New BMW brakes - Matt
I have just replaced the discs and pads all round (£180 all up -not bad for a Beemer)on my 1990 E30 318IS and am interested in a few things. Firstly, what is the best/most effective way to run in the new brakes ie. what driving style to adopt in order to prevent warping/pitting/grooving/glazing etc etc. as I have heard that these brakes a vulnerable to the above. Secondly could someone explain why having rear discs improves braking performance so much and what percentage of the braking is done by the rears? Why do manufacturers insist on fitting apparently obsolete rear drums on so many new cars? Also how does the balance valve work on my car (apparently there is one!) and would it be possible to improve braking performance by removing it - I recently saw an E30 325i (same brakes as mine) lock up all four wheels simultaneously and while I realise this is not very clever, it does at least show that maximum braking potential was available in the split second before lock-up. Probably not the most exciting question for you guys to answer but I really am very interested to find out the answers! Thanks in anticipation...

Matt
New BMW brakes - Cyd
To run in the new brakes you should brake gently for the first few hundred miles. As the mileage rises increase the amount of braking steadily. Try to avoid pulling up quickly unless you really need to.

The amount of braking done by the rear varies from car to car, but is likely to be around 20% ish under heavy deceleration from speed, but could be nil under very light braking from low speed. For this reason it will take your rears much longer to bed in than the fronts.

I've recently had the same as you - new pads all round, new front discs and skimmed the rear discs. The rears were taking ages to bed down, so I used the handbrake gently to reduce speed around town - but do be gentle, you don't want to overheat them.

I don't know the physics behind why discs give better braking than drums. In comparing discs to drums the Bosch Automotive Handbook says 'disc brakes represent the only means of achieving even response and good control on heavier and faster passenger cars'. It has a graph which shows that for the same actuation force discs give greater braking force and that the discs response is largely linear, whereas the drums response is curved. As far as I know all cars with ABS have discs, probably for this reason. Hope this helps in some way.

The function of the brake force proportioning valve is to make sure that the rear is not over braked, especially in the wet. The valve will be designed for the specific car it is fitted to, depending on various factors. If the rear is over braked and locks early in a braking manouvre the car is liable to spin out of control. I would not recommend you remove it.
New BMW brakes - Matt
Thanks Cyd, very helpful and pertinent response. The brakes do feel a bit spongey at the moment but hopefully that should pass. I can't believe discs cost more to fit to the rear for the car manufacturers and if they improve braking, surely it is mad to keep fitting drums on the rear of so many smart new cars. Thanks,

Matt
New BMW brakes - Cyd
You're welcome.

Yes, new pads often make the brakes feel spongey for the first few miles, made worse when you replace them all. It should soon pass.
New BMW brakes - madf
Rear disks are likely to cost more than drums:
1. the difficulty of getting a handbrake to work on them: often a separate drum is used!
2. Disk calipers cost more than drum wheel cylinders.
3. I suspect more power assistnce is required for the same pedal pressure: bigger servo required?

and finally
car/brake manufacturers may have lots of surplus drum machining capacity left from when drums were more commonplace..




madf
New BMW brakes - Dynamic Dave
As far as I know all cars with ABS have discs


No, they can also have rear drum brakes with ABS.
New BMW brakes - Altea Ego
>> As far as I know all cars with ABS have
discs
No, they can also have rear drum brakes with ABS.

>>

Indeed, as ABS is designed to allow you to steer while braking hard, it is only really required on the front.
New BMW brakes - Cyd
DD
They say you learn something everyday. When I had just bought my first ABS equiped car I was an engineer at Land Rover (on the Disco II Door Systems team). My ABS had a problem, which the brakes engineers helped me sort. They told me then (mid 90s) that ABS could only work with discs because drums weren't capable of having the same level of control exerted on them as discs. I guess in the 7 or 8 years that have passed technology has moved on. Would be interesting to know which cars have ABS controlled drums.

RF
you really need ABS on all four wheels - see answer above about the proportioning valve.
New BMW brakes - Blue {P}
Our BMW has ABS all round with rear drum brakes, but then I thought that the rear drum brakes were designed not to lock up anyway? I know that whenever I lock the wheels in my non-ABS car the back wheels don't seem to lock, only the fronts...
Blue
New BMW brakes - blank
I think that will be the proportioning valve in action. It will balance the force such that the fronts will lock up first, because this is generally accepted as safer than locking the rears.
I think that if you are brutal enough, you could lock all four wheels (but I can't think why you would want to;-) )

Andy
New BMW brakes - John S
Cyd

If my reading of the brochures is correct, then smaller engined Astra and Corsa models retain the rear drum brakes even when optional ABS is specified.

Regards

John S
New BMW brakes - owen
The main reason that discs are used in more high performance applications is that they are far more able to dissispate the heat generated by braking than drums, due to the improved air flow over the disc. In theory, drums are able to produce at least as much braking force as discs, since they have a "self-servo" effect, which is where the action of braking actually "wedges" the shoe harder against the drum (hard to explain without a diagram!).

I suspect that the benefit of all-round discs is only noticable if you drive very "enthusiastically" - i.e. repeated high speed deceleration. If there is insufficient time for the brakes to cool down before the next stop, then the brakes "fade", since the coefficient of friction of brake lining material deteriorates with temperature. To be honest, for most motoring needs the rear brakes make up such a small proportion of the total braking force then it is irrelevant whether there are discs or drums on the back.
New BMW brakes - Dynamic Dave
If my reading of the brochures is correct, then smaller engined
Astra and Corsa models retain the rear drum brakes even when
optional ABS is specified.


Yes John S, you're correct. My Dad specified ABS for his Astra 1.6, 16v when he bought it new and indeed it does have rear drums. Also all Vectra's have ABS as standard (as did the the Cavalier's from 1995) and the smaller engined ones of those have rear drums as well.
New BMW brakes - ShereKhan
My mates car a Pug 206 GLX has ABS added as a factory option. He also has drum brakes at the rear. I used to have a Rover 214 SEi mk2 that didn't have ABS and only had discs at the front. My current car a 306 Cabriolet has ABS EBFD with discs all round. I have yet to "properly test" the brakes (lets hope I don't need to!) I won't be getting a car ever again without rear discs. The braking is excellent. I had a few scary moments with my Rover, the rear drums locked and a huge plume of black smoke at the rear of the car at 80mph in the outside lane of the M6.


306 2.0 SE Cabriolet
New BMW brakes - eMBe {P}
in order to prevent warping/pitting/grooving/glazing etc etc.
as I have heard that these brakes a vulnerable to the
above. >>


In simplistic terms: To avoid warping, make sure you drive your car at slow speeds for some time after a fast run where you have used heavy braking which will have heated up the discs. Eg. if you come to a stop at a M-Way services straight after placing heavy loads on your discs, it is possible that the discs will still be quite hot and will warp. In that situation, you would need to drive around slowly in the car-park for a few minutes, avoiding harsh braking, until the disks have cooled sufficiently.
New BMW brakes - SjB {P}
Correct DD.

My brother had a Vectra with ABS and rear drums.
New BMW brakes - Matt
I found this in regard to disc/drums in BMWs on the car by car breakdown in this website : "4-cylinder disc/drum set ups nothing like as good as 6-cylinder's discs (318iS 1.9s had discs all round" and it does sound fairly conclusive to me! I have definitely noticed a MASSIVE improvemnet in braking performance from my old 306 disc/drum setup to my BMW's disc/disc setup.

Matt
 

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