them quickly.

ColinB - Congratulations, now that is an excellant report. Stating both your weight and the spring rate of the forks or shocks is MANDATORY.

PLEASE GUYS, ALWAYS STATE YOU WEIGHT AND SPRING RATES, EDIT: AND PRELOAD SETTING, otherwise your impressions as to ride quality are meaningless.

I was able to compare the parts listings for 06 and 07 bikes, I assume US bikes. It does not appear Harley used extra rebound springs (as called in the service manual, but called spring slider in the parts manual) to lower the forks of Lows and Nightsters. Something else is different, but I cannot tell what it is. So I have no idea if the forks of Lows and Nighsters can be raised or how much they can be raised. Someone will have to totally disassemble their forks to determine that.

One of the problems is Harley does not tell us what the spring rates are for the different springs used. In addition, the 07 Nightster had such a big preload spacer that put so much force on the spring, even knowing the free spring rate would not help much. I can tell you this, the standard FORK spring in a 07 Nightster is too light for a 180 lb rider, because the forks bottom out too easily, even with a 140 rider.

A word about spring rates. A straight rate (non-progressive) spring is rated in how many lbs of weight it takes to compress the spring 1 inch. For example, if the spring was a straight rate of 100 lbs/ inch, 100 lbs would compress the spring 1 inch, 200 lbs would compress the spring 2 inches, and so on untill the forks bottomed out at the end of their travel.

A progressive rate spring is rated by stating the initial spring rate and the final spring rate. Using shock spring rates as an example, a 75/120 lb spring indicates 75 lbs will compress the spring the first inch, but somewhere in the travel the spring rate increases to the point where 120 lbs are required to compress the spring one inch. 75 is the initial rate, this provides a smooth ride over small bumps in the road. The 120 final rate means the spring is getting much stiffer towards the end of the travel and is much better at preventing bottoming out. A 75 straight rate spring would be too light and bottom out, and a 120 straight rate spring would ride too stiff to be comfortable.

The big question is how far down the travel does the spring transition from a 75 to 120 lb rate. That is especially important to know because if you have short travel suspenion, your spring may not even fully transition to the heavier 120 lb rate, and you will bottom out all the time.

Whittlebeast and I are in constant communication about all aspects of bike set up. His has a better shop than mine. Road Chick sent him her stock shocks off 07 Nighterster, and he found the shock travel is so short, the springs are not transitioning to the higher rate. I suspect the same is happening with the forks.

A few words about fork oil. Changing to a heavier fork oil slows the speed at which the bottom fork tube can move up (compression) and down (rebound). It slows the fork movement throughout the entire range of travel. The slower the forks move, the worse the ability of the tire to track bumps in the road will be. I know two people who tried the Harley SE heavy duty fork oil. Road Chick in her 07 Nightster, it made things worse and she took it out. The other guy has a 07 Roadster. I rode his bike, his forks were mush compared to mine. In addition, while riding along side of him on the highway, I could occassionally see light under the tire because it was in the air, not in contact with the road. This illustrates why using a heavier fork oil to compensate for a weak spring is not a good idea.

It is much better to add extra oil to the forks, about 1 oz for Lows and Nightsters, 2 oz for Roadsters, which will prevent bottoming.

Got to go, heading over to a buddies and supervise him changing his suspension, more to follow.
__________________
Former Ricor test rider for IAS Shocks, Intiminators and Vibranators. Works Dual Rate fork springs, fork brace, Avon Venom X tires, loosen drive belt, and set frame rails level to floor. See "7 Pages of Suspension" thread in the Suspension Sticky Index. It's official, I am now in the "I've forgotten more than I remember club."

Last edited by XLXR; 19th September 2007 at 06:55..
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