disc brakes

What’s wrong with disc brakes for bicycles ?

Cars borrow from planes, motorcycles borrow from cars and seems like, bicycles are getting from motorcycles…

When I got my first hydraulic brake set, I immediately detest it. Yes; it was an entry-level Acera 355. Later upgraded to Deore 615’s. Guess what? Deore’s are even worse! Still, I miss my Deore V-brakes a lot. They were much more powerful, easy to adjust, lighter, and obviously, cheaper to run. When I got into hydraulic disc brakes, I did not choose to: the new frame I bought was meant for disc brakes only. Good luck finding an MTB frame, manufactured in the last 10 years, with brake bosses. You cannot. Now it’s becoming the norm for road bikes. Even pro road cyclists don’t like them, but they have to accept these because of sponsors.

Hydraulic brakes on a bike seem like a bright idea, but it does have lots of pitfalls – the bike is driven by limited power: you. So, everything has to be lightweight and efficient. To achieve this, it does have to be dead simple.

Hydraulic brakes are a stupid idea. Disc rotors are minuscule. There is a way bigger rotor than your hefty

Disc rotor and pads, compared to yours
A brake rotor in an average car has a diameter of 300mm, but thickness is at least 10 times bigger then a bike rotor. These rotors are heavier then an heavy bike, they are cast iron (generally) and sides of the rotor sandwiches hefty cooling channels. Rotor is a single piece with the hub on most applications. Brake pads are at least 15-20x thicker then a typical bike application, so they last.

203mm one, it’s far better in cooling, has a huge braking area, and can dissipate abnormal amounts of heat compared to rotors. It’s available since bikes are a reality, and that hyper technology is called “rim”.

Rotors on bikes are not just inefficient, they are also terribly designed. I’ll talk about that later.

What happens when you want to have more than one set of wheels? You just buy an extra set of rotors, of course! How convenient…But troubles don’t stop there. Since calipers are fixed but rotors are on wheels, which is another terrible design choice, in most cases, you have to adjust your calipers when you change your wheelset. Even when you just remove the same wheel, and put it back. I wish both rotors and calipers were an integral part of the fork or rear triangle. You wouldn’t need to buy an extra set of rotors with each wheel you buy or do not have to readjust calipers when you remove the wheel for a simple job like patching inner tubes.

Disc brakes are even inferior on road bikes because they throw off wheel dishing more and wheel balance, compared to MTB wheels. You have a huge rotating mass on your wheels. I won’t dive into security issues…

These are some basic problems with disc brakes, lets dive into some detail, carefully digging into every part…

Bicycle Rotors (or discs…)

Rotors account for 80% of issues about disc brakes. They are heavy, fragile, harsh on brake pads, and minuscule. If you happen to know a BMW touring motorbike owner, he will be shocked by the amount you spend on your brake maintenance, although BMW service prices are sky-high.

Here is why they fail:

1. Rotor Size

Now and then, you’ll come across some “industry media” promoting bigger discs, saying how immense their power and heat dissipation are, ending with how inferior rim brakes are.

Well, standard big-size rotors are 203mm in diameter, while a “small” wheel, 26″, is 560mm. Rims are aluminum, far superior in heat dissipation, and also have full metal surfaces compared to rotors that are

Scored and worn disc rotor
This is an average quality brake rotor, and seems well destructed under just moderate use: no heavy or prolonged braking, and not used on an MTB. I never saw such badly scored discs on everyday driven passenger cars. This disc is also badly warped, and “truing” it only solves the issue until the next braking. The main material issue is, its stainless steel, which is very poor at heat dissipation. Stainless steel does not contain high graphite content as car/motorcycle discs, which dissipates heat and provides some sort of “lubrication” that resists locking under heavy braking.

aggressively shaved off.

If the rotor diameter was a big parameter in braking, even bikes with 140mm disc would be impossible to control as they will lock up once you touch your brake levers: I owned a Citroen Saxo VTS, which had 247mm discs front and back…And that car had a shorter braking distance at the time, compared to the Porsche 911!

Being thin, these rotors do not last long. Their mass, or lack of it, also contributes to warping.

2. Materials

Cars and motorcycles have generally cast iron rotors. Cast iron has 2 obvious advantages: they do not warp under heat and wear much more consistently. They cannot be used on bikes, because they have one big disadvantage: they are very heavy.

Stainless steel warps easily under temperature, and since these rotors have a very tiny mass, the warping is highly pronounced. When you brake hard, pads create a perfectly smooth surface (not talking about scores on disc) on stainless steel, which is not a perfect surface for brake pads. Another downside is, due to warping, you may experience rubbing issues occasionally.

Unfortunately, there is no suitable rotor material for bikes. Even carbon-ceramic brakes are not well suited, because they are almost useless at low temperatures. Not to mention they are super expensive.

3. Bad design

I don’t know where to start, because every damn thing seems wrong about these things…

Let’s start with the most obvious: they’re steel, prone to warp under high temperature.

Bike brake rotors have very small surface areas to make them lightweight. Manufacturers claim these slots help maintain brake performance because they shave off brake pads (Indeed so!). In racing applications, like racing cars, you may see cross-drilled or grooved discs. These serve 2 purposes: grooves clean disc pads because they shave off a layer especially when you break hard. If you do not need such aggressive cleaning, cross-drilled rotors are a better choice. These openings especially help gases escape formed under very high temperatures. Either rain/water from the tarmac and some pad material like resins evaporating under extreme pressure can exit from the contact area efficiently. Slots in bike rotors, as I said, are about making them lighter, and for cosmetic purposes. And they shave off your pads at an incredible rate! They are not needed; because brake pads in bikes never come close to car rotors, which can get bright red under braking. No gases formed. And they are not needed to shave off your pads. Bike pads or rotors never get hot enough to decompose brake material, or gases are no big issue because they do not have such clamping power.

Bike Disc Brake Calipers

Even the top-notch racing calipers for cars are very primitive devices; not much sophisticated than a syringe. Hydraulic pressure moves pistons, pistons compress pads against the rotor. There is a rectangular-shaped o-ring in the caliper body, which retracts the piston when hydraulic pressure goes down. Simple as that.

In a proper caliper, like the one found in the most rubbish car, this square cut o-ring is the only problematic part. Even so, they barely go bad. Have seen 30-year-old cars having original brake calipers.

In bikes, calipers may leak and can crack pistons, due to the materials used and poor isolation. Like, newer Shimano XT’s with ceramic pistons cracking. A proper piston should be induction hardened steel alloy, with Nickel and Chrome, to ensure it does not crack under heat, or deform too much, which may damage piston housing. This is not possible with bike brake calipers, as the caliper would be very heavy otherwise.

A bigger issue with calipers is the square-cut seal, which ensures a seal between the caliper body and piston, but also retracts the piston when the pressure goes down. There is a huge design flaw with bike calipers: they don’t have rubber boots to protect the piston area. So, when you brake, you pump all debris from pads, dust from the air, water from the road into the caliper body. There is no excuse for such poor design. Never saw a caliper with a dust boot yet.

When this seal goes bad, and under such circumstances, it will quite quickly, calipers are paperweights, at least for Shimano. Shimano does not sell repair parts for their brakes, except cabling and some trivial brake lever parts. For me, this is more than enough to ditch Shimano brakes for good, in favor of Sram, Hayes, or even Hope, all selling replacement parts. You can get these seals from AliExpress which are not authentic. They’re not cheap too, so if you have Deore’s or cheaper series, just throw them away and buy another brand. Sorry, Shimano.

Seal damage manifests itself with rubbing issues. (well, there are soo many issues about bike hydraulic brakes, all seem to cause rubbing). When the seal starts to lose elasticity, it will fail to retract the piston fast enough. This is called “lazy piston”, in automotive-speak, at least. Your brakes will make unpleasant noises, for a brief time after braking. In time, this rubbing period will extend, and it will start to leak eventually.

Caliper issues are extremely dangerous and can cause death or serious injury – no disclaimer bullshit, this is perfectly true. You’ll lose all hydraulic pressure if the seal fails, thus having absolute zero clamping force. When you start to have lazy piston issues, I suggest either a complete rebuild if you have the parts, or ditch it altogether.

Unlike V-brakes that are bolted higher on your fork or rear triangle, calipers are bolted to the most vulnerable areas of your bike. Under hard braking, your forks have to fight against tremendous forces and try to resist bending. And forces are not distributed equally as in rim brakes, where some forces cancel each other. Also, since calipers are attached low, leverage is much bigger. That’s why it’s a very bad idea to weld/braze some steel adapters to old steel forks to fit disc brakes. Old steel forks are exactly the opposite that is needed with disc brakes: They are strong at the top and purposefully build weaker at ends to make them more compliant; providing some suspension to a degree. Some vintage steel forks may have 1cm, or even more flex vertically, acts as a short-stroke suspension. What about carbon forks? I wouldn’t buy a Chinese or second-hand carbon fork for a disc brake bike. Consequences would be haunting me.

Brake lines

Surprisingly, hydraulic brake lines are very robust; the ones I’ve seen have a protective sleeve outside and a hard(ish) plastic hose inside. Given there are no huge hydraulic pressures like in cars, brake lines are light and sturdy. Still, looks like the hydraulic lines are pretty narrow. They have about a 2mm internal radius.

Two things are bothering me about these brake lines. One is, as I said, they are too narrow. If seals disintegrate in time, these narrow lines can be clogged. I suggest replacing hydraulic/mineral oil more often if your brakes are older than 5 years; because rubber seals tend to swell, disintegrate, or be torn after 5 years.

The second one is not a big issue, but it would be worthwhile to make some if from metal, which would provide additional cooling.

Brake pads

Brake pads are annoying in hydraulic brakes because they last as long as a TikTok video. Pad material is not any thicker than the brake pad backing plate.

In the last few years, metallic pads are becoming popular. I used metallic pads in a rally-spec car many years ago. Metallic pads are not meant for everyday use, and they are DANGEROUS: they have no stiction when they are cold. When discs and pads get hot under consecutive and hard braking, they bite the discs aggressively, so you have to get used to them before using them as a replacement to resin pads. Once they are hot enough, they respond very quickly, provide the immediate stopping power and last longer under racing conditions, where resin pads start to disintegrate because they are soft and resin burns under heat.

Using metallic pads in even sportier cars in road use is not a good idea. They are harsh on rotors, diminish their life probably 3x fold. Since their reaction is immediate, not nice, and progressive as the resins, you have to be extra careful when braking at corners.

For bikes, metallic pads are the worst idea about disc brakes. Rotors are very thin and wear like candles. You have no braking force or feel when you need to brake suddenly, like a dog jumping on your way. In a way, metallic pads are also almost necessary for bike brakes, because they roughen the rotor surface, which provides a nicer surface for pads to operate. Think of it like 80 grit vs 1000 grit sandpaper: with 80 grit, you get a rough surface. 1000 grit is like a mirror finish, which is not good as a braking surface. Resin pads almost “polish” stainless steel discs. Next time when you buy a new rotor, compare its braking surface with your old rotors: they are not shiny, in fact, a little bit rough (duller in appearance).

What parts to buy?

We must accept the reality that hydraulic brakes are the future, so rather than whining about it, get the best out of them. They have some minor advantages too, like your rims are not wearing out, or they perform fractionally better in wet.

Brake sets are the things we can go cheap, and for me, the cheaper the better! I have seen more complaints about high-end Shimano’s or Sram’s, than the cheaper versions. Money seems to not buy reliability. Most people hate Magura levers and adore Hope calipers.

I am not convinced by higher-end brakes by their look. For example, 4 piston brakes…In 4 piston brakes, the brake levers look almost identical. This is my “anecdotal evidence” though; for a detailed inspection, I have to take apart brake levers and measure stroke and bore of the “master cylinder”. Still, if you have additional 2 pistons to move, albeit they’re a bit smaller, you’ll need either 2x longer stroke levers (longer pull) or 2x master cylinder piston area. (chunkier brake lever bodies). People think that by installing bigger rotors and calipers, they’ll get a big jump in braking forces. I’m quite puzzled by the fact that even some amateur rally team mechanics don’t know anything about elementary school grade physics. I’ve seen huge Brembo or AP Racing discs with 4 or 6 piston calipers, performing much worse than original brakes. Why? Simple: without changing the master cylinder (brake lever on bikes) you are applying the same force to a bigger area, diminishing the clamping force of the piston.

That means that you cannot use brake levers meant for 2 piston calipers, with 4 piston calipers. They will still work, at a reduced braking force. Using brake levers for 4 piston calipers with 2 piston calipers will also work: brakes will act instantly and will throw you over the handlebars. In extreme cases, you may blow the square-cut seals on caliper pistons.

I advise against buying Shimano brake levers or calipers because they do not sell spares. I also wouldn’t buy a “new old stock” brake set, older than a few years. You’re buying stuff that almost completed its service life. I wouldn’t be enticed to boutique stuff like Hope. Well, they sell replacements, which is nice, but also repairing/maintaining a Hope set would cost more than a Sram set. Sram looks like a good choice, and let’s hope Hayes rise again, too.

For rotors, it’s a different story. My obvious choice would be either XT or XTR rotors, second to none. Icetech? I don’t give a damn. As long as they have full-length, aluminum spiders extending to the braking surface, Shimano XT, XTR, and Saints’ are the best, because they do not deform like tinfoil, as other rotors, especially 180mm and 203mm ones, do. A huge advantage of these big-size, aluminum spiders is that they keep warping to a minimum, so your rotors do not rub to pads, causing annoying noises and burning off pads. XT, XTR, or Saint-level rotors are amongst the most expensive rotors, but they are worth it. Pads are not cheap too, and you’ll save a lot of pads due to reduced rubbing.

What about real floating discs? “Real” ones are super rare; I’ve seen one brand produced in the USA, which looks top notch, but I can’t remember the brand, or never used one. AliExpress has lots of them, but I don’t trust them: let me explain why by explaining what is a floating rotor, and why you may need one…

Normal rotors are machined from a single block of steel. This is not a problem if the rotors are thick enough, or made from a rigid material like cast iron. However, thin, steel rotors tend to flex a lot. In an ideal world, caliper pistons on either side will exert the same pressure, and push/retract at the same rate. In the real world, this never happens. Rotors receive differential pressure on each side, and calipers do sit perfectly perpendicular to rotors. This bends the discs, given that metal thickness from center to the braking surface is generally flimsy at best.

In a “real” floating rotor, the braking surface is attached to a carrying spider with elastic metal. When you brake, the rotor is rotationally rigid, but an elastic metal spring lets the braking surface move in the horizontal axis between pads. Pressure on spider relieved, which is also (have to be) made from a more rigid material, so rotors do not bend.

XT, XTR or Saint rotors are not floating rotors. The rivets that attach the braking surface to the spider (carrier) do not permit any movement, unlike the ones in motorcycles.

For the brake pads, I’ve not concluded yet, about material composition at least. Metallic (sintered) pads seem to fit stainless steel rotors better; which I wouldn’t ever use on any vehicle meant for everyday use. I think the best compromise will be to start with resin brake pads, and after a set, go for metallic. You’ll have better longevity with rotors, compared to using only metallic pads. Since resin brake pads make rotors perfectly shiny after prolonged use, metallic pads will help make it rough enough to be “sticky” again.

Brake fluids

Shimano uses mineral oil, while Sram uses DOT fluid, which is also used in cars and motorcycles. Both companies sell fluids, but Sram says you can use DOT 4 or DOT 5.1.

If your brake system uses DOT fluid, never use DOT 5! DOT 5 is silicone-based, and an entirely different thing than DOT 3,4, and 5.1, which looks counterintuitive, as normally, you would expect DOT 5 to be an upgrade to DOT 4. It’s not. They’re entirely different.

All DOT-based fluids, except DOT 5, will work with Sram brakes if you find DOT 3 these days. The difference between grades like DOT or DOT 5.1 is the amount of borate added; which raises the boiling temperature. All DOT fluids are transparent or have a slight shade of amber, EXCEPT DOT 5. DOT 5 is purple.

All DOT fluids are harmful to paint; so you need to be extra careful about bleeding brakes. Some “ghetto” approaches as in Shimano brakes are unacceptable(!).

Mineral fluids, such as Shimano’s, are different. On the plus side, they do not harm your paint. They are exceptionally expensive when compared to a proper DOT 4 fluid.

Unlike mineral fluids, DOT fluids are not hydrophobic; meaning they absorb water. So, purchase them in very small containers if you can (yet the ones I know are half a liter at least), keep the bottle cap always tight, and try to use it in 6 months at most.

…luckily, I never use Shimano fluid! People try all sorts of funny chemicals, including the absolute worst water to baby oil. Never use those. Proper replacement is ATF Fluid. It’s “Automatic Transmission Fluid”, also used in hydraulic power steering in cars. ATF, amongst other alternatives, is unmatched: its boiling point depends on its grade, but still, as high as DOT 4 fluid, sometimes better. They have additives, which lubricate, clean, and protect metal surfaces. Also includes rust/corrosion inhibitors, which are good for bike brakes, because they have steel and aluminum parts together. Baby oil can work good as a hydraulic fluid only but does not have any protection, or resistance to boiling, so must not be used!

There seems to be another alternative that fits the bill: LHM or LDS fluid. These are used on Citroen cars, for their legendary Hydractiv suspension. They have excellent properties to be used as mineral oil, but they have a big flaw: Flashpoint for LHM is only 110 degrees, and for LDS, it is 160. There is no point in using LHM or LDS when ATF is available everywhere and cheaper.

Particular ATF I use is Motul ATF VI, which has more boiling resistance than ATF IV fluids, and Motul is a good brand. But when it comes to oils/fluids, I’m quite brand-agnostic, as any brand sold in Europe or USA has to comply with the standards, so no problems there. They are generally sold in 1-liter containers, and a bottle will probably last forever. These fluids ARE hydrophobic, so you do not need to worry about moisture absorption that much.

With mineral fluid brake sets, you don’t need to flush fluid / bleed brakes as often as the sets that are using DOT, as I said, mineral oils don’t absorb moisture much.

Final words…

The biggest culprit about disc brakes in general is, they transform your trusty bike into a money pit like an old German car. Want to upgrade to even bigger rotors? Probably your fork cannot handle that, so you have to change the fork. With every rotor size upgrade, you also need to buy a new set of adapters. You’ll need a different retaining nut for the rotors if you are “upgrading” to thru-axles. Want to switch from Mavic wheels to Shimano? Bad news; your 6 bolt discs won’t fit Centerlock hubs, so you need to buy adapters, again. Seals on either calipers or brake levers eventually fail, and won’t take too long, so you have to replace them or buy repair kits that are not much cheaper than the unit itself. You need to buy a new set of tools to change your brake fluid. Mineral fluids are crazy expensive (yet you have alternatives) and pads will wear out faster than you decelerate.

Finally, disc brakes are bearable for MTB, but should not ever be used on road bikes! This is dangerous. With MTB, you won’t go over speeds like 50 km/h, and brake for a long time. With road bikes, you occasionally hit speeds over 50 km/h, and brake continuously to slow down, unlike MTB-style riding. The fluid in these brakes are very minuscule, and they boil under such conditions quite easily, which causes the gaseous mix to compress – you have no brakes at this point! Unfortunately, we’ll start to see many deadly crashes after this fashion, unless radical changes are made.


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