Adam M

i know this is being discussed but I wanted to try something, and some others too.

can you measure your stopping distance from 30 mph, ignoring reaction time.

I have done this by braking as hard as possible (after checking there is nothing around let alone behind me) on a flat road, using a lamp post as my marker to see how many car lengths it takes to stop.

Assuming everyone is careful, would you consider trying this and posting your results in terms of impreza lengths!

I am certain, I had stopped without abs on a dry day in less than a cars length from 30 mph, but, everyone in the room shouted bollocks when I disclosed this last night.

just want to get an average, thats all.

edited due to chucks point!

[This message has been edited by Adam M (edited 28 June 2001).]

chuckster

What's a flat hill?
What do you call a boomerang that doesn't come back?
C

Robertio

iTrader: (1)

chuckster, broken

I'd suggest a whole lot more than a car length, I have no problem trying this out, will get back to you.

BTW is this from the brakes being cold, or from normal operating temperature?

barge

What do you call a boomerang that doesn't come back?


A Stick.

I think the advert was a bit over the top, but if it stops people speeding in residential areas & past Schools then it OK by me.

chuckster

Well, I'd call it a stick Robertio, and I wouldn't call a flat hill a hill either LOL (enough smilies?)
Brake question, good one, kid could run out on you at any time, so better do it cold, as if you'd just pulled out of your drive and started off.
Chuck

SDB

Adam... it may be possible to do it..

I have no idea to be honest, but..

There is no such thing as a uniform stopping distance.

It depends mainly on your tyres, the height of the centre of gravity and your brake set-up..

In general stiff suspension and things like anti-dive will *increase* your stopping distance due to a high percentage of the weight being transfered to the front tyres.

In addition, wider tyres *can* *increase* your stopping distance as the create a contact patch with is wider than it is long. This is actually more complicated than that, as wider tyres spread the load throughout the tread better, meaning they can cope with longer periods of intense demand.

The major way to *reduce* your stopping distance are...

- Lower your car (a lower centre of gravity reduces the amount of weight that moves in the direction of inertia).

- Improve your tyres. The important things are the correct tyre pressures, and high friction co-efficient.

- Soften your suspension (within reason).

- Reduce anti-dive.

- Set-up the brake bias so the rears are doing as much work as safely possible (this is impossible to be set-up to be accurate in all sitations) - for instance, you want *more* brakes at the rear when it's wet (people often think it's the other way around) as there is less weight transfer.

- Learn to accurately left-foot-brake in a RWD car and set-up some SERIOUS rear biased brakes. That way you can modulate the braking on the rears by applying throttle to maintain as close to perfect braking on both end all the way through the braking zone (this one is VERY difficult!! )

Cheers

Simon

(PS sorry for the long one)

[This message has been edited by Simon de Banke (edited 28 June 2001).]

MorayMackenzie

Chuckster,

A stick?

Moray

SDB

Just thought...

also..

get your bump-steer removed!

and following from that..

I forgot to say... everything in car set-ups is a trade off...

So doing any of the above things (apart from improving your tyres) will have a negative affect on something else.

Cheers

Simon

MatthewC

Simon,
Please explain why wider tyres slow down your slowing down ability.

Just AFI.

SDB

Hi Matthew..

like I said "they *CAN* increase your stopping distance"... so this is not a concrete rule as tyres are INCREDIBLY complex things but..

The things that create grip on a tyre are mechanical locking and chemical locking at the road / tyre interface...

The better the tyre compound, the higher the efficiency of that locking...

Each square inch of rubber that is touching the road, causes some grip...

the more weight you press onto that square inch, the more grip it produces...

If the weight on the tyre is constant, then the larger the contact patch area, the lower the weight pressed onto each square inch, but the same amount of weight will be distributed throughout the contact patch...

OK... (still awake??? )...

this is (in VERY VERY VERY simple terms) what creats overall grip...

The shape of the patch is what dictates the lateral and longitudinal characteristics of the tyres performance. So the longer the patch, the more longitudinal traction is has, and the wider the patch the more lateral grip it has.

*** Like I said, there is MUCH more to it than that, but this is enough for what we're discussing ***

Widening the tyre *in general* shortens the longitudinal size of the contact patch and increases the lateral size of the patch.

This means that you increase the lateral grip (in general) and reduce (slightly) the longitudinal grip.

The reduction is likely to be VERY small indeed (it at all)... but I thought it was worth mentioning, as there are so many mis-understood aspects of chassis, tyre and suspension dynamics, including the thought that big fat tyres give you more grip.

Cheers

Simon

[This message has been edited by Simon de Banke (edited 28 June 2001).]

Adam M

simon,

I am not a statistician, and I appreciate there will be a range of values relating to this braking distance.

i just wanted a ballpark figure, as from the tv advert it appears that the car is taking several car lengths to stop.

I know it is to all intents and purposes designed to exagerrate the problem, I just want to know how unrealistic it is. for this, I would require an average. Since I only have one impreza, the benefits of other peoples experience would be useful.

Thanks in advance to anyone who tries this, but please be careful if you do!

Puff The Magic Wagon!

iTrader: (2)

I'm going to measure the distance from the telegraph pole to the 30mph limit on the approach to the local town (don't worry the road is straight & slightly downhill for 3/4 mile & the 30 sign is generously positioned away from any houses, curbs etc & it is a v.wide and empty road). I habitually use this marker to brake from 60 to the required 30 with no problem (when conditions allow). I estimate it to be no more than 30ft.

It will be interesting to see then from 30 to 0 at the same point & measure the difference.

I must admit that I am quite sad in wishing to fulfil this test & freely admit that it doesn't account for thinking/reaction time.

Also, I would say that our Vectra's stopping ability at the same point is greater than PTMW!s Vectra is a GLS TD with bog-standard high-profile tyres on steel wheels & standard brakes PTMW! wins when on track at full operating temperature for lap after lap

Kempo

Simon
I liked your explanation of the factors affecting grip and (boringly) the other day I was sat on a train thinking back to my A-level physics days. Friction=u(constant) x R(reaction) - the reaction being equal to the weight, therefore this is independent of the surface area in contact with the road.

This made me think how come a fiesta with 135 tyres can't corner the same as a scooby on 205s. I could only reason this was due to chassis / suspension factors that come into play at the point of "slip".

I would love to see a description of the factors affecting tyre grip, friction etc.

Do you know of any?

Cheers
Paul

SDB

wowzers!!

it's a MASSIVE subject and there are many aspects of tyre dynamics that we just don't understand (although we understand most of the important stuff).

It is SO complex, that even the most knowledgeable people fall over themselves with gaps in their knowledge which have been filled with tiny mistakes, etc.

For instance..

there was a link posted on here the other day to a VERY good article about tyres which proudly refuted a few myths about tyres.

There were a few ****-ups there also. lol

but I'll give you the broad strokes (that I know of)...

** In theory (bare in mind this all goes out the window when you consider bumps, little dimples in the road, slightly loose surfaces, wet roads, etc, etc, etc) ** ....

A single tyre's grip is a function of the friction co-efficient of the contact patch and the road, the area of the contact patch, and the weight on the contact patch.

The amount of lateral force produced by that single tyre is a function of the slip angle (as a graph plotted against lateral force) and the grip (see above) - (including the weight on the tyre).

Therefore...
The more weight you put on a tyre, the more lateral force it produces. BUT... the increase in lateral force diminishes the more weight you put on. So eventually you saturate the tyre and it can produce no more grip no matter how much more weight you put on it (in fact it would do all sorts of weird things!! - like explode probably!)

SO...
With this dimishing returns scenario, it is more important to "keep weight on" a tyre than it is to "transfer weight ONTO" a tyre. not sure if I've made that clear?

When you corner, you move weight from the inside to the outside.. and probably either from front to back (on the gas) or back to front (off the gas / on the brakes)..

If you take 10lbs of weight off the left and put it onto the right, you will reduce the grip on the left more than you will increase the grip on the right...

SO...

Now onto chassis dynamics..
The way you make a car go round corners quickly is by sharing the weight as evenly as possible..

ARB's do EXACTLY the oposite of this. They take weight OFF of the inside wheels and move it ONTO the outside wheels. (Most people think it's the other way round).

This means that they are making matters worse then surely???

well in some (probably most) case yes!.. but..

it's all WAY more complicated than that...

When a car rolls, the tyre rolls also, meaning the contact patch is narrowed as the tyre rolls onto it's crappy useless edge. If the reduction in useful contact patch is enough to make more of a differnce than the saving of grip due to better weight distribution then the ARB would be a benefit.

BUT...

its EVEN MORE complicated than THAT!! ..

in fact, I'll stop there, as I could go on for hours and hours and not even scratch the surface of the stuff *I* know!! and THAT is not even scratching the surface of the stuff a couple of the chassis engineers I have worked with know, and they readily admit that they have only scratch the surface in terms of the entire chassis dynamics deal.

These are guys who have spent their life on it!!!

It's a BLOODY big subject!

Cheers

Simon

[This message has been edited by Simon de Banke (edited 28 June 2001).]

CharliePsycho

I saw the ad as well... The car has locked it's front brakes, so all bets are off as everything Simon has mentioned above only applies to tyres that are not sliding.

Once you are sliding you are at the whim of inertia, not tyre dynamics

...c

SDB

LOL

good point.. although I assume the people testing this will not be locking their wheels? Certainly wouldn't be the best way to measure stopping distance!

BTW - A locked tyre still has laws of dynamics associated with it, but you are dead right.. the stuff I mentioned is all about non-locked tyres..

see.. it just KEEPS getting more complex!!

GranTurismo

Hello,

I dont want to be nasty, but the fact that someone thinks that it takes a car length to stop at 30mph is worrying.

I have seen a demo by a police driver how asked people to put cones out where they thought a polcie sierra wouls stop (over 10 years ago when I was at school) it was more like 6 car lengths.

Obviously a scooby stops quicker than a rage rover, but work to the lowest common denomiter and you cant go far wrong.

Also the way you drive in a Test is not the same way as you drive if something happens in real life, so Cadence braking goes out the window (unless you are Mr VeryCoolPerson)

So just slow down in built up areas and leave a big gap. You wont get there any slower.

GranTurismo

Next time you drive on the motorway and a bit of tread from a blown out lorry tire suddenly appears from under the car in front of you.... try imagining that it was a brick, plank, oil drum etc.

Thatlll make you leave a much bigger gap!

Use MY 2 Second Rule it only takes 2 seconds to say "Only a TW*T Drives closer than That"

Bye,

Robertio

iTrader: (1)

OK, tried this morning, conditions were not quite as required, but there is sod all I can do about the weather so:

Road: Damp / Bumpy / Reasonable Camber
Car: Std MY00 with 17"
Tyres: Toyo Proxies (Just about still legal)
Brakes: Almost cold

Avg Stopping Distance: 3 car lengths

Based on the average of 2 runs, on a slight gradient, one run down hill: 3 1/2 car lengths, 1 run up hill: 2 1/2 car lengths.

Run based on car traveling at a constant 30mph and just standing on the brake pedal from a designated marker.

NB ABS working almost constently from 30.

Will try and do a dry run on a level road at the weekend, weather permitting

GaryC

One thing to remember, all other things being equal, ABS will increase stopping distances in the dry. A skidding tyre on dry road (assuming its not dusty/'gravelly' etc) WILL be more effective than the same tyre on the same car with ABS.

Only in wet/slippy/poor friction surfaces will ABS hold the advantage.

Likewise, the balder the tyre, the better the stopping distance in the dry (again all other things being equal)!


Saw the above demonstrated at MIRA several years ago, did make for interesting viewing. They had a BMW 3-series and disconnented then re-connected the ABS with the same tyres, but with differing levels of tread and on different surfaces. Conclusion was that bald tyres and no ABS will reduce braking distance by up to 25% compared to the same car with ABS on the same tyres but fully 'treadded'!!!

mutant_matt

Adam,

Only slightly related but when I was doing my Advanced Bike course (to pass the test), we did some training in a playground to improve some bike handling skills.

One of the tests involved emergency stops. It was wet and it was raining and after a few practice runs I was stopping my bike (Thunderace, 198kg's with good brakes) from 30mph in about 4-5 metres. I had the front wheel juddering/slipping, was cadence braking and had the rear wheel in the air (I *was* trying and don't think you can stop a bike quicker than that!!!) This really opened my eyes to how quickly you can stop when you really have to - I have since carried this out in the dry and the best stopping distance I can achieve is about 3-4 meters.

I will try this with the Scoob this weekend as I'm curious now...

Matt

SDB

Hi Gary

I would expect that this was a long time ago?

Modern 4 Channel ABS will stop a car on dry tarmac quicker than a human bein could in almost all circumstances.

In addition, the retarding forces generated by a locked tyre are substantially (dramatically) less than a tyre which exhibits it's ideal % slip (breaking on the verge of locking).

ABS is not perfect, but what it DOES do is allow all 4 wheels to brake at their (approx) maximum. This is far from possible in a non ABS car as the disparity between the front and rear grip (and therefore the demand they can safely put on the road) changes constantly.

If it was within the last 4 / 5 years, was there anything else unusual about the test? - strange tyres / unusual surface / etc ?

Cheers

Simon

DavidBrown

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:<HR>Originally posted by GaryC:
<B>One thing to remember, all other things being equal, ABS will increase stopping distances in the dry. A skidding tyre on dry road (assuming its not dusty/'gravelly' etc) WILL be more effective than the same tyre on the same car with ABS.

[/quote]

Is this now believed to be somewhat of an old-wives tale. Here's an extract of what resident expert Simon says:

<B> <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:<HR>In terms of ABS...
...
modern 4 channel ABS will stop a car (as much as I hate to say it) quicker than any human being could in almost all circumstances.

The only circumstances where this would not be true would be on a pool table smooth surface with PERFECT brakes and tyres, and an INCREDIBLY talented driver using left-foot braking in a RWD car tuned to exibit lots of static rear brake bias.

It's a testiment to modern technology, and an indication that many skilled drivers are living in the past, or simply quoting something they hear 10 years ago (when it was still true).[/quote]

and I aint gonna argue with him.

DavidBrown

DOH !!

Seems like SdB and I jumped on that at the same time

Ignore.

GaryC

Trying to work out when it was....? It probably was at least 5 years ago, possibly more..... where does time go

I can remember thinking it was unbelievable. Not doubting the advances in ABS though Lets face it in a modern car, the wheels don't lock (or even trigger the ABS) in the dry no matter how hard you brake Unless your name is Stef, and you are on track

The rationale behind it at the time was that the friction (and thus slowing effect) caused by rubber sliding on tarmac was greater than the friction caused by the car's brakes going 'on and off' through the ABS.

Getting back on topic - the advert does use an old-ish car, and seems to slide a goddam long way - obviously just accentuating to prove a point


When will they show similar adverts but warning pedestrians about the dangers of walking out in front of moving cars though

BarryK

EEK! The length of an Impreza, what 12 feet ish. Approx the length of a snooker table.

Surely you weren't seriously suggesting that under emergency conditions, ie child/bike/fool walking in front of you that any vehicle would go from 30 to stationary in 12 feet? You'd have travelled nearly that far before your foot hit the brake pedal.

Oh my God, no wonder they're going bonkers about speeding.

Just expect it will take a lot longer than you think and drive with some respect for other people's space. Around every corner is an accident waiting to happen.

Everyone is not driving to work or wherever in the belief that they are taking part in a race. I personally couldn't care less who passes/razzes me at the lights or whatever.

What does get my goat is ditherers doing sub 30 on NSL roads who toot the horn and flash their lights when you overtake them.

No judgment drivers.

Ahhh, that's better...............

astraboy

The astra dont have ABS, however, I regularly take it up to the gravel carparks in Chobham common and practice my cadance braking. Mercifully, I have never had to try it for real as I would have to make a choice of mounting the pavement (******** me alloys) and having a possible head on. Neither appeal much, but I still parctice. Its quite good fun to lock em up, keep sliding, turn and release the brakes. The car shoots off to the side surprisingly quickly. Hope I never have to use it, mind.
astraboy.

SDB

It is amazing how bad our judgement of distance is when we're on the motorway, or in fairly fast moving traffic.

I used to do a lot of miles, and used give myself a reality check by picking a point as the car in front passed it and imagining that was the point I started to brake (if it hit something, etc) and often ended up almost doubling the distance between me and the car in-front. Quite scary!

robski

Simon,

dont start me on fast road distances, whenever I come across those marked sections I nearly **** myself on how far apart they say you should be.

Backing Adam up here, he said the braking distance, not the braking plus thinking time.
Lets face it, some people take so long to make (or not make) decisions they coul dbe in the next county before their brain decides they need to brake.

Again, it must be stated, speed limits are based on lowest commen denominator. Poor drivers in poor cars.

robski

Robertio

iTrader: (1)

Adam can correct me if I'm wrong, but I assume he had started braking when the front of the car passed a marker, then having come to a stop found there to be a 1 car gap fronm the back of his car to the braking point, quoted this without cosidering the length of the car on top of this - so making the actual stopping distance 2 car lengths.
This should be just about possible in good conditiond I think.