Another ABS question

[GTman]

Is some wheel lock up normal at initial onset of hard braking? I was out last weekend moving along pretty good on a country 2 lane when all of a sudden I see dirt up ahead. So, I got on the brakes pretty good. There was some dirt on the pavement where cars had been entering the road from the side. I hadn't gone off the pavemnet yet. I could have sworn my rear end started around to the front so I let off to straighten the bike. When I pulled in the garage at home I decided to nail the brake lever and the front wheel skidded. What's up with that? This is an 04 GT with ABS. I haven't had any indication from the idiot light that I have any sort of brake failure. Anyway, I'm going in for 24k service next week and plan on having it checked.

[rdfrantz]

I have done lots of braking drills with the '02+ K1200RS modles, and all do leave skid marks in one circumestance or another.

 

Particularly the rear, on a max pressure stop, whether the front lever is activated or not, will leave about three feet of tire mark on most asphalt. The line with be thicker and thinner in places indicating the ABS is pulsing pressure.

 

That there is a skid mark does not not mean the wheel/tire is locked. It only means it is skidding. In fact, maximum braking force is applied when the contact patch is travelling 20% of vehicle velocity. That model BMW ABS seems to approach those values - toward the end of a stop.

 

I have not gotten my front tire to leave more than about three inches of skid mark, though I can hear the ABS activation.

 

All of that is about braking with the bike UPRIGHT.

 

 

At even moderate lean angles, brake activation can cause the tire to slide LATERALLY.

 

All that means is that tire traction has been exceeded by the cornering load, or cornering and braking loads applied to the tire.

 

The ABS is not "failing". The laterally sliding tire can be rolling just fine as far as the ABS is conerned. AND THAT IS ALL THE ABS WILL EVER BE CONCERNED WITH !!!

 

ABS does NOT prevent LATERAL slides.

 

ABS only prevents - or moderates - halts to rolling wheel velocity while the bike is moving.

 

Best wishes.

[GTman]

Thanks for the explanation. I've never had ABS on a bike before. I was as far as I could tell, vertical when I applied the brake. I don't really know how much lateral movement there was. The feeling I got was similar to when a car starts to hydroplane. It just felt as though the rear wheel was no longer in contact with the road. No, I wasn't doing a stoppy. Next time I guess I'll know what to expect. That was my first experience with hard braking under compromised traction on the GT.

[HankB]

... When I pulled in the garage at home I decided to nail the brake lever and the front wheel skidded. ...

 

Under some speed, ABS may be disabled. I think it's something like 5 mph. How fast do you ride in your garage?

[rdfrantz]

Thanks for the explanation. I've never had ABS on a bike before. I was as far as I could tell, vertical when I applied the brake. I don't really know how much lateral movement there was. The feeling I got was similar to when a car starts to hydroplane. It just felt as though the rear wheel was no longer in contact with the road. No, I wasn't doing a stoppy. Next time I guess I'll know what to expect. That was my first experience with hard braking under compromised traction on the GT.

There's somthing involved in your stop where Rear Slewing occured I didn't mention.

 

With near viscous brake application, particularly with the GT front-linked-to-rear system, the front pressure comes up quickly, while there is a lag in applying that same pressure level to the rear. This brings on much more vigorous forward weight transfer, which unloads the rear wheel. That reduces rear wheel traction greatly, and makes sideways sliding much more likely.

 

This will occur even when rear pedal pressure is deployed... unless the rear pressure is deployed FIRST.

 

However, even non-linked systems produce similar though not quite as dramatic results (More violent weight shift).

 

That's why I emphasize in my classes, training the Rider to ALWAYS "Lead with the Rear Brake." Yes, we can be trained up to do so ESPECAILLY in "panic stops", where the traction increase gained by this Method is most needed.

 

Think the Cadence as with a drum, Ta-Dummmmmmm--taaa. "Ta" is Rear brake application, analabous to striking the left drum stick. "Dummmmm" is the application of both brakes, analagous to a "drum roll" using both drum sticks. The stop should be finished, when Prudent, with only rear brake - the repeated tapping of only the left drum stick.

 

Best wishes.

[ShovelStrokeEd]

Dick has pretty much nailed it here. ABS has no idea about lateral movement and can and will leave skid marks. We drag racers, who instrument for such things as rear wheel slip, find that the best acceleration comes with about 10% wheel speed above vehicle speed. We do, however, operate on a much different surface than the average road (mostly concrete covered in rubber) so his 20% figure may not be all that far off. Acceleration is acceleration, however, only the sign changes in braking. My '94 R1100RS with the early ABS will leave dashed lines from both wheels, easy to see due to much slower pulse rate.

 

The situation described by the OP is probably the toughest test of a good ABS system. Loose dirt on a hard surface. Kinda like trying to walk on ball bearings. The traction will be changing from none to moderate over very short intervals giving the poor brain and modulator pumps fits trying to keep up.

 

I'm also pretty sure the system looks at the differential between wheel speeds as well as the rate of slowing of each individual wheel. As always, good techique is the real key to good, safe stops whether they be of the emeergency variety or the more mundane stuff. Good on you for exploring when you got home. Learning how the system works is a good first step towards improving your braking performance.

[GTman]

Under some speed, ABS may be disabled. I think it's something like 5 mph. How fast do you ride in your garage?

 

My garage is about 100 feet across the back yard. I always pick up some dirt on the tires which I'm sure didn't help matters. My entrance into the garage can't be much over 5 mph.