Why not DISC bralkes on all the 4 wheels

Hi,

Somewhere in this forum I read that disc brakes are better than drum brakes.

But most of the cars have disc brakes for the front and drum brakes for the rear. If disc is better why it is not used for all the 4 wheels?

If the cost is a factor, how much difference does it make?

Regards

Harsha

Cost IS the factor. You will find disc brakes on all wheels only in higher-end cars because the cost increase is not that profound between a non-rear disc version v/s a rear-disc version. For economy passenger cars it makes things expensive as changes have to be made to the manufacturing process itself (and economy cars are the main bread-winners for any manufacturer) which is the primary reason for increased cost to the customer...something which manufacturers won't be willing to do given the competition in the economy car segment.

However even with cars with all wheel disc brakes, the rear wheels have drum brakes too - basically the disc rotor also has a drum. Why? Because it makes it very simple to implement the parking brake mechanism. The drum brake mechanism is such that the brake shoes have the inherent property of "biting" into the drum if the drum rotates while the shoe is in contact with the drum. This requires nothing more than a cable operated mechanism to push the shoes against the drum to engage the parking brake - something that cannot (or is very difficult) to do with the disc brake mechanism.

That said, with time all wheel disc brakes will become more and more common. For now front-disc, rear-drum seems to work quite well.

There are so many other features which will give better performance like hydraulic clutch/cable clutch;

But cost is the main factor.

Thanks for your reply guys.

One other point which I observed is, most of our cars are front wheel drives, so probably its sufficient to have the DISC brakes for front wheels only which are the driving wheels.

Regards

Harsha

i20 and Linea T-Jet has all 4 discs brakes.

Most hatchbacks in the world are front wheel driven. Again, this is to reduce the cost.

Then why bikes (150cc), which are rear wheel drive, are given Disc Break on front wheel (mostly)? What is the reason behind it?

Thanks

-Hira

Well, I don't know why the front wheel(s) are chosen for disc brakes...maybe it has to do more with the braking dynamics. Here's why I think so: As you brake, due to the forward motion, the vehicle's weight shifts towards the front, and as a result there's greater friction between the front wheel(s) and the road...and having superior braking in the front will more effectively make use of this weight shifting related friction to bring the vehicle to a stop in the shortest distance possible.

Also in 4-wheelers the engine is located towards the front and for most cases the vehicle is front heavy anyways. Imagine an elephant pulling a chariot - which is better for stopping: Stopping the elephant or trying to stop the chariot's wheels? Obviously the former. Now imagine that only the rear wheels have brakes - what will happen when the brakes are applied (with the clutch in neutral so the engine is no longer pulling)? The front-heavy portion due to inertia will pull the car forward resulting in the rear wheels getting dragged, plus the braking distance will the rather long. Now imagine ony the front wheels with brakes - the front heavy portion is now acting to slow down the car. Hence better braking has to be applied to the heavier portion of the car - in most cars the engine is at the front. In addition to front wheels having disc brakes, the force distribution between the front and rear is also biased towards the front - for a given amount of brake pedal push the force applied to the discs is greater than that on the drums giving a much better and controlled braking characteristic.

For bikes, again the rider's weight causes the front to be heavier and the same rules apply.

One more Question,Suppose If I want to get the disc brakes installed for the rear wheel also, What may be the approximate cost??