Oh, how did you do that bypassing encryption or is it still not encrypted?
Oh, how did you do that bypassing encryption or is it still not encrypted?
If you say so
Number_Cruncher
Active Member
>>CV need about twice as long to stop from a given speed as a car.
I don't think the extra stopping distance for trucks is anywhere near so large.
Stopping distances for vehicles are dominated by the level of available grip, or, phrased another way, the co-efficient of friction between tyre and road.
Owing to the fitment of relay valves on each axle, the time lag between the driver of a truck pressing the brake, and the brakes being applied isn't long.
The braking distances quoted in the highway code are based on an effective co-efficient of friction between tyre and road of about 0.7. Truck tyres are a bit harder in their rubber compound, and less grippy than car tyres, but not a factor of two!
I don't think the extra stopping distance for trucks is anywhere near so large.
Stopping distances for vehicles are dominated by the level of available grip, or, phrased another way, the co-efficient of friction between tyre and road.
Owing to the fitment of relay valves on each axle, the time lag between the driver of a truck pressing the brake, and the brakes being applied isn't long.
The braking distances quoted in the highway code are based on an effective co-efficient of friction between tyre and road of about 0.7. Truck tyres are a bit harder in their rubber compound, and less grippy than car tyres, but not a factor of two!
Dieselman
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Surely the amount of kinetic energy comes into the equation more than tyre friction.
A large goods vehicle has 25-30 times the amount of kinetic energy to dissipate though the brakes without the tyres losing grip.
In the linky it had typical calculations for car and lorry stopping distances. Did you read it?
It also mentions that air brakes take half a second to activate so further distance is travelled then.
At 60mph the distance shown is 2.5x as long.
A large goods vehicle has 25-30 times the amount of kinetic energy to dissipate though the brakes without the tyres losing grip.
In the linky it had typical calculations for car and lorry stopping distances. Did you read it?
It also mentions that air brakes take half a second to activate so further distance is travelled then.
At 60mph the distance shown is 2.5x as long.
glojo
Hardcore MB Enthusiast
Many moons ago some traffic officer tried to convince me overall stopping distances were roughly the same. Me being me wasn't convinced, but regarding recent statements then I wonder how much the all up weight would effect an HGV? Am I right in thinking some of these modern trucks have all the latest bells and whistles regarding braking gizzmo's? For example do they have ABS? My only concern regarding trucks stopping, is the load flying through the cab! 
Don't forget, truck drivers are probably car drivers.
regards
John
Don't forget, truck drivers are probably car drivers.
regards
John
Number_Cruncher
Active Member
>>Did you read it?
Yes, but I don't buy into the information on the page in its totality.
The thing that is fundamental is that the same extra mass that provides the extra kinetic energy, or requires the extra brake force to stop also provides extra vertical load on the tyres, and this extra vertical load allows larger tyre forces to be generated.
Here's a basic treatment of the maths of the situation - the vital point being that rates of braking are limited by tyre/road friction, the truck's braking system is more than capable of producing enough force to lock the wheels;
The work done by braking is equal to the kinetic energy lost.
Assuming constant braking force during the braking.
Brake Force * stopping distance = 0.5 * mass * velocity ^2
or, in letters,
F*D=0.5*M*V^2
The limiting brake force is given by the vertical load multiplied by the coefficient of friction
F=M*g*mu
gives;
M*g*mu*D=0.5*M*V^2
Now, here's the vital point, M is on both sides of the equation, and so, it cancels out
g*mu*D=0.5*V^2
with a little manipulation;
D=(V^2)/(2*mu*g)
Note, that having cancelled out, there's no mass term in the equation for stopping distance - heavy vehicles stop just as quickly as light ones. Also, notice the V^2 - double the speed, and the stopping distance goes up by a factor of four.
There are a few factors which do slightly modify this simplified presentation;
1) If the braking force isn't constant, you need to integrate the force with respect to distance rather than the simple multiplication
2) In reality, all the wheels will not be on the point of locking at the same time - this largely depends upon how well the trucks load sensing on its rear axles is engineered. In modern trucks, this optimisation is much better, as the presence of ABS allows rear locking margins to be reduced.
3) Heavily loaded tyres are a bit less efficient than tyres running at light load
4) There is a small lag due to the air brakes, but, relay valves reduce this effect significantly.
I don't see anything that gives a factor of 2, and I would expect most trucks to stop in distances that aren't very far outside those quoted for cars in the highway code. Of course, long, low sports cars with very grippy tyres on dry well surfaced roads can beat the figures given in the highway code by a significant margin, and there's no way a truck can compete with these vehicles!
Yes, but I don't buy into the information on the page in its totality.
The thing that is fundamental is that the same extra mass that provides the extra kinetic energy, or requires the extra brake force to stop also provides extra vertical load on the tyres, and this extra vertical load allows larger tyre forces to be generated.
Here's a basic treatment of the maths of the situation - the vital point being that rates of braking are limited by tyre/road friction, the truck's braking system is more than capable of producing enough force to lock the wheels;
The work done by braking is equal to the kinetic energy lost.
Assuming constant braking force during the braking.
Brake Force * stopping distance = 0.5 * mass * velocity ^2
or, in letters,
F*D=0.5*M*V^2
The limiting brake force is given by the vertical load multiplied by the coefficient of friction
F=M*g*mu
gives;
M*g*mu*D=0.5*M*V^2
Now, here's the vital point, M is on both sides of the equation, and so, it cancels out
g*mu*D=0.5*V^2
with a little manipulation;
D=(V^2)/(2*mu*g)
Note, that having cancelled out, there's no mass term in the equation for stopping distance - heavy vehicles stop just as quickly as light ones. Also, notice the V^2 - double the speed, and the stopping distance goes up by a factor of four.
There are a few factors which do slightly modify this simplified presentation;
1) If the braking force isn't constant, you need to integrate the force with respect to distance rather than the simple multiplication
2) In reality, all the wheels will not be on the point of locking at the same time - this largely depends upon how well the trucks load sensing on its rear axles is engineered. In modern trucks, this optimisation is much better, as the presence of ABS allows rear locking margins to be reduced.
3) Heavily loaded tyres are a bit less efficient than tyres running at light load
4) There is a small lag due to the air brakes, but, relay valves reduce this effect significantly.
I don't see anything that gives a factor of 2, and I would expect most trucks to stop in distances that aren't very far outside those quoted for cars in the highway code. Of course, long, low sports cars with very grippy tyres on dry well surfaced roads can beat the figures given in the highway code by a significant margin, and there's no way a truck can compete with these vehicles!
xpc316e
Active Member
Whether you understand the maths/physics, or not, Number Cruncher is correct in what he states.
Dieselman
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I understand the physics and maths but the additional kinetic energy can't all be dispersed through the tyres evenly due to weight shift.
Most of the weight will transfer to the front axle only and that has limits of braking an adhesion.
Also a lorry doesn't have 25-30 times the contact patch of car tyres so the frictional loss will be proportionally lower.
These people disagree with you.;
http://www.rospa.com/roadsafety/advice/driving/truck_braking.htm
excerpt;
The braking distance of a range of vehicle types was compared both at 30 and 45 mph. This illustrated that LGVs can need up to three times the distance in which to stop when compared to a car.
and
http://teamat.oxfordjournals.org/cgi/reprint/10/2/53.pdf
Their brief description.This is particularly true for lorries and coaches,. whose stopping distance is about double that. for cars.
If the mass wasn't an issue then it doesn't matter whether a lorry is empty or fully laden it will stop in the same distance. I doubt that.
Don't forget cars now don't require the highway code lengths to stop in.
Most of the weight will transfer to the front axle only and that has limits of braking an adhesion.
Also a lorry doesn't have 25-30 times the contact patch of car tyres so the frictional loss will be proportionally lower.
These people disagree with you.;
http://www.rospa.com/roadsafety/advice/driving/truck_braking.htm
excerpt;
The braking distance of a range of vehicle types was compared both at 30 and 45 mph. This illustrated that LGVs can need up to three times the distance in which to stop when compared to a car.
and
http://teamat.oxfordjournals.org/cgi/reprint/10/2/53.pdf
Their brief description.This is particularly true for lorries and coaches,. whose stopping distance is about double that. for cars.
If the mass wasn't an issue then it doesn't matter whether a lorry is empty or fully laden it will stop in the same distance. I doubt that.
Don't forget cars now don't require the highway code lengths to stop in.
television
Hardcore MB Enthusiast
removed
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LastMinute
Active Member
The flaw in Number_Cruncher's maths is the assertion that mgµd = ½mv².
A vehicle's braking action is a lot more complex than the "block sliding along a smooth plane" that we were taught at A level...
A vehicle's braking action is a lot more complex than the "block sliding along a smooth plane" that we were taught at A level...
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Dieselman
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Graphic demonstration.
http://www.youtube.com/watch?v=UiW_sQSbCgY
If mass made no differrence what happened here.?
http://www.youtube.com/watch?v=UiW_sQSbCgY
If mass made no differrence what happened here.?
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Number_Cruncher
Active Member
I'm sure you've a strong grasp of the physics dieselman, and I would never suggest otherwise.
I've seen the ROSPA data before, and there are some interesting features there.
If you look at the table near the lower end of the page, many of the trucks are actually stopping in distances which are close to those given in the highway code. There's a particularly bad IVECO, which takes a huge distance to stop, and I wouldn't be at all surprised to find that that particular vehicle / trailer combination had a fault.
>>If the mass wasn't an issue then it doesn't matter whether a lorry is empty or fully laden it will stop in the same distance. I doubt that.
There's not that much in it. The fundamental physics still stands, after all, you can't evade the equality between kinetic energy and work done. The strongest effect that does increase the braking distance for a laden truck compared to the same truck unladen is the non-linear reduction in the effective grip of the tyres which I mention in point 3) of my post.
Contact areas don't really come into it.
Weight transfer towards the front axle does happen in trucks under braking, although as trucks are long vehicles, this reduces the effect. The mechanism by which weight transfer reduces the total braking force is quite suble, and is related to point 3) of my post. Weight transfer happens in cars too, and front heavy, short wheelbase, tall, front wheel drive cars are heavily handicapped in that regard - think Vauxhall Meriva [shudder!]
>>Don't forget cars now don't require the highway code lengths to stop in.
Yes, I happily acknowledge that.
By the same token, Smith & Thatcher quote TRRL data from 1975, and I would expect that there have been significant improvements in truck tyre and braking technology since then.
The big picture which I would like to put across is that there's no need for alternative stopping distances for HGVs - they should be able to stop in distances similar to those given in the highway code [which are based on a mu of about 0.7 which should be achievable by both cars and trucks], although any HGV driver pushing his vehicle to these limits deserves to have his licence and liberty removed.
I've seen the ROSPA data before, and there are some interesting features there.
If you look at the table near the lower end of the page, many of the trucks are actually stopping in distances which are close to those given in the highway code. There's a particularly bad IVECO, which takes a huge distance to stop, and I wouldn't be at all surprised to find that that particular vehicle / trailer combination had a fault.
>>If the mass wasn't an issue then it doesn't matter whether a lorry is empty or fully laden it will stop in the same distance. I doubt that.
There's not that much in it. The fundamental physics still stands, after all, you can't evade the equality between kinetic energy and work done. The strongest effect that does increase the braking distance for a laden truck compared to the same truck unladen is the non-linear reduction in the effective grip of the tyres which I mention in point 3) of my post.
Contact areas don't really come into it.
Weight transfer towards the front axle does happen in trucks under braking, although as trucks are long vehicles, this reduces the effect. The mechanism by which weight transfer reduces the total braking force is quite suble, and is related to point 3) of my post. Weight transfer happens in cars too, and front heavy, short wheelbase, tall, front wheel drive cars are heavily handicapped in that regard - think Vauxhall Meriva [shudder!]
>>Don't forget cars now don't require the highway code lengths to stop in.
Yes, I happily acknowledge that.
By the same token, Smith & Thatcher quote TRRL data from 1975, and I would expect that there have been significant improvements in truck tyre and braking technology since then.
The big picture which I would like to put across is that there's no need for alternative stopping distances for HGVs - they should be able to stop in distances similar to those given in the highway code [which are based on a mu of about 0.7 which should be achievable by both cars and trucks], although any HGV driver pushing his vehicle to these limits deserves to have his licence and liberty removed.
Number_Cruncher
Active Member
>>the assertion that mgµ = ½mv².
I didn't actually assert exactly that.
However, the basic physics of equating kinetic energy with the work done during braking is unassailable.
Yes, I agree there's much more going on than basic friction suggests *but* this basic friction is the fundamental physical principle at work.
I don't read much into the you tube video link,because;
a) it's fifth gear!, and I don't trust a word they say [nor any word uttered by the idiots on Top Gear either!]
b) I don't think the test was well enough controlled to be of any use. For example, on the approach, the driver was clearly worried about the effect the sudden braking might have on his "ballast", and so, I would not be surprised to find that the brakes weren't applied as firmly during the second run.
I'm not trying to say that weight has *zero* effect, what I am saying is that effect isn't as large as you might think.
To help review the ROSPA data, here are some stopping distances at 30 and 45 mph estimated using a mu of 0.7.
>> d=((30*1609.3/3600)^2)/(2*0.7*9.81)
d =
13.0953
>> d=((45*1609.3/3600)^2)/(2*0.7*9.81)
d =
29.4644
I didn't actually assert exactly that.
However, the basic physics of equating kinetic energy with the work done during braking is unassailable.
Yes, I agree there's much more going on than basic friction suggests *but* this basic friction is the fundamental physical principle at work.
I don't read much into the you tube video link,because;
a) it's fifth gear!, and I don't trust a word they say [nor any word uttered by the idiots on Top Gear either!]
b) I don't think the test was well enough controlled to be of any use. For example, on the approach, the driver was clearly worried about the effect the sudden braking might have on his "ballast", and so, I would not be surprised to find that the brakes weren't applied as firmly during the second run.
I'm not trying to say that weight has *zero* effect, what I am saying is that effect isn't as large as you might think.
To help review the ROSPA data, here are some stopping distances at 30 and 45 mph estimated using a mu of 0.7.
>> d=((30*1609.3/3600)^2)/(2*0.7*9.81)
d =
13.0953
>> d=((45*1609.3/3600)^2)/(2*0.7*9.81)
d =
29.4644
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LastMinute
Active Member
Yes, sorry - I missed out a d.
xpc316e
Active Member
I know that for many of you the argument that mass has nothing to do with braking distances doesn't make sense, but it has been proven in the highest courts of law in the land that Number Cruncher's statements are correct. If the brakes are powerful enough to lock the wheels then any extra load on them is counteracted by a proportional increase in the frictional force. It really is that simple. A defence in a court might argue with the data collected by a collision investigator, and its method of collection, but they will not argue against the maths or physics. I suggest that to prove this you examine the figures from Autocar's annual 0-100-0 mph challenge. A large variety of vehicles are used from super-light sports cars (such as Ariel Atoms & Caterhams) to heavy saloons and GTs (such as BMW M5, Aston Martins) and their braking figures from 100-0 are nearly identical (excepting those cars on super-sticky slicks).
glojo
Hardcore MB Enthusiast
I personally think the numbers are correct but the instant a driver tries braking that severely it might be their death warrant, or if they are transporting livestock; it will be the death of the animals.
Folks that talk about not being able to apply sufficicient grip to the road surface should consider how much rubber is in contact with the road when we might be talking in excess of eighteen very large wheels\tyres!
I'm positive a modern truck would stop far quicker than an old Morris 1000, with its 'string' connected to the useless drum brakes. Having said that, it wasn't so long ago that trucks had drum brakes that would glaze over if we tried using them too often when going down a steep hill and I speak very much from experience
!
John
Folks that talk about not being able to apply sufficicient grip to the road surface should consider how much rubber is in contact with the road when we might be talking in excess of eighteen very large wheels\tyres!
I'm positive a modern truck would stop far quicker than an old Morris 1000, with its 'string' connected to the useless drum brakes. Having said that, it wasn't so long ago that trucks had drum brakes that would glaze over if we tried using them too often when going down a steep hill and I speak very much from experience
! John
xpc316e
Active Member
Braking distances for vehicles have not changed much over the years, believe it or not. Your old Morris 1000 will match a modern car in an emergency stop from 30mph because drum brakes are fine unless used repeatedly causing the drum itself to heat up and expand - this increases pedal travel and if the linings overheat too, brings about brake fade. Modern cars use discs because they are cheaper to manufacture, easier to service, better in sustained usage, etc. You need to remember that it is really the coefficient of friction between road surface and tyre that stops your vehicle (as long as the brakes are powerful enough to lock the wheels). A lot of modern roads have very poor grip, especially low confrontation roads such as motorways. The technology exists to provide high-grip surfaces, but grip costs money, so that technology is rarely used. I can well remember Vincent motorcycles from the Fifties generating better than 1g in panic stops with their drum brakes, although admittedly this must have been on an excellent surface. All the ceramic discs, ABS, EBD, etc., will not be much use to you on a greasy, wet motorway with a coefficient of friction of perhaps 0.3 - 0.4.
glojo
Hardcore MB Enthusiast
As a young boy I regularly sat in one of my father's lorries as they transported sand from Virginia Waters to the M4 which was then being built. It was a daily occurence to see lorries (plural) leaving the road and crashing in numerous locations (including the Thames) becuase their drum brakes had glazed over. I fully agree with the points being made about a locked wheel being a locked wheel, but in real world scenario's the wheel may well lock, but when it got hot, it would not
I have no idea on how many trucks still have drum brakes that are prone to over-heating, but I have noticed that the local escape roads that are located on numerous steep hills now appear to be getting over grown and not appearing to be used?
Regards
John
I have no idea on how many trucks still have drum brakes that are prone to over-heating, but I have noticed that the local escape roads that are located on numerous steep hills now appear to be getting over grown and not appearing to be used?
Regards
John
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An example of this is where a Shellgrip surface is used on a bend. Where it's used, it pretty much always starts on the approach to the bend in the braking area and it used to run right through to the exit. However, I've noticed a recent tendency for it to cease just after the apex of the bend - obviously for cost reasons. The trouble is, when riding a motorcycle the surface transition under poor grip conditions can cause a real problem
xpc316e
Active Member
I fully agree that most crashes are caused by poor driving, and not by excessive speed alone. However, kinetic energy increases by the square of the speed increase; therefore 30mph is 1.5 times faster than 20mph, but a 30mph collision contains 2.25 times more energy than a 20mph crash. A small reduction in speed results in much less kinetic energy, and therefore much damage to vehicles and injury to pedestrians.
It is far easier to get people to drive slowly than it is to re-educate them into being better drivers - that takes a lot of money, experienced trainers, time, and a willingness to improve that most drivers fail to see the need for. Hence a Government campaign to cut down on speeding drivers. When you start to do a job the lowest fruit always gets picked first - it's human nature, so when drivers can actually pay attention to the speedometer that's sited directly in front of them and obey the law we can then move on to combatting heroin addiction leading to burglaries etc.
Drivers have for years been able to speed with almost no risk of being caught; now that there are cameras they don't like it. Perhaps they'd like to go back to having Traffic Police on the road with humans (well, almost humans) who could exercise discretion, be sympathetic, offer advice and if the worst came to the worst, actually report someone. As a former Trafpol I used to also arrest folk for crimes as well as doing a good job of Policing traffic - show me a camera that can do that.
It is far easier to get people to drive slowly than it is to re-educate them into being better drivers - that takes a lot of money, experienced trainers, time, and a willingness to improve that most drivers fail to see the need for. Hence a Government campaign to cut down on speeding drivers. When you start to do a job the lowest fruit always gets picked first - it's human nature, so when drivers can actually pay attention to the speedometer that's sited directly in front of them and obey the law we can then move on to combatting heroin addiction leading to burglaries etc.
Drivers have for years been able to speed with almost no risk of being caught; now that there are cameras they don't like it. Perhaps they'd like to go back to having Traffic Police on the road with humans (well, almost humans) who could exercise discretion, be sympathetic, offer advice and if the worst came to the worst, actually report someone. As a former Trafpol I used to also arrest folk for crimes as well as doing a good job of Policing traffic - show me a camera that can do that.
