What if everything you knew about mountain biking was wrong? It's happened before (#26forlife). It's no secret that when disc brakes were first developed, the mountain bike landscape was a lot different. Hayes can vouch for that. They were the top name in disc brakes back then, and, like every contemporary brake maker, their technology evolved forward from that point based upon assumptions that may or may not be correct today. The reality is, the sport has ramped up exponentially in a relatively short time, which begs the question: Has disc brake technology kept pace?
Hayes took that realization to task. Before they even considered what their new Dominion brake would look like, their design team began gathering fresh data based upon contemporary riding styles. DH and enduro bikes were wired for data acquisition, and while real-time testing was going on, engineers back in Wisconsin began evaluating materials, friction pads, and piston volumes vs leverage rates. The mandate was simple: develop a best-in-class brake, free from past assumptions - with the only timetable being to get it right the first time.
Hayes Dominion A4 Brake
Purpose: Trail, enduro, DH Caliper: Forged aluminum, four-piston, dual-bleed ports Lever: Forged aluminum blade and master cylinder, reversible, split clamp Pads: Semi-metallic (standard) or sintered metallic (aftermarket) Adjustments: Blade reach, pad contact Fluid: DOT 4 or 5.1 Rotors: D-type resonance-cancelling 6-bolt design, 1.9mm thick, diameters - 180mm/160g or 203mm/200g Adapters: Discreet (tested) or "Peacemaker" direct mount for SRAM or Shimano shifters Weight: 310 grams (Front, including hose and hardware, no rotor) MSRP: $229 USD / €235.00 per side, plus rotor - $49.99 USD / €51.00 Contact:Hayes Brake
We actually created a brand new data acquisition system and totally re-evaluated braking, so this is a full tear up, from rotors to friction material - we ended up changing everything, based upon what we learned.—Eric Schutt - Hayes
Two views of the Dominion master cylinder. The forged-aluminum lever blade has tactile indents in its face and fits one or two fingers comfortably. They can be flip-flopped left or right.
Dominion Disc Brake Basics
According to Hayes, the new Dominion A4 brake was conceived as a development project. Without a target production date, the design team had the time to dive into every mechanical aspect of the basic disc brake, but where to begin? Ride testing and data acquisition answered that question.
Beyond the test laboratory: Hayes wanted to know where, when and how modern riders used their brakes before they designed a new one. Only real world data could tell them that. - Hayes image
Using pressure sensors on the levers and hydraulic lines, in conjunction with GPS positioning and acceleration data, Hayes engineers could track when, how hard, and in what manner riders braked into corners and for any combination of gradients. That data and feedback from Hayes' athletes indicated that feel at the lever was equal to, and perhaps more important than braking power. The consensus was a light lever pull with a precise engagement point, and a predictable and powerful ramp-up in braking force were the key targets.
Cutaway of the Dominion master cylinder shows the friction-reducing guide ring (white band). The reservoir is large so there will be little or no chance of pumping air into the system when the pads wear to their limits. - Hayes image
Lower internal pressures: Hayes also discovered that today's riders didn't squeeze the levers as hard as their predecessors, and that led to a complete redesign of their lever-blade ratio and also, the hydraulic leverage ratio created by the differential piston size between the master cylinder and the calipers. Lower internal pressures allowed Hayes to reduce friction in the brake lines, and in the master-cylinder and caliper passageways. Reduced internal fluid pressure, in turn, also lowered seal friction throughout the system.
Reducing friction: Further reductions in friction came from suspension technology. Hayes engineers added a glide ring to the master cylinder piston, which made a measurable difference, as did using ball bearings at the lever blade pivot. The Dominion lever's action is feather light and butter smooth.
Precise engagement: A special master cylinder seal, combined with an adjustable push-rod was developed to ensure that the master cylinder's engagement was absolutely consistent. Each lever is set at the factory so the master cylinder cup seal sits on the edge of the bleed port/engagement point.
This graphic shows (left to right) the master cylinder return spring. The cup-seal (black) is set at the edge of the reservoir return port (red) for instant engagement. The plastic glide ring (white) and the piston (yellow).- Hayes image
Adjustable reach: Dominion levers have a sturdy integrated reach-adjustment dial that does not stick out where it can be smashed in a crash like SRAM and Shimano. There is a dead-stroke adjustment hidden in the lever's push-rod clevis, but it is secured with a grub screw. As mentioned earlier, the dead-stroke is set at the factory to line up the edge of the piston's cup seal with the reservoir's fluid return port. Messing with the factory setting can only increase the lever's dead-stroke, so it's probably not something you'd want to do.
Stiffer hoses: Haye's K2 hose is said to be 8% stiffer. It's Kevlar-wrapped to prevent ballooning and to maintain the same feel between the left and right levers. The slight expansion of the brake line can be experienced in most systems as a softer feeling rear brake, due to the longer hose.
The lever rocks on ball bearings and its integrated reach-adjustment dial is easy to operate with or without gloves.
Hard to see, but there is a factory-set dead-stroke adjustment tucked into the push-rod clevis.
Less squeeze effort equals more braking force (red line). - Hayes image
Less dead stroke and an immediate transition to a linear increase in power (red line). - Hayes image
The Dominion's forged-aluminum A4 caliper is equipped with two ports so the four-piston unit can be isolated from the master cylinder and bled separately.
About the A4 Caliper
Most innovations and improvements are inside the Dominion A4 brake caliper. It's a two-piece aluminum forging that houses four aluminum pistons. The fluid passageways are engineered to bleed air easily, and to further enhance that an extra bleed port was added so mechanics seeking perfection can use their double-syringe kits to evacuate the caliper separately before moving up to the master cylinder (simple to do). The banjo hose fitting is angled, which makes for a cleaner installation, and the brake pads are bottom-loading, so you can change them out with the caliper in place, once the wheel is removed.
New square seals: Like all good brake calipers, the A4's pistons don't slide inside the cylinders to clamp the rotor. They are suspended on a stretchy rectangular seal which allows the pistons to advance and retract without sliding friction. Eventually, when the pads wear beyond a certain point, the seals creep in a little, which allows the caliper to self-adjust. Obviously then, stretchier seals will allow the pistons to retract farther and provide more space for the brake pads to clear wiggly rotors. Hayes worked double time on both the seal design and the elastomer material to maximize the return stroke.
DOT 5.1 fluid: Hayes has extensive experience building brakes that operate on DOT and mineral fluids, The decision to use DOT 5.1 fluid was based upon two factors: High-quality DOT fluid is universally consistent and available, and its all-temperature performance gave Hayes the feather-light lever stroke that they were searching for. Hayes says DOT 4 fluid is also compatible.
The central bolt that retains the pads doubles as a stiffener for the two-piece caliper. Hayes claims that the A4 is the stiffest caliper they have made.
Two brake pads: Factory brake pads are semi-metallic and labeled T106, and aftermarket brake sets will be shipped with a second pair of stronger-stopping T100 sintered metallic pads. The review brakes used the T106 pads.
Crosshair adjustment: Dominion A4 calipers can be micro-adjusted to perfectly center the brake pads over the rotors. Each side of the caliper has a small Allen grub screw that lines up with the caliper fixing bolts. The fixing bolts are snugged down with the caliper pulled outwards, so the inner pad is rubbing on the rotor face. Using a 2mm Allen key, the grub screws are turned in until there is no pad contact with the rotor and then the fixing screws are torqued to spec. It's quite effective and takes the guesswork out of centering the caliper.
Hayes' D-Series rotors are 1.95 millimeters thick and patterned specifically to self-cancel vibration harmonics.
"Modal Resonance Cancellation" describes a web cut-out design that blocks the natural resonant frequency of the new D-Rotor. Hayes also worked out the relationship between the rotor's natural tendency to vibrate at a particular frequency and the shape of the pad, so that the two components will cancel, not create new harmonics. If that all works as planned, Dominion brakes should not howl, which is a good thing.
D-rotors are thicker than most, at 1.95 millimeters (similar to Magura), which in my experience, helps keep the brakes cooler and the rotors running straighter. The surface finish has been scrutinized to facilitate a quick and thorough break-in period by ensuring that the pad material is more evenly distributed onto the rotor's friction tracks. D-rotors are available in 180 and 200mm diameters, in a six-bolt pattern.
Setup and first impressions: Initially, Hayes installed the Dominion brakes on a Diamondback Release that I have been riding for most of the year. Hayes assured me that my brakes were production models, fitted with standard, T106 semi-metallic pads - with the only caveat being that they were still fussing with the metallic black paint finish on the master cylinders, and that their marketing department was pushing for a mirror polish on the calipers and lever blades. I prefer the "factory team" finish like you see here - with some of the forged aluminum's irregularities showing through the polished and gold-anodized parts. A mirror finish broadcasts every ding and scratch, while these test brakes still look great after two and half months of careless use.
Early on, I had to remove the Dominion brakes to photograph the Diamondback with its original SRAM stoppers. That offered me the opportunity to install them myself - which went without a hitch (Disclosure: I did not bleed the brakes during this review). While Hayes admits that they included the possibility that users could adjust the engagement point, the official word is to leave it alone. The master cylinder's dead stroke is so short, that you'll never need it anyway. Use the reach adjustment to tune the engagement to the most comfortable point in the lever's stroke and leave it there. The ah-ha moment came when I used Hayes' Crosshair pad-alignment feature. I wish every caliper had a similar micro-adjustment.
Feel and ergonomics: As promised, the feel and action of Dominion brakes are enviable. Lever feel is comfortable with or without gloves and its action is smoother than Shimano's or SRAM's best. The two-finger blade is short enough for seamless one-finger operation, and the master cylinder's in-line spring provides just enough back pressure to maintain communication. The contact point arrives firmly, precisely at the same point, and with just enough damping to ensure you won't initiate an unwanted skid. Squeeze force and braking friction increase in direct proportion, from initial pad contact to maximum braking. The entire system operates quietly, right down to the subdued "click" the levers make on the return stroke - all of which, made braking so intuitive that I had to remind myself periodically to pay attention in order to collect impressions for this review.
Power and modulation: I never give my brakes a thought when they are working perfectly. In fact, the only times I do think about my brakes is when something isn't right. The surprise front wheel push when Shimano XT pads smack the rotor face like hammers, or half way down a big descent when the SRAM Guide's lever-blade starts squeezing my middle finger against the grip. This part of the review is going to be short, because only good things happened.
Modulation was wonderful. Braking power tracks squeeze pressure at the lever all the way to lockup. Perhaps more important is that the brake releases instantly. If you do lock up a wheel inadvertently, the release feels equally responsive and linear. There is no "elastic lag" caused by ballooning hoses or flex in the system to increase the distance that the lever has to travel before the pad breaks free from the rotor.
I reviewed the softer, semi-metallic pad compound, clamping 180-millimeter rotors and never found the end of the Dominion's stopping power. Less hand pressure is required to bring the brake to lockup compared to SRAM Guide or Shimano XT (I did not try direct comparisons with Shimano Saint or SRAM Code brakes), and although you might think that the reduced squeeze pressure would erode modulation feel during maximum braking efforts, the opposite was true. The Dominion's predictable pad contact and proportionate braking response provided next-level control during maximum braking events, especially where traction was iffy.
Heat performance: Without adhesive temperature recorders, I can't make specific claims. I regularly use two sustained descents for review bikes, however, that drop over 2500 vertical feet and stress brakes pretty hard. The verdict was that the Dominion's engagement point never wavered, and my arms and hands were more relaxed than I can remember them being at the bottom of the hills. In my experience, every organic or organic/metallic pad will fade to some degree when temperatures reach the point where you can smell them. In this case, any fade was negligible, because the brakes felt consistent top to bottom.
What could be better? It seems that with all of the engineering that Hayes put into their ground-up brake design that they could have come up with a better looking adapter than this one. Or, better yet, a caliper that fits 180mm rotors and directly mounts to standard frames and forks. I'm dreaming here, but if the industry could get together on that one it would clean up the bike a lot.
Hayes admitted that the Dominion was not the lightest brake out there, but compared with Shimano XT and SRAM Guide RSC (both use aluminum lever blades) it is competitive. The weight of a front brake and a 180mm six-bolt rotor is roughly 458 grams for XT, 529 grams for Guide RSC, and 470 grams for the Dominion A4.
My only other suggestion is that, in the future, Hayes offer a dedicated one-finger lever. With their squeeze-force-to-stopping-power ratio, one finger would be enough for all but the most wicked gravity runs - and with the addition of their sintered pads...?
The Dominion brake is a winner in every sense, and it had to be if Hayes was going to get another chance at redemption. Throwing out convention and starting from scratch must have been a tough choice for a brake maker that has been in the game longer than anyone else, but it proved to be the right decision. If you are in search of a good brake, start with this one.—RC