Knobblies + ABS question

[robc]

Morning All,
At the weekend I fitted a pair of knobblies to my non-servo ABS 1150GS. I've never ridden a bit fitted with knobblies before and I know the feeling would be different. I rode up and down my street a couple of time to make sure I put everything back together correctly and I noticed the ABS allows both wheels to lock before releasing the brakes a little. I only got upto around 20mph and braking went something like this: brake, lock wheels, brake, lock wheels, brake, stop.

Is this just a characteristic of knobbly tyres I have to be aware of? ABS works perfectly with Tourances fitted.

Regards

Rob C

 

[Number 6]

I noticed the ABS allows both wheels to lock before releasing the brakes a little. I only got upto around 20mph and braking went something like this: brake, lock wheels, brake, lock wheels, brake, stop.
Rob C

That's what ABS is supposed to do. It's activating because your tyres have less grip than the 'old' Tourances because they're new (presumably?) and because they're knobblies with a smaller contact patch. Grip will improve once you get a few miles on them.

 

[Colban]

Since the wheels have been out,it might be worth checking the ABS sensor gap to the cast ABS ring on the wheel.You should have a feeler guage tool in the toolkit to do this.If the gap isn't right for whatever reason,the ABS electronics may not be receiving the correct pulses from the wheel sensor and causing the ABS to kick in.
Taking a file to the knobbies to remove any smoothness may also help.
Which tyres are on there?.

 

[Grazoid]

As a tyre fitter of my aquaintance keeps telling me "DO NOT BRAKE HARD... until you have 100 dry miles ona new set ot rubbers to allow them to bed on the rims"... his capitals not mine.

He reckons that most accidents on new tyres are down to them slipping on the rims under hard braking now that mould release compound is not as waxy as it used to be. So do a few miles today once the road has heated up and try it again.

 

[robc]

Hi Chaps,
Let me clear something up. I know how ABS feels on old and new road tyres. The wheels never lock up as the ABS does it's job a bit before that happens.

My question was why do both wheels lock up before the ABS kicks in? Is this a characteristic of knobbly tyres I need to know about?

The ABS sensor gaps are ok but I will double check again later just in case. It would be odd for both sensor gaps to be out just because I took the wheels out.

By the way, the front tyre is a part worn Karoo and the rear is a new TKC 80. I bought the wheels from a forum member and I don't know how old the front tyre is but it looks in good shape. I replaced the rear Karoo because it was almost worn out and the TKC 80 was the cheapest tyre to get.

I'm not planning on having those tyres on full time, just when I want to do some green laning - hopefully on Salisbury Plain this weekend.

Regards

Rob C

 

[Wingit]

That's what ABS is supposed to do. It's activating because your tyres have less grip than the 'old' Tourances because they're new (presumably?) and because they're knobblies with a smaller contact patch. Grip will improve once you get a few miles on them.

Wot he said

 

[Grazoid]

Ah.... hang on so these are not the same wheels as you usually use.... so are the pads unchanged and biting on different discs?

Pads bed into the discs, and changing discs without changing pads gives different braking; you could be "over squeezing" the lever and pedal. So that could be causing it to lock.

 

[robc]

Hi Grazoid,
I transfered the discs and the ABS rings. I did change the rear pads at the same time because they were goosed so I was expecting rear braking to be poor for a while but I was expecting the front braking to be pretty much normal.

I'll check the sensor gaps and take the bike for a spin to scrub in the rear tyre and pads.

I don't use brakes very often anyway - my Versys has 58,000 miles on the original front pads and the rears were changed at 48,000

Regards

Rob C

 

[Grazoid]

In that case then I would take it for a good long run , get the discs and pads warmed up and try it all again.... but do it progressively as I have never locked a wheel except with new pads or old pads on new discs.... gently gently till you find the bite point

 

[Number 6]

. The wheels never lock up as the ABS does it's job a bit before that happens.

Ahhhhhh, you've got predictive ABS, that explains it then

 

[Colban]

Hi Grazoid,
I transfered the discs and the ABS rings.

If the wheels are different,although you have moved the discs and ABS ring,the sensor gap may well be off.I have a similar intermittent ABS cutting in problem on my own bike with the front wheel changed,but staying with dualsport tyres.

 

[Greger]

Ahhhhhh, you've got predictive ABS, that explains it then

ABS is predictive based on the rate of deceleration detected by the sensor - or so I read somewhere!

Not sure why both wheels would lock at the same time, try using only one brake at a time to test each wheel on it's own.

 

[littleredrooster]

ABS is predictive based on the rate of deceleration detected by the sensor - or so I read somewhere!

No, BMW ABS only reacts to a locked wheel AFAIK.

robc - I think the difference is down to the tyres; Tourances have much more grip than new knobblies, so you might experience a different reaction from the ABS. Because it releases the brakes momentarily when it detects a lock-up, that will occur much earlier and more noticeably with knobbly tyres, I suspect.

 

[Ed209]

The ABS controller monitors the speed sensors at all times. It is looking for decelerations in the wheel that are out of the ordinary. Right before a wheel locks up, it will experience a rapid deceleration. If left unchecked, the wheel would stop much more quickly than any car could. It might take a car five seconds to stop from 60 mph (96.6 kph) under ideal conditions, but a wheel that locks up could stop spinning in less than a second.

The ABS controller knows that such a rapid deceleration is impossible, so it reduces the pressure to that brake until it sees an acceleration, then it increases the pressure until it sees the deceleration again. It can do this very quickly, before the tire can actually significantly change speed. The result is that the tire slows down at the same rate as the car, with the brakes keeping the tires very near the point at which they will start to lock up. This gives the system maximum braking power.

When the ABS system is in operation you will feel a pulsing in the brake pedal; this comes from the rapid opening and closing of the valves. Some ABS systems can cycle up to 15 times per second.

Although this refers to car systems, the principle should be the same i think.

 

[Wrigsby1]

Knobblies can be very slippery when new and improve greatly with a few miles. New back tyres on 1100S's used to lock like mad until worn in as the weight transfer was more pronounced and there was less spring unload on the suspension.

 

[robc]

Hi Chaps,
I put 100+ miles on the new tyre with 20 odd off road. All seems to be well now. I'm putting it down to a combination of newness and the completely different feel over the Tourances.

Regards

Rob C