70MPH? The immediate memory is 1967 in the Old Man`s morris traveller.
Time seemed to depart from its Newtonian perception as a linear event. In fact I thought of lots of things and watched motes of dust float slowly in the cabin as the stationary cars in front got slowly closer.
(funny how sound seemed to diminish too, or is that just the memory?)
A *very* near thing and if time could slow like that again, I would now still be 17 and not pushing 60.
|
The "official" braking distance in the Highway Code is 70 ft Thinking + 245 ft Braking = 315 ft overall stopping distance. Arguably significantly less in these days of ventilated dual circuit disc brakes and EBA (ABS doesn't shorten the distance, of course!), but one basic law of physics which, it seems, will never penetrate certain drivers' brains - mainly because it is never taught in the first place! - is that DOUBLE the speed means QUADRUPLE the braking distance.
Add a dose of alcohol, smattering of drugs, some thumpy bumpy music, a mobile phone conversation and a talkative passenger and it's a wonder how some cars ever manage to stop at all!
|
>>Arguably significantly less in these days of ventilated dual circuit disc brakes and EBA
I don't really think so;
See
www.honestjohn.co.uk/forum/post/index.htm?t=56041&...e
and
www.honestjohn.co.uk/forum/post/index.htm?t=56041&...e
Putting in the numbers for 70 mph gives;
vmph=70; vmps=vmph*(1609.3/3600); mu=[0.7 1]; g=9.81; d_metres=(vmps^2)./(2.*mu.*g)
d_metres =
71.2965 49.9075
d_feet=d_metres*(1000/(25.4*12))
d_feet =
233.9123 163.7386
If you have a co-efficient of friction of 0.7, the fastest you could stop is in 234 feet + thinking distance reducing to 164 feet if you have a co-efficient of friction of 1.
>>is that DOUBLE the speed means QUADRUPLE the braking distance.
Agreed - that's the vmps^2 in the equation.
Number_Cruncher
|
If you have a co-efficient of friction of 0.7 the fastest you could stop is in 234 feet + thinking distance reducing to 164 feet if you have a co-efficient of friction of 1.
I think the maths needs checking. Edmunds.com in the US recorded 60mph-0 in 114.5 ft for a 2007 Impreza STi and a remarkable 106 ft for the 2008 model. I presume this does not include any thinking distance but a whole lot less than the highway code says.
|
There's nowt wrong with the maths - they *must* have been using some very sticky tyres in absolutely perfect conditions. The numbers you quote will not be attainable in day to day conditions using ordinary tyres on British roads.
Number_Cruncher
|
|
Standard tyres on ordinary dry asphalt. And the maths says 164 ft at a coefficient of friction of 1. Surely you can't have a c of f more than 1, or have Subaru achieved it? ;-)
Edited by nick on 18/12/2007 at 14:09
|
vmph=60; vmps=vmph*(1609.3/3600); d_feet=[114.5 106]; d_metres=d_feet*((25.4*12)/1000); g=9.81; mu=(vmps^2)./(2*g.*d_metres)
mu =
1.0506 1.1349
They are getting a coefficient of friction of more than one - either sticky tyres and good tarmac, or, they are getting some downforce when travelling quickly.
Either way, this isn't something you could go out and achieve in todays road conditions.
Number_Cruncher
|
The other possibility is that the measurement is flawed - were they really doing 60? relying on the in car speedo? How were they sure exactly where the braking began?
At 60, you are doing 26 metres per second, so a small error in timing / placing the start of braking can have a large effect.
Number_Cruncher
|
|
|
Surely the range for a coefficient of friction (or anything else for that matter) is between 0 and 1? Less than 0 or greater than 1 is by definition impossible? Or is my memory of a physics degree all those years ago wrong? Could well be, I admit.
The tyres are the standard tyres the car comes with and downforce at 60mph is surely negligable. I might go out at the weekend with a mate and a tape measure and try it in my Legacy and report back.
Remember, the constipated mathematician works it out with a pencil!
|
|
|
|
The Asso. of Brit Drivers www.abd.org.uk/motorwayspeedlimit.htm says
"In 1965, many cars were still equipped with drum brakes all round, stopping distances were dictated largely by tyre performance and there were no aids to prevent wheel lock-up or to maintain steering control under braking. The braking distance shown in the Highway Code at 70 mph is 75 metres, which was probably a realistic figure in 1965 under good conditions with a reasonably competent driver at the wheel. In 2001, nearly all cars tested by Autocar6 could brake from 60 mph to a standstill in 3 seconds or less. This rate of deceleration is equivalent to a braking distance from 70 mph of 55 metres. When the thinking distance is added, this means that the total stopping distance for a modern car is 76 metres, compared with 96 metres shown in the Highway Code. This represents a 21% improvement in performance since 1965."
|
|
|
Surely you can't have a c of f more than 1 or have Subaru achieved it? ;-)
You can due to the way tyres deform and grip, but I'd agree with number_cruncher and suspect bad measurement as being more likely.
|
|
I'm sure you can't. It is defined as being between 0 and 1. And Edmunds, who test hundreds of cars a year, can't measure accurately?
|
I'm sure you can't. It (coefficient of friction) is defined as being between 0 and 1.
Nick, coefficients of friction in the range 0 - 1 as discussed in Physics lessons relate to friction between rigid solid bodies: pneumatic tyres on tarmac are not so limited, as you can see at drag races, where initial acceleration may be 3g or more (i.e. coefficient of friction of > 3), achieved with sticky compounds and controlled wheelspin.
Even rigid solids can appear to exceed friction coefficient of 1 at times; metal parts on space vehicles can adhere to each other and engineers slip blocks will appear to be stuck when 'wrung' together.
|
Garethj is actually quite right - with tyres, you can have what appears to be a co-efficient of friction greater than 1. This is because in addition to pure friction, there is also an adhesive mechanism that temporarily "bonds" the tyre to the road. This contribution to grip is lost as soon as there's any significant contaminant between tyre and road.
So, I can believe 1.05 as a co-efficient, but, much higher on standard road tyres is a bit odd.
Aprilia's post, which I link to above also describes how tyres can appear to produce co-efficients greater than 1.
I agree, at 60, you wouldn't expect much downforce, I mention it as a possibility when considering how a car can produce a force greater than you might expect.
Number_Cruncher
|
So as tyres improve and braking systems become more intelligent, braking distances can be reduced quite significantly? As indeed has happened over the years.
I think the parroting of the highway code braking distances in the driving test is a waste of time as who really thinks in feet or metres when judging braking distances? Car lengths possibly but best way is to try it in your own car and then you get a feel for it.
The key to the explanation above is the word 'appears'. The c of f between two materials is surely defined as between 0 and 1. If you introduce contaminants or other mechanisms or change the temperature or whatever you have changed the parameters so comparative calculations are not valid. Or am I going mad?!?
|
This is all very interesting (really!). This has prompted me to dig out old text books from way back and I'm now remembering why I liked physics in the first place. Mind you, I also remember why I ditched applied physics and went more into the nuclear side of things.
Thanks to all for rekindling an old flame. (Maybe I should get out more).
|
>>went more into the nuclear side of things
On land, or in long black cylinders, submerged?
Number_Cruncher
|
On land or in long black cylinders submerged? Number_Cruncher
On land, in Berkshire.
|
All the classic friction you will probably have come across on a physics simply doesn't encompass tyres and rubber components in general - they are virtually a law unto themselves.
For some more on tyres, try books by Miliken & Miliken, and books by Gillespie, where tyre to road friction is explained in more detail. In there, you'll also find odd facts about tyres, like the way that the co-efficient of friction *decreases* as the vertical load increases.
For all practical purposes, what we are talking about is the relationship between normal and tangential loads - what is typically called co-efficient of friction. I use the word appear, because there is more going on in the contact patch than what is traditionally known as classic friction. Although rubber goes beyond classic friction, there's nothing there that makes the calcs invalid - it's just that you see a number for mu that's greater than 1 in some special circumstances.
For example, tyres voilate the usual teaching that friction is independant of area, because the rubber conforms, and the actual contact area is much larger than would be the case for non-conforming parts - the actual contact area can be an appreciable fraction of the apparent contact area.
In summary, tyres and rubber parts in general are weird, and if you haven't worked with them specifically, things that you think you know from normal materials will not help you.
Number_Cruncher
|
Fascinating stuff though, NC, or am I weird?
No answers to the last part please.
|
>>Fascinating stuff
It's great stuff.
One of the nice things about tyres on dryish surfaces is that they don't develop maximum grip until they're slipping by about 20%, and have a well defined maximum grip level. This means it's relatively straightforward to design a reasonable ABS system without knowing *exactly* how fast the car is travelling - you can do it all by measuring angular accelerations, via monitoring wheel rotation speed and doing sums on the results. It's all fairly benign.
Contrast this with the problem of rail vehicles. Obtaining retardations above 12% is considered to be pushing the envelope! Maximum grip occurs at nearer 2% slip, and there isn't a well defined maximum for grip. All of these conspire to make the problem of a wheelslip protection system [trainspeak for ABS/ASR] much more difficult. Added into the problem that wheelslip under braking means a flat spotted wheel, which means a trip to the wheel lathe to have the tyres machined down. Despite all of this, the press make mirth about the wrong kind of leaves, wrong kind of snow, etc. Ignorant buffoons the lot of them!
There are some trains (class 66 IIRC?) that have a doppler radar, so the true velocity of the train is known, and so obtaining an accurate 2% slip to put down the best traction is comparatively simple.
Number_Cruncher
|
I'm sure you can't. It is defined as being between 0 and 1. And Edmunds who test hundreds of cars a year can't measure accurately?
Hopefully number_cruncher has given a bit more info about the results tyres can give compared to "solid" physics, as for accurate measuring the thoughts that come to mind are: How accurate is the car speed? Wind speed? Slope of the ground? Does the driver lift off when passing a marker or hit the brake when passing a marker? Are the measurements a "best" or "average"? Tyres and brakes can work differently depending on temperature, are they both at optimum temperature or cold?
How repeatable are the results? I'd be surprised if they can get within 5% even on the same day across 10 stops, that's the difference between the 2 Subarus right there.
Gareth
|
|
|
|
|
|
Emergency braking from 70 mph to 0 is part of the advanced driving course I give for 4x4 drivers. It certainly provokes some fairly wide-eyed comments from my 'students'.
|
I'm guilty of doing this as well - in excess of 70 if I'm honest.
Every now and again I like to take the car out usually late at night when I know a stretch of dual carriage way is deserted and make sure that the ABS is all working correctly and it gives the discs a good clean as well.
I then rev it up to make sure the VTEC or VVTi is also working correctly.
Safety for me and other road users is considered while I do it of course.
My experience is as above really - in that the stopping power is very good - car stays straight etc - I hope I never have to do it with someone behind that does not have ABS, EBD and Brake Assist
|
I do it on a semi regular basis on bikes and car.
On the bikes I do a number of time from the same place and move a tin can or something to show where I stopped. It is surprising how much you can reduce your barking distance once you have confidence on how hard you can squeeze the lever. The foot (back) brake becomes useless when doing this as the wheel is probably barely touching the ground.
The car doesn't like this treatment, smoke after 2 or 3 stops.
|
Never had to do in ernest thankfully ... and have been re-ended by an HGB in a Fiesta hire car but do not recall to this day all the details and it was late July 2006. No idea how that happened.
My comment though is about temp replacement to my stolen Golf in early 1999. Fleet manager wanted to know what was acceptable. Electronic brake force distribution was one safety item I listed and as well as side and curtain airbags.... narrowed the list of what they had but they could not really aqrgue.... I'd spec'd a car like that for obvious reasons ;-)
|
Commuting on the M3, this is required with varying degrees of severity once a week on average, as the British motorway concertina effect gets under way.
The Mondeo pulls up well, but the brakes grumble for the last 20 mph or so. It's only happened since fitting the cheap Ebay pads and discs. The discs are warping after 3,000 miles as well, so the whole lot's coming off in the New Year and OEM stuff is going back on.
The scariest thing I've ever done it in was a Lotus Elise after a moron on the M25 did an emergency stop on the entry to an exit slip road and sat with his right hand indicator on to rejoin the main carriageway. No ABS and no servo assistance. I opted to jump on them for all I was worth, which seemed to be the right option as it stopped beautifully. ABS? Pah, who needs it ;-)
Cheers
DP
--
04 Grand Scenic 1.9 dCi Dynamique
00 Mondeo 1.8TD LX
|
Always do a decent brake test on any car I test drive, maybe not 70 but certainly 50-60ish. Have done the braking in all our cars at least once just to get a feel for it too.
Best brakes I came across though were on a Mazda RX8 - bumpy country roads and doing (ahem) 61mph, stamped on the pedal. Stopped with a force that felt it was pulling the skin off my face. I'm sure there are cars out there with much better braking efficiency but the RX8 really did the business.
-----------------
Happy Festivus
|
Many times in the past. On M6 motorway usually. Even with my series of new/almost new VWs it was alarming that I never knew which direction the car would swerve at the first hard application. Pre ABS the lot. On subsequent applications the cars would pull up straight. ABS cars a lot better and was convinced by the system in wet track at a Toyota demo day.
|
|
|
|
Agreed, Mk1 Elise and Mk1 Exiges possibly the nicest breaks I have ever used. No servos, no ABS to interfere with the feel.
Edited by lotusexige on 31/12/2007 at 14:11
|
|
|
|
|
|
