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Braking power, CBR250R and RR?

Discussion in 'Bike Reviews, Questions and Suggestions' started by Sweeris, May 16, 2007.

  1. I am just wondering the stoping distance of the cbr250r and the rr. The RR has 2 discs, so would this make any difference in stopping distance?

    The reason being is I kinda got into a situation where I had to use full brakes and got away with about 1-1.5 feet to spare from about 50kph. From that im wondering if the situation would have turned out dirfferent if I was on a single R?

    Needlessly to say the situation scared the crap out of me as I thought I might not stop in time....

  2. With 1 foot left, assuming bikes had same brake lines and pad quality, etc. then you probably would have had a dent or two.
  3. The difference between 1 and 2 rotors is really only going to play a part in the initial stages of braking - after then the limiting factor is the tyre (after all even a drum brake can lock a wheel). The real question is whether you were using the brakes to their full potential - I've seen experienced riders pull a Goldwing up to a stop in a fraction of the distance some learners need to stop a sports 250. I wouldn't worry too much about number of discs - just keep practicing your emergency stops and make sure your tyres and shocks are up to standard.
  4. According to physics 2 discs increase the surface area between pads and discs. More the better power. U should stop faster.
    Also the skill of breaking is very important, that would make a huge difference.
    From what i have seen with some brakes, that a good clean out of calipers, some good quality pads, and well bleed make atleast 50% difference in initial bite and i would say 25% in total breaking performance.
  5. Add stainless line/s and stopping power will also be improved.
  6. I'd expect that dual rotors would help prevent brake fade during repeated hard braking, or perhaps braking from very high speeds, more than it would in a single stop from 50kph.

    As above, stainless lines will help because of increased brake 'feel' - The brake lines distort less under the pressure, giving a more direct relationship between what you're feeling with your fingers/foot and the pressure actually applied to the pads. You'll be able to more accurately threshold brake than with rubber lines, and reach that threshold faster.
  7. I could almost guarantee that you would have had a bit more braking potential left, and could have pulled up shorter. If you had the sub/concious knowledge of hitting the vehicle ahead of you, you would have tried for a bit more brake. I doubt it would have made much of a difference in this particular example. Tire for Tire, at limits, the RR might stop 1' shorter than the R.
  8. why? the tyre is always the limit in an emergency stop. Tyre for tyre, both bikes are equivalent. You will see far more influence from the suspension and training of the rider than anything else.

    Larger pad and rotor areas are only useful for fade resistance (heat dissipation). In a single emergency stop, particularly from low speed like 50kmk/h, pad area will not make a bit of difference. This is true even for cars and the heaviest trucks, trains and aircraft in the world!
  9. Ahh but you're forgetting the time it takes for the brakes to bring the tyre to that limit. Larger rotors and/or pads will make some difference to stopping distance - but I agree with you that rider skill, suspension etc. is far more important.
  10. This was implied by at limits. If it is braking at its possible best (ie limits, which means that suspension and rider skill matters no more), i would say the RR would pull up a foot shorter, because it would take half the time to reach the braking limit.

    I agree with you completely that in reality the difference is meager, as rider training or experience will come far more into play.
  11. how the hell do you expect more pad area to "come up to a braking limit faster" ?

    any idea how brakes work fellas? pressure acting on an area gives a clamping force. That force, coupled with the radius of the disk gives a torque that slows the wheel. Nothing more, nothing less. At a minimum you are fogetting that most of the first instant of braking goes into comopressing the suspension and flattening the tyre - you can't generate a peak load at the first instant that is even close to a setup load, and you can see this in the videos you saw at the L's course. It is immaterial to the basic idea anyway.

    You think you can override basic physics? Granted you'll need more pressure for the single disc all else being equal, but the singles have a bigger radius do they not? Given that you can stoppie a single disk model all day, your fantasy idea of being able to reach peak faster is just that. You really think, tyre for tyre, a buell will pull up in a longer distance to a ds ducati? humbug!
  12. Sigh I can see this argument dragging out. Let me just say that my brother used to work for a company that manufactured and TESTED brakes - larger calipers/pads, larger rotors even different pad compound WILL reduce the stopping distance of a vehicle. Because, as I mentioned earlier, it reduces the time taken for the brakes to reach maximum efficiency. And given you can be travelling at several metres per second during this time it obviously equates to a noticeable difference in stopping distance. Whether a single disc R has a greater brake area than a twin-disc RR however is something I'm not sure on - someone should know the answer to that though.
  13. apples and oranges

    Well the second brake pad is only the tip of the iceberg!

    The R and RR have completely different swing arms and suspension components.

    Dealers will tell newbee buyers that it's only the fairings that are different! Beware.

  14. jd - brake dynos are nothing like real vehicles - the dynos tend to start with a minimum angular momentum (or constant torque if powered by electric motors) and the brake has to bring that to a stop - there is no provision for a small energy input to lock the wheel. A brake dyno has angular momentum only - the majority of a vehicle momentum is translation and not angular.

    Efficiency of a brake system comes in two parts - energy dissipation (joules/second) and heat absorbtion - (degrees/second). Sure, you can take time to reach a maximum efficiency - and you must, because efficiency increases with temperature differential - but it has no effect on a real vehicle that is tyre limited.

    Efficiency of brake systems is only important for repeated applications over an interval short enough that the system does not return to a normal (non-operating) state. In the case of brake testing in laboratory conditions, ANYTHING that increases torque will have a positive effect. This includes the things you mentioned including

    "larger calipers/pads, larger rotors even different pad compound WILL reduce the stopping distance of a vehicle"

    however NONE of these in a test environment are limited by the tyre
  15. simple and extreme example:

    large aircraft brake systems have pads all the way around the disk, AND they can have 6 or 7 rotors, with a full circle of pads between every single one.

    They have stupendous power and acutation speed (200psi hydarulics will do that) and antilock systems, but their efficiency is extremely poor - often requiring 3-4 hours to cool down before you can do another full bore stop again.
  16. Yep, that's why they tested them in real vehicles using a very sensitive (and expensive) accelerometer. Didn't want to have to get into a lengthy discusion on why so basically consider that to lock a wheel is going to require a certain amount of braking pressure. And when you apply a brake there's a time element between 0 amount of pressure - and the pressure required to lock the wheel. Now there are a number of factors that influence this time - one of which is what's referred to as the "swept brake area". More area=less time to reach maximum braking.
    Your point about the plane actually works against you :? . If increasing swept brake area had no effect on efficiency then surely they could make do with just a single pad and caliper - after all this would actually reduce the amount of heat retained. They could also reduce the number of rotors and significantly reduce the weight of the plane. Overheating isn't a huge issue given that aircraft brakes only get used during landing/takeoff - and there's usually plenty of time between those events. But they do have to do a lot of work in a short time/distance to bring a Jumbo to a halt which is why they're using the maximum swept braking area possible.
    Incidentally heat dissipation isn't an issue with many modern brake materials - there's OEM and aftermarket pads on the market that will still give 100% braking efficiency even whilst on fire (in fact if I remember right VIC and NSW Police actually stipulate this as a requirement on pursuit vehicles).
  17. swept brake area goes up much faster with an increase in rotor size than with pad area. You are also just talking about generating torque, and there are many ways of doing this and achieving the same result.

    And the aircraft example is realistic - it is a massive increase in swept area, and yet the efficiency is abysmal. You can't reduce it to a single pad and rotor because they would both disintegrate from the total energy (heat) generated. This is a condition that doesn't happen on motorcycles. Many aircraft are also limited in operation by the brake temperature they record on takeoff If it doesn't cool down to a certain point, you can't take off again.

    I agree with you that it takes time to build maximum pressure - this is NOT the same as the efficiency of a brake system. You can also instantly lock the front wheel on any single or double rotor bike on the planet with components that are not a decade old and perished. -> Acceleration testing; you need to provide proof of consistent brake application and reference controls to have that make any real sense. Having a high peak initial acceleration doesn't mean squat if the tyre breaks grip early because you didn't set up and get an impulse bounce from the carcass. ABS will also effect this behaviour by timing of pressure fluctuation of the valving and I doubt you have any sample data for equivalent systems as the bike - you are still talking single rotor with a small change in radius (packaging requirements for car wheel sizes) and a single set of pads are you not? Accelerometers also require that EVERYTHING be the same in the test - more rubber on the test track from the last pass, a bit of dust, the driver getting better with practice or change in fuel weight and brake system behaviour (pedal travel for example) means that small changes are not statistically relevant. Do you have any numbers to share? I'd love to see them.

    There is still no practical reason why a single rotor will not stop a bike as fast as a double rotor, period. If the single rotor is incapable of locking the wheel in any circumstance (and therefore achieving maximum 'power'), I would agree with you. This does not happen in reality however.
  18. ps, WTF do you mean by braking efficiency? An automotive brake has ONE purpose in life - to convert energy - ignoring tiny portions of noise and vibration, ALL energy in a brake system comes out as heat. All of it - one hundred percent. ALL friction brake systems are 100% efficient as a simple matter of physics.

    Whether the pad is on fire or not doesn't matter. Often the fire is a function of the shedding particles and not the bulk of the pad (or rotor) itself. You can still shed energy through a fire, no doubt about it - but if the system can no longer shed energy, only then does it have zero efficiency. An infinitely efficient system is one which will do all the work you need, without changing temperature from ambient. Real brake system efficiency is a measure of how hot they CAN get before their friction properties fall dramatically. A brake system does NOT have to be operating anywhere near its capacity however in order to stop a vehicle. This is typically why high capacity systems (sports bikes) can jump on the race track bog standard. Sedans cannot because they can't reject enough energy fast enough (just like the aircraft) to keep the pad temps stable.

    Adding pad area and rotor radius are simple ways to raise this limit, (repetitive application) however you are still limited by the tyre. The time required to build peak pressure will admitedly have an effect on stopping distance, but there can be no discernable difference between a single and multi rotor system if you can lock the wheel straight on application, as you can with both of the test bikes in question. For cars with lots of brake piping, lots of elastic rubber compounds and relatively flexible calipers to "eat pressure" you will see better initial performance from a large swept area as it will generate more torque from the same pressure. Bikes do not have this limitation.
  19. Case study:

    125GP bikes only use a single rotor, as the increase in unsprung weight and rolling inertia outweighs the benefits of a twin disc system in their application.

    If these bikes could stop quicker with twin discs they would have them, but grip is the limiting factor here.

    Regardless of 'efficiency' I much prefer the feel of a twin disc 4 piston setup, even over 6 piston calipers. Any front brakes that will get the back in the air at 100km/h in a controlled manner are good enough for me.
  20. :LOL: As a simple matter of physics they'll never be 100% efficient - there's always going to be some sound generated or some physical distortion (ie compression) of the pad/rotor surface ;). Incidentally it is physically impossibly for a brake pad to go from zero applied pressure to maximum pressure instantly ie you simply cannot lock a wheel instantly there will always be some time component. What I know is a larger swept area does make a measurable difference to braking when it comes to cars. Though I do agree that with a bike that already has a braking system far in excess of what most riders will ever need any difference is going to be pretty minimal. But since the brakes still operate the same a difference will exist, so it's really just a question of significance.