As one of the first things you should learn as a mechanic when it comes to disc brake maintenance, brake fluid basics are important to know – especially before working on different brake systems. On the surface it all seems pretty simple, you pick the right fluid for your brake, and bleed as necessary. However, there is more to brake fluid than just DOT or Mineral oil.
When it came to the lastest installment of our Tech Speak features, we though it might be a good idea to revisit the life blood of your hydraulic braking system. To gather as much technical information as possible, we went straight to the source and invited Formula, Hayes, Hope Tech, Magura, Shimano, TRP, and SRAM provide their response to our questions. While it may be a refresher course for some, the varying unedited responses we received from some of the biggest brake manufacturers in the industry makes for a good read especially on fluid implications towards road discs, and how your fluid choice can impact your ride (for better or for worse).
UPDATED: SRAM’s official responses have been added to the mix, after the break! Update #2: additional answers from Nick to some questions in the comment section.
What does your company use for brake fluid, and why?
Formula (Engineer): We have always used premium DOT4 fluid. Now we are using DOT4 SHELL DONAX ULTRA for all our Formula brakes. DOT4 has a higher boiling point than mineral oil and increases vapor lock resistance by being less hygroscopic than DOT5.1 and others
Hayes (Tim Abhold, Director of Engineering): We use glycol based brake fluid, commonly called “DOT Fluid”. Our systems are compatible with DOT 3, DOT 4, and DOT 5.1 fluids, but the systems are factory bled with DOT 4 fluid, which we also sell in service.
Hope Tech (Woody Hole, Progress Manager): We are currently use DOT 5.1 brake fluid in all of our brakes. This is the brake fluid with the highest dry boiling point (260°C).
Magura (Stefan Pahl, Engineer and Project Manager): Magura uses mineral oil for bicycle brakes since introduction of the very first hydraulic rim brakes in 1987. Mineral oil does not absorb water, is easier to use and does not damage paint or skin. The lower boiling point of mineral oil compared to DOT fluid is compensated over time, as DOT absorbs water, even through seals and bladders, leading to a lower boiling point, see also here. That’s why DOT has to be changed regularly. Mineral oil can stay forever in the brake without regular changes.
Shimano (Nick Murdick, Lead Multi Service Technician) : We use mineral oil for several reasons. The main discussion you usually hear between DOT fluid and mineral oil is that DOT fluid absorbs moisture and mineral oil doesn’t, it’s hydrolytically stable. As DOT fluid absorbs moisture from the air, the boiling point drops. Mineral oil doesn’t absorb moisture from the air so the boiling point never drops. Another thing you hear is that DOT fluid is regulated by the federal government so the quality and characteristics are guaranteed. Obviously this is important, which is why Shimano tests every single batch of our brake fluid to confirm that the viscosity and boiling point are perfect. We actually see this as a major advantage of using mineral oil. We don’t have to trust anyone else’s testing standards for the fluid they make. Since every Shimano brake uses Shimano brake fluid, we have complete control over the process and can assure consistent performance. Let’s take a look at those DOT standards though; they have specific targets for fluid that is both dry and wet (defined as having absorbed 3.7% water by volume):
As you can see, our boiling point starts higher than any DOT fluid and never drops.
We also use mineral oil because it won’t harm the finish on the bicycle or your skin. Along those lines, it was important to us to use a fluid with minimal environmental impact if a line is ruptured during a ride.
SRAM/Avid: We use Glycol based fluids, specifically DOT 4 and DOT 5.1 for a number of reasons. DOT fluid has a high boiling point, is manufactured to an agreed upon set of safety standards and is readily available in any auto or motorcycle store. Also, any DOT fluid residue remaining on a brake after service is comparatively easy to clean up with water instead of some potentially harmful solvent.
An important point about the hygroscopic nature of DOT Fluid is that by absorbing the water into the fluid it is preventing pockets of water from forming that remain separate from the fluid in the system. Water is heavier and settles to the lowest point in the system, such as the caliper. This means that while the boiling point of the mineral oil remains high, the boiling point of the system is now that of water, only 100C/212F.
Both mineral oil and DOT Fluid are hazardous to your health and the environment. Both fluids must be handled with appropriate protective equipment and disposed of properly.
TRP/Tektro: We also reached out to TRP, and haven’t received a response.
Can consumers use fluids other than your specific branded fluid in their brakes?
Formula: ONLY AND ALWAYS DOT4 brake fluid! Different brand can be used of course. Shell is recommended.
Hayes: We recommend only Hayes branded fluid, because all our validation and laboratory testing is performed with our Hayes fluid. Other fluid brands may perform acceptably, but without test data to verify the performance we cannot endorse their use.
Hope: Our customers can use other brake fluids with all our products, however, we generally recommend only DOT4 or DOT5.1 as alternatives. Both fluids are widely used in the car and motorcycle industry so are easily available at most good cycle and car shops. DOT3 can also be used if none of the above are available.
Magura: No, no other fluids can be used, even mineral oils, as they may damage the seals due to different chemical properties, letting the seals shrink or expand.
Shimano: Absolutely not, mineral oil is an unfortunate term to use because it is so vague and generic. The term mineral oil can describe a lot of different compounds. It’s kind of like the term salt, which refers to much more than just table salt. Most of us are probably aware that the mineral oil that you buy at the drug store is not the same as the mineral oil we use in brakes, but that extends to different brands of mineral oil brake fluid as well. We should really call it “proprietary brake fluid” but I guess that doesn’t have the same ring to it. I can tell you with absolute certainty that Magura fluid will destroy Shimano brakes in a very short amount of time. I’ve seen it happen several times. The rubber seals in the system have to be specifically designed to interact with a specific brake fluid. If you use a different fluid, the seals will interact differently. Specifically, when you put Magura fluid in Shimano brake, the lever feels spongy and the pad contact point changes because the square edge seal at the caliper is breaking free from the piston at a different time.
There are third party companies that make mineral oil brake fluid and it says right on the bottle that it is compatible with all mineral brakes. Shimano has never approved a third party brake fluid to be compatible with our brakes and we never will. How is it possible that one fluid can work in both Magura and Shimano brakes if Magura fluid destroys Shimano brakes?
SRAM: We recommend only DOT 4 and DOT 5.1 fluids, but DOT 3 can also be used without affecting the seals or anything else in the system.
Have you ever performed testing with, or produced prototypes using other fluids?
Formula: Weekly and in every weather condition. But up till now Shell is the best tested brake fluid.
Hayes: Hayes serves a wide variety of brake markets from bicycles to snowmobiles, motorcycles, heavy equipment, and military applications. We have performed testing on a large number of brake fluids, hydraulic fluids, and synthetic fluids.
Hope: We have carried out extensive with different fluids since making our first hydraulic brakes back in the early ‘90’s. Some of those tests have focused on mineral oil, however, in our experience DOT5.1 is consistently the best performing fluid to use in our brakes.
Magura: Of course we tested different mineral oils, but never DOT fluid, as that would require a new design of seals and seal seats. We keep with mineral oil.
Shimano: One of the advantages of engineering your own fluid is that you can make something that specifically fits the needs of a bicycle. So yes, absolutely, lots of different fluids were tested while developing Shimano Mineral Oil. Viscosity, boiling point, and hydrolytic stability were key factors in engineering our fluid.
SRAM: We have tested a number of different fluids, including silicone based and mineral oil based products. To date, nothing has met the performance benchmarks of our currently product.
Will the potentially higher heat buildup with road discs have any effect on fluid choice?
Formula: Sure, but not only for road disc. Heat dissipation is one of the main targets in the brake industry. You have to consider that when you brake, the torque you apply on the wheel will be transformed into heat. In addition, the job of the fluid is not the heat dissipation, but only to resist the high temperature generated during the braking action. Heat dissipation is one of the greatest areas of testing you have to perform during the design of a new brake. With higher brake performance you will have higher temperatures that need to be dissipated.
Hayes: Definitely. In fact, the heat generated in road discs should also drive investigations into performance optimized mechanical systems, to avoid the fluid boil failure mode entirely.
Hope: We have always used the brake fluid with the highest dry boiling point in our V-Twin CX/Road disc brake unit. Also heat management due to the size of road discs is more important regardless of the type of brake fluid used.
Magura: The potentially higher heat build up on road disc brakes has to be compensated on the “mechanical” side, i.e. bigger rotors, bigger surfaces for heat dissipation. The same as on road to dirt moto bikes (road bikes generally use huge double rotors and big calipers, dirt use single rotor and smaller calipers).
Shimano: I don’t really think so, because you have to think about all of the things that happen as a brake starts to overheat. It’s not just boiling fluid. The first thing that changes with pad temperature is the coefficient of friction – this is an important thing to understand. As a brake pad heats up from room temperature, the coefficient of friction goes up as well and the brakes start to work better. If your pads are designed right, the sweet spot for their coefficient of friction will match up with the normal operating temperatures of the brake. At some point though, as the temperature continues to rise, the coefficient of friction will drop off suddenly. The lever still feels good and isn’t coming to the bar, it just feels like the pad has no grip on the rotor. This is what we call “fade.” It’s a little confusing because most people use the term fade to talk about the lever pulling to the bar but that’s not exactly right.
As the brake continues to heat up, glazing can happen. This refers to the brake pad material breaking down and liquefying. This can be one of the causes of fade. When the pads cool down, the material will recrystallize on the surface of the rotors and pads, greatly reducing the coefficient of friction until the glaze is worn away. Of course, if the coefficient of friction is reduced by glazing, the brakes won’t work as good. If the brakes don’t work as good, you use them more. If you use them more, they are more likely to overheat. If they overheat, they can glaze again. It’s a vicious cycle.
Finally we can start talking about fluid turning to gas. We know that a hydraulic system works because the fluid is relatively incompressible and the pressure you put into the lever comes out at the caliper with almost no energy lost. Gasses are compressible, so if there is a lot of air in the system, or if the fluid turns to gas, the pressure you put into the lever does nothing more than compress the bubbles and the caliper pistons don’t move. To take that concept a little further, when an air bubble is squeezed, it gets smaller and the pressure goes up. If the pressure in the bubble reaches the pressure in the surrounding system the brake will work again. That’s why pumping the lever makes the brakes come back a bit.
It is possible for the fluid to boil before the pads fade or glaze if the boiling point has dropped far enough, but generally the pads start to fail before the fluid. There may be a system out there that is better for road, or there may be some road-optimized solution in the future. The important thing to realize is that you have to think about how the entire system works, not just the fluid.
I think you’re asking these questions because it’s starting to become clear that there might be some problems with just taking a hydraulic disc brake and putting it on a road bike. Even though a road bike is lighter and the terrain is smoother, heat is more of a problem on road bikes than on mountain bikes because of the way you use the brakes. Long downhills with lots of brake dragging build up a lot of heat. Also, because it is easier to stop a lighter road bike people want to use a smaller rotor, which dissipates less heat and makes things even worse. If this leads to fluid heating up and boiling, the brakes stop working. I think this is why you’re asking about different kinds of fluids and how we can change the fluid to handle the higher temperature. But the fluid is only a single piece of the puzzle. If the fluid had a higher boiling point the brakes would just fail because of fade and glazing instead, and it would happen at about the same temperature. You really have to think about the whole system, and when you do the answer becomes clear. Heat management is the solution. We shouldn’t be thinking about how to make the brake handle more heat, we should be thinking about how to keep heat out of the dangerous places, direct it to safe places, and then get rid of as much of it as possible.
People are already starting to think about this a little, they are choosing bigger rotors that can dissipate more heat. Without a complete system that can manage heat, this is about the only option. That’s a compromise though. You should be choosing your rotor size based on the power needs of the bike, not the heat dissipation needs.
SRAM: The choice of fluid is only part of the heat management equation. Proper design of the pads, rotors, pistons, and caliper as a system as well as extensive validation testing is necessary for this new application. Any fluid can be used in a braking system as long as the system is designed for it.
As far as the internals of the brake go, is there any difference in the size of the parts between DOT Fluid and mineral oil that would make it easier to fit in a road lever?
Formula: No, referring to dimensions but absolutely yes when referring to the material of the seals. The rubber compound that we use in our brakes is not compatible with mineral oil (and vice versa). If you bleed a brake that is designed for DOT4 with mineral oil, you will destroy all of the seals in few hours (and vice versa).
Hayes: No. The differences in fluid performance are slight in relation to the system in general and do not drive sizing decisions. Fluid will, however, drive internal components material selection to ensure compatibility.
Hope: No, there is no difference in terms of the size of the parts but there is a difference in the rubber materials used. Using one type of fluid instead of another would cause the seals to swell and eventually brake failure.
Magura: No, there is no difference in size.
Shimano: There are some small differences; our brakes are designed around our brake fluid, so the size of the oil ports is related to the viscosity of the fluid. The balance of those things controls how oil flows around the system, which affects bleeding, lever feel, and the ability to keep air in the reservoir where it is safe. The main differences are in the seals. DOT fluid eats seals so they have to use a specific kind of seal in their system. Even then, seals need to be replaced every few years. If you look around the bike industry, every company that makes DOT brakes offers rebuild kits and every company that makes a mineral oil brake does not.
SRAM: Seal material is critical. Porting size is also a consideration as viscosity changes due to temperature can be greater with oils than with DOT fluid. In our experience piston sizes must also be considered as mineral oil has shown higher compressibility.
Speaking of temperatures, over the years there have been various reports of extreme heat or cold affecting brake performance – is that a fluid, or an engineering thing?
Formula: Good question. To give you a short, simple and clear explanation you have to consider two brake fluid variables: 1-viscosity & 2-dry boiling point. These two variables are connected. The target is to have higher boiling point and lower viscosity (lower viscosity means a fluid like water at ambient temperature, higher viscosity means a fluid like honey). With lower viscosity the lever movement and the feeling of the brake is maximized.
Unfortunately (generally) a brake fluid with a higher boiling point has a very high viscosity at low temperature. In the same way a brake fluid with a lower viscosity has a lower boiling point at low temperature. We chose a brake fluid that combines these two characteristics in the best combination to give the customer a great performing brake in all conditions.
Hayes: Different fluids definitely perform differently as temperature changes. Compressibility and viscosity changes with temperature are a key characteristic of the fluid.
Hope: Brake performance issues shouldn’t occur with a well maintained brake system – whatever the weather conditions. In circumstances where the weather does affect performance, brakes using mineral oil brake fluids are more adversely affected by the cold as opposed brakes using DOT fluid. Over extended periods of use, because DOT fluid is hygroscopic, the boiling point will drop and affect brake performance. This is why it is important to replace the brake fluid every year or two depending on the usage.
Magura: Compensating for extreme heat is a matter of dimensioning. The bigger the surface and the mass equal weight of a brake is, the better it can handle heat (assemble a moto brake on a bike and you´ll be fine. It´s just the weight…) Compensating for cold is much more difficult, because it depends on the chemical properties of the fluid. Generally all fluids get thicker, more viscous, at lower temperatures. This leads to a slower response of the brake, the thicker fluid has to be pressed through the same small ports inside the brake. As an extreme just take water: it will freeze and the brake will not work!
Shimano: All bicycle disc brakes will stop working if they get below a certain temperature, but it’s not because of the fluid. If you want proof just take a bottle of brake fluid and put it in the freezer. The failure point is actually with the square edge seal used in bicycle brake calipers. These are designed to pull the pad away from the rotor when you let go of the lever so they don’t rub. The seals need to flex a very specific amount as the piston moves out and then help bring it back in. This seal is also responsible for the system auto-correcting for worn pads. The seal flexes out as the piston moves to a very specific point, after that it breaks free, and the piston slides past the seal. If the seal gets too cold, it loses its flexibility and the piston breaks free too soon. When this happens, the first pull of the lever goes all the way to the bar and then after a few more pulls there is almost zero free stroke in the lever and the pads rub on the rotor because they haven’t retracted. The quick fix is to ride around in circles with the brakes applied to heat up the seals, then just push the pistons back in and try again.
Brakes get much hotter from friction than anything the weather will throw out at them so the only thing I can think off in regards to a hot day is fluid expansion. If a brake sits in very hot direct sunlight the fluid can expand a bit and either lock up the brake or let fluid out of the weeping port. The weeping port is there to handle expanding fluid but it can be a little concerning to see fluid coming out of the lever. If the brake has locked up it’s just because the weeping port works best when the lever is being pulled to move fluid around. When this happens to a Shimano brake with Servo-Wave and Free Stroke Adjustment you can just turn out the free stroke screw to open the master cylinder transfer port up a little and allow fluid to flow up into the reservoir again.
SRAM: The system as a whole is designed and tested for optimum performance in a given temperature range. Extreme heat or cold can affect this performance, but generally the system can be designed to work well in any extreme one would be willing to ride in. We perform extensive validation testing to verify these limits.
Are there any fluids other than DOT or mineral oil that could be used for hydraulic brakes on the horizon?
Formula: Nothing on the horizon.
Hayes: There are a variety of fluids currently in use in other industries that could be applied to bicycles. Fluid selection is always a function of finding the right balance of performance, stability, cost, and environmental impact for the market/consumer.
Hope: Not that we are aware of at the current time.
Magura: At the moment these are the fluids of choice as they have to be good on high and low temperatures, lube the system, be incompressible, be low on costs,….
Shimano: I think that any new specially designed brake fluid would still be called mineral oil. We’ve already come up with a specially designed bicycle brake fluid ourselves and we haven’t found anything better yet.
SRAM: There are many fluids available for a variety of industrial hydraulic applications which could be used in a brake system. With that in mind, DOT Fluid was specifically designed for brakes and has been in use for years in the automotive and cycle industries.
Takeaway:
While there are basically only two categories of brake fluid in use for bicycle disc brakes, due to engineering and manufacturing differences, no two systems are alike. While that drug store mineral oil, or generic DOT fluid may save you a few bucks short term, manufacturers spend a lot of time and effort engineering the brakes as a system and using the wrong fluid could cost you dearly in the long run.
To me, one of the most interesting bits of info happens to be the quoted boiling point of Shimano’s mineral oil. I’ve always heard the argument that mineral oil has a lower boiling point than DOT fluid (reinforced above from Formula, Hope, and Magura), yet there is it. To my knowledge this is the first time I’ve seen the boiling point of Shimano’s mineral oil in print – which if true (their huge resources make it quite possible), makes a pretty strong case for Shimano’s mineral oil – higher boiling point, hydrolytically stable, and safe for your skin, bike parts, and the environment.
Overall, as we suspected, brake fluid should have little overall effect on road disc brake performance. As Nick mentioned, heat management especially for road, is more about mechanical systems than one fluid or the other.
Update #2: There were two excellent questions in the comment section regarding Shimano Free Stroke and Mineral Oil specifically, so I reached out to Nick to see if we could get the answers. Nick is actually in Japan currently continuing to learn about the new product, but he took the time to shoot back some responses below.
Matt: If you buy a bottle of Shimano mineral oil, how long does it remain good for: A. if unopened? B. if opened and then re-sealed (i.e. some poured out for a bleed, lid then replaced)?
Shimano: Mineral oil is fine to keep on the shelf pretty much indefinitely. That’s actually why we are able to sell liter sized bottles instead of just little one time use sized ones. It doesn’t make much of a difference if the bottle has been opened. The same goes for inside the brake, we recommend replacing our fluid as it gets dirty. This is about once a year for the most serious riders who mainly stick to the same bike and ride off road. If the fluid stays clean, like if the bike is in storage, it doesn’t need to be replaced ever.
Justin: I would love to hear Nick Murdick explain what exactly the Free Stroke adjustment does?
Shimano: Free stroke is defined as stroke of the lever before the pads start touching the rotor. The free stroke screw simply changes the starting point of the master cylinder piston. If the screw is all the way in, the master cylinder piston will be all the way in, and the free stroke will be the shortest. Turn the screw out a bit and the master cylinder will start further out. Because it has to travel further before it closes off the reservoir port, the free stroke is longer. In the picture of the clear brake posted above, the free stroke screw is turned all the way in.
I’ll admit that it is confusing because it seems to have the opposite effect when you turn the screw. The stock position is all the way in and that’s where most people like them. If you do nothing but turn the screw out, the pad contact point moves out. So it feels like you’ve made the free stroke shorter. The problem is that turning the free stroke screw also effects your initial reach adjustment. The pad contact point came out, but the starting position came out more. So it’s always at least a three-step process. First set the reach adjustment so that the lever starts where you want it, then adjust the free stroke screw to get the amount of free stroke you want, then turn the reach adjust knob to put the lever back where you wanted it.
I would personally like to see a lever that allows for an even shorter free stroke adjustment but as it is, when the free stroke screw is all the way in, the master cylinder is right up against the reservoir port. So really the only way to give people less free stroke would be to sacrifice clearance between the pad and rotor.
