13

I'm interested in putting a discs brake on the right side of a bicycle. The goal is to have dual front discs, similar to how motorcycles often use double front brake discs.

I can buy or make a custom hub with two discs. That won't be a problem. And I can arrange a second brake tab on the fork, or even make a custom fork altogether. The problem in question is that disc brakes are engineered to fit on the left side of the bike only. Mounting one on the right side would be straightforward, but the braking forces would be reversed compared to the normal mounting. It would be like braking while the bike was rolling backwards. Would this cause any problem? Are the brake pads designed to take brake forces in either direction equally?

Back in the days of u-brakes and v-brakes, we saw them mounted both ways by being mounted sometimes to the top or bottom of the frame stays. So we know those worked either way. But I have not seen the same variation for disc brakes.

Improve this question
7
  • 8
    Possibly a little off topic but for my own interest, what on earth are you doing where you needing dual front discs?
    – Hursey
    May 16, 2022 at 4:18
  • 1
    Yeah, I'm wondering why this would even be needed. A well adjusted disk brake is enough to send you flying over the bars. Certainly you don't need more power ,and if you do need more power than a larger rotor would suffice. If you just want to actuate the front brake with either hand, then other solutions exist.
    – Kibbee
    May 16, 2022 at 14:56
  • @Kibbee Perhaps the idea is a trandem to be used for sustained descents when thermal management is going to get problematic even with a big rotor.
    – Chris H
    May 16, 2022 at 15:57
  • 3
    Off topic, but the reasons are double: Tandem bike, where braking power is needed and braking overheating is a real problem. And second, to enable a second lever position on the handlebars....previously I ran an "interruptor" lever so that I could have two different levers on the same brake. This isn't possible with hydraulics to my knowledge. And as a side benefit, it will be another completely redundant brake system, with future potential of modifying one of the brakes into a "drag" brake.
    – BetterSense
    May 16, 2022 at 19:52
  • 1
    I would be easer to install a rim brake with a discs, but dual discs is cooler (not in thermal management sense of the word :) ).
    – mattnz
    May 16, 2022 at 21:05

3 Answers 3

Reset to default
7

Tadpole trikes have mostly had standard disc calipers running backwards on the left wheel for as long as they've existed as a category. There's not really a major concern here. How to design the mount for a given application is its own question that has ramifications on strength and reliability.

It's possible to imagine a caliper designed in a way where it's adequately strong in the normal direction but not backwards. Without doing a study on various caliper designs, it's probably pretty likely you'd find that on ultralight calipers if at all, and probably not at all.

Improve this answer
4
  • Just to clarify, when you say “running backwards”, do you mean that the caliper is effectively on the front side of the fork leg?
    – MaplePanda
    May 16, 2022 at 5:29
  • @MaplePanda Yes, on a tadpole trike the left hub has the rotor mounted on its right side, and the caliper runs flipped.
    – Nathan Knutson
    May 16, 2022 at 5:44
  • Oh, that's cool! Thank you, I will delete my answer since it contradicts yours.
    – MaplePanda
    May 16, 2022 at 6:29
  • @MaplePanda I think you made fine points about how a particular design could be optimized for loading in one direction and you should leave it up. I just think that the tadpole trike thing is decent supporting evidence that it's typically within the margin and works without issue.
    – Nathan Knutson
    May 16, 2022 at 6:49
5

You are creating a custom project, which reminded me of another custom project I saw recently:

https://www.englishcycles.com/custombikes/project-right/

The bike in the above link has a single blade fork but a "righty", not a lefty. The maker has modified an Avid BB7 to fit the trailing side of the fork rather than mounting it upsidedown on the leading side, (which is another option), preserving the smooth look.

Improve this answer
9
  • 1
    Running hydraulics upside down would seem easier - right up until you wanted to bleed them of course. Hydraulics would also split the braking force more consistently than mechanicals if used with a single lever
    – Chris H
    May 16, 2022 at 14:02
  • @ChrisH It would require a oversized master cylinder or smaller slaves to maintain a reasonable/normal braking feel, so not necessarily the easy option.
    – Noise
    May 16, 2022 at 15:14
  • I wonder if using the lever assembly for a 4-piston brake with 2x2-piston brakes would have that effect - you'd get twice the heat dissipation (or very nearly) of a single rotor, and even entry-level 2-piston brakes from decent makes have stopping power to spare (I've just put basic MT-200s on my MTB and they're an absolute joy when picking my way down difficult stuff, hardly any force needed at all)
    – Chris H
    May 16, 2022 at 15:26
  • ... or are the pistons noticeably smaller when there are 4 of them?
    – Chris H
    May 16, 2022 at 15:28
  • 1
    @ChrisH the pistons are smaller on 4-pot calipers where compatibility with 2-pot levers is required eg, Magura, Shimano etc. Often not all 4 are the same size either, to encourage one end of the pad to make contact early
    – Noise
    May 16, 2022 at 16:32
3

Edit: Note that my answer is all purely theoretical speculation. Nathan’s factual answer establishes the reality that the limitations I highlight are not problematic in real life.

From looking at XTR M9020 calipers for example, they are indeed designed asymmetrically. When one applies the brakes, I suspect the net force ends up being a torque, where the caliper wants to pivot in the leading direction with the leading bolt acting as the pivot point.

enter image description here

You can see how the caliper’s “body” doesn't appear to be centered between the post mount tabs. It looks to be more substantial on the leading side (left side in the image), which makes sense since the leading side is closer to the pivot point and therefore sees a higher force (since torque = force * distance). Accordingly, the following half of the caliper (right side in the image) sees less force, and thus the rear mounting tab can be safely located at the end of a thinner "arm" (as it is). Installing the caliper in reverse would mean that the thin arm leading to the rear tad would bear the brunt of the braking force-generated torque instead, which I don't think is a particularly good idea. (Edit: it’s fine.)

Of course, different calipers have different designs, so this may not be valid for the exact model of calipers you plan on using. However, the other calipers I own (Shimano MT200, Tektro Spyre, and Tektro HD-M275) all somewhat share this asymmetric design, so perhaps some caution is indeed warranted.

While I try to avoid overapplying the "It's your brakes, so you should be extra cautious" viewpoint, some level of hesitation is probably warranted. Do keep in mind that calipers have to accommodate reasonable levels of reverse-direction braking (eg. when someone rides backwards and applies the brakes), although I would assume the ultimate failure load is lower in the reverse direction.

Improve this answer

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.