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[mark3]

I would assume it works by seeing a difference in the speed signal from the front and rear ABS sensors and as the front wheels turn faster the the rears when cornering can i also assume it only works if it see's the rears going faster than the fronts as opposed to just a difference?

[Kevin Wood]

The algorithms involved can be quite complex but the ECU can build up a picture of the car's actual speed based on all 4 wheel sensor outputs. It will trim each wheel sensor speed to account for differences in tyre diameter and take an average. If a driven wheel exceeds that average it's an indication that it's slipping. Likewise, if a braked wheel drops below the average it might be approaching lockup. The rate of change of speed at each wheel is also part of the equation.

[2woody]

they're very complicated software routines - mainly because they have to be failsafe - indeed, there's three sets of everything in the ECU & they all have to agree before intervention. The software looks mainly for change, rather than a constant speed, to make up for muppets putting the wrong size wheels and/or tyres on.

Also, they need to differentiate between different types of surface ( snow, ice, rain, mud, etc ) so the ABS/TC knows how to intervene.

Generally there's a threshold set for intervention of either ABS or TC for each type of surface.

I do have some Bosch information if you have a more specific question.......

[TheBoy]

In addition to above, the post 1998 ones (the unreliable ones ), will monitor each rear wheel seperately, and initially try to brake the spinning wheel, before asking engine to reduce power.

Pre 1998 will monitor each wheel, but cannot actuate the rear brakes individually, so will go straight to asking engine to reduce power.

[Brikhead]

In addition to above, the post 1998 ones (the unreliable ones ), will monitor each rear wheel seperately, and initially try to brake the spinning wheel, before asking engine to reduce power.

Pre 1998 will monitor each wheel, but cannot actuate the rear brakes individually, so will go straight to asking engine to reduce power.

In addition to the above addition! I'd like to add that on the later cars tc operation is much less intrusive (therefore safer) than the earlier systems.

I find the t.c. light off putting as for me default setting is always off, as long as I've remembered to press the button when starting out...

[2woody]

yes, the early Omega TC is truly nasty of the first order....

[mark3]

Right, i was wondering how it worked as i have a set of 4 very good 205/55/15 tyres down my garden and i need new tyres on the omega but there 205/65/15 so about 20mm less in diameter.

As i don't mind the front of the car being a tad lower i got them fitted and balanced at only £5 each (result) and the results are good, very good, its a win win x 2 situation

1st win, they cost me nothing

2nd win, only a tenner to fit and balance both

3rd win, steering is much improved being a lot more direct (less side wall flex)

4th win, brakes are noticeably better due to the discs now being larger in comparison to the rolling diameter.

Its made a huge difference and all for a tenner, lol.

[TheBoy]

Right, i was wondering how it worked as i have a set of 4 very good 205/55/15 tyres down my garden and i need new tyres on the omega but there 205/65/15 so about 20mm less in diameter.

As i don't mind the front of the car being a tad lower i got them fitted and balanced at only £5 each (result) and the results are good, very good, its a win win x 2 situation

1st win, they cost me nothing

2nd win, only a tenner to fit and balance both

3rd win, steering is much improved being a lot more direct (less side wall flex)

4th win, brakes are noticeably better due to the discs now being larger in comparison to the rolling diameter.

Its made a huge difference and all for a tenner, lol.

That one will need explaining to me I'm afraid

[mark3]

Smaller overall diameter wheels/tyres but same size brakes mean improved braking efficiency, its the same equation as it would be if you fitted larger diameter discs to same size wheels/tyres.

Basically the closer to tyre tread you move the pads the better the braking will be.

[Entwood]

Smaller overall diameter wheels/tyres but same size brakes mean improved braking efficiency, its the same equation as it would be if you fitted larger diameter discs to same size wheels/tyres.

sorry .. not in my book ... larger diameter discs/pads give a greater friction surface and so increased braking. The ratio of disc size to wheel size has absolutely nothing to do with braking power. All that counts is friction area (given no change to friction material of course   which will make a major difference.)

Your comment about pads being nearer to tyres is also wrong IMHO .. what matters here is pad to axle distance .. as the pads are effectively levers .. and the longer the lever the better the force applied .. so large wheels+ large discs = very good brakes...

Just changing ratios (wheel size) makes no difference at all.... as the discs and pads remain.

[mark3]

Smaller overall diameter wheels/tyres but same size brakes mean improved braking efficiency, its the same equation as it would be if you fitted larger diameter discs to same size wheels/tyres.

sorry .. not in my book ... larger diameter discs/pads give a greater friction surface and so increased braking. The ratio of disc size to wheel size has absolutely nothing to do with braking power. All that counts is friction area (given no change to friction material of course   which will make a major difference.)

Your comment about pads being nearer to tyres is also wrong IMHO .. what matters here is pad to axle distance .. as the pads are effectively levers .. and the longer the lever the better the force applied .. so large wheels+ large discs = very good brakes...

Just changing ratios (wheel size) makes no difference at all.... as the discs and pads remain.

Sorry mate but you have all your basic laws of physics ass about face, either that or you need to throw your book away as its no good at all.

Pad friction area of course counts BUT if the pads was right out as far as the tyre tread on massive discs you would go through the screen with the slightest of pressure on the brake pedal. and distance from axle to pads is only relative if your not changing the rolling diameter of the wheels/tyres!

The lager the diameter of the disc friction area (closer to the tyre tread) the less force per same friction area needed to slow the vehicle, as you say its all to do with leverage and the larger diameter the disc the greater the leverage! basic law of pysicis is the longer the lever the easier it is.

Also as i first said since the change my brakes ARE noticeably better.

[Entwood]

Smaller overall diameter wheels/tyres but same size brakes mean improved braking efficiency, its the same equation as it would be if you fitted larger diameter discs to same size wheels/tyres.

sorry .. not in my book ... larger diameter discs/pads give a greater friction surface and so increased braking. The ratio of disc size to wheel size has absolutely nothing to do with braking power. All that counts is friction area (given no change to friction material of course   which will make a major difference.)

Your comment about pads being nearer to tyres is also wrong IMHO .. what matters here is pad to axle distance .. as the pads are effectively levers .. and the longer the lever the better the force applied .. so large wheels+ large discs = very good brakes...

Just changing ratios (wheel size) makes no difference at all.... as the discs and pads remain.

Sorry mate but you have all your basic laws of physics ass about face, either that or you need to throw your book away as its no good at all.

Pad friction area of course counts BUT if the pads was right out as far as the tyre tread on massive discs you would go through the screen with the slightest of pressure on the brake pedal. and distance from axle to pads is only relative if your not changing the rolling diameter of the wheels/tyres!

The lager the diameter of the disc friction area (closer to the tyre tread) the less force per same friction area needed to slow the vehicle, as you say its all to do with leverage and the larger diameter the disc the greater the leverage! basic law of pysicis is the longer the lever the easier it is.

Also as i first said since the change my brakes ARE noticeably better.

The power of wishful thinking ...    

[mark3]

I know my car and how the brakes was before and after, its not wish full thinking.

The most basic laws of physics apply here and i cant believe you don't see it? it really is so so simple, move the friction area nearer to the tyre tread and you will need much less pressure to brake as well as you did before so using the same pressure you will notice an improvement in braking efficiency.

[Entwood]

I know my car and how the brakes was before and after, its not wish full thinking.

The most basic laws of physics apply here and i cant believe you don't see it? it really is so so simple, move the friction area nearer to the tyre tread and you will need much less pressure to brake as well as you did before so using the same pressure you will notice an improvement in braking efficiency.

You are NOT moving the friction area .. you are moving the tyre .. the friction area/axle distance, the friction pad area, the caliper size/pressure have all remained EXACTLY the same ... therefore the braking power exerted on the disc remains exactly the same.

If the friction area moved you would have the basis of an argument ..

Your braking "may" feel different because you are going slower for the same engine RPM, you therefore have less inertia, and the same brake force "could" have a slightly greater effect.

[mark3]

Oh god!

Yes im moving the tread nearer to the brakes friction area, its the same thing and if you cant see it? well i guess there's no point me keeping on.

one last thing, bear this in mind, if you say fitting smaller wheels has no effect on braking efficiency what would happen if you fitted much bigger wheels instead?

[TheBoy]

one last thing, bear this in mind, if you say fitting smaller wheels has no effect on braking efficiency what would happen if you fitted much bigger wheels instead?

From my Physics classes, although I only did Physics to A level, nothing. But I might be missing something, hence why asking for the clarification

Bigger discs usually (but not necessarily) lead to more friction area. In addition, bigger discs allow better heat dissipation, thus can take more abuse.

But for the life of me, I can't see (beyond speed differences per revolution) what difference wheel diameter makes 


One of my bro's is a car designer, I'll have a word with him next time I see him...  ...I owe him a beer or two

[Andy H]

Smaller rolling radius will mean that the wheels spin faster for the same road speed. Power = torque x angular velocity. Increasing the angular velocity will therefore increase the rate of energy transfer (kinetic energy to heat) for the same pad pressure.

Stuff the theory though, I would prefer to fit some new pads rather than smaller tyres......

[mark3]

By fitting smaller diameter tyres the friction area of the disc is now nearer to the outer diameter of the tyre, which would be the same equation as keeping the same size tyres and fitting bigger discs, either way it means the force required to slow/stop the wheel from turning will be less.

Now im not talking about increased friction area, although this will also improve braking, im talking about reducing the distance between the friction area and the tread of the tyre, by fitting larger diameter discs OR smaller diameter tyres which ever way you choose to do it it will yield the same results.

[D]

Mark3, I think your argument would perhaps make sense if you were just using the brakes to stop the tyre whilst suspended in mid air. However in a 2 ton Omega, I suspect the smaller wheel size would have no effect at all for a given speed. If you compare the inertia of the car vs the inertia of the wheel you can see that your thought process shouldn't make a real world difference. Having said that, various trials have shown the effect of placebo in research groups.

[D]

Sorry mate but you have all your basic laws of physics ass about face, either that or you need to throw your book away as its no good at all.

Pad friction area of course counts BUT if the pads was right out as far as the tyre tread on massive discs you would go through the screen with the slightest of pressure on the brake pedal. and distance from axle to pads is only relative if your not changing the rolling diameter of the wheels/tyres!

The lager the diameter of the disc friction area (closer to the tyre tread) the less force per same friction area needed to slow the vehicle, as you say its all to do with leverage and the larger diameter the disc the greater the leverage! basic law of pysicis is the longer the lever the easier it is.

Also as i first said since the change my brakes ARE noticeably better.

Agreed, but you haven't changed your disc diameter have you? So where is the additional leverage coming from?

[2woody]

You're interested in two torques applied to the wheel.

First - the brake caliper piston applies the clamping force at the piston radius. The further out the radius from the centre the more torque the clamping force results in. Which is why bigger brakes stop better.

Second - the tractive effort. This is the force being applied by the tyre to the road. It comes as no surprise to find out that the braking force will decrease the further from the hub centre you get. But this is, of course, incompatible with bigger brakes as you run out of room to package them in the wheel.

Torque = force x distance

for the same torque, if the distance goes up, the force must come down.

[2woody]

There is another point - if the tyre sizes are mis-matched front to rear, worse braking will result.

You need to consider the ABS threshold - the point where the ABS decides that a wheel is locked and starts to modulate ( releases braking to regain control )

If the tyre size is different at one end, you'll get to the ABS threshold at lower retardation, hence the ABS will start modulating, which in turn results in a longer stopping distance.

[cem_devecioglu]

 
 
phew.. an interesting physics debate.. for exact proofs you need to solve many equations
 
I can simply say that there is no meaning to have more friction force between tire/road vs disc/pads
 
because at the final point tires will have the last word   so increasing one friction force alone on disc/pad  -may- have some effects but not bigger than tire/road friction force..
 
 

[mark3]

ABS works fine, i had it in operation on a nice wet road the other day, seems no different to before...

And i only fitted these smaller tyres as they was free, i wasn't expecting to feel any difference in braking or steering but i noticed a few days after on my way to Portsmouth that the steering felt more positive and the brakes better.

[cem_devecioglu]

By fitting smaller diameter tyres the friction area of the disc is now nearer to the outer diameter of the tyre, which would be the same equation as keeping the same size tyres and fitting bigger discs,
 
no .. cant say this directly.. if you draw a plot you will see at one point increasing disc size will make no effect until you change the tire
 
 
either way it means the force required to slow/stop the wheel from turning will be less.

Now im not talking about increased friction area, although this will also improve braking, im talking about reducing the distance between the friction area and the tread of the tyre, by fitting larger diameter discs OR smaller diameter tyres which ever way you choose to do it it will yield the same results.
 
No.. not at every application values.. there are limits which are dependant on both sides

[cem_devecioglu]

one last thing, bear this in mind, if you say fitting smaller wheels has no effect on braking efficiency what would happen if you fitted much bigger wheels instead?

From my Physics classes, although I only did Physics to A level, nothing. But I might be missing something, hence why asking for the clarification

Bigger discs usually (but not necessarily) lead to more friction area. In addition, bigger discs allow better heat dissipation, thus can take more abuse.

But for the life of me, I can't see (beyond speed differences per revolution) what difference wheel diameter makes
 
One of my bro's is a car designer, I'll have a word with him next time I see him...  ...I owe him a beer or two

it will make a difference but its too much dependant on road type/surface type and tire type so hard to predict easily..

[mark3]

Ok, imagine fitting bigger tyres to the rear, result? loss in acceleration, as the bigger wheel is harder to turn, so the smaller the wheel the easier it is to turn, and also stop.

Assuming all other variables remain the same smaller wheels are easier to manipulate as there inst as much leverage involved, this is the same for acceleration or deceleration. now if the braking diameter of the disc was also reduced there would be no difference in braking but its remained the same moving it closer to the outer edge of the tyre, meaning less effort is needed but as it has the same effort put into it the result is better braking.

[cem_devecioglu]

Ok, imagine fitting bigger tyres to the rear, result? loss in acceleration,
 
 
partially yes.. but if the engine hp is adequate result will be reverse.. example is drag cars.. so not everytime
 
as the bigger wheel is harder to turn, so the smaller the wheel the easier it is to turn,
 
yes..
 
 
and also stop.
 
No.. definitely different subjects..  in acceleration you use engine hp and some ratios from transmission/differential etc also against increasing wind force (fluid dynamics) and road friction..
but for stopping you use mostly road friction and decreasing wind force (helping)
 
now to stop better with small tires , road conditions wet /deep water/snow/ asphalt   - tire type- will change the situation and not easy to say directly.. but for a fixed road say clean asphalt bigger tires will be better..
Assuming all other variables remain the same smaller wheels are easier to manipulate
 
if you mean driving/handling yes..
 
as there inst as much leverage involved, this is the same for acceleration or deceleration.
 
acceleration and deceleration are different things
 
 
now if the braking diameter of the disc was also reduced there would be no difference
 
if you try very small diameters you will see big difference..
 
in braking but its remained the same moving it closer to the outer edge of the tyre, meaning less effort is needed but as it has the same effort put into it the result is better braking.

besides small diameter discs will heat up very quickly and will loose all efficiency..

[mark3]

Its not different at all, its the reverse of moving, ever action has an equal and opposite re-action. its the same....

Brake pad/disc friction area moved nearer the tyre thread = less force required through pedal to stop! so applying the same force means better stopping. im not talking about locking up, im talking about slowing the forward movement of the car, its mathematically and physically the case. the law of physics being the longer the lever the less force required, well my lever is effectively longer and the lever of tyre tread distance from hub has been reduced.

[cem_devecioglu]

Its not different at all, its the reverse of moving, ever action has an equal and opposite re-action. its the same....

Brake pad/disc friction area moved nearer the tyre thread = less force required through pedal to stop! so applying the same force means better stopping. im not talking about locking up, im talking about slowing the forward movement of the car, its mathematically and physically the case. the law of physics being the longer the lever the less force required, well my lever is effectively longer and the lever of tyre tread distance from hub has been reduced.

   Mark I wish I could convince you.. but seems like my language wont help.. 

[martin42]

Where's the popcorn,and other munchies 

[mark3]

I just don't see how something so simple cant be understood?

[omega3000]

Wow this is a long post ....got lost on the physics      here   

 
phew.. an interesting physics debate.. for exact proofs you need to solve many equations
 
I can simply say that there is no meaning to have more friction force between tire/road vs disc/pads
 
because at the final point tires will have the last word   so increasing one friction force alone on disc/pad  -may- have some effects but not bigger than tire/road friction force..
 
 

Hope you get it sorted .

[D]

I just don't see how something so simple cant be understood?

Simple. Because you are saying that it is easier to stop a smaller tyre. True. but at the same vehicle velocity, when you take into account the weight of the omega, then the difference you are talking about is negligble.

If on the other hand, you were on a car lift, accelerating with no car weight to carry, then yes the smaller tyres would proably stop quicker. I suspect you could prove this by using the angular velocity equation?

Comparing like for like, at the same speed, the inertia of the car is the same, irrespective of the tyre size. Braking reduces the inertia. The first limiting step is tyre contact with the road and that is what usually lets go first. The next thing is pad size/friction. Then the lever action of the brake. In your case you have changed none of these (or rather gone for a smaller tyre contact patch), so how can you explain the benefit?

Happy for you to explain it to me mathematically if you prefer?

[mark3]

What velocity makes no difference, brakes are better over the whole speed range............... and when i say better i mean they are slowing the car better when exerting the same force on the brake pedal.

Im not talking about locking up the wheels or the tyres ability to grip the road surface or heat build up in the disc, thats going beyond what i mean, its simple physics, the smaller the diameter of the tread surface in comparison to where the brake friction material is located between tread and hub the less force required to slow or stop! i cant think of a simpler way of explaining it.

As i said i never fitted these tyres to gain better brakes or more direct steering, i never even considered it but once they fitted and i noticed the difference i came to the blindingly obvious conclusion, it was along these lines "oh yeah, i never thought of that" lol

[martin42]

Why not do a before and after test,and use the brake machine at an mot station and compare the results,if as you say it has improved then surely the result would be higher braking power,or have I just lost the plot on here 

[cem_devecioglu]

What velocity makes no difference, brakes are better over the whole speed range............... and when i say better i mean they are slowing the car better when exerting the same force on the brake pedal.

Im not talking about locking up the wheels or the tyres ability to grip the road surface or heat build up in the disc, thats going beyond what i mean, its simple physics, the smaller the diameter of the tread surface in comparison to where the brake friction material is located between tread and hub the less force required to slow or stop! i cant think of a simpler way of explaining it.

As i said i never fitted these tyres to gain better brakes or more direct steering, i never even considered it but once they fitted and i noticed the difference i came to the blindingly obvious conclusion, it was along these lines "oh yeah, i never thought of that" lol

mark , why do f1 cars use wide big tires instead of using small and narrow ?

[TheBoy]

I just don't see how something so simple cant be understood?

I think you need to show us the maths, as currently, you haven't given any kind of explanation, other than said its fact.

That might help explain it. As to be honest, I still cannot see it.

[Andy H]

I just don't see how something so simple cant be understood?

Me neither

I blame the state of modern maths education