15 inch wheels on a herald

[JohnD]

Adrian, and Peter,

Friction is independant of area.     Friction is proportional to the force normal (at right angles) to the two surfaces, and the coefficient of friction between them.

Friction = Coeff.F x Force     

The force is expressed as units per unit area (eg Kilograms per Cm^2) which IS where area comes in.   In that respect, you are correct, the braking force will come from the area of the brake pads x the force squeezing them.   BUT, that force is the product of the force exerted by the brake systemm, divided by the pad area.

So,  if you make the pads bigger, and exert the same system force, then the force per unit area FALLS (System Force / bigger number).  BUT, larger pads, so multiply that by the area AND YOU GET THE SAME BRAKING FORCE AGAIN.     Bigger pads never mean better braking without other mods to the system.

 

However, actual car stopping involves tyres on track, and that is far more complex!

John

[Adrian Cooper]

John,

You are, of course, correct and you explained more tightly than I did. I had assumed that Iain's question referred to tyre width not pad width. One of the downsides of thread drift perhaps (4 wheel drift in this instance)

Adrian

[johny]

2 hours ago, poppyman said:

Personally i think it does not matter how wide or how big in diameter a wheel is, you only require the same brake pressure. You are after all only trying to stop the same weight. Yes i understand the the driveshaft to wheel theory, but that  is  going into the pulley and leveridge theory. Also stopping all depends on the amount of rubber on the ground and the surface.

Tony.

I dont know about the width factor but another way to think about the effect of overall diameter on braking is rotational speed. If you have a bigger OD then at a given speed the wheel will be turning at a slower speed and of course also the brake disc. Well the energy dissipated (heat) depends on brake design, disc speed and clamping pressure so if the first is unchanged but disc speed is reduced more clamping pressure will be required to achieve the same rate of energy dissipation (braking).

[SpitFire6]

Hi,

 I have used 145,155.165.175.185.195 & 215 tyre widths on my car. I have used 80,70,60,50 & 40 profiles on my car. I have used 13" 15" & 16" wheels on my car. I've used 4.5J, 5J, 6J & 7J wheels. I have used 1/2" & 3/8" wheel spaces. I have had long & short driveshafts fitted. Swing & fixed spring.

The +/- 5% maximum difference in diameter has never once made me want to consider the negligible effect it would have on braking due to different brake rotor speeds.

Most of my choice for wheel tyre combos were soon changed. 

Maybe I should have considered this?

Cheers,

Iain.

[SpitFire6]

19 hours ago, NonMember said:

It's the same thing. The brakes don't act on the road - they act on the hub, which then applies a slowing torque, converted into a force by the tyre. So in exactly the same way that larger tyre diameter reduces your acceleration, it also reduces your braking deceleration.

Hi,

 I think I would just brake harder without even registering if I had increased my tyre circumference 5%? Difference between a new and road legal tyre could be 6mm on the diameter?

Cheers,

Iain.

[SpitFire6]

4 hours ago, Nick Jones said:

snip

Bottom line - a narrow, taller profile tyre is likely to give a more consistent grip with a suspension that gives a wide range of camber angles.

Nick

 

So the front with double wishbones are more tolerant to a width increase?

Cheers,

Iain.

[johny]

yes on our cars the braking difference is small as we're pretty limited in possible tyre OD increase and also most people dont want to have the hassle of speedo correction but it must have an effect on those American custom motors with wheels up to 28" diameter?