why ford's rear disc size bigger than citroen's(rally spain)

[shaitan]

I saw photo of rally spain ford's rear break disc size as big as front break disc.
but citroen has smaller rear break disc than front break.

so problem is why citroen use smaller size rear break disc?
18" wheels mean no more than 380mm break disc,bigger size can produce more pressure. or I have something wrong point with breaks?

[shaitan]

sorry the photo is big...

[Mirek]

The bigger disc radius, the bigger braking momentum by the same brake pressure. But it's questionable if it's good for rear axle. Cars tend to dive their nose while braking and their rear axle is in the same time lifted up. That's why relatively small braking momentum may cause blocking rear wheels.

You can adjust ballance of pressure between front and rear axle. With bigger disc on the rear You can use more pressure in front where You realy need it. That's for sure good but there is also negative side. The bigger disc has bigger polar innertia which affects acceleration (and also braking). That's especialy the case of using same discs for both axles because those front are made for cooling much bigger heat and therefore are also much thicker and heavier than is useful for rear axle.

It's questionable what is better but I think that both have good reasons why they used their solution.

[shaitan]

]The bigger disc radius, the bigger braking momentum by the same brake pressure. But it's questionable if it's good for rear axle. Cars tend to dive their nose while braking and their rear axle is in the same time lifted up. That's why relatively small braking momentum may cause blocking rear wheels.

You can adjust ballance of pressure between front and rear axle. With bigger disc on the rear You can use more pressure in front where You realy need it. That's for sure good but there is also negative side. The bigger disc has bigger polar innertia which affects acceleration (and also braking). That's especialy the case of using same discs for both axles because those front are made for cooling much bigger heat and therefore are also much thicker and heavier than is useful for rear axle.

It's questionable what is better but I think that both have good reasons why they used their solution.

Thanks Mirek .As you say maybe they have their own reasons.
But I think bigger is better under same regulation.On Tarmac race car need more break force than gravel seems ford did a good choice .

[sollitt]

... they sometimes use a larger dick at the rear for better braking ...
And larger balls in the middle for greater acceleration?

[Daniel]

You will always have a need for more braking force in the front of the car than the rear. Bigger isn't always better as bigger discs means more unsprung weight which is bad.

[Zico]

You will always have a need for more braking force in the front of the car than the rear. Bigger isn't always better as bigger discs means more unsprung weight which is bad.

I agree with this principle, Im just wondering if its something to do with the braking dynamics and distribution being easier to manage at the different speeds when the braking performance is identical at each disc at both front and rear?

Im no engineer so its pure speculation but is it possible that a smaller disc at the rear might alter the brake distribution at different speeds?

[Zico]

Interesting question..

I agree with this principle you describe above Daniel, Im just wondering if its something to do with the braking dynamics and distribution being easier to manage at the different speeds when the braking performance is identical at each disc at both front and rear?

Im no engineer so its pure speculation but is it possible that a smaller disc at the rear might alter the brake distribution at different speeds?


Edit- Also.. with 3 or even 4 way damping control the slow bump can be increased so weight transfer is reduced allowing the rear to handle more brake bias to take some work and heat from the front... with the active centre diff and such a great weight distribution of the modern WRC cars the drivers dont have to use the weight transfer as much to turn the car in.

Just some thoughts..

[Zico]

Also.. with 3 or even 4 way damping control the slow bump can be increased so weight transfer is reduced allowing the rear to handle more brake bias to take some work and heat from the front... with the active centre diff and such a great weight distribution of the modern WRC cars the drivers dont have to use the weight transfer as much to turn the car in.

Just some thoughts..