from Rookie’s Keyboard
Hello, friends
Today, I saw something I never though I will. A customer came to bike shop asking why his rear brake is “so weak and weird”. One look is all I needed to identify a major rookie mistake.
The issue was that the had a V-brake at the back coupled with an aero disc wheel – a.k.a. a disc wheel that doesn’t have a brake track and therefore should never be used with a rim brake of any kind.
I didn’t even ask who’d put the wheel on his bike as I didn’t want to get needlessly mad.
Here’s the truth, friends:
Wheels designed for rim brakes have a rim with a brake track. The brake track is wide and vertical so that the brake shoe of a rim brake can grab it effectively.
Also, the track is reinforced (thicker) so that it doesn’t wear prematurely.
Wheels designed specifically for disc brakes have no need for a brake track. This allows the producer to make the rim lighter and give it a more aerodynamic shape.
As result, a rim brake shoe cannot effectively grab a disc-only rim. To increase the contact area, one will have to run the rim brake dangerously close to the tire. This may result in rubbing and a tire blowout.
The only way to use a rim brake with a disc wheel is to buy or build a wheel that can take both – disc and rim brakes.
Such a wheel covers two requirements:
- A Disc Brake Hub
The wheel needs to have a disc brake hub to which one can attach a brake rotor. Standard rim brake wheels do not have this option as rim brakes don’t use rotors.
- A Brake Track
As explained above, the rim needs to have a brake track, or else it cannot be safely used with rim brakes.
Some people may think that a disc wheel needs a disc-specific rim, but that isn’t the case. A rim is a rim. Thus, a rim designed for rim brakes can be used for a disc wheel.
That said, it’s recommended to stay away from rims that use fewer spokes. When braking via disc brakes, the spokes have to deal with a lot of stress. If the bike wheel has a minimal number of spokes, it may fail to withstand the extra force.
Ultimately, it’s best to stick with 32 or 36 spokes per wheel.
The downside of using a combo wheel (designed for both braking systems) is a potential weight penalty due to the extra material needed for the brake track and the disc-ready hub.
In practice, however, the actual weight of the rim depends more on the model than the brake it’s designed for.
The table below compares the weight of 28″ rims depending on brake type. As you can see, the average weight of the rims is pretty similar.
In fact, the disc brake rims in the table are 37.28 grams heavier on average.
| Rim Brake Rims | Weight | Disc Brake Rims | Weight |
| Mavic Open Pro C 28″ | 435g | DT Swiss R 470 | 450g |
| DT Swiss R 460 | 460g | DT Swiss G 540 | 565g |
| Mavic A 119 28″ | 540g | H+SON Formation Face 28″ | 615g |
| DT Swiss RR 511 | 530g | H+SON The Hydra 28″ | 455g |
| Mavic CXP 28″ | 510g | White Industries G25A | 490g |
| DT Swiss X 412 | 440g | Mavic E-XM 430 Disc | 590g |
| Ryde Trace R 20 OS | 474g | DT Swiss RR 481 | 485g |
| Average: | 484.14g | Average: | 521.42g |
The next table compares the weight of front hubs. The first column is made of hubs designed for a rim brake whereas the second consists of disc-brake ready hubs:
| Rim Brake Hub | Weight | Front Brake Hub | Weight |
| White Industries T11 | 92g | Shimano XT HB-M756 | 239g |
| DT Swiss 350 | 110g | White Industries XMR 6-Bolt Disc | 175g |
| tune Mig Front Hub | 73g | Hope Pro 4 Disc 6-Bolt Front Hub | 173g |
| White Industries MI5 | 122.6g | DT Swiss 240 Classic | 190g |
| Novatec A291 SB-SL | 60g | DT Swiss 240 Straight Pull | 111g |
| Mavic Road UB Front Hub | 103g | SRAM Predictive Steering Front Hub | 165g |
| Average: | 93.4g | Average: | 175.5g |
Conclusion: The “rim brake” front hubs are 53% lighter than the disc-brake-ready ones. Part of the reason for that is that many rim brake hubs are designed for road bikes and are thus made lighter.
Conversely, disc brake hubs are common on MTBs and are subsequently more robust to cope with the terrain.
By building a rim that’s rim and disc-brake ready, one can expect a bit extra weight (approx. 100 grams) due to the disc hub.
Until next time,
Rookie
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