[Lombard]

D

 

[nova_rider]

I don't know why you guys are worried about the surface area of the disk that the pad hits. If the rotor is connected to a plate that is the full internal diameter, that calc isn't right either. My disks have a metal heat dissipater that goes from the disk all the way into the hub, and it's vented!

The only braking is done by the pad! Which is virtually the same size on most brakes. The only thing that changes is the distance from the axle.

The larger diameter will have greater surface areas on both the rotor and cooling fins to dissipate heat, thus less fade. The slight caliper mechanical advantage can translate to less effort/better modulation.

 

[Lombard]

The larger diameter will have greater surface areas on both the rotor and cooling fins to dissipate heat, thus less fade. The slight caliper mechanical advantage can translate to less effort/better modulation.

Of course you will only notice the "less fade" part if you are in a situation where fluid may boil.

 

[nova_rider]

Of course you will only notice the "less fade" part if you are in a situation where fluid may boil.

If your fluid boils, it's not fade, but rather brake failure. Fade occurs when the rotor and/or pad overheats and lose friction during heavy application. Fluid boil is often caused by prolong dragging of pads on the rotor with light application, or a big no-no when it comes to proper braking techniques.

 

[duriel]

It is a difficult issue.
Going fast DH, overall is easier on the brakes, until they are called into duty. Then they are under a lot of stress, pressure & temperature sky rockets, oil may boil at that point but is suppressed due to thermal mass.
Going slow DH, overall is harder on the brakes. Drag braking will slowly build temperatures where the oil will boil constantly, there is no suppression due to thermal mass.

He never did answer my question on cable vs hydraulic. I think he just wants to argue.

 

[cxwrench]

I'm no smartypants, but it's surface area that is significant here. Doing the calculation Pi * R * R:

For a 140mm disc, 3.142 * 70 * 70 = 15,396 sq mm

For a 160mm disc, 3.142 * 80 * 80 = 20,109 sq mm

Difference = 4,714 sq mm

So, the 160mm disc has 30.6% more surface area than the 140mm.

Of course I am reminded of a quote from one of my favorite posters on this forum, DCGriz, who wisely said, "With bicycles in particular, it is important to separate what is merely true and what is important". A bike with 140mm disc brakes still has more stopping power than a similar rim brake bike. Unless you plan on riding your brakes down 1-mile long steep hills or longer, the 140mm disc brakes will slow and stop you just fine without melting.

I promise you the area of the entire circle isn't 15 or 20 square meters... and we're only talking about the swept area.

 

[Finx]

Such an RBR thing to do to resurrect a sleeping thread to argue the details of the math involved with a question that has already been answered.

Back to the original question, I'm not aware of a way to adapt a flat mount road disc to 180mm rotor. it might be possible, but you would be looking at some fringe hack, and not something the brake manufacturer or frame designer would likely endorse.

I'll re-iterate, speaking from a lot of experience as a heavy rider on a heavy'ish bike on steep, sketchy terrian (I live in the Cascade Mtn foothills, where fire roads are not only steep and sketchy in terms of surface, but also quite muddy due to rain), that 160mm should be adequate. If not, you might look at upgrading to a better quality brake caliper. Good quality hydraulic disc brakes with 160mm rotors stop my fat ass - they should stop yours as well...

 

[Lombard]

I promise you the area of the entire circle isn't 15 or 20 square meters... and we're only talking about the swept area.

Umm, your conversion from sq mm to sq m is off by a few decimal points. 15,000 or 20,000 sq millimeters = 0.015 or 0.02 sq meters. That IS the area of the entire circle.

And I thought YOU were a smartypants.

 

[cxwrench]

Umm, your conversion from sq mm to sq m is off by a few decimal points. 15,000 or 20,000 sq millimeters = 0.015 or 0.02 sq meters. That IS the area of the entire circle.

And I thought YOU were a smartypants.
[/QUOTE]
1000mm =1meter. How am I off?

 

[Lombard]

1000mm =1meter. How am I off?

1000mm = 1meter, correct.

1000 sq mm does not equal 1 sq meter.

Hint: 12 sq inches does not equal 1 sq foot.

Hint: How many sq feet is 1 sq yard? How many cubic feet is 1 cubic yard?

 

[cxwrench]

D'oooohhhh.

 

[duriel]

This is not a thread druggie, a guy joined to get the same question answered, then he didn't like anyone's answer, so he when back to his fake world.

 

[Chantecler]

You've forgotten your maths.
Every mm increase in radius = 2mm increase in diameter. Circumference is the word you were looking for. A 140 or 160mm disc is just that...140mm or 160mm diameter. Trying to sound smart only works when you get everything right.

Lol, I agree, I used diameter instead of circumference, but I obviously knew my maths as I used radius (being half diameter) and calculated circumference increase (which is what affects area: add 1mm to circumference and you get over 6mm squared volume extra... the non linear nature is what you missed, actual impact on area depends on depth of area heated by brake, and then actual heating depends on materials used etc). But well done, you keep on focussing on diameter and ignore the exponential difference in surface area. 140 to 160.... no big deal you say (ie what is an extra 20 on 140?). Bad maths if you are talking about either surface area (cooling) or braking impact, where again distance from hub is not linear in effect.

 

[overl0ad]

just to add fuel the the fire...

i've been toying with this idea of going 180mm in the fronts and 160mm in the rears with 4 pots both ends

OR

180mm front and rear with 4 pots in the front and 2 pots in the rear.

not that i need it, and i freely admit that i don't.

want to do it "just cos i can",

only thing i'm wondering about is that fluid displacement from the levers.

any comments about the fluid displacement?

edit: yes, i know i'll ned to go PM calipers to do 4 pot.ideally, i'd like to stick with my 2 pot FM calipers for now and just go 180/160