Gauging speed by skids
Discussion
Went round to see my gf tonight and right outside her house a maxpower saxo (roof spoiler, blackened windows, etc.) had driven into the back of a vectra. Everyone in both cars were fine albeit the saxo driver had a cut on her chin.
Where it happened the road has a slightly downhill left/right bend (it's a 30mph limit in a residential area). The impact was enough to push the front of the saxo in by a good few inches. The vectra was stationary, just about to pull out around a parked car after letting another car past in the opposite direction. The saxo hit the vectra, which hit a rover (the first of the parked cars), which then hit a white van (the second parked car). A witness, about 100yds further back up the road, reckoned it was going 50-60mph as it passed them.
The saxo had left 84 feet (by my pacing) of rubber of the road...just curious if anyone knows how to work out how fast it may have been going.
Where it happened the road has a slightly downhill left/right bend (it's a 30mph limit in a residential area). The impact was enough to push the front of the saxo in by a good few inches. The vectra was stationary, just about to pull out around a parked car after letting another car past in the opposite direction. The saxo hit the vectra, which hit a rover (the first of the parked cars), which then hit a white van (the second parked car). A witness, about 100yds further back up the road, reckoned it was going 50-60mph as it passed them.
The saxo had left 84 feet (by my pacing) of rubber of the road...just curious if anyone knows how to work out how fast it may have been going.
tja said:
The saxo had left 84 feet (by my pacing) of rubber of the road...just curious if anyone knows how to work out how fast it may have been going.
A few quick estimates for you:
A good car on the flat could be expected to brake from 49mph to 10mph in 84 feet. (0.9g braking)
An average car on the flat could be expected to brake from 46mph to 10mph in 84 feet. (0.8g braking)
The downhill has a substantial effect. A 1 in 10 hill will reduce the effective G force by about 10%. Assuming a 1 in 10 hill the average car would be able to brake from 44mph to 10mph in 84 feet.
All in all, and assuming that the brakes were not faulty (like not braking on one or more wheels perhaps) We can reasonably guess that the speed at the start of braking was between 40 and 50mph. Don't forget that the impact speed and downward slope I've used are no more than sensible guesses.
See:
www.safespeed.org.uk/braking.html
Best Regards,
Paul Smith
Safe Speed
www.safespeed.org.uk
Skid to stop can be worked out very easily on a flat surface but the problems begin when for example one vehicle has crashed into another vehicle.
To get an accurate (for Court purposes) speed at impact is very involved and you need to work out the crumple zones on the vehicle and then the various degrees of intrusion.
However, as safespeed has already demonstrated, you can make a 'guestimate' to arrive at a speed.
Its always best in these cases to simply work out a 'skid to stop' speed and forgetting about the impact damage. This way you can say that the speed quoted is his absolute minimum.
Its normally the Coroner who wants to know the exact speed taking all the factors into account because of its involved nature and obvious expense.
We regularly get called down to examine skidmarks and even Police Officers are mistaken into thinking that very long marks = big speeds.
You would be amazed to see that a set of long skidmarks in real life at an accident scene where everyone is saying that 'he must have been doing 60-70' can work out to be about 40mph. It does happen time and time again.
To get an accurate (for Court purposes) speed at impact is very involved and you need to work out the crumple zones on the vehicle and then the various degrees of intrusion.
However, as safespeed has already demonstrated, you can make a 'guestimate' to arrive at a speed.
Its always best in these cases to simply work out a 'skid to stop' speed and forgetting about the impact damage. This way you can say that the speed quoted is his absolute minimum.
Its normally the Coroner who wants to know the exact speed taking all the factors into account because of its involved nature and obvious expense.
We regularly get called down to examine skidmarks and even Police Officers are mistaken into thinking that very long marks = big speeds.
You would be amazed to see that a set of long skidmarks in real life at an accident scene where everyone is saying that 'he must have been doing 60-70' can work out to be about 40mph. It does happen time and time again.
madcop said:
Very often both!
I once rolled a Mk3 Escort and as the car flipped (due to a tree stump that I hit) I left the front seat sideways.
The handbrake lever hit me right in the ass...
...the nice officer who attended the scene of the incident described my injury to the ambulance that he was calling as "rectal trauma"!
"I would suggest that you sit down" he told me at one point "but I can see it's punched a hole all the way through so maybe best not to..."
BTW the two cars that were following me all swore that I was doing less than the speed limit *AND* that the lorry had just pulled out in front of me!
Phil
safespeed said:Wouldn't the downhill increase the G force (gravity and all that). Surely the car would be accelerated downhill and retarded uphill by the Earth's gavity("and don't call me 'Shirley'"
... The downhill has a substantial effect. A 1 in 10 hill will reduce the effective G force by about 10%. Assuming a 1 in 10 hill the average car would be able to brake from 44mph to 10mph in 84 feet. ...
)? At least, that's what my physics classes all those years ago would suggest. Ready to be shot down (or up
). Streaky
streaky said:
... The downhill has a substantial effect. A 1 in 10 hill will reduce the effective G force by about 10%. Assuming a 1 in 10 hill the average car would be able to brake from 44mph to 10mph in 84 feet. ...
Wouldn't the downhill increase the G force (gravity and all that). Surely the car would be accelerated downhill and retarded uphill by the Earth's gavity("and don't call me 'Shirley'"
Ready to be shot down (or up
Streaky
Correct if we were talking about simple gravity effect. However, we were talking about g-force in retardation of the car's speed, ie. effect on the car and driver. Downhill takes longer to slow because of the G-force (simple gravity effect) is deducted from the retardation G-force.
I hope this makes sense to you, it did to me when I wrote it, but I'm not sure now
_Al_ said:
Do you mean the effect of gravity on the car is reduced, therefore its tyres don't press on the road so hard, therefore they skid easier and brake less?
It's just that the downhill causes the car to accelerate, which counters the effectiveness of the brakes.
Going uphill gravity causes a similar acceleration, but the effect here is to reduce the speed of the vehicle (you might call it negative acceleration), which enhances the effect of the brakes.
Best Regards,
Paul Smith
Safe Speed
www.safespeed.org.uk
safespeed said:Isn't that what I said? - Streaky
... It's just that the downhill causes the car to accelerate, which counters the effectiveness of the brakes.
Going uphill gravity causes a similar acceleration, but the effect here is to reduce the speed of the vehicle (you might call it negative acceleration), which enhances the effect of the brakes.
...
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