Rear Brakes...

[Rochester TVR]

Two questions, that as PHer I really should know the answer too, but I dont...

Assuming your driving at 50mph and brake gently to a stop, what (if any) percentage of the braking force is directed to the rear brakes?

In cars without the electronic braking stability control gizmos, under what situations are the rear brakes called upon more?

[JoelRP]

Depends on the car's brake bias. I'm no expert, but for normal road cars that actually have rear brakes, the bias is usually between 60-80% to the front, for safety reasons

[JoelRP]

Depends on the car's brake bias. I'm no expert, but for normal road cars that actually have rear brakes, the bias is usually between 60-80% to the front, for safety reasons

[kambites]

I believe the front/rear brake balance is generated fixed, so the same proportion of the force is applied to the rear brakes regardless of situation (except on cars with electronic brake force distribution). This is by no means optimal, though, because it should depend on the weight shift which depends on the rate of deceleration and hence on the grip available if you're trying to stop as fast as possible.

Edited by kambites on Wednesday 1st June 15:26

[John145]

Brakes can be anywhere from (F - R) 60-40 to 80-20 ratio, depends on the age of the car and type of car. Old cars with drum brakes might not do any braking on the rear at light loads and the ratio can vary widely during heavy braking due to weight transfer and pressure relief valves in the braking system.

Modern cars use more sophisticated methods of brake pressure modulation especially with ESP systems.

[Willber]

My MK2 Golf GTI uses mainly the front brakes, I would say 80/20 split. The rears do very little apart from the handbrake.

[kambites]

doogz said:

Many cars have a little mechanically actuated load valve on the rear, that senses ride height, or the angle at which the trailing arms are resting at, or whatever, that means when the rear of the car is lower, a larger proportion of overall braking force is sent to the rear. Means you'll have more braking at the rear when the boot is full of luggage. Or dead bodies.

That's an elegantly simple solution.

[Rochester TVR]

doogz said:

Many cars have a little mechanically actuated load valve on the rear, that senses ride height, or the angle at which the trailing arms are resting at, or whatever, that means when the rear of the car is lower, a larger proportion of overall braking force is sent to the rear. Means you'll have more braking at the rear when the boot is full of luggage. Or dead bodies.

Thats interesting... the reason I ask this is that my Alfa 159 is in need of some new pads. I have decided on upgrading to Performance Friction pads for the front but considering sticking with OEM pads for the rear as they are less worked and the extra cost wouldn't be noticed.

[kambites]

Rochester TVR said:

Thats interesting... the reason I ask this is that my Alfa 159 is in need of some new pads. I have decided on upgrading to Performance Friction pads for the front but considering sticking with OEM pads for the rear as they are less worked and the extra cost wouldn't be noticed.

Yes. The risk of doing that is obviously that you'll get more braking force per unit hydraulic pressure on the front than on the back, so the front brakes will come on harder than OEM for a given pedal pressure but the rears wont, so the fronts will lock before the rears are near their limit of grip, reducing your overall braking efficiency. Much the same as running different compound tyres front to rear, but only affecting the brakes.

Personally I've never noticed it in practice, though. I don't think cars are set up that accurately in the first place.

[Monty Python]

Most cars are front biased (usually 60% front, 40% rear), but if the car is fitted with electronic brake force (EBD) distribution, then it can change depending on the road conditions, speed and wheel loading etc.

[Rochester TVR]

Im going to consult the Alfisti on the % bias, but if is as high as 60/40 then I will upgrade all round.

Thanks for the answers

[kambites]

Rochester TVR said:

Im going to consult the Alfisti on the % bias, but if is as high as 60/40 then I will upgrade all round.

Thanks for the answers

That sounds unusually rear biased to me, I would have thought around 70/30 was more common.

[dvance]

From what I've read it seems that if you're looking for safe braking then the front should lock up before the back does so from a safety point of view putting uprated pads on the front is not so bad. The efficiency point has been adequately covered.

What I wonder is what happens if you put better pads on the back and or potentially better/bigger calipers -- unstable car under braking?

[HellDiver]

The Vectra C's brakes are heavily rear biased. They'll go through rear pads twice as quick as fronts.

[littleredrooster]

kambites said:

I believe the front/rear brake balance is generated fixed,....

You believe incorrectly, then. All cars that I have come across have some kind of pressure limiting / load sensing device which ensures that the rear brakes do not lock up first under braking. That would make for an extremely unstable car.

[Gatefold]

In my untempered youth in a variety of small rear-drummed hatchbacks, spirited driving that would leave the front brakes a little cooked and limit their effectiveness, the result being that the net time one would brake hard the rear would noticeably squat as the drums became more effective than the wheezing front discs and scorched pads; a sensation not unlike pulling on the handbrake whilst moving, but not enough to lock the wheels. Seemed to be most common with small PSA and other torsion bar setups.

[kambites]

littleredrooster said:

kambites said:

I believe the front/rear brake balance is generated fixed,....

You believe incorrectly, then. All cars that I have come across have some kind of pressure limiting / load sensing device which ensures that the rear brakes do not lock up first under braking. That would make for an extremely unstable car.

Yes, but you could equally well make sure the rears don't lock up first by going for a massively front-biased static setup. I'm pretty sure that's what my MGB does, because I rebuilt a fair chunk of the braking system and didn't spot anything "active" looking in the lines.