TCMB's Quest for Better AER48 performance - DIY Valving

[TCMB371]

Lately, I've been experimenting with some various shim stacks in the AER48 forks on my MXR to improve its performance (for me). I'll start documenting my journey and perhaps some of you may find it helpful in tweaking your own suspension and finding the best setting for you.

Don't be scared! With just a few simple tools you can do this yourself as well. The AER48's are very easy to work on. Technically, if you're careful, you can change the base valve damping on this fork without even having to remove the fork from the bike! Although, i wouldn't recommend it.

For reference, i'm 6'1, 190lbs naked, and now race B class here in Southern California. I ride strictly motocross and some supercross with my MXR, so these shim stack configs are purely focused on motocross riding using the stock AER48 base valve piston and mid valve piston. Other riding types and skill levels will require different styles of damping. If you weigh less or are less skilled, you would probably need softer than my settings. If you're heavier or more skilled, you may need stiffer.

Also keep in mind that all of this is based on my current shock configuration, as well. I'm currently running a 6.4kg/mm spring with 98mm of race sag, and slightly stiffer valving than stock.

Stock Valving (-01 version AER48)

Shim Stack configuration:


Compression damping chart:

(For reference, fork velocities in the 300in/sec range can be seen in motocross riding, and 350-400in/sec in supercross)

You can see that the stock damping curve on the AER48 on the MXR is digressive, a curve you often see with offroad setups.

The total compression damping (yellow) represents a combination between the damping provided by the base valve (green) and the mid valve (red) and minor elements from the rest of the system. You can see that the mid valve contributes much more to the total damping force than the base valve, but the importance of the base valve is pressure balance between the compression chamber (in front of/above the midvalve on compression) and the rebound chamber ("behind" the midvalve as its compressing) so as to prevent cavatation of the oil. This also means the rebound shim stack on the midvalve plays a very important part in pressure balance, as well. Also notice the dashed blue line represents the damping range as a result of the position of the compression clicker. (the yellow line represents clicker at 15 out).

This yellow line will be used as a baseline to compare to the rest of the shim stack configurations. This should help you all understand how much different the other configurations are compared to the stock setting that you probably are used to by now.

For reference, here are some key velocities and the damping force at those velocities for the stock stack:


Pressure chart:

You can see that the stock valve stacks produce a pressure balanced system. A pressure drop to 0 on the rebound circuit would mean cavatation, and that's is not present. The ICS does a great job of keeping the pressure in the damping chamber on the positive side.

Personal Feedback:
The stock valving feels like it needs more damping across the board to handle big hits at the stock recommended air pressure of 10.8bar (156psi). Typically with air forks, higher pressure also means higher seal drag, which causes stiction that is most apparent in the first 1" or so of fork travel. The AER48 does much better than other air forks i've used, but its still an issue. As such, its always been my strategy with air forks use the least pressure possible that will allow the suspension to move even on small undulations but still provide deep stroke bottoming resistance, and then make up the rest of the feel by tuning the shim stacks.

Admittedly, i'm not so sure this is a good strategy to have. I think i should be taking the opposite approach, even with the air spring; Stiffer spring coupled with less overall compression damping to pair with it. More on that later...

 

[TCMB371]

Here's the first DIY modified stack i tried. I originally thought the forks didn't have enough compression damping, so my goal was to just add more across the board and see how it felt. I didn't have extra shims at the time, so i simply relied on shim stack theory and used ReStackor to reorganize the stacks in a stiffer configuration. So this stack can be made using the shims already in your forks.... if you choose.... more on that below.

TCMB Mod Stack 1

Shim Stack configuration:



Compression damping chart: TCMB Mod Stack 1 vs Stock Alta Stacks

Yellow line is the compression damping curve for the stock Alta MXR compression shim stacks, with compression set at 15 clicks out.
Blue line represents the TCMB Mod Stack 1 compression damping curve. Wow, way more compression damping than stock!
The green line shows that this base valve configuration produces almost double the compression damping of the stock base valve!
The red line shows the this mid valve configuration produces almost the same amount of damping, on its own, that the stock configuration does in total!

Here's how the damping curves compare at key velocities:



Personal Feedback:

Absolute GARBAGE. Way way way too stiff over small chop (it was beating me up), loss of front end traction, but had great bottoming resistance over bigger jumps. I'd only recommend this setting if maybe you're a freestyle rider hitting big ramp jumps. Too stiff for anything else.

 

[TCMB371]

TCMB Mod Stack 2 (stiffer than stock, softer than Mod Stack 1)

Using what i learned from the first experiment, i decided to try to aim somewhere exactly in the middle of Mod Stack 1 and the stock stack.

Shim Stack configuration:


Compression damping chart: TCMB Mod Stack 2 vs Stock Alta Stacks

Yellow line is the compression damping curve for the stock Alta MXR compression shim stacks, with compression set at 15 clicks out.
Blue line represents the TCMB Mod Stack 2 compression damping curve. Still more compression damping than stock!
The green line shows that this base valve configuration for Mod Stack 1 is the same as Mod Stack 2.
The red line shows the Mod Stack 2 mid valve configuration produces less damping force than Mod Stack 1.

Here's how the damping curves compare at key velocities:


Personal Feedback:

Better than Mod Stack 1. It still had good bottoming resistance but was still a bit harsh in the chop.

This makes me wonder 3 things.....
1. Small chop is typically a high speed suspension event. Since Mod Stack 2 has less highspeed than Mod Stack 1, i'm thinking maybe even less highspeed is needed to accommodate the small chop.
2. Since i like the bottoming resistance on larger hits, i'm wondering if making the curve even more digressive, keeping the current low speed damping forces but lower high speed damping forces would make for a better feel?
3. What if there is just too much damping and not enough spring? Perhaps a higher air pressure with less compression damping would give a better feel. Higher spring rates make for a livelier suspension feel. Softer springs with stiff damping tend to feel "dead", and this is more like what i'm feeling right now.

Maybe we should be going softer on our damping and stiffer on the spring pressure? More like the KTM setup...

 

[Don Thuren]

Thanks for doing this! I just did my second retune on my stock 2017 MX shock. I'll make a thread too once done!

EDIT: On your comment regarding air pressure(spring force) in the fork... In my opinion ESPECIALLY for Motocross/track suspension, the springs themselves front to rear need to "balance", with a lean to the front springs being on the stiffer side. I say that because on the large and frequent G-out events damping is not holding the chassis up as the time spent in the G-out is long enough to overcome it. Also, being able to be over the bars with confidence really saves on energy, and when the front is softer than the rear balance wise, you spend a ton of time more off the back in exhausting death-grip mode haha.

With the front soft coming into a choppy corner, the extra front dive steepening the head angle also promotes massive tank slap.

I think the slight extra seal drag with higher pressure would be the best bet IMO!

 

[TCMB371]

Next, i wanted to take the opportunity to compare the Alta MXR shim configuration to a stock 2018 KTM 450 SXF. I have not personally tried this setting on my Alta yet, but i may do so pretty soon just to see how it feels.

2018 KTM 450 SXF (Stock)

Shim Stack configuration:

The stock KTM 450 SXF configuration uses a single stage base valve stack and produces less damping then the stock Alta configuration, as expected by the smaller 14mm clamp shim compared to the 20mm on the Alta.
The mid valve on the KTM 450 SXF contains a single stage stack just like the stock Alta stack, but produces much less damping force than the Alta configuration. This is expected due to the smaller 10mm clamp shim on the KTM vs the 14mm clamp shim on the Alta.
The rebound circuit on the KTM 450 SXF contains a minor bleed face shim allowing for a more progressive rebound damping curve than the stock Alta configuration. Overall it, too, produces less damping than the stock Alta configuration.

Compression damping chart - stock 2018 KTM 450 SXF vs stock 2018 Alta MXR


Yellow line is the compression damping curve for the stock Alta MXR compression shim stacks, which compression set at 15 clicks out.
Blue line represents the stock 2018 KTM 450 SXF compression damping curve. You can easily see how much less compression damping the stock KTM 450 AER48 has!
Also note that compression clickers do effect the entire damping range, but the changes to high speed are much more pronounced.

Here's how the damping curves compare at key velocities:


Pressure Chart:

The stock 2018 KTM 450 SXF produces less overall pressure in the damping chamber, as expected with a softer compression stack overall. As far as i know this is insignificant, as the important thing here is that the rebound circuit doesn't drop to 0 psi, which would indicate cavation.

Personal Feedback:

(Will edit this after i've tried it).

 

[Don Thuren]

Better than Mod Stack 1. It still had good bottoming resistance but was still a bit harsh in the chop.

This makes me wonder 3 things.....
1. Small chop is typically a high speed suspension event. Since Mod Stack 2 has less highspeed than Mod Stack 1, i'm thinking maybe even less highspeed is needed to accommodate the small chop.
2. Since i like the bottoming resistance on larger hits, i'm wondering if making the curve even more digressive, keeping the current low speed damping forces but lower high speed damping forces would make for a better feel?
3. What if there is just too much damping and not enough spring? Perhaps a higher air pressure with less compression damping would give a better feel. Higher spring rates make for a livelier suspension feel. Softer springs with stiff damping tend to feel "dead", and this is more like what i'm feeling right now.

Maybe we should be going softer on our damping and stiffer on the spring pressure? More like the KTM setup...

Small chop IS a very high speed event like you say. Hitting a 3" tall curb can get the IPS way high, but rebound plays a HUGE part as if the spring rate is low and the rebound speed low, the front tire is not shooting out to grab the next bump...

If a single square edge impact is pretty smooth, but repetitive ones are rough as you go in, that's usually low speed rebound too slow...

 

[TCMB371]

I liked the feel of the stiffer bottoming resistance, but in the bumps the suspension was beating me up. This leads me to believe i would benefit from a low speed damping curve like Mod Stack 2 but high speed damping curve less than the stock damping curve. In other words, make the damping curve even more digressive.

Kreft has a damping curve chart advertised on their website (Thanks guys!) for their Revalve Control system. They recommend RC3 for Intermediate MX, which is the light blue line. So how do their damping curves compare to the stock Alta curve and TCMB Mod Stack 2?


Wow!!! Both Mod stack 2 and the Alta stock compression stack are way stiffer than even the stiffest Revalve Control setting! This leads me to believe stiffer is not the way to go here. Rather, softer in the valving with a stiffer spring.

To test this theory. I think my next move is to mimic what is listed as Revalve Control 2 with a custom shim stack and see how that feels. Probably going to need to raise the air pressure quite a bit.

 

[Don Thuren]

In the fork, more spring, less(specifically high-speed) damping, YES!!!!

 

[Don Thuren]

In the shock I like a softer spring and more damping.. Rear end squats on G-outs(more stable head angle), front stays higher braking(more stable head angle), easier to manipulate the attitude of the bike, and you can regularly put more weight on the front...

PS: If you have not played with the rebound to the extents of the clickers or shim stacks, it can be more important than compression. Faster front rebound HS and LS is a big player, and rear more rebound compared to the front adds to stability.

What specific oil are you basing all these curves around?

 

[TCMB371]

What specific oil are you basing all these curves around?

Maxima 5wt Racing Fork Oil.

 

[Philip]

Wow!!! Both Mod stack 2 and the Alta stock compression stack are way stiffer than even the stiffest Revalve Control setting! This leads me to believe stiffer is not the way to go here. Rather, softer in the valving with a stiffer spring.

I wouldn't use the Kreft's lightweight/enduro KTM damping as a reference for the Alta MXR. Remember I tried their Revalve Control, right? On the stiffest setting it was bottoming of flat landings way worse than stock, even though I ovefilled both legs with 250cc of oil and pumped up the air to 170 psi.

 

[Mark911]

Remember, the rear shock affects the front and vise versa. A soft rear makes the forks feel stiff. The opposite is also true. Changing just one end at a time (without keeping proper balance) this can throw a tuner off track even if the change would otherwise have been beneficial.

 

[Mark911]

Also, although the ultimate damping magnitudes will mathematically sum the actual “feel” of basevalve damping vs midvalve damping is quite different. There is a tremendous amount of hysteresis in basevalve damaging while midvalve damping is just the opposite. The typical spreadsheet calculator doesn’t take this into consideration. A multi-physics finite element simulation software like ANSYS can, assuming the modeling is correct.

 

[GummyBear22]

Interesting to see where this goes. As I also think stock MXR suspension is poorly set up. Especially on tight tracks with lots of square edged breaking bumps.

 

[TCMB371]

So how would we get even a more digressive curve than we already have?

A shim acts just like a spring when it bends. We know that adding preload to a spring means it takes more force to compress the spring initially. The same applies to shim stacks. Lets take a look...

With ReStackor, we can work out a shim stack configuration that produces the exact damping curve we are looking for.

Using the baseline values from the stock stack as a reference, we can target low speed damping forces a little higher than stock, and high speed damping forces a little lower than stock.

Experimental Shim Stacks - More Digressive. More low speed damping, less high speed damping.

Insert the ring shim. We have a 26mm ID/30mm OD x 0.2mm oring shim with a 26mm OD x 0.10mm shim in the middle to centralize the ring. The difference in thickness (here just 0.1mm) creates a cavity, and when you torque down the nut at the end of the shim stack it will preload the shims behind it into that pocket. This means it will take more force for the face shim to start bending away from the piston.

Midvalve shim deflection @ 0lbf:


Midvalve shim deflection @ 100lbf:


Damping Curve:

Yellow line is the compression damping curve for the stock Alta MXR compression shim stacks, which compression set at 15 clicks out.
Blue line the new digressive curve at 15 clicks out.
Red is the midvalve damping force.
Green is the basevalve damping force.

Here's how the damping curves compare at key velocities:

We can see that below about 50in/sec, this stack produces more damping than stock. But higher than 50in/sec this stack produces less damping.

For reference, your impact speed from a 5ft high freefall (less 10 inch of suspension travel) is just about 200 in/sec.

ReStackor provides a chart to demonstrate how quick the wheel will move over various square edge bumps.

A wheel impacting a 4 inch high bump while traveling 30mph is also 200in/sec.

 

[Don Thuren]

I think your quest for using a ring shim is the way to go. They are tricky, but I've used Restackor in the past to set them(on a bicycle rear shock) up with GREAT results.

Being 1:1 I completely agree with your quest for less high speed damping/more blow-off, but too much LOW speed damping and you are back to being rough on chop again. I try to focus on mid-speed so the suspension can still be active in that 0-30 IPS range, ramp up force from 30 to 200IPS, then if we could go flat all the way out haha...(edit I take that back for you and bigger jump riding you still need a decent amount of high speed or you will blow through when you don't want to)

I don't know about the AER48 internals enough to talk about the shims/flow/layout, but by the look of the shims it looks like the midvalve is the main restriction and the base valve is more shaft displacement?

If your baseline yellow tune was the same as is from 0-100IPS, and then started to curve back down after 100IPS to hit 300lb force by the time you hit 200IPS, I bet that would be a sweet tune...

 

[Mark911]

I’ve also tried all kinds of unique shim stacks trying to replicate a desired curve but the theory (restackor) is way more precise than the application in most cases. A few tenths (.0001) on a preload shim can make a difference and certainly a few thou can totally throw off a setting.

 

[TCMB371]

Here's how the stock Alta MXR configuration compares to the 2019 KTM 450SXF (EU). The 19' 450SXF (EU) has an average 30-35% less compression damping.

Shim Stack Configuration:


Compression Damping Curve:


-Yellow line is the compression damping curve for the stock Alta MXR compression shim stacks, which compression set at 15 clicks out.
-Blue line represents the stock 2019 KTM 450 SXF EU compression damping curve. You can easily see how much less compression damping the stock 19' KTM 450SXF (EU) has.

Damping force comparison - 2018 Alta MXR vs 2019 450SXF EU

 

[Philip]

So, does it looks like the Alta fork could use more low-speed damping to keep that 35% gain on the KTM at all damping speeds?

 

[TCMB371]

So, does it looks like the Alta fork could use more low-speed damping to keep that 35% gain on the KTM at all damping speeds?

I need to play around with some stack configurations and see if the fork feels better with higher spring pressure + lower compression damping, or vice versa. Right now i'm using a pretty stiff compression stack with lower spring pressure and it feels super stiff, almost SX stiff, and feels amazing hitting larger jumps, but beats me up in the rough stuff.