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I came off this morning* on a road I ride every day, in all weather conditions. Last night was frosty so the road was salted/gritted, but there was no visible frost nearby, it had warmed up from the overnight minimum, and the road is quite heavily used, both warming it and grinding the salt into the surface. But the road was damp.

So why would a salty, damp road be worse than the same road just damp?

More info, just in case it's of interest:

I was only doing about 20km/h and I've taken that bend faster in the wet before, on the same tyres when running late for my train. I was following a racing line (outside-inside-outside) so although it's a fairly tight bend my line wasn't tight.

I don't believe I just hit an oily patch. It felt slippery underfoot (walkable SPDs) when I picked myself up having slid a little way. Also I take pretty much the same line every day.

It was about 8.9 km into this ride.


* I'm OK, and so's the bike.

  • 1
    Perhaps black ice - the sort you can't see ? You did say it was damp and therefore not dry. – Criggie Dec 1 '17 at 11:09
  • I suspect it was by the railway station? Is it possible something other than water+salt was on the road? – Criggie Dec 1 '17 at 11:10
  • I'd guess either black ice, or you slipped on the grit (depending on where you are, they lay down the grit differently; in some areas it can be like riding on loose sand). In any case, you probably shouldn't be riding as if you're racing on a cold (possibly iced) damp road; thats just a recipe to have a spill. – Batman Dec 1 '17 at 11:43
  • @Batman I wouldn't call 20km/h riding like racing (I'd have guessed less but the GPS says just over 20). All other things being equal a racing line is safest at all speeds because it straightens out the bend, that's why I use them when riding gently. – Chris H Dec 1 '17 at 12:17
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    @Criggie round here they use rock salt, so there's grit in there but it's rough. – Chris H Dec 1 '17 at 22:04
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It turns out there's actually some rather interesting science behind friction in the presence of water and salt. Various model systems demonstrate that salt solutions really are better lubricants than plain (distilled) water between rubber and other materials, and by enough to matter.

Unfortunately little if any work has been done on the friction between real rubber and real wet roads in the presence of salt. But there has been some work on more idealised systems. One of the clearest results is from Experiments on the Lubrication of Raw Natural Rubber E.L. Ong and A.D. Roberts J. nat. Rubb. Res,, 1(1), 41-50 (1986). Table 5 (below) is particularly interesting. Betwen raw natural rubber and Perspex the coefficient of friction reduced from 4.1 when dry, to 2.6 when wet and 1.5 when wet with salt solution. A similar trend was observed for rubber-on-rubber friction. In other words in this system, in salt water there's about 60% of the grip compared to clean water. Fom the caption, "Stickslip motion tended to occur and maximum friction coefficients are quoted" (my emphasis). It's not clear whether this is true for all cases or just the salt solutions referred to in the preceeding sentence.

Ong & Roberts Table 5

My journal access doesn't extend to reference 3 from this paper, Rubber friction in aqueous solutions containing ions, T.P.Mortimer & K.C.Ludema, Wear Volume 28, Issue 2, May 1974, Pages 197-206. But the abstract of this latter paper states:

In the case of water lubrication of black rubber at slow speeds and low pressures the lubricating ability of water is shown to be enhanced by addition of electrolytes to the water. It is thought that negative ions from the electrolytic solution collect on each sliding surface, repel each other and prevent the close approach of two sliding surfaces. Thus a thicker water film exists between the sliding surfaces than if the ion layers did not exist. The thicker film results in a reduced viscous drag force, lower than can be accounted for by conventional hydrodynamics.

The ~40% reduction in friction from the table quoted above is enough to negate a reasonable safety margin when it comes to grip. Applying this to my case I suspect an element of bad luck came into play too: That some tiny slip became unrecoverable because the overall friction was too lowThis would be especially the case as dynamic (sliding) friction is expected to be lower than rolling friction, thus extra grip is needed to recover a skid compared to avoiding it in the first place.

Further Reading:

  • I had a suspicion there was a real effect when I posted the question, but didn't know. I'd still like to find some references that are more specific to road use (even if cars) and the lure of experimenting is strong. – Chris H Dec 1 '17 at 13:59
  • 3
    Impressive work - In my country roads are never salted, they are gritted instead. – Criggie Dec 1 '17 at 20:18
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    I did not check your references, but I would be surprised if "your" friction is not a function of concentration, with the saturation point as a singularity. In your real live situation, you do not know the salt concentration, and its variation along the road surface, due to many things. E.g. a slip of water with a salt concentration close or beyond saturation would behave like oil. – StefG Dec 6 '17 at 8:49
  • @StefG that's a good point. One of the references gives a few data points showing variation with concentration, but doesn't go to saturation. All concentrations they show are lower friction than clean water. They also don't account for road dirt mixed in. My real case actually leads to interesting phenomena which also aren't tackled: NaCl is hygroscopic and forms a powdery layer on each crystal as it draws water from the air; in addition the surface temperature had fallen below the dew point of the air overnight, and risen again making more water available. – Chris H Dec 6 '17 at 9:38
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    @Chris H: Still did not check the references, but usually such data comes from ideal boundary conditions. In your (our) case, I suspect that another difference is due to the "street limit" of the situation: a (very) thin film of salt-water mix, i.e. close to a 2D-sample. Compared to a 3D sample, diffusion processes are altered radically, leading to a higher variation of salt concentration, up to "local saturation"; likewise, the influence of the salt-water / air or - / asphalt boundary phenomena become predominant - like what you mentioned about the consequences of the hygroscopic nature. – StefG Dec 6 '17 at 14:39

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