Spring Preload Adjusters - What ARE They For?
My Ducati superbike has these things called preload adjusters and I’ve been trying to figure out what they do and (if I twiddle with them) what good or bad things to expect.
My 916 monoposto Owners Manual tells me that the front spring preload adjusters are set at the factory at 20mm (three lines showing) but can be adjusted for a range of values between 25mm (less preload) and 10mm (more preload.) Ducati makes no mention of why this adjustment is provided or needed, however.
The Manual also tells me that the rear spring preload adjustment is made by turning a ring nut on the shock, that shortens and compresses the spring. According to my Haynes Manual, no standard spring preload setting is specified by Ducati for pre-2000 models, but for 2000-on a standard compressed spring length of 151mm (Ohlins) or 160mm (Showa) is specified. So?
Further, in magazine road tests the rear spring preload setting is sometimes expressed as mm of thread showing above the top (locking) ring nut - the more thread showing, the more preload. Or, it’s effect is described - mm of sag.
I do know that all the Biposto Owners Manuals say that the rear spring should be set to its maximum preload when carrying luggage and a passenger in order to retain the loaded bike’s handling and ground clearance. They mention also that the shock’s rebound damping will also need to be increased to accommodate the increase in working forces in the preloaded spring.
Apparently that’s it. Ducati states that rear spring preload is to be used for maintaining ride height (ground clearance) when temporarily carrying extra weight. Maintaining the correct ride height preserves the height of the bike’s center of gravity, ground clearance and the angle of the front fork.
So it’s therefore interesting to me to read magazine road tests that suggest front preload settings that vary from three to seven lines showing and rear preload settings from 22mm to 13mm of threads showing. So why the variation?
Well, one reason that comes to mind is the (usually unstated) variation in body-weight of the different test riders. Instead of changing to stiffer front and rear springs, the (heavier than 160 lb.) test riders use additional preload to temporarily recover ride height until the road test is completed.
Q. So, is this really the proper use of the preload adjusters, and if so, why bother changing springs at all for heavier or lighter riders?
The only answer I can come up with is that the preload adjusters will allow you to maintain ride height - but not the correct sag. So I guess we need to discuss sag. Sag - Do I Want To?
The only reason that there’s different stiffness springs available to match-up to different rider body-weights is so that every rider sags the same amount. Seems only fair to me.
Sag is the amount of suspension travel used-up to support the weight of the bike and rider. Experience has shown that about 1/3 of the available suspension travel (spring compression) should be used initially to avoid both bottoming-out and topping-out the suspension during most riding conditions.
According to the Haynes manual a Ducati superbike has both a fork travel and rear wheel travel of 130mm. That’s why Ohlins recommends 40-45mm (1/3 X 130mm) of front and rear sag for street riding, a little less for track conditions.
Remember that he stiffness of stock Ducati springs are selected to sag this 1/3 amount for a 160 lb. rider. The ride height (that affects handling) is based on this amount of sag.
So, if a much heavier rider sits on a superbike it sags more, the ride height is too low and the spring will bottom-out (over bumps and during braking) more often. It’ll need a stiffer spring to reduce sag and meet Ducati’s ride height spec.
Similarly, if a much lighter rider sits on a bike it sags less than desired, the ride height is too high and the spring will top-out more often. It’ll need a softer spring to increase sag and reduce ride height.
But, if you just ate too many pies over the holidays, or just put in a lighter battery for the track, the smaller weight change won’t warrant a new set of springs.
Here’s when your preload adjusters become useful. Crank in a little more or a little less preload to regain (or just change) your ride height. You’ll loose or gain a little sag and available suspension travel. When it becomes more than a little you need to change springs. Kind of like a fine and coarse adjustment.
Of corsa, if you just want to change ride height without changing sag you can simply raise the rear ride height by increasing the length of the adjustable connecting rod and change the front ride height by changing the forks position in the yokes.