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PostPosted: Tue Nov 06, 2007 13:04 
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Quote:
"It was a sunny day in August. I jumped on the brakes and the car just kept going and going.

"Instead of the scream of tyre on road and a cloud of smoke there was just a gentle hiss as I passed over the road, and I skidded far further than I ever expected to."

How cold is jet black road surface on a still hot day in August?

1.The road is already hot,
2. the smoke is evidence it got hotter!

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PostPosted: Tue Nov 06, 2007 13:24 
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anton wrote:
Quote:
"It was a sunny day in August. I jumped on the brakes and the car just kept going and going.

"Instead of the scream of tyre on road and a cloud of smoke there was just a gentle hiss as I passed over the road, and I skidded far further than I ever expected to."

How cold is jet black road surface on a still hot day in August?

1.The road is already hot,
2. the smoke is evidence it got hotter!


So which was it? Evidential smoke or a gentle hiss? :hehe:

Seriously, Smoke comes from the tyres and only when the wheels are locked. The same particles of tyre are getting heated for the duration of the stop. You can't say that about the tarmac.

I can't absolutely rule out temperature effects, but I'd rate unknown 'dynamic friction' effects as a far more likely explanation.

Also seriously, I think we might be onto something here.

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PostPosted: Tue Nov 06, 2007 13:31 
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SafeSpeed wrote:
anton wrote:
But if the 1st 10mm melted under skid braking and lubricates the other 90mm of contact... I lose 90% of my braking


But there's always cold tarmac being fed in at the front, and the exposure of any particular particle of tarmac to a heating effect is very brief as you slide on.

It'd be hard to believe that enough temperature rise is present in the road surface to approach melting point.

At 60mph with a 4 inch contact patch, a particle of road suface only experiences the tyre for 3.8ms. (1/(60*1.46667*3))

Time of contact is irrelevant; the energy can be transferred quickly. If the road cannot conduct away the heat quick enough the top surface will get hot. How hot I don’t know - yet.

If a 1800kg vehicle is at 60mph, braking at 0.5G, instantaneous power transfer will be 240kW.
4 4 square inch contact patches will result with 600W per square cm, or about 2.5J over 3.8ms per square cm. Assuming in that time the heat conducted down only 0.1mm (10nM^3 volume per square cm) and thermal mass is 4kJ per litre (1mM^3) per degree (so 40mJ per degree), my really dodgy maths would suggest a rise of surface temp of 62 degrees. I guess 90C will melt tarmac.

I made a few assumptions in there, can someone check?

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PostPosted: Tue Nov 06, 2007 13:43 
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anton wrote:
Quote:
"It was a sunny day in August. I jumped on the brakes and the car just kept going and going.

"Instead of the scream of tyre on road and a cloud of smoke there was just a gentle hiss as I passed over the road, and I skidded far further than I ever expected to."

How cold is jet black road surface on a still hot day in August?

1.The road is already hot,
2. the smoke is evidence it got hotter!


Sorry Anton but you emphasised the wrong section.
Quote:
"It was a sunny day in August. I jumped on the brakes and the car just kept going and going.

"Instead of the scream of tyre on road and a cloud of smoke there was just a gentle hiss as I passed over the road, and I skidded far further than I ever expected to."

Thus there was no smoke from the tyres.

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PostPosted: Tue Nov 06, 2007 14:09 
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smeggy wrote:
Time of contact is irrelevant


Oh no it isn't!

Seriously I'd love to go over the estimates and check, but I'm dashing out for a meeting with the LaneCheck people.

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PostPosted: Tue Nov 06, 2007 14:10 
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SafeSpeed wrote:
smeggy wrote:
Time of contact is irrelevant


Oh no it isn't!

Seriously I'd love to go over the estimates and check, but I'm dashing out for a meeting with the LaneCheck people.


btw half the energy (ish) goes into the tyres.

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PostPosted: Tue Nov 06, 2007 14:27 
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SafeSpeed wrote:
smeggy wrote:
Time of contact is irrelevant

Oh no it isn't!

Well it technically isn't of course, it depends largely on the rate of dissipation, but I think it is in the context of energy transfer per unit area when the heat cannot be quickly dissipated.

SafeSpeed wrote:
Seriously I'd love to go over the estimates and check, but I'm dashing out for a meeting with the LaneCheck people.

Cool. I hope it is productive.
I would like for someone to check. I would like to know about it if I've made an error.

SafeSpeed wrote:
btw half the energy (ish) goes into the tyres.

Is that true? Take for example a case where a wheel hasn't locked, would a comparable amount of energy go into the brake pad relative to the disc? The cooler disk is conducting the heat energy away from the pad. Of course that disregards the destructive energy used to deteriorate the pad (wear); I don't know what amount that could be.


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PostPosted: Tue Nov 06, 2007 14:29 
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SafeSpeed wrote:
btw half the energy (ish) goes into the tyres.


You're not making me feel any happier about banking my motorbike over on this black treacle substitute they're using for a real road. Won't be long before I gob it at this rate. :reaper:

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PostPosted: Tue Nov 06, 2007 14:36 
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smeggy wrote:
SafeSpeed wrote:
btw half the energy (ish) goes into the tyres.

Is that true?


Yep. I'm sure of it.

And the other problem I've found with your estimate is that it only applies to the tarmac BEHIND the tyre. The tarmac at the front edge is ALWAYS at it's 'natural' (not ambient) temperature.

smeggy wrote:
Take for example a case where a wheel hasn't locked, would a comparable amount of energy go into the brake pad relative to the disc? The cooler disk is conducting the heat energy away from the pad. Of course that disregards the destructive energy used to deteriorate the pad (wear); I don't know what amount that could be.


We might as well keep it to locked wheels for now, or much of the heat will be in the brakes and we'll be lost. So adjust for 0.9g.

Now I'm late... see ya'all.

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PostPosted: Tue Nov 06, 2007 14:48 
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SafeSpeed wrote:
Now I'm late... see ya'all.


Good on ya Paul :thumbsup: Mind how you go; better to be late in this world than early for the next. (like I needed to tell you of all people) :)


Now then, I have just realised another obvious thing I missed...

"But police crash investigators have become concerned because, in certain conditions, some do not offer much grip for up to two years until they have bedded in. "

So the bedding in, as I understand, is that part of the road which is most used therefore leaving the little used or no-used parts 'un-bedded in'

So what happens when we do use these virgin parts?

Maybe the bedding in relates to time and weather too but mostly about the pounding it gets, or not, as the case may be.

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 Post subject:
PostPosted: Wed Nov 07, 2007 01:25 
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smeggy wrote:
If a 1800kg vehicle is at 60mph, braking at 0.5G, instantaneous power transfer will be 240kW.
4 4 square inch contact patches will result with 600W per square cm, or about 2.5J over 3.8ms per square cm. Assuming in that time the heat conducted down only 0.1mm (10nM^3 volume per square cm) and thermal mass is 4kJ per litre (1mM^3) per degree (so 40mJ per degree), my really dodgy maths would suggest a rise of surface temp of 62 degrees. I guess 90C will melt tarmac.

I made a few assumptions in there, can someone check?


If the car is braking at 0.5G, a lot more of its weight will be on the front tyres than the rear. Exactly how much depends on its initial weight distribution and its centre of gravity height. At 0.9G, on most modern front wheel drive road cars, I think you could (for the purposes of THIS exercise anyway) neglect the weight on the rear tyres and assume it's just about all on the fronts!

That said, it would be easy to establish whether or not the tar melted by experiment. Just go out, jump on the brakes and lock up, then get out and see if the tar has melted! I doubt it will have re-solidified in the few seconds it takes you to get out of the car and look!

We have a stretch of road not far from my house which lies in a sun trap. On very hot summer days it starts to melt. When braking (even only moderately hard) on it I have noticed wheel lockup - very like aquaplaning, but it's pretty obvious where that has occured looking at the road surface because there are big grooves in it where my tyres have been! OK, it's not quite the same situation - the sun with have heated the tar through to a much greater depth than tyres ever could, but the point is that the surface is very visibly damaged if it melts and should therefore be very easy to see.


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 Post subject:
PostPosted: Tue Dec 04, 2007 17:34 
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smeggy wrote:
SafeSpeed wrote:
anton wrote:
But if the 1st 10mm melted under skid braking and lubricates the other 90mm of contact... I lose 90% of my braking


But there's always cold tarmac being fed in at the front, and the exposure of any particular particle of tarmac to a heating effect is very brief as you slide on.

It'd be hard to believe that enough temperature rise is present in the road surface to approach melting point.

At 60mph with a 4 inch contact patch, a particle of road suface only experiences the tyre for 3.8ms. (1/(60*1.46667*3))

Time of contact is irrelevant; the energy can be transferred quickly. If the road cannot conduct away the heat quick enough the top surface will get hot. How hot I don’t know - yet.

If a 1800kg vehicle is at 60mph, braking at 0.5G, instantaneous power transfer will be 240kW.
4 4 square inch contact patches will result with 600W per square cm, or about 2.5J over 3.8ms per square cm. Assuming in that time the heat conducted down only 0.1mm (10nM^3 volume per square cm) and thermal mass is 4kJ per litre (1mM^3) per degree (so 40mJ per degree), my really dodgy maths would suggest a rise of surface temp of 62 degrees. I guess 90C will melt tarmac.

I made a few assumptions in there, can someone check?


Your maths seems fine. But as the tarmac is not perfectly smooth the contact patch will effectively be smaller, probably around 50%. Hence the surface temperature rise will be higher. And some of the energy dissipated will heat the tyre.

I have noticed the SMA effect driving a Mercedes Vito with hard compound tyres like Continentals. Pauls lower dynamic friction curves especially b) are within my own experience on muddy tarmac in competition drining. I hate tarmac as when you lose braking effect the coefficient of dynamic friction is so much lower that the car seems to accelerate under braking.


I agree that time in contact is irrelevant for this simplicity.

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 Post subject: Tarmac
PostPosted: Sat Dec 08, 2007 18:42 
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It seems that (me as a biker) wholly agrees with the statement about this tarmac being absolutely lethal and downright dangerous to ALL road users but specifically for motorcyclists.

Could this be the reason why we are seeing a plethora of theses "Slippery Road" signs popping up all over the place because councils are now playing with our lives by using a TOTALLY UNSUITABLE tarmac to resurface the roads just to save money? :evil: :evil: :x :x

Is the policy now to pass on the costs to the NHS when some poor sod is seriously injured or even killed :?: :?:

The person/s responsible for issuing the orders to surface the roads with this dangerous material should be HELD PERSONALLY RESPONSIBLE for any injuries or deaths due to this stupid act (although what good money will do for a deceased,s family I don,t know) !

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