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SSS fork spring/preload questions

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Me; 38 yrs old, 195lbs in street clothes and ride C class VET rider MX.

Bike; 2002 yz250 2 stroke with 2007 yz250F forks.

I got the forks used and only cleaned, replaced seals and oil when i got them. So i am assuming that the Springs in them are the stock 250f springs. the stock springs are .44 according to RaceTech. So i am changing the oil in them for the upcoming season and decided to check Preload, i never checked when i got them, and got 0mm of preload. 454mm is the spec free length, mine measure 452mm with 449mm being the limit, so they are within specs. According to RaceTech, the stock springs should be good at .44kg for me.

But heres my dilemma. Ive always thought that the springs were a bit soft and dive to much under heavy braking (assuming the 0mm of preload has something to do with this). And i have a set of 2010 yz450f springs on the shelf. The 450f springs are 17mm longer and put the preload at 17mm. Obviously that is ALOT of preload.

So my question is, Can i run the 450f springs? will it damage anything? Will they just be to stiff with that much preload? Or should i just get the correct preload spacers? and where can i buy them?

I am planning on hitting up practice at my local track on Wednesday and am thinking about running the 450f springs just to see how they are. But if it will possibly damage anything i wont run them at all, so i am hoping to get some good replies before i get home from work tommorrow so i can finish the fork oil change. Thanks guys...

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The longer springs will add too much preload.

4mm is a common baseline to start tuning from and it doesn't take much to make a notceable difference in handling.

 

My 2010 YZ125 (.42kg/mm springs) had 7mm in stock form,

even with a few mm shorter aftermarket .43kg/mm springs it still had close to 6mm

I've since had the spring perches machined down and can now fine tune the preload using perch spacer rings.

 

Looking to fix a front end washout problem on corner entry I've reduced the preload to 4mm and the front tire now grips much better.

 

 

Edited by mlatour
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As mlatour said, 17mm is way too much preload.

What else have you tried to reduce this bottoming under hard braking?

Perhaps you have too much reb damping and not enough comp damping. Have you given up on the clickers or so you like them as for for other parts of the track?

Perhaps your 07 250F forks are malfunctioning. When you replaced the oil did you inspect the base and mid valves and ensure the shaft nuts were tight and shims+piston rings looked ok? Also inspect the surfaces inside each cartridge?

I looked up stock 2007 YZ250F valving and it should be ok for a YZ250 2 stroke. It has a wide 0.4mm float which could allow a bit more dive, but should be ok.

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As far as valving, that i know nothing about. I've done some research on shim stacks and such, and am more confused now. Lol. I did inspect the forks when i got them and everything was good. Valve nuts were tight, shims were flat and clean. No damage to cartridge rod. The only thing i did was the free piston mod by drilling holes into it.

Otherwise i really enjoy the forks. I've only ever bottomed them out a couple times, but those were pretty hard flat landings. And i did use a zip tie to see how much of the fork travel i use, i do use 95% of it normally. My only real complaint is they feel like they are diving to much under hard braking. I have played with clickers and like how they are.

So for now ill put the stock springs back in and order some perch spacers. Unless you know of a DIY type spacer.

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And correct me if I'm wrong, but from what I've read it sounds like if i add preload to the stock springs, that should help dive. True?

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Stock 2007 fork springs are .43kg and too soft for you at 195lbs.

The YZ 250f's were '06-.44/5.1, '07-.43/4.9, '08-'13-.45/5.3

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.47 is the stock 450f springs i have. Maybe I'll machine the perch down a bit and run those.

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There is only enough material to machine about 4mm off the spring perches, 

meaning still 13mm of preload using the longer springs not designed for this application.

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There is only enough material to machine about 4mm off the spring perches, 
meaning still 13mm of preload using the longer springs not designed for this application.



I was picturing them in my head and thinking the same thing. Oh well. Guess I'll reinstall the stock springs for now and order some spacers.

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Thanks for your help everyone. I ended up making perch spacers that are 8mm tall. The thickness is what i had laying around. And am using the 250f springs. Find out tomorrow if the added preload helps. But i do plan to purchase the correct spring rates soon.
Opinions on the spring rate i should buy?

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If you add 8mm of preload, then you'll remove 8mm of dive. Probably not the best solution.

I think preload is best used to control traction and feedback from the front tire when the fork is near full extension.

If you are looking to remove more like 50mm of dive, then you need stiffer springs and/or more compression damping and/or perhaps less rebound damping if your forks are packing down over repeat bumps. Adding more oil volume could help, but if your forks are diving into the last 25% of travel then this is probably not the best solution.

Clicker adjustments are free and it takes about 30 secs.  Your 0.43 springs are obviously soft for a 195 lb rider.

Edited by numroe
195 lbs
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It does seem like closing up the compression clicker is the most effective way to solve diving.  That's one behavior that it absolutely will affect.

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I would say they are diving to about 50% of the travel under hard braking. Add braking bumps and it becomes harsh.

I did ride yesterday as planned, but unfortunately it was muddy and rutted, so i was unable to feel any difference. But i do understand i need stiffer springs. That is at the top of my list.

I really appreciate everyone's help. I never used to pay attention to what the bike was doing, i just rode. Now that I'm older, i can feel the difference in little changes and really want it set up properly. So again, thank you.

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How many clicks out are you on the fork comp (top) and rebound (bottom)?

Not enough damping can certainly be harsh. There is an optimum amount of damping for a smooth and more level ride on any terrain at some speed.

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Is the fork physically shorter? I know the 125/250f forks are shorter than the 450. I would assume the 250 2 stroke also uses the longer fork. Might contribute to the front weight bias.

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How much did it dive when you tested with the fork comp clickers on 7 or 8 out?

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Is the fork physically shorter? I know the 125/250f forks are shorter than the 450. I would assume the 250 2 stroke also uses the longer fork. Might contribute to the front weight bias.

 

I do not know if they are shorter. Never even crossed my mind. Next time I'm at the track ill see if i can measure someone else's forks. Should i try to find someone with a older steel framed yz or will the forks on a aluminum framed yz be the same length? I do have the forks raised 5mm to the line also.

 

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How much did it dive when you tested with the fork comp clickers on 7 or 8 out?


I haven't turned them in yet. Hopefully the rain will stop soon so i can feel the bike instead of fighting the muddy conditions.

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Thanks for your help everyone. I ended up making perch spacers that are 8mm tall. The thickness is what i had laying around. And am using the 250f springs. Find out tomorrow if the added preload helps. But i do plan to purchase the correct spring rates soon.
Opinions on the spring rate i should buy?


I was having the same trouble as you on your forks. My 2015 YZ250 after my first kit I had installed in the forks and shock became to soft for me. It had the 46kg springs in forks (stock were .44).
But my Forks seemed to be to spongy and wanted to dive to much so I went to a 46kg spring.........way better ......but it's a tad to stiff......however it doesn't dive anymore. I lost my clickers, I am all the way out on Compression. So what I believe I am going to do is replace one of the old 46kg spring in oe fork and the 47kg in the other fork and rake 20cc oil out of tubes from 340 to 320cc and that should give me my clickers back to about middle.

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