Stopping in the wet.

Carawaystick

On Wednesday on Pat Kenny on Radio1, there was this, I think Driving Instructor( could ave been a retired cop) , lad saying in the wet at 50 kmh your stopping distance is 30m and you could fail a driving test for passsing a red light on your test.
Story was it takes more than 3 seconds to stop in the wet at 50 kmh. amber light sequence is only 3 seconds, so if you were approaching lights and they changed to amber, you could be still moving and pass the lights after they turn red.

I thought this was a load of testicles really. I am not able to see how
a) stopping distance in the wet at 50 kmh is 30m.
b) you couldn't stop from 50 kmh to zero in 3 sec.

Am I completely off base here or are "Average" cars so bad that this stopping distance still applies?

Are people still driving anglia's on cross ply tyres? is this the average?
Are these wet stopping distances measured on pure tar covered by rotten leaves?

Same guy was telling Pat about a guard giving evidence (which was acepted) about a 2+ second reaction time to a hazard. and the average reaction time is 0.7 seconds....


Considering this guy does 0-160-0 ( kmh in 9.4 sec)
http://www.youtube.com/watch?v=f7EymE54xLw

Blitzkrieger

I've always been skeptical of the stopping distances quoted in the rotr. I used to have a 91 Fiesta and once came to a stop from ~55mph in what I would estimate as 20m on a wet day. My new car has bigger discs, all around and a sports ABS system so it'll stop much faster. The Insititue of Advanced Motorists acknowledge that the braking distances quoted are too high, but they reckon that not enough time is allowed for reaction time. They estime the advised stopping distances are reasonably accurate in real life situations.

SouperComputer

Same guy was telling Pat about a guard giving evidence (which was acepted) about a 2+ second reaction time to a hazard. and the average reaction time is 0.7 seconds....

Reaction time can be interpeted in two ways:

One is the time it takes to realise/identify there is a problem (probably .7s is in an ideal situation without distractions.

Then the 2s figure (you could call it effective reaction time) is based on:

The total time it has taken to identify the hazard
Time taken to decide on the evasive action(s) (we'll say brake for simplicity)
The time it takes for the "MOVE!" message to get to your leg & foot to start braking.
Time it takes to release pressure off the accelerator pedal, assuming yoiu were there of course.
The time it takes for your leg/foot to physically move over to the brake pedal
The time it takes for your leg/foot to apply the pressure required.

Add it all up, and 2s is very possible, in fact on average, its probably generous.

Anan1

Remember also that while ABS will allow you to steer under braking, it won't necessarily stop you any quicker.

peasant

Anan1 wrote: »

Remember also that while ABS will allow you to steer under braking, it won't necessarily stop you any quicker.

On a really greasy road (wet leaves or snow) or on loose gravel it can actually increase your braking distance significantly

Anan1

peasant wrote: »

On a really greasy road (wet leaves or snow) or on loose gravel it can actually increase your braking distance significantly

Which, as we all know, is why the Urquattro had switchable ABS.;)

Idleater

Blitzkrieger wrote: »

I used to have a 91 Fiesta and once came to a stop from ~55mph in what I would estimate as 20m on a wet day.

even allowing for a Bullsh!t factor of 10%, an over reading factor of 10% and an internet exageration factor of 10 %, I find it hard to believe that you stopped a car going near 80kmph in 20meters, considering that at 80kmph you are covering 22.22222... meters each second.

Seth Brundle

I ride my brakes so that I stop more quickly

wil

Carawaystick wrote: »

On Wednesday on Pat Kenny on Radio1, there was this, I think Driving Instructor( could ave been a retired cop) ,

That IIRC was Frank Cullinane or similar name, was mentioned on a previous thread, also on another station with same spiel.
He was talking about peole gettng unfairly done breaking red lights.

I get the impression he is a bit of a one trick pony. I wish he would put as much effort in to highlighting the number of people who blatantly, dangerously, maniacally run the red lights long after them going red.
All he ends up doing is giving them more excuse to take even bigger chances and excuse themselves by the thinking they have an extra 3 seconds on top to do so now. He may have cause for a small number of cases but that is vastly outweighed by the number of homicidal maniacs out there who run red lights.:mad:

My wife was hit by one:mad::mad::mad: some years back and was lucky to be alive to tell the tale.

Spit62500

Anan1 wrote: »

Which, as we all know, is why the Urquattro had switchable ABS.;)

The switchable ABS was for snow driving - switching it off allows a wedge of snow to build under the locked wheel and reduces the braking distance.

Poppy Cock

Anan1 wrote: »

Which, as we all know, is why the Urquattro had switchable ABS.;)

Of course it will stop you quicker. More grip = quicker stop.

Blitzkrieger

nereid wrote: »

even allowing for a Bullsh!t factor of 10%, an over reading factor of 10% and an internet exageration factor of 10 %, I find it hard to believe that you stopped a car going near 80kmph in 20meters, considering that at 80kmph you are covering 22.22222... meters each second.

Well if you want to go measure it, it was outside the (shell?) garage you pass on the way in to Carraigiline if you're coming from the Douglas side. From about the centre of the wall to about 3ft from the end of the concrete at the petrol station exit. It's hardly impossible.

Anan1

Poppy Cock wrote: »

Of course it will stop you quicker. More grip = quicker stop.

Actually, no. More friction = quicker stop.

dudara

It doesn't matter how many figures we throw around, people will still drive up your hole at 100 kmph, even when lashing rain.

I try to drive well back in bad weather, leaving plenty stopping room, and there's always someone who will use it as an overtaking gap. ALWAYS.

cson

dudara wrote: »

It doesn't matter how many figures we throw around, people will still drive up your hole at 100 kmph, even when lashing rain.

I try to drive well back in bad weather, leaving plenty stopping room, and there's always someone who will use it as an overtaking gap. ALWAYS.

Absolutely loathe the **** that do that. What time does it save on your journey? **** all. I reckon its a psychological thing more than anything. Having an acute case of wankeritis

daedalus2097

Anan1 wrote: »

Actually, no. More friction = quicker stop.

And to embellish, once the wheel stops turning relative to the road, the friction changes from static friction to dynamic friction, and all of a sudden the amount of available friction drops dramatically... And the wheel skids. That's why ABS prevents a wheel from stopping completely.

Simpified of course

domania

Once had a very bad skid in the wet, and crashed the car. I aquaplaned and lost control of the car. Aquaplaning can be a big issue, even with ABS. Tyres have a lot to do with it. Since then I am a little more paranoid about what tyres I use. Will only buy specific rain tyres since then.

cson

I'm not much of a car-head so excuse my ignorance on this, but won't narrow tyres increase your chance of locking the wheels? Therefore is it better to have wider tyres?

I'm asking this because a mate of mine ran into the back of someone at the weekend (coming around a corner doing ~80kph *under the limit* and some fecker had just stopped to look into a field). He reckons that bigger tyres would have helped him stop. (The brakes locked on him)

domania

Not neccessarily. It's got to do with thread patterns and silica compounds etc. Cheap tyres are exactly that. Don't skimp on the rubber.

Anan1

daedalus2097 wrote: »

And to embellish, once the wheel stops turning relative to the road, the friction changes from static friction to dynamic friction, and all of a sudden the amount of available friction drops dramatically... And the wheel skids. That's why ABS prevents a wheel from stopping completely.

Simpified of course

On some surfaces, the coefficient of friction drops a lot when the wheel locks. On others, from memory, the difference is so small as to be outweighed by the cycling of the ABS.

SouperComputer

cson wrote: »

I'm not much of a car-head so excuse my ignorance on this, but won't narrow tyres increase your chance of locking the wheels? Therefore is it better to have wider tyres?

I'm asking this because a mate of mine ran into the back of someone at the weekend (coming around a corner doing ~80kph *under the limit* and some fecker had just stopped to look into a field). He reckons that bigger tyres would have helped him stop. (The brakes locked on him)

Its not true, its a bit of a blanket statement. It depends on the tyre compound, the road surface and the vertical pressure on the compound. This is dictated by the mass of the car, the thread pattern, suspension geometry and the tyre pressure.

If it was dry wider tyres "may" have helped him, but that depends on the exact tyres you are comparing ie the thread pattern in conjunction with the compound means that it may or may not provide more grip. A lot of the time it does, but not always and you cannot assume that it will. It depends on a lot of factors.

Put simply a wider tyre does not mean more grip, the tyre may use a harder compound and or have less contact with the road due to an aggressive thread pattern, negating the increase in width.

In the wet, more vertical pressure on the compund is desireable than in the dry to help the thread cut through the water and make contact with the road surface. This can be acheived through a narrower width tyre, a more aggressive (IE blocky-looking) thread pattern or a change in tyre pressure.

domania

Go buy Uniroyal Rallye 680 or Rallye 550 tyres the next time your due for a change. They are the business.

Stopped me from skidding in the wet when I had to break very hard because of some muppett in front of me and since then they are the only tyres I will buy.