The problem with mechanical disc brakes…

28 01 2011

Avid BB7's on my Surly Big Dummy...

I must say upfront that this post is a bit of a rip off from an article Jan Heine posted in Bicycle Quarterly.  However, it’s something I was thinking about before I read that article and I’ll add a bit to what Jan discussed.

All of my initial experiences with disc brakes were with Avid mechanical discs.  This was simply because they were popular and easy to work on at home since they used the same brake housing and cables I was used to.  That meant I could overhaul my bike at 3am without a visit to a LBS for help bleeding hydraulic brakes. To their credit these brakes work well. I find the stopping power of a set of quality v-brakes and Avid mechanical discs comparable.  In the dry I can flip my bike over with both and in the wet they both stop the bike well albeit not as well as when dry.

I won’t argue there is no difference at all between a good set of v-brakes and a good set of mechanical discs, but the difference has not been in the range that it matters to me one bit which I use.

One issue I’ve had with mechanical discs is that after the initial part of the lever travel that does stop the bike reasonably well there is a very mushy feel to the lever as you keep pulling that seems to have little additional braking effect.  It’s worse on the rear wheel although that wheel isn’t particularly important for stopping the bike.  Since they work fine it’s not something I’ve spent too much time worrying about.  Then one day Bicycle Quarterly published an article that explained why I was experiencing this.

If you imagine that a rim brake is really a disc brake with a very big rotor.  This gives it a lot of leverage to stop the bike.  So that generating the same braking force  on a disc brake bike takes more pressure on the brake pads than for a rim brake bike.  Just the same as if you used a long and a short pry bar to open a wooden crate you have to push harder on the short pry bar to generate the same force at the far end of the lever.  In some ways the higher pad pressure of a disc brake system is a good thing since this is what is supposed to give it better wet weather braking as the pads squeeze water off the disc rotor more effectively than the lighter pressure from the rim brake pads on the bigger rotor that is the rim.

The problem is that the housing used for the mechanical brake cable is only able to resist the compression forces of the brake lever to a certain point.  Before that point most of the power you put into the lever gets to the disc pad and squeezes the rotor resulting in good braking.  Beyond that point more and more of the extra force you put into the lever goes into compressing the brake housing.  This means as you double the force you only get a small increasing in brake effect at the rotor.  This explains why after some good initial braking the mechanical disc brake lever feels mushy and doesn’t seem to have much effect.  It also explains why you can brake effectively with rim brakes since they don’t require the same high forces. Rear disc brakes also tend to use a long full run of cable housing which exacerbates the problem.

So what can you do about it?

  • if you want to stick with mechanical discs use some high quality brake housing that resists compression better.
  • if you’ve got $$ to spend install compressionless metal cable housing like the one made by Nokon.
  • if you are buying brakes go with hydraulic disc brakes since they don’t have this problem
  • the cheapest solution is to understand the issue and use the braking your mechanical discs provide…when you get to the mushy part of the lever’s pull don’t bother squeezing harder since you know not much will happen.

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31 responses

28 01 2011
dan

Do you have disc brakes on anything other than your Big Dummy? I have avid BB7s on my Big Dummy and find the rear to be a bit squishy, but that’s really only because it requires a really long run of continuous housing. IME, rear mech discs on normal frames don’t have the same problem, especially if there are housing stops over the top tube. My ancient touring bike requires a full run of housing to the rear cantis and it’s much squishier than my crosscheck with housing stops. As always, YMMV.

28 01 2011
thelazyrando

@Dan – here are my brakes:

Big Dummy – was Avid BB7s now Shimano SLX hydraulic
Pugsley – Avid BB7s
1×1 – was Avid BB7 now Shimano SLX hydraulic
LHT – Deore v-brakes
NWT Deore v-brakes
Tikit – no name v-brakes
BBC cross bike – Shimano cantis
SC Nomad – Avid Elir CRs hydraulic discs

Definitely the longer the housing run the worse the problem.

28 01 2011
UnderDaHill

I was about to give you a hard time about that list of bikes. But then I realized that they all add up to less than the cost of a new car. But your house must look like a bike shop. ;-)

28 01 2011
Chris Sorlie

Vik,
Can I ask you a NWT / LHT question? Is either bike more comfortable? How bad are Canadian customs? (I live north of Toronto)
Thanks for your input!

28 01 2011
Champs

I’m inclined to disagree. The braking performance is similar to cantis, just with a lot less setup fuss.

29 01 2011
thelazyrando

@Chris – I got my NWT sized based on my LHT and use the same saddle and similar pedals so they pretty much fit the same. Canadian customs will charge you $8+HST on the bike when it arrives…some carriers charge an additional brokerage fee of $40-$50.

They are both great bikes…the big wheels on the LHT are better on rough roads and unpaved roads….the NWT folds and is a ton of fun to ride.

29 01 2011
thelazyrando

@Champs – the physics of the compression of the brake housing is what it is…the disc does need more force on the rotor than a brake pad does on a rim. Just looking at a bike wheel this becomes obvious.

If you are happy with how your mechanical discs work that’s cool. I’ve used and do use ‘em…they work fine, but not awesome which is why I’m switching over to hydraulic discs as it makes sense.

Having said all that if you gave me a new bike with Avid BB7s I’d ride it until they wore out or someone gave me some free hydraulic discs! There is a pretty wide margin from where a set of brakes work okay to where they rock your world.

For rim brakes I pretty much use Deore V-brakes which are easier to work on than Avid BB7s and are super cheap.

29 01 2011
Avid Brake Lever

[...] The problem with mechanical disc brakes… « The Lazy Randonneur All of my initial experiences with disc brakes were with Avid mechanical discs. This was simply because they were popular and easy to work on at home since they used the same brake housing and cables I was used to. The problem is that the housing used for the mechanical brake cable is only able to resist the compression forces of the brake lever to a certain point. Before that point most of the power you put into the lever gets to the disc pad and squeezes the rotor . [...]

29 01 2011
Greg

Have you experimented with larger rotors for your BB7s?

30 01 2011
vik

@Greg – no I haven’t…I’ve invested the $$ in new hydraulic discs calipers/levers instead. I still have BB7′s on my Pugsley so I may give that a shot just to see what it’s like.

30 01 2011
Greg

Vik,

Let me know what you think if you do. I’ve bought a 185mm Clean Sweep G2 for my front BB7 on my 1×1 and have yet to swap it out. From what I’ve heard, the difference from the stock Roundagons is night and day. Treefortbikes.com is offering a price match at $30 for the 185mm G3 (similar to the G2s but lighter) right now….

31 01 2011
a

Housing compression isn’t too much of an issue if your cable run is only a few inches… as in http://www.catrike.com/catrike_trail.html

2 02 2011
MSRW

Hmm. I have found exactly the opposite–that the hydraulic rear disk on my tandem was fussy, unreliable, and didn’t ever manifest all that much stopping power (It was a Formula); but the BB7 I replaced it with has been maintenance free, works extremely well, and overall is a much more agreeable brake.

Other factors: I switched from a 185mm rotor to a 203mm rotor; and I added a device in the rear brake cable to increase the mechanical advantage of the cable pull–that device has since been removed since I switched from Chorus Ergo levers to V brake levers and bar end shifters. The rear brake felt solid and comparable to the Strange brake (a type of V brake) on the front wheel. I agree with the posters above re the lesser the amount of cable housing, the better the rear brake will perform. My tandem has no more cable housing on the rear brake than it would if the bike were a single (cable stops, long runs of bare cable etc.)

For me, there always seems to be a downside in bicycle equipment when performance improvements are predicated on added complexity–fussiness goes up, reliability can tend to go down. I find this to be true with mechanical vrs hydraulic brakes–but admittedly, have few datapoints. If the mechanical disk brake is well-engineered and the cable run is well-designed, mechanical disks can work fabulously.

2 02 2011
Tom

Some comments (based on a big dummy with 160mm rear and 185mm front and BB7s and Clarks braided cable housing)

As Vic knows I’m using this setup to get me from Banff to Tierra del Fuego, and have now done 8500km. I run pretty heavy as I’m 75kg and I carry 3/4 of our gear to even our speeds up. My non-touring mtb uses Avid Juicy 5s, so I’ll use that as comparison.

Until about 400km ago I was using the setup I’ve described – I do agree that the squishy feel is there, when compared to hydraulic disc brakes, but we’ve done some pretty full on stuff, and I’m happy to take my very heavy bike down the worst of it and I trust the brakes. This includes the descent into Copper Canyon at Urique (16km of very steep downhill over fist-sized rocks buried in fine dust).

Due to circumstances beyond my control (Mexican postal service) I ran out of replacement pads and swapped my front brake to an Avid single-digit V-brake by cutting the cable housing (no-name, the back is Clarks braided) as my front wheel has a milled Velocity Aeroheat rim for this very eventuality.

There is a very distinct difference in performance between the BB7 on 185mm rotor and the Single-digit V-brakes. The V-brakes do stop me, but with noticably more “squish” and with the weight of my setup I couldn’t lock the wheel if I wanted to, whereas I could with the BB7s.

Tom

2 02 2011
Tom

ps – the 185mm front rotor is a G2 Avid rotor and the rear is a 160mm Shimano XT rotor.

4 02 2011
AdamDZ

Hi Vik, AdamDZ here from Bike Forums. I see the same behavior even with XTR cables on my regular MTB based commuter bike with BB7s, although it’s not THAT bad to justify any significant upgrades: I can still lock the wheels, it’s just the squishy feel is kind of weird. Of course, it’s worse with the rear wheel and if the bike is loaded. I heard too before, that there are compression less cables available that are made of some kind of metal housing’ like those you mentioned, but they’re quite expensive.

So for my BD build I’m contemplating hydraulic brakes. Do you have any specific recommendations, experience? I’m looking at Avid Elixir series. They come with 190cm rear hose, is that enough for a BD?

I also worry about reliability and road repairs since I’m planning to do some touring on the BD. If this was city bike I would worry as much, but I don’t want to find myself with leaking hoses 20 miles from the nearest town that may or may not have a bike shop. So I’m undecided: should I try hydros or spend extra $$ on compressionless cables and larger rotor?

Cheers!

Adam

4 02 2011
thelazyrando

@Adam – I use Shimano SLX hydraulic discs on my Big Dummy with 160mm rotors they work fine. I have Avid Exlir CRs on my FS MTB and they work great as well. I don’t think you need a huge rotor with a Big Dummy and on sticky pavement it wouldn’t be hard to damage your fork on sticky pavement if you got too aggressive with the front brake on a heavily loaded bike.

As for reliability on tour I guess it depends where you tour. A problem is unlikely so if you are touring “normal” parts of Canada/USA the risk isn’t excessive, but if you are somewhere really remote it might not be worth it….hard to say. Personally my BD is my expedition touring rig and I don’t plan on swapping out brakes if I go to India or Baja Mexico.

If your front rim had a brake track and you wanted back up you could carry some Deore V-brakes and a lever + cable for emergencies.

4 02 2011
AdamDZ

Thanks. I now you tour on your BD, so one more reason to ask about this;) Surly claims BD accepts 203mm rotors, and the don’t warn against large rotors although they don’t specify front and/or back: http://surlybikes.com/blog/spew/getting_the_most_from_your_big_dummy/. This fork looks beefy and I use my brakes mindfully – or so I think :) But if you’re doing fine with 160mm rotors, I guess there is no reason for overkill.

I don’t plan touring outside of USA for any foreseeable future.

I might give hydraulic brakes a shot then, if only to try something new and get some new experience. Perhaps I’d carry spare parts or a spare brake as you suggested. I heard Shimano hydros come with short hoses. Have you used third party hoses? Avid supplies 190cm rear hose, which I found later after making this post, should be enough for the BD.

Chears! Adam

5 02 2011
vik

@Adam – interesting that they have removed the warning about large rotors and discs, but it’s fairly simple to figure out that if you apply too much braking force on the front wheel with a heavy load that if something has to give it will be your fork/headtube area.

If you go with 160mm rotors and decide someday you want a bigger one you can just buy the bigger rotor and the caliper mount…the rest of the brake will be the same.

I bought a longer aftermarket hose for my rear BD brake.

5 02 2011
adamdz

I’m still mulling over the options. I really don’t like the idea of having any liquids to deal with on my bike :D It just feels weird, car-like. Then I’d need the prebleed kit to replace the rear hose, it gets expensive and complicated. And I’m a bit lazy, I don’t really want to learn anything new right now, unless I really have to. Looks like only one Avid model comes with 190cm hose. Those compressionless cables actually look like cheaper way to go even with all the extensions and extra-long liner.

I’ll probably go with 185mm rotors on both ends if I decide to go with hydraulic.

Adam

9 02 2011
trip

Hey Vik,

I have a question about your article on disc brakes. If, like you wrote, “the higher pad pressure of a disc brake system is a good thing since this is what is supposed to give it better wet weather braking”, wouldn’t that also mean that putting bigger rotors on disc brakes would actually decrease their braking performance under wet conditions? Or would the rotor (which is obviously still a lot smaller than the wheel itself) still be small enough?

Best,
Trip.

10 02 2011
thelazyrando

@Trip – I don’t think changing rotor size will have much affect on how the disc brake works when it’s wet out. The rotor is still tiny compared to the size of a 26″ or 700c rim.

6 04 2011
craig

I find it funny that no one has commented on the faulty logic that forms the entire argument here. First there is the perception that the squishy feel is somehow a liability rather than just something the rider doesn’t like. One rider’s squishy feel is another’s “good modulation”. If a brake can lock a wheel there is adequate power, all the rest is ergonomic preference. If a disc brake can’t lock a wheel then something is wrong but at least there’s recourse in the form of a larger rotor. What is there for a rim brake?

There’s no doubt that compression is the culprit for the squishy feel but compression is proportional to cable tension and cable discs do not automatically require more cable tension because of the smaller brake surface diameter. They do require more clamping pressure but they achieve that through better mechanical advantage. You see, a disc brake has much less pad travel than a rim brake and that works to offset the smaller rotor size. It is quite easy to set up a disc brake to have less cable tension than a rim brake and when you do the author’s argument goes completely out the window. Disc brakes are not inherently more sensitive to cable compression than rim brakes and I have never set up a disc system that requires more lever effort than a rim brake. Someone is doing it wrong.

If lever effort is high and the lever feels too squishy, get a bigger rotor first. It’s true that hydro’s don’t suffer from cable compression but so what. Hydro’s are lighter, sexier, and they are self-adjusting. They don’t get sold because of the cable compression “problem”.

6 04 2011
thelazyrando

@Craig – I’m afraid it’s your logic that’s faulty. A rim brake doesn’t need as much force because it is a disc brake with a huge rotor. This is why housing compression isn’t an issue for a rim brake. I would also add that many bikes don’t have clearance for massive rotors and they are expensive/heavy.

10 04 2011
Matthew

I got Clarks entry level mechanical disc brakes on my entry level Diamondback bike and I’m really happy with them. The guy in the bike shop who set the bike up was so snobby and started waffling on about Hydros, ’til I explained that I use the bike 3 or so times a week for short commutes…and that when I pull my levers, my bike stops (he shut up at this point). On research, it seems that many bike snobs have experience with early mechanical disc brakes and have formed their opinions on the early offerings…but apparently they have moved on tenfold since then. BB7 obviously being the pinnacle.

I swear though, these cheap £20 Clarks calipers/rotors work so impressively, I can’t even imagine how nice the BB7′s must be. When the pads need replacing, I’m going to take the Clarks off and install Avid BB7′s instead, as they’re obviously a more robust system. But the ones I have for now are MUCH better than the good quality V-brakes on my old bike, so yeah, I’m definitely a mechanical disc convert. I have rode bikes with Hydro brakes I should point out and, yes, they’re amazingly good and modulate more efficiently. But mechanicals do the job I need them to, and I don’t need to mess with fluid and bleeding. Now if I was slinging a bike around trails, I’d have Hydros… and a FAR better bike. But_I_am_not. And mechanical disc brakes DO have a place in this world…bike snobs take heed!!!

10 04 2011
thelazyrando

@Matthew – I run several sets of BB7s – they are not that impressive in my opinion, but I suppose it depends what you compare them to.

11 04 2011
john

Vik, I realize Avid BB-7 mechanical disc brakes are not perfect. Nor is any other brake type I’ve used, be it cantilever or linear pull. Which is why I’m soliciting your opinion on a potential new bike set up….

I’m considering a new Pocket Lama. Which brakes?

More detail:

1. tours would likely take place in not too remote areas (N.America)
2. I live where it is often wet (P NW)
3. This will be an everyday commute bike
4. This bike will see off-road use, but only 20-25% of the time
5. I do not like to spend time adjusting brakes!

From another perspective:

Are there characteristics of the Lama that make it more or less suitable for disc brakes or linear pul (Fork strength, etc.) considering one’s riding will be wet 8 months a year?

Thanks,

J.

11 04 2011
thelazyrando

@John – with a travel bike I would skip the hydraulic disc brakes to make packing easier as well as the fact you are more likely to damage a hose/fitting than a standard full size bike. So you are left with two choices v-brakes or mech discs like BB7s.

I find shimano Deore v-brakes and BB7s pretty much a draw in terms of functionality and maintenance. My Tikit has seen year round use with v-brakes on 16″ rims for going on 4 years and they aren’t close to being worn out.

I live in Victoria which is part of the PNW and my goto brakes are usually v-brakes.

Having said that I ordered my new BF tandem with BB7′s and they work fine. I’ve also got v-brake studs on that frame with the idea being to add addition brakes for mountainous loaded touring and alternating through the brakes to dump heat.

Sorry to not give you a specific answer. I don’t think there is a clear winner and I don’t think you’d be disappointed by either system.

I guess if you wanted to look at it this way:

- disc brakes would result in less need to replace rims from wear
- disc brake rotors are more susceptible to damage in shipping/travel

Ultimately if you have a preference for one over the other just go for it.

16 09 2011
frank

A very interesting post, however i’m going to agree with Craig.

You are right when you state that disc brakes need a higher pressure on their brake pads because of the small diameter of the ‘disc’ compared to a rim. That however does not mean that there is more tension on the braking cables.

You can illustrate this by looking at how hard you have to pull your brake levers: assuming that you use the same brake levers for your rim brakes as your disc brakes, the amount of force you apply to your levers is directly proportional to the amount you apply to your braking cable and thus transfer to your brakes. It is not however, to the amount of force you actually apply to your braking pads, this is due to the difference in mechanical advantage between rim and disc brakes that craig mentioned. This means that you can have the higher braking pressure that is needed for disc brakes without having a higher force on your cable.

having this higher mechanical advantage in disc brakes does mean that the braking pads travel a smaller distance for every unit of distance that your cable(and thus braking lever) travels when compared to a rim brake. This is not a problem however because the space between your pads and disc is much smaller than the space between your pads and rim.

This doesn’t mean that there can be no compression in your cable housing, just that there is no difference between rim and disc brakes when it comes to cable housing compression.

If there is more slop in your disc brakes than in your rim brakes it must be because of something else. Maybe it is your braking caliper that flexes because of the higher pad pressure in disc brakes, but if that’s the case there should be no difference between mechanical and hydraulic brakes in terms of ‘mushiness’. So it might be just a quality difference in one of the many components in your braking chain.

Please correct me if I’m wrong, I just came up with this but I think it’s correct.

16 09 2011
thelazyrando

@Frank…setting up disc brakes you need to leave some room between the pads and the rotor or they’ll rub constantly since no rotor is perfectly true. Also mechanical disc brakes and mechanical v-brakes use the same levers so the extra leverage you would need to make them equivalent has to be provided solely at the caliper. You can measure the gap between disc brake pads and the rotor vs. v-brake pads and the rim and then try and work out the gap relationship that would equate to the extra force required for a 26″/29″ rotor vs. a 6″ rotor. Looking at a v-brake bike in my work stand the pad gap is ~1mm each side and at an Avid BB7 mech disc bike in my office it’s ~0.5mm each side. So the leverage reduction in each system has to use up part of the lever throw to close that gap and the rest is available to apply force. I’m too part out of uni to crunch the numbers and work out the difference, but maybe somebody reading this wants to have some fun with numbers.

Ultimately we can theorize all we want, but when you have several sets of the popular mech and hydro discs to use back to back the difference is pretty clear. Especially once you get to that second half of the lever throw where the force is getting higher.

14 11 2011
Cyclocross Magazine (@cyclocross)

Great article, and nice write-up. But I’d have to say that @craig is right though – the shorter “lever” you talk about having with rotors over rims is easily overcome by the design and mechanical advantage of the brake. Measure the pad travel of a rim brake versus a disc brake (especially a wide-profile cantilever) and you’ll see that your analogy isn’t comparing apples-to-apples.

A good analogy is if you have one ceiling-mounted pulley or two pulleys to lift up a heavy object with a rope. For 1 foot of rope, you can either move the object 1 foot, or 1/2 foot, depending on whether you use one or two pulleys. It’s a lot easier to move the object with two pulleys, but it moves less.

If you have to move your brake pads less, you can can use mechanical advantage to help overcome the higher force needed. It really is that simple. On a rim brake you may have a long lever, but then you also have the lever between the brake’s pivot and the pad. It’s that ratio that is your mechanical advantage of the brake (combined with geometries of the cable configuration like linear pull or a straddle).

Now the question is do the pads move the same fraction of a rim brake’s pads as the fraction of the rotor diameter divided by the rim diameter? I’m not sure. I know plenty of people with true wheels who like their V-brakes super close to the rim, and in that case, the answer would be no. But on average, for those who like a little room for out-of-trueness or mud, it easily could be.

The other factor is the mushyness is experienced later in braking than with a rim brake because rim brake pads are softer and compress and deform earlier and typically before the brake cable housing compresses – so you have a more constant mushyness curve that’s not so on/off, if you will. A disc brake’s hard pads don’t compress and deform as much as a rim brake, and thus once the pads are applied with decent force, all that’s left to compress is the housing…which is why they have a late-spongyness feel.

We do all kinds of crazy brake, mud and sand tests at Cyclocross Magazine, and maybe we should just go about and measure all this stuff in our next print issue. You certainly have prompted some curiosity to really measure this stuff. Issue 14 measured every component of brake setup for cantis and V-brakes, and I think you’d be surprised at the things that make a huge difference (like spring tension).

Thanks, keep these articles coming!

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