OK Jer, will ATF work with my mystery valving?

[bclapham]

Dear All, can i get some starting points for my 2001 YZ250

this is my first post in the suspension forum so go easy on me!

This weekend i rode a MXTech'ed KTM and couldnt belive the suspension. This insired me to play with my clickers and before long i had backed of the comp and rebound on my forks and got excellent results (i backed of the shock also) and before long was riding and jumping better and further (i wish i had done this weeks ago!). But i still think i could do with the forks being slightly softer and i have the clickers backed all of the way out.

So here are my questions. My bike is a 2001 YZ250, i bought it used from a dealer so i dont know what the previous owner looked like (weight wise), i am around 215 lbs and it seems like i was lucky since the bike has obviuosly been revalved and sprung (FineLine, Huntingdon Beach, CA, stickers) for my weight since all of the sag measurements (approx 100mm rider, 35 mm free) are spot on. (non OEM shock springs also)

Can anyone suggest a fork oil and a aprrox. height as a starting point? Ive had the bike for a while and time wise the fork oil definately needs changing! I have searched and found Jer suggests using ATF, but i also noticed MXtech "blends" with other oil which leads me to believe that it is too viscous in neat form.

My thinking is that if i choose a lighter weight oil, if the damping is too soft then i can turn my clickers in to slow/stiffen things up etc.
I have never bottmed the bike yet, but i am getting near to that point, but please tell me is it the oil level or the compression damping that will stop a fork from bottoming? ( i would expect it is a combination of both?)

Finally a question regarding MX Tech. Jer / socal steve, at some point i want a complete revalve service, but i really want to learn and get the best ouf of my current setup. If i send my forks and shock in for just the service seals etc. (still ok at the moment, knock on wood) can you work with the mystery vavling that this other company came up with and help me dial to my riding style?

Thanks to all in advance.

PS. just like jetting, i want to learn so speak slowly!

regards

Bruce

 

[marcusgunby]

Ahhh Bruce welcome to the world of suspension-now we have sucked you in its very hard to get out.You are correct to think the ATF is a reasonable viscosity and will not help(may not hinder) you quest for better suppleness.Now the thing to know is, no fluids however thin make a big difference.I often reccommend lighter fluid as i would for you, but it is only a stop gap-it will not make a stiff fork soft.I like the idea of people trying different weight oils as a starting point to learning more on the suspension.You get to try clicker settings and trying to feel a difference.I would 1st see how high you airgap is but this is not easy once you have the forks opened up as most tuners set the level with the outer tube to the bottom and the gap between that and the inner tube full of oil(ie fork brimmed with oil then some removed)so you cant easily compare the level with whats known to be reasonable.I would get a bottle of 2.5wt set the airgap to 100mm and go riding, if thats not good enough then at least you know youve tried.

 

[bclapham]

Ahhh Bruce welcome to the world of suspension-now we have sucked you in its very hard to get out.

yep, it my first time posting here so go slowly! But i think it will be worth it, all i did was some quite drastic clicker playing and after about 5 laps was jumping about 25% further distance, not to mention being faster and more comfortable around the track. (why didnt i try this sooner!)

With regards to the stiffness, i think i am not too far off. I know fully, that it is important to change the oil since it is long overdue, but i was hoping just to lessen the damping a bit. (However, i do accept my oil is probably gone bad and even if i put new in with the same (original) viscocity, this could probably stiffen things up also).

Its that old statement about only knowing where the limit is by crossing it! If i can bottom the fork once or twice, then i (think) i can dial in a bit more comp. and maximise the travel my forks have to offer!

In the meantime i will call the company that did the work. If i can find out what oil and levels they used this might be a starting point. I will get a manual before i take the things apart also, maybe they have a stock level for a starting point also.

so the next question is, what effect does the oil level do? Will a higher level slow the damping, or make it resist bottoming? Or does the oil level govern on which part of the stroke (top, middle bottom) at which the fork works?

remember, go slowly..i will read eric book over again a couple of times so i can get to speed with the basics.

thanks

bruce

 

[marcusgunby]

I love virgins;)suspension is such a simple concept and yet in a few sentances we can get really complicated.Ask a maths degree student to come up with a formula to work out a total stack stiffness and we soon reliase how complicated a few washers can be.Saying all that anyone with the ability to learn can get themselves along way to raceable suspension-someone of you education should have no problem;)The air gap only controls the last third of the travel, it does this by providing a air spring that in real terms is very powerfull when the fork is bottomed.Some forks have been designed which only use airsprings but these have seal problems under normal maintenance conditions.So think of the fork spring that holds you/bike up on the 1st part of the stroke then the airspring comes into play after that.

The damping is really just a device to slow the fork in its travel and allows the fork spring/air spring to do its job.We need to balance these varaibles-too little damping and we are pounding the airspring to try to resist bottoming.Too much damping and we will feel harshness and the springs will be not fully utilised.
So in order we need
1 correct springs for our weight to make the bike sit correctly to keep the geometry correct for cornering etc
2 total damping(many ways to get this correct) needs to match the riders speed/terrain
3 air gap to allow bottoming only on the hardest landing on each lap.

4 clickers-these should be set for the desired feel of the suspension.
Let the debate begin

When you come over bruce i can bore you for hours:)

 

[bclapham]

I love virgins

stop it, your making me blush! :o

OK, so gasses compress more than liquids, so to me this says that when you lower the oil level (which is an increasing number of mm measured from the top of the fork), then you are more likely to bottom...correct? However, if you raise the oil too high, then instead of the last 33% of the stroke being handled by the airspring then it might be that the last 40-50% of the stroke is determined by the air spring and as a consequence you arnt using as much of the travel as you could be. Is this the term known as riding high or low in the stroke?

I now know (i think i do anyway) why you should let the air out of your forks often. Too much pressure means the air sping is too strong and as a consequence you are using only 40-50% of the stroke and not the 66%. Where does this extra pressure come from, i assume (oh dear, an ass out of me and u) that it gets pumped in by the fork legs moving up and down? I expect this is why the oil goes bad also.

I called up fineline and the guy was very nice. He told me they use 5wt golden spectro fork oil. He said the level they use would be 125mm, but stock is 135mm.

So this far i am thinking to try the 2.5% weight at 135 mm. If i like the damping i can adjust the damping with the clickers, but if i start to bottom too much then rather than put more compression on the clickers, i should add an extra 10mm of oil (taking the level to 125 mm)

let me know if this is a good strategy!

When you come over bruce i can bore you for hours

i am still wondering if i could get my bike through customs! :p

thanks!

 

[slideways11]

I run all my Yamahas at 90mm. For me this seems to give the best bottoming resistance. For 00 and 01 yz250s Steve So Cal has a very nice setup for these. For the 02 yz 250 nobody I have talked to seems to have any idea how to set it up.

 

[marcusgunby]

It seems the 02 yz is like a 97 cr250-rigid frame causing set up problems.

 

[Vic]

Originally posted by marcusgunby
I love virgins
i can bore you for hours

:eek:

 

[bclapham]

shame on you vic!

OK lets get back on track here.

What relationship is there between valving and oil level? When i softened the compression damping with the clickers, i definately got softer landings without bottoming the fork, so i am assuming (yep, ive done it again) that my current oil level is OK? and since i spoke to fineline i could also assume (yikes) that this level is between 135-125 mm. Thus, is running 90mm as slideways suggests a good idea, is it probable that this oil level is matched to his valving, weight and riding style?

Is it a bad idea to mess with oil levels at the track, or do the forks need to be off the bike? If so, I expect it would be easier to go with a lower level and then add known volumes of oil as desired?

thanks

bruce

 

[marcusgunby]

Yes bruce each tuner may match all the variables to suit his set up.Your level sounds too high or you have too much low speed compression damping.If your level is at 125-135mm then its definatly too much LSC.

 

[bclapham]

OK, i am starting to get a bit confused here! :confused: ...i knew this would happen but please be patient, i am learning and i am very grateful for your time and help! ;)

Marcus: When you say "your level sounds too high", i think you mean that the value of mm of distance from the oil to the top of the fork is too high, and this coresponds to being a low oil level? Things alway get confusing, the oil level measurement is always a negative number, so the smaller that negative number the more oil there is in the fork.

Thus, for the application my suspension has been set up for, they have run a low oil level in the fork (high mm value) and offset this with too much low speed compresion damping? (for whatever reason)

And if this is the case :think: , (ie i have too much compresion damping) then the only real way around this is re-valving for my application, since running thinner fork oil will do little for the rate of compression damping?

And in the mean time, the best i can do is change to a thinner fluid to help a bit and suck it up. (its really not far off, so its not a massive issue).

Finally, can you explain what other effect the oil level has? More precisely, the airspace controls the last 33% of the stroke, does changing the oil levels have any major effect and move this last 33% of the stroke up or down? or is it too minisule to worry about?

please let me know if my questions start to get irritating, youve got me hooked and i want to understand and learn. I put half of my chemistry proffesors into early retirement with my questions....the rest of the class hated me for asking too many questions, but i got the last laugh on them when it came to exams! :D

thanks again.

 

[Shawn Mc]

From everything I have learned, the whole point of changing the oil level is to adjust that airspace. And it does make quite a difference because your effectivley changing the compression ratio inside the fork, making the fork think it has a stiffer spring in the bottom of the stroke.

 

[marcusgunby]

Bruce i dont think i explained myself very well, if you are not bottoming now-you either have too much oil(to small a airgap) or too much LSC.If your level is 135mm like the tech suggested then that only leaves comp damping.I would use 2.5wt at 135mm as we dont know if your forks have 135mm or even 90mm as they could have been serviced since.If they then bottom with the clickers set to middle you could increase in 10cc stages to stop bottoming.

Increasing oil levels will directly increase airspring strength by a factor of the increase in level and the firk diameter.IE a 43mm fork will have a much greater pressure rise than a 48mm. It does make a big difference if you raise the level 20mm.

 

[bclapham]

OK marcus, here we go!

The forks came out tonight, got the apart and back in the bike with no problems!

i checked the oil level before draining and one was 115 mm and the other was 125 mm (+- 10mm experimental error)

i also got some codes from the springs. indeed they were changed to Eibach, and i could swear they had the code 981.025.0, i looked up the spring rates for my weight and capabilty and i need a rate of 0.45 kg/mm, i found the codes for these springs and the range for the yam started at .037 so i am assuming i got the wrong data since the numbers were pretty worn. however i have come to the conclusion that they are 0.45 kg/mm (the number five wasnt worn) so it looks like the springs are indeed for my weight!

everything is back on the bike, but since i am without a manual i dont have the torques for the upper and lower triple clamps, can you give me a ball park suggestion (i use anti sieze), since i am well aware these can have a big effect.

also, what about fork tube overlap? I do seem to have trouble getting around the corners and i was going to move them up but the more i read into it i cant really figure whats going on since i do sometimes climb out of ruts and i have an understeering feeling so this maybe the compression damping? Should i just keep them in the middle and go with the clickers?...probably (answered my own question!), my riding is no science!:)

thats about all for now, i will read and mull over tonight and hopefully get some feedback from you!

if i dont get to ride it this weekend im gonna be mad!

thanks again

bruce

 

[marcusgunby]

Bruce i suspect the level was 135mm as its hard to get all the oil out from between the tubes.So basically you cant really lower the oil level.The springs sound about right for your weight.I run 0.41s for my 170lb.You could try a slightly softer spring but this costs money.
I would try 2.5wt at 135mm but i think you will end up at full soft with just a little bit softer feel which maybe enough for the near future.

Once the forks are a bit softer you may find the steering improves.If not try moving the forks up in the clamps in 3mm increments as it makes a big difference.Also a click or 2 on the fork rebound will have a similar effect.

On the clamp bolts i dont use a torque wrench as its hard to get good results as if you tighten one bolt the clamp deforms slightly and the other bolt comes loose-repeat this a few times and you have one crushed tube.The settings are normally about 14-16lbft for the lowers and 16-18 for the uppers.I just use my hand pressure and a sensible guess.

 

[Jaybird]

Perhaps it's time to through the Enzo sub-tanks into this mix. :)

 

[bclapham]

OK, this is where things get really wierd! (and i get more confused :confused: )

i did some more detective work regarding the springs, i called fineline and the original codes (981.025.0) i had for the eibach fork springs were correct, and they quoted them as being "0.44 kg/mm". I gave them my shock codes (992.0314.2A) and they said this was "about 4.7 or 4.8 kg/mm". so at this moment everything ties in since they dealer said "the previous owner looked about 170-180 lbs" and this all ties in nicely.

To double check, i called Eibach and since the part numbers had changed he took a while to figure things but he said the fork spings were (0.45 kg/mm) but the shock spring was "5.6kg/mm", i nearly fell off my chair. aparently the 0314 part of the code is lbs/inch! i will check his maths later on!

for a laugh, the spring rates for a 215lbs ama pro supercross with stiff preference setting would need a 5.2-5.3 spring! i think there is either some mistake, or the previous owner was a moron and took the springs out of a yz426 or something stupid!

anyway, here is what i will do. finish the fork reinstall, and then recheck free and race say and go and ride. I did mull over the cornering issue, and the more i thought about it the back of the bike has always felt a bit low, if this is the case i am sure this would take the weight bias of the front and effect cornering (correct me if i am wrong).

Jaybird, thanks for the enzo tank suggestion, but dont you thik i have enough trouble already!

thanks guys

bruce

 

[bclapham]

just did the maths, if indeed the spring weight is 314 lbs / inch this does equate to 5.56 kg/mm. :eek:

could the bike be set up for a very heavy guy or supercross? i know the fork ratings dont tie in with the shock, but could they have matched a very stiff shock spring with very stiff fork damping?

 

[bclapham]

even more bored know, i just put my values into the shock rate for a yz426, and for a regular joe application 5.5-5.6kg/mm would work!

maybe this bozo did take the shock of a 426f!

 

[marcusgunby]

Bruce some riders like some really weird set ups-i had a friend with a KX that set it up so you could feel ever ripple and it was real stinkbug so it turned on a dime and headshaked like a bugger.Check your sag numbers and see.If you have alot of rear sag it wont corner well no matter what else you do unless you are RC.

 

[bclapham]

OK this will be my last post here before riding! I have thrown the codes out of the window put some preload on and rechecked all of my settings.

95mm race, 20mm free! what ever the springs were, or are supposed to be they are good for my weight!

those measuremetns are without gas and gear so i might have to put a bit more preload on before i ride, but not much!;)

 

[bclapham]

Well, here it is....the bike was better, the bike was worse and the bike was different!

you were definately right about the compression damping, only a very small difference (i did notice it though), however, a good exercise since they did feel somewhat "smoother". The work i did with the rear sag really helped turning so that was good also...its suprising how big a difference a small adjustment can make! But thats where things went wrong, since the back end feels very stiff now....i played with all of the settings but ended up quite lost with them and it got progresively worse before running out of time!

So there we have it, my first real exploits with suspension, not a big success, but not disaster either! i will not be beat (although, i am getting beat with the current setup). In hindsight i really do think i have a stiff valving setup, i followed one guy on the same bike down the straight he was sitting but i had to stand up to keep with him (but i suppose that is good for my training!). I really hope i get to try out the CR, i dont know many people at the tracks here yet, so i havent riden any other bikes or setups so i really have nothing to compare mine with!

hopefully i will be able to speak to you in person sometime next week Marcus, i will drop you an email in the next few days.

thanks again for your help!

bruce

 

[Shawn Mc]

Bones what's his name over at Pro circus sets up yz250F's like what your bike was set up like. Im wondering if he set his bike up himself, using the numbers from the MXA article on the Procircuit yz250F? They put .45's in the front, and a 5.6 on the back of that bike. Isnt that what you found?

 

[Shawn Mc]

The rule of thumb I alway was told was 10mm of free sag with what ever race sag (95-100mm) It sounds like the rear spring is too stiff. You could probaby sell the spring you have or trade a YZF (250)guy for his stocker, and get the spring you need for free. Just an idea.

 

[dell30rb]

Sub tanks

I saw a pair of these on a bike... do they just add extra air capacity to the fork? Mabye this makes it possible to run a higher oil level without excessive harshness/hyd. lock in the last part of the stroke? Please explain!