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Discussion Starter · #1 ·
I’m using Shimano RT-99’s, 140mm front and back. I weigh 200lbs.

I haven’t noticed any trouble stopping. No brake fade. Nothing. I’m wondering why 160mm are becoming more commonplace now.

What do you folks use, and why?
 

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My road bike has rim brakes.

When it came time to build my new MTB frame, I decided to go with new "stronger" 4-piston brake calipers and gave my old brake set to my wife to replace her lower end discs. I think she ended up with a 160 mm rotor up front whereas the old one was 140 mm.

Funny thing, I always felt my old set up worked great. She felt the same about her's. After the swaps, we both had the same reaction, "wow, I didn't think it would be a big improvement but it is".

I'm not saying any brake system is inadequate, but sometimes you don't know what you're missing until you make a change.
 

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Adorable Furry Hombre
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You use larger disk rotors when you need more stopping power.

As in steeper/longer inclines (say MTB descents) or higher loads (as in say touring or on tandem bikes). My gravel/touring bike is spec'd with 160s. Which is nice when I'm hauling 13kg of bike, 78kg of myself, and another 30kg of panniers/stuff.
 

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How much stopping power you have all comes down to the tire's contact patch with the road. You can throw 500mm brake rotors on your wheels and it won't matter once the tire starts to lose it's grip. I suppose anti-lock disc brakes will be a thing on bikes at some point, but until they are, contact patch rules.

Reasons to go with bigger rotors:
  • Your total weight (bike + rider) is relatively high. My opinion would be if total weight is 200 lbs+, then use 160mm rotors.
  • You live in a hilly or mountainous area.
  • Your bike has heavy tires, like a fat bike, where most of the mass is far from the hub.
  • You ride in an urban area or otherwise busy traffic area where stopping quickly could be the difference between life and death!
Reasons to go with small rotors:
  • You're a weight weenie... logic and safety be damned! ;)
  • You're a light weight rider on a light weight bike (< 200 lbs total, IMHO)
  • You typically ride solo in rural areas where emergency stops are rare to non-existent.
  • You're trying to maximize aerodynamics, like on a TT bike.
I think this would be a good topic for the GCN Tech show on YouTube... do a test of a road bike to see when or if one ever actually 'needs' the larger 160mm rotors. GCN did do a video on this topic and showed shorter stopping distances with the 160mm front disc.

I weigh 167 lbs and have 140mm front and rear Shimano Ultegra rotors on my 2020 Trek Domane. So far, I haven't had a reason to go bigger for riding in the mildly hilly Midwest.
 

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I've read elsewhere that the 160mm discs show an improvement over the 140's when used with mechanical discs. I'm a rim brake guy so no experience, but that makes sense to me.
 

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Just generally speaking, a 160mm rotor will brake a little better and run a little cooler than a 140mm. There might also be some marginal gains in pad wear for the reasons stated previously.

The thing about this sort of stuff is, you generally don't need it... right up until you do. You haven't had any problems with 140mm rotors, and might never, but that one time you are bombing down a long fast decent and your pads and rotors overheat will be very thrilling. :)

I cooked a 160mm rotor (and pads) on my first disc brake bike. Mostly due to lack of knowledge and experience (i.e. user error), but let's just say I haven't been down that hill since that day. :)
 

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Plain and simple - more surface area, more heat dissipation. If your fluid starts to boil going down a steep hill, there goes your braking power. Some mountain bikes have even larger rotors now.
 

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gazing from the shadows
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I’m wondering why 160mm are becoming more commonplace now.
The technical aspects of rotor size are pretty simple, and mentioned. But the vast majority of people who buy a bike won't push their brakes to the limit where they will notice a difference.

Marketing reasons are also simple. Big rotor looks bigger on the showroom floor, so even noobs will believe that a bike with bigger rotors has "better" brakes.
 

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Discussion Starter · #9 ·
The technical aspects of rotor size are pretty simple, and mentioned. But the vast majority of people who buy a bike won't push their brakes to the limit where they will notice a difference.

Marketing reasons are also simple. Big rotor looks bigger on the showroom floor, so even noobs will believe that a bike with bigger rotors has "better" brakes.
Admittedly, on a road bike, I like the look of smaller (140mm) rotors.

On my mountain bike, I like the look of larger rotors. I run 203mm rotors on it.
 

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The physics of it have already been well covered, but what you need really comes down to how much you weigh, and the terrain you ride. I find that road bike discs get the worst abuse compared to mtbs because while the grades aren't always as steep, they are much more sustained, and the speeds are higher. Where I live in Colorado, you can come into a switchback at 45-50mph on the road, and repeat that over and over. On a mtb, you brake more often, but in shorter bursts and at half that speed. Accordingly, I run the biggest rotors I can fit on a road bike and can still easily turn them purple and get the pads hot enough to start to lose friction. Which, is easily remedied by opening them up for a second, but still. I'd happily run 185mm rotors up front if I could get them to fit.

If you live somewhere with rolling terrain and no long sustained descents, 140s would be ok if you're light and 160s perfect for everyone else.
 

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The physics of it have already been well covered, but what you need really comes down to how much you weigh, and the terrain you ride. I find that road bike discs get the worst abuse compared to mtbs because while the grades aren't always as steep, they are much more sustained, and the speeds are higher. Where I live in Colorado, you can come into a switchback at 45-50mph on the road, and repeat that over and over. On a mtb, you brake more often, but in shorter bursts and at half that speed. Accordingly, I run the biggest rotors I can fit on a road bike and can still easily turn them purple and get the pads hot enough to start to lose friction. Which, is easily remedied by opening them up for a second, but still. I'd happily run 185mm rotors up front if I could get them to fit.

If you live somewhere with rolling terrain and no long sustained descents, 140s would be ok if you're light and 160s perfect for everyone else.
^This.

Road bikes can really get up there in speeds. One could really put a lot of heat into a braking system on the road, MUCH more compared to mtb, reason is because mtb tires will lose traction way before the brake system gets hot, where as on the road, the road tires (in spite of their skinnier size) can really allow a lot of braking fore before they will slide out.

Also, one thing I haven't seen mentioned in here is that a larger rotor will alow for a better modulation (everything being equaled).

However, a larger rotor does have its drawback too, it's easier to warp and get distorted (thus causing pad rubbing that you will not be able to get rid off unless you buy a new rotor). I find that when the rotor gets to the 180mm range and above, that's when the noise that it makes (when it makes it) also tend to get amplified compared to a smaller rotor.

Another reason to go 160mm front and 140mm rear is that the UCI is standarizing to these sizes. This will force manufacturers to concentrate their developements and offerings in these sizes.
 

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However, a larger rotor does have its drawback too, it's easier to warp and get distorted (thus causing pad rubbing that you will not be able to get rid off unless you buy a new rotor). I find that when the rotor gets to the 180mm range and above, that's when the noise that it makes (when it makes it) also tend to get amplified compared to a smaller rotor.
Warping is a concern and is even more of an issue with rotors which have an aluminum spider. The purpose of an aluminum spider is to dissipate more heat - aluminum is an awesome conductor next to copper and silver. But aluminum bends more easily and can get hot enough to actually melt. Solve one problem, create another.
 

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^This.

Road bikes can really get up there in speeds. One could really put a lot of heat into a braking system on the road, MUCH more compared to mtb, reason is because mtb tires will lose traction way before the brake system gets hot, where as on the road, the road tires (in spite of their skinnier size) can really allow a lot of braking fore before they will slide out.

Also, one thing I haven't seen mentioned in here is that a larger rotor will alow for a better modulation (everything being equaled).

However, a larger rotor does have its drawback too, it's easier to warp and get distorted (thus causing pad rubbing that you will not be able to get rid off unless you buy a new rotor). I find that when the rotor gets to the 180mm range and above, that's when the noise that it makes (when it makes it) also tend to get amplified compared to a smaller rotor.

Another reason to go 160mm front and 140mm rear is that the UCI is standarizing to these sizes. This will force manufacturers to concentrate their developements and offerings in these sizes.
While I agree that road bikes will normally see higher speeds and generate a lot of head in braking events mountain bike brake systems definitely see a LOT of heat. 180mm Ice Tech rotor shown:

 

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While I agree that road bikes will normally see higher speeds and generate a lot of head in braking events mountain bike brake systems definitely see a LOT of heat. 180mm Ice Tech rotor shown:

That sure looks like gouging due to brake pads being totally worn down.
 

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I’m using Shimano RT-99’s, 140mm front and back. I weigh 200lbs.

I haven’t noticed any trouble stopping. No brake fade. Nothing. I’m wondering why 160mm are becoming more commonplace now.

What do you folks use, and why?
I switched from 140's front and rear to 160 front and 140 rear shortly after buying my disc bike in early 2014.

I live for twisting downhills and my decision had nothing to do with over all stopping power or heat. It had more to do with making lever pressure more equal on both levers during rapid deceleration.

It can be argued (effectively) that by adding power to the front brake you are exacerbating the loss of effectiveness of the rear--after all, we are mostly riding on the front wheel during heavy deceleration on steep downhills anyway so why make it worse? The answer relates to the willingness to move ones' ass aft and below the seat to regain some of that rear wheel contact.

Admittedly this MTB technique is not widely accepted by roadies.
 

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From my experience with a decade or more on MTBs with discs and just a couple of years on disc equipped road bikes, it's less about stopping power, and more about the effort required to stop.

On my MTB I have a pair of 180mm discs, on the road bike it's 160/140 F/R. On my first real road bike it was regular rim brakes and I hit a hairpin on a fast road descent (that I'd ridden before on my MTB) and ran very wide on the corner - why? because you need to apply much more effort with rim brakes vs discs to get the same stopping power.

What I love about discs is not that I can stop quicker, but that it takes less effort. The larger the discs, the less effort you need to apply. Light controlled 1 finger braking means I can focus instead on my line, I'm more comfortable, more confident.

You need smaller on the rear because it locks up easier than the front, so 160 back there isn't necessary. Until a recent frame change I was 160/160 on the Defy, now 160/140 on the Supersix. Where I ride, I'd not want to drop to 140 up front.
 
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