Adding more preload to stock forks?

[RC390 Fan]

Thinking I could simply remove the fork caps and put longer preload spacers in, but I have never seen forks quite like this. I have done many spring swaps on my dirt and street bikes in the past but they were different from these. Is there an assembly drawing avail anywhere?

 

[psych0hans]

Look at this thread on another forum I'm a member of. It's not too difficult, if you are well versed with tools, to make your own custom preload adjusters. https://www.ninjette.org/forums/showthread.php?t=101897 I actually made my own for my Ninja and they work great.

 

[gbabyracing]

What was done on the Ninja 250 isn't going to work on the KTM. Very different designs. The KTM is a cartridge unit, but has the springs below the cartridge assembly instead of above. The caps are attached to the damper rod with no contact with the springs.

This is NOT a beginner task and I have no responsibility if you attempt this and damage your bike or yourself.

To change preload you will need to unscrew the fork caps from the black outer tube.
Carefully loosen the jam nut on the damper rod and unscrew the cap from the damper.
Drain the oil and remove the lower chromed tubes from the outer black tubes. This could get very messy. Get a bucket to empty in to. Try to keep the outer tube from pulling up or down too far or you will have a real mess on your hands.
You will need a 5mm adjustable pin tool and a propane torch to remove the cap from the top of the chromed tube. Use just using enough heat to soften the thread sealant used on the cap.
Remove the cartridge assembly from the lower tube. The springs and a 5mm preload spacer are in the bottom. I would take this opportunity to upgrade the springs to straight weight units. You can add preload spacer of washers between the washer that sits join the spring and the existing 5mm part.
Re-assemble in reverse. 4 wt oil comes from the factory. Upgrade to 7wt would be a good recommendation. It is difficult to measure oil height on these forks so just go back with the factory recommend 450mm per side. Pour in in small increments and push the damper rod up and down to get the oil to circulate in the cartridge lower. Tighten the caps with the pin tool and maybe use a bit if blue locktite.

First picture shows the cap on the chrome tube notice the small holes and the 2 larger 5 mm holes. Use these with the pin tool.


Second the assembly removed from the tube. This shows the fork cap removed from the damper rod. The first black ring between the cartridge and the spring is the 5mm spacer.


The stock progressive springs are 36mm OD 27mm ID 345mm long came with a 5mm preload spacer.

 

[RC390 Fan]

Thanks, gbaby. This is what I needed.

Hmmm I think I will take your advice and order up some new springs from Race Tech. Who knows I may not need to preload them if I go a bit stiffer. The only reason I wanted to preload the stockers is to help with excessive dive under breaking.

Maybe I can live with the shock considering this bike will be mostly used for 'spirited' street riding and a handful of track days,

 

[gbabyracing]

If you do the straight weight springs, you should preload them at a minimum 10mm for best action.

 

[Bagwell]

gbabyracing

That proceedure is on the cup bike forks or the street bike forks? I dont know the differences between the two forks exactly, but wondered. It seems many dont specify what exact bike when referring to things on the forum. Easy thing to forget for sure.

 

[gbabyracing]

Street bike

 

[Bagwell]

cool, thanks.

 

[DCMoney]

Swapping out the springs sucked! Took way more work than I expected.

It will require several special tools or some rigging to get then apart.

 

[gbabyracing]

Here is a tool to make this go a bit easier . One purchased (race tech) - one made. While holding the lower in a soft jaw vise (where the axle holes are), use the pin tool. The home made is an adjustable wrench with two holes drilled using a carbide masonry bit to accept 5mm punches. Both are adjustable to pin width.

 

[DCMoney]

Great idea drilling the adjustable!

 

[Chris The Welder]

I've had good results on several bikes I've owned by just changing the weight of the fork oil.

 

[psych0hans]

Replacement fork springs from a 03-04 ZX6R .85kg/mm. Didn't have time to take measurements, but will do it ASAP.

 

[psych0hans]

What was done on the Ninja 250 isn't going to work on the KTM. Very different designs. The KTM is a cartridge unit, but has the springs below the cartridge assembly instead of above. The caps are attached to the damper rod with no contact with the springs.

This is NOT a beginner task and I have no responsibility if you attempt this and damage your bike or yourself.

To change preload you will need to unscrew the fork caps from the black outer tube.
Carefully loosen the jam nut on the damper rod and unscrew the cap from the damper.
Drain the oil and remove the lower chromed tubes from the outer black tubes. This could get very messy. Get a bucket to empty in to. Try to keep the outer tube from pulling up or down too far or you will have a real mess on your hands.
You will need a 5mm adjustable pin tool and a propane torch to remove the cap from the top of the chromed tube. Use just using enough heat to soften the thread sealant used on the cap.
Remove the cartridge assembly from the lower tube. The springs and a 5mm preload spacer are in the bottom. I would take this opportunity to upgrade the springs to straight weight units. You can add preload spacer of washers between the washer that sits join the spring and the existing 5mm part.
Re-assemble in reverse. 4 wt oil comes from the factory. Upgrade to 7wt would be a good recommendation. It is difficult to measure oil height on these forks so just go back with the factory recommend 450mm per side. Pour in in small increments and push the damper rod up and down to get the oil to circulate in the cartridge lower. Tighten the caps with the pin tool and maybe use a bit if blue locktite.

First picture shows the cap on the chrome tube notice the small holes and the 2 larger 5 mm holes. Use these with the pin tool.
View attachment 927

Second the assembly removed from the tube. This shows the fork cap removed from the damper rod. The first black ring between the cartridge and the spring is the 5mm spacer.
View attachment 928

The stock progressive springs are 36mm OD 27mm ID 345mm long came with a 5mm preload spacer.

Hi, do you have more pictures of the fork internals? It would be awesome if you could share them.

 

[Bagwell]

If possible, could someone list some tools required to change fork springs and oil on the RC390? Just want to make sure if I plan on doing springs that I am prepared.

Thanks,
Vaughn

 

[Bagwell]

Hmmmmmmm, I may have it straight sounds like an adjustable pin tool for tube caps on lowers and a propane torch.

 

[psych0hans]

Hmmmmmmm, I may have it straight sounds like an adjustable pin tool for tube caps on lowers and a propane torch.

Why do you need a propane torch?

 

[Bagwell]

heat up the threads with loctite on em to get it loose

 

[gbabyracing]

Here are a couple more pics

 

[psych0hans]

Adding a pic of the RC8 internals for comparison. I wonder if they can be made to fit in the stock forks. But you would lose compression damping adjustment because the rc8 has the adjusters at the bottom.