TimothyH

Anyone using 4 piston calipers for gravel such as the Hope RX4?

I realize that it might not be needed apart from racing but what are the benefits from a purely academic perspective?

Other brands besides Hope?


-Tim-

DrIsotope

Shimano XT 8020, Shimano Saint M820, SRAM Guide T, and the over-killing-est of all, Magura MT5/7/8.

Four piston is pretty much seen only in downhill, the same guys that don't flinch at a 203mm rotor.

But those Saints are pretty, and I'd run 'em just because.

gregf83 

More piston area equals greater force. The pressure applied by the master cylinder in the brake handle doesn't change but if you make the pistons in the calipers bigger, or increase the area by adding another piston, the force applied to the pads will be higher.

Basically, it increases the leverage and gives you more powerful brakes.

redlude97

That is not how it works at all, see Pascal's Law. Additional piston area just distributes the force applied at the lever over a greater area. This could help with heat distribution but not pad force.

TimothyH

These appear to be only post mount, no?

Anything for flat mount besides Hope?


-Tim-

DrIsotope

...probably not. Flat mount is pretty much road specific, I think only Cannondale is putting flat-mount on a MTB frame-- in MTB, there are still frames that are IS-- weight and aero are really considerations when bombing down the side of a mountain on a full-suspension bike. Give it a couple years, and we'll probably have dual caliper flat mount setups. If only so they can sell us 4x as many brake pads.

vinuneuro

Posted by a user on ww about the Hope's:

"I've fitted up a few of these brakes to replace the usual leaking Shimano ones…..
I had high hopes for them based on their excellent MTB brakes.

The Hope calipers fit directly onto the Shimano hoses, using the supplied Hope fittings. I've found them to be pretty awkward to bleed. If you follow their instructions exactly, then you get there in the end, but it's definitely not a case of popping the bleed pot on the lever reservoir and pumping one lot of oil through. Once bled they're super powerful, and don't squeal or rub. I would say feeling almost too powerful for a road bike…

I also had some problems with side to side clearances, the caliper body is wider than a Shimano one, and I've had to do some annoying hub respacing as the callipers wouldn't move sideways enough to centre up over the rotor…."

fietsbob

Magura

gregf83 

If I push with 1lb of force on a 1 inch master cylinder I create 1psi of pressure in the hydraulic lines. If I have a slave cylinder with 10 square inches I can apply 10 lbs. Increase the slave to 100 sq in and you'll get 100 lbs.

A side effect of a larger slave piston is the master piston will have to travel proportionately farther.

This is the principle of how hydraulic jacks work. The mechanical advantage of a hydraulic jack is the ratio of the slave piston area over the master piston area. Make the slave bigger and you can lift a heavier load. Same principle for brakes, i.e. more braking force.

redlude97

But if you hold that constant...(ie lever travel distance) you get less force if you increase piston area, or the same force if the piston area stays the same.

gregf83 

The force applied has nothing to do with the lever travel distance. The force your fingers apply is constant as is the master cylinder size in the shifter handle.

As I said before, increasing the size of the slave, or doubling the number of pistons at the caliper, results in greater braking force for an equivalent force on the brake lever. In a properly adjusted braking system there shouldn't be much movement of the lever. Ideally the pads are sitting right next to the rotor. On bikes they allow a little space between the pads and rotor that needs to be taken up initially but it's a small amount.

redlude97

Its clearly not that simple, lever travel is the limiting variable on road discs, not force. Doubling the number of same sized pistons, while keeping the force the same means doubling the lever travel then, which would be unacceptable on either shimano or sram hydraulic road levers. If you doubled the pistons while decreasing size so the area is the same, then you would have the same force exerted on the pads.

gregf83 

Sorry, but you don’t appear to understand how hydraulic brakes work.

fietsbob

4 pistons got them able to use a longer disc pad , for more surface to make the friction..

IDK if the fluid goes to 1 cylinder-pair , before the other , so modulated application, not as sudden..

redlude97

Why don't you explain how a hydraulic system is anything more than a two ended fluid displacement that changes work done moving one piston(the MC) and to the other end of the system(2+ pistons in caliper)?It's been awhile, but I'll try to follow along with my PhD in Chemical Engineering.

Or maybe you can explain why Hope, the only 4 piston road caliper manufacturer would bother decreasing piston sizes for their 4 piston and 6 piston calipers compared to their 2 piston calipers if they could magically have more clamping force if they simply increased the number of pistons? https://www.hopetech.com/wp-content/u...eals-chart.pdf

gregf83 

I don't know the specifics about the examples you provided but I do understand how hydraulic brakes work and have explained it in my earlier posts. I presume they reduced the size of the pistons because they didn't need double the braking power and that there were other design factors at play. Some of the two piston version they offered had 25mm pistons which require very large pads and rotors.

That said, Hope's 4 piston caliper with 16/18 mm pistons provides about 20% more braking force than the typical calipers used by Shimano (22mm). I believe Shimano also offers a 4 piston caliper with 16/18mm pistons for MTB.

I think you're getting confused with fluid displacement, lever travel etc. Those items are relevant only when the pads are moving. In an ideal system the pads would be right next to the rotors and not have to move at all. In the real world rotors warp and calipers are not perfectly aligned so the calipers are designed to pull the pads back a little, say .5mm.

When you initially pull on the brake levers very little force is required and there is some fluid displacement and piston movement while the pads are pushed up against the rotors. Once they are touching there is no more fluid movement and the force applied to the pads is simply a multiple of the force applied to the brake levers. The pressure in the lines is the force applied to the master piston divided by the area of the master piston. The force applied to the pads is the pressure multiplied by the total area of the slave pistons within the calipers.

In a well adjusted and aligned system the pads will be very close to the rotors and lever travel to move the pads will be minimal. In a poorly adjusted system or one that contained some air it is possible for the lever to bottom out. If you increase the size of the slave pistons, the risk of this happening is higher but shouldn't be an issue in a properly adjusted and bled system.

P.S. - If you're serious about understanding how a hydraulic braking system works it is typically covered in 1st year physics. Here is an open textbook https://d3bxy9euw4e147.cloudfront.ne...Physics-OP.pdf which discusses hydraulic brakes on page 400.

redlude97

See this is where your 1st year physics book is an oversimplification of the system. Perform a simple experiment with your hydraulic road brake lever, pull the lever until the pads make contact with the rotor. Now try to increase the force onto the lever without moving it any further. Possible? No. On both properly adjusted Shimano rs685 or Sram HRD Red 22 levers this occurs about 1/3 of the way through the lever arm travel. To actually stop the bike however, the lever needs to move at least an additional 1/3 of the travel distance. The lever arm and piston aren't flexing so more fluid displacement is occuring in the MC.

Draw a free body diagram around the piston in the caliper, while the pad side of the piston doesn't move any more relative to the rotor once it makes contact, the whole caliper body flexes an equal amount to counter the pressure exerted on the piston. There is additional fluid displacement as the pressure/force increases on the lever and subsequently at the pad/piston interface. What you need to do to determine lever travel is to integrate the change in force and volume displacement to determine the total amount of work done and subsequent lever travel. Your physics books exerpt even implies this at the bottom of the page in the box discussing conservation of energy, but it is beyond the scope of a 1st year physics student to perform the calculation. There is no free lunch, the same amount of work needs to be done.

Take your example to the extreme, and hook up a road brake lever to an automobile caliper, which has essentially no gap between the pad and rotor and according to you should be able to just multiply the force by 10+x based on the change in piston area size, but I would put money on it that you couldn't slow even a free moving rotor with the road lever, you would simply run out of lever stroke before any meaningful pad pressure would occur.

Still don't believe me? How about we go straight to the source, racing brake manufacturer Stoptech's white paper on the subject
Brake System and Upgrade Selection

gregf83 

At least you've back pedaled from your initial statement in this thread.

Yes, the braking system on a bicycle is not ideal and flexes considerably more than in a vehicle. I'm sure the hydraulic brake lines used on a bike are far from ideal as well. In a car the lines are normally steel with the flexible portion limited to 12" or so. On high performance cars it isn't uncommon to replace the rubber hose with braided stainless steel in order to minimize expansion. A bicycle has fairly long flexible lines which will expand with increasing pressure on the lever.

The other big source of imperfection in a bicycle system is air. It's not easy to properly bleed the hydraulics on a bike so there will always be some amount of air that needs to be compressed.

None of this changes the fact that the force applied to the pads on the caliper is directly proportional to the area of the caliper pistons. Provided your levers aren't bottoming out, the force on the pads will be multiplied the the caliper/master piston area ratio.

At this point I don't know what you're point is. My original statement that pad force is directly affected by piston area still stands.

Perhaps your only point now is that hydraulic brake systems on a bicycle are not ideal and there are elements of the system that flex or expand. On this I agree and have not said otherwise.

fietsbob

Magura' Gustav disc brakes was an earlier 4 piston disc brake , the company makes motorbike stuff too..

redlude97

You're right I backtracked from that in the first reply i made, what i meant was that the force at the pad is proportional to the work done(force and distance of lever travel). That should have been clear from all the subsequent statements, but if not I hope it is now. Your statement that you get more powerful brakes isn't true without a large caveat because there is a limited amount of work that the lever MC can do which is a combination of force and lever pull. To increase that force to double for example you'd have to pull the lever an additional amount that corresponded to the MA of the lever/piston assembly.

Yes, the braking system on a bicycle is not ideal and flexes considerably more than in a vehicle. I'm sure the hydraulic brake lines used on a bike are far from ideal as well. In a car the lines are normally steel with the flexible portion limited to 12" or so. On high performance cars it isn't uncommon to replace the rubber hose with braided stainless steel in order to minimize expansion. A bicycle has fairly long flexible lines which will expand with increasing pressure on the lever.

The other big source of imperfection in a bicycle system is air. It's not easy to properly bleed the hydraulics on a bike so there will always be some amount of air that needs to be compressed.

None of this changes the fact that the force applied to the pads on the caliper is directly proportional to the area of the caliper pistons. Provided your levers aren't bottoming out, the force on the pads will be multiplied the the caliper/master piston area ratio.

At this point I don't know what you're point is. My original statement that pad force is directly affected by piston area still stands.

Perhaps your only point now is that hydraulic brake systems on a bicycle are not ideal and there are elements of the system that flex or expand. On this I agree and have not said otherwise.[/QUOTE]
You seem to continue to think that the lever travel isn't an issue because fluid movement stops once the pads contact, and that a larger piston area results in a more powerful brake. But you can't just apply a set force at a fixed lever position, you have to pull the lever until the resulting force has increased to a sufficient level. Where we disagree and my point is that lever travel is the limiting factor in road bike hydraulics so given you are limited in terms of the total amount of lever travel you have which is proportional to the pressure you can apply to the MC. but there is a reason Hope uses two different pairs of piston sizes for the different MC sizes in shimano vs sram road levers in the Hope RX4, 18/16 and 16/14 respectively. Based on your reasoning, they should have just used the larger caliper with SRAM because it is more powerful