[BBC News] Driver dies in crash on main road

[WTF]

nonsense - look at http://pisa-sq.acer.edu.au/Media/Pisa/Math...raking/M215.jpg - his stopping distance at 150km/h was around 190m - no way could he have dropped to 30mph unless he had applied full brakes (as per emergency stop)

 

thats what full brakes are for.

 

so your saying he was doing 90mph and then was going to perform an emergency stop with full brakes so he could get down to 30mph in time for the speed limit?

 

Because that's the only way he could have done it.

 

 

exactly, i was only disputing the point that it couldn't be done according to a previous post , not how it could be done. as far as i'm aware there are no G force or other calibrated specifications for accelleration and decelleration in the highway code or law? it may well severly annoy who you've just overtaken and be perceived as dangerous but that wasn't the issue. as to the little chart in the linky, it does not specify a vehicle? some cars, evo's for example, have fantastic brakes and nice wide wheels to grip the road. on an episode of top gear berkson was in i believe a maclaren slr mercedes and it stopped in half the stopping distance specified in the highway code. the stopping distances are different depending on the vehicle used and whether or not it has anti-lock brakes and not what the highway code says. another point is that stopping is not the same as reducing speed by 60 mph from 90 to 30, and at those speeds resistance through the air plays a part too, not much but some.

[Frances]

no the only thing are the laws of physics - decelleration is limited by coefficient of friction between tyres + road (0.8 approx for dry road, much lower on wet/poor roads) - the mass of the car doesn't come into it as the key idea of brakes is to dissipate the energy of the car as heat in the brake pads - the retarding force depends on mass & coefficient of friction however the kinetic energy of the car depends on mass and the square of velocity thus mass cancels out and you can see why braking from high speed needs a much greater distance. Once the skid marks start he's lost it as the tyres not the brake pads are now dissipating the energy as heat and melting the rubber - unfortuneately the coefficient of friction of molten rubber is a lot lower hence skids and an increased braking distance.

 

I don't have to worry about g-force as in a controlled brake you cannot exceed about 0.8g - now if you hit a stone wall the g-force will be rather high and usually triggers an air bag - thus I still claim nonsense - given the point of impact IMO he could not have nor was intending to meet the 30mph speed limit.

[WTF]

i did an edit before i saw your response, but surely the tyres in use ( width and compound ) and the brakes ( i could not lock up my drum braked mini with wide wheels all those years ago ) would change the available grip? why would racing cars use different tyres if it didn't change the grip available? the more rubber on the road, the more friction area you have so more braking?? how far would a ton sheet of metal slide if it had a sheet or rubber glued to it? and you don't take into account wind resistance? and a formula 1 car can put over 4 G's on a person in heavy braking. i realise it wasn't an F1 car but on the same note it wasn't an 1100 fiesta either.

[Frances]

different tyres will have different coeffs of friction - racing cars also use a large aerofoil to increase the force of the tyres (ie effectively increasing the mass) for high speed manoevres (those on non-racing cars are merely an upmarket go-faster stripe having near zero effect) - if you want to brake at high speed use a paracute (eg as in rocket cars);the tyre area will have some effect in allowing the coeff to reach its maximum possible but beyond a certain area will have no further effect as basically you are relying on gravity (g) to provide the force to stick you to the road (ie g x mass of car) acting against the energy given by 0.5mass x v squared - no way you can get round this - obvously second order effects such as wind resistance may help but for high performance cars this is near minimum - possibly if you could throw up a large vertical sheet then this would also provide brakeing.

 

steel on asphalt will have a lower coeff of friction thus less braking (steel on steel as in a railway has very low friction thus the long braking distance for trains)

 

Likewise acceleration limited by same arguments - burning your tyre is giving you nothing except R-plate satisfaction.

 

[edited to add that the highway code stopping distance is conservative both on reaction time + coeff of friction (poor roads,tyres etc) - obviously a skilled, non distracted driver with a top of range car will better them - the figure was I think based on theory for best possible given usual coeff of friction]

[WTF]

i agree basically but the amount of rubber on the road must make a difference otherwise performance cars wouldn't have wider tyres, it is not just weight and gravity issue. if anyone out there has an evo9 and a mate with a fiesta do a comparrison please?

[stookie]

As this is getting scientific I am willing to bet the coefficient of grip of many road surfaces in the dry or the wet (especially in the Island where there are many sh*t road surfaces) varies considerably, further complicated by the presence/absence of contaminents on the surface.

 

You also omit other vital variables in the braking equation - that of the evenness of the road surface and the ability of a car's dampers to continue to exert the transferred weight of the car as a positive force during braking at the same time as dealing with inconsistencies in the road surface. Then we have the relative compliance of the tyres themselves and how much they distort/compress under heavy load (also significantly affected by tyre pressures, temperature and design) thus affecting the overall braking efficiency and finally (as far as I can figure) the weight of the vehicle itself. These are all (I think) very large variables and any tables will be based around assumptions of some kind of average vehicle. While I'm not here to sing the praises of the Evo or any of its drivers, I'd have to agree it can hardly be the "average" that might be used in such a simplistic and theoretical diagram!

 

Having said that, anyone who drives expecting to stop in the manner decribed or likewise bring themselves up in time for a speed limit is driving too dangerously for the road and that they crashed is unfortunate and lamentable proof they did not have full control of the vehicle.

[Albert Tatlock]

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