- This topic has 51 replies, 28 voices, and was last updated 3 years ago by thols2.
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4 pot brakes are better because…
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inthebordersFree MemberPosted 3 years ago
But the 4 pots I’ve used (zee’s)didn’t seem to offer any more power compared to my 2 pot xt.
On steep stuff I found swapping to a 4 pot Zee on the front (from a 2 pot XT) enabled me to actually stop, rather than just slow down. I’ve still a 2 pot on the rear.
DaffyFull MemberPosted 3 years agoBut if we’re saying the only difference is between a 2pot caliper and a 4pot caliper (same levers, same fluid, same rotor, etc) the only difference will be in the effect on the pistons. The rest is identical. When there is a smaller piston area, the pressure acting on them and causing them to move is greater.
pdwFree MemberPosted 3 years agoMore piston area for a given size of pad…
So more piston area for heat disposal
Except that the total piston area is essentially the same, as 4-pots use 4 smaller pistons.
4-pots give you larger pads with the force more evenly distributed for the same piston area.
pdwFree MemberPosted 3 years agoBut if we’re saying the only difference is between a 2pot caliper and a 4pot caliper (same levers, same fluid, same rotor, etc) the only difference will be in the effect on the pistons. The rest is identical. When there is a smaller piston area, the pressure acting on them and causing them to move is greater.
No. For a given force on a given master cylinder, the *pressure* in the caliper piston is exactly the same: it’s the pressure of the hydraulic fluid.
If you use a smaller caliper piston, you’ll get less force (but the piston will move further).
4-pots have smaller caliper pistons, but with more-or-less the same total area. Each piston exerts less force, but the total force acting on the pad is the same just more evenly distributed over a larger pad.
thols2Full MemberPosted 3 years agoWhen there is a smaller piston area, the pressure acting on them and causing them to move is greater.
No, unless you are talking about the master cylinder piston. The hydraulic pressure is determined by how hard you squeeze the lever, the mechanical leverage of the lever, and the size of the master cylinder piston. If the first two are constant, then a smaller master cylinder piston will give increased hydraulic pressure. If all those three are constant, then the hydraulic pressure will not change.
If you have the same hydraulic pressure, then a larger wheel cylinder will provide more force against the pads, so it will give more power. Two small pistons with the same total area as one large piston will provide the same force against the pad, but will let you run longer pads. The larger pad area should be less prone to overheating.
DaffyFull MemberPosted 3 years agoYes, but that’s what I’m saying – the piston area of 4 small pistons is smaller than that of 2 large pistons, it’s 20-25% less in most 4pots. So force applied to the fluid is the same (tech 3 levers are identical for 2 or 4 pot) and the release area for the fluid is smaller, thus they move further or exert greater pressure on the pad/rotor. No?
Agreed that a a pad with the same area but a different aspect ratio will provide better contact to the rotor and thus make better use of that pressure and friction at the expense of slightly higher heat generation in the rotor.
chiefgrooveguruFull MemberPosted 3 years agoI think almost everyone is missing the important points. There’s two main scenarios to think about:
1. The start of the braking phase when the pad approaches and contacts the disc, under low force.
2. Peak braking force, when there are big loads on the pad from three directions.At the start of the braking phase, a 2 pot caliper moves the pad perpendicular to the disc, so when it touches the disc, all of the pad hits at once. However, in the real world a 4 pot caliper never has both pistons on a side moving in perfect synchrony, so the pad always hits the disc at an angle. This gives a more gentle start to the braking, hence 4 pots have better modulation (all else being equal).
At peak braking force, the pad is under very high shear force from the disc. This shear force causes leverage of the pad surface vs where the piston exists the caliper. Worst case scenario the pad will lose partial contact as its rotated from being parallel to the disc and could even start juddering/skipping. It’s unlikely to get that bad with a modern brake but it is clear that having two pistons supporting the pad will hugely reduce that problem, just as standing on two legs is more stable than standing on one.
So a 4 pot brake is better at both light braking and heavy braking, if executed as well as a 2 pot brake.
thols2Full MemberPosted 3 years agothe piston area of 4 small pistons is smaller than that of 2 large pistons, it’s 20-25% less in most 4pots. So force applied to the fluid is the same (tech 3 levers are identical for 2 or 4 pot) and the release area for the fluid is smaller, thus they move further or exert greater pressure on the pad/rotor. No?
Technically, the pistons don’t exert pressure on the rotor, they exert force. Pressure is measured in units like pounds per square inch or newtons per square meter (which is one Pascal pressure). Mathematically, that’s written N/m^2. There’s a unit of force being divided by a unit of pressure. One atmosphere is approximately 14.7 psi or 101 kPa
Force is measured in Newtons (N) in science, but we often use pounds or kilograms in everyday life. If you multiply pressure by area, you get something like this:
1 m^2 x 100 kPa = 1 m^2 x 100 kN/m^2
= 1 x 100 (kN x m^2/m^2)
= 100 kNSo, when you multiply pressure by area, you end up with force.
If you have smaller pistons, you will have less force applied to the rotor for the same hydraulic pressure. That’s a brute scientific fact.
Assuming you have the same rotor/pad clearance, the smaller wheel cylinders will result in less travel at the lever.
However, Shimano levers and calipers are generally interchangeable, so any differences in piston area must be relatively minor. A 25% difference would make a fairly big difference in brake power and feel, so Shimano obviously keep them reasonably similar. However, you can’t assume that all manufacturers have the same policy of levers and calipers being interchangeable.
chiefgrooveguruFull MemberPosted 3 years agoFYI, the current Hope piston diameters are:
X2 – 22mm (380 sq.mm)
E4 – 16mm and 16mm (402 sq. mm)
V4 – 16mm and 18mm (456 sq. mm)A lot of reviews claim the E4 and V4 pistons are 2mm smaller than they really are.
thols2Full MemberPosted 3 years agoSo, the Hope DH brake has a bit under 20% more piston area than their XC brake. Which means they will be noticeably more powerful with the same levers and rotors, but have a little bit more lever travel and probably be slightly spongier. That makes sense.
DaffyFull MemberPosted 3 years agoOkay real life example – Hope X2 (2*22mm) vs E4 (4*16mm) vs V4 (2*18mm, 2*16mm)
Okay so the input force (someone pulling the lever) is say 25N acting on the master cylinder. That force is then applied via the master cylinder to the fluid in the form of input pressure. For simplicity let’s say it’s a square piston with each side 5mm, so 25mm2, which gives us 1mPA (P=F/A) of pressure in the fluid. On the other end we have either 760mm2 (X2) 805mm2 (E4) or 910mm2 (V4) which means that assuming the same travel/work done to move the pistons, the V4s generate 20% more force than the X2s and the E4s generate less than 10% more than the X2a. This just relates to force applied from the calliper to the pad. But a given the brake track on a rotor is usually less than 22mm in depth, I’m guessing that not all of the force from the X2s actually make it to the rotor, whereas for the 4 piston callipers , the longer pad with less depth means that all of the generated force is applied to the rotor.
EDIT – damnit, beaten to it. Distracted by actual work again.
thols2Full MemberPosted 3 years agoI’m guessing that not all of the force from the X2s actually make it to the rotor, whereas for the 4 piston callipers , the longer pad with less depth means that all of the generated force is applied to the rotor.
The rotor is the only thing stopping the pads. All the force is applied to the rotor. It doesn’t just magically dissipate into the air.
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