What are the advantages of 4-piston brakes?

Giraffoto

Just that - what are the advantages/disadvantages of four piston brakes compared with two pistons? Why do Shimano put four pistons in their Downhill groups and two in the others?

rockmonkeysc

More power and sharper bite. Possibly a bit too sharp for xc.

poah

pressure is spread over two smaller pistons rather than one large one so you can have a bigger pad because brake wear is spread out more, you should get better feel from the brakes and less chance of brake fade. However, this assumes they have been designed good. Downside you generally have to move more fluid to push the four pistons to more lever travel and slightly less overall braking force for the same pressure applied. You should get more modulation with 4 pots over 2 as well.

They look more bling too

The Rookie

Just to get it right.....
More pistons spread the load more evenly on the pad so you use the pad surface more evenly, most 4 pot calipers (such as Zee/Saint) keep a similar piston area (or only a very small amount larger) as the 2 pots (XT) so the lever remains the same and no/little more fluid is moved. the larger the piston area the more brake force you get (not less POAH!) but the longer lever travel you get, adjust the lever to suite and you loose any ratio advantage you got from upsizing the piston at the calliper.

rockmonkeysc

Larger total piston area means more force for the same lever pressure assuming the rest of the system is identical. Lever travel stays the same as well if the rest of the system is the same.
There is no effect on the cooling other than a slightly increased fluid volume to deal with the heat.

poah

if the fluid required to move the pistons the same distance then yes the force would be the same but if the 4 pots require more fluid then you are going to have to pull the lever more (greater force) to get the same movement on the pistons.

if it takes 5cc to move two pistons 1cm but 7.5cc to move 4 the same then you are obviously going to pull the lever further. this would translate into more force being required to pull the brakes on harder. The hope tech levers are the same for M2s or M4s. I don't know the dimensions of the pistons so can't calculate how much fluid would be required for each caliper.

The Rookie

Pulling the lever more is not greater force, it's greater distance and maybe more work energy (force x distance), as RM points out the lever creates a fluid pressure, the larger the piston area the greater the force (in kg or lbs) is created by the pressure (kg/cm^2 or PSI whatever you like).

Besides with positive servo levers (like Shimano) greater travel = greater mechanical advantage giving you even more benefit.

rockmonkeysc

You get the same pressure on the piston so the greater area = greater total force.

cooldad

There is more to go wrong, so more people will have even more random brake issues than usual.

slindborg

Assuming they then take a bigger pad it means the pads will last longer the a smaller pad of the same compound.

prawny

Bragging rights.

mrmonkfinger

Slight improvement in using the pads more evenly. Little else.

Brake manufacturers tend to make their more powerful brakes with 4 pistons. The 4 piston bit is not that important, TBH. My Codes have about 15% more piston area as compared to Elixirs. The pads can be correspondingly bigger. Its the difference in total piston area that makes the difference in power.


All that aside, the single most cost effective thing you can do to make your brakes more powerful is stick on a really big rotor.

rockmonkeysc

Its easier to get more pad contact with a 4 piston caliper. The pad can be longer than with a two piston design.

tobamory

Higher number so gotta be better right?

But seriously though as above more stopping power. Like a cars really, 4 pot brembos vs 6 pot brembos much more stopping power on the 6 pot, but more expensive


Subarus have 4 pot

Nissan GTRs have 6 pot

Kowalski675

Bikes had a phase of six pots in the nineties / early noughties, but it was mainly a fashion thing (the Tokico six pot used most was a crap caliper). Six pots aren't about increasing stopping power - the idea is that the increased pad area means that you can use a smaller (and therefore lighter) disc.

Giraffoto

Well, I’ve done my own bit or research, looking at cars, motorbikes and proper bikes and here’s the explanation (I think) . . .
If you want to make a brake more powerful for a given input force on the lever, you need to make the pistons in the calliper larger. however, the size of the pad is a limiting factor – the piston needs to be roughly the same size as the pad. A limiting factor on the size of the pad is the size of the brake track on the disk – so the width of the brake track places a limit on the size of pistons that can be employed in the calliper. You can circumvent this limitation by having the piston area spread over four rather than two pistons, and making the pads a bit longer. You can even do a clever trick by making the pistons different sizes – it gives them a sort of toe-in effect, and can improve the modulation. The resulting brake is a bit heavier though.
Can anyone who does Downhill add anything to this?

And of course, bragging rights come into it somewhere.

The Rookie

That conveniantly ignores the fact you can't just increase piston area as it increases lever travel, and if the make the lever cylinder a larger diameter to compensate you loose the levererage advanatge you gained in the first place, so it's back to making better use of the pad.

nicklouse

The Rookie wrote:

That conveniantly ignores the fact you can't just increase piston area as it increases lever travel, and if the make the lever cylinder a larger diameter to compensate you loose the levererage advanatge you gained in the first place, so it's back to making better use of the pad.

or to put it another way. the use of a longer pad increases the swept area which means more braking power. but you also need to control the pads contact with the Rotor the best way to do that is to have a piston the same shape as the pad. not practical, but some MCs do have oval brake pistons but i might be getting confused with oval engine pistons. so the next best thing is to use a number of round pistons. and if you have them with different diameters they will act on the pad at different times during the lever operation (in an ideal world) allowing a progressive aplication of pad to rotor.

what longer pads can also do is give you way too much braking power.

thelonegroover

I'm sure we had a thread recently, where the conlusion was (amongst some members at least), surface area did not affect friction, and it's friction that is required to make a brake work.

nicklouse

Thelonegroover wrote:

I'm sure we had a thread recently, where the conlusion was (amongst some members at least), surface area did not affect friction, and it's friction that is required to make a brake work.

swept area is not surface area.

and you are going back to the rotors with big holes in them.

thelonegroover

I was refering to the pad area rather than the disk area, the sweapt area may well effect braking due to being further from the axle and cooling may be better because of the larger disk, but the actuall stopping power of the pads should be the same.

nicklouse

Thelonegroover wrote:

I was refering to the pad area rather than the disk area, the sweapt area may well effect braking due to being further from the axle and cooling may be better because of the larger disk, but the actuall stopping power of the pads should be the same.

nope, more surface more braking.

The Rookie

Not sure I agree, no area in the standard friction calc, double the area and half the (average) load per unit of area.

nicklouse

The Rookie wrote:

Not sure I agree, no area in the standard friction calc, double the area and half the (average) load per unit of area.

yes that is for the pistons.

The Rookie

No, the standard friction calc has no adjustment for area, for the same applied force (from the pistons), double the pad area and you halve the applied force per unit of pad area so friction stays pretty much the same Friction versus applied force is a curve not linear which may gain or loose you a small amount of friction), the benefit of larger pad area is more thermal inertia and lower wear rate (in terms of pad thickness) you don't get more brake force unless one of those factors is critical (pad overheats or wears out).

nicklouse

arg feeking forum crashing and losing that post.

reply will be coming later.

Kowalski675

nicklouse wrote:

but some MCs do have oval brake pistons but i might be getting confused with oval engine pistons.

I think you probably are. I've never heard of any motorcycle using oval brake pistons. Honda (being Honda) dabbled with oval engine pistons (shaped like a tin if spam, with two conrods per piston) with their NR750 GP bike (and subsequently the outrageously expensive NR750 road bike in 1990, that was just a corporate willy waving exercise), but it was a pretty disasterous technological dead end (the race bike earning the nickname "Never Ready". Standard practice for multi pot MC brakes to use pistons of differing sizes though, to offset the increased wear on the leading edges of the pads, as you can see from one of my Nissin 4 pots here:

The Rookie

Not so much to counter leading edge wear, but leading edge effect which sees the leading edge doing more work (thus wearing faster) so more evenly using the whole pad and stabilising temperature and hence performance across the pad.

Leading edge effect can be beneficial though, made the Mini on drum brakes a lot less unsafe and makes 'drum in disc' handbrakes very effective when they work properly (are you reading this BMW?)

Kowalski675

The Rookie wrote:

Not so much to counter leading edge wear, but leading edge effect which sees the leading edge doing more work (thus wearing faster) so more evenly using the whole pad and stabilising temperature and hence performance across the pad.

Longer way of saying the same thing, lol - as you say, the leading edge wear is a symptom of the leading edge doing more work, rather than the problem in itself, and the different sized pistons (as you also said) equalize the braking performance, pad wear and temperature over the whole pad area (and reduce the chances of hot spots warping the disc).

Leading edge effect can be beneficial though, made the Mini on drum brakes a lot less unsafe and makes 'drum in disc' handbrakes very effective when they work properly (are you reading this BMW?)

My first bike was a '75 RD 200. It had a big twin leading shoe front drum brake that was far more effective (at least until you cooked it) than the first disc brakes on the '76 model - they had the right idea, but not the materials technology to make it work effectively yet.

supersonic

Some mtb brakes have oval pistons.

Chunkers1980

Yeah some Formulas.