how effective is 'skid testing' by cops?

**bugjuice** · 3rd May 2005, 18:17

just watching the news now about an unfortunate accident in Ponsonby, Auckland. A pedestrian was hit by a Skyline. The news showed a cop driving down the road and skidding to try to figure how fast the Skyline was going.

I have often wondered how effective that is, bearing in mind that there's so many varibles to consider, is it even worth bothering?

The varibles I can think of off hand would be of the suspect vs cop car:

Tyres - different grip between cars, pressure, brand of tyre, wear on the tyre

The car - ok, so a Skyline and Holden could be similar in weight, but some cars are way heavier or lighter which would effect the stopping distance

Was the driver braking before the wheels started to lock up causing the skid?

Brakes - are they defective on the back, causing the front load more..

Just so many things to think about, I can't see the point in trying.. I even once saw (this is what got me on about it years and years ago) on TV, a huge old Rover 3200 trying to find the speed of a mini by its' skid marks. Gotta be kidin me..

not bashin cops here, just trying to understand how it could even be thought to be done. I can (at a realistic guess) imagine that it'd give a very rough idea which could be +/- 15kph or so of the impact/skid speed, but again, what about braking before the skid?

/rant

**scumdog** · 3rd May 2005, 18:21

Can't quote precise figures but was really surprised at how close the braking distances were in a variety of unrelated car, the results were put on a graph and it was easy to follow the speed/distance thing..

**Ixion** · 3rd May 2005, 18:25

But surely that is completely meaningless ? A (very) good driver - or one with ABS brakes - will stop without *any* skid marks. Once you are leaving skid marks you have less than optimal retardation.

So does that mean that a driver who is driving very fast but is very skilled , holds the brakes just under lock up point until just before impact, then locks up and skids, would be assumed to be going very slowly ?

**Motu** · 3rd May 2005, 18:28

After the Boi Racer ''accident awhile ago outside our place...the Cops blocked off the road and did some skid testing to try and get an idea of his speed...I hear they backed of at 120kph,so they know he was going way faster than that.I don't see they wasted their time...but they coulda asked me..I reckon 160kph.

**Sniper** · 3rd May 2005, 18:31

Hmmm, Im with Buggy on this one, way too many variables to consider

**tl_tub** · 3rd May 2005, 18:45

A device they use sometimes is a piece of tyre (from memory around 1/4-1/3), with a certain mass of concrete in it, tyre is pulled (with tread side down ofcourse) with a gauge attached to measure how much force is required.
Ofcourse the more scientific way is to test the surface using a british pendulum.

**madboy** · 3rd May 2005, 19:07

The data I've seen in various car mags over the years showed huge variances between cars. Falcons outbraking MX-5s, even though 500kg heavier (almost 50%) Falcons had ABS, the MX-5 tested didn't. Who woulda thought? And I know from personal experience that a performance tyre is not even remotely comparable to a budget tyre.

As BJ alluded to, some poorly maintained piece of junk with Jap import tyres (harder compound) vs a well maintained more modern vehicle with quality tyres... not a particularly accurate nor scientifically comparable result.

I'm assuming the serious crash units know a bit more of the science behind this and can actually do a decent job of allowing for the variables in their scientifically sound calculations. But it is always a big assumption to make that the cops are as on to it as they make out (not bashing, just commenting on my experience).

**marmel** · 3rd May 2005, 20:00

Next time you see a cop doing skid testing have a look to see if there is a very expensive looking piece of kit stuck to the inside of the windscreen. It is used to find out just what the friction rating of the surface is, they don't start laying down rubber and measuring it, that is just a by-product of testing the surface friction, it's all calculated electronically. I have been involved in measuring a few scenes and speed was normally calculated down to 93.73 etc by measuring the length of the original skidmarks and using a formula including the friction rating of the surface. ABS on vehicles does negate this however, you can't measure speed of a vehicle accident scene without skid marks....

**Skunk** · 3rd May 2005, 20:23

Originally Posted by marmel

Next time you see a cop doing skid testing have a look to see if there is a very expensive looking piece of kit stuck to the inside of the windscreen. It is used to find out just what the friction rating of the surface is, they don't start laying down rubber and measuring it, that is just a by-product of testing the surface friction, it's all calculated electronically. I have been involved in measuring a few scenes and speed was normally calculated down to 93.73 etc by measuring the length of the original skidmarks and using a formula including the friction rating of the surface. ABS on vehicles does negate this however, you can't measure speed of a vehicle accident scene without skid marks....

I was going to guess that they would be measuring the surface friction and apply a formula to that to work out the other cars speeds. Sort of geting a baseline to measure from to plug the variables into.
:unsure:

**marty** · 3rd May 2005, 21:12

but it's so much more fun slagging off the cops for doing their job than it is actually doing some research before opening one's un-informed gob

**madboy** · 3rd May 2005, 21:20

Originally Posted by marty

but it's so much more fun slagging off the cops for doing their job than it is actually doing some research before opening one's un-informed gob

Damn right - never let an opportunity to slag off the 5-0 go wasted!! (PT)

**ricksta** · 3rd May 2005, 21:22

i saw the fotage on the news... the witness said that he said that he saw the woman was hit and landed about 17-18 metres away... thats insane!

**StoneChucker** · 3rd May 2005, 23:15

Originally Posted by ricksta

i saw the fotage on the news... the witness said that he said that he saw the woman was hit and landed about 17-18 metres away... thats insane!

Didn't the witness (the guy) say 80 meteres? (He said it may have been 70).
When you think about it, 20 meters is not far really...

I'm sitting here thinking it must of been 18, not 80. But then, why would someone give two figures, so close together. I'm sure he said 80. Either way, thats a witness account, and people always exaggerate. (Like the fish I caught the other day, it was THIS big...)

I do know, that at motorway speeds, people can be catapulted MANY meters, possibly 100? SD, SC, Marty? Is this correct?

**bugjuice** · 3rd May 2005, 23:22

Originally Posted by marmel

Next time you see a cop doing skid testing have a look to see if there is a very expensive looking piece of kit stuck to the inside of the windscreen. It is used to find out just what the friction rating of the surface is, they don't start laying down rubber and measuring it, that is just a by-product of testing the surface friction, it's all calculated electronically. I have been involved in measuring a few scenes and speed was normally calculated down to 93.73 etc by measuring the length of the original skidmarks and using a formula including the friction rating of the surface. ABS on vehicles does negate this however, you can't measure speed of a vehicle accident scene without skid marks....

good call, but then that only takes off a few of the varible factors. There's still the effectiveness and condition of the tyres on the suspects car, the weight of the car, the braking before locking up etc...
I can see how it'd work, but I can still see how it'd be flawed..

SC, 80m is a fuk of a way to be bounced.. you'd be mush at that speed, surely? I can't remember what I heard myself, but even -20m would be more than enough to kill, and this is in a very busy road.. I'm thinking the 17-18m..