In this article, we’ll be discussing a Braking System for a Bicycle by SRAM, US Patent 10,913,510. The publication date is Feb 9th, 2021 and the filing date is March 6th, 2019.
This is a fairly complex and intimidating patent, but I’ll try to keep it simple and just speak about the high-level points. And it seems like SRAM are using a bike from the 00’s compared to Shimano’s early 90’s bikes.
Brief Summary (tl;dr)
SRAM are introducing a mechanical anti-dive/pitch system that utilizes a dual-use brake lever, which actuates the front and rear calipers. The rear caliper pivots and contains a valve that opens and closes the fluid flow to the front caliper. When the dual-use brake lever is depressed, the friction between the rear pads and rotor rotates the caliper forward, and the valve is pressed against a bumper, allowing fluid flow to the front caliper. If the rear wheel comes off the ground, the rear wheel stops, and the friction between the rear pads and rotor does not exist. Therefore, the pivoting caliper moves backward and closes the valve, preventing flow to the front caliper. With flow cut off from the front caliper, the front brakes can’t increase pressure and will actually release pressure, allowing the rear tire to move back to the ground.
We’re talking about anti-dive/pitch again. But this time, we’re talking about a mechanical anti-dive/pitch solution. I recently wrote an article about a Shimano anti-dive system that utilizes sensors in the lever and caliper and suspension actuators to maintain a level bike under braking. The rider hits the brakes, the bike senses dive and applies some level of dampening or stroke adjustment to to the suspension. It’s an expected solution that any company will be able to figure out.
A mechanical solution is very different compared to an electronic system, primarily because this mechanical system is passive rather than active. These systems have been used in motorcycles in the past and older mechanical systems would hydraulically connect the brake fluid to a damper control, and they typically worked well enough from what I can find.
Copied from my other article: In electronic-based active systems, sensors can sense braking forces, speed, and pitch of a bike, among other inputs. The system is programmed with algorithms, where super smart engineers have figured out ideal settings for the suspension to prevent the bike from diving and staying in a flatter state. Valves will open and close to prevent or allow the suspension to move at certain rates to keep the bike in place during braking.
Here’s a cool article about an Aprilia patent, where they created a linkage structure to alleviate the anti-dive situation. It’s important to note that Aprilia is solving the issue with a passive system, where Shimano’s system is active. But why are Aprilia using a passive system? Because MotoGP banned active electronic systems in 2010, just like the FIA in 1994. Hmmm… wonder why. Off on a tangent, but it appears Ducati has figured out a way around that ban using a secondary hydraulic system that the writer is saying is similar to Canyon’s Shapeshifter. Secondary meaning no rider input, and hydraulic meaning no electronics. It’s active but not electronic, so it gets around MotoGP rules. Nice.
SRAM also have some mentions of known prior art. They’re stating a known system for cable-actuated braking only, not fluid actuation.
Some known anti-pitch brake systems exist for cable pull rim brakes. In these known systems, one brake lever is used to directly apply the rear brakes. As the rear brakes generate braking force at the rim, movement of a sliding brake pad transmits this force through a cable to a front brake caliper, thereby applying the front brake. As such, both brakes are applied to minimize stopping distance. If the rear wheel 20 leaves the ground in a possible pitch-over event, the rear brake force at the rim goes to zero, the sliding brake pads retract, and the front brake force reduces to zero. The rear wheel then returns to the ground, eliminating the possibility of a pitch-over event. However, these known systems are only applicable for cable pull rim brakes. Further, these 25 known systems only offer one brake lever that actuates both brakes simultaneously.
SRAM are introducing a mechanical anti-dive/pitch system that utilizes two brake levers. The rear brake lever will actuate the rear brake only and the front brake lever (hereinafter the ‘dual-use lever’) will actuate the front brake – through the rear caliper. In short, as the rider hits the dual-use lever’, the front and rear brakes are both actuated. Here’s where the magic happens: when the back tire lifts off the ground (rendering the rear tire completely still), the fluid pressure to the front caliper is closed, and the front caliper cannot increase braking pressure. In fact, the fluid pressure is lowered, releasing the front rotor. Therefore, the rear tire is placed back on the ground, and you don’t smash your face into the ground. Pretty cool little system they’ve developed.
The intended novelty here is about the rear brake caliper. The rear brake caliper has a valve that is actuated by a spring-loaded articulating rear brake caliper, where there are two fluid ports. One port is connected to the dual-use brake lever, and the other port is connected to the front caliper. The valve is opened and closed as the caliper articulates, which allows and prevents fluid flow to the front caliper.
This invention is obviously to keep a rider in a safer situation and not to be bucked over the bar, but SRAM have a problem statement:
A bicycle and its rider, as well as similar two wheel vehicles, have a relatively high center of gravity and a short wheelbase length. The greater stopping power and high center of gravity of the modem bicycle put the rider at risk of a front wheel pitch over during emergency or aggressive braking events… when excessive front braking is experienced, the rear wheel may rise off of the ground. This reduces control of the bicycle. Further, in extreme cases, the rider may pitch over the handlebars of the bicycle, which can lead to serious injury to the rider and/or others around the rider.
The system is set up for this process:
- The rider hits the dual-use brake lever
- The rear pads grab the rear rotor
- The rear caliper is forced forward due to friction, the valve hits a stop, and opens the fluid path to the front caliper. With flow of fluid to the front rotor established, the rider has the front and rear brakes actuated when depressing the dual-use brake lever only. So, they’ll have good stopping power.
- With too much braking, the bike starts to dive/pitch, and the rear wheel comes off the ground. With the rear wheel off the ground, the caliper stops the rear wheel from rotating. When the rear wheel stops rotating, the caliper moves back to its static position, which closes the valve connecting the rear caliper to the front caliper. Therefore, the front caliper cannot increase braking pressure.
SRAM state this very concisely:
…the valve may switch to the closed state when the rear wheel is lifted from the ground and/or otherwise has reduced traction force. In the closed state, the valve fluidly isolates or disconnects the first port and the second port (and, thus, fluidly isolates the first fluid line and the second fluid line).
This is an important line:
Thus, further actuation of the [dual-use lever] does not increase the braking pressure to the front wheel. Rather, because braking pressure from the [dual-use lever] is cutoff by the closing of the valve, the brake fluid in the front brake caliper and the second fluid line rebounds or flows back in the opposite direction (toward the valve) and decreases in pressure, thereby reducing braking force at the front wheel. With less braking force at the front wheel, the front wheel is able to rotate slightly faster. As a result, the rear wheel is lowered back down to the ground.
Figure 9 shows the system in question. The top tube is the dedicated rear brake, bottom tube is the the input line to the rear brake from the dual-use lever, and the middle tube is the output line to the front caliper.
Figures 17A and 18A are the important images here. Valve 316 is the component that controls the flow to the front brake. Figure 17A shows the system in the ‘open’ state, where the fluid flow is open to the front caliper. The arrow shows the direction the rotor is spinning. As the pads grab the rotor, the friction between the rotor and pads allows the caliper to be pivoted forward, and the valve is pressed against the stop 802, and you get dual braking.
Figure 18A shows the caliper in a closed state, where the back tire has stopped moving. Without going too into the weeds here, the valve has a ‘neutral cavity’ that applies a biasing pressure to the backside of the valve, moving the valve to the closed position when the back tire stops moving because there are no forces applied from the rotor when the wheel stops. Therefore, flow is prevented from moving to the front caliper. Figure 17A shows the system in an ‘active’ state, and 18A shows the system in a ‘static’ state.
The spring force was throwing me off until I found this line. The spring does not apply enough force to keep the caliper forward.
However, the spring 622 alone does not provide enough force to maintain the [valve] 316 in the open position.
Figures 17B and 18B show a more detailed view of the valve itself in the opened and closed positions. The neutral cavity 318 is what is applying forward pressure to the valve.
Therefore, the force from the pressure in the neutral cavity 318 acting on the first spool 322 (on the left side of the first spool 322 in FIG. 18B) is greater than the force provided by the stop 802 on the first spool 322 (on the right side of the first spool 322 in FIG. 18B). As a result of this force differential, the [valve] 316 moves to the closed position shown in FIGS. 18A and 18B.
In the end, the friction between the rear pads and the rear rotor allows the front brakes to be actuated. When you lose the friction between the rear pads and rotor, the front brakes are released automatically.
I’ve said this before, but there’s something about a creative mechanical solution that gets me excited. Anyone can put some sensors and and servos on anything, to do anything, but these ideas are so much more fun to read about. And before the internet comes at me, there was probably someone, somewhere that came up with something very similar. That doesn’t mean this isn’t new.
I will assume this system won’t be on your new $6000 bike park slayer. Rather, this is probably going to be on their entry-level bikes to keep beginners a little safer. Little Johnny 4-fingers, hitting the brakes with everything he’s got, may be able avoid losing some face-skin with this invention.
That then leads to the next obvious question, something something maintenance. Bleeds will probably be simple enough, but there are a lot of new moving parts in this system that needs to be maintained. And what about dirt in the valve? What if it gets such in place (open or closed)? I’m sure the final product will address these issues. It is SRAM, they ain’t stupid.
I might be missing something here, but if the rear pads are still grabbing the rear rotor and the rear tire comes off the ground, does the pressure from the neutral cavity need to be large enough to overcome the static inertia of the wheel to move the caliper away from the stop and move the caliper to the closed position?
Anyway, thanks for reading!