4CS fork oil levels?

[SuburbanDad]

Has anybody found the factory spec for fork oil levels in upper and lower Chambers on the 4cs forks? I searched the service manual PDF and did some googling, etc - but I can't find any spec for oil volume/height for the 2017 MX.

I am contemplating using the 2016 KTM XC fork oil spec since they are the same fork, but I know Alta made some changes to the valving and am wondering if they did the same to the oil levels.

Fwiw, the 2016 XCs get 510ml in the lower chamber and 130ml in the upper. Anybody have better info?

 

[Philip]

The 4CS is filled not by volume but up to a level.

The Alta 4CS oil level spec is 65mm from the top. You measure it with the spring removed and the fork fully bottomed out.

The lower chamber needs to be filled completely full and then burped or bled. There are a couple of WP videos on YouTube explaining this bleeding procedure -- one manual and the other one by pressurizing the fork with a high air pressure, I think 6 bars, IIRC. The fork will bleed itself over time anyway, but you won't be able to set an accurate oil level if you do not bleed it properly.

I think a 2016 KTM XC fork will be a downgrade from the stock Alta's fork. The KTM uses a much softer spring and is not known for a well-tuned suspension.

 

[SuburbanDad]

The 4CS is filled not by volume but up to a level.

The Alta 4CS oil level spec is 65mm from the top. You measure it with the spring removed and the fork fully bottomed...

Thanks for the info, where did you find the specs?

 

[Philip]

Alta sent me some MX suspension info earlier this year.

They also sent me the stiffer MX shim stack specs for riders weighing more that 100 lbs. PM me your email address if you want that spreadsheet.

 

[Monza]

100 lbs ? Many of you are under 100 lbs ?
It was 200 lbs or 100 Kg i imagine @Philip

 

[SuburbanDad]

I figured he was implying the factory mx suspension was too soft for anyone over 100lbs.

For motocross the 17 MX is definitely way too soft, but it is ideal for my weight and the woods/enduro riding I usually do.

It would be interesting to see that sheet however. Will pm

 

[Monza]

I imagine no one here is less than 100 lbs or 45 Kg ? So why Alta made a setup instruction for guies more than 100 lbs (99% of the owners) instead than doing it as standard set up on the MX ? Curious.

 

[Philip]

I figured he was implying the factory mx suspension was too soft for anyone over 100lbs.

Yes, exactly.

The MX stock suspension is too soft for motocross. The springs are barely stiff enough to support the bike's own weight during jump landings. But it is okay for trail riding. It shouldn't have been called MX, it should have been called EX.

BTW, does the EX have the exact same springs as the MX? If so, then the two suspensions must be identical. It works for the EX but not the MX.

 

[Philip]

For motocross the 17 MX is definitely way too soft, but it is ideal for my weight and the woods/enduro riding I usually do

If so, then why are you not happy with the stock damping? It seems to match the stock soft springs pretty well. The stock 4CS works way better than any other stock 4CS forks that I have tried.

 

[SuburbanDad]

Oh I was/am quite happy with the stock damping. But all my damping was leaking down the fork leg and onto the brake pads... I was doing fork seals and bushings.

FWIW, other 4CS resources I found on the net agreed with the 65mm oil level for the 4cs fork. The 510ml/130ml figures came from a Rocky Mountain ATV vid on fork servicing the 4CS. The lower chamber figure is actually correct. 510ml fills up the bottom of the fork. 130ml for the top is pretty far off though. with 130ml in the upper, I was about 120mm from the top of the tube. Thanks for the spec @Philip

This was the first time I have had to work on a 4CS fork, and I am happy to report it is quite simple. The only special tool necessary was for the fork cap.

 

[Philip]

I must have mis-read your original post. You were going to use just the XC oil level spec, not the XC fork.

 

[Butch]

is the fork oil just "KTM WP fork oil"? Is this a dumb question?

 

[snydes]

It would be any comparable brand of the same weight, but the the weight they specify I do not know off the top of my head.

 

[Philip]

is the fork oil just "KTM WP fork oil"? Is this a dumb question?

Yes. But I use KYB oil, it is much cheaper.

 

[Butch]

It would be any comparable brand of the same weight, but the the weight they specify I do not know off the top of my head.

...on one site I see three weights of Motul fork oil, five for Red Line Suspension Fluid...

 

[Philip]

...on one site I see three weights of Motul fork oil, five for Red Line Suspension Fluid...

I think the default oil for all dirt bikes is 5W. Someone please confirm.

 

[Don Thuren]

I prefer the Silkolene PRO RSF 2.5w.. Silkolene Pro RSF (2.5wt Liter) – SilkoleneStore

Compared to most 2.5w suspension fluids is thinner when cold and thicker when hot, so you get more consistent feel...

 

[Don Thuren]

I think the default oil for all dirt bikes is 5W. Someone please confirm.

Most newer oils these days can vary so much between 2.5w and 5w, when comparing the true CST flow spec, which is what really matters..

I attached a cool chart for reference that I DL'd a while ago..

 

[Judaslefourbe]

It would be any comparable brand of the same weight, but the the weight they specify I do not know off the top of my head.

The way weight is calculated between brands is not common: a 5 weight motul will be different than a 5w belray for instance. Unless you can get the WP oil (difficult without being a WP specific dealer), I recommend to whatever brand and stick to it until the end of your road.

 

[Butch]

Yo! Thanks. I wonder what Alta05 thinks...