Fat guy suspension/spring rate questions 1999 400f

I recently purchased a basket case 1999 yz400f and spent a small fortune bringing the powerplant back to life and would now like to address some suspension short comings. 


Im 28 6'3 and around 280 or so pounds.  Ive been to the racetech website and it seems they dont offer fork springs in anything close to a spring rate i could use.  Does anyone know of another manufacturer who sells fork springs in the .55 to .57 range?   Perhaps springs for another bike?  It only shows up too .48 and the stiffest ive found is .49.  I was surprised how soft the forks were, its hardly stiffer than my 1994 kdx200 with the stock springs.  I do like the valving better though.  I will be doing some lite motocross and some fast trail riding.  Im really depressed about this, i bought this bike in the hope i could set it up so i could actually jump something for once.  


Thanks to this forum for helping me get the motor back together!  Lurkin and searchin

Am i to understand that all previous and current owners of the 400f have been slim, trim and happy with the stock spring rates?  

I think you'll find most of them a bit lighter than 280, yes.  But it's more likely a matter of no one having an answer to your question of where to find heavier springs for that old fork.  You might try contacting Dave at SMART Performance, see what he can find.


And .52/.54 is probably closer to what you want.

Thanks for the advice, i never really thought about it being 14 years old.  I remember watching mr Henry kicking butt on the prototype and in my head this bike just came out.  Since the bike and i are pretty evenly outdated i think im going to stick with it even if i need to do some type of fork swap.   Time for more searching!



You can install the complete fork from an '06 - '07 model with relative ease and have something to work with.  Expensive solution, though.  A cheaper way will likely be to find someone who has done this with a late ('05+ ) WR450 and is selling their stock forks. Your front wheel and brake will fit either one.

Thanks for the awesome info!  Going off racetech's site 05 is the oldest yz  that .5 up springs are offered. Would i be correct to assume that 05-07 will work but 06-07 has is a better fork?  


Im thinking about just buying .49 fronts and cutting a couple coils off with a longer spacer (do these bikes have preload spacers?).  If i go this route i guess ill just match the rear spring to the front off racetechs calc.  Maybe ill end up with a .5 or.51 and save up for a future fork swap since it seems 06-07 forks are worth their weight in gold!


Thanks again Greyracer


That's about right, yes.  The '05 YZ fork was the AOSS, the first KYB twin chamber design.  It was completely revised top to bottom the following year, and the '06 design continues unchanged up to the current models.  There were changes to brake hardware and axles in later models, though, so the '06 and '07 (except the '07 YZ250F) are the two that directly fit.

I pulled apart one of the forks tonight and found it to be surprisingly clean,  Im going to cut the current springs unless someone talks me out of it. I figure the cut spring will fatigue very quickly and this will be a short term fix but should buy me some time untill i can find the right deal on some updated forks (and let me know if i like stiffer suspension).  


Ive gotta do some more math before i cut and confirm the current spring rate and see how close im coming to coil bind.  I counted 28 active coils and plan on losing 4-4.5 (maybe less if im too close to coil bind).  I found a formula greyracer posted a few years ago to figure the current spring rate so I need to bring my caliper home from work to measure spring diameter and calculate true coil bind.


Should i put my spacer down in the fork leg or up under the cap?   

grayracer, do you happen to know if the '06 YZ250 and 450 forks were the same?  Any differences in springs or valving?  Thinking about looking for a set for my '11 WR450, which I've already gone through with springs and gold valves, but haven't quite dialed in yet.  I've been reading up on swapping over to '06 YZ450 forks and the benefits sound great - found a pair of '06 250 forks for a decent price.


Also, thanks to your son for his service -- I'm down range right now (only one month left to go).

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