How fast can you stop. Have you tried?

[Mikkel]

I am not talking about pulling a stoppie, thats why I said more rubber on the road (terminology for have wthe wheel on the road... it ain gonna stop you if it is in th eair is it???)... and what you are saying is basically what I said...???? having both wheels on the ground... you will stop faster... (more rubber on the road I wasn't actually saying the rubber stops you... though it helps when you are in full lock and you have smoke coming from that rubber on both tyres...)

What you said, in relation to not stopping as efficiently doing a wheelie was this:

mainly because you have less rubber on the ground

I read that as you saying that the amount of rubber on the ground is the main factor in determining how fast you can stop.
But then, I am not certain I am reading what you write in the right way.

The rubber does have some to do with your stopping power as well... ride round on your rims at 100kph do snap brake you will be in a skid before you know it...

...or for that matter trying to brake with a flat tyre.

Yes, the rubber is crucial when talking stopping power. It is however not how much of it that is touching the ground which is important. And that is all I am saying.

[imdying]

Hey that's interesting... but are you sure you don't mean that 'friction coefficient' of 1?

Reason I say that is because any light bike with good tyres can pull 1g of force in cornering on the skid pad test. I suspect braking would yield far more force right?

F1 car can pull just over 5g in braking force. From watchin both motogp and f1 i'm guessing a motogp bike can surely pull about 2.5g on the brakes?

interesting.

I'm sure in a reasonably recent PB mag, they did some bolt on upgrades to a GSXR600, which included using iron discs. Took it from 0.9g deaccelerative force to 1.6g. I'll try to remember to look it up. (So it'd be interesting to see what the max a slicked up MotoGP bike with Rossi on board can pull, plenty I'm guessing...). Having said that, doesn't help skiddy's fairytale fantasy.

[Jantar]

Some very interesting scientific test results at: http://www.msf-usa.org/imsc/proceedi...ngDistance.pdf

Have a look at the table on page 6.

[Badjelly]

Some very interesting scientific test results at: http://www.msf-usa.org/imsc/proceedi...ngDistance.pdf

Have a look at the table on page 6.

Thanks. I was just about to do some sums on them.

Caution: the high-speed results from the BMW F650 look very impressive compared to the others, until you read the fine print: it was tested at 118 km/h, whereas the others were tested at 129 km/h.

[mikeey01]

[QUOTE=trademe900;1904588]

They took a Honda GL1500 and a Honda CBR929 and made hundreds of braking attempts. Basicallly, they did stops from 100kph. Amazingly the CBR929 and the GL were very similar and there was no real difference between the 2 in stopping distance. They got 38m as an average for the good runs.

Hey I'm not a maths guru, nor am I a word smith and I am certainly not a doubter when it comes to the years of knowledge u fellas have collectively got, but bear with me for a shake...
Lets apply some pure logic to the above for a second... something to me doesn't gell!

Honda GL1500 specs..
792 pounds dry (360 kg)
Front brakes Dual full-floating 296mm discs with LBS three-piston calipers

Honda CBR929 specs..
374.8 pounds dry (170 kg)
Front brake Dual 320mm discs with 4-piston calipers

So the GL weights twice as much, has less braking effort and stops in near the same distance as something that's half it's weight and has a greater braking effort? I'm lost!

I'm not as clever as most of guys on here so I'm thinking like a big train takes years to stop and a car takes bugga all, yeah no real comparison but something doesn't gell with what you said...

Don't shoot me down as I'm really finding it hard to come to terms with how this all works.

[Jantar]

[QUOTE=mikeey01;1909086]

...Don't shoot me down as I'm really finding it hard to come to terms with how this all works.

Like most tests, there isn't enough information here to explain exactly why, but the results are not surprising. Just giving the number of pistons per caliper and the disk diameter doesn't give the swept area which does affect braking performance. I also wouldn't be suprised to find that there is more allaoy in a sports bike disk and more iron in a tourer's disk whiach again will affect the braking performace. Also there is the amount of rubber in contact with the road, the air resistance etc. etc.

There are so many factors that affect braking performance that it isn't possible to calculate to the meter what any bike will achieve. It is possible to tell when claims are fancifull, or to tell when improvements could be made, but is about the limit.

[The Stranger:Me]

Some very interesting scientific test results at: http://www.msf-usa.org/imsc/proceedi...ngDistance.pdf

Have a look at the table on page 6.

Interesting why?

[Jantar]

Interesting why?

Mainly because the test results are exactly in line with the theoretical results we have been discussing in this thread. The stopping distances are all in the 0.7 - 0.9 G range.

[Mikkel]

So the GL weights twice as much, has less braking effort and stops in near the same distance as something that's half it's weight and has a greater braking effort? I'm lost!

I'm not as clever as most of guys on here so I'm thinking like a big train takes years to stop and a car takes bugga all, yeah no real comparison but something doesn't gell with what you said...

The over-the-top brakes on sportsbikes are there to allow hard braking into each and every corner at very high speeds for hours on end, while maintaining consistent high performance and feel. For any imaginable single braking action such brakes are overkill and so are the ones on the GL1500 as well. After all, the GL1500 would have to be able to drive down a steep mountain pass without the brakes overheating and fading.
So, for a single braking action you shouldn't see too dramatic a difference.

The reason that trains take a long time to stop has more to do with the fact that it is steel-on-steel not rubber-on-tarmac that provides the friction that facilitate the stopping. Also, for some freight trains only the locomotive is braking, which makes a huge difference. But the steel-on-steel factor is also the reason why you only see fairly mild gradients on railroads - on the other hand, the fact that the steel wheels doesn't really deform during rotation is what makes railways so efficient.

[racefactory]

The over-the-top brakes on sportsbikes are there to allow hard braking into each and every corner at very high speeds for hours on end, while maintaining consistent high performance and feel. For any imaginable single braking action such brakes are overkill and so are the ones on the GL1500 as well. After all, the GL1500 would have to be able to drive down a steep mountain pass without the brakes overheating and fading.
So, for a single braking action you shouldn't see too dramatic a difference.

The reason that trains take a long time to stop has more to do with the fact that it is steel-on-steel not rubber-on-tarmac that provides the friction that facilitate the stopping. Also, for some freight trains only the locomotive is braking, which makes a huge difference. But the steel-on-steel factor is also the reason why you only see fairly mild gradients on railroads - on the other hand, the fact that the steel wheels doesn't really deform during rotation is what makes railways so efficient.

thats exactly how i would want to put it- thanks dude. The reason why those GL1500 and CBR were nearly the same stopping distance was mainly because there is no advantage of having those overkill brakes on the sports bike from those relatively low speeds. The CBR brakes would be designed for caning it around a track lap and lap again with minimal damage and fading.

Like i said before, limiting factor for these single runs is not the brake system but the tyre and in some cases- the CG.
Most brakes in proper spec and looked after will be able to lock the front wheel at these speeds (and that is all that is needed for this) therefore it is nothing to do with the brakes but how much friction the tyre can provide before it skids.

Lastly, on the average sporty bike, when trying to get best stopping distancem, rear contributes absolutely NOTHING except from the very first instant in the braking where the load is still on the rear wheel. If you are using rear brake whilst doing this stuff then you are clearly not applying front brake as hard as it can go.... because if you were, your rear wheel will literally be floating above/skimming the ground- providing no stopping power whatsoever.

Of course this assumes conditions are dry, good surface, reasonably good tyres.

Cool thread!

[racefactory]

Also about the car stuff that was said earlier and rear brakes.

Cars set up for track and very sporty cars will have a brake balance set of around 67% front brake bias. In real life that equates to driving at highspeed and applying brakes to the point where front will lock just before the rear does.

Compare that to a bike... it's 100% front brake bias. except the very first instant where the weight is on the front tyre.

That's how the racers say they do it don't they... rear brake first, load the front, take off the rear and 100% front brake.

Reason is because the car has better CG rear weight bias under brakes so can still put good force on rear tyres.

But then, bikes have a lot more aerodynamic force to help them.

You'd be surprised, a sports road car and bike will brake nearly about the same distances... there is so much aerodynamic help from braking on a bike at high speed. Sitting up on the bike is something like putting a great big bloody airbrake on top of the car. In the end, i think it balances out.... At least from the stats they are near the same. Until you have the super road cars like Ferraris and Lambos..extremely low to the ground with rear mounted engines.....


...but only until you start putting on great big fecking wings/extremely low cgs/mammoth sized tyres would there be any massive difference I reckon.


Bikes:

+tons of aero force

sporty car:

+ cg rear weight bias meaning more weight on the rear
+ slightly bigger contact patch vs weight.



race car:

+ big aero forces

+ massively big tyres

+ perfect CG rear weight bias.

[racefactory]

One more-

Is there formulae that can be used in conjunction to calculate braking distances and forces including the important aerodynamic effects at high speed???

[Mikkel]

sporty car:

+ low and well placed CG meaning more weight on the rear
+ slightly bigger contact patch vs weight.

Bullshit, that has nothing to do with braking. The contact patch depends upon the tyre's pressure and the load it is carrying. You can achieve exactly the same contact patch vs weight on a bike compared to a car. That still won't change a thing. The size of the contact patch has a lot more to do with feel, stability and handling than actual grip (that is while keeping the pressure within the recommended range for the given application of course).

By CG I suppose you're meaning the centre of mass (or gravity as some errant folks like to put it). It is true that the CoM is important in relation to all of this - however, it has nothing to do with how much weight it puts on the rear wheel. It has everything to do with the vertical height of the CoM compared to the centre of your front wheel. If the CoM is higher, or a lot higher, than the centre of the wheel you will be, very, limited in how much braking force you can apply at the front tyre before making a somersault.

Is there formulae that can be used in conjunction to calculate braking distances and forces including the important aerodynamic effects at high speed???

Yes, but it is a 2nd order non-linear differential equation and as such it is very difficult to work with:

m*d^2x/dt^2 - k*(dx/dt)^2 = F - F_fric

m is of course the mass.

d^2x/dt^2 is the acceleration, a.

k is a constant given by the cross-sectional aerodynamical profile of your vehicle times the drag coefficient and the atmospheric density.

dx/dt is the velocity, v - it is squared in this case because for systems on the scale of motorcycles travelling at considerable speeds the Reynolds number indicate that we are in the turbulent flow regime and thus the drag scales with the square of the velocity.

F is the applies force at the rearwheel and F_fric is the constant rolling resistance due to bearings, deformation of tyres, etc.

You asked for it

Note - this is the simple case of a body travelling in a straight line on a completely flat surface in a completely still atmosphere. Fluid mechanics are in the larger field of black magic as far as I am concerned.

[racefactory]

http://www.youtube.com/watch?v=wWCAtS_Mr-g

watch this video of motogp bike stopping from 291kph...

let's see how much force he's pulling and what distance he stops in.

I downloaded the video off youtube and he goes from 291 to 146 in 3 seconds...

He loses 145kmh in 3 seconds...

So.. what are the figures here then?

[Badjelly]

Is there formulae that can be used in conjunction to calculate braking distances and forces including the important aerodynamic effects at high speed???

The aerodynamic drag force can be quantified with a simple equation, see

http://en.wikipedia.org/wiki/Drag_equation

The tricky part is coming up with good numbers for Cd (the drag coefficient) and A (the cross-section area).

Jantar suggested a good rider on a faired bike could get to Cd = 0.4, which seems reasonable. A has to be in the vicinity of 0.5-1 m^2. When the rider sits up, both Cd and A will increase, especially Cd.

I threatened earlier on this thread to do some calculations and report the results, but I haven't got around to it. I thought I'd start to try to get a handle on CdA by relating top speed to power on a few bikes.