Discs Or Drums?

[james3004]

I'm not sure if this has been discussed before, but here goes:

What is better rear discs, or rear drums?

[Tatey]

Rear discs obviously look better, and have a bit more stopping power. But they are brakes from a 1.2 tonne car going on a 500kg car, so you wont really notice it.

So it all depends on what you have to hand or what you would prefer to buy.

Thats my thinking anyway.

[james3004]

My preferance would be discs cos i hate all the fiddly springs and auto adjuster type bits in drums.

but by using drums my choice of donor cars opens up.

[twinturbo]

And people are having problems with drum rears being too effective already

I have 10" drums for mine, and will be using 260 vented disks up front.

TT

[twinturbo]

And..

If you have ever owned a disk brake sierra you will probably have suffered the sticking handbrake. On a car that gets limited useage it will be even more of a problem.

TT

[Bonzo]

I have 10" drums for mine, and will be using 260 vented disks up front.

TT


Surely 2 disks up front will be plenty :D :D :o

Drums for me :)

[HandyAndy]

if using drums opens up the donor options then use drums, they are more than good enough in such a light car, why make things more difficult with your choice of donor ;)

the average sierra was approx 1100kgs, the Roadster is between 500 to 650kgs , so the brakes are plenty capable of stopping a Roadster:)

cheers
andy

[gingea1pom]

Adrian,

I am thinking we are talking internal measurement? Otherwise I have got 9 ½ “ drums.

Do the 19.05mm slave cylinders come from the 8” Sierra drums?

Cheers Ginge

[Davey]

A lot depends on what you plan to do with the car. The major difference between drum and disc brakes is the rate at which heat can be dissipated to the air. with drum brakes the shoes have no air flow over them and can very quickly overheat when used hard, discs on the other hand have much better airflow and that air also passes over the pads so keeping them cool for longer. Anyone who has ever experienced severe brake fade will almost certainly want discs all round.

There is another advantage to discs on the rear that may have escaped some of us. Drum brakes have a "self servo" action. What this means is that the leading shoe, once it contacts the rotating drum will tend to be pulled into tighter contact with it thereby increasing the braking effort without increasing pedal force. Disc brakes don't do this as they rely on clamping the rotating disc between the friction pads.

Anyone who is old enough to have driven cars from the sixties (or earlier) will probably remember that the brakes were crap going backwards, that's because the front brakes on an all drum system had a twin leading shoe setup. When leading shoes are run backwards they become trailing shoes and lose the "self servo" action. Moving on to disc braked cars I'm sure anyone who has been towed in a disc braked car will understand just how much pedal force is required when the vacuum servo isn't operating.

D.

[Davey]

Hi Adrian, sorry I didn't explain it very well. In old cars with drum brakes all round you'd have twin leading shoes on the front for maximum braking effort and single leading/single trailing on the rear. Later cars with discs up front would typically have single leading/single trailing shoes on the rear. disc brakes require a lot more pedal effort to give the same braking force as a twin leading shoe drum setup hence the use of vacuum servos on disc braked cars. with a disc drum setup you should be fine for "normal" road use but as soon as you get on track the brakes take a real hammering.

I remember way back in 1980 we had a customer who was returning for new brake pads every week on his new XR2 under warranty, after week three "we" started to ask some bigger questions ("we" being the dealership management and "we" weren't even a Ford dealer). turned out he was doing some stage rallying at weekends and could totally trash a set of pads in 300 miles!

I've driven big vehicles and had brake fade after a relatively short (1/4 mile) but very steep (Wynnatts pass in North Derbyshire) descent and its just not funny arriving at your expected stopping point with no brakes:eek: !

Vehicle mass isn't everything, your brakes have to convert kinetic energy (movement) into heat energy and dissipate it to the air. The energy that has to be dissipated is not just related to mass but velocity as well. does a 500KG car travelling at 100MPH have more, less or the same energy to dissipate as a 1000KG car doing 50MPH?

D.

[Davey]

OK I did miss understand the drum brakes bit, I remember our old Triumph Herald That was drums all around.


For a bit of fun Energy is Kinetic energy KE = (M*V*V)/2 (can't figure how to get squared!).

So a Sierra at 1000 Kg doing 50 mph, must change to metres per sec. errr. 22.35 mps

KE is 249761.25 Joules

A 500 Kg car at 100 mph is 499522.5 Joules

So our little car at 100 compared to a Sierra at 50 has twice the energy because of the square, but half the energy of the Sierra at 100 mph.

Adrian

Errr yeah, I think:D :D . I threw the question out to see what would come back but I have a sneaky suspicion that your sums are better than mine. I shall be erring on the side of caution and using discs all round, probably with a servo;) .

D.

[AshG]

trust me you really really really really really really DONT need a servo.

i was dubious until yesterday when i drove the car properly for the first time with sorted brakes.

due to the increased pedal ratio the breaks were very very effective with very little foot pressure. it took absolutely nothing to lock the car up

the ideal setup for the roadster to get through iva without any problems is

sierra 22.2mm master cylinder
240x24mm vented front sierra discs
standard sierra 53.9mm front callipers
203x46mm sierra drums with 20.6mm slave cylinders.

as far as i know this is the only break set-up for a roadster that has passed iva first time without any tweaking of the rear pressure.

[Bonzo]

Duly noted & written in my book Ash :cool: :) :)

[Davey]

Thanks for the input Ash but mine isn't exactly as per the book, for one thing it will be significantly heavier, despite my aims for a 650KG kerb weight its looking like I'll be lucky if its under 750KGs. I'll hold fire on the servo till I've tested it all (I have a private road available adjacent to the build location:p ).

D.

[flyerncle]

Drums it is then, 1 Snare and a Kettle :D

[spud69]

as far as i know this is the only break set-up for a roadster that has passed iva first time without any tweaking of the rear pressure.

I had the standard sierra setup, rear drums, and mine passed fine. Stops on a sixpence and no brake fade at all on a tight and twisty track, and that's rally booting it.....:D

There is no need to be concerned about upgrading brake setup the Sierra disc and drum works perfectly adequate on the 500kg roadster. You can always uprate them after IVA if you feel them to be insufficient.

Just Have Fun....AndyH

[james3004]

Thanks for the advice Ash:)

Looks like drums is the easier/better option for the roadster