180mm vs 160mm rotors

[Windbreaker]

So I assume that the 180mm rotors provide better stopping power in theory but is there a difference in reality?

[rad006]

I actually found by using a 160mm with a smaller surface area it works great as it doesnt build up as much heat

[100Tours]

if you get onto anything steep you're going to want the bigger rotor. 180mm has so much of stopping

[Wyatt Earp]

I am running a 140 rear and 160 front on my Epic 29'er, I have plenty stopping power.

[Johan Bornman]

I actually found by using a 160mm with a smaller surface area it works great as it doesnt build up as much heat

 

This doesn't make sense. I'm not arguing that 160mm disks stop well. There is nothing wrong with them.

 

But, if you have two disks, one with X surface are and the other with 1/2 X 2 surface area, the one with less surface area will be hotter after stopping the same rider on the same bike over the same distance.

 

It has to. It cannot dissipate the heat as fast. Heat generated at the pad/disc interface can only go two places - into the disc or backwards into the calliper. It goes both ways where it is dissipated into the air  by the larger surface areas of the hub, spokes, calliper and fork.

 

 

[rad006]

I actually found by using a 160mm with a smaller surface area it works great as it doesnt build up as much heat

 

This doesn't make sense. I'm not arguing that 160mm disks stop well. There is nothing wrong with them.

 

But' date=' if you have two disks, one with X surface are and the other with 1/2 X 2 surface area, the one with less surface area will be hotter after stopping the same rider on the same bike over the same distance.

 

It has to. It cannot dissipate the heat as fast. Heat generated at the pad/disc interface can only go two places - into the disc or backwards into the calliper. It goes both ways where it is dissipated into the air  by the larger surface areas of the hub, spokes, calliper and fork.

 

 

[/quote']

JB is got to me as well. Before I had XT rotors which when doing some hectic downhill gave me brakefade, then I put on some Alligator Starlite rotors which are pimp but have less surface area and when I competed at the Mankele downhill race I had no fade whatsoever

[Christopher Brunsdon]

 

Don't use discs but using my knowledge of heat sinks from PCs here. You need to consider the entire surface area. Drilled holes and decor will increase it. This will lead to faster cooling down of the disc.

 

It is just very possible that the design of one 160mm has a far greater surface area than a 180mm disc of a different design.

 

 

UPDATE: Here you can see the difference: The Alligator has a larger surface are due to its zig-zag design:

 

 

 

Alligator Starlite:

 

 

And XT:

 

 

 

 

 

cbrunsdon2010-01-18 13:55:05

[SwissVan]

I actually found by using a 160mm with a smaller surface area it works great as it doesnt build up as much heat

 

This doesn't make sense. I'm not arguing that 160mm disks stop well. There is nothing wrong with them.

 

But' date=' if you have two disks, one with X surface are and the other with 1/2 X 2 surface area, the one with less surface area will be hotter after stopping the same rider on the same bike over the same distance.

 

It has to. It cannot dissipate the heat as fast. Heat generated at the pad/disc interface can only go two places - into the disc or backwards into the calliper. It goes both ways where it is dissipated into the air  by the larger surface areas of the hub, spokes, calliper and fork.

 

 

[/quote']

JB is got to me as well. Before I had XT rotors which when doing some hectic downhill gave me brakefade, then I put on some Alligator Starlite rotors which are pimp but have less surface area and when I competed at the Mankele downhill race I had no fade whatsoever

 

rad006 - You need to compare apples to apples, perhaps there is something better about your starlite setup compared to the XT or maybe the one downhill required longer braking.

 

Fact is the smaller the rotor the quicker the heat build up.

 

 

[Christopher Brunsdon]

Another point my engineering boet-in-law would say here would be the heat transfer properties of each disc. Many factors to consider at the end of the day.

 

[Flea]

Summit - are you trying to save weight by going smaller - or is there some other advantage?

[Wyatt Earp]

Summit - are you trying to save weight by going smaller - or is there some other advantage?

 

Not at all, sure I like a fairly light bike, but my XX was supplied as such and there is more than enough braking power.

[Thug]

On my Scandal 29er I'm running 180mm up front and 160mm in the rear.

 

 

 

I just think that on a 29" wheel, a 160mm up front looks totally puny.

 

 

 

 

[NotSoBigBen]

On my Scandal 29er I'm running 180mm up front and 160mm in the rear.

I just think that on a 29" wheel' date=' a 160mm up front looks totally puny.

[/quote']

Then imagine that 140 on Boris 29" rear wheel ....

[AndreZA]

Summit - are you trying to save weight by going smaller - or is there some other advantage?

 

Not at all' date=' sure I like a fairly light bike, but my XX was supplied as such and there is more than enough braking power.[/quote']

 

 

 

and the 140 at the back looks freakin' awesome.

[Johan Bornman]

Don't use discs but using my knowledge of heat sinks from PCs here. You need to consider the entire surface area. Drilled holes and decor will increase it. This will lead to faster cooling down of the disc.

It is just very possible that the design of one 160mm has a far greater surface area than a 180mm disc of a different design.


UPDATE: Here you can see the difference: The Alligator has a larger surface are due to its zig-zag design:



Alligator Starlite:



And XT:

 

Lets consider the classic drilled disc.

 

The diameter of the hole is say, 6mm. Thus the surface area removed by the hole is 28.27mm^2

 

The surface area of the cylinder created by the 6mm hole in a 2mm disc is

simply circumference x 2 which is 18.85 x 2 = 37mm^2

 

Thus, there is a larger total surface area created by drilling holes. The question now is how useful that extra area is. Does air flow well enough over/through it?

 

Clearly the area gain increases as the disc gets thicker, but the airflow will decrease.

 

I dunno....where are the heat exchanger experts. Chickenrunforme - your phone is ringing. Come chip in.

 

 

 

Johan Bornman2010-01-19 02:01:56

[jayz]

J.B. Your equations are a bit higher grade for me but I get exactly what you are saying. It boils down to like for like, the same make rotors with the same brakes and pads you will always get better performance out the larger rotar. Just look at Ferraris etc. The faster the car the larger the discs.Also of course if you have ceramic pads you will even get better performance as they build up much less heat. The XX is 160 front and 140 rear as it is geared towards the race snakes where any amount of weight saved is better plus these riders are almost always better technical riders and thus use far less of their brakes when negotiating downhills.Lastly from the write ups I've read about the Xx it seems the new taper bore technology and pistons etc makes these brakes far superior to the Juicy ultimates of the past and yet they are much lighter.