Xantia Rear Brakes (Low Force on OSR) and NSF Bearing

This is the Forum for all your Citroen Technical Questions, Problems or Advice.

Moderator: RichardW

harryp
(Donor 2022)
Posts: 687
Joined: 07 May 2011, 16:03
Location: Telford, United Kingdom
My Cars: Forte HDI 110 hatch in wicked red, SORN
Exclusive HDI hatch in quartz, SORN
C4 Picasso Exc E, Wifes
Peugeot 207cc
x 73

Re: Xantia Rear Brakes (Low Force on OSR) and NSF Bearing

Post by harryp »

'Fraid I dont know about what/how the front brakes were disconnected; it was just said that is had been done. The inference was there was no braking from the front at all, only from the rear - and so it jacknifed!

On the 16TS, should have added that it had larger dia torsion bars and I also fitted heavy duty shocks. It was of it's time, but was much faster than any 2 litre I came across in my boy racer years, not bad for 1565cc.
Regards, Harry

2000 Nov Forte HDI 110 hatch in Wicked Red; currently SORN
2000 Mar Exclusive HDI hatch in Quartz; currently SORN
2013 C4 Picasso Excl E
Peugeot 207cc 1.6 Roland Garros
KP
Posts: 3980
Joined: 10 Jul 2006, 12:11
Location: Warrington
My Cars:
x 27

Re: Xantia Rear Brakes (Low Force on OSR) and NSF Bearing

Post by KP »

Mandrake wrote:
Hell Razor5543 wrote:As the weight of the rear of the car directly affects how much braking force is felt on the rear brakes, so the rear suspension also should drop slightly (as fluid gets diverted from the suspension to the rear brakes), and this will help to keep the apparent weight roughly centralised, which should help keep all 4 wheels firmly in contact with the ground, improving overall braking.
While you're right that the rear brakes prevent the rear suspension rising up under braking, its not because of fluid diversion. Press the brake pedal hard while the car is stationary - does the rear go down at all ? Not visibly, as the quantity of fluid removed is infinitesimal.

Its the action of the brake torque on the arm geometry. Imagine viewing the car side on from the right hand side. The rear wheel is rotating clockwise, you then apply the brake which tries to clamp the rotating wheel to the wheel hub imparting a clockwise torque on it proportional to the braking force.

This torque tries to rotate the trailing arm clockwise also, since the pivot point is at the body end of the arm the arm must rotate around this point and a clockwise rotation moves the wheel end of the arm up into the body - eg the ride height drops. Simple as that :)

If you can keep the rear height at or below the normal ride height during braking the amount the body weight throws forward dynamically is greatly reduced so braking is improved.

As a test to this put the car in low.
put your foot as hard down on the brake pedal as possible.
Put suspension in high.
Keep foot on brake pedal.
Car will creak like a nutter being told the sky isnt falling as it tries to act against the brake clamping force.
If you now release the pedal the car will lurch up under the built up pressure in the system. Do it for long enough and it really pops up!

Its a way i've tested rear brakes in xantias of old and works on the other hydro citroens and why the handbrake on ever does the front wheels and never the rear on hydro cars. If a car was left in gear and a rear handbrake left on and the suspension slowly sank it would put a lot of stress on the gearbox and braking system, not good.

If the rear brakes or rear accumulator is dead then the rear brakes likely wont be able to act against the rear suspension pressure and the rear will rise :)

Pauls method of weighting up the boot is good for the fact that it makes the rear brakes actually do some work. The Xantias i've bought and come across in hatchback for where the rear pads almost fall apart when removed to check due to age shows how little use they get. Estates dont seem as bad but they do get loaded up a lot with things like engines and gearbox's and pepsi max ;) (cough cough CHRIS!!) lol
Hell Razor5543
Donor 2023
Posts: 13726
Joined: 01 Apr 2012, 09:47
Location: Reading
My Cars: C5 Mk2 VTX+ estate.
x 2993

Re: Xantia Rear Brakes (Low Force on OSR) and NSF Bearing

Post by Hell Razor5543 »

The other reason the handbrake operates on the front brake is that the rear wheels are on a trailing arm. If you were to pop a green blooded Citroen on high (whilst on a level surface), chock the rear wheels, take the handbrake off and set the suspension to low the car will move forward slightly. Imagine what would happen on a BX, for example, if you parked up overnight with the rear wheels locked and the front wheels free to move.
James
ex BX 1.9
ex Xantia 2.0HDi SX
ex Xantia 2.0HDi LX
Ex C5 2.0HDi VTR
Ex C5 2.0HDi VTR

C5 2.2HDi VTX+
Yes, I am paranoid, but am I paranoid ENOUGH?
Out amongst the stars, looking for a world of my own!
User avatar
Dommo
Posts: 1191
Joined: 11 Apr 2009, 09:43
Location: Stoke-on-Trent
My Cars: Current
07 C5 VTX+ 2.2 HDi 173hp
97 S1 Activa
06 Boxster S
93 XM 2.1 Turbo SD

Previous cars
91 Toyota Soarer UZZ32 Active Suspension
97 S1 VSX 1.9 Turbo D
99 Xantia Activa
98 2.1TD Xantia
99 306
x 19

Re: Xantia Rear Brakes (Low Force on OSR) and NSF Bearing

Post by Dommo »

Adding to this, if you stop in traffic on a downhill slope, put the hand brake on (and the handbrake is on the rear wheels in this scenario), the rear will sink (try it in a 306/zx or similar), the xantia suspension will think someone has sat in the car and raise the ride height. So when you take the handbrake off, the suspension will shoot up and take a few seconds to correct.

Or vice versa on an uphill slope.
KP
Posts: 3980
Joined: 10 Jul 2006, 12:11
Location: Warrington
My Cars:
x 27

Re: Xantia Rear Brakes (Low Force on OSR) and NSF Bearing

Post by KP »

The thing is the brake on the rear would have no real affect when the engine is running as long as the front wheels are free to move. It's only when the wheels on each corner are prevented from moving it causes an issue.

In the downhill light scenario very little would be pulling against the rear brakes as once you moved off any excess pressure in the system would be allowed to bleed away, but having a slightly higher rear when the lights go green would actually help with weight distribution anyway :)
KP
Posts: 3980
Joined: 10 Jul 2006, 12:11
Location: Warrington
My Cars:
x 27

Re: Xantia Rear Brakes (Low Force on OSR) and NSF Bearing

Post by KP »

In the bx or non anti sink xantia scenario for example there would be about 2" of foreword movement but this is not the reason the handbrake does the front. When a bx sinks the rear is usually first to go(suspension system in healthy state!) but doesn't move backward or forward and the front is helped kept up by the handbrake. Only succumbing after a long period left stood or the brake pad and discs cooling down enough to allow a little bit of movement of discs and pads.

Don't left a hydraulic sprung citroen parked with just the handbrake on, I always have left mine in gear. Saab even saw sense with leaving a car in gear and you couldn't take the key out without leaving it in gear on a manual box :)
Post Reply