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Old 04-15-2002, 11:08 PM   #1
bjteel
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Default ABS Variables?

Can we start a thread listing mods you have made and how they affect the ABS braking problem?

For example, some people have swapped out only tires, some wheels and tires, some springs only, some coil-overs, and some may have done all of the above.

List your mod, and how it affected the ABS situation. Maybe we can find the culprit ourselves ...
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Old 04-15-2002, 11:33 PM   #2
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Just a quicky on ABS.

All ABS is made up of is wheel speed sensors. An ABS ECU, an ABS HCU (which together are called the ICU)

ECU- electronic control unit (The Computer)
HCU- Hydraulic Control Unit (that AL block)
ICU- Intergrated Control Unit

Plus the ABS pump.

There is 4 channel and 3 channel systems (Subaru uses 4 channel). The difference is that a 4 has a sensor for each rear wheel.

The sensors send a signal to the ECU letting it know that wheels speed. If the speed drops to a point at a certain rate where the ECU is programmed to assume a soon to lock up wheel it will activate the pump and valves in the HCU to modulate brake pressure to that wheel.

Now changes that cause the rear wheels to lock up sooner will cause the ABS to act differently (or the fronts for that matter). Also changing overall wheel diameter will change how the ABS reacts.

However, ABS is a "dumb" control, it only reads one parameter, wheel speed, and then does only one thing. It's not like ESP or even traction control that use multiple sensors and then can use multiple control logic routines to "decide" what to do.
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Old 04-16-2002, 08:43 AM   #3
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Thanks for the info, but you didn't list any mods.

I like the theory that bumps are causing the wheels to lift off the ground enough to cause the wheel speed to slow, thereby fooling the ABS into thinking there is about to be slippage.

This may be compounded by stiffer springs. It may be helped by a heavier wheel/tire combo or any heavier sprung weight. I would just like to see what changes people have made and how they have affected the brakes. If everyone with a heavier wheel/tire combo get an improvement, and everyone who went to a lighter wheel/tire combo have the problem occur more often, we may find out the culprit.

Do stiffer springs help or hinder the ABS problem?

Does a heavier wheel/tire combo help or hinder?

Does a lighter wheel/tire combo help or hinder?
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Old 04-16-2002, 08:46 AM   #4
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I didn't see your name until after you posted. I am an engineer as well. (I saw that you were in another post)

My dealer has a hydraulic pump on order and they plan to replace mine as soon as it gets in. I want to get some more background information on this. Every post usually complains about the problem, but no one is trying to eliminate the non-essential variables.
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Old 04-16-2002, 09:12 AM   #5
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Quote:
Originally posted by bjteel
Thanks for the info, but you didn't list any mods.

I like the theory that bumps are causing the wheels to lift off the ground enough to cause the wheel speed to slow, thereby fooling the ABS into thinking there is about to be slippage.

This may be compounded by stiffer springs. It may be helped by a heavier wheel/tire combo or any heavier sprung weight. I would just like to see what changes people have made and how they have affected the brakes. If everyone with a heavier wheel/tire combo get an improvement, and everyone who went to a lighter wheel/tire combo have the problem occur more often, we may find out the culprit.

Do stiffer springs help or hinder the ABS problem?

Does a heavier wheel/tire combo help or hinder?

Does a lighter wheel/tire combo help or hinder?

I hate to say this but it depends.

Anything that would get the wheel off the ground would make the ABS act more "aggressively." The weight of the wheel/tire won't make a difference unless it is causing the wheels to either hold contact better or lift. So, I guess it depends on how the weight of the wheels effects the suspension.

But the only things that effect ABS are:

1. Change in overall diameter of wheel/tire
2. The suspensions ability to keep a good contact patch. A lifting tire under braking will lock up ASAP and set off the ABS.

So it is possible that a stiffer suspension may cause lift. Example, you brake into a turn and hit a bump. The tire(s) lift and the ABS goes off. Wheel weoght will only effect ABS indirectly through the changes that weight makes to the suspension.

Like I said the ABS is a "dumb" device. It has no idea what's going on, it just looks at the wheel speeds and that's it.

I guess it depends on the car, but yes a stiffer suspension can cause the ABS to act differently. Also, an "over-braked" car will cause the ABS to go off more and sooner just because better brakes are more likely to lock-up the wheels.

A lot of how a system acts depends on how it was designed to act. A lot of OEMs (too many) are have their lawyers do ABS development (and especially TC and ESP development), so the things have a very low threshold. Not much you can do about this unless you have access to a development track and a few test engineers!
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Old 04-16-2002, 09:16 AM   #6
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I think a heavier sprung weight (heavier wheel/tire combo) will affect the ABS in that it will be easier to lift off the ground over a bump. F=m*a
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Old 04-16-2002, 09:17 AM   #7
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I meant it a heavier combo will make it less likely to lift off the ground.
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Old 04-16-2002, 09:18 AM   #8
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Quote:
Originally posted by bjteel
I think a heavier sprung weight (heavier wheel/tire combo) will affect the ABS in that it will be easier to lift off the ground over a bump. F=m*a
It's a possibility.
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Old 04-16-2002, 03:52 PM   #9
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Default just a thought....

wouldn't tire pressure be a factor as well? if the tires had less pressure in them, they should be softer with less bounce, and therefore not lift as much...like a flat tennis ball.
running higher pressure should give it more bounce, and cause it to lift more often, possibly activating the ABS.

any thoughts?
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Old 04-16-2002, 03:58 PM   #10
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Makes sense. I hadn't thought about that.

SOA could then blame two problems on tire pressure (clutch and abs) haha
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Old 04-16-2002, 04:08 PM   #11
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But in my opinion this problem some have with ABS is a Subaru problem and one see more often.

Basically, the Subaru system is over-sensitive. The threshold is just too low. Suabru (and the ABS supplier) have just set the control software to activate ABS at every little thing that may be impending wheel lock.

Some systems will allow a little bit of lock to compensate for hitting a bump while braking or having a tire lift. Subaru doesn't seem to.

All an owner can do is try and minimize wheel hop and go to tires that offer better grip to postpone wheel lockup.

I had a BMW with ESP (DSC) whose traction control part made it impossible to drive in snow even with winter tires. It was developed so it went off (aggressively) at even a hint of wheel spin.

You can thank the lawyers for this one!
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Old 04-16-2002, 04:14 PM   #12
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Well, my dealer is going to install a new hydraulic pump in my Rex. I am assuming that it will be a different design. Does anyone think this will make it "less sensitive?"
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Old 04-16-2002, 04:16 PM   #13
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Quote:
Originally posted by bjteel
Well, my dealer is going to install a new hydraulic pump in my Rex. I am assuming that it will be a different design. Does anyone think this will make it "less sensitive?"

Nope. It's the ECU that decides that. The pump just does what the ECU tells it to do.
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Old 04-16-2002, 04:30 PM   #14
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That's what I was thinking. Unless the pump has a different flow rate or something .... ?
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Old 04-16-2002, 04:40 PM   #15
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Quote:
Originally posted by bjteel
That's what I was thinking. Unless the pump has a different flow rate or something .... ?
That is possible, but it doesn't change when the system goes off.
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Old 04-16-2002, 04:47 PM   #16
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My car has stiffer springs and slightly heavier wheel/tires. The ABS "issue" seems to have improved marginally.

Did I mention that I think that Subaru's ABS is more DANGEROUS than anything? Well, I DO!
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Old 04-16-2002, 08:28 PM   #17
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The WRX system is not as "dumb" as stated here. IMO that might be exactly why we experience the problem. The WRX incorporates a G sensor that monitors vehicle deceleration. I don't know if anyone has found out how the ABS computer actually uses this information. One post mentioned that the ABS computer calculates what the vehicle speed should be based off the G sensor and then compares it to the wheel speed sensors. This would cover the situation of all wheels skidding in snow/ice/etc. Most G sensors I have seen are very touchy when subjected to bumps and tend to oscillate a bit. If the ABS computer relies on this calculated speed for anything, the computer is going to freak out when the G sensor gets any sharp bumps unless there is very good filtering of the signal. IMO, this may be why the "no brakes" situation only happens under very specific conditions. If the ABS computer software did not take into account one specific set of driving conditions, the computer may not know what to do or at the least may do the wrong thing. This seems to fit the experience that lifting and reapplying the brakes usually solves the problem.

Zak
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Old 04-16-2002, 08:40 PM   #18
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Quote:
Originally posted by Zak
The WRX system is not as "dumb" as stated here. IMO that might be exactly why we experience the problem. The WRX incorporates a G sensor that monitors vehicle deceleration. I don't know if anyone has found out how the ABS computer actually uses this information. One post mentioned that the ABS computer calculates what the vehicle speed should be based off the G sensor and then compares it to the wheel speed sensors. This would cover the situation of all wheels skidding in snow/ice/etc. Most G sensors I have seen are very touchy when subjected to bumps and tend to oscillate a bit. If the ABS computer relies on this calculated speed for anything, the computer is going to freak out when the G sensor gets any sharp bumps unless there is very good filtering of the signal. IMO, this may be why the "no brakes" situation only happens under very specific conditions. If the ABS computer software did not take into account one specific set of driving conditions, the computer may not know what to do or at the least may do the wrong thing. This seems to fit the experience that lifting and reapplying the brakes usually solves the problem.

Zak
Bumps would not effect a long-A sensor as it doesn't care what is happening in the vertical. But long-A sensors are usually only used with AWD AND ESP systems.

The long-a maybe used by the AWD system, but it wouldn't be worth the extra (high) cost to have one just for ABS. ABS doesn't really need it.

ABS only works with your foot on the brake, so all it needs is you braking and wheel locked up. Know anything else would be useless to an ABS only ECU. It's not as if ABS can self activate like traction control or ESP.

Long-A are used to distribute power on AWD drive cars. So the Subaru probably has one, but it's used to figure out where to send engine power.

Now if the Impreza had a full ESP system, one that acts on torque delivery and can activate the brakes without driver input then a long-A (and a bunch of other sensors) would be of use. And long-A sensor is almost always used on AWD cars with ESP as it is possible for an AWD to be sliding while all 4 wheels are acting normally in relation to each other because all 4 are being powered. On two-wheel drive this is not possible because as soon as the non-drive wheels lose traction they stop spinning.

The other sensors in an ESP are also useless to an ABS only ECU, lateral-g, yaw rate, steering wheel angle, throttle body position, brake pedal position, and on some a ride height sensor.

Last edited by romanom; 04-16-2002 at 09:29 PM.
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Old 04-17-2002, 06:54 PM   #19
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Two things,

First, a heavier wheel/tire combo is probably more likely to lift a wheel over bumps. The tire has to get over the bump somehow. The tire will get kicked up no matter what. A heavier one will have more force when kicked up (F = M * A as stated), making it harder for the suspension to properly control it. Hence the quest to lower unsprung weight - so that the suspension can make the tire follow the terrain better.

Second, are we sure about the WRX using a g-sensor for the ABS? The article I read about the new STi in England sited the incorporation of a g-sensor as a big improvement in the STi. Of course they were comparing it to the regular English WRX, not the US WRX. But they get better brakes over there anyway. Well, better calipers anyway.

Better distribution of power? Manual WRXs have "dumb" viscous diffs.
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Old 04-17-2002, 07:32 PM   #20
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A heavier wheel and tire combo wouldn't get "kicked up" more.

Since the combo is more massive, that same force exerted by the bump on the wheel/tire combo would result in a wheel and tire that didn't get "kicked up" as much.

However, the suspension would be slower to react and therefore slower to put the tire back on the ground since it is more massive.

Either way, we don't really know if the tire is actually lifting or not. I doubt it is at 20mph.
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Old 04-17-2002, 08:18 PM   #21
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I agree that an ABS system doesn't "need" the G sensor. According to the service manuals it has one anyway. I have attached the page from the wiring diagram. I don't know what they use it for, but I believe another volume of the service manuals may explain it somewhat. I remember someone posting an excerpt a while back that the ABS computer calculates the theoretical vehicle speed during braking using the G sensor (just like a Gtech or similar device does). It then compares this to the wheel sensor readings to help it decide what actions to take during the stop.

In theory, this could help out the ABS system obtain better stopping distances. In testing FAILED brake systems (required by government for new vehicles) with the front brakes disabled, MANY pickups with rear ABS would actually take over 100 feet longer (about a 25% increase in distance) to stop from 60 mph with the ABS on rather than threshold braking with the ABS off. The constant firing of the ABS would kill deceleration and make the stops MUCH longer than they could have been. Front brakes failing isn't likely to happen, but 100 feet of distance is a lot of distance to give up. With a G sensor, this type of system would know the decel rate that triggered the ABS activity and then only allow the pressure to increase back up to a level that gave a decel rate just below that trigger point. It would keep that lower pressure for a short time (I have recently seen a dynamic rear proportioning system that holds it for over a second) before trying a higher pressure again. This would be a pretty close approximation to what a driver does while threshold braking. Obviously this exact set of circumstances wouldn't apply to diagnal split systems typically used in cars, but I would guess that there are other similar situations that would.

It is nice to say that a G sensor can only react to lateral motion, but a sharp pothole type bump that seems to trigger the "no brake" situation is going to have quite a bit of lateral motion to it. Most of the G sensors in vehicles I have seen are very prone to reading errors from bumps. They, at the least, oscillate a bit. I'd suspect one used for this pupose would be pretty cheap and small. This would also work against it properly isolating motion from a single direction.

Zak
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Old 04-17-2002, 08:20 PM   #22
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Well, it's a momentum thing. Anyway, this isn't the place to debate physics. Although I think we're actually saying the same thing.

The tire probably is coming off the ground a bit. On sharp bumps, even at slow speeds, the spring just can't put the tire down on the otherside quick enough. Otherwise you'd have too much rebound on more rounded bumps.

The one place I repeatedly see this problem, I'm preety sure the tire is coming off the ground. I've gone through the same spot in non-abs cars, and they get all messed up - it's easy to lock up repeatedly on threshold braking. The bumps are really harsh and continue throughout the braking zone. The Rex is actually a bit more comfortable across these because all I have to do is "reset" the ABS a couple of times versus fighting all the way to get the most braking force without locking up.

But that brings me to what I think is the real question: why does the ABS just freak out? Why doesn't it recover? I've driven other ABS cars where you can feel the ABS "click" a couple of times over sharp bumps, but it then recovers and brakes normally, without having to be reset. On snow, my WRX does just fine, transitioning immediately from ABS action to normal braking when, for example, you run out of snow and hit a dry patch.

BTW, "real" snow tires did wonders for snow braking.
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Old 04-17-2002, 08:24 PM   #23
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Gotcha, we are saying the same thing. BTW, I don't get to use physics in my job like I thought I would, so why can't I use it here if I want to?
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Old 04-17-2002, 08:57 PM   #24
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My ABS was flaky with both:
Stock Suspension + Stock 16" wheels + Stock RE92's
And
Stock Suspension + 17" OZ wheels + 215/45/17 S02s

I think it was worse with the 17's on, as they tended to be a harsher ride and the chassis generally seemed to be more affected by bumps in the road and the ABS seemed to kick in more easily and more often.

Now that I've pulled the ABS Fuse it's like Night and Day, I'm just so sick and tired of calling SOA and having them do nothing about it.

- Richard
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Old 04-29-2002, 01:57 PM   #25
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Like I posted elsewhere, I got a new hydraulic pump installed today and I will let you know if it helps or not.
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