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Fork Spring Preload (cartridge forks)

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I got a set of '03 DRZ400E forks and was doing some measurements. Does a factory fork preload of 15 mm sound right? Seems like a lot... I did some quick sag measurements (in street clothes @ 190 lbs) and got:

  • 1.25" static
  • 2.6" race

I have a '00 DRZ400S with a clarke 3.9 tank (that is about 3/4 full). My concern is if I go to a higher rate spring as typically recommended (0.46 - 0.48) then I'll end up with even less static and maybe just the right race (since I probably have 15 lbs of gear). RT springs are the same free length as stock springs (maybe a couple mm shorter) so the preload would be about the same.

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I don't recall having any preload spacers in my forks when I stripped them, however I do now run 15mil of preload with .48 springs, essentially this is to compensate for the extra fuel load of the 17 litre tank.

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I don't recall having any preload spacers in my forks when I stripped them, however I do now run 15mil of preload with .48 springs, essentially this is to compensate for the extra fuel load of the 17 litre tank.

and the 220 lbs rider weight........ 😛

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I don't recall having any preload spacers in my forks when I stripped them, however I do now run 15mil of preload with .48 springs, essentially this is to compensate for the extra fuel load of the 17 litre tank.

I too didn't see any preload spacers. What I did was loosen the fork cap and slide down the inner tube till it bottomed. Then I measured from the top of the inner tube to the fork cap edge where the spring rests and with the cap removed to the top of the spring. That difference is 13 mm. Now if my measurements are correct than that is preload. I often hear with the right spring, preload should be around 5-6 mm (for a mx/dirt bike). So I don't know if that's in addition to the factory set preload due to the install spring length.

Edited by bmwpowere36m3

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I remeasured my sag in riding gear... I guess I'll need to go back and check for any preload washers or maybe the spring is aftermarket. Don't know since I bought these forks used.

Forks:

  • Static 1.375" (per dwight rudder target 1.582")
  • Race 2.325" (target 2.825")

Shock:

  • Static 1.625" (target 1.276")
  • Race 4" (target 3.944")
  • I know I need a stiffer spring

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I remeasured my sag in riding gear... I guess I'll need to go back and check for any preload washers or maybe the spring is aftermarket. Don't know since I bought these forks used.

Forks:

  • Static 1.375" (per dwight rudder target 1.582")
  • Race 2.325" (target 2.825")

Looks like the rate is pretty darn close but you have a bit too much preload...

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Looks like the rate is pretty darn close but you have a bit too much preload...

Yea that's the thing, there are no preload spacers in the forks. Essentially the spring would need to be ~13 mm shorter for there to be 0 preload. I'll go back and measure the fork spring length, but my other measurements (coil diameter 0.205" and weight ~620 g) are similar to what cal_tony posted (other than weight?):

The fork springs
have
no identifying marks on them. However, I can give you the weights of the springs

Stock S fork spring is 44 kg/mm it weighs in at 572 gr.and is made from .205 wire

I do have a 48 kg/mm spring. It weighs in at 640 gr. and is made from .215 wire.

So when people talk about preload I wonder if they just mean the thickness of any spacer installed (for example if there was a 6 mm spacer in the forks, you could say 6 mm of preload... but truly it would be 19 mm). However I did find a problem with one of the forks (dented fork lower and a sticking fork tube about 1/2 down in the stroke), so that might be partly to blame... in addition to fork oil that was yellow/mucus green in color and pretty thick (probably never changed).

Edited by bmwpowere36m3

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Found a topic where a member reported the same preload ~13 mm with his factory springs, so I'm not the only one. Maybe I'm just over thinking it, but if you go by static & race sag then the fork spring is okay for me at 215 geared up (according to Dwight Rudder's recommendations). However I saw an older post by Eddie S where he states that 20-30 mm of static sag is desired front and rear.

have checked the the static sag of both ends?should be around 20-30mm on both ends,just the wieght of the bike.

what about fork spring preload?

do the forks feel like they have stiction?could the be in bind on the axle?

My perception is the front is too soft and the general consensus says to use a 0.48 spring at my weight. I'm guessing people you have 0.48 springs (RT FRSP 4351 series 508 mm vs. stock 510 mm length) installed must have very little sag in the front.

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