Shazaam's Tech Library - Suspension Springs and Sag

A useful technical article from guest contributor Larry Kelly of San Diego CA (aka Shazaam!).

 

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.

Extract taken from this thread at Speedzilla.

Section 8 Superbike offer the following advice

Shock Springs

Below you will find our recommended spring chart for shocks. All rider weights are to be calculated with gear. For example if you weigh 180lbs you would add 15lbs (average gear weight) for a total of 195lbs.

All rates are published in Newton Meters. We do this because that is the unit of measurement that is printed on the springs. Some people talk about rates in Kilograms (which is very close to Newton meters) and LBS. For your convenience we have added a conversion chart to assist you if you are trying to convert a rate in a different kind of measurement.

You will notice that some of the fields under OEM spring rate are left blank, this is because we have not had a chance to get those springs off of the bike and test the actual rate (most of the time it’s different than advertised). We will be adding more stock rate information as we get it.

We included Ohlins base rates in the chart for the springs (OEM) that we have not had a chance to test yet. The Ohlins base rates are usually close to what Ducati ships the bikes with, but that is not 100% of the time. As we test more springs we will add them to the chart.

Below you will find our shock spring chart. The chart outlines what springs we think you should run based on experience, research and a lot of testing.. These can be considered a good starting point, and for most people they will be spot on. But, springs can be tailored to specific riders and styles so this spring chart can not and should not be considered “LAW” but it is definitely going to be spot on for 95% of the rider/bike combos out there. When tuning for that last 5% of the perfect suspension setup we have found that you may end up using a spring that is one step in either direction from your starting point, but this can’t be figured out without some trackside testing and a lot of laps. Please be aware that Ducati sometimes ships the bikes with different rate springs for some reason no one can answer. The stock spring rates in the chart below are most likely correct, but there is a bit of a grey area.

You will notice for example on a 748 that the stock spring rate is 6.4nm. If your weight dictated that you needed a 9.0nm spring then you would be in the position of needing to have your shock re-valved. The valving on the shocks are set up to cope with the spring rate that was originally spec’d out for it. If you increase the spring rate dramatically you will now have a spring that is operating outside the adjustment range of the shock. For example, if you switch a spring from a 6.4 to a 9.0 the rebound circuit will not be able to control the spring as it unwinds and the rebound will be way too fast. On the flip side the increased spring rate will be added to the available compression range and you will end up with a shock that is “too stiff” There is no hard and fast rule as to when the valving needs to be updated, the necessity of valving depends more on the application or specifically what bike/shock/spring change you are making. Some shocks have a larger range and therefore are better at handling a large jump in spring rate. Of course just to make this a little more vague we have not had a chance to test all of the shocks and therefore don’t have data on every combo. But below is a list of what we do have tested and some broad assumptions about some of the ones we have not:
749/999 Showa shocks can handle a 3nm change without needing to be re-valved
749/999 Ohlins shocks can handle a 2nm change without needing to be re-valved
748-998 Showa shocks can handle a 4nm change without needing to be re-valved
748-998 Ohlins shocks can handle a 2nm change without needing to be re-valved

Most of the other models, that consequently come with lower spec shocks, and most of the time need completely different spring rates than stock will need to be re-valved and too add insult to injury some of them are not serviceable and therefore you will need to simply replace the shock.

Click table for bigger image

 

How To Read An Ohlins Spring Code

1091-34/100 is an example of a code you will find on an Ohlins shock spring. Using the below charts you can “decode” what you are looking at.

The 1091 refers to the diameter and the length of the spring. Almost all Ohlins springs have a diameter of 57mm so that’s easy, the 1091 in this case means it has a 160mm length (when not on the shock)

The -34 is the spring rate. For some unknown reason Ohlins uses it’s own numbers to label rate, but below on the rate conversion chart you can cross reference this number. In this case a -34 is a 10nm or 10.19kg or 571lb spring

The /100 is the springs rate in Newton meters, but without the decimal. In this case the /100 means it’s a 10.0nm spring.

Spring Length Chart

1093 - 150 mm - 5.9"

1091 - 160 mm - 6.3"

1092- 170 mm - 6.7"

More from Shazaam!

1091-31 is the Ohlins code for a 160mm (6.3 in) long 57mm I.D. spring having a spring rate of 95N/mm (542 lbs/in). Suitable for a monoposto rider+gear weighing more than 265 lbs. or a biposto rider+gear weighing 235-245 lbs.

where:
/95 = 90N/mm stiffness
L090 = production batch number

1091-29 is the Ohlins code for a 160mm (6.3 in) long 57mm I.D. spring having a spring rate of 90N/mm (514 lbs/in)
Suitable for a monoposto rider+gear weighing 245-265 lbs. or a biposto rider+gear weighing 220-235 lbs.

where:
/90 = 90N/mm stiffness
200 = production batch number

The following table lists the Öhlins rear springs for Ducati applications and rider weights:

Click the table for a bigger image

Ohlins says that the allowable deflection of their 160mm-long springs is between 75 and 85mm total. The lower value is for the stiffer springs. Preloading to 18mm (or any other number) proportionally reduces the available suspension travel.

Please note that Ducati-UpNorth.com cannot accept any liability for the accuracy or content of this section. Visitors who rely on this information do so at their own risk. If you are unsure it's worth contacting your local Ducati dealer who will be able to help. Do not attempt a repair or modification if you do not have the correct tools or knowledge to do so.