Nerding out on Brakes shall we? Not another tech deraliment

iceman2058 wrote:

Speaking to this point: "Downsizing one set of the pistons decreases the amount of fluid trying to return so may not create the same restriction or...

Speaking to this point: "Downsizing one set of the pistons decreases the amount of fluid trying to return so may not create the same restriction or at least alleviate it some. "

I would have to think it has more to do with smaller seals creating less friction. The amount of oil moving back and forth is dictated by the master cylinder, not the size of the caliper pistons (the smaller the pistons, the more distance they move for the same amount of lever travel). Also, if a big piston gets stuck even a little bit, it would affect proportionally more of the fluid in the system, thus creating more of an inconsistency at the lever end for the same amount of piston stickiness. If that made any sense...

HexonJuan wrote:

I see what you're saying, but as an example from a past career we had an issue with cavitation at the actuating piston (which would be...

I see what you're saying, but as an example from a past career we had an issue with cavitation at the actuating piston (which would be the MC piston in a brake system) in a new system under rapid engagements, no problem under slow actuation. Sat in a review meeting where an engineer mapped out the inner diameters of the entire system. Only one dimension changed compared to previous generations which had no issue, 1 banjo bolt. It was slightly smaller through the throat and the cross drill. I latched on it and modified one of them, full cross drill rather than just to throat, and dilated the throat. Cavitation issue disappeared. Cavitation is a worse-case scenario, but cavitation can occur due to fluid being unable to respond to the release of an input. 

Ideally, seal roll and slip should be the same between the 18 and 19.5mm pistons. Manufacturing of course is kosher with 99% equal action so it shouldn't cause an effect. Now if we look at rough displacements between 4x18mm and 2x18, 2x19.5, we see roughly a .05cc difference if we follow 63expert's math. for an orifice that's maybe 1.5-2mm max (think hose insert/barb), that's a significant change in volume trying to get through the door, especially if the oil is cold (molecules more packed together). As the fluid heats up and thins out, it becomes less of an issue, as shown by Engel's last review. At least that's how I'm perceiving it. 

iceman2058 wrote:

You're addressing two issues at the same time here. Absolutely the inner diameter of the hoses and ports matter in this case, if you create a...

You're addressing two issues at the same time here. Absolutely the inner diameter of the hoses and ports matter in this case, if you create a choke point you can easily end up with inconsistent bite point etc due to the fluid not managing to move fast enough pull the caliper pistons back before they are activated again. However, the the point about the larger caliper pistons moving more oil back through the hose to the master lever is not correct. The amount of oil that moves up and down the hoses is solely dictated by the master cylinder. The pistons in the the caliper are not responsible for pushing the oil back when you let off the lever, the spring in the lever is. It pulls back exactly the same volume of oil as it pushed out when you pressed the lever. The resulting MOVEMENT of the pistons in the caliper will be different between caliper pistons of different diameter (which is how hydraulic leverage is created to begin with), but the AMOUNT of fluid flowing back and forth is the same, if the master cylinder is the same in both cases.

Slavid666 wrote:

Cavitation occurs when a pressure drop causes the local pressure to fall below the vapor pressure of the fluid. In the scenario you described, fluid was...

Cavitation occurs when a pressure drop causes the local pressure to fall below the vapor pressure of the fluid. In the scenario you described, fluid was being aspirated by retracting a piston, which introduced a pressure drop due to flow restriction. At low actuation speeds, the flow rate was low enough that the vacuum generated could still draw the fluid through the system without issue. However, at higher actuation speeds, the pressure drop became large enough for the local pressure to fall below the fluid’s vapor pressure, releasing dissolved gases into the closed system and introducing bubbles.

I’m not sure what fluid you were using, but for reference, the vapor pressure of both DOT fluid and mineral oil used in our brake systems is very low—typically less than 1 mmHg. It’s fairly straightforward to estimate when cavitation might occur using basic fluid dynamics principles.

To illustrate this, we can use the Hagen–Poiseuille equation to estimate pressure drop through a tube. While it relies on several simplifying assumptions and isn’t perfect, it’s usually close enough for evaluation and is something I use frequently.

The equation can be arranged to solve for pressure drop as:

Q = (ΔP*π*r^4)/(8*μ*L)

where
• ΔP = pressure drop
• μ = dynamic viscosity
• L = tubing length
• r = tubing radius
• Q = volumetric flow rate

Using the pressure drop required for cavitation (~14.67 psi or 759 mmHg of vacuum), you can solve for Q, which comes out to roughly 2189 mL/min. For assumptions, I used an inner diameter of 0.082", tubing length of 35", and a dynamic viscosity of 1.44 cP (based on a density of 0.96 g/cm³ and a listed kinematic viscosity of 1.5 cSt).

A 10 mm master cylinder piston has a surface area of 0.785 cm². Using the volumetric flow rate (~300 cm³/min for ease of units) and the relationship V=Q/AV = Q/AV=Q/A, the piston’s linear velocity would need to be around 2789 cm/min, or about 46.5 cm/s, to generate a sufficient pressure drop to induce cavitation under the stated conditions.

I thought this was an interesting way to look at the problem. Cavitation in our brake systems often comes up in discussions, and while it sounds plausible in theory, once you work through the equations, the flow rates and piston velocities required—given the fluids we use—simply do not support the physics.

HexonJuan wrote:

I see what you're saying, but as an example from a past career we had an issue with cavitation at the actuating piston (which would be...

I see what you're saying, but as an example from a past career we had an issue with cavitation at the actuating piston (which would be the MC piston in a brake system) in a new system under rapid engagements, no problem under slow actuation. Sat in a review meeting where an engineer mapped out the inner diameters of the entire system. Only one dimension changed compared to previous generations which had no issue, 1 banjo bolt. It was slightly smaller through the throat and the cross drill. I latched on it and modified one of them, full cross drill rather than just to throat, and dilated the throat. Cavitation issue disappeared. Cavitation is a worse-case scenario, but cavitation can occur due to fluid being unable to respond to the release of an input. 

Ideally, seal roll and slip should be the same between the 18 and 19.5mm pistons. Manufacturing of course is kosher with 99% equal action so it shouldn't cause an effect. Now if we look at rough displacements between 4x18mm and 2x18, 2x19.5, we see roughly a .05cc difference if we follow 63expert's math. for an orifice that's maybe 1.5-2mm max (think hose insert/barb), that's a significant change in volume trying to get through the door, especially if the oil is cold (molecules more packed together). As the fluid heats up and thins out, it becomes less of an issue, as shown by Engel's last review. At least that's how I'm perceiving it. 

iceman2058 wrote:

You're addressing two issues at the same time here. Absolutely the inner diameter of the hoses and ports matter in this case, if you create a...

You're addressing two issues at the same time here. Absolutely the inner diameter of the hoses and ports matter in this case, if you create a choke point you can easily end up with inconsistent bite point etc due to the fluid not managing to move fast enough pull the caliper pistons back before they are activated again. However, the the point about the larger caliper pistons moving more oil back through the hose to the master lever is not correct. The amount of oil that moves up and down the hoses is solely dictated by the master cylinder. The pistons in the the caliper are not responsible for pushing the oil back when you let off the lever, the spring in the lever is. It pulls back exactly the same volume of oil as it pushed out when you pressed the lever. The resulting MOVEMENT of the pistons in the caliper will be different between caliper pistons of different diameter (which is how hydraulic leverage is created to begin with), but the AMOUNT of fluid flowing back and forth is the same, if the master cylinder is the same in both cases.

Nobble wrote:

“The amount of oil that moves up and down the hoses is solely dictated by the master cylinder.”The fluid is moved by the master cylinder, but...

“The amount of oil that moves up and down the hoses is solely dictated by the master cylinder.”

The fluid is moved by the master cylinder, but the amount is defined by the caliper. It’s the volume required to take the pistons from retracted to contacting the rotor. You could calculate it with the piston diameters and the rotor clearance.

That’s why if you put a Magura caliper on a shimano lever you have increased lever throw over a shimano caliper. You have to move more fluid into the caliper for the pads to move from the retracted position to the contact point.

Slavid666 wrote:

Cavitation occurs when a pressure drop causes the local pressure to fall below the vapor pressure of the fluid. In the scenario you described, fluid was...

Cavitation occurs when a pressure drop causes the local pressure to fall below the vapor pressure of the fluid. In the scenario you described, fluid was being aspirated by retracting a piston, which introduced a pressure drop due to flow restriction. At low actuation speeds, the flow rate was low enough that the vacuum generated could still draw the fluid through the system without issue. However, at higher actuation speeds, the pressure drop became large enough for the local pressure to fall below the fluid’s vapor pressure, releasing dissolved gases into the closed system and introducing bubbles.

I’m not sure what fluid you were using, but for reference, the vapor pressure of both DOT fluid and mineral oil used in our brake systems is very low—typically less than 1 mmHg. It’s fairly straightforward to estimate when cavitation might occur using basic fluid dynamics principles.

To illustrate this, we can use the Hagen–Poiseuille equation to estimate pressure drop through a tube. While it relies on several simplifying assumptions and isn’t perfect, it’s usually close enough for evaluation and is something I use frequently.

The equation can be arranged to solve for pressure drop as:

Q = (ΔP*π*r^4)/(8*μ*L)

where
• ΔP = pressure drop
• μ = dynamic viscosity
• L = tubing length
• r = tubing radius
• Q = volumetric flow rate

Using the pressure drop required for cavitation (~14.67 psi or 759 mmHg of vacuum), you can solve for Q, which comes out to roughly 2189 mL/min. For assumptions, I used an inner diameter of 0.082", tubing length of 35", and a dynamic viscosity of 1.44 cP (based on a density of 0.96 g/cm³ and a listed kinematic viscosity of 1.5 cSt).

A 10 mm master cylinder piston has a surface area of 0.785 cm². Using the volumetric flow rate (~300 cm³/min for ease of units) and the relationship V=Q/AV = Q/AV=Q/A, the piston’s linear velocity would need to be around 2789 cm/min, or about 46.5 cm/s, to generate a sufficient pressure drop to induce cavitation under the stated conditions.

I thought this was an interesting way to look at the problem. Cavitation in our brake systems often comes up in discussions, and while it sounds plausible in theory, once you work through the equations, the flow rates and piston velocities required—given the fluids we use—simply do not support the physics.

HexonJuan wrote:

Fair, my point with bringing up the cavitation was as a primer for my line of thinking. In the cases here, I don't suspect we're seeing...

Fair, my point with bringing up the cavitation was as a primer for my line of thinking. In the cases here, I don't suspect we're seeing cavitation, but fluid restriction(s) in the system.  It's my thinking that manufacturers may need to adjust hose and fitting diameters if they want 110% problem free operation in all conditions. The changes Sram put forth seem to align with a bit of that.

 

And thanks for the detail analysis. That's the good stuff right there.

iceman2058 wrote:

But the fixed point is the rotor contact. The pads will then retract from the rotors, as determined by the amount of fluid the master cylinder...

But the fixed point is the rotor contact. The pads will then retract from the rotors, as determined by the amount of fluid the master cylinder pulls back. A bigger caliper piston will roll back less distance but the same amount of fluid will be pulled back by the master cylinder. The pads stop moving when the timing port in the master cylinder is open again.

iceman2058 wrote:

But the fixed point is the rotor contact. The pads will then retract from the rotors, as determined by the amount of fluid the master cylinder...

But the fixed point is the rotor contact. The pads will then retract from the rotors, as determined by the amount of fluid the master cylinder pulls back. A bigger caliper piston will roll back less distance but the same amount of fluid will be pulled back by the master cylinder. The pads stop moving when the timing port in the master cylinder is open again.

jbfiets wrote:

Pads retract based on piston seal rollback, not on master cylinder volume. It's a common myth, one I believed until I started deep diving into brake...

Pads retract based on piston seal rollback, not on master cylinder volume. It's a common myth, one I believed until I started deep diving into brake bleeds because of persistent issues bleeding brakes. Here's a video explanation: https://youtu.be/mPptmoy0u3s?si=t8CNIRP7x5uGnBrf

iceman2058 wrote:

But the fixed point is the rotor contact. The pads will then retract from the rotors, as determined by the amount of fluid the master cylinder...

But the fixed point is the rotor contact. The pads will then retract from the rotors, as determined by the amount of fluid the master cylinder pulls back. A bigger caliper piston will roll back less distance but the same amount of fluid will be pulled back by the master cylinder. The pads stop moving when the timing port in the master cylinder is open again.

jbfiets wrote:

Pads retract based on piston seal rollback, not on master cylinder volume. It's a common myth, one I believed until I started deep diving into brake...

Pads retract based on piston seal rollback, not on master cylinder volume. It's a common myth, one I believed until I started deep diving into brake bleeds because of persistent issues bleeding brakes. Here's a video explanation: https://youtu.be/mPptmoy0u3s?si=t8CNIRP7x5uGnBrf

iceman2058 wrote:

Thanks for that, enlightening indeed! I *think* some of my original point still stands though, in that the amount of fluid traveling in either...

Thanks for that, enlightening indeed! I *think* some of my original point still stands though, in that the amount of fluid traveling in either direction is still determined by the master cylinder volume being pushed, not the size of the caliper pistons.

But yes, at equal seal strength, I can see how the larger caliper piston will have less "power" to push the fluid back (because of it's larger surface area moving more fluid), if the seals are at equal strength/elasticity between the two different calipers, which explains how the reduction in piston size could make for a more consistent brake. Q.E.D.

Solid discussion, learn something every day!

thresh wrote:

Anyone knows if they will sell the B1 cams independently, without the lever and all the other useless faff like a torx screwdriver? Or is the...

Anyone knows if they will sell the B1 cams independently, without the lever and all the other useless faff like a torx screwdriver? Or is the lever an integral part of the update - e.g. the interface on the older one is different?

 

Tom wrote:

Just fitted the Maven lever tuning kit to my A1 silver brakes. I thought I would put this up here as I  surprised there is no test...

Just fitted the Maven lever tuning kit to my A1 silver brakes. 

I thought I would put this up here as I  surprised there is no test info on this considering it’s a big selling point. 

First of all I personally haven’t had a problem with them from start. 
This silver set have been faultless but I have got another set of ultimates on my trail bike and it did take about 3 bleeds and lot of massages to get them working consistently like the silvers. 
But I do understand what people say about the stiffness in lever pull. 

Anyway the new lever and cam feel loads better and it’s an improvement, for me that initial resistance in the first part of the pull has completely gone. The lever shape feels better too and hopefully not a slippery ( I ended up running lizard skin cover on the old ones). 
No riding experience yet but that big bite of power still seems there, testing one with the new parts and the other with the old, the power feels similar. 

The deadband has changed as to what Sram said. But I have been able to set them up how I like. 
On previous Code brakes I always ran the contact adjustment all the way out (close as possible bite point). 
I found on the Mavens I was running it about half way. 
So now with this set up I am back to running it all the way out and that gives me the same bite point as the full A1 set up set half way. 

Just some numbers from my set up. 

Full A1 
With the lever reach at 65mm from the tip of the lever to the centre of the grip and contact set halfway 23 clicks from in. The bite point is at 45mm 

A1 brake with tuning kit

With the lever reach at 65mm and contact set halfway 23 clicks from in. The bite point is at 40mm 

With the lever reach at 65mm and contact set fully out. The bite point is at 45mm 

I don’t know if pump out is better. I have been measuring this on a lot of different brake makes and surprisingly find in to be quite similar. If a brake level is pulled from resting it bites at a set point, in my case 45mm then if you do repetitive fast pulls in a row the bite point changes by 5mm so in my case 50mm. As I said I have measured this on a lot of brakes and they all do a similar thing. I am wondering if it more noticeable on the A1 Maven as you do have that initial resistance to over come when the bite point changes to 5mm out are you hitting that strong bit point with more leaver force from your finger? Anyway testing the new set up pump out seems to be about 2mm better.  

So my takeaway is that if the power is still there when riding then is a good upgrade to the A1 as long as you are not already running the contact adjustment all the way out. 

 

Evwan wrote:

SRAM just downgraded all maven brakes to 18/18mm piston calipers. All new mavens are going to be a glorified maven base moving forward. If you like the...

SRAM just downgraded all maven brakes to 18/18mm piston calipers. All new mavens are going to be a glorified maven base moving forward. 

If you like the stopping power of mavens, too bad, they just got weaker. Super disappointing decision by SRAM. 

 

seanfisseli wrote:

They increased power at the lever which led to needing to downsize the pistons. Also like @yeahboiwahoo said they are keeping A1 (for now, at least…) 

hamncheez2003 wrote:

TRPavens??

MoldyMTB wrote:

Not sure if this is the right spot to ask this. But I am rather new to higher end brakes, and just got a set of...

Not sure if this is the right spot to ask this. But I am rather new to higher end brakes, and just got a set of hope tech 4 v4's installed. I have only rode them a couple times when we had some nice weather, as i installed them during the winter in canada. 

Recently i have noticed they are rubbing and ive been trying to figure out how to fix it, I have done some alignment but in the process of doing that i have noticed the pistons push out inconsistently, like one side will come almost all the way out and the other side hardly comes out at all. I have used some nice silicone lubricant (like the stuff hope recommends) and that doesnt seem to have done much to help. 

Anyone able to help me figure out how to get them more consistent? Is this normal? 

MoldyMTB wrote:

Not sure if this is the right spot to ask this. But I am rather new to higher end brakes, and just got a set of...

Not sure if this is the right spot to ask this. But I am rather new to higher end brakes, and just got a set of hope tech 4 v4's installed. I have only rode them a couple times when we had some nice weather, as i installed them during the winter in canada. 

Recently i have noticed they are rubbing and ive been trying to figure out how to fix it, I have done some alignment but in the process of doing that i have noticed the pistons push out inconsistently, like one side will come almost all the way out and the other side hardly comes out at all. I have used some nice silicone lubricant (like the stuff hope recommends) and that doesnt seem to have done much to help. 

Anyone able to help me figure out how to get them more consistent? Is this normal? 

Whattheheel wrote:

Hello fellow brake nerds!  Just a quick question on who is making the best aftermarket brake bleeding kit hydraulic and mineral these days that includes most...

Hello fellow brake nerds!  Just a quick question on who is making the best aftermarket brake bleeding kit hydraulic and mineral these days that includes most of the fittings for the major brands?  Thank you for your time. 

MoldyMTB wrote:

Not sure if this is the right spot to ask this. But I am rather new to higher end brakes, and just got a set of...

Not sure if this is the right spot to ask this. But I am rather new to higher end brakes, and just got a set of hope tech 4 v4's installed. I have only rode them a couple times when we had some nice weather, as i installed them during the winter in canada. 

Recently i have noticed they are rubbing and ive been trying to figure out how to fix it, I have done some alignment but in the process of doing that i have noticed the pistons push out inconsistently, like one side will come almost all the way out and the other side hardly comes out at all. I have used some nice silicone lubricant (like the stuff hope recommends) and that doesnt seem to have done much to help. 

Anyone able to help me figure out how to get them more consistent? Is this normal? 

Evwan wrote:

@Jason_Schroeder I saw your maven video where you discuss the "pump out" sensation. Good to hear you liked the brakes.I have experienced the pump out on...

@Jason_Schroeder I saw your maven video where you discuss the "pump out" sensation. Good to hear you liked the brakes.

I have experienced the pump out on my shimano / maven setup and wasn't sure if it was due to the xt lever or the maven caliper. 

Given that SRAM changed both the caliper size (18/18) and the swinglink on the new mavens, do you think the "pump out" is more related to one or the other? I'm curious if SRAM has said anything about the 19.5/18mm caliper causing issues. 

Perhaps there is a reason we haven't been told about that is making SRAM ditch the 19.5/18mm caliper? 

FWIW when I ran trp evo dhr levers with maven bronze 19.5/18mm caliper, there was zero pump out. I’m inclined to believe that pump out primarily comes from lever design, but I do not understand the mechanics of what causes it in the slightest. 

TheKaiser wrote:

Regarding mechanics/causes of the pump-out sensation, it has always seemed to me to most likely be due to a mismatch in the speed at which the...

Regarding mechanics/causes of the pump-out sensation, it has always seemed to me to most likely be due to a mismatch in the speed at which the fluid can flow back through the hose and fittings when the lever is released, and the "demand" for fluid to take up the expanding space in the M/C as the lever returns to the starting position. In other words, the fluid is moving too slowly, and so the return spring in the lever creates a partial vacuum in the M/C, which causes it to then draw in extra fluid from the reservoir when the piston seal exposes the timing port. Once the fluid coming back from the caliper "catches up" and then you quickly squeeze the lever again, you essentially have an overfilled system.

Things that could potentially slow the fluid return include (working from bottom to top): 

1. Relatively weak caliper seal return force. As someone pointed out earlier in the thread, assuming the same seals, the 18mm pistons will push back harder relative to the fluid moved 2. Really gunky caliper seal/bore/piston interfaces, such that drag is being exerted on the pistons as the seal tries to retract them. All else being equal, the smaller pistons will have less surface area where gunk could exert drag. 3. Pinched/restrictive hose ID or fittings 4. Overly viscous fluid relative to the hose and fittings.

If the sum of those factors leads to a slower fluid flow than the pull of the master cylinder piston return spring, you have a potential problem. It seems to me that putting a weaker spring in the M/C could be one way to restore balance to the force (as well as reduce finger fatigue) but for some reason that doesn't seem to be a thing.🤷‍♂️ Anyway, if one were to assume that the stock configurations are designed in such a way that the forces are balanced, with these DIY mashup combos it seems like you have greater potential for there to be a mismatch. Having a mismatch where the fluid is returning faster than required wouldn't create a problem so, if you have a choice, it seems like a safer bet to choose the individual parts that will supply more oil back to the lever rather than less. For example, if we knew what the flow rate under a given pressure was on Maven vs. Shimano hose and fittings, or the viscosity of their respective fluids, pick fastest flowing hose/fittings and thinnest fluid.