best shim stack for 235lbs??


9 replies to this topic
  • bigtooth

Posted August 01, 2007 - 08:52 AM

#1

I'm going to be re-valving a pair of forks and shock for my 2003 xr650r and wanted to see what was recommended for shim stacks.

I am 235lbs with gear and have the .47kg fork springs and 11kg shock spring. I ride mostly rocky/woods terrain. I would like a plush ride for the small chatter but a setup that can also take on the big hits. I don't jump that much but would still like to be able to.

I was thinking about drilling the fork compression valves out like Borynack recommends, and going with the lighter fluid.

I guess i'm confused with all the different shim stack possiblities and if my spring rates will change what I need for shims. :crazy: I would love to hear what kind of setups everyone has tried and what you all think of them.
:ride:

  • Denn10

Posted August 01, 2007 - 11:39 AM

#2

you may want to put this in the suspension forum im sure youll get some responces there cuz i havent a clue.

  • bigtooth

Posted August 01, 2007 - 11:44 AM

#3

you may want to put this in the suspension forum im sure youll get some responces there cuz i havent a clue.


Hey good idea, I posted this over there also.

  • XR680RR

Posted August 01, 2007 - 04:05 PM

#4

With the stiffer springs you will need more rebound control. Thinner fluid is the hot ticket with the XR650R but, now you need more shim to control the easier fluid flow. The compression valve being drilled will bring the forks much closer to their origonal concept design. The compression valve holes have been made even smaller on the 2005+ bikes to help stop (compromise) the forks from bottoming with the big guys that buy the bike. This makes for a harsher (less plush) ride. I have seen the fork and shock pages that you are talking about change many times over the years but, mostly changed more to be usable for the average rider with only a few setups that are out of touch with most XR650R riders. For a guy that can understand those pages, are good at turning a wrench, there is no better way to go. The Baja 500 setup is the one I think is in Gene Lanes bike and I road that with a bunch of others and we all decided it did everything any riders should ever want. It fixed so much in both directions that it wasn't even close to feeling like a stock BRP. It was ten times more plush then a stock bike and soaked up anything the Baja 250 could throw at it. David Gray had a stiffer setup (I think it was the Baja 250 setup) and it was too stiff for any sane riding and about as plush as the stock setup. It did not bottom on a five foot from the bottom of the tires drop off. 70 foot table top, whoops going North out of San Felipi to Zoo road. Gene's could easly handle the 55 foot table top at Third Gear Pinned and a slow drop (faster would be much easier to handle) off a 4+ foot drop off. I have the desert setup and I think the shock can handle more then the forks. I guess the reasoning is to make the forks more plush to help stop fatigue and the shock handle the big whoops and sudden drop off's found in the desert. More for the guy that rides at the back of the seat and keeps the front end light. I found that there is some of the shims being moved from the rebound stack to the compression and the other way around, making it so that there are only a few shims needed to make the changes on those pages. The charts on the pages do not give into acount for very heavy spring change (it might say something in the writing) or real big guys. If you studdy the charts they are not that big of a deal. Mostly the slow stack being stiffened to handle the fluid change, ported holes and to control the rebound of the spring. It seams that the shims do much more work on the rebound side by the rebound adjustment being turned out with a lot less turn, which also slows down the compression. The compression adjustment is opened more helping give a much plusher ride but once a bigger hit comes along the stiffer shim stack handles it. The by pass can only flow so much fluid then the fluid has to pass through the valve, bending the shims. More slow stack shims you can handle a bigger hit without bottoming. Thinner fluid, more slow shims, cross over shim stack change, flowing more fluid through the compression valve (forks), how many shims where, how they and when they bend, the change in the adjustments, more nitrogen to help stop stictation (it also changes the way the rebound and compression adjustments work/flow), and all the other things have to work in harmony to get what you want.

  • Kritter

Posted August 01, 2007 - 08:46 PM

#5

more nitrogen to help stop stictation (it also changes the way the rebound and compression adjustments work/flow), and all the other things have to work in harmony to get what you want.


Increasing nitrogen increases seal friction, thats how a rod seal works, the more pressure it has the harder it pins itself against the shaft. Thats why on a fork which isnt charged it uses a spring loaded seal...

You want to run the least amount of nitrogen possible but without having access to a shock dyno I dont recommend you play with pressure to gain any advantage.

The only thing more or less nitrogen (assuming you have enough to begin with) does is change your spring force (pressure x shaft area). If you dont have enough to begin with your shock fluid will boil/foam/cavitate from the pressure differential(vacuum) and your shock will be worthless after a few hard hits.

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  • bigtooth

Posted August 02, 2007 - 08:22 AM

#6

Sounds like i'll try the Baja 500 setup, It sounds like it would be a better comprimise towards a plusher setup for the rocky terrain I see rather then the baja 250 setup.
On this page: http://www.xr650r.bo...r650r_shock.htm there is a setup for rock/tight trails for the shock, is there a matching fork setup somwhere? Do you think this setup would sacrifice to much bottoming resistance.

I take it you can re-use shims that are stock, so once you decide on a setup you can just compare it to stock setup and buy the ones you need? Has the stock shim stacks ever changed?

  • XR680RR

Posted August 02, 2007 - 10:07 AM

#7

Increasing nitrogen increases seal friction, thats how a rod seal works, the more pressure it has the harder it pins itself against the shaft. Thats why on a fork which isnt charged it uses a spring loaded seal...

You want to run the least amount of nitrogen possible but without having access to a shock dyno I dont recommend you play with pressure to gain any advantage.

The only thing more or less nitrogen (assuming you have enough to begin with) does is change your spring force (pressure x shaft area). If you dont have enough to begin with your shock fluid will boil/foam/cavitate from the pressure differential(vacuum) and your shock will be worthless after a few hard hits.



Yes, pressure makes the shaft seal work but, there is more to it then that. It also changes how the fluid responds to the bladder and transfers through the compression adjuster. It also (to a point ) helps releave stictation of the valve to shock body. Nitrogen has almost nothing to do with the shock spring force, but, a lot to do with the fluid rebound force. There is a lot wrong with the statment above and so right but, it needs to be in context.

  • BWB63

Posted August 02, 2007 - 04:07 PM

#8

First you have the swing arm to shock travel ratio. Our bike with its linkage is close to 3:1. Wheel moves 1" the shock shaft moves about .3" that isn't the real numbers but, close. A CR250 or bike like that is closer to 2.4:1

This makes a big difference in the spring that is used for the weight of the rider and weight of the bike. 200 pound ride would need a 10kg/mm shock spring for our bike (XR650R) and a 5.6kg/mm shock spring for the CR250R. The springs are the support for the bike and rider’s weight, end of story. How fast the shock valve travels in relationship to the swing arm movement. What size ports you need in the valve and the shims used with the valve. Fluid, bypass of the fluid, is all part of dampening the fluid flow that dampens the spring’s movement, or the control of the spring’s rate of compression or rebound.

Damping adjuster, I'll call it the compression adjuster on the reservoir. Is more like a one way volume/flow valve as the shock is compressed it regulates the fluid flow into the reservoir pushing on the bladder filled with nitrogen pressure. The Nitrogen pressure does affect this flow greatly and is a big part of the picture. Most all the shops I have dealt with that do Pro rider setups use 180~200 psi of nitrogen, not the 145 psi that is stock. It also changes the flow back into the shock body through the compression adjuster effecting rebound. The bladder that is full of nitrogen is like a pillow. It adjusts to the flow in and out of the reservoir, keeping a constant pressure on the fluid. Keeping pressure of the shaft seal and making up for the vacuum as the fluid is sucked out. The stictation of the valve ring/seal to shock body wall is also affected as was said before. As the shock rebounds it will pull the fluid from the reservoir and start a vacuum pulling the valve seal/ring into the shock body, more nitrogen pressure relieves this to a point.

Rebound adjuster screw on the bottom does not adjust only the rebound. It should be called the bypass adjuster. It adjusts the amount of fluid that bypasses the valve assembly. It is not a one way valve. It is a very long rod with a needle point on the end that fits into a hole at the end of the shaft inside the shock. This rod is inside the shock shaft and is moved up and down by the rebound adjustment screw. With the spring off you can test the shock by compressing the shock and then let it go. You should not hear the sound of air bubbles in the fluid. Kind of a grinding noise. Compressing the shock and while compressed you turn the rebound adjustment screw in till it stops and the shock should not open. Once you turn the screw out the shock should open.

Getting a plusher ride with thinner fluid makes you have to change some of the things in a stock shock and all work for your benefit. It lets you close the bypass (rebound) more letting you control the flow of fluid with the valve and shim stack more. When the shock is rebounding the pull on fluid from the reservoir is much less, letting the bladder respond better sooner. There will be much, much less hydrolicing of the fluid as it is forced through the ports/valves. You will need to use more shims to control this easier flow but, most want a plusher fast stack anyways (fast as in quick movement of the suspension, quick curb hit, ruts) but, need more control of the slow stack (big slow hits like a table top jump). This is where you need the extra shims and the slow stack is the bigger shims. There is a cross over shim stack in there and it is usually moved up for a compromise and more shims are added after that. I hope I didn't make this more confusing.

  • bigtooth

Posted August 03, 2007 - 05:00 AM

#9

Thanks BWB, that makes a lot of sense and paints a good picture of how everything is supposed to play together.
But, back to the shim stacks, I really don't know where to begin. I know that currently with the heavy springs
the back seems to swap a little to much and front could be a little softer over the choppy stuff, but really,
I dont know what is wrong with the suspension because I've never been on something that was correctly setup. I
always rode my xr600 stock so that couldnt have been right for me. All I know is that if there is somthing I can do
to improve the ride/handling of my bike I would like to do so. Especially now that I have a spare set of forks on the
way(I need to replace my left fork slider because its dented), I figured theres no reason not to try the re-valve myself.

I was thinking the baja 500 setup, but do I need to make any changes to the stacks listed here:
http://www.xr650r.bo...r650r_shock.htm
http://www.xr650r.bo...r650r_forks.htm

to accomadate my spring rates of .47kg and 11kg.

I was thinking the baja 500 and not the baja 250 setup with the thinking that the 250 might be to stiff.
If anyone has other suggested setup please post away.

Is it wrong of me to be asking for shim stack setups? Thats not like asking for a well kept secret family
recipie or something...is it?

  • BWB63

Posted August 03, 2007 - 06:05 AM

#10

People make money off the idea of shim stacks being a "well kept secret family recipie " I was blasted many time by two different suspension shop/guy for making the suspension pages. "What do you think you are doing?"

Well, to get what you really want in the end you would need a good suspension guy to ride with you and piant a picture of what you realy need but, inless you are a pro or know and ride with a suspension guy, that is not going to happen. I know for a fact that if you send you suspension into a shop you are going to get one of their generic setups for what you want and like you are saying, ' how are to know what that is'? Well, the Baja 250 setup is to stiff for me and I can get some good air with the BRP. Your main concern here is you are using stiff springs for your weight so, needing more rebound control will be an issue. You may need an extra shim in the rebound stack of the Baja 500 shim stack to fine tune for where and how you ride your beast. PM me and I'll give you my number and we can go over it. In the end there is more to it then just charts but, compared to what you spend your money on if sending it in, you are way, way ahead doing it yourself. The hard part is knowing what you want. I am not a web site maker so, that makes it more confusing. I had more setups and narrowed it down to help. I may change it again (next time I get hurt) and make it better.





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