ABS plumbing

[AZPete]

To Brake Gurus,
Installing the OE ABS system, my first thought was run a line from the master cylinder primary to the proportioning valve and then a line from the proportioning valve to the MC1 inlet of the ABS module. But, the ABS module sends fluid to diagonal wheels so the proportioning valve would be restricting flow to diagonal wheels - not good!

The Factory Service Manual includes this: (ABSCM = ABS Control Module)

E: EBD (ELECTRONIC BRAKE FORCE DISTRIBUTION)
The EBD system utilizes the function of the conventional ABS. This system prevents premature locking of the rear wheels by using electronic control instead of a proportioning valve. The system provides a feature to optimize the brake force distribution to the front and rear wheels according to change in loading conditions or displacement of the center of gravity during deceleration.

The ABSCM calculates the optimum brake force distribution from the difference in speed between the front and rear wheels. Based on the results, the ABSCM controls the rear wheel brake pressure through the ABS hydraulic unit (H/U) to achieve the optimum brake force distribution suitable for the driving condition.

The proportioning valve is disused on EBD equipped vehicles.

Now my question: Assuming I install the OE ECU, am I correct in not using the proportioning valve? Should I plumb the master cylinder primary line to the ABSCM MC1 port and the master secondary line to the ABSCM MC2 port?

[RM1SepEx]

in theory, yes with a huge but... there is no way that it will be expecting the amt of brake proportioning required. So can it deal with a wide range or not? Who knows. I'd use the proportioning valve too...

[Xusia]

So how would I know if my WRX has EBD?

[Bob_n_Cincy]

I had my ABS plumb in earlier this week. Then I ran into leaky flare syndrome.
While getting things cleaned up, I Was surprised when I sprayed brake cleaner in to the MC1 port, It came out MC2. I thought they would be separate circuits.
My thought was to put a proportioning valve on each front wheel.
Bob

[skullandbones]

Hi Pete,

Although I am not a brake expert, I did quite a bit of research on the roadster starting with the donor MC. I went from stupid to "it works" mode. I have the Wilwood bias bar system and it works perfectly with the initial adjustment. I also have the proportioning valve installed but probably will not need ever. If you install it between the primary and ABSCM 1 and it will be there if you need it. I think in your case, the chances of you actually needing it are greater than mine because of the extreme change in weight distribution and the ABS logic. It will be great if it works for you like in the OE scenario, but if it doesn't you have an immediate working solution. Since your "experiment" with this has not been tested, you will be the test pilot. It would be nice to be able to adjust on the fly. Even with the ABS installed, you can still do panic stop testing like others do on the non ABS systems to see if it behaves as designed. The ECU isn't going to know the PV is there until you adjust it. IMO, WEK.

[RM1SepEx]

What he said +1

:-)

[AZPete]

B & C: "My thought was to put a proportioning valve on each front wheel."
Yes, I thought of that but if the proportioning valve is before the ANSCM diagonal wheels are effected - not good. If I put a proportioning valve on each front line after the ABSCM then the valve controls the fluid, not the ABSCM, so no anti-skid function. Right?

WEK: "If you install it between the primary and ABSCM 1 and it will be there if you need it."
Same problem - fluid sent to ABSCM 1 goes out to 2 diagonal wheels, either Rgt Frt & Lft Rear or Lft Frt & Rgt Rear, which I think would cause some scary stopping and might involve a tree. Right?

I'm not sure I should use the proportioning valve. I'm only sure I should wait until I can figure it out.
Pete

[Xusia]

Hi Pete,

I have a couple thoughts:
1. I don't see where it says the brake force is distributed diagonally. Is that listed in some text that isn't posted here?
2. You could always completely bypass this issue by getting an ABSCM that doesn't utilize EBD. I'm sure there are plenty of people who aren't going to use ABS. I'm assuming there exists models with ABS but without EBD. <-- Some expert care to verify??

[skullandbones]

Pete, I was thinking the proportioning valve might give you an adjustment point to begin exercising it at different pressures to reduce the front bias if it was evident when you were testing. However, I don't believe the diagonal wheel process of controlling that the system uses is an issue. The ABS should be doing permutations in very quick time: much faster than human reaction times (e.g. pressure on left front and right rear then pressure on right front and left rear........) in the same corner or slide. So if it is balanced to correct for weight bias, you would expect it to function as designed. The real tough problem is you don't have a similar car that has been setup with the Subie components unless some racers have done it on one of these exotics with similar weight distribution. WEK.

[AZPete]

The factory service manual describes the ABS system as having "two diagonally split brake circuits". It sounds to me like a proportioning valve installed between the master cylinder and either inlet of the ABSCM would effect one front wheel and one rear wheel, hence it would not effect front bias. I believe the reason for the diagonally split brake circuits is in case one circuit looses pressure the car would handle better than either just the fronts or just the rear, as in many other cars. Or, maybe it's designed like that because they wanted to screw with me.

Plan B: don't install the proportioning valve and let the EBD feature figure out there's more weight in the rear. (Holy crap, this guy has a load of bricks in the trunk!)

Got to go home now. My wife called to say there's a heavy box at the front door! Yippee! Backordered parts.

[tmoretta]

To Brake Gurus,
Installing the OE ABS system, my first thought was run a line from the master cylinder primary to the proportioning valve and then a line from the proportioning valve to the MC1 inlet of the ABS module. But, the ABS module sends fluid to diagonal wheels so the proportioning valve would be restricting flow to diagonal wheels - not good!

The Factory Service Manual includes this: (ABSCM = ABS Control Module)

E: EBD (ELECTRONIC BRAKE FORCE DISTRIBUTION)
The EBD system utilizes the function of the conventional ABS. This system prevents premature locking of the rear wheels by using electronic control instead of a proportioning valve. The system provides a feature to optimize the brake force distribution to the front and rear wheels according to change in loading conditions or displacement of the center of gravity during deceleration.

The ABSCM calculates the optimum brake force distribution from the difference in speed between the front and rear wheels. Based on the results, the ABSCM controls the rear wheel brake pressure through the ABS hydraulic unit (H/U) to achieve the optimum brake force distribution suitable for the driving condition.

The proportioning valve is disused on EBD equipped vehicles.

Now my question: Assuming I install the OE ECU, am I correct in not using the proportioning valve? Should I plumb the master cylinder primary line to the ABSCM MC1 port and the master secondary line to the ABSCM MC2 port?

Since the ABSCM calculates brake force distribution from diff. between front and rear wheels, does that then prohibit the use of a staggered size wheel setup like most 818 builders want to include? Do the front and rear wheel/tire diameters have to be exactly or near the same?

[AZPete]

tmorreta, ABS works off wheel speed and not wheel size, so it should not be a factor. There's an interesting thread in this forum that explains much more, but does not address the plumbing question I asked here:

http://thefactoryfiveforum.com/showt...es-on-the-818R

[WIS89]

AZ Pete-

Have you hit bottom on your question yet? Unfortunately I don't know the answer, but I am curious about it.

Have you considered calling a dealership and talking to one of their ABS guys? I think your question is general enough that they may be able to help you; i.e. it is not vehicle specific, it is specific to the ABS system, and its operation. It seems to me that someone well versed in the mechanics of the ABS system could probably answer this and provide you with the peace of mind you are seeking.

I'd be willing to call someone around me to see if I can find an answer for you if you would like. It's an interesting dilemma...

I hope you are able to solve the mystery!!

Regards,

Steve

[Bob_n_Cincy]

If I put a proportioning valve on each front line after the ABSCM then the valve controls the fluid, not the ABSCM, so no anti-skid function. Right?

I don't agree here. if the ABSCM reduce pressure lets say from 1200 to 800 psi. The after the proportional valve the pressure to the wheel would change from 1000 to 666 psi.
Having a proportional valve at each front wheel would be like just having smaller brakes on the front.

The circuit is diagonal see picture


plumbing.jpg

The way FFR is plumbing, the primary is going to the back wheels and the secondary is going to the fronts through the Prop. valve. they get by one valve and one adjustment point. This is a more traditional setup.
Bob

[AZPete]

Steve and Bob, you guys understand my question. Thanks for the Subaru info, Bob. As Steve suggests, I think I'll look for some smart brake guys online. It's really an ABS question more than a Subaru question. I've met some of the mechanics at the local Subaru dealership and I don't think they would understand the question as all they do is replace parts per instructions.
When I find the answer, I'll post it.
Pete

[Goldwing]

Just to be clear, EBD uses the ABS system, they aren't interchangeable terms. EBD on many cars is an electronic driving aid that uses the ABS system to help distribute brake forces in dynamic driving conditions to prevent losing control. ABS by itself is just basic individual wheel skid protection. So a decision has to be made whether to just have ABS or have the added features of EBD as well. I haven't looked into how you utilize only ABS without EBD, perhaps just not using the G sensor would isolate to ABS only? Are there other sensors that help determine loading? The idea of 2 portioning valves after the ABSCM seems to make sense to me. Definitely not before because of the diagonal distribution issue. So here's a question for someone smarter than me. For a build using only Basic ABS, not EBD, could you plumb the primary so both outputs from the ABSCM go to the rears and the secondary through one portioning valve first through the ABSCM then to the fronts, like the FFR manual? Of course making the same adjustments to the wiring so the abs matches the tone wheel sensor with the brake line? I'm more inclined toward the two portioning valve approach due to too many unknowns, and the possibility of having EBD work too. Not really answering questions, sorry, but hopefully moving us closer.

[Goldwing]

Are there metering valves in the wrx with discs all around? If so where? If its integrated into the ABSCM, then scratch the plumbing change idea.

[Bob_n_Cincy]

Hey Pete,
Here is my ABS plan.
1. plumb abs per donor car without the donor double rear proportioning valve. prop valve.jpg
2. test car with abs unplugged.
2a. if premature front lock up occurs, add donor double proportioning valve to 2 lines going to front calibers.
2b. if premature rear lock up occurs, add donor double proportioning valve to 2 lines going to rear calibers. (OEM plumbing)
2c if good balanced braking, plug in abs, test abs.

3. If donor prop. valve not enough. remove it and add 2 adjustable valves to front or rear calibers.
3a. test and adjust as necessary.
3b if good balanced braking, plug in abs, test abs.
Bob

[AZPete]

Bob, I like your plan. To save some work later however, I'm planning to install a prop valve on each front brake line, after the ABSCM. The Wilwood prop valve info says, "Rotating the adjuster knob clockwise until it is all the way in will provide full pressure delivery to the calipers." Hence, when the prop valves are open it's like they are not there so if the fronts lock up in testing I can adjust. If the OE EBD system compensates for the front/rear braking then I can leave the prop valves open. Bob's plan covers all conditions.
Thanks. Now I can start flaring and bending.
Pete

[auburn2]

I don't think the aportionment matters and if you are going to use EBD I would not use a portioning valve (like the original). The EBD is a control system it is designed to account for traction variances far more significant and diverse than what the different weight bias will cause . The forward/rear difference in traction between an 818 and impreza due to weight bias is miniscule compared to the difference in traction which is encountered when different wheels are on different surfaces (example one wheel on ice one on dry pavement) and EDB is desgned to deal with those variances.

A bigger issue to me is if it uses data from the yaw sensor at all. I don't see how it can work with the Subaru VDC if it doesn't. If it does in fact use the yaw sensor input then placement of that sensor relatve to the yaw center will be critical to proper functioning.

[AZPete]

Thanks, auburn2. I'll keep this in mind. When I get on the road I'll do brake tests to learn more and then adjust. . . . or start completely over.
Pete

[mn_vette]

Hey guys, I read this post and I wanted to set a few things strait.

The ABS module is the proportioning valve for an ABS car. The stock brake setup should be balanced pretty well, but in some braking conditions the ABS unit will decrease the braking pressure to the rear wheels using the valves inside the ABS unit. This is done electronically, so Bob_n_Cincy, you need the ABS unit powered to have it work.

The typical setup for a braking system is that there are two sides with the separation starting at the master cylinder, each side does one front and one rear, in a cross pattern. That way if one of the circuits fail you don't have braking on only the front/back or left/right. You get a diagonal which is safer. A proportioning valve only reduces the pressure, so you would want to put them on the rears only if you were adding a valve. And you would have to add two of them, one per rear tire. If you are keeping the stock front/rear brake setup then I would suggest not adding an additional proportioning valve.

If you are adding heavier brakes on the front, then I would think you would want more pressure going to the rears to balance it out, and a proportioning valve would not make sense for the rears. You could put them on the front at that point, but why would you upgrade the front brakes just to reduce the amount of pressure going to them slowing your braking back down.

The different weight distribution of the 818 vs the WRX will change things according to the brakes. With more weight in the back the rear brakes should be doing more of the braking. The EBD is calibrated with the WRX dynamics in mind but without some good testing we may not know how much of a difference the weight distribution effects the EBD.

The other thing I saw mentioned was something about the wheel size. Wheel size(total diameter) changes will cause the wheel speed readings going to be different going into the ABS unit. However, if the change is not too dramatic then things should be ok. The ABS modules are required to deal with things like low tire pressures and curves, so non-matching wheel speeds should not be a problem. Just remember to fix the speedometer. The actual algorithm that sets off ABS is the speed delta per wheel. If it slows down too quickly(approaching lock up, speed = 0mph) then the ABS unit reduces the brake fluid pressure to the part and keeps things rolling.

I hope some of this info helps.

[Xusia]

I was actually wondering if the proportioning valve was needed when using ABS, because it did seem to me that the ABS would essentially perform that function. Thanks!

[Quiny]

Hey Pete,
Here is my ABS plan.
1. plumb abs per donor car without the donor double rear proportioning valve. prop valve.jpg
2. test car with abs unplugged.
2a. if premature front lock up occurs, add donor double proportioning valve to 2 lines going to front calibers.
2b. if premature rear lock up occurs, add donor double proportioning valve to 2 lines going to rear calibers. (OEM plumbing)
2c if good balanced braking, plug in abs, test abs.

3. If donor prop. valve not enough. remove it and add 2 adjustable valves to front or rear calibers.
3a. test and adjust as necessary.
3b if good balanced braking, plug in abs, test abs.
Bob

How did this work out? Did you need the proportioning valve?

[wleehendrick]

because it did seem to me that the ABS would essentially perform that function. Thanks!

You would not want the ABS controller to try to cover for an improperly balanced system (one that would otherwise lock-up the front or rear prematurely without ABS active). The Subaru system was all designed to work together in an Imprezza/WRX; when adapting the system to an 818, there are lots of variables (different wheel diameters, brake upgrades, weight distribution, etc... to consider). I like Bob's plan... he knows what he's doing and I'm curious to see his result. Note that he's going electric and will have greater front weight distribution than a typical 818.

[AZPete]

Warning: this photo may give you a headache.
Does it work? I won't know until road testing. But, following Bob's plan I added two proportioning valves - one in each front line. If the OE system compensates for the front-rear weight difference then prop valves are not needed and I'll leave them open, but I added them now in case they are needed. Plus, I had triple the flaring fun!



The spaghetti of brake lines: 2 from master to ABSCM, 2 from ABSCM to rears, 2 from ABSCM to prop valves, 2 from prop valves to fronts, clutch line.

[Xusia]

You would not want the ABS controller to try to cover for an improperly balanced system (one that would otherwise lock-up the front or rear prematurely without ABS active). The Subaru system was all designed to work together in an Imprezza/WRX; when adapting the system to an 818, there are lots of variables (different wheel diameters, brake upgrades, weight distribution, etc... to consider). I like Bob's plan... he knows what he's doing and I'm curious to see his result. Note that he's going electric and will have greater front weight distribution than a typical 818.

Can you expound on your logic? I certainly understand the system was designed for a different car, but at the same time it was designed to do a very specific thing: To prevent a wheel from locking up under braking. The system does not take into account why that would happen; it just takes action to prevent it. In other words, the system doesn't know or care if the reason for impending lockup is ice, gravel, or a different weight bias. Therefore, if the brakes are unbalanced, would it just activate more often on whatever end needed it? I don't understand why that would be bad.

I'm not trying to argue; I'm seeking understanding.

Warning: this photo may give you a headache.
Does it work? I won't know until road testing. But, following Bob's plan I added two proportioning valves - one in each front line. If the OE system compensates for the front-rear weight difference then prop valves are not needed and I'll leave them open, but I added them now in case they are needed. Plus, I had triple the flaring fun!



The spaghetti of brake lines: 2 from master to ABSCM, 2 from ABSCM to rears, 2 from ABSCM to prop valves, 2 from prop valves to fronts, clutch line.

If the valve is needed, this makes the most sense to me - AFTER the ABS module.

[wleehendrick]

Can you expound on your logic? I certainly understand the system was designed for a different car, but at the same time it was designed to do a very specific thing: To prevent a wheel from locking up under braking. The system does not take into account why that would happen; it just takes action to prevent it. In other words, the system doesn't know or care if the reason for impending lockup is ice, gravel, or a different weight bias. Therefore, if the brakes are unbalanced, would it just activate more often on whatever end needed it? I don't understand why that would be bad.

I'm not trying to argue; I'm seeking understanding.

I have no doubts that the ABS controller will work fine in an 818; all it's doing it's detecting any wheel which is turning dramatically slower than the others, then releasing that brake for a short time interval. The issue is that to maximize braking performance the system still needs to be balanced first, and ABS can't do anything about the balance.

Bob's plan spells it out... you want to balance the system so that the front/rear reach their traction limits equally. If the rear locks up first, then ABS kicks in to release the rear tires before full braking potential is achieved on the front (or vice versa).

The WRX has nearly 60% of weight on the front axle; the 818 nearly opposite; this is why the kit includes a BPV for the front brakes. If transferred over without modification, and using the same running gear, the 818 brakes will be front biased and the fronts will lock up way before the rears are doing much braking. ABS cannot fix this unbalanced condition, all it can do is release the front brakes when they lock up, but this occurs long before the rears. Installing the BPV on the front does not reduce the front braking capacity, it just means to you need more pressure to get the same response, so front and rear lock up at nearly the same time, meaning ABS can function properly, releasing a single locked wheel when the other three are near the traction limit.

Make sense?

[AZPete]

Xusia, nothing sounded like an argument so don't worry. The complexity stems from the diagonal control in the Subaru system. Bob and some other guys said proportioning valves on the front lines made sense, and I agree. But, will the EBD system (post #1) compensate for the heavy rear and light front? I think it will, but I'm not sure. When I get my 818 on the street I'll pull the ABS fuse to deactivate it and then test the brakes for lockup, pulling, etc. with the prop valves open. If I get front lockup I'll adjust the prop valves until it's right. Then, I'll then test again with the ABS active. It will be interesting. Fortunately I live where I can try lockups on private pavement and gravel roads. You're right that the ABS only cares about wheel rotation, not the cause. From what I read, ABS reacts to each wheel independently so it would work on a unicycle.

I think Erik Treves installed the donor ABSCM but I haven't heard if he has tested it. If his ABS works without dual front prop valves then I've over-thought, over-built this. It's all part of being a pioneer builder and a year from now the answers will be obvious.

[Xusia]

OK. I think I understand that. If I read it correctly, you are implying ABS activation is a bad thing? Or somehow results in less braking force? I don't get that.

Let's say in the 818 the brakes are biased toward the fronts, and therefore they would lock up before the rears. If I understand ABS correctly, it would intervene and reduce and then reapply braking force (technically, they stop intervening for a very short period) keeping effectively keeping the brakes just shy of the limits of traction (more on that limit in a sec - hang with me...). Meanwhile, rear braking is unaffected, so if additional pedal force continues to be applied, rear brake force increases unaffected until they also reach their limit. Problem solved??

Regarding the limits of traction & ABS... I understand that humans can achieve braking force closer to the limit of traction than an ABS system. I'm using ABS because I believe it can do a better of that than I personally can, so I don't want to get into an ABS vs. human driver argument when discussing this particular topic. I just want to know if I really need to install a proportioning valve or if I can rely on ABS to solve this problem for intended street use.

[wleehendrick]

If I read it correctly, you are implying ABS activation is a bad thing? Or somehow results in less braking force? I don't get that.

No, just premature ABS activation. I think ABS kicking in at 0.8G deceleration with unbalanced brakes is a bad thing if you could brake at 1.0G without ABS, if the system were properly balanced. ABS does reduce braking force because the brakes have to be released to restore traction. Yes, it's only for a small amount of time, but it does reduce the duty cycle of the time the brakes are applied. This is why, in ideal conditions, threshold braking outperforms ABS. If the system is really unbalanced, using ABS is a crutch that does not address the root cause of the problem (imbalance).

Let's say in the 818 the brakes are biased toward the fronts, and therefore they would lock up before the rears. If I understand ABS correctly, it would intervene and reduce and then reapply braking force (technically, they stop intervening for a very short period) keeping effectively keeping the brakes just shy of the limits of traction (more on that limit in a sec - hang with me...). Meanwhile, rear braking is unaffected, so if additional pedal force continues to be applied, rear brake force increases unaffected until they also reach their limit. Problem solved??

Yes, that's correct, you could apply more pressure, but that implies that all hard braking would be done with ABS kicking in on the fronts all the time; which would reduce performance as previsously discussed. It would be far better to balance the system, and only rely on ABS when absolutely needed.

Regarding the limits of traction & ABS... I understand that humans can achieve braking force closer to the limit of traction than an ABS system. I'm using ABS because I believe it can do a better of that than I personally can, so I don't want to get into an ABS vs. human driver argument when discussing this particular topic. I just want to know if I really need to install a proportioning valve or if I can rely on ABS to solve this problem for intended street use.

I agree that in most real world conditions (not a controlled track test), ABS can brake better than most drivers, me included. However, I believe its far better to have it kick in only when absolutely necessary. Even if you're using ABS, I think it's a really bad idea to not balance the system, particularly in the case of Subaru brakes (designed for a 60/40 weight distribution) on an 818 (with 40/60 distribution).

[Xusia]

You are the man! I understand what you saying. Thank you for taking the time to explain it to me.

[Bob_n_Cincy]

How did this work out? Did you need the proportioning valve?

I am not running in go kart mode yet. So I have not tested ABS yet.
During my glider test I did lock up tires but I have no Idea which one.
It would be bogus info as the glider only weighed 446 front 394 rear for a total of 840 Lbs.

[AZPete]

Wleehendrick, nice explanation. Thanks.
Erik Treves, have you been able to test your ABS yet?

[wleehendrick]

You are the man! I understand what you saying. Thank you for taking the time to explain it to me.

Wleehendrick, nice explanation. Thanks.

Glad I could help!

[Junty]

Interesting reading - thanks for all the various views...
Please could someone who has already elected to use ABS please post an update, I'm still dismantling my donor, and very keen to keep ABS for my 818s road car. However some of the above posts and the theory has left me wondering if it is worth it or possible? I'd appreciate an update from someone who has been successful at completing an ABS install, are they happy?
Sorry to be very naive, but my original logic suggested that because of the significant weight loss from full Subaru WRX 4x4 to 818 2wd, very little alteration would be needed to the standard brakes and the 818 should stop as well if not significantly better than a full WRX - even once power assist was removed?

[AZPete]

Junty, since I started this thread I should answer you, but to my knowledge nobody has yet tested their ABS installation on an 818. There are only a few 818s, if any, that are licensed and on the road. I've installed ABS, but my 818 is not yet running. I suggest that you keep the ABS control module and any proportioning valves from your donor, as well as the toner rings on the hubs, but discard the brake lines. Hopefully by the time you are installing your 818 brake system enough guys - hopefully me - will have tested it so we will all know how it works and how it should be installed.

You are correct in that the donor brakes will perform better on the 818 than on the WRX due to less weight, even without power assist. For instance, the FFR roadsters with Mustang brakes stop much quicker than the heavy Mustang. ABS comes into play on the 818 or WRX only in panic stops or slippery conditions.

(I'm jealous of you living in such a wonderful place! I've really enjoyed visiting New Zealand.)

[JeromeS13]

What's holding you up, Pete? I thought you would have at least driven yours by now!

[Junty]

Thanks AZPete, I wish you all very best and I trust(I'm quietly confident) that the ABS will be just fine in the 818. I look forward to further posts regarding this.
Thank you for kind comments of our beautiful country. As a Kiwi we are very proud of this reputation - please PM me if/when you return.

[Junty]

Happy new year AZPete, Any further update on ABS working?