YZ Leaf spring mid valve conversion

Mr. Robinson
Edited Date/Time 6/3/2022 3:19pm
Just wondering if anybody has messed with YZ250 / 125 leaf spring mid valve conversions and if it's really worth the pain?
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cwtoyota
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cwtoyota 9/15/2021 8:16pm
9/15/2021 8:16pm
Mr. Robinson wrote:
Just wondering if anybody has messed with YZ250 / 125 leaf spring mid valve conversions and if it's really worth the pain?
I am running them in my YZ125, YZ250 and my KX500.

It makes it very easy to get all the hold-up you want without making the fork harsh on small chop.
It's really the best of both worlds on the YZ SSS stuff.

I know SDI sells the cups for a really good price, but I've never messed with theirs. I buy my shims from SDI, but I make my own leaf-spring conversion cups in my shop. If you valve your own stuff, you should give the leaf spring mod a try for sure.
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Rider 5280
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Rider 5280 9/15/2021 9:05pm
9/15/2021 9:05pm Edited Date/Time 9/15/2021 10:15pm
What is this mod you speak of?

Do tell (please)!
2
cwtoyota
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cwtoyota 9/15/2021 10:51pm
9/15/2021 10:51pm
Rider 5280 wrote:
What is this mod you speak of?

Do tell (please)!
A standard mid-valve in a standard fork (Showa twin-chamber or KYB AOSS or SSS) has a stack of shims below the mid valve piston for compression at mid and high stroke speeds. There's a small cup with a small spring beneath it. That spring has a lower spring rate than the Bic pen in your pocket. The shims are allowed to move away from the piston on a small bump impacts and initial compression movement of the fork. There's a stop so they don't go too far away from the piston. The maximum gap between the piston and the shim-stack is referred to as "float" by most guys.

A woods rider might have half a millimeter of float or more in their fork while a fast guy racing motocross might have as little as one tenth of a millimeter or even zero float. Float allows the shims to move to take the edge off of some small bumps and chatter. A firmer compression valving spec can be used with a reasonable amount of float.

The trouble with that float is that it adds some wallow to the fork and makes the middle of the stroke feel hollow or allows the fork to dive if there's too much. Taking away all the float makes the fork harsh on small chop and doesn't allow the bike to settle into corners as easily.

So, leaf-spring mid-valves are intended to be a best of both worlds scenario.

Basically you have a cup underneath the mid-valve shim stack where at bic-pen spring was. Now increase the diameter of that cup to about 20mm and throw out the spring. bridge across the cup with some 20mm shims and build up a small shim-stack with nothing underneath (inside the cup) to stop it from bending back down inside. Those shims bridge across the cup like the leaf-springs on your truck. Your mid-valve shims can push back against that leaf spring setup kinda like the conventional float described above, but a good tuner can shuffle the shims around and add or remove some hold-up while allowing the mid valve to blow off pressure on some small chop.

I don't know who created this image, but it compares conventional to leaf spring mid valve.



Here's the standard cup from a YZ and that light little spring I mentioned.
The ID of the OE spring is 8mm and the OE cup is 6mm ID with a 17mm OD.




Here's the 20mm cup I make in my shop with and without a 20mm x 0.10mm shim sitting on it.
The cups are 1mm deep, so I place small limit shims under that 20mm to set the maximum travel of the leaf-spring. I've found that somewhere around 1/2mm of travel works really well.

The next photo is that leaf-spring setup installed under the mid valve for a 2006 - 2021 YZ125 or YZ250.


19
murph783
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murph783 9/16/2021 5:25am
9/16/2021 5:25am
Rider 5280 wrote:
What is this mod you speak of?

Do tell (please)!
cwtoyota wrote:
A standard mid-valve in a standard fork (Showa twin-chamber or KYB AOSS or SSS) has a stack of shims below the mid valve piston for compression...
A standard mid-valve in a standard fork (Showa twin-chamber or KYB AOSS or SSS) has a stack of shims below the mid valve piston for compression at mid and high stroke speeds. There's a small cup with a small spring beneath it. That spring has a lower spring rate than the Bic pen in your pocket. The shims are allowed to move away from the piston on a small bump impacts and initial compression movement of the fork. There's a stop so they don't go too far away from the piston. The maximum gap between the piston and the shim-stack is referred to as "float" by most guys.

A woods rider might have half a millimeter of float or more in their fork while a fast guy racing motocross might have as little as one tenth of a millimeter or even zero float. Float allows the shims to move to take the edge off of some small bumps and chatter. A firmer compression valving spec can be used with a reasonable amount of float.

The trouble with that float is that it adds some wallow to the fork and makes the middle of the stroke feel hollow or allows the fork to dive if there's too much. Taking away all the float makes the fork harsh on small chop and doesn't allow the bike to settle into corners as easily.

So, leaf-spring mid-valves are intended to be a best of both worlds scenario.

Basically you have a cup underneath the mid-valve shim stack where at bic-pen spring was. Now increase the diameter of that cup to about 20mm and throw out the spring. bridge across the cup with some 20mm shims and build up a small shim-stack with nothing underneath (inside the cup) to stop it from bending back down inside. Those shims bridge across the cup like the leaf-springs on your truck. Your mid-valve shims can push back against that leaf spring setup kinda like the conventional float described above, but a good tuner can shuffle the shims around and add or remove some hold-up while allowing the mid valve to blow off pressure on some small chop.

I don't know who created this image, but it compares conventional to leaf spring mid valve.



Here's the standard cup from a YZ and that light little spring I mentioned.
The ID of the OE spring is 8mm and the OE cup is 6mm ID with a 17mm OD.




Here's the 20mm cup I make in my shop with and without a 20mm x 0.10mm shim sitting on it.
The cups are 1mm deep, so I place small limit shims under that 20mm to set the maximum travel of the leaf-spring. I've found that somewhere around 1/2mm of travel works really well.

The next photo is that leaf-spring setup installed under the mid valve for a 2006 - 2021 YZ125 or YZ250.


Jesus. The more you learn, the more you realize you don’t know. Thanks for that breakdown man, that was great.
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Rider 5280
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Rider 5280 9/16/2021 7:28am
9/16/2021 7:28am Edited Date/Time 9/16/2021 7:30am
Rider 5280 wrote:
What is this mod you speak of?

Do tell (please)!
cwtoyota wrote:
A standard mid-valve in a standard fork (Showa twin-chamber or KYB AOSS or SSS) has a stack of shims below the mid valve piston for compression...
A standard mid-valve in a standard fork (Showa twin-chamber or KYB AOSS or SSS) has a stack of shims below the mid valve piston for compression at mid and high stroke speeds. There's a small cup with a small spring beneath it. That spring has a lower spring rate than the Bic pen in your pocket. The shims are allowed to move away from the piston on a small bump impacts and initial compression movement of the fork. There's a stop so they don't go too far away from the piston. The maximum gap between the piston and the shim-stack is referred to as "float" by most guys.

A woods rider might have half a millimeter of float or more in their fork while a fast guy racing motocross might have as little as one tenth of a millimeter or even zero float. Float allows the shims to move to take the edge off of some small bumps and chatter. A firmer compression valving spec can be used with a reasonable amount of float.

The trouble with that float is that it adds some wallow to the fork and makes the middle of the stroke feel hollow or allows the fork to dive if there's too much. Taking away all the float makes the fork harsh on small chop and doesn't allow the bike to settle into corners as easily.

So, leaf-spring mid-valves are intended to be a best of both worlds scenario.

Basically you have a cup underneath the mid-valve shim stack where at bic-pen spring was. Now increase the diameter of that cup to about 20mm and throw out the spring. bridge across the cup with some 20mm shims and build up a small shim-stack with nothing underneath (inside the cup) to stop it from bending back down inside. Those shims bridge across the cup like the leaf-springs on your truck. Your mid-valve shims can push back against that leaf spring setup kinda like the conventional float described above, but a good tuner can shuffle the shims around and add or remove some hold-up while allowing the mid valve to blow off pressure on some small chop.

I don't know who created this image, but it compares conventional to leaf spring mid valve.



Here's the standard cup from a YZ and that light little spring I mentioned.
The ID of the OE spring is 8mm and the OE cup is 6mm ID with a 17mm OD.




Here's the 20mm cup I make in my shop with and without a 20mm x 0.10mm shim sitting on it.
The cups are 1mm deep, so I place small limit shims under that 20mm to set the maximum travel of the leaf-spring. I've found that somewhere around 1/2mm of travel works really well.

The next photo is that leaf-spring setup installed under the mid valve for a 2006 - 2021 YZ125 or YZ250.


murph783 wrote:
Jesus. The more you learn, the more you realize you don’t know. Thanks for that breakdown man, that was great.
Dayuuuum - amazing! THANK YOU!
2
luckyguy19
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luckyguy19 9/16/2021 7:39am
9/16/2021 7:39am
Rider 5280 wrote:
What is this mod you speak of?

Do tell (please)!
cwtoyota wrote:
A standard mid-valve in a standard fork (Showa twin-chamber or KYB AOSS or SSS) has a stack of shims below the mid valve piston for compression...
A standard mid-valve in a standard fork (Showa twin-chamber or KYB AOSS or SSS) has a stack of shims below the mid valve piston for compression at mid and high stroke speeds. There's a small cup with a small spring beneath it. That spring has a lower spring rate than the Bic pen in your pocket. The shims are allowed to move away from the piston on a small bump impacts and initial compression movement of the fork. There's a stop so they don't go too far away from the piston. The maximum gap between the piston and the shim-stack is referred to as "float" by most guys.

A woods rider might have half a millimeter of float or more in their fork while a fast guy racing motocross might have as little as one tenth of a millimeter or even zero float. Float allows the shims to move to take the edge off of some small bumps and chatter. A firmer compression valving spec can be used with a reasonable amount of float.

The trouble with that float is that it adds some wallow to the fork and makes the middle of the stroke feel hollow or allows the fork to dive if there's too much. Taking away all the float makes the fork harsh on small chop and doesn't allow the bike to settle into corners as easily.

So, leaf-spring mid-valves are intended to be a best of both worlds scenario.

Basically you have a cup underneath the mid-valve shim stack where at bic-pen spring was. Now increase the diameter of that cup to about 20mm and throw out the spring. bridge across the cup with some 20mm shims and build up a small shim-stack with nothing underneath (inside the cup) to stop it from bending back down inside. Those shims bridge across the cup like the leaf-springs on your truck. Your mid-valve shims can push back against that leaf spring setup kinda like the conventional float described above, but a good tuner can shuffle the shims around and add or remove some hold-up while allowing the mid valve to blow off pressure on some small chop.

I don't know who created this image, but it compares conventional to leaf spring mid valve.



Here's the standard cup from a YZ and that light little spring I mentioned.
The ID of the OE spring is 8mm and the OE cup is 6mm ID with a 17mm OD.




Here's the 20mm cup I make in my shop with and without a 20mm x 0.10mm shim sitting on it.
The cups are 1mm deep, so I place small limit shims under that 20mm to set the maximum travel of the leaf-spring. I've found that somewhere around 1/2mm of travel works really well.

The next photo is that leaf-spring setup installed under the mid valve for a 2006 - 2021 YZ125 or YZ250.


Great job explaining that. From what I understand, this is similar to the modification WP made to the 2021 and up AER forks to make them better.
2
Mr. Robinson 9/16/2021 5:15pm
9/16/2021 5:15pm
Great write up cwtoyota. What I'm wondering is what sort of shims stacks guys are coming up with. I have a few of the SDI cups and would like to start working with them. If the '22's have them before to long we'll know what KYB is doing for them, but mid valve stacks that are used in the F's seem to be too much for the YZ's. I'm wondering if a stock YZ MV stack sitting on a mild leaf spring stack that allows the same amount of float as the standard MV would be a good starting point?
1
cwtoyota
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cwtoyota 9/16/2021 8:04pm
9/16/2021 8:04pm
Mr. Robinson wrote:
Great write up cwtoyota. What I'm wondering is what sort of shims stacks guys are coming up with. I have a few of the SDI cups...
Great write up cwtoyota. What I'm wondering is what sort of shims stacks guys are coming up with. I have a few of the SDI cups and would like to start working with them. If the '22's have them before to long we'll know what KYB is doing for them, but mid valve stacks that are used in the F's seem to be too much for the YZ's. I'm wondering if a stock YZ MV stack sitting on a mild leaf spring stack that allows the same amount of float as the standard MV would be a good starting point?
Thanks.

What's your skill level and weight? You could be on the right track.

I weigh around 160lbs am pretty competitive in 40A and a mid pack guy in 30A at the bigger events on my 250.
I'm on 0.47kg/mm fork springs and a 2009 YZ250F swing arm with a 5.3kg/mm shock spring and a 143.5mm link.
That sets the rear end about 6mm lower than stock and I run the front at the 5mm groove in the fork tubes.

My current mid valve stack is similar to the OEM spec sitting on the leaf spring with some 11mm diameter shims to take up any free-float. Here's the leaf setting I'm running. It's firm, but never harsh.

[Piston]
[MV Stack]
11x.30
14x.10
16x.10
18x.10
20x.10
[CUP]

I like to add one more face shim on the fork rebound stack so I can open up the rebound clicker into the 8 to 12 range. With the stock rebound stack I always end up at around 4 or 5 clicks. There's a bit of cross-talk between that rebound clicker and the mid-valve compression.
5
Mr. Robinson 9/17/2021 3:43pm
9/17/2021 3:43pm
The interesting thing is my bike is set up pretty much like yours. I'm on .47 / 5.6 springs with the '09 F swingarm and 143.5 linkage, but I'm about 180#, riding all XC stuff. I like a more active and plush suspension in the first part of the travel to hide all the trail trash, but firms up for the faster sections. Tall order to fill, I know.

I will probably do something like a 250X stack with a similar leaf stack as you show and "stop stack" ( not sure what else to call it ) that sets the free float about equal to a 125x, then go from there.
1
cwtoyota
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cwtoyota 9/17/2021 5:04pm
9/17/2021 5:04pm
Mr. Robinson wrote:
The interesting thing is my bike is set up pretty much like yours. I'm on .47 / 5.6 springs with the '09 F swingarm and 143.5...
The interesting thing is my bike is set up pretty much like yours. I'm on .47 / 5.6 springs with the '09 F swingarm and 143.5 linkage, but I'm about 180#, riding all XC stuff. I like a more active and plush suspension in the first part of the travel to hide all the trail trash, but firms up for the faster sections. Tall order to fill, I know.

I will probably do something like a 250X stack with a similar leaf stack as you show and "stop stack" ( not sure what else to call it ) that sets the free float about equal to a 125x, then go from there.

I should have mentioned that I am limiting the travel of the leaf-spring to about half of a millimeter. That works really well for my riding style in motocross.

I check that leaf-spring travel measurement using a feeler gauge set. I slide them between the piston and the first face shim. The largest feeler that slides all the way in against the 8mm sleeve is the number I record in my notes.

I am careful to check that my stack is not pre-loading the leaf-spring, but that my face shims are up against the piston ports.

Your SDI cups will probably feel just a tad lighter than the cups I make. My cups are 18mm inside diameter at the rim and I believe SDI copies the OEM KYB leaf-spring posts which are about 19mm. The leafs pivot on that inside edge as they are deflected down into the cup. The larger diameter provides a bit more leverage, but it is not a linear relationship between force and diameter.
Mr. Robinson 9/17/2021 6:48pm
9/17/2021 6:48pm
Yeah I really don't think I want any preload on the stack, but at this point I'm not for sure. I guess the first set up will more or less be a crap shoot.
1
Mr. Robinson 9/18/2021 8:01am
9/18/2021 8:01am Edited Date/Time 9/18/2021 11:40am
The SDI "valve stopper", as KYB calls it, is close to yours, but much different than the one used in the F's. The SDI is 20mm OD and 18mm at the bottom of the cup and only .030" deep. But, has a sleeve recess in the top and and no relief in the bottom.



cwtoyota
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cwtoyota 9/18/2021 9:55am
9/18/2021 9:55am
Mr. Robinson wrote:
The SDI "valve stopper", as KYB calls it, is close to yours, but much different than the one used in the F's. The SDI is 20mm...
The SDI "valve stopper", as KYB calls it, is close to yours, but much different than the one used in the F's. The SDI is 20mm OD and 18mm at the bottom of the cup and only .030" deep. But, has a sleeve recess in the top and and no relief in the bottom.



That second one you have pictured looks like the 17mm standard cup designed for the light coil spring.
It's the OEM KYB/YZ part from 2006 - 2021 and is in the photo above with my calipers measuring the spring.

As far as I know KYB does not offer a conversion part for the standard mid valve posts.
The KYB system for the current YZ and the old 2012 KX450F with leaf spring has the cup integrated into the steel post.

Here's a photo of one of my conversions next to a completely stock 2012 KX450F mid valve.


Mr. Robinson 9/18/2021 10:49am
9/18/2021 10:49am Edited Date/Time 6/3/2022 3:19pm
Yeah my bad. I know they don't make a conversion, but I was thinking that was the cup they used on the post. I didn't realize it was integrated into the post, I thought it was removeable. Just a senile old man.
1
Mr. Robinson 6/12/2022 9:20am
6/12/2022 9:20am
Been working with this SDI conversion cup plate but I'm not sure it's apples to apples with the OE KYB design. I thought the KYB OE cup that's machined into the holder had a 1mm steep into the cup and the gap between the cup edge and the piston face was 2.65mm, but I'm not sure if that's right.

The SDI step is only 0.8mm and the installed gap width is 2.25mm with the standard 5.3mm sleeve and no spacing shim. So, if an OE stack uses a 0.2mm bottoming shim then the SDI should not need any shims to have the same amount of leaf shim flex travel that an OE stack would.

Likewise, if the GAP is 0.4mm narrower, then you would need to remove that amount in spacing sims to get the stack to function the same as a comparable OE stack.

I'm looking for the installed GAP WIDTH as in the picture, but I'm not even sure if all the OE KYB gaps are the same.

Does anyone know for sure what those specs are?


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