Why is it always reccomended that brake disks should be replaced when they become grooved.
To my way of thinking grooving increases surface area between pad and disk.
Is there something I'm missing here.
Alvin
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The grooves in the pads have to match the grooves in the discs for Alvin's theory to stand up. It's usually rear discs that go groovy first. The reason is localised corrosion. All discs start rusting as soon as you park the car in winter. But the surface rust is cleaned off the fronts first time you brake afterwards. It isn't cleaned off the rears unless you brake hard from quite high speed because otherwise the rears don't get clamped by any force by the pads. So the surface corrosion goes deeper and the lumps of rust rust cause scoring which is how you ger groovy rear discs.
HJ
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HJ, in your opinion are rear disc brakes the way to go or should the industry stick to rear drums?
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If you have rear discs at least you can see when they are worn.The more worn your discs the more fluid is required in the system also heat disapation is less and added together you could lose considerable amount of braking efficiency.
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honest john wrote:
It isn't cleaned off the rears unless you
> brake hard from quite high speed because otherwise the rears
> don't get clamped by any force by the pads.
Cool! So my habit of doing handbrake turns in the Company carpark if I arrive early or leave late is doing my car untold good!!!
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Alvin
The actual surface area increase is marginal, and doesn't improve the brakes. Whilst in theory there could be a wedging effect for deep grooves, the downside is increased pad wear, and new pads take a while to bed in. So, the best all-round solution is flat discs and pads.
regards
john
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On my bike my discs are shiney and polished. I don't want to invest in new discs but I am certain that taking th epolish of th epads would improve braking.
Anyone care to comment?
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I meant off the discs.
I also meant to type properly!
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Dave, Your bikes discs are all nice and shiny because you operate them independantly, unlike a car. And this gloss is a work hardened surface that give your discs a longer life. S/S for eg. is quite soft and requires this surface to have much longevity...that's why they say to not brake hard on news ones for the 1st 100 miles or so...
As an aside, at home...NZ..we are allowed to machine grooved dics provided you do not exceed the manu. min. thickness...bit tricky as the two faces are not parallel as I found out the first time I did some...
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On the subject of brakes, my Honda Integra sticks every time I release the hand brake. Does anybody know a solution?
Andy
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Sorry to have to tell you, but friction force is independent of surface area - the equation is:
Friction force = 'coefficient of friction' x normal reaction.
In other words, the friction force depends only on the friction abilities of the materials multipiled by the force squeezing them together. (Obviously, the bigger the disc, the better the heat dispersal - a very small disc will simply melt!). All grooves will do is reduce the effectiveness of the 'normal reaction' as much of the force does not act perpendicular to the disc surface and the effectiveness is thus reduced according to the angle. Thus, believe it or not, the braking force is reduced as the discs become scored.
Also, believe it or not, I also have a life.
Martin.
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Martin -
So does that mean that the size of the pad is *only* related to heat dissipation ? Surely the surface area must be inversely proportional to the force required ?
> Also, believe it or not, I also have a life.
Wish I did.
Mark.
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Merk,
Yes and no....
Unfortunately, it is somewhat more complicated (like life!). The heat dissipation is just one (major) factor.
Pressure = Force / Area (over which the pressure acts). So, yes, a bigger pad area requires less force per unit area.
The bigger the surface area, the more that you 'spread out' the force required to achieve the same braking effect (assuming all other things are 'equal') and so heating effects, wear and so on become reduced. But the (seemingly) greater surface area available to a scored disc isn't all perpendicular to the normal reaction - in this case the direction of the caliper force - and so generates lots of extra heat for less braking force. This has all been proven in brake tests (I used to do them on jet engines!).
Also, a bigger diameter disc, with the pad acting at a greater radius, will also require less force than a smaller disc to achieve the same braking effect.
This also applies to tyres, where a wider tyre can handle more power before letting go and thus ultimately provide more grip.
The limit in both cases is the materials used.
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Mark (Brazil),
Sorry, I didn't mean to call you 'Merk'....
Martin.
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> Sorry, I didn't mean to call you 'Merk'....
that's alright, Mertin.
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Mark
What you're saying is that larger diameter discs alllow greater braking torque to be applied for any given pad preesure.
Regards
John
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John,
Yes, that what I was trying to say (in gobbldygook....).
Martin.
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John Slaughter wrote:
>
> Mark
>
> What you're saying is that larger diameter discs alllow greater braking torque to be applied for any given pad preesure.
> Regards
>
> John
If I was saying that, I don't think I meant to. I thought I was talking about an increase in the area of contact between the disc and the pad ?
Force*area etc. etc.
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I had a letter from a Porsche 996 owner a few months ago. His rear discs were so badly corroded at 10,000 miles he needed new ones. But the guy used his car exclusively in London and on motorways. Never took it to a track day or anything like that. Since the brakes on his car are designed to stop it from 172mph (or whatever) the rears don't do anything unless braking from quite high speeds. So the surface corrosion never got cleaned off his. I don't know where design can go from here. If the car is designed to brake safely from 172 on the autobahn, then the rear discs are going to rust badly if the car is used as a town cruiser.
HJ
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Don't handbrakes work on the rear wheels ?
If so, is it not a good idea to put the handbrake on very lightly and travel a short distance ?
Wouldn't this have a similar cleaning effect ?
Clearly I am not talking about jamming it on at high speed.
M.
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Sad to think of a Porsche being so little used.
I'm no maniac driver but I do keep front and rear discs very clean on all my cars. Brakes are there to be used as part of the whole driving thing not saved for an emergency!
The Land Rover is an exception, I expect the linings on that to last 10 years plus....lowish mileage and the fact that taking your foot off the throttle soon drops you 20mph with all that drivetrain drag. No need for brakes at all really.
David
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Mark (Brazil) wrote:
>
> Don't handbrakes work on the rear wheels ?
>
> If so, is it not a good idea to put the handbrake on very
> lightly and travel a short distance ?
Yes, my dad got a horse lorry through it's MOT recently after driving 7 miles with the hand brake on to clean them up.
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Dave,
Off topic for the rest but remember the friend's aunt who used to type up the books for Fred.
Was your friend the watchmaker/repairer's son?
If so this is getting very very close.
David
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David W wrote:
>
> Dave,
>
> Off topic for the rest but remember the friend's aunt who
> used to type up the books for Fred.
>
> Was your friend the watchmaker/repairer's son?
>
> If so this is getting very very close.
I'm glad you got round to that post.
It's a girl.
I think we may be off track here. Her father is v keen on repairing clocks as a hobby rather that business. So it's just possible we could be talking about the same person but I suspect our paths may have diverged a little.
OTOH how many secratary's/helpers could FA have had?
It's all a v. freaky cooincidence!
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Dave,
Actually didn't think it through, the chap who was Dad's mate was JC and lived in a cottage in Grafton. The lady who was his sister lived in Beckford I think.
JC had a job but was a watch/clock repairer/enthusiast for a very serious hobby.
I remember the had three wheeler bubble car things many many years ago.
David
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David W wrote:
>
> Dave,
>
> Actually didn't think it through, the chap who was Dad's mate
> was JC and lived in a cottage in Grafton. The lady who was
> his sister lived in Beckford I think.
>
> JC had a job but was a watch/clock repairer/enthusiast for a
> very serious hobby.
Ok, sit down and pour yourself a stiff drink. This coincidence thing could be going crazy.
Are we talking about ***n Cl*****d?
He used to live in Grafton now lives next to his sister in Beckford?
I know his daughter v. well.
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Sorry Dave, its going too far!
Yes he was one of my Dad's pals in the 70s when Dad was in the antique business. I spent hours and hours at the Grafton cottage with Dad, J and M (his good lady. They had just bought it and we watched much of the renovation process.
Haven't seen them for 15 years of more. Remember J as a very nice interesting chap.
David
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David W wrote:
>
> Sorry Dave, its going too far!
>
> Yes he was one of my Dad's pals in the 70s when Dad was in
> the antique business. I spent hours and hours at the Grafton
> cottage with Dad, J and M (his good lady. They had just
> bought it and we watched much of the renovation process.
>
> Haven't seen them for 15 years of more. Remember J as a very
> nice interesting chap.
Incredible. I see JC & M a few times a year. They and their Son and Daughter are v. well. I know the Daughter from school. He's just renevated a cottage for said daughter.
JC has retired and spend a lot of time pottering with boats etc. Top bloke. At one time they had e-mail. I'll find out if they still have and put you in touch!
I think we may be living in 'the matrix' and someone is trying to tell us through astonishing co-oincidence!
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Dave,
Very interesting. "Top bloke" would be my memory of J. I saw him a lot in that stage when I was still going about with parents prior to having a licence so all my contact was "second hand" if you see what I mean.
Remember me to them (bet they remember an idiot teenager!) and if they have a mail address do let me know. You can send it to me direct.
Thanks,
David
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David W wrote:
>
> Dave,
>
> Very interesting. "Top bloke" would be my memory of J. I saw
> him a lot in that stage when I was still going about with
> parents prior to having a licence so all my contact was
> "second hand" if you see what I mean.
I know exactly what you mean!
> Remember me to them (bet they remember an idiot teenager!)
Well if you want to impress them with how far you've come I could tell them that Dr. Woolard sends his regards. ;-)
> and if they have a mail address do let me know. You can send
> it to me direct.
Will do!!!
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Mark
The handbrake rarely uses the main brake pads - it is usually a separate system, usually being a small drum brake. Using the main pads would be a mistake. The disc and pads heat up and expand with use. If the pads were applied as a handbrake, the clamping pressure would reduce as the components cooled, resulting in a relaxation of the braking effect.
A drum brake on the other hand increases the load on the shoes as the drum cools.
regards
john
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John Slaughter wrote:
>
> Mark
>
> The handbrake rarely uses the main brake pads - it is usually
> a separate system, usually being a small drum brake. Using
> the main pads would be a mistake. The disc and pads heat up
> and expand with use. If the pads were applied as a
> handbrake, the clamping pressure would reduce as the
> components cooled, resulting in a relaxation of the braking
> effect.
>
> A drum brake on the other hand increases the load on the
> shoes as the drum cools.
>
> regards
>
> john
But doesn't that happen ? Or at least used to. That a car would roll away as the brakes cooled ?
Now, I have to admit its been a while since I messed with a car, but I don't remember any secondary set of pads or a small, additional drum brake on the back.
Now, the "drum brake" to one side, even if it was an additional smaller set, surely a few minutes with the handbrake on would still clean the disk, which was my point.
M.
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John,
You've been looking at posh cars with the separate brake drums inside the disc. Citroens and the like have the handbrake working on the normal pads via a lever/screw arrangement inside the caliper.
Hence the aggro in the early nineties with the Xantia's rolling away some time after being parked.
David
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Porsche are starting to offer optional ceramic brake disks which should nobble this problem. I believe they will be offering them as a retrofit option. I'm not to clear on the details but I remember seeing them at the motor show last year.
Also provides a significant reduction in unsprung weight.
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Replies to two threads here.
1) The sticking honda brake is not unusual. On a drum brake it usually indicates there is a lot of dust inside the drum. Some cars suffer more than others.
2) Applying the handbrake may not clear the overnight corrosion from rear discs if like on certain Vauxhalls and Mercs the handbrake is independent of the rear disc brake. Not forgetting the transmission brake on L-Rover and the handbrake works on the front of some Citroens and Saabs!
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