Noob BX Question

[retrodriver]

Now this is going to seem bizarre, but im new to all this so stick with me.

How is the back end supposed to react under braking? The reason I ask is that since doing the rear spheres the back end has risen up under hard braking but since changing the rear pads it now drops down to keep the car level. Which is correct?

[Kitch]

I've always found a BX with efficient rear brakes tends to squat very slightly under braking, although doing an emergency stop the back still lifts slightly.
It behaves oddly as the front has the anti-dive system, which is different to the one on the Xantia's, because it actually works.
But if you think about the position of the calipers, and the angle of the rear trailing arms, and then apply the brakes in your head and work out which way the arm would move, it does make a bit more sense.
As the rear brakes grip hard, the rear arms of the car are restricted in their travel. Thats why the handbrake is on the front wheels....if it were on the rear you'd pull it up, leave the car for a few hours, return, get in the car, release the handbrake and the back end would suddenly collapse!
If you stand hard on the brake pedal and get someone to get in, as they do the rear end won't sink. Once they're in, release the brake and the rear end rises. Works in reverse when they get out. The front works instantly.

Saxo's and Peugeots have a trailing arm setup like the BX, although they're suspended by torsion bars not spheres, but likewise the rear end on those doesn't react like a coil sprung car under braking. They have the handbrake on the rear though, as theres no concern about it sinking once you've switched it off!

[MULLEY]

As long as the car stops, i'm not bothered whether the rear is up or down. Perhaps new pads means the rear's are braking better which is now controlling the lift as the brake balance has shifted to lessen the load from the front to the rear??

[Way2go]

IIRC the rear brakes work off rear suspension pressure which means the greater the load in the back, the greater the rear braking effort. So a change in spheres could well give a slightly different attitude.

Also when the rear pads are more efficient there will be a greater restriction on the turning moment of the rear arms (much as Kitch said) so it will likely resist the lift more.

So both of the results you have seen are legitimate in the circumstances.

[Philip Chidlow]

And what is it when you stop and hold the brake (in 'drive' - I have an auto) and the rear sinks? Take the foot off the brake and the rear rises a few inches as you creep forward/pull off - not very 'composed', so I wonder if it's something to do with rear brake doseur valve?

[Way2go]

That may be just 'stickiness' of your rear height corrector as the rear brakes take fluid from the suspension.

I've just cured the sluggish reaction of my front corrector by spraying it and the linkages with a progressive cocktail of WD40, 3in1 penetrating oil and lithium spray grease. Works a treat now and is both fast rising on start-up from low setting and holds its attitude when parked.

[retrodriver]

thanks for the thorough responses guys, I guess if the thing stops quickly (which it does!) I shouldnt worry. The pedal is pin sharp and not mushy at all so I guess I will just leave it, feels quite good actually, like its trying to remain composed at all times.

[DavidRutherford]

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.

[citronut]

hydraulic citroens usualy sit down square front and rear when braking hard, if every thing is working as it should,

IMO this restrains the forward force's fare better than cars which nose dive on hard braking,

i thought citroen put the hand brake on the front so they could sell more handbrak cables

regards malcolm

[Tim Leech]

My 16v does sink at the back when I brake hard but, I fitted new pads etc last year and now they are bedded in the brakes are fantastic.

[Way2go]

My 16v does stink at the back

Too much information!

[DavidRutherford]

IMO this restrains the forward force's fare better than cars which nose dive on hard braking

The attitude of the car under braking makes absolutely no difference to the level of braking available. All it does it allow the passengers more comfort, and the driver more confidence.

[Aerodynamica]

Well I suppose an amount of the Kinetic energy that goes into the motion of the front of a car lowering under brake dive might otherwse have been kinetic to friction energy in non diving front axles as the front might be seen as a rigid body.

The analysis of the trailing arms as moment arms makes no mention of the force at the point of contact of the tyre to the road or the force at the arm bearings by the car's weight (unless these are being taken as resolved forces but in a dynamic system surely that's not the case)

I still think the reason the rear doesn't squat under braking is because the frint brakes are already providing the torque force that the rear's can't match even with the long moment arm of the wheel span (neglecting the 1/2 trailing arm moment)

Anyway, I agree overall too. There should be no lift or squat at the rear on a BX under normal sedate braking, if it lifts then the rear brake force is too low or too late, if it squats then the rear brake force is greater than the front or there's air compressing in the rear suspension/brake circuit.

I had a squatting problem on the CX a few years ago caused by a fault with the FRONT brakes - they were contaminated and the rear was biting as normal but the trailing arms were pivoting down due to there not being enough force in the front brakes to 'brace' agaist the horizontal part of the rear brake force that normally stops the squat at the rear brake action. New disks and pads cured it in an instant.

[citronut]

IMO this restrains the forward force's fare better than cars which nose dive on hard braking

The attitude of the car under braking makes absolutely no difference to the level of braking available. All it does it allow the passengers more comfort, and the driver more confidence.

if the front of a car is diveing and the arss is raissing then it stands to reason the car is not braking eficiantly, and the wheight of the car will still be trying to go forward,

then if the front and the arss are going downwards together this sudgests to me a fare more eficiant braking attitude, and less wheight is being shuved forward

regards malcolm

[citronut]

IMO this restrains the forward force's fare better than cars which nose dive on hard braking

The attitude of the car under braking makes absolutely no difference to the level of braking available. All it does it allow the passengers more comfort, and the driver more confidence.

if the front of a car is diveing and the arss is raissing/raised then it stands to reason the car is not braking eficiantly, and the wheight of the car will still be trying to go forward,

then if the front and the arss are going downwards together this sudgests to me a fare more eficiant braking attitude, and less wheight is being shuved forward

regards malcolm