servo on rear drums as well as front discs???

[wibble_puppy]

(curses i can't type today , excuse any tupos, you don't want to know how many i made just in the title of this thred )

opinions and advice wanted, as ever...

I'm restoring the van's brakes (van is undergiong restoration, all in pieces atm). I want to stick with the current set-up of drums on rear, Marina discs on front, master cylinder with remote reservoir, and servo.

Now.

I've asked the question halfway through another thread, about whether I should apply the servo to the rear drums as wel as the front discs. People I have loads of respect for gave me snesible advice, both on the forum and back-channel, some saying yes, some saying no.

I'm reading up about it and doing all the research I can; and wile I am at it I also want to pull the subject off into a new thread to gather all the opinions and advice I could (hopefully with nice long, simple, explanations of why the advice is correct )

so, please, give me the benefit of your experiences, advice and opinions on this It's so important to get it right.

Cheers everyone

wibble

[simmitc]

A servo does not increase efficiency or performance per sae, it reduces the amount of effort required from your leg pressing the pedal.

Brakes should be matched or balanced so that you get the proper split of effort between front and rear. During braking the weight of the car is transferred to the front of the car (talking simple terms here, not physics masterclass) and front brakes are usually more powerful than rears (think standard Minor front drums (later cars) vs rear drums.

If you fit the servo only to the front then you are distorting the balance since no matter how hard you press the pedal (acting on rear brakes) the servo will always press a bit harder on the front. Therefore operate all four brakes from the servo. BTW that's how I fitted my first set of discs, and I've been using them for the past 18 years.

[wibble_puppy]

thank you very much for posting, simmitc, and you get a big fat gold star for the clarity of your explanation, which is hugely appreciated

[simmitc]

My pleasure, thank you for your kind words. Now we'll just wait for someone to come along with a different explanation and a different point of view

[jonathon]

Well you will not from me.

[wibble_puppy]

ooooh simmitc, before you go, i have a question:

have you had an opportunity in those 18 years of trying out the brakes in an emergency stop situation?

[bigginger]

I'm in full agreement with the above. The only reason (contentious here) for not servoing the lot is that just doing the fronts is a fair bit easier, but it strikes me as an unsafe compromise that I've never been prepared to make.

[bmcecosse]

The rear drums were designed to work without a servo - and the Marina discs were designed to work WITH a servo. Ergo - I say you should only need to servo the discs!! With a servo on all 4 wheels - it may be that the rears will lock up under hard braking long before the discs are doing all they can to stop the car. The way round this on the Mini (and 1100 range etc) was to fit a pressure limiting valve in the circuit to the rear brakes - various spring pressures were available to fine tune the rear brakes to match the fronts. In rallying it was common to only fit the servo to the front discs on the Mini - although my Rally Elf S had no servo at all - and stopped just fine even with DS11 and VG95 pads/shoes. So - it depends on users experience of whether the servo allows enough force on the front discs before it locks up the rears!

[patgarrett]

Hi I agree with simmitc that you should use the servo on all the brakes eg front and rear. This how mine is set up and it works great. The balance between front and rear is just about right, even under heavy braking. Hope this helps Pat.

[simmitc]

have you had an opportunity in those 18 years of trying out the brakes in an emergency stop situation?

Generally I try to avoid them but sometimes other road users do make them necessary On a couple of occassions I have had to brake heavily. Everything worked fine, and the power of the discs is demonstrated by being able to lock the front wheels (mind you, that can be done with drums) before the rears. Discs don't give you ABS, so the old cadence braking technique is still needed on some road surfaces; but generally speaking everything simply works, and works perfectly

[Packedup]

My Triumph has a servo (standard fit, between pedal and master), and works as usual - ie on all wheels. There is no pressure limiter/ regulator for the rears at all, although I believe it was fitted to the Sprint (which also had larger rear drums).

There's nothing I can see in the rear drums of a Minor that say they were "designed" to work unservo'd. Specced in terms of size and cylinder bore maybe, but then they'd have also been specced to compliment the original front drums - Which aren't fitted any more in this case. So any matching of parts as an engineering process has already gone out the window, and the servo'd front discs will outperform servo'd rear drums more than adequately to remain safe IMO (the danger is when the rears brake harder than the fronts and potentially lock).

[bmcecosse]

Indeed - that was my point - that with a servo on the rears they may lock before the discs are doing all the stopping they are capable of. But if users have found that not to be the case - then go for it!

[wibble_puppy]

Thanks so much to all of you for letting me have the benefit of your experience I'm aware of how little I know, and I can see you've all been really careful to explain your opinions in simple, untechnical language, for which I'm extremely grateful!! It really does help us learners to get to grips with the issues involved... Gold stars all round It's great also to be able to discuss different opinions on such an important topic so positively cheers everyone, I feel I'm learning a lot here

oh lord i'm sounding really pious. Bottoms.

[picky]

(this post is only my opinion but I think it makes sense)

If you want to get better braking, then since the standard drums can lock the wheels anyway, fitting "stronger" brakes will not help much unless you upgrade your rubber - wider tyres and or wider wheels. If you are skidding on a road with vented discs holding your wheels locked, or standard drums, what difference will it make to your stopping distance??

[wibble_puppy]

thanks for posting, picky

i've always driven my vans on standard width wheels, the wider ones aren't for me although I know a lot of people like them. I've had to do emergency stops and never found a problem - saved a child's life once, who ran out into the road in front of me. I tend to drive more cautiously in my van, partly because I'm aware of the scope of the braking system, and partly because I'm stupidly protective of it! I'm quite good at the whole "pumping the brakes while coming to a controlled stop" business *touches every kind of wood within a five-mile radius* - not my peerless talent and genius at driving, but just because I was taught well and did a lot of practice.

As I understand it, and borrowing from simmitc's post, "A servo does not increase efficiency or performance per sae, it reduces the amount of effort required from your leg pressing the pedal" - which is jolly useful for me as I'm not very strong due to my M.E.

[Willie]

I know of at least one regular on this board who suffered from rear wheel lockup and had to fit a modified rear pressure limiter to cure it.
The reason I chose to servo only the front discs was because the increased efficiency was likely to increase the forward weight transfer
which could lead to premature rear wheel lockup, it certainly wasn't because I was lazy!
Picky, You are correct when the wheels are locked but, if you have been using the brakes in anger then you are much less likely to be able to lock the drum brakes until they cool down again. I also wonder if the
different models ( convertible, Traveller etc) would differ in their leaning towards rear wheel lock up?

[simmitc]

Willie, an interesting question. I've fitted discs front, drums rear on all three of my Travellers, but kept the saloon drums all round. I also carry a a lot of useful bits in the boot, which will add a bit of weight. I think that everyone has made some very constructive comments on this thread. Thinking things through, how about this as an argument:

Marina front discs were designed for use with Marina rear drums, which are larger than than Minor ones. The Itals certainly had a pressure limiter in them, but the smaller Minor rear drums must provide less braking effort than the Marina ones, and so restore a perfectly balanced system without the need for the limiter. Whatever, I reckon that there's enough evidence of working examples, so as said above "go for it".

[Willie]

SIMMITC, You can only go by your own experiences so obviously you are satisfied. I too am perfectly satisfied with servoed front discs only.
The consensus seems to be in favour of servo'ing all four. I do not see how any Minor can have a 'perfectly balanced brake system' if discs have been fitted because you have fundamentally altered the original,
under heavy usage the discs will still be ok but the rears will have faded
(come to think of it that is why you don't get rear wheel lock up, the rears have stopped working until they cool down)!!

[wibble_puppy]

come to think of it that is why you don't get rear wheel lock up, the rears have stopped working until they cool down

Geeeeenius!

can I ask a really ignorant question?

I thought that the Minor had a hydraulic brake system which was fully compensated, ie the pressure originating in the M/C is distributed evenly among the brakes on the four wheels?

So that whatever servo assistance you put into the equation, each brake would still receive the correct amount of power?

Have I misunderstood that?

*ducks*

[Packedup]

As I understand it, the Minor system as standard will apply the same line pressure to all four wheels.

However, the slave cylinder bores, drum sizes etc will have been specced to give the desired braking characteristics (ie which end brakes harder for a given foot/ line pressure).

The moment you introduce discs to the equation you have altered the fundamental original calculations.

Now, drums are self servo to an extent by their nature, discs are actually not massively better (and possibly worse!) at sheer stopping power (as you're clamping the disc between two pads, so the only force in play is the pressure in the calipers) but what they are far better at is cooling themselves. Meaning they will brake for longer before fading, something that is very important IMO (though sensible normal UK driving will seldom if ever lead to fade).

Unservo'd discs are actually quite hateful IMO - I had a Triumph with them (and an identical model with servo, I'll come to that later) and I've now got another car with them which takes a lot of shove to bring to a halt. So, if fitting discs, I'd always contemplate fitting a servo to help with that.

When you fit a servo to a single line car such as the Minor, if you fit it before the line splits front and rear, you will increase the line pressure for a given pedal pressure on all wheels. Depending on the discs, rear weight, weight transfer under braking etc this might be too much and cause the rears to lock, unless there's a limiter/ reducer in the system.

Now, my Triumph experience. The 1300 I had with a servo had it plumbed into the fronts only as far as I could tell (never really looked to be honest!), and it worked fine. The one without had exactly the same discs and drums, the only difference, from the factory, was teh remote servo.

However, I've also had a smattering of Dolomites and 1500s, and the servo lives between bulkhead and master (and these have been single circuit). This means the line presure is "servo'd" between all four wheels, yet the discs are the same, calipers are the same, and drums are if anything more effective, than on the near identical 1300!

So I would say unless you have very grabby shoes in a very light reared car, plumbing the servo in to run all four is just fine (subject to usual disclaimer and reminder to see how it works for you). However, I would also say there is no problem fitting it to power just the fronts either, as the fronts will be doing by far the greatest proportion of the braking so there's likely to be next to no increase in stopping distances fitting it that way.

Is that clear? cause I've confused myself!