why does my bike "hop" while braking over a bump?

[NoHeat]

In city street riding, it often happens that when I'm braking and hit a small bump the bike gives me a very noticeable "hopping" sensation. The happens while I am braking with the clutch pulled in, and speed reduced typically to about 20 mph, while riding over the small bump. It seems weird that instead of absorbing the bump, the suspension transmits it to me with much more force than without the braking.

 

I'm wondering if this is a peculiarity of shaft-driven bikes, linked brakes, the paralever, or something else. Anybody know?

[Bob Palin]

Dies it feel as though you've hit a small patch of ice or is actually a bounce?

[NoHeat]

No ice here.

 

An odd thing about the sensation is that it feels more like a forward bump than an upward bump.

 

Usually if you ride over a bump that isn't absorbed by the suspension you feel yourself bounced up, but this momentary sensation is more of a forward acceleration (or lessened decelleration, since I'm undergoing braking at the time). I think the sensation is more a forward bump than an upward bump. It's a very brief sensation.

[Mark K]

It feels like a forward bump because your rear is momentarily losing contact with the road surface, negating some of the braking forces you may be under (assuming you use both front and rear break simultaneously). How are the shocks?

[NoHeat]

Yes, I'm braking with both wheels simultaneously. Always, because the brakes on the 1150RT are linked.

 

I had thought of the possibility that the rear wheel is momentarily losing contact with the ground. If that's it, shouldn't I notice something unusual when I'm not braking, too? Shocks are original with 18k miles, but I've noticed this since I got the bike last year at 8k miles.

[Mark K]

Yes, I'm braking with both wheels simultaneously. Always, because the brakes on the 1150RT are linked.

 

I had thought of the possibility that the rear wheel is momentarily losing contact with the ground. If that's it, shouldn't I notice something unusual when I'm not braking, too? Shocks are original with 18k miles, but I've noticed this since I got the bike last year at 8k miles.

 

Straight-line? Probably not. On a curve? I would think so.

[Tony_K]

It's just ABS stuff. Go find a nice washboard surface and do some braking over it. You will feel way more hop, skip and jump than you could ever imagine... afterwards a little bump wont even phase you.

[567guy]

I agree with abs, I feel when on the brakes on rough road same feeling and hear sound that must be the abs engaging when back tire is slightly airborn.

[swilson]

My guess is its an ABS activation caused by a momentary rear wheel slip. Could you describe it as a lurch forward instead of a bump?

 

I get this on my work bike on occasion... usually on the same manhole cover in the left turn pocket for the turn back into the station... but also when braking hard over bumpy pavement. The front tire always has plenty of traction with so much of the braking force on the front end. The rear end is of course light from the braking and the bump(s) cause the rear tire to either slip or momentarily come off the ground, either of which tells the ABS system there is a difference in wheel speed between the front and rear and the ABS engages... which causes a lurch forward as it lowers the braking pressure on the front and reapplies. Usually I get only one ABS pulse. Over really choppy pavement I've gotten more... not a happy feeling.

[Bob Palin]

No ice here.

 

An odd thing about the sensation is that it feels more like a forward bump than an upward bump.

Yes, exactly like riding over a patch of ice, that's why I asked. It's the ABS and TADT. You can eliminate it most of the time by using both brake controls, I know the brakes are linked and that is actually the problem. The rear bounces a tiny bit over the bumps which allows the brake to grab, the brake computer thinks it is locking so releases all the brakes (stupid, stupid, stupid design). If you use both controls it seems to be able to just release the rear brake, maybe because it knows you are pressing it.

[NoHeat]

Maybe it is a single ABS pulse.

 

I didn't think of mentioning that I use only the hand lever.

 

As a test I'll try using both controls instead of just the hand lever to see if that changes it as you describe, Bob. It happens predictably on the way home every day on one particular bump, so it should be easy enough to test.

[Keith S]

I was going to ask this very question. For me it was washboard just before a stop sign. I almost went through that stop sign with the ABS cutting in.....freaked me out!!! Never had this happens in a car or truck.

 

Keith

[Eschelon1]

Your shocks might be worn.

[Keith S]

Your shocks might be worn.

I can see where your coming from but this wasn't a shock situation. I pulled even harder on the brakes and if it was shocks then when the tire made contact with the road the tire would of either squealed or the ABS would kick in. The sensation I felt was the ABS kicking in.

 

Keith

[kiwiaudio]

Bob is right on the money here. Both John and Keith are describing a typical BMW ABS design flaw that started with the first linked servo abs brakes on the early '02s.

Folks with these early adopters were bitterly complaining about both brakes releasing whenever the rear was unloaded (commonly described as a sphincter puckering experience), and the early '02s were one to avoid.

This was supposedly corrected with '02s later than VIN ZE87455, but still you hear of people sliding through intersections on stutter bumps.

Both your bikes are '03s that should have gotten the fix !?

I'd be checking !

[BluegrassPicker]

My '02 does that too at one spot on the way home from work.

[co_g30]

It's just ABS stuff. Go find a nice washboard surface and do some braking over it. You will feel way more hop, skip and jump than you could ever imagine... afterwards a little bump wont even phase you.

 

same symptoms on my 04 1150RT, still trying to get used to it.

[Keith S]

Bob is right on the money here. Both John and Keith are describing a typical BMW ABS design flaw that started with the first linked servo abs brakes on the early '02s.

Folks with these early adopters were bitterly complaining about both brakes releasing whenever the rear was unloaded (commonly described as a sphincter puckering experience), and the early '02s were one to avoid.

This was supposedly corrected with '02s later than VIN ZE87455, but still you hear of people sliding through intersections on stutter bumps.

Both your bikes are '03s that should have gotten the fix !?

I'd be checking !

 

How would we check to see if our bikes have the fix?

 

Thanks, Keith

[Michael_T]

Both your bikes are '03s that should have gotten the fix !?

I'd be checking !

 

How would we check to see if our bikes have the fix?

 

Thanks, Keith

 

More importantly is the "fix" going to cost you or would it be covered by the dealership? If not, anyone have an estimate of cost?

 

Michael

[Ken H.]

While the descriptions of what is happening is correct, is it a design flaw? I'm not so sure. After all, releasing the brakes some when traction is reduced or reducing is what an ABS system is suppose to do. If the rear wheel looses traction because a bump/washboard puts it in the air, even for a microsecond, no braking/slowing action can happen of course. ABS or no ABS. A tire in the air can not stop the bike during the time it is out of contact with the pavement. The ABS sees the wheel decelerating excessively and just like it is suppose to releases the brakes some to regain traction. The front is also released some to attempt to force the rear back into contact with the pavement, again to regain traction so braking, slowing down of the bike, can continue.

 

While the sensation is disconcerting at first, it's important to remember that as a result of the surface irregularities creating reduction or loss of tire to pavement contact, you are already in a reduced or no braking/slowing action available situation.

 

The rider aggravates the situation too as when (s)he feels the 'sliding forward' effect the instinctive reaction is to squeeze the lever / pedal harder, which increases the lockup, and thus the action of the ABS system to eliminate the lockup by releasing the brakes even more. Our reaction thus increase the time period when the braking action is reduced.

[Bob Palin]

On my commute in San Diego there was a hill with a stop sign at the bottom of it, the last 20 feet or so were bumpy and the brakes would release every time if I just used the hand lever. The first couple of times it caught me by surprise and I ended up halfway out in the cross road, luckily there was no traffic. My KLR and SV650 could stop on those bumps with absolutely no excitement but the BMW never would until I learned the trick of using both controls. A braking system that can "launch" me out into the cross street when two other much less sophisticated systems stop me with no problem is poorly designed.

 

Undoubtedly somebody will say it's my fault for not always using both controls and we've discussed that at length before, however, I don't accept that the above is acceptable braking system behaviour no matter what input the rider gives.

[ShovelStrokeEd]

Ken speaks the truth.

 

I no longer ride a bike equipped with ABS, I own one, a '94 R1100RS but I'm not riding it at present due to some mechanical issues. My strategy when dealing with stutter bumps is to ease back off the brakes and modulate as necessary if the bike gets a little squirrelly. Does this extend my stopping distance? Sure it does. Is it, or will I be, able to stop in the same or shorter distance? I dunno, I never measured or compared. I have had other bikes, my '04 GS Adv, that did the surge forward thing to me a couple of times, and I know I didn't like the sensation. There is more than just the sudden loss of apparent braking, the suspension gets all confused in this situation as well and it can be downright disconcerting. I much prefer to leave the modulation to my right hand and foot. I want the bike to react to my inputs, not what some engineer sitting a couple of thousand miles away and only riding a chair thinks the bike should do.

[kiwiaudio]

The "fix" pertaining to'02s later than VIN # ZE 87455 involved the following parts:

 

34517667322 Pressure Modulator

34317654901 Brake Ma. Cyl

34517661591 Gasket Ring (x4)

34517654982 Clamp (x4)

Renewing i-ABS pump (4.1 hrs labor)

 

Early '02s that exhibited ABS failures were fitted with these replacement parts according to a BMW service bulletin.

Every unit after this VIN code was supposed to be "fixed", but as we can see, this is not the case.

 

As recently stated, the very nature of ABS brakes is to release braking pressure to any wheel that has lost traction with the ground. As was also stated, this is barely noticeable in a vehicle with four wheels. The BMW linked system has been known to release the front wheels braking pressure when only the rear is slipping.

 

My '98 is not servo assisted or linked, and only the slipping wheel enacts the ABS system. I'm not sure, but if you use both the front lever and the rear pedal on a linked system, I'll bet they act as if independant !

 

Incidentally. the ABS system is designed to react to a slipping wheel many times faster that a human can, so the industry rule is to keep the pressure on !!

[co_g30]

well I tried it today on a stretch of bumpy road just before a stop sign....held down both the brake lever and the brake pedal and no "forward lurch" as I'd consistently experienced before. I'll keep trying this method, good habit to try and use both controls anyways when stopping.