DavidRutherford wrote:Time to dispell a few myths about the BX braking system.
The position of the rear caliper has no effect on the braking at all. It is put where it is for ease of manufacture more than anything else. When brakes are applied, the overall effect is torque on the rear wheel, with an equal an opposite torque on the trailing arm. This will be the same regardless of the caliper location. The actual force generated by the pads is balanced by an equal and opposite force at the wheel bearings. The distance between the pads and the bearings generates the torque. This is why bigger discs mean more braking effort, as the further there is between the two equal and opposite forces, the greater the torque is.
With good brake pads and a working brake balance system, the equal-and-opposite torque applied to the trailing arm balances the desire for the rear of the car to rise under braking. If your brakes are a bit rubbish, the rear will still rise, but if your brakes are working really well, then the rear will drop. In an ideal world it would remain level. This is also the reason the rear rises under heavy braking, as the rear brakes simply cannot generate the level of torque needed to keep the back down.
The handbrake is a very different matter. There are two reasons why it's on the front axle. Primarily, it's safer to do that. In the event of total brake failure, you still have a very effective emergency brake. The other reason is nothing to do with when the car is parked, it's to do with stopping on hills. With trailing rear arms and a rear handbrake, the rear of the car will sit very high when parked facing uphill and very low when parked facing downhill. Not a problem when actually parked, but it is an issue when waiting at traffic lights. If you had a rear handbrake, and were stopped for long enough, the rear suspension would try to compensate. Then you move off, and the suspension will either top or bottom out (depending on which way you were facing). Having a front wheel handbrake avoids this. It's not a problem on conventionally sprung cars, as the suspension doesn't actively try to correct. When sitting at traffic lights on a hill with the handbrake on, the back just sits either high or low for a while.
Would Citroen honestly think they could sell a car that thumped to the ground every time you left it an hour and then returned and released the handbrake then?
Of course it's got something to do with the parking David, come on!!
I fully agree with your theory there too however....it makes complete sense and it's probably the primary reason behind the design. It's something I never thought of, although maybe thats because I don't use the handbrake on hill starts?
I've re-read what I wrote and it does look like I mentioned the position of the caliper on the disc....I meant the area of braking resistance on the arm, being from the caliper/disc itself. If the braking came from the area where the rear arm bearing is, the car wouldn't sink at all at the rear under braking.
Mind you if thats where the wheel was too, it'd be a fairly hard ride!
