Anatomy of an AHC shock (1 Viewer)

[TeCKis300]

@grinchy - I see no non-destructive way to get inside the shocks. It is what it is and there's very limited opportunity to mod. I think the opportunity would be to look at the LX600 AHC dampers. The LX600 being a lighter vehicle isn't great from the surface, but it does seem it has opportunity for more travel.

Anyone interested in contributing to get a couple to rip apart?

As for using the rear shocks for the front, it's possible but not without a lot of external surgery. Then applying things like external limiting straps. It seems like the major aspects are similar enough, but the rear shocks operate on closer to a 1:1 motion ratio with the front shocks closer to a 2:1. It's hard to judge from the shim stacks, but they probably account for some of this and the tuning would be questionable.

Interesting.. that diagram doesn't show the actuators mounted on the rear shocks on some LX600s. Toyota denotes those as F-sport and "AVS".. @TeCKis300 wasn't that the name of the system on 200-series AHC? Or was that AVSS?

There's at least 3 flavors of suspensions applied to the LX600. I'll say Lexus really fails at marketing and distinguishing as this was hard to find, and I had to distill it out myself. 3rd party reviews and marketing are inconsistent as well. AVS generally refers to adjustable dampers and lots of Lexus models have this, usually as an actuator directly on the shock, which is likely the component you referenced. AHC inherently has AVS but it's accomplished differently in that that's built into the remote damping valve. The LX600 then has a unique performance tuned coilover setup for the F Sport flavor, but can be optioned over by AHC, which itself is performance capable. Wonder if it gets slightly different tuning?

LX600 Trim	Performance Suspension​	AVS​	AHC​
Base	-​	-​	-​
Premium	-​	x​	-​
F Sport	x​	-​	Opt​
Luxury	-​	x​	Opt​
Ultra Luxury	-​	x​	x​

 

[TeCKis300]

Are there fluid volume limitations in the system? In other words adding stroke would imply adding fluid, but can the system handle the excess?

Sorta, not? The volume limitations I assume you're asking about is potentially bottoming out the accumulators. In the AHC system, the concern is combined with load.

With normal load, the accumulator diaphragms will not be very compressed against the ~400psi charge, allowing for much more volume before bottoming out. Compared to at high load, with diaphragm much more compressed, there is a remote possibility it could bottom out. The loads would have to be extremely high and accumulators worn in my experience.

 

[lx200inAR]

Went for a jog and was tossing this around. There is still a possible stroke limitation that is set by the volume of the globes. The shock can only extend if volume of something else in the system can shrink. There are only 2 ways for it to shrink. The accumulator expanding due to less pressure against it and the center cylinder shifting. The accumulators are 400cc I think. If the rams are 45mm and you could use every bit of the accumulator that’s a total shift of about 10”.

There’s a lot to unpack here if someone really wanted to. We know the starting pressure of the globes and we know their volume. Plus we know that ideally a stock truck is around 8mPa when at N. So we should be able to calculate the volume of the accumulator displaced fluid from the system charge.

 

[TeCKis300]

Went for a jog and was tossing this around. There is still a possible stroke limitation that is set by the volume of the globes. The shock can only extend if volume of something else in the system can shrink. There are only 2 ways for it to shrink. The accumulator expanding due to less pressure against it and the center cylinder shifting. The accumulators are 400cc I think. If the rams are 45mm and you could use every bit of the accumulator that’s a total shift of about 10”.

There’s a lot to unpack here if someone really wanted to. We know the starting pressure of the globes and we know their volume. Plus we know that ideally a stock truck is around 8mPa when at N. So we should be able to calculate the volume of the accumulator displaced fluid from the system charge.

Great point.

From the system diagram on page 1, there is a gas relief chamber on the front axle that's purpose is probably to add some more margin of expansion. As the system is cross linked similar to KDSS, each gas chamber probably supports both front and rear axle in a cross fashion.

Of course, to your point, there's going to be some threshold that'll eventually limit expansion.

 

[TeCKis300]

Well... in cleaning up the cut-up shocks, now that the oil has mostly bled off, the shims are separating more readily and multiplying! I didn't catch all of them in the first pics.

Here's a comparison of them side by side. Fronts top, rears lower. Each having about 9-11 shim elements stacked up differently.

Fronts: ~11 element compression shim stack. ~9 element rebound stack.
Rears: ~10 element compression shim stack. ~11 element rebound stack

@grinchy , these shims are .1mm thickness.

 

[bloc]

Spring rate would start increasing as globe gas volume gets compressed, if I’m thinking about this correctly. I don’t remember boyle’s law well enough to recite whether it’s logarithmic or exponential.. but rate would go up quick.

Perhaps something to consider.. truly using all of the globe volume probably isn’t going to happen without pressures getting dangerous, at which point I’d expect the system to protect itself.

Are you sure the shock you are looking at is the AHC shock and not the standard suspension one. Or I guess possibly some crazier F-sport version.

I’m not, in fact I remember the various parts diagrams being lean on the AHC plumbing, but that’s also confusing as it’s in a different section as well. I’ll dig into it more later when I have a real computer in front of me.

Side note: I understand they used shaft diameter to get the pressure/force relationship they needed, but it’s nice to see something seriously heavy duty under the vehicle we like so much. Good luck bending one of those shock shafts.

 

[lx200inAR]

Spring rate would start increasing as globe gas volume gets compressed, if I’m thinking about this correctly. I don’t remember boyle’s law well enough to recite whether it’s logarithmic or exponential.. but rate would go up quick.

Perhaps something to consider.. truly using all of the globe volume probably isn’t going to happen without pressures getting dangerous, at which point I’d expect the system to protect itself.


I’m not, in fact I remember the various parts diagrams being lean on the AHC plumbing, but that’s also confusing as it’s in a different section as well. I’ll dig into it more later when I have a real computer in front of me.

Side note: I understand they used shaft diameter to get the pressure/force relationship they needed, but it’s nice to see something seriously heavy duty under the vehicle we like so much. Good luck bending one of those shock shafts.

Right, I was only calculating the theoretical max on globe. Where it gets interesting is if you preload your spring or add a heavier spring so that it takes less pressure to reach a certain height (remember that when setting height the amount of fluid is variable). Then the globe will be less compressed and therefore you will have less bandwidth for droop. And to your point you definitely dont want to maximize that pressure giving you more droop because then you will essentially have no spring on the upstroke, the shock will just hit a “wall” against the pressure in the globe.

Great point.

From the system diagram on page 1, there is a gas relief chamber on the front axle that's purpose is probably to add some more margin of expansion. As the system is cross linked similar to KDSS, each gas chamber probably supports both front and rear axle in a cross fashion.

Of course, to your point, there's going to be some threshold that'll eventually limit expansion.

The center cylinder shifting is what I was referring to as far as the cross linking is concerned.

I forgot about the expansion accumulators. I’d have to go back and read the FSM. But I thought those ran at pretty high pressure. I thought they were a “safety valve” for hard hits. If so, then probably at typical AHC system loads they are not compressed and therefore wouldn’t have any volume to give back to allow extra droop.

when I, or someone has some time they will have to do the math on how much the volume of the accumulator changes at 8-9MPA.

Also the cross linking might be the important part of all this equation. It may easily allow for the droop we are looking for in most situations depending on how much volume is displaced by each of its chambers.

 

[AnyMal]

Right, I was only calculating the theoretical max on globe. Where it gets interesting is if you preload your spring or add a heavier spring so that it takes less pressure to reach a certain height (remember that when setting height the amount of fluid is variable). Then the globe will be less compressed and therefore you will have less bandwidth for droop. And to your point you definitely dont want to maximize that pressure giving you more droop because then you will essentially have no spring on the upstroke, the shock will just hit a “wall” against the pressure in the globe.

I wouldn’t exactly go that far though. There very limited damping in the shock itself from my experience to begin with. The exact amount of resistance is pretty much what happens when you hit L. Not much at all. Bit of resistance sure but nothing meaningful when considering a wheel impact for example at speed.

It would be true though if the gas chamber and piston were directly linked. There is the entire damping control system in between that does exactly what you are describing here. And thats where the beauty of the ride modes shine.

The system damping is purely a combination of internal pressure and fluid dynamics. With most of the rebound IMO being attributed to pressure and not gas displacement volume, for which I think there is comparatively minimal influence. I can see and feel it when i jack on the truck for example. And by the ferocity at which wheels return to pavement after a curb or bad speed bump. If we think of it another way, the entire load of the vehicle is being actuated back on the pavement and that pressure is always looking to fill that gap. It is why the truck feels so planted to me.

But where displacement is used instead I think is cross link, through the core etc. and body control.

I really think the gas charge is there to absorb the minor vibrations, not provide rebound, and thats why the truck tends to ride like s*** compared air or mag systems where you have much more gas to compress and rebound (and heat up). You don’t get this kind of payload and durability without a tradeoff…

 

[AnyMal]

I wouldn’t exactly go that far though. There very limited damping in the shock itself from my experience to begin with. The exact amount of resistance is pretty much what happens when you hit L. Not much at all. Bit of resistance sure but nothing meaningful when considering a wheel impact for example at speed.

It would be true though if the gas chamber and piston were directly linked. There is the entire damping control system in between that does exactly what you are describing here. And thats where the beauty of the ride modes shine.

The system damping is purely a combination of internal pressure and fluid dynamics. With most of the rebound IMO being attributed to pressure and not gas displacement volume, for which I think there is comparatively minimal influence. I can see and feel it when i jack on the truck for example. And by the ferocity at which wheels return to pavement after a curb or bad speed bump. If we think of it another way, the entire load of the vehicle is being actuated back on the pavement and that pressure is always looking to fill that gap. It is why the truck feels so planted to me.

But where displacement is used instead I think is cross link, through the core etc. and body control.

I really think the gas charge is there to absorb the minor vibrations, not provide rebound, and thats why the truck tends to ride like s*** compared air or mag systems where you have much more gas to compress and rebound (and heat up). You don’t get this kind of payload and durability without a tradeoff…

That being said, i still do want to know how much influence yall think the shock valving has through the stroke. Im not pretending like ive figured any of that out.

And also would be cool to dismantle one or both of the damping/control stages!

 

[AnyMal]

@daneo what say you?
can you do us a diagram? And some other fun stuff?

Im entirely in over my head but i did use the ahc shock as a super soaker in my garage to shoot fluid across the room!

 

[AnyMal]

Ok i might be going stupid tonight, its been a long day. But. Part of me thinks in the upside down world of AHC the coil will provide some of the other remaining rebound effect, past the gas compression stage…

That means, while my preload is doing its job, it may be making my globes last LESS time instead of more as I once thought… given that the coil is stiffer, it will bottom out the nitro membrane earlier before it compresses on a crash, considering none of the fluid is obv compressing and no longer has anywhere to go. Instead of being more slinky, it’s stiffer and its losing synergy with the globe.

Why did I ever touch any of this is long behind me. I give up.

Edit +
This could also explain why nothing i do has dramatic impact on ride quality. Im constantly robbing peter to pay paul. Trading pressure for preload at the expense of compression, trading lift for droop, adding droop @lx200inAR and losing… something?

 

[LX Cruiser]

This makes me wonder, if I’m replacing globes at some point in the near future (just rolled over 154k miles btw), should I go ahead and replace the shocks as well?

I decided against it because I read on here that the shocks don’t actually do any damping on their own, but now it looks like they do contribute to it, and I’m wondering now if replacing them while I‘m tackling the globes makes more sense.

 

[grinchy]

This makes me wonder, if I’m replacing globes at some point in the near future (just rolled over 154k miles btw), should I go ahead and replace the shocks as well?

I decided against it because I read on here that the shocks don’t actually do any damping on their own, but now it looks like they do contribute to it, and I’m wondering now if replacing them while I‘m tackling the globes makes more sense.

the damping the shocks do is not subject to degradation, excepting an actual leak at the seals. The shims stacks are metal and cycle life is irrelevant.

 

[TeCKis300]

This makes me wonder, if I’m replacing globes at some point in the near future (just rolled over 154k miles btw), should I go ahead and replace the shocks as well?

I decided against it because I read on here that the shocks don’t actually do any damping on their own, but now it looks like they do contribute to it, and I’m wondering now if replacing them while I‘m tackling the globes makes more sense.

Agreed with @grinchy.

The major wear item on a normal shock is leakdown of pressure. One of the most important qualities to make sure the oil doesn't aerate and still can provide damping. The AHC system is actively pressurized which removes that factor.

The second major wear items is seals. Unless there is a leak, and this probably depends on how/where the car is used (i.e. dirt vs mall crawling), the AHC shocks can go 200-300k+ without leaks. Often, it's a globe rupture the precedes and then causes shocks to leak as the pressure in the system spikes. Even with a minor weep, the AHC system has so much fluid, it won't affect performance really.

Other than taking advantage of the flush when doing globes, there's not much overlap in replacing shocks. Might as well address that when it becomes an issue, and do a separate flush, as that's suppose to be done every 60k anyhow.