YZ400F Fork Help!

[Imho]

G'day All,

Our local MX club recently modified one of the table-tops on our track so that it has a steeper up-ramp. This has resulted in higher but shorter jumps for a given approach. That's OK, except now if I don't hit the up-ramp hard enough to make the distance to the down-ramp, the resultant drop will cause my forks to bottom out hard. This happened when I tried to reach the down-ramp a couple of times but hurt my wrists so much when I bottomed the forks that I thought discretion was the better part of valour and stopped trying.

I weigh about 95kg (210lb) and the forks were set up by RaceTech for the previous 85kg (185lb) owner. I know that a set of springs is probably the go but I would like to either change the fork oil or oil level (air-gap) first but I'm not sure which I should try.

Can anyone suggest the ramifications of changing to heavier weight fork oil vs. changing the oil level, particularly in relation to the bottoming problem and the overall fork action.

Any help would be appreciated.

I forgot to include that the compression settings on the forks are at max and I'm using standard weight fork oil.

Also, apologies for posting in the wrong area but, having realised, I can't delete this post and I can't move it either.

 

[bclapham]

the fork oil viscosity will have little effect on damping, but oil level will have a huge effect on bottoming. get a small plastic syringe and add 10cc's of oil to each fork and keep addding untill it stops bottoming.

ps. this is the suspension forum so you are in the RIGHT place :thumb:

 

[russ17]

Imho
If the oil level does not help as Bruce discribed, (and springs)There could be a possibility that the active valve was converted into a check plate That is the usual practice with race tech mods. If not the mid shims could be stressed.

Russ

 

[Imho]

Thanks. I will try the raising the oil level first. I have found a couple of references about doing this as the first step but also one that says the bottoming system in the YZ400F fork is not the best. The main thing I don't want to lose is the nice fork action I currently have.

BTW, I started this thread in the Mods & Performance Forum by mistake and someone kindly moved it for me.

 

[Imho]

Does anybody know what 10cc's would equate to in the normal fork oil level measurement for a YZ400F? It would be handy to know to ensure I avoid exceeding the 80mm maximum oil level listed in the service manual.

 

[Imho]

Also, based on the above, would the following points be correct if all other things, such as rebound and compression settings, remain the same:

1. If you increase the oil level, the fork will tend to ride higher in the stroke and have added resistance to bottoming.
2. A heavier weight oil will slow the fork action down.
3. A heavier weight oil will have more of an effect on high speed damping than low speed damping.

Thanks.

 

[bclapham]

the wight of the oil will have little effect on damping.

dont worry about adding too much oil, i think you will have a long way to go before you get to 80mm!