Disc vs Rim brakes, and why I'll stick with rim for now

SHOULD YOU TRASH YOUR RIM BRAKES? 

• January 31, 2017
• Published in Issue No. 747

By Jim Langley

If this week’s Tech Talk title caught your attention, it was supposed to. But I didn’t really make it up – I pretty much stole it. I paraphrased the title of the online Outside magazine article that ran recently, Why You Should Throw Your Rim Brakes in the Trash.

The author basically says that discs on road bikes are so much better than rim brakes that you... well, you already know what he wants you to do with your rim brakes.

Rim brakes are disc brakes, too

The problem with this point of view is that it ignores the basic fact that rim brakes are essentially disc brakes. Instead of the separate discs (called “rotors”) attached to the hubs with disc brakes, rim brakes rely on much, much larger discs, the rims.

This much larger disc found on all road bikes with rim braking actually has a huge mechanical advantage over even the finest disc brakes. The first bicycle engineers way back in the 1870s already understood this.

The earliest bikes were directly driven by pedals attached to the front hub. Braking was done by holding back on the pedals the same way fixies without brakes are stopped today. It was and is a form of hub braking a little like disc braking.

But after only a few years of foot braking, the limitations were obvious and mechanical braking arrived. These first brakes weren’t very good yet because they rubbed on the tire, and only one tire. But they were attached to the top of the wheel, showing that engineers were taking advantage of the leverage principle.

Let’s fast forward to 2017 and hear from Santana Tandems, a company that has done a lot of brake analysis. When you’re designing the fastest descending bikes in the world and responsible for the safety of two riders per bike (or more), maximum speed control is all-important.

3 times the leverage

Quoting Santana, “On a 700c road bike rim brakes apply power at a leverage point that’s over three times more effective than that of an 8-inch disc. While rim brakes may seem crude or old-fashioned, a 300% difference in leverage (and braking power) can’t be ignored.”

That’s not to say that disc brakes aren’t powerful enough for single road bikes. Just that rim brakes offer even more power. Santana points out, for example, that a rear disc on a tandem cannot cause the rear wheel to skid. Yet rim brakes can easily do it.

So you might ask why some roadies love their new disc brake bike and swear that they brake so much better than their former rim brake bike. It’s entirely possible that’s the case. However, it’s probably because the rim brake system on the old bike is of lesser quality, old, worn out or damaged.

Disc brakes add weight

The other issue with disc brakes is that they make road bikes heavier. Yes, maybe it’s not enough additional weight to matter to some riders. But it’s undeniable that they’re heavier. And more weight means more energy is required to ride the bike.

One reason disc brakes are heftier is that they’re composed of more parts, mainly the rotors. Which also require a mounting place on the hubs, making them heavier, too. Plus, the cables/hoses are longer and for hydraulic discs there need to be fluid reservoirs.

The other thing that adds grams is beefing up the frame at the front and rear so it can withstand braking forces at the hubs. Note that rim brakes are attached at two of the strongest parts on the frame (seat stays and fork).

And even though cool mountain bike technology like thru axles are making their way onto disc brake road bikes to add the strength needed for the braking forces, these changes add even more weight.

Rim brakes, disc brakes, it’s all good

All of this is not to say that there’s anything wrong with disc brakes on road bikes. They work great, especially in wet weather. They also keep working even if the rims get damaged. And disc brakes don’t drag on and wear rims like rim brakes do.

But if the bike industry were to lose its marbles and follow Outside magazine’s recommendation and nix rim brakes for good, it would come at a cost in stopping power and pedaling efficiency. Which is a high cost if you’re looking for maximum road bike performance. Instead let’s just keep improving rim and disc brakes and let riders choose which they prefer.

For further reading on the pros and cons of disc brakes, the best article I’ve seen recently is by Jan Heine on his wonderful blog Off The Beaten Path.

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Jim Langley is RBR's Technical Editor. He has been a pro mechanic and cycling writer for more than 40 years. He's the author of Your Home Bicycle Workshop in the RBR eBookstore. Check out his "cycling aficionado" website at http://www.jimlangley.net, his Q&A blog and updates at Twitter. Jim's streak of consecutive cycling days has reached more than 8,000. Click to read Jim's full bio.