Mountain bike discs have to be big and small? The reverse is better

The front of the brake disc is large and the rear is small. This is the setting we have been used to for many years. In addition, at most discs of the same size before and after are used. However, the latest practice has found that it is better to use a larger rear disc. Why?

If you like to ride at high speeds, then you must also stop fast, at this time the disc will be under great pressure. In order to save weight, manufacturers now try to reduce the size of the disc under other conditions. For example, Shimano RT86 discs, compared with the size of 203mm and 180mm, increase a part of the size, only a few grams of weight are sacrificed, but it can greatly improve your braking performance. So instead of replacing a more advanced brake system, you can use a cheaper way to replace larger discs.

If the rider is lighter and doesn't have long downhill slopes, most of the original discs will be enough. However, if you weigh more than 80 kilograms, or like long-distance downhill, you should pay attention to the disc. Under normal circumstances, after a trip down the mountain, the rear disc will overheat, and even change color after cooling, while the front disc has basically no change.

Usually the front wheels require more braking power than the rear wheels, which is why the front disc is larger than the rear disc. In theory, when you brake before entering a corner, the braking force of the front wheels is guaranteed to be more than 70%, and the braking force of the rear wheels accounts for 30%. However, the decisive factor that causes your disc to overheat is not those short and violent sudden brakes. But when going downhill, you need to rely on the rear brake to adjust the speed of the car, so as to make the front wheels easier to control and produce maximum grip, so the continuous braking of the rear wheels will bring higher temperatures and the braking force will be weakened. .

Why does the vehicle brand dislike original large-size discs? Although large discs can generate greater braking power while reducing heat, they also have shortcomings, one of which is weight. Nowadays, many consumers buy cars using lightweight as an important reference factor. On the other hand, large discs are prone to deformation and noise. Users don't want to adjust the discs twice in three days.

Replacing a 180mm disc with a 203mm disc can increase the braking area by about 11%. For example, replacing it to 220mm will increase the braking area by 24%, which makes the brakes more effective in heat dissipation. A 180mm SRAM Centerline disc weighs about 151g, and the 200mm version is 207g, an increase of 37%, but the difference in weight (223g) between 220mm and 200mm is only 11.5%. The temperature on the disc is a key factor in determining the braking force. According to the measurement, under the same conditions, the temperature of a 200mm disc to 220mm drops by 39°C. This reduced temperature can increase the braking force by about 10%.

The large discs not only provide stronger braking power, but also reduce the pressure on the hands and limbs of the driver, because you don't need to pull the brake lever as hard as using small discs, so the car will be better controlled. Of course, in addition to choosing the right disc size, there are other ways to improve the braking performance, such as choosing the front resin rear wheel metal to make the disc. The resin is softer, the initial braking force is better, the overall linearity is better, and the metal is more heat-resistant, which is more suitable for the rear wheel.

At present, MAGURA, SRAM, Trickstuff and GALFER have all introduced 220mm or 223mm discs, but users need to check whether their front forks and frames support such large discs.

Generally speaking, the use of the front brake is short and powerful, and the rear brake is used almost all the way downhill, so it will generate more heat. So for aggressive Enduro drivers and 29-inch models, 200mm discs are a must. For heavier riders, 220mm discs are better.


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