Bahnstorming Speeds

[Julian Scott]

8IKERDAVE said:

Speeds will definitely drop this year as I'll be in a hire car with around 100bhp.

Nothing is faster than a hire car ;-)

[DarkVeil]

Gary C said:

braddo said:

Gary C said:

Yes, a completely unbiased article isn't it...

Well, good luck finding an 'unbiased article' that says unrestricted autobahns are safer than the limited ones.

Don't shoot the messenger. I enjoyed some autobahn last weekend, although only got to 130mph very briefly due to the masses of traffic even on a Sunday. And rain, and roadworks... To be expected in the Rhine-Ruhr region however.

Sorry, not pointing a finger

Germany deaths on Autobhan in a year is 30 / 1000 km and the Uk is 25 / 1000 km

So it is worse.

UK deaths per KM are distorted due to traffic density on motorways being far higher than in other European countries, so much more distance is driven in heavy traffic at speeds far too low to possibly result in fatalities.

According to that same list Ireland has the by far the lowest motorway death rate in Europe, but this is only due to most of their motorway miles being covered on the M50 around Dublin which is just as clogged up as the M25.

[Olivergt]

DarkVeil said:

Gary C said:

braddo said:

Gary C said:

Yes, a completely unbiased article isn't it...

Well, good luck finding an 'unbiased article' that says unrestricted autobahns are safer than the limited ones.

Don't shoot the messenger. I enjoyed some autobahn last weekend, although only got to 130mph very briefly due to the masses of traffic even on a Sunday. And rain, and roadworks... To be expected in the Rhine-Ruhr region however.

Sorry, not pointing a finger

Germany deaths on Autobhan in a year is 30 / 1000 km and the Uk is 25 / 1000 km

So it is worse.

UK deaths per KM are distorted due to traffic density on motorways being far higher than in other European countries, so much more distance is driven in heavy traffic at speeds far too low to possibly result in fatalities.

According to that same list Ireland has the by far the lowest motorway death rate in Europe, but this is only due to most of their motorway miles being covered on the M50 around Dublin which is just as clogged up as the M25.

Does that mean that going slower is safer then?

[LunarOne]

Olivergt said:

DarkVeil said:

Gary C said:

braddo said:

Gary C said:

Yes, a completely unbiased article isn't it...

Well, good luck finding an 'unbiased article' that says unrestricted autobahns are safer than the limited ones.

Don't shoot the messenger. I enjoyed some autobahn last weekend, although only got to 130mph very briefly due to the masses of traffic even on a Sunday. And rain, and roadworks... To be expected in the Rhine-Ruhr region however.

Sorry, not pointing a finger

Germany deaths on Autobhan in a year is 30 / 1000 km and the Uk is 25 / 1000 km

So it is worse.

UK deaths per KM are distorted due to traffic density on motorways being far higher than in other European countries, so much more distance is driven in heavy traffic at speeds far too low to possibly result in fatalities.

According to that same list Ireland has the by far the lowest motorway death rate in Europe, but this is only due to most of their motorway miles being covered on the M50 around Dublin which is just as clogged up as the M25.

Does that mean that going slower is safer then?

Not existing is safer, because it excludes all possibility of death or injury while also mitigating to need to be anywhere. Global mass sterilisation will fix all future safety concerns.

[DarkVeil]

Olivergt said:

DarkVeil said:

Gary C said:

braddo said:

Gary C said:

Yes, a completely unbiased article isn't it...

Well, good luck finding an 'unbiased article' that says unrestricted autobahns are safer than the limited ones.

Don't shoot the messenger. I enjoyed some autobahn last weekend, although only got to 130mph very briefly due to the masses of traffic even on a Sunday. And rain, and roadworks... To be expected in the Rhine-Ruhr region however.

Sorry, not pointing a finger

Germany deaths on Autobhan in a year is 30 / 1000 km and the Uk is 25 / 1000 km

So it is worse.

UK deaths per KM are distorted due to traffic density on motorways being far higher than in other European countries, so much more distance is driven in heavy traffic at speeds far too low to possibly result in fatalities.

According to that same list Ireland has the by far the lowest motorway death rate in Europe, but this is only due to most of their motorway miles being covered on the M50 around Dublin which is just as clogged up as the M25.

Does that mean that going slower is safer then?

Not unless you are suggesting all motorways are as slow as the M25 in rush hour. The point is that if you exclude the amount of time spent crawling in congestion, the Autobahn actually has a lower fatality rate than UK motorways.

[braddo]

DarkVeil said:

Not unless you are suggesting all motorways are as slow as the M25 in rush hour. The point is that if you exclude the amount of time spent crawling in congestion, the Autobahn actually has a lower fatality rate than UK motorways.

Has anyone actually tried to demonstrate that with figures and research, or are you just guessing?

The M25 is a tiny part of the overall UK motorway network. Germany has a similar area of massive population density and congestion in the Rhine-Ruhr region.

[Blown2CV]

LayZ said:

I would urge some caution on the tone here, a lot of blustering. It takes a lot of mental effort to drive at 200km/h+ safely. It can be done, and I have done it, but it isn't a relaxing way to make progress. As per the Reg video you have to be on high alert all the time.

More contraversial statements - most people don't have the driving skill to safely do 200km/h+ either, and it probably shouldn't be allowed for those people.

but you can develop the skill, and that skill is really useful even for 'normal' UK motorway driving. I strongly believe it is not that some individuals are just too thick to cope, just that their driving ability is ste, but can be improved. In many ways it's not their fault as the standard of driving tuition in this country is atrocious and doesn't even teach the slightest thing about safe motorway driving.

[DarkVeil]

braddo said:

DarkVeil said:

Not unless you are suggesting all motorways are as slow as the M25 in rush hour. The point is that if you exclude the amount of time spent crawling in congestion, the Autobahn actually has a lower fatality rate than UK motorways.

Has anyone actually tried to demonstrate that with figures and research, or are you just guessing?

The M25 is a tiny part of the overall UK motorway network. Germany has a similar area of massive population density and congestion in the Rhine-Ruhr region.

The M25 was just the most obvious example, my point was that a higher proportion of the UK motorway network travels through very busy urbanised areas. Of course stretches of Autobahn exist like this too, but as a much smaller proportion of the overall network.

Also the Rhine-Ruhr region has multiple different Autobahn routes, where as around London all of the traffic is forced onto 1 orbital route. I've driven to Cologne numerous times and only had problems with congestion when there are roadworks, so in my anecdotal experience it's not comparable to the M25 in terms of traffic.

[herebebeasties]

Aphrabehn said:

herebebeasties said:

So the energy your brakes need to dissipate to decelerate from 140mph to 70mph is three times what you need to decelerate from 70mph to zero. (OK you'll also benefit from a bit more air resistance slowing you down from higher speed, but it's not significant compared to maximum braking effort.)

Sorry that's completely wrong, air resistance is hugely significant at higher speeds and is obviously generally the limiting factor in Vmax. Not many published graphs for road cars covering braking from 200kph, but if you take the most extreme case of Bloodhound SSC, (with a far smaller drag coefficient of 0.1165), then you can see that the greatest braking (3G) occurs simply at engine shut off from 1000mph. Three stage chute used for small reductions, then disc brakes doing very little later on except helping with the stopping location.

You absolutely do NOT need anything like three times the braking effort to get from 140-70 as 70 to 0, the graph for road cars will actually be close to linear. You can even see that in the last 200kph of the Bloodhound graph below.

I don't dispute that air resistance is significant at speed, nor that it's the limiting factor for top speed. That's obviously true, but does not alter the correctness of my assertion that you absolutely do need to absorb much more energy into your brakes/tyres from 140mph to 70mph than from 70mph to 0mph. The basic kinetic energy calculation (v^2) says so, and the energy dissipation is about 20x more from braking than air resistance, even at this speed, so yes, the air resistance component is "not significant" in this instance.

It's basic physics:

Air resistance force is given by the drag equation F = ½ * ρ * Cd * A * v²

Where:
ρ = air density (approx. 1.2kg/m³ at sea level)
Cd is drag coefficient (typically 0.3 or so, maybe less these days)
A is the frontal area of the car (typically 2.5m² or so)
v is the velocity (155 mph ~= 69m/s)

So deceleration you get from F=ma as usual => a = F/m
Where:
F is force
m is the mass of the car (let's say 2000kg for a modern 155mph bahnstormer with a passenger or two)
a is acceleration (deceleration)

So plugging in the values above, drag force at 155mph comes out at ~2142 Newtons.

So initial deceleration of a vehicle with the above characteristics due to air resistance alone from 155mph comes in at almost exactly 1 metre per second per second (2142 Newtons divided by 2000kg mass). I.e. ~0.1g.

A car's brakes will do 1g any day of the week (unless they are cooked or the tyres are ancient), and more if you have decent tyres and/or downforce.

So air resistance is only about 10% of the braking effort at 155mph, and progressively less as you decelerate. By 70mph it's down to only 2% of the effort. I can't be bothered to do the integration to get the total from 155 to 70 mph, but it's going to be ~5% or something across the range. I.e. "Not significant". Even the 11% at the initial 155mph is not what I would call "significant". Even in a G-Wagen, it's only ~15%.

(Comparisons with charts for Bloodhound seem very misguided: You really cannot compare the effect of drag and aluminium wheels on sand at 1000mph to that of drag and rubber on tarmac at 155mph given the non-linearity of air resistance and the massively increased friction the rubber/tarmac combo is capable of. Any smaller fixed drag coefficient and mass terms for Bloodhound are going to be absolutely dwarfed by the squared velocity term at 1000mph. And even below 150 mph you're not going to be generating 1.0g from aluminium wheels on sand, and it's not like Bloodhound is likely to have been threshold braking in the chart you provide - I wouldn't fancy having a lock-up in that on sand at even 50 mph let alone much faster!)

Edited by herebebeasties on Thursday 27th June 15:52

[Aphrabehn]

herebebeasties said:

Aphrabehn said:

herebebeasties said:

So the energy your brakes need to dissipate to decelerate from 140mph to 70mph is three times what you need to decelerate from 70mph to zero. (OK you'll also benefit from a bit more air resistance slowing you down from higher speed, but it's not significant compared to maximum braking effort.)

Sorry that's completely wrong, air resistance is hugely significant at higher speeds and is obviously generally the limiting factor in Vmax. Not many published graphs for road cars covering braking from 200kph, but if you take the most extreme case of Bloodhound SSC, (with a far smaller drag coefficient of 0.1165), then you can see that the greatest braking (3G) occurs simply at engine shut off from 1000mph. Three stage chute used for small reductions, then disc brakes doing very little later on except helping with the stopping location.

You absolutely do NOT need anything like three times the braking effort to get from 140-70 as 70 to 0, the graph for road cars will actually be close to linear. You can even see that in the last 200kph of the Bloodhound graph below.

I don't dispute that air resistance is significant at speed, nor that it's the limiting factor for top speed. That's obviously true, but does not alter the correctness of my assertion that you absolutely do need to absorb much more energy into your brakes/tyres from 140mph to 70mph than from 70mph to 0mph. The basic kinetic energy calculation (v^2) says so, and the energy dissipation is about 20x more from braking than air resistance, even at this speed, so yes, the air resistance component is "not significant" in this instance.

It's basic physics:

Air resistance force is given by the drag equation F = ½ * ? * Cd * A * v²

Where:
? = air density (approx. 1.2kg/m³ at sea level)
Cd is drag coefficient (typically 0.3 or so, maybe less these days)
A is the frontal area of the car (typically 2.5m² or so)
v is the velocity (155 mph ~= 69m/s)

So deceleration you get from F=ma as usual => a = F/m
Where:
F is force
m is the mass of the car (let's say 2000kg for a modern 155mph bahnstormer with a passenger or two)
a is acceleration (deceleration)

So plugging in the values above, drag force at 155mph comes out at ~2142 Newtons.

So initial deceleration of a vehicle with the above characteristics due to air resistance alone from 155mph comes in at almost exactly 1 metre per second per second (2142 Newtons divided by 2000kg mass). I.e. ~0.1g.

A car's brakes will do 1g any day of the week (unless they are cooked or the tyres are ancient), and more if you have decent tyres and/or downforce.

So air resistance is only about 10% of the braking effort at 155mph, and progressively less as you decelerate. By 70mph it's down to only 2% of the effort. I can't be bothered to do the integration to get the total from 155 to 70 mph, but it's going to be ~5% or something across the range. I.e. "Not significant". Even the 11% at the initial 155mph is not what I would call "significant". Even in a G-Wagen, it's only ~15%.

(Comparisons with charts for Bloodhound seem very misguided: You really cannot compare the effect of drag and aluminium wheels on sand at 1000mph to that of drag and rubber on tarmac at 155mph given the non-linearity of air resistance and the massively increased friction the rubber/tarmac combo is capable of. Any smaller fixed drag coefficient and mass terms for Bloodhound are going to be absolutely dwarfed by the squared velocity term at 1000mph. And even below 150 mph you're not going to be generating 1.0g from aluminium wheels on sand, and it's not like Bloodhound is likely to have been threshold braking in the chart you provide - I wouldn't fancy having a lock-up in that on sand at even 50 mph let alone much faster!)

Edited by herebebeasties on Thursday 27th June 15:52

There is a hole in your assumptions, for example the trace for an F1 car around Monaco, completely linear braking from 80 to 20m/s

https://www.researchgate.net/figure/Velocity-of-a-...

[Aphrabehn]

Aphrabehn said:

herebebeasties said:

Aphrabehn said:

herebebeasties said:

So the energy your brakes need to dissipate to decelerate from 140mph to 70mph is three times what you need to decelerate from 70mph to zero. (OK you'll also benefit from a bit more air resistance slowing you down from higher speed, but it's not significant compared to maximum braking effort.)

Sorry that's completely wrong, air resistance is hugely significant at higher speeds and is obviously generally the limiting factor in Vmax. Not many published graphs for road cars covering braking from 200kph, but if you take the most extreme case of Bloodhound SSC, (with a far smaller drag coefficient of 0.1165), then you can see that the greatest braking (3G) occurs simply at engine shut off from 1000mph. Three stage chute used for small reductions, then disc brakes doing very little later on except helping with the stopping location.

You absolutely do NOT need anything like three times the braking effort to get from 140-70 as 70 to 0, the graph for road cars will actually be close to linear. You can even see that in the last 200kph of the Bloodhound graph below.

I don't dispute that air resistance is significant at speed, nor that it's the limiting factor for top speed. That's obviously true, but does not alter the correctness of my assertion that you absolutely do need to absorb much more energy into your brakes/tyres from 140mph to 70mph than from 70mph to 0mph. The basic kinetic energy calculation (v^2) says so, and the energy dissipation is about 20x more from braking than air resistance, even at this speed, so yes, the air resistance component is "not significant" in this instance.

It's basic physics:

Air resistance force is given by the drag equation F = ½ * ? * Cd * A * v²

Where:
? = air density (approx. 1.2kg/m³ at sea level)
Cd is drag coefficient (typically 0.3 or so, maybe less these days)
A is the frontal area of the car (typically 2.5m² or so)
v is the velocity (155 mph ~= 69m/s)

So deceleration you get from F=ma as usual => a = F/m
Where:
F is force
m is the mass of the car (let's say 2000kg for a modern 155mph bahnstormer with a passenger or two)
a is acceleration (deceleration)

So plugging in the values above, drag force at 155mph comes out at ~2142 Newtons.

So initial deceleration of a vehicle with the above characteristics due to air resistance alone from 155mph comes in at almost exactly 1 metre per second per second (2142 Newtons divided by 2000kg mass). I.e. ~0.1g.

A car's brakes will do 1g any day of the week (unless they are cooked or the tyres are ancient), and more if you have decent tyres and/or downforce.

So air resistance is only about 10% of the braking effort at 155mph, and progressively less as you decelerate. By 70mph it's down to only 2% of the effort. I can't be bothered to do the integration to get the total from 155 to 70 mph, but it's going to be ~5% or something across the range. I.e. "Not significant". Even the 11% at the initial 155mph is not what I would call "significant". Even in a G-Wagen, it's only ~15%.

(Comparisons with charts for Bloodhound seem very misguided: You really cannot compare the effect of drag and aluminium wheels on sand at 1000mph to that of drag and rubber on tarmac at 155mph given the non-linearity of air resistance and the massively increased friction the rubber/tarmac combo is capable of. Any smaller fixed drag coefficient and mass terms for Bloodhound are going to be absolutely dwarfed by the squared velocity term at 1000mph. And even below 150 mph you're not going to be generating 1.0g from aluminium wheels on sand, and it's not like Bloodhound is likely to have been threshold braking in the chart you provide - I wouldn't fancy having a lock-up in that on sand at even 50 mph let alone much faster!)

Edited by herebebeasties on Thursday 27th June 15:52

There is a hole in your assumptions, for example the trace for an F1 car around Monaco, completely linear braking from 80 to 20m/s

https://www.researchgate.net/figure/Velocity-of-a-...

Or take a much lower Cd Lemans Hypercar, totally linear again:

https://www.pmw-magazine.com/features/tech-insight...

[herebebeasties]

Aphrabehn said:

Or take a much lower Cd Lemans Hypercar, totally linear again:

https://www.pmw-magazine.com/features/tech-insight...

Err, you seem to be telling me I'm wrong while saying literally the same thing as me now? I.e. That air resistance doesn't play much of a part in energy dissipation under max braking effort, unless at very much higher speeds than 150-odd mph. Otherwise you would expect a quite non-linear, much higher initial deceleration due to all the air resistance, per your Bloodhound chart from much higher speed. Which... isn't happening from lower speeds, as your charts are pointing out. QED.

I don't know why you keep trotting out charts that are not for road cars. Your F1 and Le Mans Hypercars generate more than their own weight in downforce at those sorts of speeds. Downforce acts approximately linearly with speed, so directly counteracts the squared term for the kinetic energy. I.e. They generate more braking effort to dissipate the higher KE at high speeds more rapidly at those higher speeds, due to the linear downforce multiplier, so end up looking fairly linear.

Did you do GCSE physics? Does KE = 0.5 × m × v² not ring a bell? Why do you think performance cars have serious brakes? It's because of the squared component in that kinetic energy equation. Energy is conserved by turning nearly all of the kinetic energy into heat energy in your brakes. Some disappears to drag, but that is a "not significant" component like I said, and have even proved with the calculations above.

Go look up literally any stopping distances chart. They typically only go up to 70mph, but even for those you can see that it is distinctly non-linear. Project out to 150mph and that velocity squared term starts to really hurt, unless you have very significant downforce to counteract it, which of course we don't because we're talking about road cars at autobahn speeds.

[Jimjimhim]

Adding graphs always makes a topic even more exciting.......

[Aphrabehn]

herebebeasties said:

Err, you seem to be telling me I'm wrong while saying literally the same thing as me now? I.e. That air resistance doesn't play much of a part in energy dissipation under max braking effort, unless at very much higher speeds than 150-odd mph. Otherwise you would expect a quite non-linear, much higher initial deceleration due to all the air resistance, per your Bloodhound chart from much higher speed. Which... isn't happening from lower speeds, as your charts are pointing out. QED.

I don't know why you keep trotting out charts that are not for road cars. Your F1 and Le Mans Hypercars generate more than their own weight in downforce at those sorts of speeds. Downforce acts approximately linearly with speed, so directly counteracts the squared term for the kinetic energy. I.e. They generate more braking effort to dissipate the higher KE at high speeds more rapidly at those higher speeds, due to the linear downforce multiplier, so end up looking fairly linear.

Did you do GCSE physics? Does KE = 0.5 × m × v² not ring a bell? Why do you think performance cars have serious brakes? It's because of the squared component in that kinetic energy equation. Energy is conserved by turning nearly all of the kinetic energy into heat energy in your brakes. Some disappears to drag, but that is a "not significant" component like I said, and have even proved with the calculations above.

Go look up literally any stopping distances chart. They typically only go up to 70mph, but even for those you can see that it is distinctly non-linear. Project out to 150mph and that velocity squared term starts to really hurt, unless you have very significant downforce to counteract it, which of course we don't because we're talking about road cars at autobahn speeds.

Again, your pure physics assumptions remind me of the classic "perfect sphere travelling in a vacuum" physics/engineering horse racing joke.

Your original claim, to remind you, (which I completely disagree with, as does Porsche), was that "the energy your brakes need to dissipate to decelerate from 140mph to 70mph is three times what you need to decelerate from 70mph to zero"

In reality, as my graphs show, all sorts of frictional losses occur at higher speeds which dissipate that kinetic energy faster than at lower speeds.

You are also criticizing my well researched referenced graphs (with three hugely different Cds) and then claiming there are other graphs for road cars disproving them, but not being able to find or link them...

Tell me you have a physics BSc rather than and Engineering BEng without telling me.....

Edited by Aphrabehn on Tuesday 2nd July 09:57

[Jimjimhim]

[herebebeasties]

Aphrabehn said:

Again, your pure physics assumptions remind me of the classic "perfect sphere travelling in a vacuum" physics/engineering horse racing joke.

Your original claim, to remind you, (which I completely disagree with, as does Porsche), was that "the energy your brakes need to dissipate to decelerate from 140mph to 70mph is three times what you need to decelerate from 70mph to zero"

In reality, as my graphs show, all sorts of frictional losses occur at higher speeds which dissipate that kinetic energy faster than at lower speeds.

You are also criticizing my well researched referenced graphs (with three hugely different Cds) and then claiming there are other graphs for road cars disproving them, but not being able to find or link them...

Tell me you have a physics BSc rather than and Engineering BEng without telling me.....

"All sorts of frictional losses" are rolling resistance and air resistance and, errr, that's it. And those are "not significant" compared to friction brakes. Why do you think they are? Have you never lifted off the throttle at say 100mph vs. hitting the brakes hard? Do you imagine that effect is suddenly ten times more pronounced at 150moh? Even without looking at the physics that just makes no sense.

I do not bear the burden of proof here to educate you, but seeing as you are being belligerent and won't just Google "stopping distances", here is a link to hundreds of graphs that show non-linear stopping distances: http://www.google.com/search?q=stopping+distances&...

E.g.



It's even in The Highway Code - https://assets.publishing.service.gov.uk/media/65f...

Why do you think they are non-linear with velocity? It's because kinetic energy is. If the energy required to decelerate a car from 2x to x were the same as from x to 0 then stopping distances would be linear. They are not,

Further proof this is all basic GCSE Physics - https://www.gcse.com/fm/braking_energy.htm

I have yet to come across a degree level engineer who doesn't believe in GCSE Physics, but whatever dude - you just wave your hands and believe whatever you want to.

Edited by herebebeasties on Wednesday 3rd July 00:33

[dcb]

herebebeasties said:

It's even in The Highway Code - https://assets.publishing.service.gov.uk/media/65f...

It's worth mentioning that stopping distances in the Highway Code
haven't changed in 55 or more years, so are *very* out of date.

While reaction times haven't changed much across the decades,
cars have a lot. Radial tyres, disc brakes & ABS systems to name just three.

Shame the distances don't get updated every ten years or so
with a current average car.

That would be far more useful, but this is the UK Gov Department
of Transport we are dealing here. Not folks to let reality get in the way ;-|

[Olivergt]

dcb said:

herebebeasties said:

It's even in The Highway Code - https://assets.publishing.service.gov.uk/media/65f...

It's worth mentioning that stopping distances in the Highway Code
haven't changed in 55 or more years, so are *very* out of date.

...

But the non linear relationship between the stopping distances has not changed.

[Capitan Obvio]

In Germany I find 200kph about the right cruising speed on quiet sections.

[cerb4.5lee]

dcb said:

herebebeasties said:

It's even in The Highway Code - https://assets.publishing.service.gov.uk/media/65f...

It's worth mentioning that stopping distances in the Highway Code
haven't changed in 55 or more years, so are *very* out of date.

While reaction times haven't changed much across the decades,
cars have a lot. Radial tyres, disc brakes & ABS systems to name just three.

Shame the distances don't get updated every ten years or so
with a current average car.

That would be far more useful, but this is the UK Gov Department
of Transport we are dealing here. Not folks to let reality get in the way ;-

This always niggles me as well. Brakes and tyres have improved and moved on so much over the years, yet the ancient stopping distances are still being used though.