Ending the rotor size myth – Why you should have a bigger brake rotor at the back than the front

Large brake rotor up front, small at the rear: that’s the setup we’ve been riding on our mountain bikes for years. At best, you’ll have the same size rotor front and rear. However, we’re of the opinion that a large rotor up front and an even bigger one at the rear would be better still. Why is that?

If you like to ride fast you want to know that you’ll be able to slow down just as quickly. Disc brakes on mountain bikes are subjected to extreme forces and frequently they’re not up to the task. The problem is that manufacturers try to save weight and an easy way to do this is downsizing rotors and speccing lighter brakes. However, comparing SRAM CODEs and G2s, we found that SRAM CODEs are actually the better option for almost every rider. Only a few grams of extra weight bring significant benefits in terms of performance, fun and safety. An easier (and cheaper) way to increase the power of your brakes is to fit larger rotors.

Heavy rider and steep terrain: a nasty combination

Lightweight riders who avoid long descents usually won’t have any problems with the standard brakes on most bikes. But, if you weigh upwards of 80 kg and often tackle descents of several hundred metres or more, you should take a closer look at the colour of your rotors. In almost all cases, it’s the rear rotor that takes the brunt of the braking, overheating and discolouring after cooling down. In contrast, the front disc usually retains its normal silver colour.

Braking force distribution myth – how do you really brake?

You need more braking power at the front than at the back: this is the usual argument many riders make for having a bigger rotor up front compared to the back. It’s true, in principle, If you hit the brakes before going into a corner, you’re guaranteed to have upwards of 70% braking force at the front and a maximum of 30% at the rear. However, the decisive factor that leads to an overheated rotor is not those short, hard braking manoeuvres, but what you do throughout the descent. While engine braking on a motorcycle slows down the rear wheel as you let go of the throttle, the only thing keeping your speed in check while you descend on a mountain bike is the brakes. That means they have to dissipate a lot of energy in the form of heat. Usually, we use the rear brake to maintain our speed, because that way the front wheel remains easy to control and is able to generate the most cornering grip. Unless you want to do a stoppie to lift your rear wheel around a switchback, you wouldn’t normally want to lock up your front wheel. The constant friction at the rear results in significantly higher temperatures, which leads to fade and ultimately to overheated brakes and discoloured rotors.

Why bike brands don’t like speccing large rotors

Although large rotors have the advantage of delivering more braking power while building up less heat, they also have disadvantages. First of all, weight. A larger rotor is heavier and many customers still watch the scales closely when choosing a new bike, meaning bike brands are hesitant to make the sacrifice. We at ENDURO have long believed that we should stop weighing our bikes and we’ve written an article about this in the past. Another reason that large rotors are less commonly used is that they tend to warp more easily, resulting in rubbing pads and noise. It can be a real problem for a dealer if a customer repeatedly has to return to the store to have their rotors straightened.

All things being equal, changing from 200 to 220 mm rotors reduces the temperature at the pad by 39 °C, which simultaneously results in a 10% increase in braking power!

Larger rotors for improved heat management

Replacing a 180 mm rotor with a 200 mm model increases the braking surface by around 11%. Go straight to 220 mm and the increase is 24%. This allows the brake to dissipate heat much more effectively. The difference in weight depends heavily on the rotor itself. A 180 mm SRAM Centerline rotor weighs about 151 g. The 200 mm version is about 37% heavier at 207 g. The difference between a 200 mm rotor and a 220 mm rotor (223 g) is only 11.5%. The temperature at the brake pad is a crucial factor in determining a brake’s stopping power. According to SRAM, the temperature drops by 39 °C when going from a 200 mm to 220 mm rotor under the same conditions. This 39°C temperature reduction, in turn, is said to deliver about 10% more braking power.

A smaller rotor up front offers better modulation

There is a lot to consider when looking for the perfect rotor size, including your body weight, riding style and also the terrain on which you ride. In his own experiments, our editor Christoph has always ended up going to the largest rotors he can get. He quickly upgraded from 200/180 mm to 200/200 mm and since 220 mm rotors have become available, to 220/220 mm. The larger rotors not only provide more power but they also put less strain on your forearms and shoulders since you don’t need to pull the brakes as hard, keeping you fresher for longer and allowing you to ride more actively. However, depending on the terrain and the ground you’re on, huge rotors up front can be too powerful, making them difficult to modulate. Since the power of a 200 mm rotor up front in combination with a powerful four-piston brake is generally sufficient, Christoph has actually ended up downsizing the front rotor. It’s also worth noting that many of the current forks on the market aren’t officially approved for use with 220 mm rotors and struggle to deal with the forces, making them creak. For this reason, Christoph now rides with a 220 mm rotor at the rear and a smaller 200 mm rotor up front.

Additional tips for more power and improved modulation

In addition to choosing the right size rotors, there are other things you can do to improve the performance of your brakes. We’ve summarised six of the most useful tips for more stopping power in a separate article. For example, Christoph usually installs organic pads at the front and sintered pads at the rear. Due to their softer structure, the organic pads have better initial bite and are easier to modulate, whereas the metal pads at the rear are more heat resistant. An important factor that increases power regardless of rotor size is properly bedding in the pads.

Who offers 220 mm rotors?

Currently there are 220 mm or 223 mm rotors available from MAGURA, SRAM, Trickstuff and GALFER. Before fitting them on your bike, it is important to check if your fork and frame are compatible and approved for that size rotor. FOX’s new 38 fork, which we’ve already reviewed in a separate article, is approved for 220 mm rotors.

Conclusion

After having ridden every brake on the market with almost every rotor combination you can think of over the last few years, our conclusion are as follows:

• We continue to encounter many bikes specced with brakes that aren’t powerful enough, especially on the larger frame sizes intended for heavier riders.
• You usually brake harder and shorter at the front. In contrast, the rear brake gets dragged a lot to control speed, generating a lot more heat.
• For aggressive enduro riders with large 29” wheels, 200 mm rotors are mandatory and upgrading to 220 mm rotors is worthwhile for heavier riders.
• Trail bikes also benefit from powerful brakes. Unless you’ve got powerful brakes such as SRAM CODEs or a four-piston Shimano model, we would always resort to 200 mm rotors.