[kjmccarx]

After replacing my brakes recently (upgrade from Juicy Three to Shimano XT - front, and SLX - rear), I began wondering where the actual stopping power comes from.

I know the basics behind how hydraulic brakes work, but what happens when I mix and match different brake levers and calipers?

The XT brake has a bit more power than the SLX brake, and the lever is smoother. Hypothetically, if I were to swap the calipers on each brake, so the XT caliper is with the SLX lever (and vice-versa), how would the feel and power compare to how they are now?

 

[Cleared2land]

Just do it! My initial thought would be that they are about the same.

 

[kjmccarx]

Just do it! My initial thought would be that they are about the same.

I don't actually want to switch my calipers/levers, I'm just interested to see what it would be like. Since XT and SLX are so similar I'm guessing there wouldn't be much of a difference at all. But what if someone bought a Deore lever and Saint caliper, would they have the stopping power of a Saint break, or a Deore break?

 

[thickfog]

What data do you have proves the slx are less powerful than the xt?

 

[006_007]

I have a set of XTR trails on my bike, and SLX on my wifes bike. Both have same size rotors(ice-tech) Both using finned pads (one resin, one semi-metalic). Both bikes are running same tires. Power and modulation wise I have a really difficult time telling the two of them apart.

Only difference I can tell with a scale is the XTR are lighter.

The XTR also have a "free stroke adjust" screw - which whenever I use it I never really notice any difference with the damn thing.

If they are not feeling nearly identical, I would suggest a bleed to see if that sorts it all out (and confirm they are using the same type of brake pads)

 

[kjmccarx]

What data do you have proves the slx are less powerful than the xt?

This isn't about which brakes are better. It's a question asking about where the stopping power comes from.

 

[thickfog]

This isn't about which brakes are better. It's a question asking about where the stopping power comes from.

Where do I say it's a post about what brakes are better? I'm asking you where you got the data in which you state slx are less powerful than xt.

 

[kjmccarx]

Anyway, to get back on track, does anyone know how how mixing and matching brake parts would affect performance? Instead of my XT/SLX example, what about pairing a Deore brake lever with a Saint caliper, and a Saint lever with a Deore caliper? How would the performance from each of those setups compare to stock Saint and Doeore brakes?

 

[ca_rider]

Anyway, to get back on track, does anyone know how how mixing and matching brake parts would affect performance? Instead of my XT/SLX example, what about pairing a Deore brake lever with a Saint caliper, and a Saint lever with a Deore caliper? How would the performance from each of those setups compare to stock Saint and Doeore brakes?

Below is one way to approach braking power.

The master cylinders convert your applied braking force to pressure. The calipers convert pressure back to force. The hoses allow the fluid to flow in order to transmit pressure. We can assume the viscous energy losses in the hoses are negligible.

Therefore the operative mechanism is this: When you pull the lever, a piston converts that force into pressure. The pressure is related to the force based on the leverage ratio of the lever hinge and the width of the master cylinder piston. Pressure varies with force according to P=F/A where P is pressure, F is force, and A is area of the piston =pi*radius^2. This relation is true in both the master cylinder and in the caliper.

To put it all together:
You put a force F on the lever at some distance D from the pivot. The resulting force on the master cylinder is F*D/d, where d is the distance from the pivot to the master cylinder piston pushrod. The master cylinder has a piston diameter Dp. The caliper has a pistion diameter dp. The pads apply drag on the rotor according to some constant Uk. Thus the drag force on the rotor (F_rotor) is

F_rotor=F*D/d*dp/Dp*Uk

To convert this to Power (P), consider P=energy/time = force*distance/time. To calculate braking power, take F_rotor*s/t where s is 2*pi*r where r is rotor diameter and t is time you have been applying brakes.

To get fancy, work everything as integrals and consider brake fade where Uk is a function of T (temperature) and T is a function of power and airflow and conduction, resulting in coupled equations. Within that power function can be convective and conductive heat transfer effects and pretty soon you need a Nusselt number and things start getting interesting (see: Nusselt number - Wikipedia, the free encyclopedia )

What this all means is you have to consider the ratio of area of the master cylinder to the area of the caliper pistons. For a given applied lever force, the braking power scales inversely with the area of the master cylinder piston and linearly with the area of the caliper piston. So Saint calipers, which may have more caliper piston area than some other caliper, will have more braking power. To find how much more power is presented, you need to measure the total piston area.

That is, to a first approximation, the braking power scales with dp/Dp and you need to find the ratio of those dimensions to compare various setups. Lever excursion will vary with the inverse.

 

[Cleared2land]

Geezzz...All this time I just thought that you pull the brake lever and the bike stops.

 

[Procter]

I think ca_rider nailed it. The only thing I'd add is that the caliper design (aka slave cylinder design) and master cylinder design must also minimize flex of the pistons, piston cavities, etc. If there is any flex in the piston cavity under pressure it will diminish force applied at the pad.

You must also minimize overall flex in the caliper body as it takes forces tangential to the rotor and forces at the post mounts. Likewise the lever must minimize and resist flex from the hand gripping it, and from the attachment to the handlebar. This must all be accomplished thru careful design of the physical dimensions of the levers and calipers and the selection of metals.

But all that is probably secondary to ca_rider' s observations about piston area.

 

[J.B. Weld]

Ultimately "stopping power" comes from the tire/earth interface.

 

[411898]

Unless there is any form of internal difference in design between Shimano levers/caliper models, they should stop exactly the same when paired up with the same tire and rotor and the pads are the same thickness and all of the systems are bled properly. That doesn't answer anthing, does it?

 

[Cleared2land]

Unless there is any form of internal difference in design between Shimano levers/caliper models, they should stop exactly the same when paired up with the same tire and rotor and the pads are the same thickness and all of the systems are bled properly. That doesn't answer anthing, does it?

I concur.

 

[ca_rider]

Ultimately "stopping power" comes from the tire/earth interface.

Good point, if one is in a 100% skid then the tire is bleeding energy instead of the brake. However if one is not skidding, the tire merely transfers the energy to the brake. You can trace which components are providing braking power by finding which are shedding heat.

 

[ca_rider]

Unless there is any form of internal difference in design between Shimano levers/caliper models, they should stop exactly the same when paired up with the same tire and rotor and the pads are the same thickness and all of the systems are bled properly. That doesn't answer anthing, does it?

The designs absolutely vary. For example, Servo Wave is case where D/d is a function of lever angle so it is not even fixed in a single design.

 

[411898]

The designs absolutely vary. For example Servo Wave is case where D/d is a function of lever angle so it is not even fixed in a single design.

Well then there you have it.

HOWEVER, none of that matters unless the different brakes in comparison are properly bled, run on the same disc, same wheel, same tire, same tire pressure, same exact stretch of terrain, under the same exact weather conditions,.... shall I go on?

 

[ca_rider]

I did my best to try to explain braking power concisely. Sorry you think what I said doesn't matter. Yes, please go on.

 

[411898]

I did my best to try to explain braking power concisely. Sorry you think what I said doesn't matter. Yes, please go on.

Oh, sorry. I did not mean it like that. I was simply trying to say that comparing any items would only provide accurate results if the conditions were exactly the same during each test. I was just trying to enhance your statement.

 

[ca_rider]

That makes sense thanks. Still working on my first coffee, I should have waited a few hours to wake up so as to interpret better. As you can see I haven't quite got into work mode yet today

You are right that innumerable variables affect braking and tires are an important component too.