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[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..