Let's talk rotors

Discuss light weight issues concerning road bikes & parts.
fromtrektocolnago
Posts: 1140
Joined: Sat Dec 07, 2013 10:15 pm

by fromtrektocolnago

on my gravel bike i use 160 mm rotors front and back
on my road bike i'm 160 front , 140 back

I'm not a big guy. what's the current thinking, and what are you riding? i know both are done.
Colnago C-59 (Dura Ace)
Firefly(Ultegra)
Colnago C-64 disc(ultegra) with Bora 35 wheels

Hexsense
Posts: 933
Joined: Wed Dec 30, 2015 12:41 am

by Hexsense

you can discuss in here:
viewtopic.php?f=3&t=145230
In addition to rotor model choice, there are a few discussions about rotor size too.

by Weenie


moyboy
Posts: 447
Joined: Thu Aug 18, 2016 12:19 am

by moyboy

170 lbs... 160mm front and rear for both road and gravel.

TheRich
Posts: 415
Joined: Tue Jan 01, 2019 1:36 am

by TheRich

At 175lbs, 160/140 is sufficient for fast paced intermediate mountain biking on my CX bike. Without suspension and big tires, road and CX bikes just can't go as fast as mountain bikes, so you don't need big braking power...and you don't have the traction to use it if you had it. But I ride in the drops off road where the leverage on the lever is much higher when I'll be doing lots of braking.

TobinHatesYou
Posts: 4169
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

Consider the scenarios, TheRich. On a road bike down alpine descents with long straights and switchbacks, you are going to be braking hard from high speeds in bursts. This is a lot of kinetic energy being converted into heat in a very short amount of time. In contrast on a MTB you might be dragging your brakes more often, but I doubt you are frequently braking down to 20mph from 50mph like you might on the road.

All in all, in order to reach the bottom of the mountain on either requires a certain amount of friction. On the MTB you are slowing down more from rolling friction and aerodynamic drag friction. On the road bike, you have less rolling resistance and aerodynamic drag, so a higher percentage of your braking must come from your actual brakes. Then again, the slower you come down the mountain, as you might on a MTB, the less dramatic the aero drag effect will be and a greater amount of time will be spent fighting a constant acceleration (gravity.)

fromtrektocolnago
Posts: 1140
Joined: Sat Dec 07, 2013 10:15 pm

by fromtrektocolnago

TobinHatesYou wrote:
Thu Mar 07, 2019 8:12 pm
Consider the scenarios, TheRich. On a road bike down alpine descents with long straights and switchbacks, you are going to be braking hard from high speeds in bursts. This is a lot of kinetic energy being converted into heat in a very short amount of time. In contrast on a MTB you might be dragging your brakes more often, but I doubt you are frequently braking down to 20mph from 50mph like you might on the road.

All in all, in order to reach the bottom of the mountain on either requires a certain amount of friction. On the MTB you are slowing down more from rolling friction and aerodynamic drag friction. On the road bike, you have less rolling resistance and aerodynamic drag, so a higher percentage of your braking must come from your actual brakes. Then again, the slower you come down the mountain, as you might on a MTB, the less dramatic the aero drag effect will be and a greater amount of time will be spent fighting a constant acceleration (gravity.)
Since I'm more likely to descend at 20 mph than 40 mph. I'm guessing I'm fine with the 160/140 blend even if coming down Gallibier
Colnago C-59 (Dura Ace)
Firefly(Ultegra)
Colnago C-64 disc(ultegra) with Bora 35 wheels

TobinHatesYou
Posts: 4169
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

That just means you are braking more than the guy going 40mph and converting more energy in the moving system into heat...(unless you’re on a gradual descent and his speed is achieved by putting out high watts.)

fromtrektocolnago
Posts: 1140
Joined: Sat Dec 07, 2013 10:15 pm

by fromtrektocolnago

TobinHatesYou wrote:
Thu Mar 07, 2019 8:34 pm
That just means you are braking more than the guy going 40mph and converting more energy in the moving system into heat...(unless you’re on a gradual descent and his speed is achieved by putting out high watts.)
sadly you are correct
Colnago C-59 (Dura Ace)
Firefly(Ultegra)
Colnago C-64 disc(ultegra) with Bora 35 wheels

Finx
Posts: 67
Joined: Wed Jun 15, 2016 1:14 am

by Finx

All of my bikes are 12mm thru axle with 160mm front and rear.

I could go 140/160 or even 140/140, but I want all of my wheels to be interchangeable between bikes, and because I do a lot of gravel these days, I settled on the 160/160 option for all of them.
Last edited by Finx on Thu Mar 07, 2019 9:49 pm, edited 1 time in total.

andreas
Posts: 176
Joined: Thu Jun 12, 2014 10:21 pm

by andreas

Edit: wrong

TobinHatesYou
Posts: 4169
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

That said, I did switch from 160/160 to 160/140 mostly because of the miniscule weight savings, but also because it does "balance" out the front and rear brake feel. Then again I also weigh next to nothing.

TheRich
Posts: 415
Joined: Tue Jan 01, 2019 1:36 am

by TheRich

TobinHatesYou wrote:
Thu Mar 07, 2019 8:12 pm
Consider the scenarios, TheRich. On a road bike down alpine descents with long straights and switchbacks, you are going to be braking hard from high speeds in bursts. This is a lot of kinetic energy being converted into heat in a very short amount of time. In contrast on a MTB you might be dragging your brakes more often, but I doubt you are frequently braking down to 20mph from 50mph like you might on the road.

All in all, in order to reach the bottom of the mountain on either requires a certain amount of friction. On the MTB you are slowing down more from rolling friction and aerodynamic drag friction. On the road bike, you have less rolling resistance and aerodynamic drag, so a higher percentage of your braking must come from your actual brakes. Then again, the slower you come down the mountain, as you might on a MTB, the less dramatic the aero drag effect will be and a greater amount of time will be spent fighting a constant acceleration (gravity.)
I did consider the scenarios, since I am a rather avid mountain biker and racer, and have taken my CX bike mountain biking...and I don't tiptoe around the trails either. BTW, my MTB has Saint brakes and 203/180mm rotors. They are insanely powerful, and I am more than confident that hydro brakes with typically sized but good rotors are up to the task of the speeds that a bike with skinny tires can handle off road.

That said, you're kinda making my point here that the braking requirements of road riding are quite high, the main difference in braking on the road or MTB descents is in the timing of the hard braking events. On the road, they're more spread out, just like they would be on a dirt road, so the required heat capacity of the rotors is different. Typical dirt road usage is that you'll acheive a speed that's too fast, and then slow down a little (with light braking because of lower traction) to a speed that's more comfortable. That isn't very taxing, definitely not as taxing as you'd see on the road or on a MTB. If you find yourself on a steep, tight, descent on dirt, you're going to be going slow anyway because you don't have the braking or cornering grip to allow high speeds in the first place, and the same applies if the surface is especially rough.

The majority of braking is from your front brake, if you cook your rear pads but not your front, you're doing it wrong. If you aren't cooking your brakes, the rotor size isn't the issue. Use both brakes, and if you want more power, get in the drops. Basic technique, no need to run around changing parts.

Karvalo
Posts: 735
Joined: Fri Aug 10, 2018 6:40 pm

by Karvalo

TobinHatesYou wrote:
Thu Mar 07, 2019 8:12 pm
Consider the scenarios, TheRich. On a road bike down alpine descents with long straights and switchbacks, you are going to be braking hard from high speeds in bursts. This is a lot of kinetic energy being converted into heat in a very short amount of time. In contrast on a MTB you might be dragging your brakes more often, but I doubt you are frequently braking down to 20mph from 50mph like you might on the road.

All in all, in order to reach the bottom of the mountain on either requires a certain amount of friction. On the MTB you are slowing down more from rolling friction and aerodynamic drag friction. On the road bike, you have less rolling resistance and aerodynamic drag, so a higher percentage of your braking must come from your actual brakes.
And yet I've never seen a road bike with 203mm rotors :wink:

You can and do brake way harder on a true MTB than a road bike. IMO rotor choice on the road is going to be more to do with heat management, so the OP who'se apparently gently braking all the time is more of a candidate for a 160mm rear.

youngs_modulus
Posts: 575
Joined: Wed Sep 20, 2006 1:03 am
Location: Portland, OR USA

by youngs_modulus

Karvalo wrote:
Fri Mar 08, 2019 8:53 am

You can and do brake way harder on a true MTB than a road bike.
What makes you say that?

On flat ground, both road and mountain bikes can lift the rear wheel under braking. When that happens, you’ve reached the max deceleration possible for a given bike/rider/position. (The location of your center of mass is what drives this, and that’s why tandems can brake harder than single bikes—they’re more resistant to endo-ing).

Mountain bikes have incrementally longer wheelbases than road bikes (which increases max braking before pitchover) but also put the rider/bike system’s center of mass higher than it is on the road (which decreases max braking before pitchover). To a first approximation, these cancel out.

So road bikes and mountain bikes can stop equally fast. This is true until one or the other locks the front wheel without pitching over—then, the max deceleration rate is limited by tire traction, but not before.

I often hear people say things like “mountain bikes stop faster than road bikes because the contact patch on a road tire just isn’t big enough.” That’s not true. First of all, the area of a contact patch has almost nothing* to do with how much grip a tire can generate. More importantly, neither tire is traction-limited: both will pitch you over the bars (barring snowy or wet metal surfaces) before they’ll lock the front wheel.
Karvalo wrote:
Fri Mar 08, 2019 8:53 am
IMO rotor choice on the road is going to be more to do with heat management, so the OP who'se apparently gently braking all the time is more of a candidate for a 160mm rear.
Yep; that’s pretty much right. A 160mm disc produces a little more braking power for a given lever pull force than a 140mm disc because the caliper is farther from the center of rotation, but the bigger rotor doesn’t let you stop you any faster.

Rather, it dumps heat more effectively into the air because it has more surface area than the smaller rotor. The bigger rotor also doesn’t get quite as hot after a given stop because it has more thermal mass to over which to distribute the energy that pad friction dumps into it.

If your rotor isn’t overheating, then there’s not much reason to switch to a bigger one; it won’t stop you any faster in an emergency. (It will reduce lever effort by a small-but-perceptible amount, which some might prefer).



* Contact patch effects are both hard to measure and quite small for road bike tires; they’re hard to measure and a little bigger for mountain bike tires because of the way the tread interacts with soft trail surfaces. Undergrad engineering students are taught that frictional force is a product only of the coefficient of friction and the normal force at the contact patch. This is basically true. It falls apart at certain extremes, such as at the nanoscale, so sometimes people decide that that ignoring the contact area is a Big Lie rather than the reasonable heuristic it actually is. Of course, engineers also usually ignore relativistic effects because they’re so small at human-scale speeds, and no one other than paranoid schitzophrenics complains about the Big Lie of Newtonian mechanics. At any rate, the contact patch’s effect on traction doesn’t matter in this case, as it isn’t traction that’s keeping you from stopping harder—pitchover is.

Karvalo
Posts: 735
Joined: Fri Aug 10, 2018 6:40 pm

by Karvalo

youngs_modulus wrote:
Sat Mar 09, 2019 12:04 am
Karvalo wrote:
Fri Mar 08, 2019 8:53 am

You can and do brake way harder on a true MTB than a road bike.
What makes you say that?

On flat ground, both road and mountain bikes can lift the rear wheel under braking. When that happens, you’ve reached the max deceleration possible for a given bike/rider/position. (The location of your center of mass is what drives this, and that’s why tandems can brake harder than single bikes—they’re more resistant to endo-ing).

Mountain bikes have incrementally longer wheelbases than road bikes (which increases max braking before pitchover) but also put the rider/bike system’s center of mass higher than it is on the road (which decreases max braking before pitchover). To a first approximation, these cancel out.
My MTB has a front centre of 749, my road bike has a front centre of 577. I addition the much slacker head angle and shorter stem put the bars much further behind the axle on my mtb, and the dropper post allows me to weight shift much further, which I do much more agressively on the MTB than the road bike.

So what exactly is your definition of 'incremental'? :wink:

by Weenie


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