Grooves in discs (rotors)

[Alan Cherry]

OK Gents and Ladies,
A quick question. When looking at a front disc from the outside, should the grooves go from the centre out, or the outer edge in when the disc is rotated in a forward direction.
Reason for asking - i picked up a new set of discs from Hi-spec Brakes ( Monster Brakes ) this afternoon, as my existing ones have developed a nasty crack since the last race. I'm sure poster on sales office wall showed grooves going in to out,but on checking those already on the car, they go out to in ! - have to get car ready for Lydden on sunday- any directions much appreciated.

p.s. I've been told that drilling discs is mainly for boy racers, and can cause the disc to crack- old discs were drilled , so I've specifically got vented grooved discs that have not been drilled - good move ? or not

Many Thanks
Alan

[GORDON STREETER]

If you look at the direction of rotation the groove at 12 oclock should go from the center to the front of the car (if that makes sense)

[Tim Falce]

They can go either way depending on the application but Jeff at Hi Spec says have the groove going from the inside to out going backward so any water or crap is thrown out.
I don't see any point in drilling them either.
Word of warning with Hi Spec, check both discs for symmetry from one side to the other and disc to disc. I had one disc of a pair that had one face machined differently to the other so one pad started to wear with a shoulder on it.
Were your old discs from Hi Spec?

[Tony C]

I have run rotors in both directions depending on the event duration and whether I want to encourage cooling of the rotor or retain the heat (short events or cold/wet days).

Think of the rotor as a centrifigul air pump.

[Alan Cherry]

Originally Posted by falcemob

They can go either way depending on the application but Jeff at Hi Spec says have the groove going from the inside to out going backward so any water or crap is thrown out.
I don't see any point in drilling them either.
Word of warning with Hi Spec, check both discs for symmetry from one side to the other and disc to disc. I had one disc of a pair that had one face machined differently to the other so one pad started to wear with a shoulder on it.
Were your old discs from Hi Spec?

Old discs were from Rally Design ( The only people who had them in stock at the time - as usual needed them quick ) I'll check the machining -never thought it may be different.
Thanks
Alan

p.s. when are you or your uncle coming back out in the fester to play falcemob?
p.p.s. No slur against Rally Design intended, but some of their stuff seems a bit 'cheap and cheerful' ahem

p.p.p.s - I like cheap !

[Notso Swift]

I have bought different brands that have gone different ways, they have been coded left or right
(unless it is the right as you look at the car, or the right as you sit in the seat!)

[phoenix]

Originally Posted by Alan Cherry

I've been told that drilling discs is mainly for boy racers, and can cause the disc to crack- old discs were drilled , so I've specifically got vented grooved discs that have not been drilled - good move ? or not

No stratight answer, but Porsche have run drilled disc on competition cars for decades, which they would not do if there was no advantage. However Porsche also seem to be able to avoid cracks, which not many others can claim I believe.

[morrisnutter]

Originally Posted by phoenix

However Porsche also seem to be able to avoid cracks, which not many others can claim I believe.

They probably stress relieve them after drilling which is time consuming (thus expensive). I'm not sure of the benefits of cross drilled or grooves. The only reason they're there is to allow the gases to escape.

[AU N EGL]

Originally Posted by phoenix

No stratight answer, but Porsche have run drilled disc on competition cars for decades, which they would not do if there was no advantage. However Porsche also seem to be able to avoid cracks, which not many others can claim I believe.

I do believe a lot of the drilled, milled or cast in holes has to do with marketing and give what the customer wants, not really any benifit at all. Rotors with holes in where originally designed for outgassing of asphestous brake pads back in the late 50s and 1960s.. While asphestous is no longer used in brake pads, there is no longer a need for rotors with holes in.

Most manufactures state the air should flow into the center of the hub and out the veins. This is also were the brake duct hoses from the fenders should point too, the center of the hub to cool the hub and then the air is pulled out though the rotor too cool the rotor.

So when you look at a rotor from the side, the veins should curve back at 12:00 O'clock to pull the air OUT though the rotor, Not forward and pull the air down onto the hot hub and bearing.

Does it really make a difference which way the veins curve? for the most part, no.

I have lots and lots of photos of crached rotors, drilled, slotted and plain sided, only the rotors with holes in do crack faster.

[Dave Brand]

Originally Posted by AU N EGL

Rotors with holes in where originally designed for outgassing of asphestous brake pads back in the late 50s and 1960s.. While asphestous is no longer used in brake pads, there is no longer a need for rotors with holes in.

The basic composition of an organic friction material is a phenolic resin & a fibrous material, together with other ingredients, mainly in powder form, to modify the friction characteristics. Until the 1980s the main, almost universal, fibre used was asbestos. The banning of asbestos forced the use of different fibres, such as Kevlar, rockwool, glass fibre & several others. However, the resins remain the same; gassing is the result of the action of heat on the resins, so the requirement to provide an escape path for the gases is just the same for non-asbestos pads as it was for asbostos pads.

[AU N EGL]

Originally Posted by Dave Brand

The basic composition of an organic friction material is a phenolic resin & a fibrous material, together with other ingredients, mainly in powder form, to modify the friction characteristics. Until the 1980s the main, almost universal, fibre used was asbestos. The banning of asbestos forced the use of different fibres, such as Kevlar, rockwool, glass fibre & several others. However, the resins remain the same; gassing is the result of the action of heat on the resins, so the requirement to provide an escape path for the gases is just the same for non-asbestos pads as it was for asbostos pads.

That debate can go on for ever.

If some one want to use Drilled and slotted or cast in holes that is their choice. From our reseach and many other race teams, rotors with holes in vs the same rotor without holes in has a shorter life and will crack much sooner then a disk without holes.

Plus when you really dig deep into manufactures technical pages with holes in, they say rotors should be check for damage before each race, and or for marketing purpose only.

All rotors should be check in a pre-event tech inspection anyway.

[Dave Brand]

Originally Posted by AU N EGL

That debate can go on for ever.

There's nothing debatable in my original post. All I was doing was stating the fact that the requirement to provide an escape path for the gases is the same irrespective of whether or not the friction material is asbestos-based. (Unless, of course, it's a sintered metal material!) I made no comment as to how that should be achieved! It is the reason why many pads are grooved.......

[dtype38]

Cross drillings in the discs prevent the pads aquaplaning for a couple of revolutions of the wheel when you want to brake in the wet. Not noticable on cars, but most bikers can tell you about how heartstopping that lag can be when need to slow down urgently. Most bikers use drilled discs. Slotted face discs do the same sort of thing, they just instantly clear the water from behind the pad, but aren't as pretty imho. Don't think it matters much what the pattern of holes is or what direction the grooves point, just so long as they're evenly distributed across the pad. As for the gassing thing, not got any hard info on that, but sounds likely in high temp situations like racing, and obviously needs to be cleared from behind the pad for similar reason. Do remember though, that the pad can only work on metal, not air, so don't go for discs with too many holes or slots. That'll just reduce your braking capacity. :-)

[Dave Brand]

Originally Posted by dtype38

Do remember though, that the pad can only work on metal, not air, so don't go for discs with too many holes or slots. That'll just reduce your braking capacity. :-)

Friction is independent of area, so that statement doesn't hold true.

[MGDavid]

Originally Posted by Dave Brand

Friction is independent of area, so that statement doesn't hold true.

I've never quite understood that, if the area is nil (i.e. no disc) they still work ?

[johnny yuma]

My understanding is apart from allowing gases to escape,another function of slots, grooves or holes in a rotor is to allow a scraping clean of the deposits which can be embedded on the rotor surface when the gases which may not escape reconstitute back to a deposit on the rotor,in an irregular pattern which then builds on itself and leads to a mistaken diagnosis of warped rotor,as well as inhibiting braking function.
Apart from that the other truisms are centrifugal force is best not fought against so let the waste gases,debris whatever go outwards and ,with gravity downwards,and with airflow backwards,and make ducting help this.
Anyone got clues on little "scrapers" that treat the rotor ?

[tristancliffe]

Originally Posted by Dave Brand

Friction is independent of area, so that statement doesn't hold true.

On smooth body analysis that may be true (the sort you do for A-Level maths). But in real life surface irregularities mean that surface area does come into it. Otherwise cars couldn't corner or accelerate harder than 1g, wider tyres would just slow you down, and race brake pads would be tiny.

As it is, friction DOES depend on surface area, so we have wide slick tyres on formula cars, chuncky brake pads etc etc. Also, a larger surface area will reduce the wear rate.

[JohnD]

From the webpages of StopTech, that have attracted some most authoritative writers on braking matters.
See: http://www.stoptech.com/tech_info/wp...lections.shtml

___________________________________
DRILLED VS SLOTTED ROTORS
For many years most racing rotors were drilled. There were two reasons - the holes gave the "fireband" boundary layer of gasses and particulate matter someplace to go and the edges of the holes gave the pad a better "bite".

Unfortunately the drilled holes also reduced the thermal capacity of the discs and served as very effective "stress raisers" significantly decreasing disc life. Improvements in friction materials have pretty much made the drilled rotor a thing of the past in racing. Most racing rotors currently feature a series of tangential slots or channels that serve the same purpose without the attendant disadvantages.


John

[dtype38]

Mmm... I don't want to contradict that site, but I would like to point out that they make numerous statements of fact that rely on some rather large simplifying assumptions. Personally I'm not completely convinced. For example, the link you gave starts with the following as a given...

1) The brakes don't stop the vehicle - the tires do. The brakes slow the rotation of the wheels and tires. This means that braking distance measured on a single stop from a highway legal speed or higher is almost totally dependent upon the stopping ability of the tires in use - which, in the case of aftermarket advertising, may or may not be the ones originally fitted to the car by the OE manufacturer.

So the article is going to start with the assumption that we are only concerned with cars with braking power far in excess of the traction between the tyres and the road?

Doesn't apply to most racing cars then

And especially not to old Jaguars!

[GORDON STREETER]

Just to put my twopence worth in . The aftermarket drilled discs that I am asked to fit sometimes on road cars normally last five mins before warping and finish up putting std ones back on. I would think that some are just cosmetic rubbish probably made for 10 bob or less in some sweat shop and sold to the masses to look the part in the car park !

[Al Weyman]

I drilled a set of discs once on my Chevy and they cracked and as they form the front hub as well not a good idea. I also tried grooves but did not think it worth the effort and now just use the LE1 12" jobs from the chevy special parts bin as they come from the factory which bolts straight on and does the job and I can genuinely say I notice absolutely no difference whatsoever infact I think the brakes are remarkable good on my cars for what they are and remember the bloody car is like heavy with all steel and glass and goes on the scales at about 3200lbs!

[Alan Cherry]

Originally Posted by GORDON STREETER

Just to put my twopence worth in . The aftermarket drilled discs that I am asked to fit sometimes on road cars normally last five mins before warping and finish up putting std ones back on. I would think that some are just cosmetic rubbish probably made for 10 bob or less in some sweat shop and sold to the masses to look the part in the car park !

Just a quick note : The cracked disc had 4 disc exploding type cracks in it, each one starting from a drill hole nearest the centre of the hub. In their favour, they didn't warp like the ordinary just vented discs used to - but I'd rather have a bad vibration under braking telling me I have a problem rather than an exploding disc. p.s The grooved discs with the slots running 'outwards' seemed fine at Lydden on sunday in a 15 min race - pity about the driveshaft though !

[boyracer]

If I recall correctly, the V8 Supercars here have run different brake discs in qualifying to racing. The qualifying versions having a much more aggressive pattern presumably to clean the pad more effectively and retain that sharp bite for the duration of the qualifying lap. Must be fairly aggressive on pads to not use it for racing. Either that or they run a much thinner/lighter disc in qualifying.

[AU N EGL]

Here is something related to rotors:

So you still think holes in rotors are good?
This is for the engineering geeks out there like me! Try this little calculator that lets you know the stress concentration factors of a plate with holes. Put in the radius of the hole, the distance to the hole from another hole or edge and see how much faster the holes will make your rotors fail. We all know all rotors will fail, it is a matter of when and this will give you a factor of 2-4x less life before cracking. BTW, slots do a similar concentration factor. This is especially true in a brittle material like cast iron. Use the case of an infinite plate with 2 circular holes. For the case of 2mm radius holes 25mm apart, the stress is 2.91 times the "usual" rotor stress. Or to put it another way, you will crack 2.9 times sooner.
http://www.fatiguecalculator.com/finders/findkt.htm

Besides, the co-efficient of friction of a brake pad on air is very low as compared to cast iron. ( this one is funny )

[phoenix]

From the AP Racing site:

Disc Choice
AP Racing Formula Brake Kits come supplied with large diameter discs. The ventilated discs have either 24, 30 or 48 cooling vanes depending on the application, to draw air through the centre of the discs. They are handed left and right, and are cross drilled or grooved, again, depending on the application, to allow gasses that build up on the surface to escape. Where cross drilling is used it is more restrained than on the face of our full race discs, as pad longevity is more important on a road car than weight saving. The discs are wider and of a larger diameter than standard. The extra material controls heat build up and the larger diameter means that the caliper can be mounted further away from the centre increasing the leverage effect, which increases braking torque while decreasing effort required on the pedal.

Grooved Discs : G4, G8, T2, Grooves improve 'cleaning' of the pad surfaces and result in a more consistent brake performance. Grooved discs have a longer life than cross-drilled discs.

Radius Drilled Discs : RD, Radius Cross-Drilled discs offer improved 'bite' and 'feel', but are less resistant to cracking than grooved or plain discs.

http://www.apracing.com/roadcar/brakekit/choice.htm