Disc wear.

Post by **wanderinstar** » Thu May 17, 2007 6:00 am

Now as many of you will know I don't have discs on my Moggy.
However this question is on of curiosity. Do you people with discs suffer from disc wear/rusting. My reason for asking is we have an 03 reg Clio which has just been for its MOT and the tester advised changing discs shortly due to wear, on a 3 year old car. We used to have a Ciroen AX before that and that needed new discs every year due to rust.
Has anyone else suffered the same?
Ian.

Post by **Alec** » Thu May 17, 2007 6:33 am

Hello Wanderinstar,

rust on the discs isn't usually a problem on cars that have regular use. However, modern friction materials are quite abrasive so regular disc replacement is required.
I'm sure that the current view on driving doesn't help either, I'm referring to the gears for go and brakes to slow philosophy. The oft touted reason being that brakes are cheaper to repair than gearboxes\clutches. I'm not so sure that this hold true any more.
I still use the throttle\gears a lot and find that I brake for far less distance than most of the traffic arround me. (I am aware of following traffic and take account that they are not surprised by this technique.)
Apart from that it is more economical on fuel rather than keeping on the throttle and then braking.

Alec

Post by **8009STEVE** » Thu May 17, 2007 7:05 am

On trucks, it is usual to change the disc/ drum every other pad/shoe change.

Post by **Alec** » Thu May 17, 2007 7:15 am

Hello Steve,

I notice that truck drivers like to follow each other very closely, so I guess their brakes get a lot of work, that and the load they carry, of course.

Alec

Post by **8009STEVE** » Thu May 17, 2007 8:13 am

I notice that truck drivers like to follow each other very closely

That is because they are idiots! and want to die.

the load they carry

any thing up to 44 tonnes gross weight.

Post by **bmcecosse** » Thu May 17, 2007 8:31 am

Errr - Wanderinstar has a monster truck!!!
Ian - the pads these days are loaded with iron dust - hence the mess they make of the wheels - and the rate of wear can be high. However - it is a standard 'rip-off' at the garage to get you to pay for new discs as well as pads. The good news is that discs are cheap - and dead easy to fit - I recently put new discs and pads on my daughter's 3 year old Seat Arosa - 25k miles - and they cost less than £50 all in and I had them fitted in an hour!

Post by **JimK** » Thu May 17, 2007 8:46 am

There are a variety of reasons, both for disc wear and the "brakes for slow" thing:

Discs that aren't used for a few days gain a surface layer of rust, which you'd expect. When the brakes are next used, that rust is scraped off, resulting in accelerated wear relative to discs that weren't allowed to go rusty. Rust between the disc and the hub can push the disc out of line, too.

As has been noted, modern asbestos-free pads are more abrasive, so wear the discs quicker. My Fiesta had three sets of front discs in 120k miles.

The "brakes for slow" comes to a large extent from the fact that modern engines don't provide nearly as much engine braking as older engines:
In an A-series, when you lift off the throttle the butterfly closes but the engine is still trying to suck air through. The resistance caused by the closed throttle slows the engine considerably.
In modern engines, when you lift off the throttle and the intake buttefly closes, another valve opens that draws exhaust gas around to the intake, reducing the resistance felt by the engine and consequently reducing the engine braking effect. The only engine braking you get now is the internal friction and inertia of lumps of metal moving, and even the friction is less than it used to be.

So, if you try to slow a modern car by lifting off the throttle and changing down, you don't get any appreciable braking effect unless you change down earlier than the engine would normally be comfortable with.

Taking that along with the simple fact that brakes are more effective and reliable now, you end up with the "gears for go, brakes for slow" mantra. I'm not saying I entirely agree with it, just that there some good technical reasons for it.

Post by **bmcecosse** » Thu May 17, 2007 8:50 am

Except that many cars now are diesel - with NO throttle. They have excellent engine braking!

Post by **Rob_Jennings** » Thu May 17, 2007 8:59 am

I run a modern diesel and as Jim says it has very little breaking effect when the throttle is lifted.

I guess its the engine design and the fact that the car must weight 1.5 tonnes yet the power to weight ratio from the engine is very good, so there really is a lot more body work and equipment to slow down compared to the minor.

Post by **JimK** » Thu May 17, 2007 9:07 am

bmcecosse wrote:Except that many cars now are diesel - with NO throttle. They have excellent engine braking!

You're sort of right when you say they have no throttle (in that there is no control over the air intake volume) but of course they have a throttle if we mean a control over engine speed.

In a diesel the energy required to compress the intake air by 16 to 20 times has to come from somewhere, and in the absence of any fuel it must come from the kinetic energy of the car and engine components. The programmer can mitigate this effect by adding small amounts of fuel at the correct time, enough to compensate for some but not all of the resistance.

Rob is experiencing exactly as much engine braking as the ECU programmers want his car to have.

Post by **alex_holden** » Thu May 17, 2007 9:11 am

JimK wrote:In a diesel the energy required to compress the intake air by 16 to 20 times has to come from somewhere, and in the absence of any fuel it must come from the kinetic energy of the car and engine components.

But the air acts as a spring and returns most of that energy during the power stroke even in the absence of fuel.

Post by **Peetee** » Thu May 17, 2007 9:53 am

Rob is experiencing exactly as much engine braking as the ECU programmers want his car to have

That is probably true as the emmission control equipment on the car will do what it can to eliminate unburned fuel whilst 'backing off' the throttle.

Post by **plastic_orange** » Thu May 17, 2007 11:40 am

My father (80 next year) is old school and seldom uses his brakes. The result is that every car (modern) he has owned has required new discs due to rust and ridging of the disc face. I tend not to have that problem as I make sure the brakes are used hard - ish on every journey to keep then clean and bright.
Have a look at parked cars and if you can see the disc you can tell how it's driven.

And yes, they are very cheap and easy to replace and are a must do if worn.
Pete

Post by **Peetee** » Thu May 17, 2007 12:54 pm

The trend for open alloy wheels also adds to the problem.

Post by **bmcecosse** » Thu May 17, 2007 1:51 pm

The diesels I follow all have good engine braking - my petrol engine car keeps trying to run into them as they lift off the throttle! Anyway - what happened to the good old Free-wheel that Rovers had as standard. Always seemed like a really good idea to me!

Post by **wanderinstar** » Thu May 17, 2007 5:57 pm

Alec, this car is used every day. Do people who have fitted discs to Minors suffer the same problem?

Post by **Alec** » Thu May 17, 2007 6:14 pm

Hello Wanderingstar,

we have had discs on our minor for a few years now, and this has not been a problem, pad wear even, is minimal so far.

Alec

Post by **Packedup** » Thu May 17, 2007 6:26 pm

JimK wrote: As has been noted, modern asbestos-free pads are more abrasive, so wear the discs quicker. My Fiesta had three sets of front discs in 120k miles.

I've heard it's not unusual for moderns to need new discs every other set of pads!

I suspect the Midget is on the original discs (as they're half the min recommended thickness - New ones going on very soon when I rebuild the front suspension) and I'll bet it's had more than 2 sets of pads in the last 42 years...

The Triumph I've got will probably have good used discs go back on, and again, I can't see why they wouldn't be as old as the car they came off (the ones currently on have loads of metal left, but have warped).

Whether or not brand new pads will start to eat original discs has yet to be seen...

Post by **M25VAN** » Thu May 17, 2007 8:21 pm

I've had discs for at least 15 years now. Pads get changed every 20-25k but the discs themselves have got plenty left on them. Mainly busy motorway driving so they do get stood on quite alot. Not sure why modern discs don't last as long.

Post by **jonathon** » Thu May 17, 2007 8:30 pm

Depends on the quality of the discs as to how long they last and the composition of the pads also the suitability to each other. One could buy a pair of vented discs for £1.00, ours are over 30 times that because we insist on top quality for a reasonable price, however we could easily reach £90 for a pair and some cases £150 for the very best quality all based on the same initial specification/application.