Skid Marks

2008 ZX10R Owner · Derestricted Danger Joined: 05 Aug 2008 Karma :

How do the police calculate the speed you were going, or how much speed you managed to scrub off, from skid marks left in the road???
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deanoet · World Chat Champion Joined: 04 Jul 2007 Karma :

i would guess at:

same car / bike, same surface, same tyres, same pressures
go 30mph slam on, till stop, measure the marks left behind, work it out from that

You can calculate force of impact from damage caused to vehicle or what you hit. With that you can work out the speed you were going when you hit
measure the length of the skidmarks and using the same method as above you can work out how much speed was lost during the slide

Can prob be done using calculations and witchcraft to make it easier, but i would guess at that being the general method used
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0ddball · World Chat Champion Joined: 15 Jul 2005 Karma :

We could tell you but then we'd have to kill you Twisted Evil

TUG · World Chat Champion Joined: 12 May 2007 Karma :

Takes you alot longer to slow down the faster your going, so if you lock a wheel, then they measure that mark to calculate a rough estimate of speed.
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MarJay · But it's British! Joined: 15 Sep 2003 Karma :

Wafer_Thin_Ham · Super Spammer Joined: 18 Nov 2005 Karma :

Noxious89123 · World Chat Champion Joined: 10 Jun 2007 Karma :

Makes you wonder how this works... There's far too many variables that would affect the length of a skidmark. The tyre itself. Grippy sports tyre, or hard tourer. Tyre pressure. Load/luggage on the bike / car. How fast you were going when the wheel locked. How fast you were going when you unlocked the wheel. Etc. Confused

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Wafer_Thin_Ham · Super Spammer Joined: 18 Nov 2005 Karma :

They can still estimate the speed, which is good enough for the courts I guess.
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bazza · World Chat Champion Joined: 27 Aug 2004 Karma :

plugger147 · World Chat Champion Joined: 13 Mar 2005 Karma :

I don't think its as accurate as they make out, during my brothers court case we listened to both defense and prosecution accident experts and both disagreed with what the traffic officer had said in terms of speed and how he'd lost control of the car.
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c-m · World Chat Champion Joined: 12 May 2006 Karma :

don't most new cars have ABS which would mean no skidmarks
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AngelGrinder · World Chat Champion Joined: 09 Jun 2007 Karma :

When you do think about, it does seem very unreliable....

As mentioned, if your using road-legal racing tyres, your going to stop alot quicker, giving alot shorter skid.

Also, if your braking progressively, then panic brake at the last second....you'll have a short skid...

Another time, you may panic, break hard and lock your brakes...leaving you skidding a long distance....

Hmmmm....
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NiteMare · Spanner Monkey Joined: 12 Sep 2008 Karma :

my citroen bx will skid at almost the slightest hit on the brakes, you really cannot afford to panic brake in these cars as the brakes are operated by opening a valve in a pre-pressurised system....

it's a horrendous braking system that takes quite a bit of getting used to, as once the brakes lock the car appears to lose no speed at all and just keeps sliding
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Dav1son · Trackday Trickster Joined: 24 Jun 2006 Karma :

qpop · Nova Slayer Joined: 11 Aug 2008 Karma :

You learn about it in A level maths/physics...

If you know the co-efficient of friction on the road, you can calculate a rough initial speed for a vehicle. If anything the calculations give underestimates, rather than overestimates, due to the fact that braking is usually done before the skid is entered, as well, and other similar factors
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Vinyardo · Scooby Slapper Joined: 16 Sep 2008 Karma :

Skudd · Super Spammer Joined: 01 Oct 2006 Karma :

I would think that they use the skid marks as a guide and not as an accurate to two decimal points measurement.

I used to be able to lock the rear for over a hundred yards at relatively low speed but would indicate a higher speed just by not using the front brake, but at higher speed and greater use of the front brake the rear would lock and drag, unlike a car that digs and pushes. But i'm sure there is some boffin at Police headequaters that has thought of this.

I now have dual linked brakes with ABS so can't play anymore.
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