Grip qualities of road surfaces

Topics relating to Advanced Driving in cars
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EasyShifter
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Grip qualities of road surfaces

Postby EasyShifter » Fri Jun 17, 2016 1:47 pm

Does anyone know of any information available about the relative grip qualities of different road surfaces?
Michael

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Horse
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Re: Grip qualities of road surfaces

Postby Horse » Fri Jun 17, 2016 2:45 pm

Try searching for SCRIM values or "coefficient of friction". Also DMRB may have minimums permitted.

However, any values you find for particular surface treatments will be 'best possible' - they'll really only deteriorate, or 'minimum permitted' (e.g. mways should have higher grip than other roads.
Your 'standard' is how you drive alone, not how you drive during a test.

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Horse
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Re: Grip qualities of road surfaces

Postby Horse » Fri Jun 17, 2016 2:47 pm

Your 'standard' is how you drive alone, not how you drive during a test.

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GTR1400MAN
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Re: Grip qualities of road surfaces

Postby GTR1400MAN » Fri Jun 17, 2016 3:00 pm

Horse wrote:(e.g. mways should have higher grip than other roads.

I always thought motorways had the lowest allowed grip.

DESIGN MANUAL FOR ROADS AND BRIDGES (PDF)

Here's what page 41, A.1.8 says "For some Site Categories, no statistically significant relationship, or only a weak relationship, is observed between skid resistance and crash risk. A good example of this is motorways, where the road design has effectively reduced the potential for conflict between road users. Although the skid resistance is still important, because of the need to provide uniform road characteristics, the level of skid resistance can be lower than other categories."

There's also a grid of road types and their allowed grip levels on page 22.
Mike Roberts - Now riding a Triumph Explorer XRT. My username comes from my 50K miles on a Kawasaki 1400GTR, after many years on Hondas of various shapes and styles. - https://tinyurl.com/mikerobertsonyoutube

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EasyShifter
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Re: Grip qualities of road surfaces

Postby EasyShifter » Sat Jun 18, 2016 8:40 am

Thanks, folks - as always it's a more complex subject than I'd given it credit for! That should keep me in bedtime reading for a while!
Michael

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Re: Grip qualities of road surfaces

Postby Astraist » Sat Jun 18, 2016 8:25 pm

The coefficient of friction can be measured through the amount of force it can supprt, which for a good, dry road just tops 1g for a passenger car on decent tyres.

Asphalt qualities do vary in terms of how abrasive they are, and it pays to bring it into account in your "safely stop well within the area seen to be clear" calculations, because the coefficient of friction does flactuate somewhat.

The abrasive tarmac tends to be porous so that the difference in traction is magnified when wet: I've heard of figures of 0.7-0.5 being thrown around, which obviously also depends on the film of water and the ability to clear water through the tread at speed.

On the other hand, light rain on heavy-traffic stop-go environments tends to result in greasy runoff floating over the water and the coefficient of friction drops even further.

With tarmac there is also the issue of how much bumpy it is. Some of the car's potential goes to dampen the undulations of the surface and you get less of what the coefficient of friction suggest you should be capable of getting.

Soft surfaces like gravel and sand are even less grippy: 0.5-0.4. Occasionlly I see people getting it wrong (and date the scenery) due to this. The looser, the less grippy but it also retardates the slipping car more, so it tends to even out.

If anything more slippery appears on the road itself, it tends to coincide with a "moment". Pebbles, a big diesel spill (thankfully very rare), mud and snow, drop the coefficient of friction to 0.3-0.2.

fungus
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Re: Grip qualities of road surfaces

Postby fungus » Sat Jun 18, 2016 8:37 pm

I don't know about the science, but after yesterdays deluge there was a hell of a lot of gravel and detritus washed out on to the roads locally.

Nigel.

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Re: Grip qualities of road surfaces

Postby Astraist » Sun Jun 19, 2016 12:42 pm

To each his own, I guess.

It's very hot and dry here nowadays, but with some of the fen roads the binding tar in the road tends to boil which reduces tyre traction all the same. It also creates a Fata Morgana over the road surface. Thankfully, most roads are quite good and this doesn't tend to happen that much.

Also, there are plenty of other places where water can accumulate on the road and it tends to be very slippery when that happens.

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EasyShifter
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Re: Grip qualities of road surfaces

Postby EasyShifter » Sun Jun 19, 2016 2:44 pm

It's obviously a very complex subject and not one that can be analysed on the move - so what it amounts to really is that any change of road surface is to be noted and taken account of, recognising that it can potentially affect the handling more than might be predicted. Add to that that, in the event of actually having to 'stop well within the distance that can be seen to be clear' the car may well be half-way round a bend with implications for stability. and the cautionary implications are clear.
The post was prompted by a loss of feel through the steering on a bend where the road surface had changed colour significantly. There were no dramas or nasty moments, but it was noticeable enough to give me pause for thought.
Michael

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Re: Grip qualities of road surfaces

Postby TheInsanity1234 » Sun Jun 19, 2016 9:49 pm

I do notice the Citigo has particularly sensitive handling. It seems to grip perfectly well on certain surfaces, but you can really tell when the car seems to be perfectly happy or a bit more cautionary.

There have been times where I round a bend at a sensible speed, and halfway through the bend the surface seems to change and the steering suddenly goes from being quite firm to being a bit lighter and even though the car continues on the path I've chosen, I do get a feeling I should be taking it easy or whatever.


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