ABS and braking in a corner

[ajf]

The benefits of ABS have been debated endlessly, but there's one hanging issue that I haven't seen resolved or even discussed intelligently.  Now that I've bought a bike with ABS, I would like to get a definitive explanation.

Consider the following situation:  You're cornering fairly hard - say, 5 mph below the speed at which both tires would start to drift -  and you decide you need to slow down a bit, so you feed in a bit of front wheel brake.  You squeeze slightly too hard and the wheel starts to slide.  Without ABS, you have to be really quick at getting off the brake and on the gas to get the wheel to hook up.  If you had ABS, would it detect the impending lockup and prevent it?  Would this prevent the front end from starting to wash out?

Every ABS thread has a few people who say dogmatically that ABS will not help while braking in a corner. Why not?  Can anyone support this position with a plausible explanation?  Can anyone report losing the front end during mild trail braking with no apparent help from ABS?

For bonus points, describe what you feel through the bars in the above scenario when the front wheel hooks up.  (If you don't know, you haven't been there and your position on this issue is probably is just an opinion, not knowledge.)

[kevin_stevens]

It'll work pretty much like it does without ABS.  Current ABS isn't very good at interpreting sliding forces.

The reason is that the wheel rotation (which is what ABS senses) isn't slowing at a rate consistent with the traction loss, which is now happening laterally.  That basic assumption is what ABS is predicated on, so it turns into a GIGO situation.

KeS

[UFO]

ABS systems on bikes monitor wheel rotation near the hub via pulses generated from a sensor.  ABS won't save you from washing out your front end because your front wheel is still rotating, especially at speeds where your front end might wash out.

Short answer:  No  

[OmegaSix]

No, but ABS will flat save your ass riding down a steep gravel road!    

[chornbe]

[thatguy]

Damn that's a nasty photo.

However braking in a corner when the front end is going will put you into contact with the pavement.You'll finish the job you've started by piling more load onto an already overloaded tire.Apply throttle gently and you'll have more chance of riding on.It's one of the hardest things to do because it goes against what your brain is telling you.You need to unload the front tire and there's only one way to do it.Gas it.Practice cornering in the rain.It's easier to feel the tires message.Or crash more often,your choice.

[jeremy556]

As others have stated, ABS only senses wheel rotation, no way for it to know about lateral acceleration. It will not help you from losing the front end while trailbraking.

To answer your quiz, when you lose the front end it starts off as a slight oscillation in the bars.

I have used my VFR with ABS at about a dozen track days, as well as completed one road racing school with it.

[tomek]

Like everybody else pointed out ABS won`t help you here,at least current systems on production bikes.

If you are already cranked over to the max and "think" that your sped is too high the best chance is to ride it out.

Having said that modern radial tires will take huge amount of trail braking before loosing a grip.Way more the most of the riders think.The key is a smooth controls allowing gentle weight transfer/increasing contact patch.But ABS won`t help you here.And yes,the bars wiggle when front tire is loosing its`s grip in the corner,hopefully it is not followed by that nasty,light feel of front end washing out,,,,,,,,,

[ajf]

Thanks for the thoughtful replies, but I'm still not convinced.  In the scenario I described, the front wheel initially has plenty of traction.  For the brake to cause the front end to slide, first it has to slow the wheel down enough for it to begin to slide forwards.  Then it can start to slide sideways as well.  Of course on time scales humans can detect both things appear to happen together, but slowing the rotation of the front wheel is what starts the problem.  Why won't ABS do the same thing in this situation (ie. prevent the front wheel from slowing down enough to initiate a slide) as it does when braking upright?

One way this might happen is if there's a switch in the system that keeps ABS turned off until sufficient brake pressure is applied.  I know such switches don't exist in cars - the ABS in my cars works fine on ice with the slightest brake pressure.  And it works on ice just as well when turning as when going straight.  What's different about motorcycle ABS?

Another possibility is that the percent reduction in RPM of the wheel that is enough to cause it to begin to slide sideways is below the threshold that ABS detects as impending lockup.  On my Corvette the ABS appears to work well when I have to add a bit of brake while cornering near the limit of traction (although it is a bit harder to interpret what's going on because the stability control can intervene as well), so again, what's different about motorcycle ABS?

Anyone have any other (better) explanations?

jeremy556 & Tomek:  The quiz question was what do you feel at the instant the wheel hooks back up, not what you feel while it's sliding.  Actually, I should withdraw this question.  It's kind of silly to demand that people try to prove their competence on the internet.

[FJR1300]

Paging Mr Bird, Mr Snowbird... please pickup the white ABS phone.

4 - 3 - 2 -

 

[rockinjc]

Similarly when I take off on a sweet jump, and am about to overshoot the landing my abs system (Bosch), does not seem to be able to re mediate the fact that my tires are in the air. Go figure.

As for Vettes, does anything take more side G's that those things?

j

[tomek]

[quote author=ajf link=topic=30855.msg697552#msg697552 date=1221410756

jeremy556 & Tomek:  The quiz question was what do you feel at the instant the wheel hooks back up, not what you feel while it's sliding.  Actually, I should withdraw this question.  It's kind of silly to demand that people try to prove their competence on the internet.
[/quote]

Acually you feel both "things" in this cycle,first bars go light when the front begins to slide, it is  followed by gentle tug,,,that is when tire hooks up again.If bars stay light it means you have just crashed.

[vulcanbill]

"so again, what's different about motorcycle ABS?"

Given your description that you're already laying over really good into the turn, the difference is that there is very little input required to take up that last 5% of available traction.  I think there is an assumption that braking force in this scenario is going to cause the wheel to "skid" or lock up in the traditional sense.  I think that a very small amount of braking force is simply going to hasten the slid out or exacerbate the already near terminal side force causing the wheel to lose grip.  Also, consider that your contact patch at that point is extremely small and you may very well be riding on the very edge of the tire if you're going to overwhelm a modern tire in good conditions.  Tough question to consider without factoring in a hundred variables.

[Global Rider]

Every ABS thread has a few people who say dogmatically that ABS will not help while braking in a corner. Why not?

Not when leaned over to the limit as you've described.

Look up something called a "traction circle".

[thatguy]

For the brake to cause the front end to slide, first it has to slow the wheel down enough for it to begin to slide forwards.  Then it can start to slide sideways as well.

Nope.All it has to do is lose traction which you assist by applying the brakes.End scenario.

It's kind of silly to demand that people try to prove their competence on the internet.

I'm more than willing to show you.I'm in central Maryland,you are in for a long ride.

[ajf]

it is  followed by gentle tug,,,that is when tire hooks up again.

Bingo!

I take that tug to mean that I turned the bars a bit while the wheel was sliding, and hooking up straightened them abruptly.  Of course, a really competent rider would not turn the bars . . .

[Windblown]

Everybody has a different way to say the same thing.

ABS will absolutlety prevent the front wheel from locking up. However... at some degree of lean angle the lateral forces get high enough that any increase in load on the front will induce a lateral slide.  Application of any braking force can trigger this overload and ABS cannot prevent an overload of lateral forces. Therefore yes, it's possible to wash the front end of a bike equipped with ABS just as it is possible to wash the front end of a bike without ever touching the front brake.

At moderate lean angles where the lateral forces are low enough and traction is high enough is where ABS can assist.  When lateral forces are low enough that the only way you could loose the front is to lock up the front tire then ABS will prevent this.  

Clear as mud, eh? LOL.

[BobW]

Fastest car lines on the track are not always the same for a motorcycle. A car at rest is stable. A motorcycle requires movement to be so. One is steered, the other counter steered. One is driven, one is ridden. A pattern develops that could indicate they are different in key areas.

Until you can appreciate/visualize/interact within the dynamics of a four wheel vehicle negotiating maximum turning VS those of a two wheel vehicle and how the effects of exceeding the point of an incipient skid and how the stability of these two very different platforms respond at that point, the various yaw, abs, brake actuation, traction control devices, fuel control, etc., as well as inherent impact the drivers lack of stability (seat belted in proper supportive seat VS hanging off) in assisting all the above becomes even clearer than mud.

This ? would best be researched and or conducted in a quality shop where real time conversation as well as presentation material/examples could be used to help you feel more comfortable with the technologies you are trying to make the same(?) in application and end result.  JMHO/YMMV

[kevin_stevens]

Thanks for the thoughtful replies, but I'm still not convinced.  In the scenario I described, the front wheel initially has plenty of traction.  For the brake to cause the front end to slide, first it has to slow the wheel down enough for it to begin to slide forwards.  Then it can start to slide sideways as well.  Of course on time scales humans can detect both things appear to happen together, but slowing the rotation of the front wheel is what starts the problem.  Why won't ABS do the same thing in this situation (ie. prevent the front wheel from slowing down enough to initiate a slide) as it does when braking upright?

One way this might happen is if there's a switch in the system that keeps ABS turned off until sufficient brake pressure is applied.  I know such switches don't exist in cars - the ABS in my cars works fine on ice with the slightest brake pressure.  And it works on ice just as well when turning as when going straight.  What's different about motorcycle ABS?

Another possibility is that the percent reduction in RPM of the wheel that is enough to cause it to begin to slide sideways is below the threshold that ABS detects as impending lockup.  On my Corvette the ABS appears to work well when I have to add a bit of brake while cornering near the limit of traction (although it is a bit harder to interpret what's going on because the stability control can intervene as well), so again, what's different about motorcycle ABS?

Anyone have any other (better) explanations?

jeremy556 & Tomek:  The quiz question was what do you feel at the instant the wheel hooks back up, not what you feel while it's sliding.  Actually, I should withdraw this question.  It's kind of silly to demand that people try to prove their competence on the internet.

Several of your premises above are incorrect.  In particular, the notion that the tire "sees" a braking force and then a sideways slip, sequentially, is not accurate.  It sees a compounded vector of all forces applied at all times.  Also, automobile ABS doesn't work with lateral loads either; if that Corvette is completely sideways the ABS will sense no wheel motion and reduce brake application.  I've raced C4 and C5 Corvettes with ABS.  Basically as long as the wheel is turning and pointed more or less straight in the direction of travel, ABS will work correctly.  If the wheel is travelling significantly sideways, it won't.  Car ABS works better simply because there are three other wheels to compare to so the computer can make an informed decision about what's going on.

I agree with the guy who said that the initial feel of the front end leaving is a slight oscillation in the bars, but then as I said that's exactly what you feel with no ABS as well.

KeS

[kevin_stevens]

Similarly when I take off on a sweet jump, and am about to overshoot the landing my abs system (Bosch), does not seem to be able to re mediate the fact that my tires are in the air. Go figure.

As for Vettes, does anything take more side G's that those things?

j

Actually, what you mention is a very real problem with some auto ABS systems.  On both my 1989 Corvette and 1994 Camaro (whatever gen Bosch ABS that is), the way it worked was that when the ABS sensed wheel slip, it would lock the line with that amount of brake pressure and cycle between that amount and zero.  Well, if you were racing, and got into the ABS when the front wheels were in rebound from a big bump and effectively weightless...  then it was cycling between whatever the pressure was needed to lock the wheel at that point (very little), and zero.  You got a high, hard, firm pedal with absolutely no braking going on, and it wouldn't reset until you got *out* of ABS for approximately a lifetime (a time estimated with stationary objects approaching rapidly, it might not be completely accurate   ).

The C5 Corvette never did that, so I assume they changed the programming rules for the system.

KeS