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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.

Michael come lately
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Chris H
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    Perhaps black ice - the sort you can't see ? You did say it was damp and therefore not dry. – Criggie Dec 01 '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 01 '17 at 11:10
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    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 01 '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 01 '17 at 12:17
  • @Criggie I'm starting to think black ice must explain it, even though both wheels seemed to go at the same time, and I didn't think it was that cold. The grit can be a bit uneven there; I'm used to dodging where there's too much, which is partly why I take an easy line. I'd normally have winter tyres by now. On the other hand it felt slippery - but not as slippery as ice - when I got up. I reported it (as it's on station premises) in case of a spillage or something,and they didn't know of anything. – Chris H Dec 01 '17 at 12:39
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    @ChrisH if the grit is round that could explain things - round pebbles and round beach sand can act as bearings and roll under you. This is more of a problem on smoother surfaces. – Criggie Dec 01 '17 at 20:16
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    @Criggie round here they use rock salt, so there's grit in there but it's rough. – Chris H Dec 01 '17 at 22:04
  • When you write "the same road just damp", do you imply "at equal temperature"? The decreased performances of rubber is given as one of the reasons to swap summer tires for winter tires on cars, it's likely that the same can be said for bike tires. Unless I'm wrong, salt is spread when temperature is low, when rubber performance is lowered. This phenomenon can also contribute to your observation. – Rеnаud Jan 25 '22 at 13:59
  • @Renaud it was a while ago, but equal temp to within a degree or two. With temperatures just above freezing, the road is likely to be salted if clear skies are expected overnight, not if there's full cloud cover. I ride it almost daily so would certainly have taken the same road, even the same line, at about the same speed and almost the same temperature without salt. From my comments, I was running Marathon Plus, which are meant to wear well more than to grip well. My studded winter tyres also seem to be a more appropriate compound for cold wet roads – Chris H Jan 25 '22 at 14:04
  • A further point is that (several years later, different bike, tyres, and maybe shoes) a bike can feel squirrelly but rideable on ice bad enough that stepping off the bike leads to a fall from lack of shoe grip. At the time I said it felt slippery underfoot, but not as slippery as ice. – Chris H Jan 25 '22 at 14:09

1 Answers1

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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. Between 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.

Table 5. Lubrication by Aqueous Salt Solutions

Lubricant 'Perspex'/rubber
Friction coefficient		 Rubber/rubber
Friction coefficient
Dry surfaces 4.1 10  
Distilled water 2.6 7.4
Sodium nitrate 2.6 7.3
Sodium sulfite 2.1 4.9
Sodium chloride 1.5 4.6
Sodium carbonate 1.1 2.1

Normal load 3.43 N, sliding speed 0.2 mms-2, temp. = 22°C-24°C, RH = 55%-65%. SMR CV hemisphere (R = 21 mm) slid on smooth 'Perspex' track or on smooth sheet of SMR CV (5 mm thick) supported on the 'Perspex' track. All salt solutions were 0.1 M concentration. Stickslip motion tended to occur and maximum friction coefficients are quoted.

From 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 preceding sentence.

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 low. This 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:

Michael come lately
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  • 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 01 '17 at 13:59
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    Impressive work - In my country roads are never salted, they are gritted instead. – Criggie Dec 01 '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 06 '17 at 08:49
  • @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 06 '17 at 09:38
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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 06 '17 at 14:39
  • @StefG given what's in the literature (not much, all idealised) I think a full model would take most of a PhD -- and I'm not vounteering even if I could get funded for another one (in a field where I'm not an expert at that)! – Chris H Dec 06 '17 at 14:58
  • There is one thing that bugs me with the data: The low normal force that was applied. This makes the data next to pointless in the context of bicycles. For a bicycle, normal pressure at the contact patch is always near the tire pressure, which can be as high as 7 bar = 0.7 MPa = 70 N/cm^2 That's about 20x the total force they used in the experiment applied to every square centimeter. – cmaster - reinstate monica Dec 14 '23 at 21:01
  • @cmaster-reinstatemonica good point. It was a long time ago but a slight bump can certainly reduce the normal force momentarily - maybe that was enough for it to break free – Chris H Dec 14 '23 at 21:43