Replacing fork oil with heavier stuff?

[Dragunov-21]

My front forks are softer than I'd like - diving under braking etc...

Is it possible to firm them up a little by replacing the fork oil with a different grade? Is it difficult? What effect would it have on the bike's handling? (Obviously if it'll adversely affect it, I don't want to do it).

Cheers - Alex

[Chewie]

How long has it been since you changed your fork oil? Your forks may just need to be serviced rather than changing oil.

Changing fork oil is a fairly easy job.

[Cbr1k]

Fork oil only last a few years at most so always a good idea to change it.

However if you are too heavy for the current springs then changing the fork viscosity is a bad way to fix it.

[Ryder]

Have used thicker fork oil in my old CB250 and ZZR250 to great effect. This works a treat in bikes that have no adjusters. Also, if you need to increase your preload (which it sounds like you do if it's diving under brakes), you can do it by adding a couple of thin (1mm ish) spacers on top of the springs just under the caps.
Appropriately sized washers will do this, or if you're lucky and own a 250 with small forks then you might find a 20c coin is a perfect fit. Again, if your bike has no adjusters then this works an absolute treat.

[Dragunov-21]

How long has it been since you changed your fork oil? Your forks may just need to be serviced rather than changing oil.

Changing fork oil is a fairly easy job.

The oil's done 2 years, 9000kms. (Original oil)

Fork oil only last a few years at most so always a good idea to change it.

However if you are too heavy for the current springs then changing the fork viscosity is a bad way to fix it.

I'm 70kg, so the springs should be about right, yeah?

Have used thicker fork oil in my old CB250 and ZZR250 to great effect. This works a treat in bikes that have no adjusters. Also, if you need to increase your preload (which it sounds like you do if it's diving under brakes), you can do it by adding a couple of thin (1mm ish) spacers on top of the springs just under the caps.
Appropriately sized washers will do this, or if you're lucky and own a 250 with small forks then you might find a 20c coin is a perfect fit. Again, if your bike has no adjusters then this works an absolute treat.

Cheers - might give it a shot.

[Arwon]

you can increase the weight of the fork oil and also decrease the airgap. Don't use thicker than 10 weight, and check the airgap in the manual and reduce it by 10mm, this will act as a proportional spring. make sure your seals are in good condition before you reduce the airgap.

[Veefore]

^^^^What Arwon said.

From my fading memory, and someone please correct me if I'm wrong, increasing the volume of oil, or reducing the air gap increases the rising rate of the fork. The air in the fork acts as a supplemental spring that increases in stiffness as it is compressed, unlike a coil spring which has a linear rate. By reducing the volume you actually bring the increasing rate of the "airspring" into effect earlier in the stroke.

[Arwon]

that is correct, it is a good way to avoid bottoming during had braking.

[bigmak]

I changed the fork oil from 10w to 15w in my stunta but that was only because i was unhappy with how bouncy it was mid stoppies and i tried everything else i could think of. If your going to do it, do it properly, take your forks off and apart, make sure you clean the internals properly and take your time with it all. it wouldnt hurt to change seals also. In my experience it didnt make the forks 'dive' any less but slowed the process down, which was perfect for what i wanted.

[Marty MOOSE]

These are damper rod forks with very little control you can go way heavier than 10w (sorry Arwon ) I've even mixed up oil for these with a bit of engine oil to get enough damping You can preload with 20c pieces as ryder said and yes reducing the air gap will help heaps mainly over the last 1/3 of travel. What bigmak has suggested is worthwhile if you want to do it properly.

Cheers
MM