Low speed rebound issue / PSF1 (CRF 450 2013)

[REAL-C]

53 minutes ago, mog said:

Is you mid valve bleed hole clear 

Yes everything is clear on both sides. 

 

 

[mog]

The check plate on the base sounds like the issue then 

Maybe ring a tuner and ask them how it should be ? I remember it being a different setup than normal 

[Corsicanbiker]

By memory from the one I did recently the rebound check plate had no float like usual fork 

[REAL-C]

Thanks to both of you.
I think I'm gonna go in that direction. I'll leave the rest of the stacks I think.
I'm supposed to go to a KYB specialized tuner tomorrow to get some parts, I'll try to ask a few questions if he's in the mood or not, first time I go there...    



 

[REAL-C]

So, I've reassembled the forks with a more orthodox check plate (BV), the clamp is a lot higher than what is used to be 0.5 mm now vs 0.114 mm before so the shims have more space to move up.
Like I said before by pulling it with my nail I couldn't see the piston holes, now by applying less force I can uncover like half or 2/3 of the surface. Looks a lot better to me.

I've bounced the front end of the bike and it seems faster to me despite the ground at my buddy's place lacking grip (tire going front to back and compensating the upward motion of the fork).
But, like I said I think it's better now and worth a try on the race track.

I'll keep you posted.

PS: if the issue is still here I'm either burning his shed down or drilling the rebound tap by-pass holes (2 to 3 mm).

I'm putting copies of my excel files here next to each other so reading and comparing is easier :

previous tune vs MX (OG) set-up
then
latest iteration vs the MX set-up
then
previous tune vs the latest set-up
 

   

              

    

            

 

 

            

  

Edited May 12, 2023 by REAL-C

[REAL-C]

Also, I've got the PSF1's manual and it reminded me that apart from the top-out spring there's also a balance spring which must be pretty long judging by the fact that before putting pressure in the fork, the chrome only protrudes by like 170 mm or something, then you fill it with air and the fork goes back to its normal length (I gotta check it back but I think there's like 320 mm of visible chrome when it's pressurized). 

 

So I'm wondering if this doesn't make for a false positive when trying to get a feel for rebound like with normal forks, I mean forks with coil springs.

Looks like the balance spring is effective right in the area where I bounce the bike, on the rebound stroke on at least 100 or 150 mm it slows down the return of the fork.

 

 

 

 

 

Edited May 12, 2023 by REAL-C

[mog]

5 hours ago, REAL-C said:

Also, I've got the PSF1's manual and it reminded me that apart from the top-out spring there's also a balance spring which must be pretty long judging by the fact that before putting pressure in the fork, the chrome only protrudes by like 170 mm or something, then you fill it with air and the fork goes back to its normal length (I gotta check it back but I think there's like 320 mm of visible chrome when it's pressurized). 

 

So I'm windering if this doesn't make for a false positive when trying to get a feel for rebound like with normal forks, I mean forks with coil springs.

Looks like the balance spring is effective right in the area where I bounce the bike, on the rebound stroke on at least 100 or 150 mm it slows down the return of the fork.

 

 

 

 

 

Mine used a longer softer balance spring I believe 

[DEATH_INC.]

9 hours ago, REAL-C said:

So I'm windering if this doesn't make for a false positive when trying to get a feel for rebound like with normal forks, I mean forks with coil springs.

Looks like the balance spring is effective right in the area where I bounce the bike, on the rebound stroke on at least 100 or 150 mm it slows down the return of the fork.

 

 

 

 

 

Yes it would. It's the same as the negative chamber on the wp fork, messing around with it would have a big effect on how the fork feels at the top of the stroke.

[REAL-C]

18 hours ago, mog said:

Mine used a longer softer balance spring I believe 

Was it any good in MX?

[mog]

43 minutes ago, REAL-C said:

Was it any good in MX?

Best fork I have ever ridden 

[REAL-C]

Quick follow-up

I haven't been able to attend any trackdays with the CRF 450 2013 in question, so no news about that.

I was wondering, is it possible to remove those springs and still make the fork work? Has somebody ever done that back in the days?

Edited January 31, 2024 by REAL-C

[GHILL28]

Can you confirm that the balance spring is OE?  From the video it looks more like it was super stiff but with a normal air setting.  If it had been tuned before then that's a likely variable.  Try running a lot more PSI and see if it feels like it would be normal for someone heavier.

[REAL-C]

On 1/31/2024 at 10:56 PM, GHILL28 said:

Can you confirm that the balance spring is OE?  From the video it looks more like it was super stiff but with a normal air setting.  If it had been tuned before then that's a likely variable.  Try running a lot more PSI and see if it feels like it would be normal for someone heavier.

No I can't confirm anything about the balance spring unfortunately.

The "linearity" (if that's a word) of the rebound feels better with more pressure in static in my opinion, yes.
But the guy is already on the high end of the scale on sag at the rear so I can't amplify this effect on the geometry and push more weight at the back.

I would like to see and check a stock PSF1 just for my own knowledge...
I'll keep posting here if I learn anything new about that.

I can't wrap my head around how these forks would feel on a motocross track on braking pumps for exemple with the symptoms I have.
I mean if a standard out of the box PSF1 acts the same as this one.
It can't, magazines would have destroyed it in the post tests articles.

Edited February 2, 2024 by REAL-C

[GHILL28]

31 minutes ago, REAL-C said:

No I can't confirm anything about the balance spring unfortunately.

The "linearity" (if that's a word) of the rebound feels better with more pressure in static in my opinion, yes.
But the guy is already on the high end of the scale on sag at the rear so I can't amplify this effect on the geometry and push more weight at the back.

I would like to see and check a stock PSF1 just for my own knowledge...
I'll keep posting here if I learn anything new about that.

I can't wrap my head about how this forks would feel on a motocross track on braking pumps for exemple with these symptoms I have.
I mean if a standard out of the box PSF1 acts the same.
It can't, magazines would have destroyed it in the post tests articles.

I'd imagine all the different rate balance springs are the same length and would vary more by wire diameter and active coils.  Someone somewhere should have specs on each.

[REAL-C]

This doesn't  solve my problem but if somebody reads this in the future and needs balance springs for these forks here are the balance springs that you can order from KYB for the PSF1 forks (2013 + 2014 KXF 450 and CRF 450:

Part number     ---> Rate in kg/mm ---> d(1) ----> D(2) 

110480002601 ---> 3.1 ----------------> 24.00 --> 30.00
110480002701 ---> 3.2 ----------------> 24.00 --> 30.00
110480002801 ---> 3.3 ----------------> 24.00 --> 30.00
110480002901 ---> 3.4 ----------------> 24.00 --> 30.00
110480003001 ---> 3.5 ----------------> 24.00 --> 30.00

1: Diameter of the coil wire in mm
2: Overal diameter (exterior)
Nota: Free length is unknown from the KYB parts book I've found.
Nota 2: They all appear to share the same dimensions so the difference must be in the active coils.

I don't know which one is the stock spring for the 2013 CRF 450.

Edited February 6, 2024 by REAL-C