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6 Aug 2007, 11:13 (Ref:1982396) | #51 | ||
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I guess I will have to ask Porsche! |
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6 Aug 2007, 11:14 (Ref:1982398) | #52 | ||
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6 Aug 2007, 11:28 (Ref:1982414) | #53 | |||
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"When the fear of death out weighs the thrill of speed, brake." LG |
6 Aug 2007, 14:01 (Ref:1982568) | #54 | ||
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14 Aug 2007, 12:59 (Ref:1988371) | #55 | ||
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when you are ultimately trying to make a car as light as possible drilling may be a valid option if the risk of a new disk cracking is minimal. If you are changing everything between races then theres only an advantage to be had..
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14 Aug 2007, 16:08 (Ref:1988547) | #56 | |||
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Most classes have minium wts. Yes removing wt from some areas, especially rotational wt and putting the balast near the CoG would help. When we look at the mathmaics of braking, holes in rotors hinder braking more then they help. What is the friction co-effecnt ( u ) of a hole? ZERO Outgassing no longer happens with the organic brake pads. Heat disipation comes out the top of the rotors. Holes only cause very uneven heating and cooling of the rotor thus causing cracks 3x faster then a solid sided rotor. Slots, groves or j-hooks help clean pads of debris and glazing from overheating ( poor brake technique) Yet still are starting points for cracks. Then what is your budget? can a team or indvidual. afford to replace rotors every race? That is up to the individual, Most can not. |
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"When the fear of death out weighs the thrill of speed, brake." LG |
14 Aug 2007, 17:38 (Ref:1988636) | #57 | |
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How is brake overheating down to poor braking technique? in simple terms, the temperature of the brakes is a result of the energy to be dissipated over the time of the brake application. The harder you brake (in simple terms) the hotter they get, right?
If you brake too hard (poor technique) the brakes run cooler (the tyres do the energy dissipation and go square). If you brake too early/gently (poor technique) they run cooler as the energy is being converted over a longer time, during which the disk et al radiates some of the energy, reducing peak temp. The only poor technique I can think of that would increase temps is riding the brake with your left foot. Am I wrong? G |
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14 Aug 2007, 18:03 (Ref:1988887) | #58 | ||||
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Show me a formula for braking effort which takes surface area into account. Quote:
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14 Aug 2007, 21:57 (Ref:1989167) | #59 | ||||||
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http://www.fatiguecalculator.com/finders/findkt.htm Quote:
No need for holes in rotors. Quote:
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Unforutnilty many ppl as you have mentioned brake too soon and too little and the other I can think of, drivers braking where no braking is needed, but a slight lift of the throttle will do. Some ppl will continue to use rotors with holes in, drilled, milled or cast in, and that is their choice, or there brake sponsors choice. Some of us found that solid side rotors better for braking and last longer. I dont have a brake sponsor and I buy my own rotors. Cheers |
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14 Aug 2007, 22:42 (Ref:1989196) | #60 | |
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So grooved discs are a no-no too.
I am so glad I spoke to Porsche. Longevity of rotor life is not expected from grooving or cross drilling of discs, but improved braking (and therefore better lap times) are. So, if you want 35,000 miles out of your discs, make sure they are not machined in any way. If you want quicker lap times, machining becomes either a budget consideration or you make your discs out of material and with dimensions that are able to cope with the increased stress. Or better still, a combination of both! p.s. anyone interested in the reason Porsche still use cross drilled discs can PM me - this open forum argument stuff is not for the faint hearted like me. |
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14 Aug 2007, 23:17 (Ref:1989212) | #61 | ||
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Braking too hard is when you exceed the grip of the tires - i.e. one or more (loaded) wheels lock. When a loaded tyre is close to the point of losing grip, but hasn't, is the most effective use of the grip available - just as in cornering (ignoring slip angles for the smart bottoms around here). So the brakes should be able to create the level of retardation (a function of friction and time) that will match the tyre grip, plus a bit, regularly, every corner, for one race/one season/a lifetime (budget dependent) WITHOUT exceeding the temperature (or heat cycles) a) at which the pad loses it's effectiveness, or becomes irreversibly damaged b) that will damage the disc. The killer of brakes is heat above the design parameters of the braking system - of what the pad and disc materials will tolerate, whatever their surface area, construction or design. For those prepared to think a bit, here is an brain exercise: If you take a disc of metal 5 mm thick and 20 mm in diameter and drill a hole in it, 6mm in diameter, what property or propeties of the disc will change, and by how much? Now change the disk to 28 mm thick. What property or properties of the disc are changed, and by how much, and can you see a benefit in either drilling or not in drilling the hole? What is/are the benefit(s)? Answers on a postcard to: Porsche AG, Stuttgart, Germany. (I can't promise a prize for the winning answer) A clue: if the disc of metal is only 3mm thick there is no benefit....... |
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14 Aug 2007, 23:40 (Ref:1989222) | #62 | ||
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Bye the way, are our rotors in parallel or perpendicular tension - and is it one of these rather than the other that makes the cracks appear? |
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15 Aug 2007, 07:58 (Ref:1989350) | #63 | |||
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Now answer my question: Show me a formula for braking effort which takes surface area into account. |
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17 Aug 2007, 11:11 (Ref:1990857) | #64 | ||
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Best explantion I've heard is 'Transient Molecular Adhesion' at least it sounds techy
Why J-Hooks in a disc, what's the advantage over convential slots/cross drillings ? It's a more complex profile to machine (though I guess CAD?CAM?CNC probably makes that argument null and void), so there's got to be something in it. |
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19 Aug 2007, 16:49 (Ref:1992115) | #65 | |||
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Just so as the less knowledgeable of us can follow this properly, could you just confirm your definition of "braking effort" and "surface area". I ask because my brake supplier seems convinced that my pad area has a lot to do with how quickly I'll be able to stop. The thing is, that could be for all sorts of reasons not directly related to any formula for force vs area, such as thermal and wear considerations. Thanks. Last edited by dtype38; 19 Aug 2007 at 17:04. |
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19 Aug 2007, 17:24 (Ref:1992128) | #66 | |||
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I spent nearly a year trying to stop the brakes on a big Jaguar from overheating when I was racing. I tried all sorts of high temperature pads and fluids and various ducting and nothing helped... until one day I read that braking too gently could be the cause. So I went out and instead of gently progressing the brakes on, modulating them over bumps and and surface changes, then rolling gently off them, I tried nailing them as hard and short as I could without locking the tyres. I then had all sorts of suspension instability to deal with, but one thing was for sure, the short hard braking completely sorted my pad/fluid/disc overheating problems. I then even managed to change to a softer pad and get better feel and still didn't overheat them. I belive the explanation for all of this is quite simple. Time may not be a factor in the equations which relate the total energy disappated to the total retardation, but it is certainly a big factor when it comes to transmitting heat through materials, and for disappating heat from surfaces. Put simply, consider these two things... First, the more of a lap you spend on the brakes, the less of it is spent cooling the discs down, and the latter is one of the main controlling factors in how hot the brakes run. Secondly, I'm pretty sure (please don't make me go get my old thermodynamic books out on this) that exposing the surface of an object to a very high temperature for a very short time transmits less heat into its interior that a lower temperature for a longer time. So short hard braking doesn't heat the interior of the calipers and discs up as much as longer braking. Ducks and waits for verbal lashing Last edited by dtype38; 19 Aug 2007 at 17:28. |
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20 Aug 2007, 04:40 (Ref:1992452) | #67 | ||
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Well dtype, you have put your left one out there with that post Kudos to you for asking the question, and then backing it up with your personal experiences.
I have no idea about the answers, by the way, but I hope you don’t get flamed because I would like more of this type of constructive, practical input into any of our Tech discussions. Personally I know that I slam the brakes on as hard as I can when I am driving, I then often find that I am backing off during the process to modulate the brakes as you slow down, though I am sure that part of this is due to my compounds. Generally, I work on the basis that you are either trying to go faster or slower, there are very few times where you want to be in between. |
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20 Aug 2007, 07:49 (Ref:1992521) | #68 | |
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Usually only lurking, but thought I'd comment on dtype's experience (not flaming, just discussing!): Braking energy is absorbed by track surface, tyre, rotor, and pad. By braking harder you are probably shifting energy absorbtion to the tyre. E.g. because it is deflecting more. I don't think time is such a big factor, if it was it would be the other way round.
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20 Aug 2007, 11:50 (Ref:1992660) | #69 | |||
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Where F is the friction force mu is the coefficient of friction between the two mediums and objects R is the normal force acting on that contact patch You might say that I haven't answered your questions of "Show me a formula for braking effort which takes surface area into account.", but consider that in the real world, mu varies with heat, surface area, surface roughness etc (either averaged over the area of contact, or even more accurately considered in small areas within that area). It's far too complex a science to have 'an equation' given to it (just like there isn't an equation for "how much fuel does it take to get from England to America), so you have to give some thought to at least as many things as possible. It's the reason wide tyres give more grip than narrow tyres (generally speaking). Last edited by tristancliffe; 20 Aug 2007 at 11:54. |
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20 Aug 2007, 13:12 (Ref:1992749) | #70 | ||
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20 Aug 2007, 13:23 (Ref:1992766) | #71 | |||||
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Reduced to basics, a brake is a device to convert kinetic energy into heat. Heat is generated at the pad-disc interface. If the pad is very small the heat will be concentrated into a very small area, thus for a given rate of energy dissipation the temperarture rise will be much higher than it woould be for a large pad. Pad wear, measured in volume of material per unit of energy dissipated, is fairly constant. It follows, therefore, that the bigger the pad area the less the reduction in thickness & therefore the longer the life of the pad. A lot of road car pads have large chamfers to reduce the tendency to squeal. As a result of this, the effective pad area can vary by probably as much as 15% over the life of the pad, with no discernable variation in braking effort. Quote:
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20 Aug 2007, 16:46 (Ref:1992913) | #72 | |||
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Surface area has no effect in smooth body problems - but it does in real life problems. Whilst drilling and grooving are unlikely to have a noticable effect on either surface area or braking torque (indeed, I'd imagine it's so negligable that this discussion is a bit silly, but it was a question and this is what happens), it's still an effect. Of course tyres deform more than brake pads (we hope), and of course the problem is slightly different, but the comparison is still valid - surface area changes the force due to friction. A tiny brake pad (say 1mm^2) will give you less friction (and a hell of a lot more wear) than a pad with 500mm^2 area (which will wear less). That's before you even consider the thermal issues of the pad. |
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20 Aug 2007, 17:58 (Ref:1992953) | #73 | |||
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First, although the comparison with tyres might be interesting, I think it might be clouding the issue a bit. The original question was about grooves in brake rotors which expanded to a discussion of putting grooves or holes in rotors and their theoretical and practical effects, which has then moved onto pads and brake performance in general. It's producing some interesting discussion, so can we keep it focussed there please chaps (and chapesses if there are any reading), thanks....
Takes "mod" hat off and puts "techy" hat on.... Quote:
This would suggest that the argument about braking force vs pad area only holds true while well within the operating parameters of the pads. This obviously falls down when comparisons are made between 1mm sq pads and 500mm sq pads (not getting at you trist) so means that this sort of discussion can't be made by considering extremes. Anyway, it seems that to get a nice linear relationship of area to braking force we'd need to pick a pad with temperature resistance well above where we want to use it... but (as I'm told, and from my own experience) high temperature resistance = very hard and that I'd get no "feel" from them (an issue we haven't covered yet). My brake supplier says that I should aim for the softest pad I can without reaching the critial high wear temperature. This is supposed to give me the best "feel" (NOT braking force) even though I'll probably get through pads quite quickly. That suggests that I could quite happily buy some really hard pads that would last me all season, but my braking wouldn't be as effective. (Hmm, seems I've been told something similar about tyres, but lets not go there please ) So, I think what I'm trying to say is that our two arguments aren't actually contradictory, but apply to different operating temperatures. Ie, if you have very hard pads, then grooves, holes etc make no difference, if you're right on the temparture limit of the pads, then they can have beneficial effects, but if you're right on the temperature (or structural) limit of the rotors then they could be bad news. Am I making any sense at all or should I go and hide my head now? Last edited by dtype38; 20 Aug 2007 at 18:08. |
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20 Aug 2007, 18:13 (Ref:1992962) | #74 | |||
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I once bought a Cortina GT which had been fitted with DS11 pads & had no brake servo......practically needed both feet on the brake pedal to stop it first thing in the morning! |
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20 Aug 2007, 18:41 (Ref:1992987) | #75 | ||
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