Drums vs. discs

[Sweeney921]

I was just wondering, what are the advantages/disadvantages of drum brakes and disc brakes? Is there anything that drum brakes do better than discs?

[megotmea7]

the major advantage of disc brakes is their ability to quickly cool during at after heavy use. drums are a more contained design so heat soak is a big issue as there is very little air circualtion to cool them down. most of a disk brakes friction surfaces are exposed to a moving stream of air so cooling is much les of an issue, esp. on commuter cars. the advantage of drums however ar they can be setup for no drag which can help in things like drag racing where every tenth of a second counts and repeated hard braking isnt an issue.

[Sweeney921]

interesting, but do disc brakes improve braking distances?

[jcp123]

As far as I know, yes. They also have more swept area, as I understand it, than drums do, and fading even inside the same braking run (start to finish) is far less. Having driven Mustangs with manual AND power drums and discs (gives a fairly even basis for comparison), the discs also seemed to have much better responsiveness vs. the drums. Also, even with power drums, I was never able to lock up the wheels, where the power discs I have now have no trouble doing it, so it seems the discs just have more bite, too.

Personally, I liked the feel of the manual discs best; good responsiveness and feedback.

[Egg Nog]

For the past many few decades, most cars used discs at the front and drums at the back. I believe this was because it was easier to put a powerful brake up front and a weaker one at the back to get the desired brake application than it was to have powerful ones all-around and have a finicky proportioning valve. A lot of sports cars (including the 914 :)) would use 4-wheel discs just because it would give better performance overall.

[jcp123]

If it weren't so expensive, I'd love to ger 4-wheel discs on my car...

[Sweeney921]

A lot of sports cars (including the 914 :)) would use 4-wheel discs just because it would give better performance overall.
and the volvo 850 :D. It's very surprising how big the diameter of the discs are, I might go outside and measure them tomorrow, since it's dark outside here.

[Niko_Fx]

Drums are also cheaper. So on a Honda Civic, which is not a heavy car and with poor acceleration they fit well. My car breaks pretty good I must say, no problem whatsoever when braking.

[TheOne]

my eclipse breaks fine, it has discs at the front and disc/drum at the back(dunno why but yeh, its a disc/drum at the back)

[KnifeEdge_2K1]

the internal geometry of drum brakes allows an easier hand brake/ parking brake setup since there'll be no need to have internal drums inside each hub or axle where as the handbrake mechanism can be intergrated to the drum brakes themselves, also this setup (drum brakes) provide alot of the braking power themselves which means a vacumm (i think thats what its called) booster isnt needed, however the disadvantages drumb brakes have outweigh the advantages by far

the reason why most cars (in the not so distant past) used drums in the rear are because on the road you want more braking power in the front then the rear because of the weight transition, locking the rear tires under braking can be possible if the brake bias isn't right, however 4 wheel disk brakes and abs system in today's cars can accomplish the same feat which is why we're seing dum brakes being faded out

[PerfAdv]

Major advantages/disadvantages given above but also consider:
-drum brakes are cheaper to service and in a commuter vehicle where high performance is not an issue, are an acceptable substitute.
-drum brakes, as the name implies is sort of an enclosed cylinder. Heat buildup is an issue and they can also hold water. In a vehicle that might have to operate in water at a boat dock for example, will flood their brake drums with water and braking power will be reduced until the water drains. With street flooding this would also apply.
-For alloy wheels designs which show through, a drums setup in the back is unsightly.

[henk4]

For the past many few decades, most cars used discs at the front and drums at the back. I believe this was because it was easier to put a powerful brake up front and a weaker one at the back to get the desired brake application than it was to have powerful ones all-around and have a finicky proportioning valve. A lot of sports cars (including the 914 :)) would use 4-wheel discs just because it would give better performance overall.

The Citroen GS was already fitted with 4 discs when it got into production in 1970.

[jcp123]

'71 and up Cadillac Eldorados featured 4-wheel discs, with other Cads not far behind and I could be wrong but Corvettes might have had 4-wheel discs before '70, too

[henk4]

'71 and up Cadillac Eldorados featured 4-wheel discs, with other Cads not far behind and I could be wrong but Corvettes might have had 4-wheel discs before '70, too

I mentioned the GS because it was a mass produced small car. There were alerady many European top sportscar fitted with four discs and also the Renault R8 from the sixties, but I can't remember by head wether it had 2 or 4.

Anyway, the use of drums at the rear was considered to be cheaper at the time and that's why they kept on being used so long.

[jcp123]

Oh, ok.

Yeah, until today even. My car even uses the disc/drum combo straight off the '77 Granada we jacked the brakes (and a bunch of other stuff) from.

[PerfAdv]

Dunlop developed the Disc brake in 1952. They made it into some british production models in the early to mid-50s.

[Matra et Alpine]

Drum brakes can put more friction material into contact with metal for braking effort than similarly sized discs can. So you still see Drums on large commercial trucks where LOTS of material is needed for the HUGE braking effort to bring a few tens of tons to a stop !! Trucks get around the limitations with HEAVY thick drums and air-assisted braking. Neither of these are practical in a sports car or small family saloon :)

Discs give a much better 'feel' to braking and don't need as complex adjustment.

Disc brakes are more costly to desisgn and manufacture BUT are MUCH, MUCH cheaper to service. Nothing needs dismantled and adjusted to replace friction material ( well unless it was a Citroen and there you had to remove the manifold to get to the rear pad of the inboard discs :) )

[PerfAdv]

Disc brakes are more costly to desisgn and manufacture BUT are MUCH, MUCH cheaper to service. Nothing needs dismantled and adjusted to replace friction material ( well unless it was a Citroen and there you had to remove the manifold to get to the rear pad of the inboard discs :) )

Speaking from personal experience with Mazda sedan I used to have. Whenever, I had the brakes serviced, it had front discs and rear drums, the discs would cost more. The brake pads were more expensive than the shoes required for the rear drums. Also, the drums didn't need resurfacing where as the discs did, adding to the overall cost as well.

The rotors in a disc brake setup are also more likely to need maintenance than a drum. As I have had rotors warp, which then required resurfacing. This then made the rotors thinner and even more susceptable to warping. The need for resurfacing, has atleast for me, cost extra on replacement rotors. In my experience discs cost more than drums to maintain.

A possible explanation: disc brakes on front wheels take most of the abuse, especially in a FWD car.

[Matra et Alpine]

Speaking from personal experience with Mazda sedan I used to have. ....
A possible explanation: disc brakes on front wheels take most of the abuse, especially in a FWD car.
Wow, another example that convinces me that you guys get sold a piece-of-shit at every turn. Your price for a new car may be cheaper than over here, but you clearly are "getting what you paid for" :(

Only ever had one car where the front rotors needed replacement and that was a FORD and the service manager said that it was cheaper for Ford to replace the rotors for those who complained about the warp than to spend the extra pound getting them made from better material. I've not owned another Ford since :)

With decent pads and standard rotors I've done 80-100,000 miles.
I've done upwards of 10,000 miles of competitive driving and not had the problem.
On the drums v discs wearing debate, most folks don't bother to check drum wear untill it is VERY bad. Discs are easier to check and hence are changed at the right time rather than later.

Left-foot braking puts HUGE loads on discs much more that 'normal' driving and I've not had rotors warp or wear enough to need resurfaced. Course I change the pads as soon as they wear to THEIR limit - if you don't the risk is higher to damage the rotors.

ah, well it'll be interesting when Chrysler start importing POS to see how many rotor replacements we get before they realise we won't buy POS more than once !!! :)

[PerfAdv]

It's funny you mentioned a Ford having rotor problems. As Ford owns part of Mazda....cheapest parts is not a good way for quality: job #1 or whatever. I didn't have any brake problems with an Integra I had, which was also driving with more verve than the Mazda.

Something else I've noticed is that European cars tend to get a lot more brake dust buildup. Showing they use softer pads designed to wear away while not harming the rotor. Whereas a harder pad material would cause more rotor wear.

We definitely get suckered with inferior products in the US. Especially those that feel they have to buy American are doomed to defending their actions/problems. Even the gasoline is bad, it caused compression loss in the first BMW V8s of the mid-90s. The block composition couldn't handle the high-sulfur content, it was corroding the cylinder wall. Since then BMW changed to a high nickle and silicone alloy(Nikasil) for the engines.

Give the consumer the cheapest product regardless of possible drawbacks. Its not just automotive, witness the hormone and antibiotic fed poultry and livestock, genetically moded(GMO) grains and veggies, the list goes on... If a politician points to any of these things he's a socialist, marxist, pinko... Don't mess with the CORPs...

[Blue Supra]

Iv got discs on the front of my car and drums on the back, as hard as i stand on the brakes ive never been able to lock up the rears but have the fronts. Can someones explain this?

Also when i brake hard i a shudder through my steering wheel. What does this mean?

[henk4]

Also when i brake hard i a shudder through my steering wheel. What does this mean?

That may mean that your discs are slightly warped.

I changed my front discs after 160.000 km's.

[Blue Supra]

Well this cars got 280000ks on it and i dont think the rotors have EVER been replaced.

[KnifeEdge_2K1]

Iv got discs on the front of my car and drums on the back, as hard as i stand on the brakes ive never been able to lock up the rears but have the fronts. Can someones explain this?

Also when i brake hard i a shudder through my steering wheel. What does this mean?

well since weight transfers to the front during braking usually it would be the back that locks (assuming 50/50 brake bias which is very unusual), newayz drums dont have as much "bite" as disks and since they arnt the best at getting rid of heat they will wear much faster then your disks, this coupled with a front brake bias is probably why you locked your front wheels more often

and also manufacturers dont really want you to lock ur rear wheels as opposed to ur front because that would be the equivalent of pulling the handbrake ... which means losta oversteer

[Blue Supra]

Im thinkn im going to replce the drums with discs anyway out of a TRX bluebird for the hillclimbs and downhills i do, the brakes feel yeh bad when it shudders like that. Nothing wrong with a bit og oversteer as long as u can control it:D

[Matra et Alpine]

Im thinkn im going to replce the drums with discs anyway out of a TRX bluebird for the hillclimbs and downhills i do, the brakes feel yeh bad when it shudders like that. Nothing wrong with a bit og oversteer as long as u can control it:D
Knifeedge is spot on, good post :)

You dont' want rear brake induced oversteer on roads.
It's helpful and necessary on loose surface rallies and in autotests.
BUT on the open road it is bad
WHY ? Because you don't have the engine on-throttle at the time it breaks loose. So it's more likely that as you try to feather power to keep it in the drift or to bring it back in line that it will either not pick up and you go off backwards as the rears are getting engine breaking OR that it snatches and you go into a tank slapper !!
Either way is NOT clever on public roads and just plain embarrasing on track - I know :)

If you're going to put discs on the rear, go for a smaller rotor, piston and pad AND fit a variable brake bias to allow you to set it up for the best handlng.

Good luck.

[Blue Supra]

good thing i read that Matra, i was just about to open a new thread asking what needs to be done with it...

I user understeer a lot on the track though, get it through the tight corners.. i js oversteer it so the back slips out and i kinda half drift half flick the car through, you know what i mean? it took me a while and a few embarressing moments to get it right but it works well for me now... is that something i shouldnt be doing?

[Matra et Alpine]

good thing i read that Matra, i was just about to open a new thread asking what needs to be done with it...

I user understeer a lot on the track though, get it through the tight corners.. i js oversteer it so the back slips out and i kinda half drift half flick the car through, you know what i mean? it took me a while and a few embarressing moments to get it right but it works well for me now... is that something i shouldnt be doing?

Well that's EXACTLY how I drove when I came back to track nearly 9 years ago. I was lucky and got some great tuition from some seriously good drivers. One of them after 3 laps asked me if I'd driven competitively before and when I said yes he asked if I'd rallied. 'owned' :( He said I drove like I was rallying and that track needed me to learn different lines, techniques and to THINK more about the 'weight' of the car. He got 3-4 seconds OFF my lap time by the end of the first 20 minutes !!

Scandinavian flicks are spectacular and necessary on twisties and rough surfaces. On most corners though they are the SLOWEST way round :(
( They're also SOOOOOOO much fun - until you lose it :) )

Anyway, on track it is smoothness that matters if you want to reduce lap times. Getting the weight transfer of the car to move smoothly and to transition at the right points to maximise turn-in and traction is key to moving quickly.

One of the GREAT movies of the 70s had Jackie Stewart driving a Ford Capri round a track and he was showing different techniques. The classic "club racer" he called it with fast in, hard braking, chucking it round the corner and scrabbling on exit and then the "smooth" one - his :) No guesses what was faster.

It's also less stressful and effort to drive it smoothly. I used to be knackered after a 20 miunte session on trackdays. Arms throbbing, wrists hurting and sweating like a pig. With a better race technique it's a helluva lot easier as well as faster :0 ( I still come in throbbing, hurting and sweating - but now I've gone faster :) )

I still keep both styles when I'm out having fun. So I'll do the 'controlled' drive and then do a couple of sessions of getting it sieways, flicking it and coming out on FULL power smoking tyres. Guess which one my kids used to love when they came to watch :)

[fpv_gtho]

My dad had this 1971 F250 that had god knows what done to it, but by the end of a mega customisation, the original F100 body had an F250 chassis with the original 300 Canadian and 4spd..with i think the drum brakes off an F350. the thing literally locked all the wheels from a slight tap on the brakes, and could crack your neck

[Blue Supra]

Well that's EXACTLY how I drove when I came back to track nearly 9 years ago. I was lucky and got some great tuition from some seriously good drivers. One of them after 3 laps asked me if I'd driven competitively before and when I said yes he asked if I'd rallied. 'owned' :( He said I drove like I was rallying and that track needed me to learn different lines, techniques and to THINK more about the 'weight' of the car. He got 3-4 seconds OFF my lap time by the end of the first 20 minutes !!

Scandinavian flicks are spectacular and necessary on twisties and rough surfaces. On most corners though they are the SLOWEST way round :(
( They're also SOOOOOOO much fun - until you lose it :) )

Anyway, on track it is smoothness that matters if you want to reduce lap times. Getting the weight transfer of the car to move smoothly and to transition at the right points to maximise turn-in and traction is key to moving quickly.

One of the GREAT movies of the 70s had Jackie Stewart driving a Ford Capri round a track and he was showing different techniques. The classic "club racer" he called it with fast in, hard braking, chucking it round the corner and scrabbling on exit and then the "smooth" one - his :) No guesses what was faster.

It's also less stressful and effort to drive it smoothly. I used to be knackered after a 20 miunte session on trackdays. Arms throbbing, wrists hurting and sweating like a pig. With a better race technique it's a helluva lot easier as well as faster :0 ( I still come in throbbing, hurting and sweating - but now I've gone faster :) )

I still keep both styles when I'm out having fun. So I'll do the 'controlled' drive and then do a couple of sessions of getting it sieways, flicking it and coming out on FULL power smoking tyres. Guess which one my kids used to love when they came to watch :)

Youd probably have to show me, im a visual learner...
but put simply (i know im going off topic...)
ie. in a paragraph
whats the fastest way through a corner?

[Matra et Alpine]

Youd probably have to show me, im a visual learner...
but put simply (i know im going off topic...)
ie. in a paragraph
whats the fastest way through a corner?
'racing line' has been discussed here in UCP before ( but sorry I can't find the tread ATM :( )

However, I'd been sent THIS in planning a trip with mates.
http://www.bmw.com/generic/com/en/fascination/bmwm/automobiles/specials/_pdf/bmwm_nordschleife.pdf
It covers all the issues in taking the corners of Nordschliefe :eek: , so you can see the importance of how the line changes for the corner conditions away from the ideal smooth breaking point->apex->wide exit flattening of a curve that is the 'traditional' view of perfect line.
Take these 2 options for a hairpin - car coming from right to left....
http://phors.locost7.info/images/17image01.gif
So if looking at fastest LAP time the theoretical lines non-drivers talk about is on the left and according to physics gives the fastest speed through the corner. However the line on the righ is actually the track that woudl give fastest lap time because EXIT speed is higher. So it;s slower just before the apex but the better traction on exit gives faster spped on the upcoming straght :)
But one corner doesn't make a fast lap and it's often been described as "Zen-like" the line a fast driver will adopt through a series of corners :) Comes with experience :)

[fpv_gtho]

I saw something about this in the handbook for Gran Turismo 2.....looked a bit like this

[Alastor]

The advanced racing line is discussed in more detail in The Physics of Racing Series, Part 17:

http://www.miata.net/sport/Physics/Part17.html

[Matra et Alpine]

The advanced racing line is discussed in more detail in The Physics of Racing Series, Part 17:

http://www.miata.net/sport/Physics/Part17.html
yeah, that's where the images are from.

I have a problem with that guys series as it's all the maths of the right corner. It tempts folks into thinkinf you can look at a map of a circuit and KNOW the fastest line. In reality, that gets you an approximation and then you need to 'feel' the best line as camber affects slip - something he just doesn't account for ( and yet is so precisely mathematical in everything else a major flaw ) He also avoids the difference the CoG of the car has on it's handling which affects the best line. FWD and RWD cars of same weight will have totally different lines !!!!

Rather than all the maths it's easiest to read a book from any of the great racers. They dont' cover the math, they describe a line and talk about feeling for the optimal. Just can't find any of THOSE on-line. Driver rather than mathematician view :)

[Alastor]

yeah, that's where the images are from.

I have a problem with that guys series as it's all the maths of the right corner. It tempts folks into thinkinf you can look at a map of a circuit and KNOW the fastest line. In reality, that gets you an approximation and then you need to 'feel' the best line as camber affects slip - something he just doesn't account for ( and yet is so precisely mathematical in everything else a major flaw ) He also avoids the difference the CoG of the car has on it's handling which affects the best line. FWD and RWD cars of same weight will have totally different lines !!!!

Rather than all the maths it's easiest to read a book from any of the great racers. They dont' cover the math, they describe a line and talk about feeling for the optimal. Just can't find any of THOSE on-line. Driver rather than mathematician view :)

The series is the physics behind racing and I don’t think it is intended to teach people how to drive, but if that is the idea they get then I agree it is wrong. The series is geared toward non-technical people who may or may not race and want to understand some of the vehicle dynamics behind racing. Too much complexity and the reader base will be discouraged from trying to understand.

Although he doesn’t mention everything governing vehicle dynamics there is still enough data to build a reasonable vehicle model and because of that I think it is worth reading.

[Matra et Alpine]

Too much complexity and the reader base will be discouraged from trying to understand.
All that math already does that for 80% of the population :)

Although he doesn’t mention everything governing vehicle dynamics there is still enough data to build a reasonable vehicle model and because of that I think it is worth reading.
Yep, it's all the hallmarks of a game developer rather than a real world drive :)
An excellent intellectual exercise and for engineers a good read. But - again - for the purpose of racing line and real-world driving most of it is irrelevant and better described in books on racing rather than mechanics.

[KnifeEdge_2K1]

damn it i was the one that brought that whole topic/website up man !!!

[Matra et Alpine]

damn it i was the one that brought that whole topic/website up man !!!
:) Any engineer who races sims and thought about writing one has read that guys papers. It's one of those well-known papers in the development community :) It appears EVERYWHERE :)

[Blue Supra]

Too much complexity and the reader base will be discouraged from trying to understand.


i was discouraged...:o
anyways im waiting for my suspension to get fixed up before i test these lines, after a nasty 70kph accident over a traffic island! stupid dog! what was it doing in the middle of the road at 6.30am!:mad: my cars been in the shop all day for the past consecutive 4 days and it has to go in for a re check tomoro, again for a check in a week and it may even need new parts :mad: , underneath it looked pretty scraped when it was up on the hoist and my gearbox and tailshaft are loose:eek: so thats why it clunks through 1st and 2nd apparently, and theres a leaking seal on my diff too, wow i didnt know so much could go wrong at once! still im looking at a 2L engine and a 5 speeder for it on monday so im hoping it will all be fixed by then!:) and i can try out those lines properly!

[Matra et Alpine]

and i can try out those lines properly!
Joking, right, please ????

Those lines are for race circuits.
Lines for fastest/safest on roads are totally different !!

ALL roads should be driven to their "vanishing point".
This is the point where you don't see the road anymore because it's over a hill or round a corner. You need to drive so that you can stop within that distance. ANYTHING else is dangerous to you and other road users.

This image and full description at http://www.sabernet.biz/~bobkarol/ridingin4d/ridingin4d.6.pdf

Please don't do 'racing lines' on public roads. Especially sngle track as their might be somebody coming in the opposite direction :(

Also,, by learnign these on track you get the opportunity to try a different line every couple of minutes, so the learning is fresh in the memory and you get to try to 'connect' them into a flowing, fast movement.

[Blue Supra]

clearly on a track... im not a complete idiot:p
i know what you mean by vanishing point, im always looking ahead of where i am to gauge speed and stuff, even when im just driving a nice windy piece of road...

[danno]

this is a very interesting/helpful/useful thread.

[KnifeEdge_2K1]

:) Any engineer who races sims and thought about writing one has read that guys papers. It's one of those well-known papers in the development community :) It appears EVERYWHERE :)

:P to u :D

[Blue Supra]

^^^
pwned:)

[crisis]

'racing line' has been discussed here in UCP before ( but sorry I can't find the tread ATM :( )

However, I'd been sent THIS in planning a trip with mates.
http://www.bmw.com/generic/com/en/fascination/bmwm/automobiles/specials/_pdf/bmwm_nordschleife.pdf
It covers all the issues in taking the corners of Nordschliefe :eek: , so you can see the importance of how the line changes for the corner conditions away from the ideal smooth breaking point->apex->wide exit flattening of a curve that is the 'traditional' view of perfect line.
Take these 2 options for a hairpin - car coming from right to left....
http://phors.locost7.info/images/17image01.gif
So if looking at fastest LAP time the theoretical lines non-drivers talk about is on the left and according to physics gives the fastest speed through the corner. However the line on the righ is actually the track that woudl give fastest lap time because EXIT speed is higher. So it;s slower just before the apex but the better traction on exit gives faster spped on the upcoming straght :)
But one corner doesn't make a fast lap and it's often been described as "Zen-like" the line a fast driver will adopt through a series of corners :) Comes with experience :)
Another point to consider is the camber on the road on either the approach and/or departure.

[crisis]

Drum brakes can put more friction material into contact with metal for braking effort than similarly sized discs can.
You can , for a price, equal this with dual calipers. I would think large discs would also be lighter than equivalently performing drums and as such offer less inertia and unsprung weight. As stated previoulsy though I think cooling is one of the main benifits. Brake fade with drums comes quick and fast.

[Blue Supra]

what is brake fade in drums? ive heard it refer to and i have a slight inkling but im not sure.

[Matra et Alpine]

You can , for a price, equal this with dual calipers. I would think large discs would also be lighter than equivalently performing drums and as such offer less inertia and unsprung weight. As stated previoulsy though I think cooling is one of the main benifits. Brake fade with drums comes quick and fast.
Problem with disks is the more force you put through them it becomes nearly impossible to stop the buggars from warping.
Hence why big heavy trucks use drums and why competition cars replace rotors on a regular basis.

[Matra et Alpine]

what is brake fade in drums? ive heard it refer to and i have a slight inkling but im not sure.
It's exactly the same as for disks.
the firction material heats up beyond it's operating range and loses it's friction properties.
As drums are a LOT harder to cool than discs this is a bigger issue with drums. ( Hence why truck drivers will ALWAYS brake using the gears and engine as much as possible )
This has become a more sever problem with the loss of asbestos as friction material as metal sintered material isn't all that clever in drums. I saw an early set of metal sintered shoes STUCK to the drums as they had overheating so much that a layer of metal was melted onto the drum interior and stuck to the metal still in the shoes. Another one of the reasons for NEVER putting on the parkign brake when doign track days ( the main one is the brake can stop working as it cools )

[KnifeEdge_2K1]

maybe this has been said b4 but drum brakes have more friction material so theoretically they have more power but again the fade is an issue as well as their tendency to flood if they're submerged in water (like a big puddle of water or something)

[Rob Mann]

there is no comparison in a race car. big disk brakes will out perform drum brakes any day. however there are some slight advantages with using drum's but theses are not big enough and do not have enough effect to warrant drum brakes in a car.

[jcp123]

i got front disc/rear drum in my car. it's enough to lock 'em up.