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Cross-Drilled and Slotted Disc Rotors 

 

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 Post subject: Cross-Drilled and Slotted Disc Rotors
Posted: Fri Jun 27, 2008 1:24 pm 
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The recent rise in popularity of these discs has brought some confusion as to the right products for particular applications.

As we know, most brake discs have a smooth, flat surfaces. Holes and slots in the face of the disc have a number of purposes.

The first is to remove gases from the face of the disc. Disc pads, when hot, expel gases. These gases form a cushion between the face of the disc and the pad, and can greatly reduce the coefficient of friction.

It takes a tenth of a second to squeeze these gases out on normal rotors. This does not sound like a long time, but consider this. When a vehicle is traveling at 100 km/h, it is moving at a rate of 30 metres per second, therefore a tenth of a second is three metres. So when the brakes are applied the vehicle travels for three metres squeezing out gases, and not creating friction to slow the vehicle.

Another problem that occurs when the build-up of gases is not released is that the pad material becomes hardened and glazed, greatly reducing the amount of grip between the pad and disc. Cross-drilling and slotting allow these gases to be removed immediately, and also helps to deglaze the pads, increasing the grip between the pad and disc, hence shortening the braking distance.

Cross-drilling and slotting makes the disc surface uneven so water and dust cannot develop into a thin layer that becomes a smooth, glass like surface and can greatly reduce the coefficient of friction.

Cross-drilling and slotting works effectively to reduce the main problems that occur in brake systems. But there are some tradeoffs, such as reducing pad life by approximately 10 per cent (if your getting 40,000 km from a set of pads this can be reduced to 36,000 km, though most would agree this is a a small price to pay for better braking performance). Also, the cross-drilled rotors are more prone to cracking under extreme conditions, such as racing.

In normal motoring the slotted-only and cross-drilled and slotted rotors have similar performances qualities. So the question is: do I fit cross-drilled and slotted discs, or just slotted discs?

This question has to be asked: what is the main purpose of the vehicle? Is the vehicle used for racing, or driven extremely hard? Does the vehicle go off road? If the answer is yes to either of these questions, slotted-only discs are the best choice.

If a vehicle is only used on the street, but is occasionally driven hard, and has nice open wheels where the rotors need to look good as well as perform, cross-drilled and slotted rotors are the way to go.

 

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Posted: Fri Jun 27, 2008 2:59 pm 
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nice. Here's my input:

Manufacturing:

Most cheaper disc rotors (ie those used with stock brake set ups) are generally manufactured as solids, with the slots and cross drillings made later. The stress of making these incisions inherently makes these rotors weaker, however cost justification wins here. This is more noticable on with cross drilled rotors due to drilling placing more stress on a concentrated area than slot incisions (as well as greater weakness in general due to cross drilling design).
Brembo's and other higher end brake rotors have slots cast into them (I'm not sure about cross drillings) and I know OEM Porsche brakes that are slotted and cross drilled have both slots and cross drillings cast into their design.


Warping & Cracking

Probably the most likely of scenario's to cause the death of rotors on falcons. It is caused by two things, firstly sharp changes of thermal energy within the rotor and secondly the difference in temperature between frictional area and the inner area of the rotor. Now note that the total amount of thermal energy the disc can absorb is dependant solely upon the disc material and volume. Also cross drilling decreases the ability of the disc to transform kinetic energy to thermal energy by reducing the surface area of contact between the disc and pad, resulting in harder braking force needing to be applied to the remaining rotor face for similar braking. The effect when brakes are used with cross drilled (and slotted to less effect) is this:

For the same braking effort, the same kinetic energy will become thermal energy, however as there is a lower thermal capacity of cross drilled and slotted rotors the temperature will raise higher, faster. This both creates a sharper change in temperature and creates a greater difference in temperaturee between the outside and inside of disc. Result: more prone to warp (not good in a car that has brakes prone to warping in the first place).

I have more to type but cbf atm lol.......motivation in the last 2 minutes just plummeted

 

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Posted: Thu Jul 03, 2008 10:32 pm 
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from DBA slotted only rotors can take more than the slotted and cross drilled ones. i had a customer who had a VT bombadore and the SX ones kept cracking up, with advise from DBA we went to S, problem solvered

SX-slotted and cross drilled
S - slotted only

 

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 Post subject: brakes
Posted: Fri Jul 04, 2008 12:24 am 
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Practically every thread i read about slotted and cross drilled is that nearly every person using DBA or RDA cross drilled and slotted have had problem eg stress crack & warping the 2 main things i noticed.

Most ppl using slotted dont seem to have as many problems so i choose DBA slotted all round with Bendix ultimat pads and what a difference in braking. Well worth the money

 

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Posted: Sat Jul 05, 2008 12:43 am 
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I fitted DBA slotted to my el ghia with bendix ultimate pads. Within 4000 km they had started to shudder when braking at 80+ km/h. I had them machined and after being a bit noisy for a while are now fine. The only thing is that between each slot half of the rotor face is shiny silver and the other half is much darker.

i don't know why, maybe someone else does.

 

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Posted: Sat Jul 05, 2008 1:43 am 
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Sharpyelghia wrote:
I fitted DBA slotted to my el ghia with bendix ultimate pads. Within 4000 km they had started to shudder when braking at 80+ km/h. I had them machined and after being a bit noisy for a while are now fine. The only thing is that between each slot half of the rotor face is shiny silver and the other half is much darker.

i don't know why, maybe someone else does.

contact DBA - they will help you.
I called up p****d right off about it, they told me to bring the car arround, had a look and said that it was the pads. i had bendix CT's - and apparenlty my driving style was making them leave meterial discharge on the rotors - and the CT pads were good for doing this, they contacted bendix and got me a different set of pads sent out - been good ever scince.

 

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 Post subject: brakes
Posted: Sat Jul 05, 2008 2:26 am 
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Mitch_

What pads are you using now?

Apparently DBA recommend to use some sort of cevlar coating pads or something?

 

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Posted: Sat Jul 05, 2008 11:35 am 
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they sent me some bendix heavy duty - not overly impressed by them, really dusty, but they do the job for the moment.
I'm thinking of trying EBC Green Stuff...

 

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Posted: Sat Jul 05, 2008 11:43 am 
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From my experience with the change from standard to slotted discs and ultimate pads, you need to leave braking real late and stand on them, as they need to be heated to work efficiently. Light touches only glazes up the pads and discs.

 

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Posted: Sat Jul 05, 2008 11:46 am 
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Thats ridiculous to have to drive like that on the street!

 

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Posted: Sat Jul 05, 2008 11:56 am 
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Not really,-I perhaps overstated the case, but there is a need for heat, more so that with standards. Anyone with a bit of experience driving will handle it with ease!

 

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Posted: Mon Jul 07, 2008 2:58 pm 
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Do you think a day's worth of solid hard braking (track day, etc) will 'deglaze' the rotors? I do 400km of highway driving a week, so light braking is all i do.

This is the front drivers side. She still pulls up smoothly and consistently but opinions would be appreciated.

 

 

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front right rotor.JPG
front right rotor.JPG [ 17.29 KiB | Viewed 13467 times ]

 

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Posted: Thu Jul 10, 2008 9:47 pm 
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I installed DBA 4000 series slotted with Bendix SRT. They stop now as before had no steering after going down a long hill. Don't leave must dust either.

 

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Posted: Sun Aug 03, 2008 11:05 am 
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I found EBC Green pads to work extremely well with cross drilled and slotted rotors (until they started leaving deposits and shuddering under light braking). With just slotted rotors they felt no different to Ultimates but wore the discs much less.

I fitted DBA Drilled front discs several years ago along with EBC Green pads (all new) - the braking was nothing short of A-M-A-Z-I-N-G .... I swear my eyeballs felt like they were going to pop out when braking hard from high speed and round town I found myself braking late and hard without even thinking about it. Tyres were Dunlop FM901's. PROBLEM was, the car developed slight brake shudder under light braking - there were visible material deposits on the discs - so I went to slotted only - with a huge noticeable loss of performance but there was no more deposit buildup and no shuddering. I've since wondered if the deposit buildup was just the EBC pads breaking in and maybe I just needed to sand it off and all might have been well.

From reading people's experiences etc. I'm going to try another set of drilled / slotted rotors and QFM pads next.

 

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Last edited by Greenmachine on Fri May 06, 2011 3:59 pm, edited 2 times in total.

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Posted: Sun Aug 03, 2008 2:11 pm 
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Sharpyelghia, my brakes behave exactly the same as yours. My discs have exactly the same markings. I have had shuddering no matter what I try. New discs, machined discs, new bearings, I only get about 10 k of driving before they start shuddering.
I have tried a few different Bendix pads, Advance, Ultimate, CT. The Ultimate's brake well but dusted too much. Advance are great low dusting with resonable braking. I am wondering about EBC greenstuff but $140 a set is a lot of coin for pads. They better be worth the price.
Everyone who has used them on here does seem to recommend them

 

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