No braking at rear

chrisstdt · Feb 8, 2021

Was pulling a trailer in the snow with my tdi 90 today and found there is very little braking at the rear
Fronts lock up but rear dont
Plenty meat on the pads and discs are being cleaned by the pads just very little braking
Do they have a bias valve or similar ?
Thanks

Europa486 · Feb 8, 2021

Did you have the trailer tail heavy? Cannot tell you about a bias valve on yours but I have one on my
Discovery 200tdi 1990 so you may have,

flat · Feb 8, 2021

The rears wouldn’t lock up normally, under heavy braking there is a valve that deliberately stops that happening

It’s called the G valve, and is on the drivers side (RHD) chassis dumb iron, below the master cylinder. Follow the lines and you will find it. Cant be adjusted and doesn’t normally fail.

when you emergency stop you won’t lock up all 4 wheels, by having less brake effect on the rear it keeps the vehicle straight under heavy braking

chrisstdt · Feb 8, 2021

Trailer had a quad in the front so was nose heavy if anything
If I get chance tomorrow il jack the rear up and see if I can turn the wheels under braking
I have noticed before that the fronts lock up when driving in snow and ice

kevstar · Feb 8, 2021

flat said:
The rears wouldn’t lock up normally, under heavy braking there is a valve that deliberately stops that happening

It’s called the G valve, and is on the drivers side (RHD) chassis dumb iron, below the master cylinder. Follow the lines and you will find it. Cant be adjusted and doesn’t normally fail.

when you emergency stop you won’t lock up all 4 wheels, by having less brake effect on the rear it keeps the vehicle straight under heavy braking

Never heard of a G valve heard of G spot tho

.....load sensing valve or brake bias valve.

Rougharse Racing · Feb 8, 2021

There is a pressure limiting valve to prevent the rear end locking up under heavy breaking. If the back end locks up before the fronts then the rear wheels will have less grip and will try and overtake the fronts.

Al2O3 · Feb 9, 2021

I thought the weight of the engine and box apply more downward force on the front axle than there is on the rear. This creates far greater friction between the tyres and the road making it much harder to stop the front wheels turning. The rear wheels have less force pressing down on them and so would lock up if the same braking force was applied. So a bias valve is used to proportionately split the required force between front and back. If you had an old standard car and pulled the hand brake on the rear would not overtake the free running front unless you slowed the front by turning the steering, this would also create a turning force about the front wheels causing the car to pivot about that point and the rear end would start to rotate past the front.

flat · Feb 9, 2021

kevstar said:
Never heard of a G valve heard of G spot tho .....load sensing valve or brake bias valve.

instriggle to find them all no matter what SWMBO calls it

flat · Feb 9, 2021

Al2O3 said:
I thought the weight of the engine and box apply more downward force on the front axle than there is on the rear. This creates far greater friction between the tyres and the road making it much harder to stop the front wheels turning. The rear wheels have less force pressing down on them and so would lock up if the same braking force was applied. So a bias valve is used to proportionately split the required force between front and back. If you had an old standard car and pulled the hand brake on the rear would not overtake the free running front unless you slowed the front by turning the steering, this would also create a turning force about the front wheels causing the car to pivot about that point and the rear end would start to rotate past the front.

on a perfect surface only surely? Perfectly flat in all directions with, perfectly centralised steering , perfectly matched tyres and balanced brakes etc but in reality camber, undulations, slightly different tyre pressures etc, one brake fractionally stronger than another, mass slightly off centre (driver in seat etc.) and all could make you spin if you yank that handbrake

Al2O3 · Feb 9, 2021

flat said:
on a perfect surface only surely? Perfectly flat in all directions with, perfectly centralised steering , perfectly matched tyres and balanced brakes etc but in reality camber, undulations, slightly different tyre pressures etc, one brake fractionally stronger than another, mass slightly off centre (driver in seat etc.) and all could make you spin if you yank that handbrake

Yes, but theories are based on a set of standard conditions. It's not my theory, just how I thought it worked.

No braking at rear

chrisstdt

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Europa486

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flat

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chrisstdt

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kevstar

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Rougharse Racing

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Al2O3

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flat

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flat

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Al2O3

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