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09-19-2014, 03:32 AM | #23 |
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Yeah I know it seems counter intuitive.
On dry tarmac during braking the maximum friction between the tyre and road surface develops when the tyre is rotating at around 80% of road speed - ie slightly skidding. If you hit the M3s brakes at 62mph and pressed as hard as you could, the ABS system would activate and hold the tyres at the preset slip rate (~80%) for maximum deceleration and you stop in 34.1m. Repeat the test with a passenger (adds 200lbs), hit the brakes, the ABS activates , the tyres are held at 80% slip and your stopping distance will be the same at 34.1M. The only way that adding weight to the M3 would make the braking distance longer would be when the ABS could no longer make the tyres brake at the optimum slip rate (all else being equal). Note that a little VW Polo brakes from 62mph in the same distance as the M3 and the Porsche GT3 at 34.1m...braking is not really about weight and brake disc size (within reason) - but mostly about the tyre compound and the force pushing it into the tarmac. You can strip out weight, throw whatever brakes you like at a car but if its on road tyres the best you can hope for is a little over 1g in deceleration. The other part of braking is the disc hardware. Braking a car is the conversion of kinetic energy into heat. A moving vehicle has energy calculated as KE=1/2mv^2 (a half of the mass times its velocity squared). When you brake, the pads press against the disc and the friction produces heat which is removed by the air flowing past the disc. The discs themselves have a "thermal capacity" which basically means the amount of heat that it can hold. Imagine them as being a bucket with holes in the bottom - as you brake the discs fill with heat which drains out of the bottom, if however you keep adding more and more heat (repeated hard braking at a track, driving down a mountain on the brakes) the bucket fills up with heat faster than it can drain out of the bottom and the whole system starts to overheat and you get brake fade. Which is why bigger brake kits are fitted by people who track their cars - its to handle the extra heat, by having brakes with a bigger thermal capacity and better cooling (bigger bucket with bigger holes). The alternative would be to improve the cooling to the standard brakes - the effect would be the same. Last edited by SenorFunkyPants; 09-19-2014 at 07:42 AM.. |
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09-19-2014, 01:46 PM | #24 | |
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Do you have a reference for your information, I would be interested to read it? I looked for the above article but can't find it. |
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09-19-2014, 01:57 PM | #25 | |
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If weight were not relevant to braking distance, then all cars would have roughly the same size brakes except for applications where heat build up is an issue. And yet, SUVs and heavy vehicles tend to be fitted with very large brakes.
Brakes have to overcome inertia. Inertia is a function of speed and mass. The coefficient of friction of the two surfaces (tire and road) is a factor, but so, too, is weight. This is basic physics: http://www.physicsclassroom.com/clas...ertia-and-Mass Quote:
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09-19-2014, 04:18 PM | #26 | |
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It's true if the car has zero toe or toe out, it could worsen braking performance due to suspension geometry and added deflection and in turn reduce the tire contact patch while going straight, but both the regular M3 and GTS are toe-in at the front suspension.
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09-19-2014, 04:35 PM | #27 |
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Yes a heavier car will have more Kinetic energy (or momentum if you prefer) so the brakes will have to do more work to stop it at the same deceleration rate as a lighter car but as long as its within the ability of the ABS to work as designed then the stopping distance will be the same...otherwise little lightweight cars would stop in a much shorter distance than heavy ones yet the stopping distance from 62mph for a typical 850kg small car is little different to that of a 1700kg M3.
Look at the braking distance for any modern car (with decent brakes) and the very first thing you will see is that the typical 60-0mph braking distance is consistently the same (within a small margin) regardless of vehicle weight. Last edited by SenorFunkyPants; 09-19-2014 at 04:51 PM.. |
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09-19-2014, 10:20 PM | #28 |
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Senor,
I think you are focusing on the ability of the brakes where I am talking the ability of the tires to bring the car to a stop. I am assuming most cars have the ability to lock the tires or activate the ABS for at least one "panic" stop. Lets forget about various ABS setups and different cars for a minute just to make a apples to apples comparison. If my M3 did one stop, full ABS, at minimum weight then with the same conditions tried it again with max weight shouldn't it take a longer distance to come to a stop? I hope I am not coming off a antagonistic. One of the reasons I am interested in the topic is I think you have Ross Bentley (Speed Secrets) on your side and I could not figure his point either. At any rate for me it is a fun discussion. |
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09-20-2014, 07:12 AM | #29 | |
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Absolutely...I think I am right but I if I'm wrong I would rather find out so I'll know better for next time. Discussing and learning stuff is the best part of M3post. Last edited by SenorFunkyPants; 09-20-2014 at 07:34 AM.. |
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09-20-2014, 05:59 PM | #30 |
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I think it is waste of money putting a BBK if you do just one club track event a year. Visually it would be great and will give you bragging rights but much lighter in your wallet. I think for street driving the standard breaks are just perfect. Keep the money because you will need to spend if you happen to keep this car for a long time.
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09-21-2014, 10:03 AM | #31 | |
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09-21-2014, 10:30 PM | #32 | |
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Thanks |
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09-22-2014, 04:33 AM | #33 | |
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It is true that lighter cars are *easier* to stop, you can use smaller brakes and tyres, the brakes need to do less work, they produce less heat and the tyre contact patch will tend to deform less under hard braking than a heavy car. Yes top of the braking list will always be cars like the 500kg Ariel Atom track car on road legal track tyres - but only a few meters behind will be near 2 ton performance Mercedes and even a 2.3 ton Bentley Continental can stop in a staggering 33 metres from 60mph. The principles always remains the same, its the friction produced between the tyre and the road surface that determines the stopping distance all else being equal. If the brakes can hold the tyre at the optimum slip rate and the tyre friction coefficient is unchanged then adding a few hundred lbs of weight will not make a difference to stopping distance. Take an otherwise standard M3 and put on a set of carbon ceramic brakes from a Mclaren P1. Make a stop from 60mph. Add 300lbs and make another stop from 60mph. The brakes are surely easily up to the job of stopping the car, the weight difference is not sufficient to put the tyres outside of their operating range so what mechanism could be occurring that would cause the stopping distance to change? |
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09-22-2014, 11:31 AM | #34 | |
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Let's try it this way: Assume an infinite length straightaway. Two identical trucks take off at the same time, reach 80 mph in the same distance, shift into neutral, and then coast to a stop without hitting the brakes. The only difference is that one of the trucks is loaded with four tons of cargo. Which one will come to rest sooner than the other? |
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09-22-2014, 03:19 PM | #35 |
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See http://hyperphysics.phy-astr.gsu.edu/hbase/crstp.html Its not that heavier cars stop in the same distance just that the brakes need to do more work to make them stop in the same distance. Also http://en.wikipedia.org/wiki/Braking_distance Last edited by SenorFunkyPants; 09-22-2014 at 03:33 PM.. |
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09-22-2014, 10:49 PM | #36 | ||
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I sense that you and I actually agree: For the most part, the chief limiting factor on stopping distance is friction, as a factor of the tire (or "tyre") grip and the road surface. Most cars are built with brakes that have ample extra braking force, so that if you load the car with passengers or cargo, the braking distance will not suffer much if at all (assuming similar surfaces and tire conditions). But as you pointed out, the brakes have to do more work to stop a car that weighs more. This work will translate to additional heat, which will cause the brakes to heat up faster than those same brakes on the same car but with less weight in that car. (The same would be true of the cars, tires, and weights were the same, but one car was traveling 80 miles an hour faster than the other car when the brakes were applied.) The extra heat starts to build up on the track and braking distances will start to grow. It's probably true that additional weight will not increase stopping distance significantly if at all in a single-stop scenario, assuming dry tarmac and good tires. But this is because cars are built with ample braking capacity, not because weight has no impact on braking distance. As the Wikipedia page you linked to said, the vehicle's mass is a critical component of the stopping-distance formula: Quote:
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09-23-2014, 02:39 AM | #37 |
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Ah OK so we are not that far apart at all.
Interestingly an M3 can stop from 100mph in a little over 4 secs. Turn that round and consider how many BHP you would need to accelerate a 1700kg M3 from 0-100mph in 4 seconds...I'm guessing easily over 1000bhp. That indicates how just much work the brake system has the potential to perform. Braking from 60mph is hardly stretching them at all...the brakes are only handling a quarter of the kinetic energy a car has when at 120mph. Last edited by SenorFunkyPants; 09-23-2014 at 07:41 AM.. |
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09-23-2014, 08:15 AM | #38 |
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The answer to this question is excellent ABS calibration and Michelin PS2's, which when the M3 came out were the best all-around street tire available, and still were right up until the PSS came out. Top performers as street brakes yes absolutely. Most people here want more.
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