Motorbike ABS.

[Swin]

Originally Posted by SoulKiss

However at high speed I would argue that if you held the lever to the bar, then the stopping distance would be a lot longer than if you treated the ABS engaging as a lock and disengaged the brakes and then re-applied progressively.

About 100 years ago, when some BMW's came with ABS the bike journos testing them were able to brake in a shorter distance by pre-empting the ABS in just such a manner as you say.

I always assumed that ABS means your wheels will be turning when you smack into something, rather than being locked up, thus saving you from burning some rubber on your way in

[SoulKiss]

Originally Posted by muffles

The one thing I got confused about when you were explaing it it yesterday was the amount of pressure transmitted through to the brakes.

I imagined that it would be that it would reduce the amount of braking pressure going to the brakes to the maximum level possible that would not cause a lock. That's where I thought the adage that ABS brakes will always stop the quickest came from.

But from what you were saying (about having to disengage and reapply anyway) it sounded like it would reduce the braking pressure going to the brakes to zero.

Did I misunderstand/am wrong/etc?

As far as I am aware (on my Kwak, the new, 3 times plus more expensive system some manufacturers are using may be different) the rider sets the input level, the ABS just backs that off to zero if a lock is detected then re-applies to the input level set - so if that is high enough to cause it to kick again, it would

Example

Lock point is 80%

Rider hits brakes at 100%, ABS kicks in, you get a braking up to 80%, then nothing, the system backs it off to 0% and has another go, if you are still applying > 80% it will hit that and go through the cycle again until the input is < 80%.

As I said the OP is from experience/reasoning, and I accept I may be wrong in some of my conjectures, and I am happy to have those errors pointed out.

[DarrenSV650S]

I don't know much about how bike abs works exactly but I have a question...

Let's say I grab too much front brake in an emergency situation, the abs then kicks in and tries to switch the brakes on and off. If I continue to hold the same amount of front brakes, will the abs do anything? Will it somehow apply less pressure than I am applying? Or will it try to engage the brakes, realise I am still applying them too hard, then disengage them again? Resulting in next to no braking at all

My recent off was due to panic breaking > locked wheels front and rear > skid > fall.

Without the locked wheels and skid I would probably have stayed upright

[Sir Trev]

Originally Posted by SoulKiss

ABS is NOT the brainchild of Satan, but neither is it a Guardian Angel, its a tool and its up to the rider to learn how it works, and to learn to use it.

Sounds well sensible to me. Having dropped a bike from front wheel lock-up myself at slow speed in a straight line it would have been avoided if I'd had ABS. It may not be too helpful in some situations but as we all know you cannot pick where or when these sorts of things crop up!

You don't have to fully disengage after locking up you just dial it back a bit.

Personally ABS is not something I will ever subscribe to, with correct tyre pressures etc you can brake to the point of feeling the loss of traction - something that ABS takes away and therefore you can never find the true limit of the brakes.

One thing I will say is that braking is a skill, dirt biking helped me sort it and not be afriad of locking and the reaction to just let the lever go.


Having said all that MCN wrote a raving review on it didn't they!

[Sir Trev]

Originally Posted by DarrenSV650S

Or will it try to engage the brakes, realise I am still applying them too hard, then disengage them again? Resulting in next to no braking at all

Yes and no. It happens so quickly that it effectively keeps your brakes at maximum application for the situation. Using the 80% Soulkiss mentions above it modulates your braking down to that level and effectively keeps it there until the situation changes. You'll feel it happening through the lever just as you do the pedal in the car.

My question, still stands, can you feel it engage?

yes you can on a bmw not sure of others

This graph shows the hydraulic pressure in an ABS system when ABS is in operation



The red line is the speed that the controller thinks the bike is travelling at calculated using an average of lets say the rear and front wheel speed.

If the front wheel speed drops significantly below the calculated speed (see the dips in the blue line) the controller reduces the pressure in the braking system via the modulator (see the corresponding dip in the black line). This drop in pressure will cause the front wheel speed to increase again. When the wheel speed approaches the actual speed again the controller increases the brake pressure again until the wheel speed starts to dip again.

As you can see the brake force does not reduce to 0%, what the controller tries to do is balance the force so that the wheel speed stays close to the actual vehicle speed.