Braking distance ramble

[susser]

Here's a ramble I had this morning

here on the VOC site.(bear in mind Volvo's are more about stopping quickly)

(Something that hasn't been pointed out so far, and answers 5lab's question;

If you double your speed then you will travel twice as far in a given time, ie 0.7 seconds reaction tine.

If you double your speed then you will quadruple the kinetic energy your car has and therefore the amount of energy thet the brakes have to convert to heat.

1500kg of 760 at 30mph has 114.7 kJ of energy. Brakes which can dissipate 60kW of heat can stop the car in 2.44 seconds.

1500kg of 760 at 60 mph has 578.8 kJ of energy. The same brakes can still dissipate 60kW of energy and can stop the car in 9.6 seconds.

This assumes no brake fade and no loss of efficiency due to cold brakes of course. It just explains why it looks like it does in the Highway Code.)

On the side of the fence that on sit on here, it also explains why it is so hard to go fast. ie 0-60 in 5 secs needs 4 times the power that 0-30 in 5 secs needs.

Therefore to back up what Davey Bee was on about here

Don't I run on 🤔

 

Edited by - susser on 29 Jun 2005 08:14:14

[Myles]

Don't I run on

 

You'll find that if you turn your fan off first, you'll avoid the back-EMF that causes that situation...

 

Project Scope-Creep is live...

 

Alcester Racing 7's Equipe - 🙆🏻™

 

Alcester-Racing-Sevens.com

---

[Colin Mill]

The Highway code stopping distances are based on a thinking time of 0.67 seconds followed by 2/3 g deceleration and have not changed since the days when the highway code also had a section covering the signals given by horse-drawn vehicles with the whip!

 

Actually, around here these signals are still in use - one that is well worth knowing is " I do 'specially dislike that little g!t in the noisy sporscar and if 'e do get close enough I will 'av im wi me wip"

 

Edited by - Colin Mill on 29 Jun 2005 09:04:52

[MartinWoodham]

I think anyone who frequents the volvo site should have their blatchat ID terminated immediately....

 

whatever next

 

caravans?

 

😬

 

Martin

Roadsports B

If everything's under control you're going too slow (Emmerson Fittipaldi)

[susser]

Martin

Yes I got a caravan. Pick the bones out of that. Anymore of that and I've got a rather good picture of you exiting Russel backwards.

Myles. I've often wondered about my EMF.

Colin. 2/3 g ??

If your energy is quadrupled by doubling your speed, then clearly to halve it you gotta quarter it. If your brakes work at 2/3 g then they would produce a speed/time graph of Y=mX+C. But your energy/time graph would be Y=X Squared. (Can't do superscript). On the other side of this argument. Acceleration due to gravity is 8.8m/s/s. but kinetic energy aquired is 1/2 mv squared (still can't)

8ugger, I'm running on again.

If I'm not careful I be demoted to 1/8 🙆🏻 physicist.

susser (1/2 🙆🏻 physicist)

[Colin Mill]

Hi Susser

Close - Acceleration due to gravity (g) is 9.81m/s/s or 21.94 mph/s (interesting but useful mix of units there I think)

 

I think the way to look at this one is to consider the work done by a constant force acting on the car. Ignoring air friction etc the force will equal the rate of change of momentum of the car or F= d/dt (m.v) so the momentum changes linearly with time for a constant force. However the work done by a force is the force times the distance moved. The rate of doing work is the power which is the force times the speed or P = F.v

 

So the higher the speed the greater the work done by the constant force. So while the momentum increases linearly with speed the kinetic energy increases as the square of the speed.

 

I guess this means that the standard stopping distances assume that the brakes have the ability to dissipate the heat fast enough that they don't become the limiting factor and that it is the grip of the tyres that is the limiting factor. If the ability of the brakes to dissipate the heat was to become the limiting factor you would see a reduced deceleration at high speeds in line with a constant rate of loss of kinetic energy.

 

 

[susser]

Colin

I concur with that. The reason I quoted 8.8 m/s/s/ is because my brain was being outrun by my fingers. It's usually my mouth that does that, so that's a change. I realise that it is of course 9.81 and I usually shove the additional 27 on the end for the hell of it.

It's this constant dissipation of kinetic energy thing which should not produce a straight line speed/time graph which is doing my head in, (to use the Parlance.)

As you imply, at high speed we would see a reduced deceleration so that would satisfy my theory and leave my head intact.

I'll quit now before I blow any semblance of cred that I still have.

susser (still 1/2 🙆🏻 physicist)

[neil.cavanagh]

I normally shove a .19 on the end of the G figure, but hey each to their own! 😬

 

As for the kinetic energy vs linear graph issue... I don't think it is linear.

 

The force applied by the brake caliper to the disc is a constant. This force will be as high as you can go without locking the wheels. NOw, at 90mph the ENGERY being produced through friction by this force will be far higher, then at 30mph. So the brake system has two things to do:

 

1) apply a force to the disc to slow it down

2) disapate the heat produced by 1.

 

Task 1 is a constant no matter what the speed as (I believe) it is directly related to the grip the tyre can produce.

Task 2 will vary with speed. At high speed the brake system, for a given force applied for a given time will need to disapate far more engery than at a low speed. (square rule).

 

So whilst a distance/speed graph can be linear, the distance/energy graph for the same braking event would be a curve (1/x2 I think).

 

so there.

[stands back and waits to be proved tottally wrong! 😬]

[susser]

Oright, maybe it's 0.001927 but consider this;

At standstill or low speed rolling contact, the force required to break static friction of tyre/road is higher than that to cause tyre slip at 100 mph. Therefore the brakes can do less work ?

The greatest grip a tyre can give was 15% tyre slip (once)

At high speed, the amount of force it takes to exceed this is less than at low speed due to the static/dynamic friction thing. (bit like the principle which causes brakes to squeal and chalk to squeak on the board.)

(This last was a series of statements with brain in idle mode.)

I've seen video of a NASCAR car at high speed getting the tiniest nudge, and going into 4 wheel slide with no evidence of any grip at all. This, I assumed was because the tyre slip was much much bigger than that 15% and dynamic friction was pretty negligible.

And I did say I was only 1/2 🙆🏻d

Apart from this bit of frivolity I'm not having an easy day. Don't know how long I can keep this up,,,,,,

 

[Colin Mill]

A pal of mine (don't be silly Mill you don't have any 😬) told me about a recent TV prog. where they said that some recent road surfaces in the UK will melt very easily once you lock up or skid and that the grip drops off dramatically. Did anyone else catch the prog.?

[susser]

Colin

Yes I did. There was a bit about a biker who died as a result of sliding on some overbanding, if that's the one you mean. Can't remember what it was called, but there was a bit of what you might call journalist science in it. They were wittering about the tyre melting or was it the tar melting and a total loss of grip once the tyre broke traction.

Was that it ?

[Colin Mill]

Hi Susser

Yes, that sounds like the prog. I guess you could melt the tar with the friction of a skid - one time they "re-surfaced" the road I lived in and the tar stayed soft for months - like human size fly-paper: if you stood in one spot for long the heat through your shoes glued you down.

 

 

[susser]

Don't you just love BC 😬

[Colin Mill]

Yes, I can't believe how much technical info there is on here for what must be about the simplest car still in production (I use the term production advisedly 😬)

[Davey Bee]

Susser, why bring me into it, I never said a word 😬

 

TRY 5S

Buzzin' Bee It's never too late for a happy childhood *cool*

 

 

 

 

[susser]

Davey Bee.

Threw me there for a minnit. You're right. That was a bit of non literal communication. The less said the better. 😬

[Tony Martyr]

Just so the pedants are represented - acceleration under gravity changes with geographic location, although it is probably a little difficult to detect it in a Caterham. The figure of 9,81 m/s2 is calculated for sea level at 47 degrees north and south of the equator . It increases towards the poles and decreases towards the equator.

It is a problem when we calibrate dynamometers with 'dead weights' as a unit calibrated in London will read 0.13% high recalibrated in Sydney and 0.09% low if recaibrated in St Petersburg unless the local value of g is used. If you buy a geological map of your area you will find the local variations of g marked.

I also have a theory that the value of g expereinced by the human body increases with age

[Colin Mill]

Yes, to be pedantic the value 9.80665 m/s/s is defined as the "standard" acceleration due to gravity at sea level and of course is a misnomer as it is due to both gravitational and centripetal contributions.

 

I guess you already heard the story of the Engineer, the physicist and the Mathematician on a train going to Scotland. On crossing the border they see a black sheep in a field. The engineer says " I didn't know all sheep in Scotland were black". The Physicist says "You can't say that - you can only say that some sheep in Scotland are black". "Actually" says the mathematician, " you can only say that in Scotland at least one sheep is black on at least one side"

 

[susser]

Tony

I concur with your age/g theory. However I would hypothecise that the value of age/g is not a constant . It is affected by the local OH group. I usually find that after 5 pints of the stuff, it reaches a critical value and I fall over. 😬

[Colin Mill]

Hi Susser

I think there are two slightly different effects here. Age causes an increase in g with, I think, a highly non-linear g/age curve (at least a cube law). The OH group seems to cause a precession of the gravitational vector. Beyond a critical concentration it is impossible to track the precession and one falls down.

[Tony Martyr]

susser - You make an interesting point, but I believe you are confusing the chemistry of the brain cortex with the physics of external forces.

After a critical amount of R-OH group chemicals you are more at risk from gravity but it remains constant - you don't. Interestingly the chemistry modifies the effect of the sudden deacceleration in that you only notice the full effect when the its worm off.

I went climbing with my son on Saturday and it is quite clear that I was under far earthwards greater force than he after taking difference in mass into account.

I intend to use this theory as excuse number 17, after the wrong tyre choice, during the Le Sept trip

[susser]

Guys

I'm impressed. You must be much more 🙆🏻d at scientisting that me.

susser (1/2 🙆🏻d scientist)

😬 😬

[Colin Mill]

I'm hoping that, should I be silly enough to ask for my old job back, they would not have me. (A bit like Groucho Marks who would not want to join a club that would have him as a member)