/ Resistance on a wet road.

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ablackett - on 23 Jun 2013
A friend of mine took about 5% off his Strava time on a "half climb, half flat ish" segment last week, on about his 10th attempt, he has had marginal gains over the previous attempts.

5% was a huge leap forward and gave him the KOM (out of 1500), on Ullswater Matterdale climb segment, so he isn't some punter who has just got his cadence/pacing right for the first time.

The only thing that was different was the road was wet, but it wasn't raining.

My theory is that a wet road = higher humidity, water vapour has a molecular mass of 19, air has a molecular mass of 29, so an increase in humidity could significantly reduce air resistance. But it would also reduce your aerobic capacity because you get less O2 in every lungfull, it's not obvious to me which effect would be stronger.

This chap thinks it is about rolling resistance and the water filling in the gaps.
http://velonews.competitor.com/2007/06/bikes-and-tech/technical-faq/technical-qa-with-lennard-zinn-r...

Lance thought wet road = quick times.

Another chap I read about reckoned he could hit a baseball 5-10% further when it was foggy.

Thoughts? Anyone want to get out and try it today?
Ghastly Rubberfeet on 23 Jun 2013
In reply to ablackett:

My ride was about 20% slower today in similar conditions to those you describe. Last time I did it it was bone dry.

I think 3 factors brought about this inferior performance.

1) It was significantly more windy today.

2) I have not been out on my bike for nearly a year.

3) I have put on about 2 stone!

I doubt the wetness of the road made much difference!

;~))

andy - on 23 Jun 2013
In reply to ablackett:

>
> 5% was a huge leap forward and gave him the KOM (out of 1500), on Ullswater Matterdale climb segment, so he isn't some punter who has just got his cadence/pacing right for the first time.
>

Isn't the lad who's got KOM on Hardknott a punter who doesn't own his own road bike and did it on someone else's? ;-)

http://app.strava.com/activities/58134551
Pocoyo - on 23 Jun 2013
In reply to andy:
> (In reply to ablackett)
>
> [...]
>
> Isn't the lad who's got KOM on Hardknott a punter who doesn't own his own road bike and did it on someone else's? ;-)
>
> http://app.strava.com/activities/58134551

Yeah that's right on a borrowed bike that was too big for him, pretty sure he beat some times on 'Outrigg climb' - St Bees section on his MTB causing a bit of a stir!

ablackett - on 24 Jun 2013
In reply to Pocoyo: I can see that an absolute hero could get KOM's on a borrowed bike, but that's not what I'm talking about.

Anyone able to do the sums on how much difference a wet road should make to the humidity and therefore the air resistance and therefore the time up Ullswater Matterdale?
http://www.strava.com/segments/ullswater-matterdale-climb-1144239
andy - on 24 Jun 2013
In reply to ablackett:
> (In reply to Pocoyo) I can see that an absolute hero could get KOM's on a borrowed bike, but that's not what I'm talking about.
>

No, but my (tongue in cheek) reply was in response to your assertion that the fact your mate's got a KOM on a popular segment is evidence that he's an experienced rider and not a "punter" - the lad who's just got KOM on another popular segment is actually a very inexperienced rider (I think it was his second time on a road bike), and presumably by that definition, a "punter".

Tom the tall on 24 Jun 2013
In reply to ablackett: I think humidity might be a red herring. I've found wet roads, especially on certain surfaces with small cracks, feel faster and put it down to a reduction in rolling resistance between tyre and road, but not sure why.
Toby_W on 24 Jun 2013
In reply to ablackett:

I have also found that where my intense fear of road rash is not an issue (no technical cornering) that wet roads produce faster times for me. No idea of the physics.

Cheers

Toby
Henry Iddon - on 24 Jun 2013
In reply to ablackett:

Wet weather often occurs during low barometric pressure - hence less air 'density' = faster ride.

I've seen fast times in time trials as thunderstorms / ridges of low pressure pass through and am sure that when Boardman did his hour records the air pressure and temp in the velodrome was taken into account.

ablackett - on 24 Jun 2013
In reply to Henry Iddon: Air pressure when he did it was 1000 (not sure about units), but that's about average as far as I know but if he only normally does it on high pressure still days then it could count for the change I guess.

Would a 5% reduction in air pressure = a 5% reduction in times? Assuming everything else is constant.



Henry Iddon - on 25 Jun 2013
In reply to ablackett:

No idea - guess we need an aerodynamics guru in this parish !
elsewhere on 25 Jun 2013
In reply to ablackett:
I'll assume this is correct as I can't be bothered to calculate it myself.
http://www.denysschen.com/catalogue/density.aspx

21C at sea level
Humidity -> density
0% -> 1.196 kg/m3
100% -> 1.185 kg/m3

So air density reduced by 1%.

Somebody who knows something about lungs may know if 1% density change will have any impact in inhalation/exhalation.

Air resistance - power requirement proportional to density and cube of speed (very basic formula from dimensional analysis).

Density down by 1% so speed up by 0.3%.
JLS on 25 Jun 2013
In reply to ablackett:

Let me guess, it was June 15th due to the strong tailwind rather than an air pressure drop...

http://www.wunderground.com/history/airport/EGNC/2013/6/25/MonthlyHistory.html#calendar

http://www.analyticcycling.com/ForcesAirDensity_Disc.html
ablackett - on 25 Jun 2013
In reply to JLS: No, it was June 12th, about 7pm

http://www.wunderground.com/history/airport/EGNC/2013/6/12/DailyHistory.html?req_city=NA&req_sta...

No wind. We had considered that as a possibility!
ablackett - on 25 Jun 2013
In reply to JLS:
> (In reply to ablackett)
>


That link is interesting though, he suggests that the effect of humidity is small. Perhaps it's the rolling resistance then?

Greased bearings roll better than dry bearings, so why not wet tyres running better than dry ones?

Double Knee Bar - on 25 Jun 2013
In reply to ablackett: greased bearings actually dont roll better than dry ones. They just last a lot longer.
Timmd on 25 Jun 2013
In reply to andy:
> (In reply to ablackett)
> [...]
>
> No, but my (tongue in cheek) reply was in response to your assertion that the fact your mate's got a KOM on a popular segment is evidence that he's an experienced rider and not a "punter" - the lad who's just got KOM on another popular segment is actually a very inexperienced rider (I think it was his second time on a road bike), and presumably by that definition, a "punter".

A punter or somebody very fit already from MTBing...
JLS on 26 Jun 2013
In reply to ablackett:

>"No wind. We had considered that as a possibility!"

How normal is that? Have the other rides been into a slight head/cross wind?

I'm not saying rolling resistance isn't a factor but when you see big speed differences it's generally air resistance that's the variable at work.
Ian Broome - on 26 Jun 2013
In reply to ablackett:

Doesn't a wet road have a lower coeffecient of friction then a dry road? Surely this is why you skid when its wet?

If your riding a road bike its unlikely you'll loose traction through force pedalling, the water in between the tyre and road must smooth out the texture of the road and reduce the friction between them.

I've really noticed the difference riding in the wet, and the fear you could stack it on a bend!
Bob_the_Builder - on 26 Jun 2013
In reply to ablackett:

A wet road definitely decreases sliding resistance, but when going (rather than stopping) you care more about rolling resistance than sliding resistance which works slightly differently. If anything I imagine a wet road would increase rolling resistance but I'm not sure.

Air pressure would have an effect on drag but I reckon it is negligible compared to rolling friction.

Having said that I feel like I ride faster when it is damp. My (very unscientific) theory is that a bit of moisture in the air makes breathing more comfortable.

(mechanical engineer but didn't do any maths for this)

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