Do rear discs on a sierra matter. Will drum brakes do or will i have to upgrade them?
Chris

In terms of stopping power and weight, drums will be absolutely fine. Some people prefer discs for looks but either will be ok and more than up to the job of braking the rear end of a seven.

If you go for drums and then decide you'd prefer discs, you can always get a conversion.

Sorry, but rear discs are essential. Are there drums on a Veyron ? I think not.

Seriously though, buy the cheapest Sierra and find a rear beam from a 4x4.

quote:

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Sorry, but rear discs are essential

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I agree with Helfire on this one. Most of the breaking is done by the frount breaks anyhow and these are disks....

Also durms (for the smaller ones, can't remeber the size of the top of my head) can reduce the unsprung weight of the car, as they are lighter than serria rear disks.... So you could say they are better.....

[Edited on 18/12/05 by Bluemoon]

Piddy,

Because they look so much better than drums

Oh yes, I must agree with that.

[Edited on 18/12/05 by piddy]

quote:

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Originally posted by fesycresy
Sorry, but rear discs are essential. Are there drums on a Veyron ? I think not.

Seriously though, buy the cheapest Sierra and find a rear beam from a 4x4.

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id use a pair of inboard bike discs, so there less weight than either, especially unsprung, and less bias probably needed.

and drums are more than enough for a 7.

[Edited on 18/12/05 by JoelP]

Discs do look much better though

9" Sierra drums have more than enough braking capacity for a road Locost unless you intend going over the 200 bhp mark. Very few front engined road cars actually get any advantage from rear discs. The weight distribution on a Locost is usually around 50% front- 50% rear because of weight transfer this means that the rear brakes must do between 30% and 33% of the work in a full 1g brake application --- to put it another way even on a heavy build each rear wheel is only slowing about 100kg

Doing the sums
To slow a ladden Sierra estate from 70 mph to zero the each rear brake must sink around 190 KJ of energy.

To slow a Locost from 70 mph to zero each rear brake sinks less than 60 KJ

Of course where the Sierra is doing 70 mph the Locost is at 100 mph so doing the sums to slow from over 100 mph to zero each rear brake sinks less than 120 kJ --- still well below the Sierra at 70.

There are other issues with drum brakes -- pedal response, recovery time and how they respond when you push them beyond the fade point this implies for pure circuit work the disc brake will always be the first choice but for road use drums are more than adequate.

Something else to consider, drum brakes tend to be more reliable rear calipers

To add my 2 penny worth.

There is absolutely nothing wrong with the stopping power of drums and they are lighter than the disc setup. There is however not much in it.

They are cheaper to replace the friction material and offer a more responsive handbrake.

I have gone to considerable effort to use a drum brake set up with a LSD (normally only compatible with discs) due to the (minimal) weight saving and the fact I got a drum brake back end for a great price.

I wouldnt be swayed violently either way, but if its looks youre after - discs win every time, if its lightweight and perfectly adequate stopping power - its drums.

Rgds

Rich

You could fit drums and put some of those classy looking false disks on, they fool everybody.

I've never owned a seven, but all the cars I've had with drums at the back have given me some grief:
They never seem to adjust properly (esp the self adjusting ones) leading to too much pedal movement before they bite.
if you have steel wheels, the flex in the wheel can distort the drum slightly leading to slight rubbing noises on cornering.

The cars I've had with discs all round have had a much better feel. This might not be because of the rear discs, but I believe it has an effect.

My choice would be discs

Drum brakes have the same stopping power as disc brakes for the same area of friction material used.

Disc brakes cool better and so have less fade, assuming your car is heavy enough to heat them up in the first place.

Disc brakes drain better when they get wet, inless you have cross drilled drums.

Drum brakes make better handbrakes

Drum brakes are lighter (at least as taken off the back of a sierra)

Drum brakes are not as fashionable and that is a big part of why they are not on a bugatti. Steel rims are usually lighter than alloys but you won't see them on a bugatti for the same reason.

Area has little to do with stopping effect. The friction of disc brakes isn't quite good old O grade physics Coulomb friction but close enough. Disc brakes generally have a friction coefficient of between 0.25 and 0.41, this of course varies with temperature.

With drum brakes the friction model is more complex because both the drum and brakes shoes distort as a result of mechanical forces much more than comparable discs and calipers, so the mechanical performance of drum brakes falls off more at high pedal pressures. Drum brakes with a leading shoe have a self-servo effect built in.


Increasing brake diameter on both discs and drums increasing the stopping effect and heat dissapation. The stopping effect increases because the friction force is applied at a larger radius. The heat dissapation increases because of the increase in swept area.

Increasing the width of a drum brake dosen't increase the braking effect however it increases the surface area available to cool the brakes.

With drums when the brakes get hot the drum diameter increases due to thermal expansion this causes incresed pedal travel long before the fluid starts to boil. Disc brakes don't suffer from this problem.

Self adjuster on drum brakes are generally better than discs although the self ajusters on modern floating rear caliper disc brakes such as used by Ford, Rover, VW and PGA is pretty good. Any problem with self adjusters on either type of brake are usually due to not giving the hand brake cable enough slack to allow the adjuster to work.

Drum brake shoes tend to last for at least twice the mileage of disc pads.

All good stuff.

I really wouldnt get hung up on which is best for this application. As you can see, there isnt a lot in it, its more important to see what comes along at the time for a reasonable price.
To put it in the context of your original question, no, discs dont matter

Rich



[Edited on 18/12/05 by RichieC]

I'm using discs just because that's what happend to be on the donor I bought.

When I asked at halfords about rear bearing prices they said it was £40 a side if you had ABS and £10 a side if you didn't. I'm guessing the difference is disc/drums rather than ABS/no ABS.

So having discs might end up costing me an extra £60 Im hoping there's somewhere cheaper though or that they come down in price at some time.

I wouldnt put too much emphasis on what halfords say, they quoted me £19.99 a side for bearings for drums the other day, when Id just bought the fronts for £6 odd a side.
The conversation went something like this:
can you give me a price for a rear wheel bearing kit for a 1990 Sierra please (cautious of not giving him too much info to begin with)
Ford?
yes
what model?
sierra
petrol or diesel?
petrol
carb or injection?
carb
what year?
still 1990
they didnt make them in 1999
I said 1990
oh
front or rear?
rear.....still
ABS or non?
non

This was just a couple of days after telling me that most cars dont have CV joints as they dont have rear wheel steering < honestly and why would I want to replace them anyway. When I asked the chopper what he thought I meant, he said, Oh my car doesnt have CV joints - its a Saxo..........riiiiiiigggggggggghhht I said and left.

Apologies to anyone who works at the god forsaken place and actually knows their ar5e from their elbow, but I cannot understand why the management of some stores put such buffoons on the parts counter, surely they could be better employed selling overpriced go faster bits to like minded knuckle draggers



[Edited on 18/12/05 by RichieC]

i went for a rear disc set-up, because i think they look a lot neater, but in doing so, ihave had a lot of problems.
i failed the 1st sva on therear balance being well out, and handbrake trouble. after some thought, i bought some new rear calipers, and while the balance problem was sorted out, it failed sva no.2 on the handbrake wither not being effective enough, or binding.

im still happy i went with discs, but im not sure they were worth the hassle in the long run.

i think the best approach would be to stick to drums till all the paperwork is out of the way, and if you still prefer discs at a later stage, upgrade (which is pretty simple to do, only needing a bracket and calipers)

Tom