"Extreme" tailgating
Discussion
That's occurred to me bfore as well. If you were only a couple of inches away then the closing speed would probably be less than 1 mph, not enough to dent the bumper.
But then the bumpers would have to resist the slower compressive force as your car presses against the one in front.
On the other hand you'd be on your own anchors before the compression got too big.
Hmm, you could actually run some maths and work out how close you'd have to be, let's see ...
I'll be back!
>> Edited by JohnL on Monday 7th October 22:01
But then the bumpers would have to resist the slower compressive force as your car presses against the one in front.
On the other hand you'd be on your own anchors before the compression got too big.
Hmm, you could actually run some maths and work out how close you'd have to be, let's see ...
I'll be back! >> Edited by JohnL on Monday 7th October 22:01
OK I'm sure I've read that bumpers can sustain an impact of 3mph so lets take that as a starting point. That's about 1.33 metres/second (roughly 2.25mph = 1 m/s).
Let's assume that you're travelling at 80 mph which is about 35 m/s.
Let's also assume that you won't have time to brake before you hit the car in front.
Let's assume you're not on your bike ...
Hmm, mmm, ...
So first we need the time taken to decelerate from 35m/s to 33.67m/s.
Then we need the distance travelled by the car behind, which remains at 35m/s, in that time.
The first bit's the hard bit ... Ok , high school physics ... v=u+at (see www.xrefer.com/entry/214932 - I had to look it up
)
where
v is final velocity (33.67m/s)
u is inital velocity (35 m/s)
t is time in seconds, the answer that we want.
a is the rate of acceleration in m/s2. Haven't got a clue what that's going to be, let's make some gross approximations (ie total stab in the dark guesses
)
From this load of sh!te I find the official highway code stopping distances: www.roadsafetyuk.co.uk/speed2.htm#dist which tells me that the stopping distance for a Morris Marina (or something) at 80mph is 121m.
So to approximate the deceleration rate:
I'm going to arbitrarily take a figure of 2/3 of that which is 81m.
Now another useful formula is v2=u2+2as, or to twist it around, a=(v2-u2)/2s. s here is the distance, ie 81 metres. v=0m/s, u=35m/s, a is what we want to know. So a=(0x0 - 35x35)/(2x81) = -7.6m/s2. (The minus means it's slowing down). Well that's a starting point, and it is around 0.75g which sounds credible. Air resistance is higher at high speed and lower at low speeds so in theory deceleration will be higher at high speeds - but I'm going to call that neglible for this purpose (consider: how long would it take to coast to a stop with no rolling resistance? A: Fcuking ages) The decelerative force will be constant (ish) from the brakes, and accleration (or deceleration) is dircetly proportional to mass and to force, so ... I'm going to take that -7.6m/s2 as the acceleration to use in the above equation.
So now we have:
v=33.67; u=35; a=-7.6; t=the answer.
v=u+at, or t=(v-u)/a
so t=(33.67-35)/(-7.6)=0.175 seconds.
And the car behind travelling at 35m/s will cover 35*0.175 = 6.1m in that time so you have to be a surprisingly big 6.1m (a car length or so) or closer for this to work.
You know that was actually quite fun
Anyone care to try it?
>> Edited by JohnL on Monday 7th October 23:02
Let's assume that you're travelling at 80 mph which is about 35 m/s.
Let's also assume that you won't have time to brake before you hit the car in front.
Let's assume you're not on your bike ...
Hmm, mmm, ...
So first we need the time taken to decelerate from 35m/s to 33.67m/s.
Then we need the distance travelled by the car behind, which remains at 35m/s, in that time.
The first bit's the hard bit ... Ok , high school physics ... v=u+at (see www.xrefer.com/entry/214932 - I had to look it up
) where
v is final velocity (33.67m/s)
u is inital velocity (35 m/s)
t is time in seconds, the answer that we want.
a is the rate of acceleration in m/s2. Haven't got a clue what that's going to be, let's make some gross approximations (ie total stab in the dark guesses
) From this load of sh!te I find the official highway code stopping distances: www.roadsafetyuk.co.uk/speed2.htm#dist which tells me that the stopping distance for a Morris Marina (or something) at 80mph is 121m.
So to approximate the deceleration rate:
I'm going to arbitrarily take a figure of 2/3 of that which is 81m.
Now another useful formula is v2=u2+2as, or to twist it around, a=(v2-u2)/2s. s here is the distance, ie 81 metres. v=0m/s, u=35m/s, a is what we want to know. So a=(0x0 - 35x35)/(2x81) = -7.6m/s2. (The minus means it's slowing down). Well that's a starting point, and it is around 0.75g which sounds credible. Air resistance is higher at high speed and lower at low speeds so in theory deceleration will be higher at high speeds - but I'm going to call that neglible for this purpose (consider: how long would it take to coast to a stop with no rolling resistance? A: Fcuking ages) The decelerative force will be constant (ish) from the brakes, and accleration (or deceleration) is dircetly proportional to mass and to force, so ... I'm going to take that -7.6m/s2 as the acceleration to use in the above equation.
So now we have:
v=33.67; u=35; a=-7.6; t=the answer.
v=u+at, or t=(v-u)/a
so t=(33.67-35)/(-7.6)=0.175 seconds.
And the car behind travelling at 35m/s will cover 35*0.175 = 6.1m in that time so you have to be a surprisingly big 6.1m (a car length or so) or closer for this to work.
You know that was actually quite fun
Anyone care to try it?
>> Edited by JohnL on Monday 7th October 23:02
Now what really is interesting (well a bit) is that that road safety website spends most of its time saying that low speed limit areas are where most accidents occur, and that 70mph zones have fewest deaths and serious casulaties by a long way, with 60 mph not far behind, and 30 mph zones being higher than the other two put together. But speed kills, boys and girls, and don't forget it!
quote:
John where do you liveive got my old BX on the drive we could experiment with
A few too many miles away to make it easy
. Still, we could find an abandoned airfield somewhere in between (Manchester's probably halfway ) - We'd need a second car though ... Best I could offer is my soon-to-be-on-the-road-for-a-grand-total-of-less-than-£300 MR2
, but I want to keep it on the road ... >> Edited by JohnL on Monday 7th October 23:18
My instinct tells me someone who doesn't spend their time writing the prose of an iron fairy and hence is more in touch with their Newtonian Mechanics (ie my antithesis) is going to debunk you.
I just cannot believe that 6 metres is a realistic figure.. my instint says half a metre.
I may well be wrong, but I have a good instinctive sense about these things sometimes..
Trust me.. I'm a Docked Awe.
I just cannot believe that 6 metres is a realistic figure.. my instint says half a metre.
I may well be wrong, but I have a good instinctive sense about these things sometimes..
Trust me.. I'm a Docked Awe.
[even-nerdier-mode]
Think you've calculated how far the two cars will move before they impact, rather than the initial separation between them. It's all relative, see?
Acceleration/time figures are about right (I think you could count on 1G under braking, but .76 is close enough). Using half-A-T-squared with A=7.6, T=0.175 I make it a smidge over 0.23 meters travelled. Which feels about right, imagine accelerating hard from a standing start, and smacking a wall eight inches later. You'd hit at a few mph, that's the sort of thing we're talking about. A gap of 0.23 meters would take about seven milliseconds to cover at 35 ms-1.
[/even-nerdier-mode]
God I can't believe I actually sat down and worked that out. Still, it beats working eh? Night all! :wave:
Peter Humphries (and a green V8S)
Think you've calculated how far the two cars will move before they impact, rather than the initial separation between them. It's all relative, see?
Acceleration/time figures are about right (I think you could count on 1G under braking, but .76 is close enough). Using half-A-T-squared with A=7.6, T=0.175 I make it a smidge over 0.23 meters travelled. Which feels about right, imagine accelerating hard from a standing start, and smacking a wall eight inches later. You'd hit at a few mph, that's the sort of thing we're talking about. A gap of 0.23 meters would take about seven milliseconds to cover at 35 ms-1.
[/even-nerdier-mode]
God I can't believe I actually sat down and worked that out. Still, it beats working eh? Night all! :wave:
Peter Humphries (and a green V8S)
quote:
That's occurred to me bfore as well. If you were only a couple of inches away then the closing speed would probably be less than 1 mph, not enough to dent the bumper.
I agree that if they're so close to the back of you, they (the numpty tail-gaiting you) probably wont cause that much dammage to your car.
*BUT* surely the principle of tail gaiting being dangerous is that if a kid jumps out infront of your car and you've got someone 1 inch from your back, they aint gonna have the time to react and brake - they are gonna hit your car. The force of their car hitting yours is gonna nudge your car further than i otherwise would have taken. Basically they're gonna push you into the kid, when the poor blighter should have lived.
There is no excuse for tail gaiting, you watch people tailgaiting each other, they never get to where they are going any faster.
NUMPTIES 
John, the maths work. At 80mph, a gap of 6.4m gives a 3mph impact speed.
3mph is a fair whack, though. While at skool, the police brought a sort of sledge thing that was propelled against a wall at 3mph. The idea was to demonstrate how important seatbelts are. 3mph was enough of a whack to throw you forward into your belts with some force. Enough force, easily to unsettle the car in front and start a News at Ten worthy pile-up. Imagine driving your car (or better still someone else's) into a wall at walking speed...
So, 1mph might be a better impact speed (having watched too many American films as a kid, 1mph seems to be a good ramming speed anyway).
To generate that impact, the gap required is 2.1m. That's quite close. Most of us wouldn't park less than 70cm from the car infront. So, 80mph at 3 times parking seperation. Works for me.
3mph is a fair whack, though. While at skool, the police brought a sort of sledge thing that was propelled against a wall at 3mph. The idea was to demonstrate how important seatbelts are. 3mph was enough of a whack to throw you forward into your belts with some force. Enough force, easily to unsettle the car in front and start a News at Ten worthy pile-up. Imagine driving your car (or better still someone else's) into a wall at walking speed...
So, 1mph might be a better impact speed (having watched too many American films as a kid, 1mph seems to be a good ramming speed anyway).
To generate that impact, the gap required is 2.1m. That's quite close. Most of us wouldn't park less than 70cm from the car infront. So, 80mph at 3 times parking seperation. Works for me.
Gassing Station | General Gassing [Archive] | Top of Page | What's New | My Stuff