clausti

Hey, I am building a winter commuting/play in the mud cyclocross-ish bike, but since this question is about disc brakes, I thought I might find more information in the mountain bike forum.

My question is this: I want to put 140mm disc brakes front AND rear, but obviously they only make "rear." In my googling around, I've noticed a lot of 160mm rotor/calipers are marked front OR rear. and they mount in the same direction, either way, right?

So is there any physical reason that I can't buy two 140mm "rear" calipers and mount them front and rear on my no-suspension bike? Other than that I shouldn't? I weigh 110 lbs, and live in Ohio- not so much with the ever riding down "hill."

Thanks for the input!

kenhill3

Which disc brake uses 140mm rotors?

clausti

avid makes 140mm in the road version of both bb5 and bb7.

sirtigersalot

idk about 140mm stuff (didn't even know they existed) but with the 160s the only diff is the little adapter piece that attaches to the 2 bolts on the frame and fork, this piece https://www.jensonusa.com/store/produ...r.aspx?sc=FRGL
so if you put fronts on the rear then the caliper woudln't line up with the rotor, you need new adapter, same if you wanna run larger rotors, you can change out that little piece and run 180mm rotors on some mtn bikes (not cross bikes and some forks only rated to max rotor size of 160)

as I said idk if this is same for 140 rotors,

jcook1989

Not sure if you could get a bracket to get the caliper close enough unless you have a fork designed for 140mm rotors. My advice to you is to get 203mm and be done with it.

BearSquirrel

This is a bad idea. When you brake, you transfer your weight to the front axel. This means the front takes the brunt of the braking. You want a larger rotor on the front than the rear.

ca7erham

For cyclocross a 140 up front and back would do fine. Besides, in theory a larger rotor will have no more actual stopping power.

Cyclist30907654

You should take some physics classes.

ca7erham

What I'm saying is that a larger rotor should offer more modulation and heat dissipation, but won't have any more stopping power (perhaps "locking power" would be a better term?). I'm not going on physics knowledge, but on what my limited experience has been. I can lock up the front wheel of a bike with 6 inch rotors just as easily as I can with 8 inch rotor (Avid J3s), and same with disc Vs V (but the modulation on Vs sucks). Just my 2 cents.

Cyclist30907654

Torque equals the amount of perpendicular force applied multiplied by the distance from the pivot (lever arm).

An 8 inch rotor means that the torque applied to the hub should be approximately 33% more than the same caliper with 6 inch rotors.

ca7erham

...

taylor p

[QUOTE=ca7erham;7557752 I'm not going on physics knowledge, but on what my limited experience has been. [/QUOTE]
your limited experience is wrong

kenhill3

Sorry, wrong. The braking force applied to the wheel (at the hub) is greater for an 8inch rotor because there is more distance (moment arm) from the center (hub). This is basic physics as BenLi pointed out. You're young, yet. Plenty of time to get some physics study under your belt. I guarantee it will help you in understanding the world happenin' around you, as well as your bike.

ca7erham

My experience is very limited. Limited to avid J3s on used bikes and various Vs and mechs

ca7erham

Bad set up for the big rotor maybe? Because I am 100% positive that the big rotors gave almost nothing over the small ones.

the_mac

The thing is, even if the 8-inch CAN offer more stopping power, as long as the 6-inch can lock the front, then they're both limited by the traction of your tires, not by the power of your brakes. In this way, a 140 is the same as a 203. They can both lock you up, so they both stop you just as quick, and they're both just as strong. Sure, takes more hand strength with the 140, but you're not a weenie, right? And the 203 can dissipate heat better and has better modulation, but if you're limited by front traction, then your brakes are all equal.

ca7erham

I might be wrong, but I think that is whatish I was trying to say(ish), but I cant talk very well and my brain seems to be a bit slower than usual this weekend.

Cheeto

^true.
I can lock up my front bb7 and its not a huge rotor.
tires is where its at, as my V's can do the same.

taylor p

for the love of peter fagerlin