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I would like to upgrade brakes on my bike but I have a dilemma / question - currently, on my front wheel there is 180 mm rotor and on back wheel there is 160 mm rotor. Is there any reason why bike manufacturer did this (is it assumed that bike doesn't need that much stopping power on rear wheel?). Would I gain anything by installing same sizes rotors (180 mm) on both wheels and is there any risk to bikes frame?

  • There's not a lot to gain - your calipers need adapters, but they still bolt to the fork and frame in the same place. Common thought is rotors of 200mm and over are only for bonkers downhill riders. Remember, its the tyre/ground contact patch that matters the most. Plus bigger rotors weigh more, and are more vulnerable to accident damage, and to theft. – Criggie Mar 27 '16 at 10:20
  • Check with the fork manufacturer, some limit rotor size as a liability issue. If you exceed their maximum they will likely void any warranty. – mikes Mar 27 '16 at 15:15
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    This is one case where "Its not the size that counts" has never been truer. Better quality brakes will trump bigger disks 100% of the time. – mattnz Mar 28 '16 at 8:34
  • @mattnz - I agree with you about quality over size but I'd like to know if it is recommeneded to change 180 mm rotor on front wheel with 160 mm (for example SLX/XTR) rotor? – False Identity Mar 30 '16 at 19:58
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Advantages of smaller discs

  • Lighter
  • Less prone to damage
  • Less prone to warping

Advantages of larger discs

  • Dissipate more heat
  • More stopping torque

The calliper and pad choices are more important than the disk as far as performance. Bigger discs increase performance cheaper than better callipers. At some point, bigger discs are needed to get enough performance, but mostly disc size choice is about the cost vs weight trade off.

In your case, I expect the current 180F/160R is a nice balance of cost and performance. Better callipers and 160 on the front would save grams but either loose performance or cost $$$. 180 on the rear (if it fits) wont help much with braking power, so the cost to change is pointless

Given the front and back wheels are not interchangeable, there is no disadvantage of different size discs. As such, its probably a lot easier (and cheaper) for you to keep your existing discs

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This is a common set up, as the majority of your stopping power is assumed to come from the front wheel, and many frames, especially older frames, had less clearance on the rear triangle of the frame, so often the 180mm rotor wouldn't fit.

You can change it if you like, but there is little necessity to do so.

If you choose to change it, you will only need a rotor, and the appropriate adaptor for you brake caliper type.

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  • Can you expand your answer regarding part of the question related to the risks of putting bigger rotors - if there is a chance of deforming frame, wheels, fork? – False Identity Mar 30 '16 at 19:49
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    @FalseIdentity As the rotor increases in diameter, the calipers have to move out, and since most forks only have one set of mount holes, there has to be an adapter fitted. This increases the leverage that the caliper applies to the fork when braking because its gripping further out (a longer lever) – Criggie Mar 30 '16 at 21:03
  • There is little risk involved in using a larger rotor, unless it touches frame or fork somewhere. However, there is little benefit, either, and larger rotors can mean a more difficult setup on the bike. – zenbike Mar 31 '16 at 12:02

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