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Why are the rear brakes disks larger than the front brake disks on the Model S?

Discussion in 'Model S: Driving Dynamics' started by DrStrangelove, May 21, 2013.

  1. DrStrangelove

    DrStrangelove New Member

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    Reviewing the model S specs I came across this:

    front: 355 mm x 32 mm; rear: 365 mm x 28 mm
     
  2. PRJIM

    PRJIM Member

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  3. lolachampcar

    lolachampcar Active Member

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    It has a lot to do with why the car stops from 60-0 in 108 feet with no drama. Normally the center of gravity on most cars is much closer to the center line of wheel rotation. When you stop, a lot of of the car's weight "rolls" onto the front wheels which plants them down on the pavement harder and lets them generate more braking force before they break loose. Manufacturers know this and thus most normal cars have larger front brakes and have a forward bias on the brake line pressure (sometimes done with the mechanical link between your pedal and the master cylinders that drive the front and rear brakes but mostly done by having different sized cylinders on the front and rear master cylinders to provide the differential pressure).

    In Tesla's case, they have good static weight distribution AND a very low center of gravity. That leaves a lot more weight on those rear wheels when you brake thus they need a brake size (and probably pressure) bias that is more neutral or even to the rear. I've not seen this short of a few GTP cars and NEVER on a street car (if someone else has, please chime in here). It is just one more example of how Tesla has thrown out the "how we do things" book for the better :)

    Who'd thunk a big ole lux EV would out brake a Maranello (114 feet)?
    For reference
    599 105 feet
    Carrera GT 101 feet
    GRT Black 101 feet
    Diablo 109 feet
    R8 96 feet

    There seems to be a lot of carbon brakes in the following listing-
    22 Cars that Stop from 60 MPH in Less Than 100 Feet - Motor Trend
     
  4. TEG

    TEG TMC Moderator

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    Yeah, I think low center of gravity, and rearward weight bias (compared to competitors) are the most likley factors.

    Also, I have seen many cars where the rear discs are bigger because they have less airflow than the fronts.
    In some cases cars will use solid rear discs, and ventilated fronts.

    Also, the rear has a 2nd disc for the parking brake mechanism which may influence the size.

    Why are the rear brake discs larger than the front discs on the model s? : teslamotors

    Tesla Vehicle Engineering - Part 2 - YouTube
     
  5. mfennell

    mfennell Member

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    Not so fast. :)

    Not disputing the exceptional results but on the surface, it looks like the Model S has the same brake package as a Camaro SS and early CTS-V. Rotor sizes are the same as are the front calipers (couldn't find a pic of the rear Chevy calipers). The front may be smaller to compensate for the larger front caliper and keep the wheel clearances similar front-to-back. It may be as simple as the Tesla engineer who spec'd it came from GM (or Brembo) and was intimately familiar with that collection of components.

    Brembo does a good job of reselling the same components for a long time across many manufacturers. A '99-'05 Ferrari 360 has the same front caliper as a 1995-97 Lotus Esprit, early Viper, and even some Porsche 944 Turbos.
     
  6. lolachampcar

    lolachampcar Active Member

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    ah, I was not aware of that. I stand corrected.
     
  7. MaxG Tech

    MaxG Tech New Member

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    Rotor Sizing considers both the Temperature Management and the Torque Management. The overall required Torque of the brake is governed by the weight distribution as described by others. Each Brake must deliver its Torque by a product of both the rotor diameter and the caliper piston size. The Rear brakes are required to provided less than 50% of the Torque. This allows a smaller caliper. Because the caliper is smaller it allows a larger rotor to be packaged in the wheel. General design practice is to use the biggest rotor you can fit and then pick a caliper size to deliver the overall required Torque. It is very common to see a larger rear rotor diameter due to this.
     
  8. JST

    JST Active Member

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    It actually has to do with traction control. More and more RWD cars are using larger rotors in the rear than in the front, because the rear brakes are playing a larger role in power management, both in terms of traction control and automatic electronic "limited slip" differentials (which basically mimic an LSD by applying the brake on the wheel that has lost traction).

    Bigger rotors allow more heat dissipation. At the same time, TC and ABD don't require the same clamping force as braking to a full stop, which is why you don't need a giant caliper.
     
  9. zwede

    zwede 2013 P85+

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    And the parking brake is a factor. A larger diameter rotor allows a smaller parking brake caliper & pad, and it also requires less clamping force which allows a smaller parking brake motor.
     
  10. tezco

    tezco Sig P85

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    I assume RWD cars would also need to plan for a worse case scenario where braking needs include stopping force plus force needed to counteract full engine power (in the event you have a run-away engine).
     
  11. yobigd20

    yobigd20 Well-Known Member

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    Is this thread even relevant anymore? OP was from early 2013. Since that time, to address the "brake squealing problem" Tesla is now using larger and heavier rotors on the front. As I had the squealing brakes I had a Tesla apply the TSB for this and they replaced my front rotors with new bigger heavier ones. Much better now with no more squealing. I think they said this is standard on cars from the factory now.
     
  12. zwede

    zwede 2013 P85+

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    I guess you didn't see the old and new rotors side by side. The diameter and brake surface thickness are identical. There is extra material in the hub area, increasing the total mass of the rotor.
     
  13. yobigd20

    yobigd20 Well-Known Member

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    no I'm certainly no rotor expert or guru. I thought they said it was bigger and heavier but I guess by bigger they didn't mean increased diameter.
     
  14. tezco

    tezco Sig P85

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    If I remember correctly, I think it had to do with changing the harmonic resonance characteristics of the rotor to help reduce the squeal, which was accomplished by adding mass.
     
  15. wycolo

    wycolo Active Member

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    So I have less unspring weight on the front than you do? Nyah, nyah!! :tongue:
    --
     
  16. tezco

    tezco Sig P85

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    Uh oh, sounds like a squeal developing...:wink:
     

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