Updating AHC system

[dfrelich]

Since this topic is active. Thought I'd throw this in. I had new globes last summer, and my ride has improved. Was in love with it all over. So I thought... it wasn't until I changed end links and sway bar bushings. My love has jumped into another level. FYI -- That's couple of weeks ago.

Now these days, I started to wonder if I did the other way around. Change out bushings should have been the first step in tracking AHC problems (after techstream). Would this be my answer - or at least temporarily because of graduation test indicates that my globes were dying though.

Anyway, my hypothesis is that the AHC sensors constantly monitors the condition and adjust pressures. So with dried up bushings it sends wrong message to the sensor thus it firms up or loosens up the system. Either way it gives you unwanted results.

What you think? Try bushings next before trying anything else? I bought everything for less than $100 (including new end links).

 

[whitenoise]

I was referring more to this attached procedure with Point 3. Its a fair assumption that the truck loses ride height as the springs sag and the AHC system ages. My contention is that you should be using ride height and not neutral pressure as a way of baselining the system. Once thats done, the neutral pressure should automatically be in spec. Do you agree?

Of course, for many people who have heavy aftermarket stuff in their trucks this will not work, unless they also measured ride height delta due to the weight they added.


Its interesting to note from this document that the AHC-equipped truck sits 7mm F / 40mm R lower than the base suspension, and this 40mm is also the amount that the rear raises up in H mode (40-55 is the Toyota spec). This is one of the advantages of AHC - not having to start with an initial pitch to compensate for rear axle load. I'm pretty certain this has a measurable influence on highway fuel economy as well.

 

[PADDO]

Since this topic is active. Thought I'd throw this in. I had new globes last summer, and my ride has improved. Was in love with it all over. So I thought... it wasn't until I changed end links and sway bar bushings. My love has jumped into another level. FYI -- That's couple of weeks ago.

Now these days, I started to wonder if I did the other way around. Change out bushings should have been the first step in tracking AHC problems (after techstream). Would this be my answer - or at least temporarily because of graduation test indicates that my globes were dying though.

Anyway, my hypothesis is that the AHC sensors constantly monitors the condition and adjust pressures. So with dried up bushings it sends wrong message to the sensor thus it firms up or loosens up the system. Either way it gives you unwanted results.

What you think? Try bushings next before trying anything else? I bought everything for less than $100 (including new end links).

Absolutely agree that end link bushings and cushions make a huge difference, best bang for buck and very easy to change! Did my end links and cushions maybe 20k miles ago and the improvement in ride and overall reduction in NVH was very noticeable.

 

[PADDO]

I was referring more to this attached procedure with Point 3. Its a fair assumption that the truck loses ride height as the springs sag and the AHC system ages. My contention is that you should be using ride height and not neutral pressure as a way of baselining the system. Once thats done, the neutral pressure should automatically be in spec. Do you agree?

Of course, for many people who have heavy aftermarket stuff in their trucks this will not work, unless they also measured ride height delta due to the weight they added.


Its interesting to note from this document that the AHC-equipped truck sits 7mm F / 40mm R lower than the base suspension, and this 40mm is also the amount that the rear raises up in H mode (40-55 is the Toyota spec). This is one of the advantages of AHC - not having to start with an initial pitch to compensate for rear axle load. I'm pretty certain this has a measurable influence on highway fuel economy as well.

Yes totally agree that pressure needs to be adjusted to height, or set height first then adjust pressure. I think we are saying the same thing. I'm from the Southern Hemisphere so I see thinks upside down. Don't necessarily agree with the contention that when the height sensors are set to spec the neutral pressures automatically fall in line though. If you take an AHC truck straight off the street and if it hasn't had its neutral pressures tuned in you'll most likely find that, though it's front and rear heights are in spec (cause the height sensors are working fine and haven't slipped) the neutral pressures, both front and rear will be high. That's where TB cranking (after height/cross level has been set) and new coils and /or spacers come into their own. Anecdotally, In a previous post where guy1 installed new coils and 10mm packers we were going to and fro to get that rear pressure down some more when we established his rear end was about 25mm higher than ideal, he adjusted down the rear height sensor lever arm about 1/4 inch and lowered his rear height by 25mm and pressure by about .3.

 

[Jack90210]

OK, bringing this thread back from the dead. I need new springs for an AHC-equipped LX, and I want to see if the King springs are still the best way to go. My concern is that they'll be too stiff so I've been hesitant to pull the trigger.

 

[Moo87]

I'm wondering also, as I'm in the "keeping AHC camp" who will have heavy load (1500 - 1600 LBs) here and there that would exceed the 1200LBs payload cap. I would like to know how much the King springs can increased my payload to, and if there're any other similar options out there (Custom?). My ideal springs would meet or exceed OEM durablity and be able to handle 1600 LBs without being too bouncy when the rig is empty.

 

[bamabrock]

Did anyone ever come up with the actual spring rates of the king springs compared to the factory LX and LC springs or did this thread die of hurt feelings from differences of opinion?

 

[PabloCruise]

Absolutely agree that end link bushings and cushions make a huge difference, best bang for buck and very easy to change! Did my end links and cushions maybe 20k miles ago and the improvement in ride and overall reduction in NVH was very noticeable.

What is NVH?

 

[Ramathorn15]

What is NVH?

Noise, vibration, and harshness

 

[PabloCruise]

Noise, vibration, and harshness

Whew! I was afraid I forgot some important acronym!

 

[Fuge]

Understood, point well taken. Based on my understanding of the system, i feel this is the most efficient route for refreshing the system:
1. Bleed and refill system with fresh fluid.
2.. Measure baseline pressure with truck at curb weight (full fluids). When I measured, I was outside the vehicle and used a broom stick to reach the height switch - procedure says to lower to L, then back to N for reading pressures.
3. Measure ride height using Toyota's procedure and correct to lower end of range (I.e. truck sits at highest recommended ride height) using torsion bar adjustment and replacing coils/packers. (Haven't done this yet)
4. Adjust ride height sensors to read zero offset at this height.
5. Remeasure neutral pressure, now it should be close to the low end of the scale.

FYI, you can hit the button with the door open. Once you shut the door after hitting the button, it will work. Or a broom stick works too.

 

[Fuge]

I would like to second Bamabrock's question. Does anyone know what the spring rates are or where to find that information? There is bound to be a way to do the math based on how much armor and gear a vehicle is carrying to figure out the appropriate spring rate that will get the AHC into spec. Does anyone know what this equation is?

I have a 99 LX with an ARB front bumper with winch, Slee dual swing rear, roof rack, slides and steel skids. I carry a recovery bag, small tool kit, xjack, and a few other odds and ends at all times. Best estimate is 1000lbs for all of that. My factory LX springs were shot when I bought the vehicle and the pressures were obviously way off when I put all the stuff on. To fix that, I turn the torsion bars and got the pressures in the front right. For the rear I replaced the springs with new factory LX springs and added 30mm spacers. That brought the rear pressure down to 7.1 but not quite in spec. The system works great now. Except when we load up the whole fam with camping gear. I think that is just too much weight for it. I can trick the system to go to high mode by getting everyone out of the car once off road and then hit the button. It will go up and stay up then. Best estimate for family, dog and camping gear is 800lbs.

The way I understand it is the springs and TBs are suppose to carry 60% of the weight the AHC is supposed to carry the other 40% plus up to 1200lbs of additional weight (people and gear). So if we call that the neutral position of the system, with my LX being slightly out of specs, it is putting more weight on the back AHC than it should. Which I feel then cuts into my 1200lbs of people and gear. The ride is not as good either. The computer system was supposed to operate with the TBs the way they were and not extra turns and springs that aren't squished with spacers. My goal would be to find a TB and spring combo that would bring the system back to how it was designed to operate while accommodating the extra static weight. If I can get the correct stronger TBs and rear springs, the AHC essentially wouldn't know I had added 1000lbs. Most of the variables are known I just don't know the equation or the spring rates. I haven't studied Mechanical Engineering since college 16 years ago so I pretty much don't remember any of it other than I know this should be a solvable puzzle with out using opinion, guess or trial and error. Come on smart people on here, show me your magic.

 

[dfrelich]

Or just hit height button and hold the door pin on the jamb in. Just make sure you stay away from the truck.

FYI, you can hit the button with the door open. Once you shut the door after hitting the button, it will work. Or a broom stick works too.

 

[Perfectmax]

Question related to the AHC...

Goal - 2-3" lift with 305/65/18 BG

I've had both an 2005 and 2007 LC. The 2005 absolutely road better without the AHC but it is fun to be able to adjust ride height. This said though, I'm looking to purchase bigger tires and am wondering if it makes sense to either buy the AHC override that will keep the truck in H at all times or if I should make the switch to an OME suspension. If I make the switch, how much of a bear will it be to yank out the AHC entirely? Also, does anyone ride in H all the time? Is it comfortable or does it ride like crap?

 

[uHu]

I would like to second Bamabrock's question. Does anyone know what the spring rates are or where to find that information? There is bound to be a way to do the math based on how much armor and gear a vehicle is carrying to figure out the appropriate spring rate that will get the AHC into spec. Does anyone know what this equation is?....

This page contains the equation for coil spring rates, explained: Performance Alignment
(Only problem is that this one works in inches and pounds, to which I cannot relate...)
Then we need some number from the gas springs (the nitrogen accumulator) as well, I suppose. Or, since the coil springs will carry the whole extra load, we may need only the change in coil spring rate to match the increased load.

 

[Fuge]

BLUF (bottom line up front): LX OEM spring rate 58 lb/in and KTRS-79 is 130 lb/in.


uHu Thanks for the equation. Assuming that equation is correct which it seems to be, I did some math to compare the spring rate of the OEM LX spring and the King Springs KTRS-79. Then I made gross assumptions about my vehicles weight distribution and assumed a linear relationship of how the load is bared to see how much more spring would be needed to neutralize the effects of the added armor weight on the AHC system. What I came up with is if you add 1000lbs of armor and gear plus 800lbs of people and camping equipment, the KTRS-79 should be able to handle that with room to spare.


Below is the math I used if you care. I have purchased these and am waiting for them to arrive and install them on my LX so this is only theoretical and definitely makes some liberal assumptions. I got the springs from here New King Springs Suspension Standard Rear Springs KTRS-79 fits Lexus LX 470 for $188 ($137 + $51 shipping) if you anyone else is curious (no affiliation just the cheapest I could find including shipping. King Springs can sell them to you directly but their shipping was significantly more.) This discussion assumes your AHC is working properly and your front pressures are correct. I do not know what the unintended consequences are though in relation to ride quality, handling or articulation. The question was asked what the KTRS spring rate is so here it is and how I interpreted it in relation to using spring rate to compensate for added weight with AHC.


I emailed King Springs about the dimensions of the KTRS-79 spring and this is what they sent back. I converted the mm to inches myself.


Wire Diameter - 14.5mm (.5709in)

Internal Diameter 140mm (5.5118in)

Free height of coil - 430mm (16.9291in)

Spring rate - 130lb/in

Number of coils – 6.5


Here is a synopsis of the link from above uHu sent for the spring rate equation:


k=(d^4G)/(8D^3N)


k = spring rate

d = wire diameter

D = coil diameter

G = the sheer modulus of the spring steel

N = the number of active coils


Since they sent the spring rate I didn’t really need the equation for that but I did need to find out what they used for G. I solved for G and got 10,655,819 (unknown what unit of measure this is).


Using that information, I calculated the spring rate of the LX oem spring since after scouring Mud and the net I could not finding it anywhere. Here are the numbers I used in the LX equation. I got these numbers from searching Mud, the internet, and rough measurements of the oem springs on my LX.


LX OEM

d = .485in

D = 6.1417in

G = 10,655,819 (I used the same the G as the KTRS for simplicity. If you use 11,500,000 instead it adds 5 lb/in)

N = 5.5


If I got those numbers right, the spring rate for the oem LX rear left hand spring is k = 58 lb/in.


If I remotely got this math right that would mean the KTRS-79 is more than twice as strong as the stock spring. Which led me to my next question, will that be enough for the extra weight? Assuming the stock LX weighs 7000lbs (ish) and my armored LX weights 8000lbs (not actually measured), that would mean the spring rate would need to increase by 14.2857% (8000/7000 = 1.142857). That assumes the armor puts an equal amount of weight on all four corners. That obviously depends on your setup but for me front and rear bumpers are about the same, the sliders are exactly in between the wheels, and the more forward skid plates offset the more aft roof rack. Getting the distribution perfect exceeds my level of nerding out for one thread so I am calling those even.


So the 14% increase covers the armor. Now put 800 more pounds of gear in the back/on top. Let’s say worst case the rear suspension handles all of that. Assume the stock vehicle carried 1750lb per wheel (7000/4=1750) and that 800 is split per wheel to 400lbs. That requires a 37.3% increase in spring rate which comes to 80lb/in.


So with all that, it would seem theoretically that the King Spring could handle armor, people, gear, and possibly something on the hitch and keep the pressures in spec. The negative consequence I feel could be if you don’t carry a bunch of extra weight it could have the inverse effect making the pressures too low or the articulation too stiff. From what I have read on the forums it seems like most people with weight and/or trailer seem happy with their choice of the King Spring. I was also curious if OME made a spring that could do this. Based on this chart Cruiser Outfitters, I don’t see a comparable OME spring to the KTRS-79 using my limited knowledge to compare them. Let the questions and spears through the holes in my research fly. I have no doubt I have missed something here but potentially could be a good start.

 

[PADDO]

@Fuge this is all good stuff. I'd like to throw a few data points into the mix:
I've previously measured LX AHC coils at between 90-95 lbs/inch - higher than the calculated 58lbs/in and in line and reasonable (in my view) with the published 170lbs/in for standard non AHC coils and Kings stated 130lbs/in.
I've found that 210lbs directly over the rear axle ~ 1 MPa increase in rear pressure
1 inch increase in rear height ~ 0.6MPa
1 inch increase in front height ~ 2.2MPa
@Ramathorn15 recent install of the Kings saw his rear pressure drop ~3.9 MPa and my own experience with these springs netted a drop of 2.5MPa. Averaging these gives a drop of 3.2MPa which if you multiply by my 210lbs per MPa you come out with something like 670lbs increase in capacity over standard coils. The objective always being to return and keep the rear neutral pressures in their design range, ideally the lower end of the range. And as we are measuring to one decimal place for pressures with a similar accuracy for height there's going to be some variations.
A few more pre and post King install pressure readings (with no other variables thrown in to confound) would help build out the data set.

 

[ihadmail]

I just did some more digging, and it appears the LX450 springs that I tried in my LX470 had a spring rate of 150lbs/in. I haven't done the math to confirm this as I don't have all of the info needed at work with me.

It makes sense though, as I've read multiple places they aren't as stiff as the 100 series non-AHC coils. I'm still keeping my fingers crossed that after I add the rear bumper and drawer system that the 80 series springs aren't too stiff still.

 

[txtortoise]

To fix that, I turn the torsion bars and got the pressures in the front right. For the rear I replaced the springs with new factory LX springs and added 30mm spacers. That brought the rear pressure down to 7.1 but not quite in spec."

You're the first report I've seen that experienced the same thing I did, i.e., new springs and spacers and still didn't get rear pressures down to stock or below. That's with stock weight in rear though, while you have rear bumper.

 

[Fuge]

txtortoise, I think for me it is just the extra weight. I changed the AHC fluid last summer but that is all I have done so I don't know the status of the globes or other components other than it seems to work fine without people and gear and it has 190k on it.

Paddo, 58 does seem low to me but I figure the AHC is doing a lot of work so maybe that was plausible. How did you measure the spring rate? I don't know how to do that so I would be curious to learn how. Using 90 instead of 58 would yield a spring rate of 123 lb/in for a 37% increase so the King Springs theoretically would still be good for the parameters I was using.

ihadmail, good luck and sounds like you have a great excuse to buy more cool stuff for your rig.