Disc brake question

[Tickdoc]

Been a while since I've asked another stupid bike question, so here it goes:

Disc brakes with various sized rotors....how do you tell which size rotor goes where? Is there a standard? And then, how do you know which size rotor for which brake? Does the rotor brand have to be the same as the brake?

[foo_fighter]

Bigger one in front.
The calipers don't care about rotor size but you will need adapters, either IS or Post mount.
You can mix and match rotors and calipers from various companies.

[dustyrider]

Rotors should be marked with their size, so you can be sure to buy the right adapter. Got to know which type of brake mount you have too in order to get the right adapter if needed. Typically larger rotor would be found on the front since most braking force is generated there. Rotors and calipers don't seem to know the difference between brands as shimano's ice tech seems to be the go to rotor these days.

[IFRider]

Check for max rotor size if deviating. I know on some "Race" bike, larger rotors don't fit. Running 180mm front and 160mm rear off road. Got 203mm on the tandem.

Warren

[Tickdoc]

Thanks for the info. My brain wants the bigger rotor in the back, tho.

[ColonelJLloyd]

Thanks for the info. My brain wants the bigger rotor in the back, tho.

People do that. Downhill MTBers.

[R3awak3n]

Why would they have it in the back?

Even with a 140 you can easily lock the wheel, specialy in gravel.

[IFRider]

Why would they have it in the back?

Even with a 140 you can easily lock the wheel, specialy in gravel.

I was not aware of DH racers using larger in the back, but based on the Tandem with big 203mm in this makes sense ...

[sales guy]

Many times the size of the rotor is dependent on the fork or frame specs. We spec a 140 max for the rear on a couple bikes. Mainly due to frame shapes and type of riding. Fork manufacturers are the same way.

Traditionally, it's 160 front, 140 rear. Many ROAD bikes, they will be 140/140.

You will usually need an adaptor whether flat mount or post/IS mount.

[bikinchris]

I was not aware of DH racers using larger in the back, but based on the Tandem with big 203mm in this makes sense ...

Tandems CAN'T flip over no matter how hard you squeeze the brake. There is also plenty of weight in the rear since the stoker rides there. You can make a front wheel skid on a tandem.

To make the answer clear to the OP. The caliper doesn't care what size rotor. is squeezes. You pick a rotor size and then pick a mount that matches it. But like Salesguy said, some frame just won't accommodate the largest rotors.
My next touring bike is going to have a 203mm front and 180mm rear rotor. Overkill? Yes.

Maybe the OP might consider the same size rotor font and rear?

[Tickdoc]

Tandems CAN'T flip over no matter how hard you squeeze the brake. There is also plenty of weight in the rear since the stoker rides there. You can make a front wheel skid on a tandem.

To make the answer clear to the OP. The caliper doesn't care what size rotor. is squeezes. You pick a rotor size and then pick a mount that matches it. But like Salesguy said, some frame just won't accommodate the largest rotors.
My next touring bike is going to have a 203mm front and 180mm rear rotor. Overkill? Yes.

Maybe the OP might consider the same size rotor font and rear?

Too late, 160 front and 140 rear already here. Avid bb7 road. To be clear, already had 160 avid front, and ordered 140 for the rear, just wondered why. Waiting on wheels, then I need some cables. And then it's almost go time.

[HillDancer]

Having cooked 140mm rotors and glazed pads often with that size, I strongly advise against 140mm size rotors for road use.

Especially with mechanical brakes, I recommend the largest rotor a frame & fork will allow. Larger rotors perform better than smaller. Large diameter rotors provide a longer lever arm than small rotors, so less effort applied at the lever for the same work at tire/road interface. Thermal capacity increases also with an increase in rotor size.

Note objective measurements show rear braking is useful. http://off-road.cc/content/feature/talking-disc-brake-technology-with-sram-663

Wheel lock/skid is not and indicator of power or ultimate performance capabilities.

[ergott]

Typical road/gravel setup (not tandem) is 160 front and 140 or 160 rear. Both are fine for 99% of riders out there.

I agree with above, I didn't like 140 front.

[marciero]

I just went through this on my first disc brake single bike; a gravel/all road. I wound up with 160 front and rear. I figured that would be safe, i.e. conservative choice. So far so good. On the tandem it's 203 front and rear, for the reasons HillDancer gives. Heat dissipation is critical on tandems. Many internet horror stories of melting of plastic caliper parts and even discs on long descents.

Also keep in mind that the larger rotor sizes put a great deal of torque on the fork so it has to be designed to handle that.

Edit: Oops. Smaller rotor puts greater stress on the fork for given braking force. I've also confused torque and force on the fork here. See later post by MarkMcM.

[choke]

My brain wants the bigger rotor in the back, tho.Look at any sport motorcycle....two huge discs up front, one tiny disc in the back.

[Mark McM]

I was not aware of DH racers using larger in the back, but based on the Tandem with big 203mm in this makes sense ...

Another reason for the larger disk in the back of a tandem is that traditionally the rear brake is used as a drag brake (to limit top speed on downhills). Continuously dragging a brake generates a LOT of heat, and a larger rotor helps dissipate it better.

[Louis]

I recommend the largest rotor a frame & fork will allow. Larger rotors perform better than smaller. Large diameter rotors provide a longer lever arm than small rotors, so less effort applied at the lever for the same work at tire/road interface.

So, you're saying I should go back to using calipers and rim brakes? ;)

(sorry, I couldn't resist)

[marciero]

Another reason for the larger disk in the back of a tandem is that traditionally the rear brake is used as a drag brake (to limit top speed on downhills). Continuously dragging a brake generates a LOT of heat, and a larger rotor helps dissipate it better.

Sounds like a plausible reason but I think larger disc on the back would be uncommon on a tandem. I've not seen or heard of anyone doing this. I think about all production tandems are spec'ed with 203 front and back, though I've heard of one team using 180 front and back for racing in flat terrain. A drag brake was typically a drum brake on the rear in addition to two rim brakes in order to control speed on long descents without overheating the rims. They've sort of gone out of fashion and out of production with the advent of discs. I think the only disc recommended for use as a drag brake is Santana's 10" rotor "...recommended for riders who do sustained, steep descents or like to drag their rear brake to control speed on moderate descents." They make a good argument for V-brake front and 10" rear disc, especially now with the advent of mini-Vs like the Paul minimoto, which I've found stop as well or better than 203 mm disc brakes.
http://santanatandems.com/Techno/UnderstandingBraking.html

[zap]

Edit

I think the only disc recommended for use as a drag brake is Santana's 10" rotor "...recommended for riders who do sustained, steep descents or like to drag their rear brake to control speed on moderate descents." They make a good argument for V-brake front and 10" rear disc, especially now with the advent of mini-Vs like the Paul minimoto, which I've found stop as well or better than 203 mm disc brakes.
http://santanatandems.com/Techno/UnderstandingBraking.html

Santana is not exactly known for sound technical solutions.

Brake pads for discs are not designed for drag applications. Agree that rim brakes stop better but for safety, I'm willing to give up rims superior braking performance for discs front and rear for tandem mountain riding.

[bikinchris]

I just went through this on my first disc brake single bike; a gravel/all road. I wound up with 160 front and rear. I figured that would be safe, i.e. conservative choice. So far so good. On the tandem it's 203 front and rear, for the reasons HillDancer gives. Heat dissipation is critical on tandems. Many internet horror stories of melting of plastic caliper parts and even discs on long descents.

Also keep in mind that the larger rotor sizes put a great deal of torque on the fork so it has to be designed to handle that.

Read a tandem club article where they rode up and down Mt. Ventoux. The stoker wondered if the rotors were hot and before the captain could say anything she touched it. 3rd degree burn instantly.

[Mark McM]

Also keep in mind that the larger rotor sizes put a great deal of torque on the fork so it has to be designed to handle that.

Actually, just the opposite. For a given braking force, the bigger the disc, the smaller the force/torque on the fork. The wheel and disc act as levers - the longer the lever, the less force you need to generate a given amount of torque. With a bigger disc (longer lever) there is less force at the caliper.

[marciero]

Actually, just the opposite. For a given braking force, the bigger the disc, the smaller the force/torque on the fork. The wheel and disc act as levers - the longer the lever, the less force you need to generate a given amount of torque. With a bigger disc (longer lever) there is less force at the caliper.

yes. My bad.

[marciero]

Actually, just the opposite. For a given braking force, the bigger the disc, the smaller the force/torque on the fork. The wheel and disc act as levers - the longer the lever, the less force you need to generate a given amount of torque. With a bigger disc (longer lever) there is less force at the caliper.

...and yes, is the same torque but a greater force.