Relfy Posted June 19, 2010 Share Posted June 19, 2010 All brakes are engineered for a certain situation and to exercise a specific coefficient of friction on a brake drum or disc. Under normal circumstances, the brakes designed for the application should be sufficient for normal use. Now, the problem often arises where brakes don't perform to specs and overwhelmingly the most common cause of this is brake fluid that is not doing it's job and this is governed by age, type, quality and use. Brake fluid absorbs moisture and for every application of your brakes, degrades in performance. Hard frequent use will effectively 'wear' out your brake fluid. Just as common also is the situation where brakes are not adjusted properly. I've seen this so frequently on all types of motor vehicles and this is so important. These two things will effect your braking ability and if you have changed your brake fluid and adjusted your brakes properly, you shouldn't have any dramas. As someone esle has already mentioned, perhaps a greater braking ability may induce other problems ie, grass strips etc. Link to comment Share on other sites More sharing options...
facthunter Posted June 19, 2010 Share Posted June 19, 2010 brakes for aircraft. If you really think about it in the U/L field they are underdesigned. Cost and weight consideratons most likely the reason. On grass you get away with a lot . On tarmac you roll for ever and if you are heavy and fast, you have a lot of kinetic energy to dissipate. If you get your touchdown speeds back to a minimum it helps, Tiny wheels cannot have the right sized discs alongside them. Although it seems a way out concept, good brakes with ASB are the way to go eventually. The 45 Kts stall makes this solution a matter of choice as most aircraft touch down at about 55 kts. (airspeed) . We are on the borderline of needing it. Differential braking should be the norm. You do after all need to control your aircraft directionally. nev Link to comment Share on other sites More sharing options...
Vev Posted June 19, 2010 Share Posted June 19, 2010 Quote Now, the problem often arises where brakes don't perform to specs and overwhelmingly the most common cause of this is brake fluid that is not doing it's job and this is governed by age, type, quality and use. Brake fluid absorbs moisture and for every application of your brakes, degrades in performance. Hard frequent use will effectively 'wear' out your brake fluid. --------------- Hi Relfy, Can you explain what happens to brake fluid during "hard frequent" braking that "wears out" brake fluid? Cheers Jack Link to comment Share on other sites More sharing options...
Guest ozzie Posted June 19, 2010 Share Posted June 19, 2010 Brake fluid is really not suitable in aircraft as one of the reasons is it amonst other things, attacks aluminium. use the correct hydralic fluid. it also does not absorb moisture and that in turn reduces corrosion to the internals of the components. not related but an interesting approach when it comes to getting out of trouble out in the bush i was taught that if you need a brake fluid but do not have any is to use urine. the high salt content raises the boiling point. can get you home. old army trick. Link to comment Share on other sites More sharing options...
Relfy Posted June 20, 2010 Share Posted June 20, 2010 'Can you explain what happens to brake fluid during "hard frequent" braking that "wears out" brake fluid? Cheers Jack' Jack, brake fluid when new or near new is not compressible. It is rated according to boiling point and when hard braking is applied, the brake fluid can and will boil in the caliper inducing gas bubbles within the fluid, as all fluids have a boiling point. Gas is compressible so the pedal will go soft and lead to a greater travel range. Brake systems have breathers and will, when brake fluid boils from hard braking induce water into the system through the breathers and caliper seals. This effectively draws in moisture, and reduces the effective non compressibility of the brake fluid and in turn 'wears out' the brake fluid. The effectives this has on brake fluid are quite detrimental. Brake fluid should be replaced often and you should use a good quality product. Link to comment Share on other sites More sharing options...
facthunter Posted June 20, 2010 Share Posted June 20, 2010 High temp brake fluid. Disc brakes have the piston(S) that actuate the pads right up against the pad. The high boiling point that new fluid has, is rapidly diminished when water is absorbed (The fluid is hygroscopic, like alcohol) If the temp is raised too far the fluid boils and the previously INCOMPRESSIBLE fluid becomes compressible and the brakes are spongy and ineffective. Bleeding the brakes using new fluid (fresh container, not an old opened one), will restore the efficiency of the brakes.. Most brake fluids are made by one manufacturer and are synthetic. Corrosion of aluminium is not as bad as it used to be and some master cylinders are made of aluminium and some aeroplane calipers likewise. You can use other oils IF permitted by the brake manufacturer but one that I use doesn't have the lubricating capabilities of the more common type. I am not going into the dot 3 and dot 5 etc, things look them up. The main thing is to REPLACE the fluid and bleed the system at least every 12 months. Nev Link to comment Share on other sites More sharing options...
Modest Pilot Posted June 20, 2010 Share Posted June 20, 2010 Call Matco; they are very helpfull. The package they will recommend was almost impossible to find in their catalog. Be aware that it has a different axle bolt pattern to standard Jabs and you will have to fill and rebore the U/C legs. Also you have to specify a special axle nut it you want to fit wheel spats. The recommended tire is narrower larger diameter 6 ply aviation tire, but is as hard and thick as 10 ply Trellborgs and similar in price. You will also have to decide to change the O rings in either the Master or the Slave cylinders, if you go with the slaves it's only a couple of bucks and saves the hassle of getting at the master, you will then be stuck with Dot 4/5 fluid which might not be a bad option anyway. Package will cost around $A1000 delivered plus tyres. Link to comment Share on other sites More sharing options...
ianboag Posted June 21, 2010 Share Posted June 21, 2010 Going back to the physics of all this, I would expect two sets of calipers to be better than one ... here's why ... If you put two bodies into contact, there is a friction force that acts to prevent relative motion between them. This friction force is (the amount of force pressing the two surfaces together) times (the friction coefficient between the two materials). The friction coefficient is a property of the two materials involved. So if you have one set of calipers there will be a braking force that is proportional to the squeeze force exerted by the calipers. Two sets of calipers = two lots of squeeze force = twice the braking force. So it should all work ... However, twice the braking effect will give twice the rate of heat generation at the pad/disk level. So the disks could become very hot which could stuff up the disk <-> pad friction. Also there are the points that have been made about water in brake fluid etc So - on the one hand this - on the other hand that. I would expect INITIAL braking to work a lot better with two sets of calipers compared to one. I would expect the benefit to taper off some as the disk heats up. Link to comment Share on other sites More sharing options...
jetjr Posted June 21, 2010 Share Posted June 21, 2010 at $1K plus install that makes this option pretty costly. Redrilling legs also isnt something Id be that keen on. I just had the flock from behind stub crack and come out after brakes overheated. No doubt it is a better setup if done from new. I have a spare set of Jabiru calipers somewhere if someone would like to buy them. then just need pads, mount plate, springs and bolts, some hose and tee piece Link to comment Share on other sites More sharing options...
Guest Andys@coffs Posted June 21, 2010 Share Posted June 21, 2010 Ian Im not sure its that simple. Let me use the womans high heels shoes as examples:- Im told that in the early jet aircraft the floor was aluminium and there was a problem with high heel shoes where the weight of a woman was exerted on the floor over a a very small surface area. There was perferations of the floor. A man, who in general is heavier than the woman had no perferation because the heavier load was spread across a larger surface area. In the brake scenario the pressure that is applied to the single caliper solution is that which the masetr cylinder can apply. In the dual caliper solution which as I understand uses the same master cylinder the pressure applied is exactly the same (I pressume???), but over a larger surface area. And thats where Im out of my depth. I dont understand if the pressure applied is the same, or effectively less because its shared. If less over a larger area does the heat generated remain the same as a single solution or is it less... Is there anyone reading this who is an engineer and can provide the principles that are in use? Andy Link to comment Share on other sites More sharing options...
facthunter Posted June 21, 2010 Share Posted June 21, 2010 Pressure of fluid. The pressure is the same in both cases because the master cyl piston area and the force applied to the pedal is unaltered. The extra caliper piston gets the same pressure as the other because the fluid is all interconnected and confined fluids transmit pressure equally and undiminished, in all directions. The master cyl piston may TRAVEL further because the oil will now feed more caliper piston area displacing more VOLUME of fluid, when those pistons move. Nev Link to comment Share on other sites More sharing options...
Tomo Posted June 21, 2010 Share Posted June 21, 2010 Yep. Pressure = Force / Area Force = Pressure x Area Link to comment Share on other sites More sharing options...
jetjr Posted June 21, 2010 Share Posted June 21, 2010 Yep same pressure over larger area, means you will have to pull back harder to get same pad/rotor force as single setup. From this I reckon the second caliper will mainly reduce temps in pad and reduce brake fade, not necessarily brake harder but longer. New rotor is also designed to disperse heat better, also allows clearing/cleaning of pad. This increases wear rate but still works much better Link to comment Share on other sites More sharing options...
Guest rocketdriver Posted June 21, 2010 Share Posted June 21, 2010 Yep.Pressure = Force x Area Force = Pressure x Area Pressure = Force/Area ........ Kg/sq m ........:thumb_up: Link to comment Share on other sites More sharing options...
Tomo Posted June 21, 2010 Share Posted June 21, 2010 Pressure = Force/Area ........ Kg/sq m ........:thumb_up: Tah! :thumb_up: I knew what I meant... just musta had a juniors moment Link to comment Share on other sites More sharing options...
facthunter Posted June 21, 2010 Share Posted June 21, 2010 Another possibility. Any play in the wheel bearings will cause the disc to run out of true, with bumps etc and keep pushing the pads away or distorting the disc *temporarily). Keep a bit of preload on the bearings, ( don't overdo it especially with aluminium wheels) but avoid having "play", if possible. Stainless steel discs don't work as well as other metals, and it might be a good idea to make sure that the pads are good motorcycle quality. You might be able to modify a motorcycle pad to fit. You can cut and drill the steel and friction material. Nev Link to comment Share on other sites More sharing options...
Guest rocketdriver Posted June 21, 2010 Share Posted June 21, 2010 You can cut and drill the steel and friction material. Nev Be careful tho' ..not sure about now but brake pads used to have a lot of asbestos in them and I don't know if they have found a replacement yet ..... Link to comment Share on other sites More sharing options...
ianboag Posted June 21, 2010 Share Posted June 21, 2010 Yep same pressure over larger area, means you will have to pull back harder to get same pad/rotor force as single setup. Nope. Same pressure, larger area = larger force. It's FORCE at the pad that determines the friction force. So same pressure (force on the lever) will generate more braking force with two caliper sets than it would with one. From this I reckon the second caliper will mainly reduce temps in pad and reduce brake fade, not necessarily brake harder but longer. Nope again. Second caliper set will result in more friction force than just one caliper set = more braking = more heat generation = greater possibility of heat-induced fade. It will brake harder but maybe not longer. New rotor is also designed to disperse heat better, also allows clearing/cleaning of pad. This increases wear rate but still works much better Better heat dispersion is a good idea because there is a whole lot more to disperse. Final answer - do two caliper sets give better braking than one? Yes. If in doubt, just stand the question on its head - if the pad area was halved would it make things worse? Of course it would ..... Link to comment Share on other sites More sharing options...
jetjr Posted June 21, 2010 Share Posted June 21, 2010 Ian, your probably right BUT How can we get double the friction force for free? If you double the pad area and keep hydraulic force the same the end result is same braking ability (spread over two pads). Overall temps are same as you say but each caliper may be cooler and work better? Dunno? Point is you can easily increase lever pressure and the dual pad setup can apply the force. Braking harder if required unlike single setup which overheats and fades fairly quickly Is the fading caused by pads not working or what?? My experience with the swap to two is exactly that, you can brake harder which allows you to back off before fade really sets in. Singles meant much longer braking events and worse fade - nearly to zero at times Link to comment Share on other sites More sharing options...
Relfy Posted June 21, 2010 Share Posted June 21, 2010 Just looking at some formulas I have and some other literature about break forces and caliper setup, has anyone noticed their jabiru pulling to the right after hard braking, especially larger pilots one up? Link to comment Share on other sites More sharing options...
Guest rocketdriver Posted June 21, 2010 Share Posted June 21, 2010 Ian, your probably right BUT How can we get double the friction force for free? Its not free Jetjr ... at the foot you press just as hard and have to move the pedal twice as far. This to fill up (move) the second caliper at the same pressure as before ...... you put in twice the effort (same force * twice the distance moved) and in return you get about about twice the braking effort .... RD Link to comment Share on other sites More sharing options...
facthunter Posted June 21, 2010 Share Posted June 21, 2010 Double for Free? You can't get anything for nothing, You are correct there, but each pad does push on the disc as hard as the other but once the pads move, (work is done) W = F times D, the master cylinder piston moves further, and could prove to be too small IF the pistons in the calipers have a long travel, the MC piston could bottom out. (Go to the floor in a car). Nev Link to comment Share on other sites More sharing options...
ianboag Posted June 21, 2010 Share Posted June 21, 2010 At the pedal where what you see is "pedal force" -> hydraulic pressure = "pedal force"/(master) piston area. At the pad -> "squeezing force"=hydraulic pressure x (slave) piston area. Braking drag is proportional to squeezing force. If there are two slave pistons in the circuit there will be twice as much "squeezing force". You could achieve the same increase in squeezing force by just having a bigger (diameter) slave piston - even with the same pad area. But - what you need extra pad area for is to dissipate the extra braking energy (heat) from the fact that you are slowing down faster. So it makes sense to double up the pad area to dissipate the extra heat. However there will be some heating issues because you don't get to double the disk area .... One would expect much faster initial braking - for the same lever pressure - but some possibility of fade later. IB Link to comment Share on other sites More sharing options...
JG3 Posted June 21, 2010 Share Posted June 21, 2010 I just finished fitting dual caliper Matco brakes in place of the single caliper that came on my Savannah. The improvement in braking effect is dramatic. Same master cylinder. And also no fade now, which used to be very noticeable when trying to stop downhill. The disc is much thicker and is steel instead stainless. Stainless is a very poor conductor of heat, so doesn't conduct heat away. JG Link to comment Share on other sites More sharing options...
Tomo Posted June 21, 2010 Share Posted June 21, 2010 Has anyone thought of doing a master cylinder for each side? Keep it in the same place, just put the two levers side by side so you can grab both with one hand, have a locking device if you think you may pull on one more than the other - bit like most tractors. That way you get differential brakes, plus the added benefit of not needing to upgrade to a bigger M/C for double calipers. Just a thought? i_dunno Link to comment Share on other sites More sharing options...
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