From Rookie’s Keyboard
Hello, friends
It’s midnight where I am, and I can’t sleep due to “unfavorable” circumstances. But I will use my insomnia to fuel another post for the rookie cyclists out there.
Yesterday, I began reassembling my old MTB and had the painful realization that my disc brake rotors were under their minimal thickness (1.58mm out of 1.8mm).
I am not a pedantic bro so you won’t see me hyperventilating because a part of my bike isn’t in the most optimal condition.
But the examination gave me an idea for a post – small vs. big rotors. Today, I will focus primarily on 180mm and 160mm models as those are the two most common sizes out there.
Let’s begin the analysis before I fall asleep.
IMO, there is no debate that 180mm rotors are simply better than 160mm when the option is available.
Why? Where do I begin?
- Supreme Stopping Power
180mm rotors have a larger diameter and position the brake caliper further away from the center of the rotor. The additional 20 millimeters give the caliper better leverage on the rotor resulting in supreme stopping power.
- Breaking With Less Effort
When all variables are equal (brake type, pads, levers…etc.), the same squeeze of the lever produces more braking power when the rotor is bigger.
In other words, 180mm rotors require less physical effort to match the stopping effect of 160mm.
But in the name, of truth, this doesn’t mean much in practical settings.
- Superior Heat Dissipation
On long descents, the brake rotors overheat due to constant friction against the brake pads. Larger rotors have greater thermal mass and need more friction to experience overheating.
Overheated rotors are a problem because they can melt the upper layer of the brake pads.
The melted brake pad gets on the rotor, precisely at the point of contact, and greatly reduces the friction forces between the rotor and the pads.
The result is a notable loss of braking power.
Another downside of overheating is disc rotor warping or in other words uneven wear of the rotor. Overheating speeds up this process by damaging the pads at random places. The downside is once again reduced stopping power.
- Better For Larger Wheels
Bigger wheels have a greater rotational mass and are more difficult to stop. Hence large rotors are welcome here. If you have 29″ wheels, definitely go for at least 180mm rotors. The difference in stopping power will be noticeable.
- Fat Riders Approved
Don’t get angry. This is how physics works. The larger the mass, the bigger the inertia. A truck carrying lots of cargo needs brakes far more powerful than what you find in a VW Polo. Bikes are the same way.
If you are on the fat side and/or transporting cargo, 180mm rotors are a better choice than 160mm… let alone the pathetic 140mm.
At this point, you are probably asking yourself whether 160mm rotors have any advantages at all.
Well, they actually do, but it’s nothing massive.
First, 160mm are lighter since they require less material. But the difference isn’t impressive as shown in the table below:
| 180mm | Weight | 160mm | Weight |
| NOW8 Float | 108g | NOW8 Ultralight disc | 67g |
| Magura MDR-C – 6-Bolt | 162g | Jagwire Sport SR1 | 129g |
| Magura MDR-C – Centerlock | 190g | Trickstuff Dächle HeavyDuty | 111g |
| FSA Afterburner | 155g | Rotor UNO | 95g |
| NOW8 Ultralight disc | 93g | NOW8 Float | 108g |
| Magura Storm Rotor | 152g | Magura MDR-C | 140g |
| SRAM Centerline Round Edges Rotor | 112g | SRAM Centerline Round Edges | 112g |
| Shimano Deore XT SM-RT86 | 136g | FSA Afterburner | 125g |
| Jagwire Sport SR1 | 153g | Carbon-Ti X-Rotor SteelCarbon 2 | 77g |
| Trickstuff Dächle HeavyDuty | 154g | Shimano RT86 Ice-Tech | 113g |
| Average: | 141.5g / 4.99oz | Average: | 107.7g / 3.79oz |
So, on average 160mm rotors are 30 grams lighter. In other words, you will save 60 grams total if you go for two 160mm units.
Sorry but in what world does that matter? Certainly not in mine.
That said, there is one advantage of smaller rotors that is worth talking about.
Smaller rotors are less aggressive and don’t “bite” as quickly.
The lack of aggression increases the brake’s modulation.
Modulation is a fancy word for control over the braking force.
Bigger rotors initiate high braking forces faster. That’s considered aggressive modulation and in some situations it’s not ideal.
For that reason, some people prefer to run a smaller rotor on the back of the bike and a larger one at the front.
Never forget, friends, that most of the braking done by any vehicle happens via the front wheel(s).
Why? Well, when you initiate braking, there’s a weight shift to the front. That weight shift increases front end traction at the expense of the rear end.
The increased front traction results in a higher braking power initiated by the front wheel. Hence the importance of having a strong front brake.
Meanwhile, the less aggressive rotor at the back will give you better brake modulation and thus control when you just want to slow down a bit on technical terrain.
This combination gives you the best of both worlds.
+ Bonus Guideline
Sadly, I still don’t feel like sleeping, but luckily for you, this allows me to talk a bit about how to switch from 160mm to 180mm rotors if you so desire.
Below is a short guideline that explains just that.
Step 1: Identify The Disc Mounts On Your Bike
There are three types of disc brake mounts – International Standard, Post Mount, and Flat Mount.
International Standard(IS) is the norm for older forks and frames. The brake caliper connects via bolts screwing perpendicularly to the mount. Modern fork manufacturers have completely abandoned the International Standard.
If you have IS mounts and want to switch from 160mm to 180mm rotors, you will have to buy +20 IS to Post Mount adapter.
Post Mount (PM) is the most common today. The caliper attaches via bolts going straight into the mount.
If you have PMs and 160mm rotors and want to upgrade to 180mm, you will need a 20mm post-mount adapter.
Flat Mount(FM)
Flat mount is a fairly new disc mount standard found on road bikes. The caliper attaches to a flat part of the fork or frame. The goal is lighter weight and simplicity.
Currently, flat mounts are limited to 160mm rotors. So, you won’t be able to upgrade to 180mm.
Step 2. Install the Calipers
Once you have the right adapter on, install the brake calipers.
Step 3: Replace the Old Rotors
Remove the wheel, unscrew the rotors, and screw on the new ones. Get the bolts to the torque required by the brake manufacturer.
Step 4: Install the Wheels
Note: I realize that the guide above isn’t the most detailed. But at the moment (1 a.m.) where I live, this is all the information that I can provide. I plan to come back to the post and update it when I have the time.
Well, friends, I hope this post has been helpful to you.
Stay tuned for more.
Until next time
– Rookie
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