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Old 05-Jun-2005, 18:08
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DSC Member Shazaam! Shazaam! is offline
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I don’t know the length and stiffness of a DU8440 but your front and rear springs are likely still too stiff for your bodyweight. Also, you shouldn’t use increased damping to compensate for an incorrect spring rate. If your damper has incorrect damping rates for your bike it will add to the handling problem and ride harshness. You first need to set sag correctly, front and rear.

Ducati’s superbikes are designed to use a 160mm long rear spring that will allow 75 to 85mm of deflection. If you’re too heavy for the spring, the spring will sag too much under your weight and use up the suspension travel needed to absorb bumps. For severe bumps the suspension will bottom-out (hit the bump stops) if the spring stiffness isn’t in the proper range for your weight.

Conversely, if the spring is too stiff for your weight, the spring won’t be compressed enough and the suspension will (too often) fully extend and top-out.

The rule-of-thumb is that you want the spring to be about 25% compressed when you are just sitting on the bike at a standstill. Ducati recommends 40mm of rear sag for street riding, 30mm for the track.

In the table below, the sag under just the bike weight without rider is called static sag, with rider it’s called rider sag.



I’m assuming here that when you use the term “free” sag you mean static sag. Using your rule of thumb (free sag = 90% rider sag) is incorrect. For use on the street, the free/static sag should be (30/50 to 30/45) 60% to 66% in the front, and (10/40) 25% in the rear.

Using a percentage is incorrect, you need to achieve BOTH static and rider sag measurements. If you can’t, you need to change your spring.

Here’s an example:

The factory OEM rear spring on a 996 has a spring rate of 64N/mm (365 lbs/in, Ohlins’ stiffness code -16) which is adjustable for rider-plus-gear weighing between 155 and 165 pounds.

So, the first thing you need to do is to adjust your spring preload to achieve the proper rider sag.

If you fall within the 155 to 165 pound weight range then when you adjust your preload to get the correct rider sag, when you check your static sag it’ll also be correct.

However, if your weight is more than 165 pounds, you’ll need to increase the rear spring preload to get the correct rider sag, so when you check static sag you won’t have enough because you have preloaded the spring too tight. So you need to replace the stock unit with a stiffer spring.

Conversely, if your weight is less than 155 pounds, you’ll need to decrease the rear spring preload to get the correct rider sag, so when you check static sag you’ll have too much (i.e. more than 10mm) because you have removed too much preload from the spring. So you need to replace the stock unit with a less-stiff spring

The same logic applies to the front springs. Front springs just have more travel to absorb bumps better.



Ohlins Spring Code

Example: Ohlin's spring marked 1091-29/90 090

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)

/90 = 90N/mm stiffness
090 = production batch number

This would be the correct spring for a Ducati pre-999 superbike monoposto rider weighing between 245 and 265 pounds (including gear), or a 155 to 165 pound biposto rider plus passenger (with the spring preload increased to obtain 30mm sag)

The first number refers to the length:

1093 - 150mm
1091 - 160mm
1092 - 170mm
1095 - 180mm
1096 - 190mm

The next number is a code for the spring rate, and the next is the spring rate in N/mm. So, 1093-26/85 is a 150 mm spring with an 85N/mm spring rate.

The following table lists the Ducati standard Öhlins rear springs:



Notes:

• 160mm (6.3 in) 57mm I.D. springs have the Ohlins’ length code 1091

• 64N/mm (365 lbs/in) springs have the Ohlins’ stiffness code -16

• 66N/mm (377 lbs/in) springs have the Ohlins’ stiffness code -17

• 68N/mm (388 lbs/in) springs have the Ohlins’ stiffness code -18

• 70N/mm (400 lbs/in) springs have the Ohlins’ stiffness code -19

• 75N/mm (428 lbs/in) springs have the Ohlins’ stiffness code -21

• 80N/mm (457 lbs/in) springs have the Ohlins’ stiffness code -24

• 85N/mm (485 lbs/in) springs have the Ohlins’ stiffness code -26

• 90N/mm (514 lbs/in) springs have the Ohlins’ stiffness code -29

• 95N/mm (542 lbs/in) springs have the Ohlins’ stiffness code -31

• 100N/mm (571 lbs/in) springs have the Ohlins’ stiffness code -34

• 105N/mm (600 lbs/in) springs have the Ohlins’ stiffness code -36

• 110N/mm (628 lbs/in) springs have the Ohlins’ stiffness code -39

As far as I know, Ohlins doesn’t make a stiffer spring than 1091-39








[Edited on 6-5-2005 by Shazaam!]
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