ABS....B.S. or not B.S.
Road Warrior
Joined: Nov 2008
Posts: 1,296
Likes: 3
From: Cheltenham, UK
There was a HUGE thread on ABS just a couple of weeks ago and the topic sure was flogged. One issue described in this thread does need some clarification however.
Going back to basics, the brake force applied through the pads is the product of pressure and the piston area (F=PxA).
ABS does not "apply more pressure" than a non ABS system can or does. That is the equivelant of saying you can push harder against an open door. The fact is that a specific pressure gives a specific brake force, and that force may result on a wheel lock state which is a function of a whole host of variables such as loading, road surface conditions etc etc. The fluid does not know how this pressure was generated or the route that was taken to get there - end of story.
So, what does happen then. The difference between ABS and non ABS is that between the "pump" (could be a master cylinder in many cases) and the brake caliper piston a modulating valve is added. This valve (modulating at high frequency) connects the brake piston with the high pressure supply or the low pressure return. By switching quickly between these 2 modes the "optimum" pressure can be achieved.
How do we know what the optimum pressure is. Well, a feedback device is used which detects angular velocity of the wheel. The real trick is knowing exactly what to do with that info, and whilst I have a reasonable idea and can speculate, I don't know for sure so I won't. Suffice to say, this feedback device tells the valve what to do, and this is why the ABS system adjusts automatically to various road conditions.
So what is the "high pressure" that is being discussed in the context of ABS systems. This is the pressure between the pump and the modulating valve, NOT between the valve and the brake calliper. By having a higher pressure in the supply line you get a much quicker response through the modulating valve.
So, in summary, there is high pressure from the pump to the modulating valve, and the pressure between the valve and the calliper varies depending on "ABS optimum" braking conditions. Could be virtually zero on ice if you take it to an extreme. The same pressure would exist in that particular line without ABS.
Personally, I think ABS is a great tool.
Going back to basics, the brake force applied through the pads is the product of pressure and the piston area (F=PxA).
ABS does not "apply more pressure" than a non ABS system can or does. That is the equivelant of saying you can push harder against an open door. The fact is that a specific pressure gives a specific brake force, and that force may result on a wheel lock state which is a function of a whole host of variables such as loading, road surface conditions etc etc. The fluid does not know how this pressure was generated or the route that was taken to get there - end of story.
So, what does happen then. The difference between ABS and non ABS is that between the "pump" (could be a master cylinder in many cases) and the brake caliper piston a modulating valve is added. This valve (modulating at high frequency) connects the brake piston with the high pressure supply or the low pressure return. By switching quickly between these 2 modes the "optimum" pressure can be achieved.
How do we know what the optimum pressure is. Well, a feedback device is used which detects angular velocity of the wheel. The real trick is knowing exactly what to do with that info, and whilst I have a reasonable idea and can speculate, I don't know for sure so I won't. Suffice to say, this feedback device tells the valve what to do, and this is why the ABS system adjusts automatically to various road conditions.
So what is the "high pressure" that is being discussed in the context of ABS systems. This is the pressure between the pump and the modulating valve, NOT between the valve and the brake calliper. By having a higher pressure in the supply line you get a much quicker response through the modulating valve.
So, in summary, there is high pressure from the pump to the modulating valve, and the pressure between the valve and the calliper varies depending on "ABS optimum" braking conditions. Could be virtually zero on ice if you take it to an extreme. The same pressure would exist in that particular line without ABS.
Personally, I think ABS is a great tool.
Road Master
Joined: Apr 2008
Posts: 866
Likes: 1
From: FT BRAGG, NC
Problem with bleeding brake lines from the ABS Module and the brakes themselves is that if air enters the system by mistake when you try to work on the system...you're stuck with using Harleys Digital Tech to bleed the system. The high pressure is only when the system is running...it's allowing the air in the system between the module and the brakes is when you run into taking it to Harley with ABS!
Ride Safe, Mike
Ride Safe, Mike
Outstanding HDF Member
Joined: Feb 2005
Posts: 2,925
Likes: 5
From:
This is interesting reading. Yes, there is a pump with high pressue that actuates, but there should be no problem replaceing the line. I wouldn't have the key on and pull the brake lever, but i'm ure that wasn't part of the plan.
Think of this: If you were doing 80 and grabbed the brake lever hard enough to activate the ABS, the system starts pulsing pressure to the wheel, allowing pressure than bleeding, over and over many times a second. Without the pump, the lever would step down each time fluid was bled until the lever was all the way to the grip and now useless. In the older ABS cars, you would have to manualy pump the brakes up to 100 times with the key off to depressureize the system before service. Of course this is all safety precautions to protectyou and them in case of an incedent.
Those that call ABS itself BS either don't have it or have never tried it.
Think of this: If you were doing 80 and grabbed the brake lever hard enough to activate the ABS, the system starts pulsing pressure to the wheel, allowing pressure than bleeding, over and over many times a second. Without the pump, the lever would step down each time fluid was bled until the lever was all the way to the grip and now useless. In the older ABS cars, you would have to manualy pump the brakes up to 100 times with the key off to depressureize the system before service. Of course this is all safety precautions to protectyou and them in case of an incedent.
Those that call ABS itself BS either don't have it or have never tried it.
Road Warrior
Joined: Jun 2005
Posts: 1,324
Likes: 522
From: Dutchess County, NY
I have ABS on my FLHX and am very happy with it.
However not all ABS is created equal. I had a 1995 BMW Automobile and the ABS only kicked in once in 110,000 miles. It worked very well and most of the time you never new the ABS was there. Now I have a 2003 Subaru WRX wagon and the ABS is no where near as good as my BMW. On the Subaru the ABS will go off when braking on bumpy roads and does not install allot of confidence. As a matter of fact Subaru had a technical bulletin out to upgrade the ABS software which improved the ABS but is still no where as good as what I had on my BMW.
However not all ABS is created equal. I had a 1995 BMW Automobile and the ABS only kicked in once in 110,000 miles. It worked very well and most of the time you never new the ABS was there. Now I have a 2003 Subaru WRX wagon and the ABS is no where near as good as my BMW. On the Subaru the ABS will go off when braking on bumpy roads and does not install allot of confidence. As a matter of fact Subaru had a technical bulletin out to upgrade the ABS software which improved the ABS but is still no where as good as what I had on my BMW.
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