robski

Ok, starter for 10

why do people always say, if your going to increase the power, you better do your brakes?

I can see that if you are going to go on track you would want to due to brake fade etc.

But, bearing in mind that your not aiming to increase the top speed, but purely to accelerate faster to 70 then why do you need upgraded brakes?

Im not asking why you would want upgraded brakes as opposed to OE brakes as I know they are better. Just what is the link that causes people to make that statement?

robski

IntegraR

Well, when an object accelerates faster, the amount of effort required to slow it down is greater.
Therefore, we can conclude, that should you upgrade your performance, the little old lady in the Corsa will be approached at a far greater rate of knots, and to avoid your Scooby becoming a Suborsa, you will need to have a more effective method of slowing down.
Besides which, you can never have brakes that are too efficient.

Gary Foster

I think your simplifying that too much Robski.

Take your favourite coutry road, theres lots of corners etc which you probably can't go round at full speed - ie 60 so you spend most of your time acclerating then braking for the next corner.

With more oomph you will be braking from a higher speed each time to go round the corner at the same speed as before.

Ps Of course, if you upgrade your brakes you'll just end up braking later (might as well use them) so they will still overheat, it's a vicious circle

Bajie

I thought it was so the passengers didn't have to try too hard to get out of the seats.

robski

IntegraR,

yes but, on the road with twistyish A roads, surely unless they are mightily twisty, you are going to be holding at 60 or 70

the fact that you accelerated faster to get there doesnt affect the weight of the car that you need to slow down.

What Im trying (poorly) to say is that, I know better brakes slow me faster, so if I want to improve my A to B performance then better brakes will do so. If I am in a situation where I will be constantly accelerating or braking then I may need better brakes as I may exceed what the OE ones can handle.

The link I cant make is that generally I cant think more than a few roads that I know where with the acceleration of the scoob you would be travelling any faster in an F1 car than the scoob (complying with the speed limits) by the time you came to a corner.

So, why do you need better brakes just because you have increased the power a bit!? get my drift?

Like I said, Im interested in why people make the comment about get better brakes as soon as someone says they have increased the power a bit. Massive power increases obviously can totally change the dynamics of the car, but a MY99-00 has got better brakes than the early hot versions anyway. No one says oh the My99 has better brakes than your MY95 WRX, so you must improve your brakes do they!

robski

fast bloke

Robski,
As a general rule manufacturers will fit brakes that provide a sufficient amount of stopping power to handle the go power under the bonnet (Thus no 6 pot brembo's on a reliant robin.) I would be generally advisable to keep the brakes at a standard that will be able to cope with the performance available. If you are driving on roads where you are travelling at a constant 70 MPH, you won't need the increased power, so you are totally correct in that you wont need uprated brakes. People make the statement based on the assumption that you will be using the extra power you have paid for.

IntegraR

Okay, I understand more what you meant, but you mentioned that you would like to improve say a 0-70MPh time, and by doing that you would have to have better brakes to better slow down the improved acceleration.
I am inclined to believe it is closely related to Einsteins Theory of Relativety, but I keep coming back to Newtons Concept of Motion: Acceleration = time rate of change of velocity in a particular direction,
and veiwing this theory, we can only conclude that the better brakes would be a neccesity to slow down the time rate of change, as the forward force would become very much stronger.
PS: in case you mistook the dumb comments, Im full of **** with the theories, just having a laf, and not a Science type at all, but the jist of it is true, the faster you accelerate, the more force it takes to slow you, which means if you accelerate at a tremendous speed towards a corner, but still only reach 70MPh, as opposed to reaching 70MPh at a slow rate, there is a difference in the amount of force requiered to slow you to a safe speed to take the corner.
Damn, this really IS rocket science

"PS: in case you mistook the dumb comments, Im full of **** with the theories, just having a laf, and not a Science type at all" no, really? wow IntegraR, you really had us fooled for a sec, thought you were some kind of a Science genius!! I know, Id have to try a lot harder than that you convince anyone, ho well

[This message has been edited by IntegraR (edited 26 January 2001).]

robski

fastbloke,

I guess thats it, they are assuming that you are going to totally change your driving style, and hence need the extra braking

and what do assumptions make? thats right, an *** out of u and me

IntegraR,

now this may make some sense! does an accelerating object take more to stop than a constant velocity one? I would say no, the force is speed x weight. So it shouldnt matter if at the point you hit the brakes you were accelearting or at a constant velocity, but maybe Im wrong? Would an accelerating vehicle have some extra force hidden away?

I still suspect its a thing that people have had ingrained into them through years of people saying it, I was trying to get to was there any real reason back in day 1 when the first bloke/bird said those immortal words


robski

IntegraR

No, it does!! think about it like this. The greater the change in speed over a distance, the greater the amount of force needed to slow that object.
Like, when you accelerate fast to 60, in say a Lamborghini, you get pushed back into your seat, implying there are greater forces acting on your body. Now in the Metro, you still get to 60 (just ), but the forces placed on your body are less, its the whole equal and opposite idea, so there are entirely different amount of forces in motion, even though the end result is the same.
The weight of the car is being flung towards a spot at a greater increasing velocity, and because of this , the thing requiered to stop it has to be more.
Well, thats how I view it in my head, but I may be wrong. Greater acceleration must mean a greater stopping force required.

chiark

Doesn't inertia and momentum come into this equation too?

Well out of my depth here, but a damn good question now I think about it...

IntegraR

Chiark, you think you're out of your depth, I was just lucky to have a snorkel, goggles and water wings at the ready, cause Ive been treading water since this thread began

fast bloke

I guess the statement was initially made by someone who putting a Cooper engine in a standard 1979 Mini... Loads of power, excellent handling, press middle pedal 10 times to start slowing down. You really did need to upgrade the two front drum brakes

Robertio

iTrader: (1)

Right, I may be wrong, but

force = mass * acceleration OK?

So the force for a mass of 100 at a constant velocity = 0

Where as the force for an acceleration of 1= 100.

In order for the car to slow down a force must be applied to counter the above force, in this case friction from the brakes. But for simplistic terms we will say just a force in the opposite direction.

So to reduce the velocity of the first object we apply acceleration of say 2, this will require a force of 200. (force = 100 *2)

With the second object we have to cancel out the force of the acceleration as well as the force to stop the object. This means the force will have to equal the 200 + the 100 in order to stop in the same distance.

I think this is right , will check with a physicist tonight and correct if required on Sunday.

DavidRB

Ummmm, the only way you can decelerate an "accelerating body" is by pressing both pedals at once!

At the point you lift off the loud pedal, the car very quickly stops accelerating. It isn't instant because the accelerator doesn't snap shut, but it's probably no more than a quarter of a second.

If you've just accelerated to 60, or whether you've been travelling at 60 for some time, then the same amount of braking is still required.

Brakes simply turn the Kinetic energy of the vehicle's motion (1/2 m v^2) into heat. At <I>exactly</I> 60mph, the kinetic energy of the two cars is identical, ignoring the small effects of the fuel pump & the turbo vanes.

The reason you need bigger brakes on a more powerful car is that people typically accelerate <I>for a given distance</I> and <I>not</I> up to the same speed.

For example, a slow and a fast car come out of one corner and accelerate towards the next corner that is 200 yards away. By the time the two cars cross the 100 yards marker, the more powerful one is travelling faster.

Cornering speed is still the same for both cars, so by the time they reach the braking zone, the faster car has to lose more speed than the slower one.

Better brakes allow you to get rid of the extra speed more efficiently and more repeatably.


Robski put us onto a red herring at the start by saying "But, bearing in mind that your not aiming to increase the top speed, but purely to accelerate faster to 70".

A more powerful car can reach 70 where a less powerful car might only reach 50, so the more powerful car has to get rid of more kinetic energy - and a car at 70 has almost <I>twice</I> as much kinetic energy than one at 50.

And that's why you need better brakes - to spend longer on the accelerator. QED.

[This message has been edited by DavidRB (edited 26 January 2001).]

Jay m A

iTrader: (2)

I agree with David. Momentum is the thing that the brakes are reducing, which is based purely on the cars mass and velocity. So if you had been travelling at 60 for some time, or just finished accelerating to 60, the braking force required is the same. I think...

Justin

GEJL

DavidRB - spot on with the maths, I reckon.

James Douglas

Just to throw something into the debate. The people who say you need to uprate your brakes are the same people who are modifying your car. I had an old car that i got tuned, and one tuner said i must upgrade for the reasons you say, whilst another said unless i was going to the track the ones i had were fine.

Could there be a conspiracy to get you to part with more cash.

I agree with the comments that the quicker you are going, the more stress the brakes take so they (may) need more bite to get the speed down.

Aparently your tyres have a lot to do with the amount of braking force you can apply effectively. If theyre ****e they wont stop whatever youve got!

James

Rikki23

Superb explanation David.

Integra to explain more simply ( I hope that doesn't sound patronising - it wasn't supposed to - I spent lots of time in A-Level physics trying to get my head round stuff and simple explanations are often the best), the car braking straight from accelerating to 60 and the one travelling at a steady 60 would both require the same braking force. This is because as the driver who is accelerating to 60 removes his foot from the gas, there is a brief moment when the car is neither accelerating or decelerating before the brake is applied. At that point the car is travelling at a constant velocity, quite similar to the car travelling at the steady 60.

Rikki


Edited for crap spelling

[This message has been edited by Rikki23 (edited 26 January 2001).]

106rallye

If you can lock your wheels and/or cause the ABS to function are your brakes not already good enough? I know there must be a good reason for this but apart from fade what else is there? just a thought no doubt many people will be able to tell me!

boomer

Robski,

in a road car your can almost always increase your safety by improving your brakes, but there are very few circumstances when increasing power (in a Scooby, not a Fiat 126) will make things safer.

Sure you can initially _appreciate_ an increase in power a lot more, but it is the *few* times that you need more braking that makes it all worth it!

mb

scoobysnacks

Ok, but if the car is leaning back on it's suspension, which it will be while accelerating, the sudden braking will throw the car foward even more onto it's front suspension than it would go if it had been at a constant speed, just like a pendulum hanging inside the car would swing further forward when going from acceleration to deceleration. But does this actually happen and does this mean more forward force for the brakes to act against because of the extra momentum of the car shifting on it's suspension or am I talking rhubarb? Quite probably. There is also the lighter front end to consider at the moment the brakes are applied due to the acceleration, maybe reducing the braking efficiency for the first few moments. That may mean better brakes will be required to 'catch up' on the deceleration.

[This message has been edited by scoobysnacks (edited 26 January 2001).]

hooksin

hey guys - I'm sat here @ 8am just getting ready to go view some houses(moving to Leeds/Wakefield soon), & thought I'll check the scoobynet for a bit of light read through the threads.....

JESUS....I am now ready to sit my pure Maths/Physics A level @ 9am after reading these posts
just kidding btw.......

TonyBurns

iTrader: (3)

Look, we all know the reason people get bigger brakes, its because they look better under 17's than the standard 4 pots

Tony

Rikki23

Hmmmm, interesting point scooby

I would guess that the lighter front end issue at first would not be too significant because the deceleration as you lift off the gas and switch to the brake, although brief, would be very rapid and by the time braking pressure is applied the weight will have already shifted too the front.

As for the momentum shifting forward I am not too sure, that could possibly play a significant part but my limited knowledge of kinetic physics forbids me to estimate without a large amount of speculation involved

Taxi for Lovelock....

scoobysnacks

I agree. I was just trying to think of some other factors that might come into play. Just speculation though. I'm away for a week now, so I'll be interested to check out any other theories when I return. See ya.

BOB.T

70 mph is the same whether you got there in a matter of seconds or if you had been there for a week therefore the braking effort required is the same. I can also eliminate the weight transfer malarkey aswell. Any tuner worth his salt won't let you leave his workshop until you've bought X amount extra horsepower, bigger brakes and uprated dampers and springs, therefore eliminating weight transfer! Obviously! Bob

lokokkee

Momentum that needs to be retarded by the brakes = 1/2mv^2, so if max speed is restricted to say x mph all the time before and after modification, then braking force required remains the same. But if car is accelerated harder than before, then the AVERAGE speed has been increased and bigger braking force will be required to bring down braking distance on AVERAGE. If old brakes can generate greater braking force than the coefficient of friction between tires and roads, i.e. lock up the wheels, then limiting factor would be the tyres, not the brakes. (just trying to apply A level physics AFAICR)

[This message has been edited by lokokkee (edited 28 January 2001).]

Mike@PD

Just to be a tw*t - when you brake, you are actually converting kinetic energy into heat energy and its your ability to get rid of the heat that dictates how efficient your brakes are! Bigger brakes with efficient cooling really are better. Thats why we liquid cool rally car brakes. Have a look at a Subaru WRC in tarmac trim and the words, BIG, SERIOUS, WATER COOLED, EXPENSIVE will all come to mind and they weigh less than your road car. Need I say more?

DavidRB

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:<HR>Originally posted by lokokkee:
<B>But if car is accelerated harder than before, then the AVERAGE speed has been increased and bigger braking force will be required to bring down braking distance on AVERAGE.[/quote]

Are you saying that a car that accelerates to 60mph in 6 seconds will be harder to stop from 60mph than one that took 12 seconds? If so, then I'm afraid that's not correct.

Kinetic energy (momentum) is not related to time, unlike power. The average speed of a car along a straight has nothing to do with the braking effort required to slow it down. Try it yourself on the open road. Accelerate quickly to 30mph and brake, then accelerate slowly to 30mph and brake. You'll find that the car stops in roughly the same distance given the same pressure on the pedal.

I had forgotten about the compression of rear springs and the extension of the front springs on an accelerating vehicle, but these are orders of magnitude less than the vehicle's kinetic energy.

An accelerating car will be carrying extra energy in its springs, which will be transferred through the spring/damper units to the wheels when you brake. Stiffer springs store less of the energy and transmit more of it directly to the tyres.

Understand the significance of this and you'll be well on the way to setting up race-car suspensions!

Dippy

You've all missed the key point.

Cars are not driven by computers they are driven by people, and people have reaction times which are crap enough to make your theories irrelevant.

If you make your car better at accelerating (and then use that acceleration) then you are increasing the RISK of placing yourself in a position where you need to emergency brake at a higher speed than if the car hadn't been modified.

Unless your reaction times are reduced along with the modification, you'll need to be able to brake more quickly in order have the same chance of avoiding an accident as before.