This probably sounds like a stupid question: why it is advisable to switch off the ABS when riding on dirty roads? I ride motorcycle for 30 years, but I’m not able to understand the reason.

If the reason is to use the rear brake for skidding a bit in the bends, it looks to me a kind of maneuver which is fine for a competition or for fun, but not for a regular traveling on dirty roads, especially in two up.

If ABS works well on slippery wet roads, why it shouldn’t work well on dirty roads as well?

One example is when the rear locks up the brakes are released, leaving you with no brakes. You might like to drag that rear a bit coming down Widow Maker Hill.....but now have no brakes.

Earlier this summer I was riding on some dirt roads in Blegium on my R1100GS and as I approached the end of a dirt section I needed to take an immediate 90degree left turn onto tarmac. My bike has no ABS , so with the back wheel locked and in a controlled skid on the dirt I was slowing down fine for the tarmac and the left turn. To my amazement someone on a nice shiney new R1200GS flew past me on the right, went straight on and ended up front wheel down in the ditch which was full of muddy water.
the reason was very simply - he hadn't turned his ABS off so when he braked , not enough happened on the dirt and when he hit the tarmac which was dirty with sand and dry mud nothing happened at all - so he had no control and went staright on.
It's good to practice riding on some dirt and locking up the back wheel. By shifting your weight you can easily control it and if you get good you can steer the back round with yoru weight too. Try it and see how quickly you can stop in a controlled skid and then turn on your ABS and try it again.....it's actually pretty scarey how bad ABS is at stopping you on anything but tarmac .
Don't misunderstand me, I am not maligning ABS, it is terrific and I wish I had it for normal road riding but turn it off when you go off the black stuff otherwise you lose your ability to stop when you want to !!

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Gecko has it absolutely right - but perhaps a little clarification WHY:

When you stop on dirt, the ONLY reason you really slow down quickly is the build-up of dirt in front of a semi-locked wheel.

Since the slightest touch of brake in the dirt locks the wheel up - at least partially - almost instantly, the ABS turns the brake OFF because it detects a locking wheel - and keeps doing so whenever the brake comes back on and tries to "lock" the wheel slightly to slow down. In effect, there is no brake at all if you are braking aggressively.

Braking VERY gently, without need of ABS, will work fine - but you slow down VERY gradually.

So Turn the ABS off for the dirt!

I also think there should be an easy switch instead of the nonsense procedure currently required.

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Hmmh...all very informative and interesting. I does make me wonder, however, how this aspect of ABS fits with the almost routine inclusion of ABS on four wheel vehicles these days. Since ABS cannot be turned off in the typical car does it mean that four wheel braking on dirt roads or on loose gravel in ABS equipped cars is actually rendered ineffective?

Norm

I think the assumption there might be that you are in low box and have no need for agreesive breaking as the engine will slow you down to a crawl anyhow...If you look a ralley cars, they still have the ability to lock wheels...

Horses for Courses is what I say.

Jens

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Very interesting and exhaustive indeed.
But now I have another doubt: how comes that the ABS we all have on our cars works properly (I hope...) on snowy and/or icy roads, much more slippery than dirt roads?
Do your answers imply that the brake system of cars equipped with ABS is ineffective on snow or ice?

In a word " YES " ..... if you know what you are doing you can stop as quickly or possibly even quicker on a none ABS equipped vehicle. The point is most riders/drivers don't know how and in the heat of the moment panic sets in so the electronic trickery kicks in and helps us out. If he hit the brakes in an ABS equipped car on the ice you won't stop any quicker but you may be able to steer out of the situation which is hard to do without ABS as all 4 wheels will lock . Modern ABS on cars is able to figure out which wheel needs how much braking power.

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...The answer to your question comes down to surface area. A car has four wide wheels on the ground, a bike only two with a minimal surface area. Although the braking on gravel or dirt roads is not quite as effective on cars with ABS, it is still adequate.

...i would just like to add to this. Ice and dirt are like apples and walnuts, completely diferent subjects. When you brake on ice, the friction creates heat, which creates a thin film of water. Ever walk on wet ice? Anti-lock brakes prevent this by regaining your traction by pulsing the brakes. When i was younger and growing up in northern Canada before the days of anti-lock brakes, we quickly learned to pump our brakes to maintain controll. Personally, i would prefer an ABS system in the winter. Applying firm preassure to the pedal stops you quite effectively, and at the same time you can still steer the car and stay out of the ditch.

O.K. Here's a question that spins off from this discussion. It concerns the relative tire size that makes a motorcycle handle well on dirty roads or on dirt generally. I understand that narrow tires are desireable off pavement and simply glancing at dirt bikes vs. street or even dual purpose bikes confirms this. But then there's the Yamaha TW200, a small dual sport which runs on manifestly fat tires and is claimed by Yamaha and reputed by many to handle very well on trails, dirt etc. What gives? Is this science or art?

Norm