[vivekiny2k]

Originally Posted by Rollin' Thunda

In any situation when the bike is moving with speed, it will either (a) be going in a straight line or (b) going along an arc of a circle (instantaneously, when it weaves, the circle keeps changing radius and position). In either case, the wheels will be moving at the same angular velocity if they are not skidding. Even in the case you mentioned, if you think about it, the bike will be moving along the arc of a circle.

obviously you didn't even understand the example, so let me back up to the example again.

take a bike if you have at your disposal. turn the handle bar to one side as far as it would go. then tie a rubber band to the wheel spokes and a stationary point. move the bike a little and check which wheel rotated more by checking the extension in rubber bands. if your bike allows a complete 90 degree turn (BMX?), the front wheel will keep moving leaving the rear wheel completely stationary.

If you have an ordinary bike, it would be less than 90 degrees and the rear wheel will move slightly, going thru a very small radius of circle as opposed to the front wheel which will traverse a large circle and hence a higher angular velocity.

[Rollin' Thunda]

Originally Posted by vivekiny2k

if your bike allows a complete 90 degree turn (BMX?), the front wheel will keep moving leaving the rear wheel completely stationary.

It won't be stationary, it will be rotating sideways around the contact patch. This is a skid, by my definition. [Skid= when the relative motion between the contact patch and the ground is non-zero]. So it does not qualify as normal rolling motion, which is when the angular velocities of the two wheels will be equal.

For anything less than 90 degrees, rolling motion is possible and the two angular velocities will be equal.

[vivekiny2k]

Originally Posted by Rollin' Thunda

For anything less than 90 degrees, rolling motion is possible and the two angular velocities will be equal.

question, are you measuring the angular velocity arond the axis that passes the the wheel axles? or a point at the center of the arc of movement?

Also, now that you understand the movement, there is always a sideways skid (twist) on the rear wheel when a bike turns. It is much more visible when the angle is 90 degress though. Considering the fact that the tire rubber is flexible, it's likely that it does not result in a real skid.

[dhanushs]

Originally Posted by SPIKE ARRESTOR

^^ Angular velocity / steerability, what exactly??

Angular velocity.

When angular velocity = 0, the wheel locks and you lose steerability.

Doo.. aint the topper supposed to enlighten me about all this?.

Originally Posted by SPIKE ARRESTOR

Dhanush, are you sure on this? Does that mean ABS kicks in only when it realizes loss of traction? How is this sensed?

AFAIK, ABS kicks in only when it realizes loss of traction. Well, wheel lock directly translates to loss of traction, and is sensed obviously by speed sensors on the wheel.

Originally Posted by Rollin' Thunda

...

Rollin' Thunda, I'll leave it to you guys to sort out the angular velocity 'difference'.

[vina]

Originally Posted by vivekiny2k

question, are you measuring the angular velocity arond the axis that passes the the wheel axles? or a point at the center of the arc of movement?

Also, now that you understand the movement, there is always a sideways skid (twist) on the rear wheel when a bike turns. It is much more visible when the angle is 90 degress though. Considering the fact that the tire rubber is flexible, it's likely that it does not result in a real skid.

For ABS to be relevant the vehicle (bike or car) will have a large linear velocity (otherwise why the hell do you need ABS) - in all such cases your example of 90 degree turn is irrelevant and the two wheels (rear and front) will have very similar (within a few percent) angular velocities.

Even if a vehicle is going in a straight line (braking/skidding or not) the angular velocities will not be exactly same - simple because circumference of the two tyres in question will not be exactly same.

Thus comparing angular velocities is a very viable way of checking lock-up. for cars and even for bikes though a control system based on this would become less stable in case the two wheels being compared start locking up simultaneously - this is unlikely on the time scales in question.

Another simple way a lock up can be detected is by measuring the linear acceleration (or deceleration in this case) with (angular acceleration of wheel)*radius . If the latter starts exceeding the former heavily that means the wheel is locking up - you don't need to know about other wheels (and linear acceleration can easily be measured by accelerometers)

[vina]

A lot of people speculated why bikes have independent front and rear brakes - I think the question can be framed thus - why do cars NOT have independent braking systems.

I think it may have to do with the fact that prior to power-steering becoming common, plenty of force was needed to move the steering (and in any case, unlike in a bike, you can not do complete rotation of the steering wheel that easily even in a power steering car) and given that we have only two feet, adding more controls (read independent front/rear brakes) would complicate things more.

Of course this is purely conjecture.

[heatednemz]

Hello All,
while i rarely post on team-bhp, I do visit this site very often to read ownership reviews and i think its great we have such a platform to share and discuss our knowledge, joys and sorrows about indian motoring.
As the topic suggests i had a few questions about the ABS on motorcycles. We have a few members on the board who recently purchased honda's new CBR 250 i would love their input on this.
Now the story starts a few hours ago when i was having a discussion about the bike with a friend who had booked his cbr250 ABS in Jan and has been patiently waiting for delivery which is schedueld in the second week of august.we started discussing how useful ABS on bikes really is? I am sort off convinced that while it not a bad thing to have, its not worth the extra wait/money that he has invested in. He obviously thought otherwise. Now while i know how to ride a bike, i am not a bike enthusiast hence my narrow mind will only allow so much interest in them.
But now after considering a lot of scenarios where i am convinced ABS on a bike is not really needed and skilled and experienced bikers would successfully bail out of a dangerous situation without any help from the ABS, There has to be a Good reason casual and pro bikers would want them, which is why i am here to get your opinion.

I have a few questions about the ABS on bikes and looking for reliable answers.

1) Does the Honda CBR250 have abs on both wheels?

2) Are they independent of each other?

3) If they arn't, then in a panic braking situation if you press the front
brake hard, will the front wheel lock up or does the ABS compensate
that?

4) I read somewhere that ABS (on bikes) works in a straight up position
only, If you try to lean the bike out of harm's way (say a car) , you
wont be very successful at it as opposed to maneuvering a car while
ABS has engaged.
So is it really worth the extra money ?

If any information on this has already been posted elsewhere on this forum kindly link me there, If this thread is in the wrong section , Moderators Feel free to move it to the correct one.

[salunkhe.vizz]

Originally Posted by heatednemz

1) Does the Honda CBR250 have abs on both wheels?

2) Are they independent of each other?
.

CBR250 has a combined ABS on both wheels - meaning even if you pull the front brakes in a panic situation, the ABS will balance out the braking between both rear and front brakes and avoid locking of either wheels.

Having said that, I am an amateur myself in understanding the "real world" benefits of ABS on a 2-wheeler. I opted out on the ABS model and got myself a STD version CBR to save on cost. So really looking forward to expert comments

[Sutripta]

^^^
AFAIK, in the CBR250, the front will not activate the rear. But the rear will also activate (partly) the front. ABS on both wheels.

ABS on a bike is a different kettle of fish compared to that of a car. And a significant part has to do with philosophy rather than technology. That actually is what I wanted to explore! In vain.

Regards
Sutripta

[Haroon]

Originally Posted by salunkhe.vizz

CBR250 has a combined ABS on both wheels - meaning even if you pull the front brakes in a panic situation, the ABS will balance out the braking between both rear and front brakes and avoid locking of either wheels.

Having said that, I am an amateur myself in understanding the "real world" benefits of ABS on a 2-wheeler. I opted out on the ABS model and got myself a STD version CBR to save on cost. So really looking forward to expert comments

My response is not exactly on the CBR250, but since a few queries on front & rear brake activation, ABS usefulness etc have come up, I shall give you a few inputs from my side purely from experience, since I have been using ABS equipped bikes for more than 12 years now (BMW to be specific).

-ABS- A real life saver esp in WET conditions. I dont think an accomplished rider can really do the job as good as an ABS in high speed situations.

-Yes, ABS works best in straight line braking only. Any which way, if you go to any rider course, they will tell you to engine brake or apply regular brakes before a curve and you are not supposed to brake inside the curve as you can easily lose your rear. So simple laws of physics overrule ABS in a curve.

-As for ABS on front or rear, all ABS equipped bikes have them on both wheels as it does not make sense to have it only in one wheel (unlike that idiotic ABS mechanism that is advertised for Rs. 2,000 that can be fixed on Pulsars, ZMAs etc!).

-As for the reference to applying the front brakes & that taking care of the rear brake as well- thats called "linked brakes" pioneered by BMW and thats a different concept not related to ABS. Yes, some bikes like BMWs have linked brakes that also have ABS. My present BMW also has a third additional thing called 'servo assisted brakes', where a small motor pump kicks in everytime you apply the brakes making the braking more abrupt.

-I have a had few close calls and the ABS has been marvelous. On a recent trip thru Tamil Nadu, riding in medium rain, myself & wife were riding with fully loaded sidecases & topcase on our R1200GS and had to brake suddenly when a dog popped out of the grass filled road divider. With 600kms to cover that day I was easily doing about 120+ kph as the arrow-straight highway was empty due to the rains. There was no time for engine braking and hence I had to slam the brakes hard. The bike just stopped right there with not even the slightest indication of a skid or any other drama. Those sure were some terrifying seconds, but the bike stayed its course purely due to the superior ABS capability.

BTW, just for the records ABS was introduced for the first time on production bikes by BMW in 1988 on their K1 bike. So ABS has been around on bikes for a long time now and has evolved into a very advanced stage now.

[Sutripta]

^^^
Didn't Moto Guzzi pioneer 'linked' brakes?

Given a choice, I would buy a bike with ABS, and learn to live with its idiosyncrasies (if any). If it saves me from even one spill, it has more than earned its price.

However, i would still like to go through some white papers about motorcycle ABS. Not so much the what and how, but why?

Regards
Sutripta

[sharanvenu]

I think we missed out a point from our 8th standard physics text book.

'Limiting Friction is greater than sliding friction and rolling friction'

It is this theory that our ABS uses. It holds the tyres in this limiting value so that maximum friction is made available and helps us stop in a shorter distance & time.
Yes it will be very useful on wet roads but will reduce friction to a nightmarish extend while off road. This is because on gravel we can stop only with a slight slide.

And I believe that combined brake system will kill the fun of riding. For example, I tap rear brakes to regain balance while riding in bumper to bumper traffic. Likewise, I have had an instance couple of months back when I had to do an emergency stop in the middle of the road and bring it to halt diagonally to shield a couple who had fallen down from a 2 wheeler in front of me. All these are possible only with independent brakes.

Of course, if ABS is working properly, you may be able to brake even while negotiating a curve; but if you cannot shift your body to adjust balance in such a maneuver, then better get prepared for a fall. Shifting weight in this scenario is not very easy.

I have heard stories of confused Insurance investigating officers desperately searching for skid marks on the road in accidents involving vehicles with ABS. My point is, such fool proof & fail proof systems are available in the market.

So, for me its a Yes for ABS and a big No for Combined Brakes. Its meant for rookie drivers and super heavy motorcycles.

[Sutripta]

Originally Posted by sharanvenu

So, for me its a Yes for ABS and a big No for Combined Brakes. Its meant for rookie drivers and super heavy motorcycles.

Hi,
Offhand, I'd agree with you,
BUT
Honda also use this on their non-novice bikes (CBR1000RR, in fact in a lot of their bikes). And they sell it in litigation happy countries with product liability laws. Maybe they know something more than us.

I agree that a driver aid should help a driver, rather than over rule him. Esp. in the case of a motorcycle.

Regards
Sutripta

[sharanvenu]

Very true. The C-ABS used in CBR1000R and likes are evolutions of the system in the revolutionary CBR600RR prototype in which the Combined ABS used a computer control unit to ensure the correct balance of front and rear brake use and also controlled when the ABS should engage, unlike th RTR's system which gives a fixed 50% bias to the front brakes.

And for people who don't like these system bossing over them, we have options to switch off the ABS, we've got paddle shifts in automatic cars etc., right?

[Sutripta]

^^^
Hi,
Can't make out whether you are praising or panning the C-ABS system!

unlike th RTR's system which gives a fixed 50% bias to the front brakes.

Don't understand. RTR has two independent channels, AFAIK.

And for people who don't like these system bossing over them, we have options to switch off the ABS, we've got paddle shifts in automatic cars etc., right?

Not always.

Regards
Sutripta