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Senoj
Shanghai


Joined: 23 Jan 2006
Posts: 4894
Location: Sussex


PostPosted: Sat Dec 24, 2016 8:46 pm    Post subject: Reply with quote

Force applied is the same, torque is greater due to the greater diameter. Try stopping a merry go round from the hub as opposed to the outer edge for eg. Heavier, probably/possibly depends on brand and materials but a net gain in stopping power if you have enough tyre and surface grip to cope. Heat and to a certain extent gyro effect gets managed through floating mechanics of the disc and bell set up that allows radial and axial expansion.

Premature cracking of the porsche drilled discs are prime example of how they cant cope with heat build up. One or two laps no problem but prolonged use and they have cracked too much to stay safe. (10 mm is max from memory) that's why a slotted disc will last a lot longer as you can use it all up.

None of this really applies on the road btw.
 
  
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Disco
Paul Ricard


Joined: 13 May 2008
Posts: 3426
Location: Hertfordshire

2010 Porsche 997 GT3

PostPosted: Wed Dec 28, 2016 7:46 pm    Post subject: Reply with quote

IMI A wrote:

In fact logic tells you that through giroscopic effect i.e. weight of the heavier 380mm rotor perhaps becomes even heavier when spinning hence the 380mm rotor may need more force to stop it than say for example a lighter 350mm rotor if using exactly the same caliper and pads.


Erm - as I alluded to earlier - the single piece 25.5lb (11.6kg) 350mm iron rotor isn't actually lighter than the 2 piece 380mm rotor on its alloy bell (which comes in around half a kg lighter per corner). There is simply no performance argument in favour of the stock 350mm setup.

If weight alone is the goal then you want a 362mm floating rotor and appropriate aftermarket wheels which can be significantly lighter than stock (doing that actually takes more gyroscopic mass out of each corner than ceramic rotors behind stock wheels and you can genuinely feel the difference through the steering. Cheaper than optioning ceramics too).
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Colin

Black 997 GT3 3.8 CS - Acquired for the 2012 season and beyond
Black 996 GT3 Mk2 - Gone, but will never be forgotten
 
  
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IMI A
Österreich


Joined: 02 Aug 2014
Posts: 976
Location: United Kingdom


PostPosted: Sat Dec 31, 2016 5:07 pm    Post subject: Reply with quote

Disco wrote:
IMI A wrote:

In fact logic tells you that through giroscopic effect i.e. weight of the heavier 380mm rotor perhaps becomes even heavier when spinning hence the 380mm rotor may need more force to stop it than say for example a lighter 350mm rotor if using exactly the same caliper and pads.


Erm - as I alluded to earlier - the single piece 25.5lb (11.6kg) 350mm iron rotor isn't actually lighter than the 2 piece 380mm rotor on its alloy bell (which comes in around half a kg lighter per corner). There is simply no performance argument in favour of the stock 350mm setup.

If weight alone is the goal then you want a 362mm floating rotor and appropriate aftermarket wheels which can be significantly lighter than stock (doing that actually takes more gyroscopic mass out of each corner than ceramic rotors behind stock wheels and you can genuinely feel the difference through the steering. Cheaper than optioning ceramics too).


Yes although you can buy light weight aftermarket two piece 350mm rotors. 997 turbo with steels comes with 350mm as standard. The brake bias and ABS all calibrated to 350mm size rotor. All I am trying to point out is that the key to a good braking by system is not necessarily just having big rotors. Balance far more important and TBH I find the stock OEM steels fantastic hence have been reluctant to swap without understanding the pros and cons.

If I had 380mm PCCB I'd have no problem just swapping out for 380mm two piece steels.
 
  
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NXI20
Approved Trader


Joined: 02 Feb 2008
Posts: 3159
Location: South Bucks

2004 Porsche 996 GT3 Mk2

PostPosted: Sat Dec 31, 2016 6:35 pm    Post subject: Reply with quote

IMI A wrote:

Yes although you can buy light weight aftermarket two piece 350mm rotors. 997 turbo with steels comes with 350mm as standard. The brake bias and ABS all calibrated to 350mm size rotor. All I am trying to point out is that the key to a good braking by system is not necessarily just having big rotors. Balance far more important and TBH I find the stock OEM steels fantastic hence have been reluctant to swap without understanding the pros and cons.

If I had 380mm PCCB I'd have no problem just swapping out for 380mm two piece steels.


Do you really think that there's a different ABS unit for models with 350mm steel & 380mm PCCB? Here's a clue: there isn't.

In the last year, we have fitted 380mm discs to 996 Turbo, 996 GT3 & 997.1 GT3 vehicles, all of which previously had 350mm OEM steel rotors & 6-pot calipers. Nobody has reported anything untoward; indeed, they have been unanimous in their praise of the improved braking efficiency. This isn't theory, this has been proven in the real world. My own 996 GT3 has had 380mm rotors since 2012. It started life with 350mm PCCBs all round and has had 350mm, 362mm & finally 380mm steel rotors on the front. On the rear, I've had OEM & Alcon 350mm & currently Alcon 360mm steels. The brake bias & ABS has been absolutely fine during all those changes.

If you feel that 350mm steels are enough brake for you then great, you probably won't get the benefit of 380mm so might as well save your money. There is a huge difference in brake performance with the larger rotors but you really do need to be doing VMAX events or trackdays to fully appreciate how much...
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Nick
Need Alcon discs or Pagid pads? https://uber9s.com

2004 GT3 CS in Atlas Grey with too many mods to list!
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