Posts Tagged ‘bicycle’

Bicycling and the permanent headwind

July 30, 2019

cyclinglaw

Discussion on Facebook

Interesting that the permanent headwind which blows in your face whatever direction you cycle still operates in the hot weather. Clearly its intelligence does not yet equal its malice and agility.

I find that crosswinds are treacherous because: a) they have a similar drag effect to a headwind and b) they have a vastly greater angular range than just head on. Unfortunately that pretty much encapsulates any wind direction. Get out early if you can, before the thermals kick in.

Have you seen the study that shows the wind is always against you? It’s summarized in New Scientist here and the original paper is here.

Further analysis

OK so now we’ve got onto the purely hypothetical case where you cycle from A to B and then back from B to A and the wind continues to blow from B to A as though nothing had happened.  Even so, the analysis in the references above looks rather complicated.

Let’s ignore rolling resistance and consider a cyclist who wants to cycle on the flat from A to B and back again.  He cycles with a power P which gives a limiting velocity V relative to the air. (In the absence of rolling resistance we ignore interactions with the ground.)

So let’s say as an example B is 10 km from A and V is 20 km/h.  In the absence of any wind it takes 0.5 hours to go from A to B, another 0.5 hours to go from B to A, for a total of 1 hour or 60*60 =3600 seconds.  Then with P measured in joules/second, the energy expended will be 3600*P J.

Now a wind of 10 km/h blows from B to A.  The cyclist’s speed relative to the ground is then 10 km/h on the outward leg and 30 km/h on the return leg.  So the time taken is 60+20  = 80 minutes or 4800 seconds and the energy expended is 4800*P J.

The cyclist has spent more time and expended more energy even with a completely unchanging wind.

This analysis clearly applies to any situation where there is a component of the wind along the cyclist’s direction of motion, even if it doesn’t turn round on you…

 

BikeHut track pump @ Halfords

October 1, 2009

I decided I’d get a track pump for my bicycle.  I went down to Halfords and they had 3 types, with no indication of the prices.  I thought the BikeHut one was the cheapest, so I took it to the checkout and it turned out to be £38.  Then when I was out of the shop I found the head was missing!  So I took it back and changed it.

pumpSo then I found it wouldn’t work with my Presta valves (there were no useful instructions with it).  Eventually I found some discussion here with the final answer

Thanks for all the replies guys,

I think I have figured it out now. I simply wasn’t pushing the valve
into the connector hard enough. The ‘schrader push pin’ is spring
loaded and pushes back quite a way. Initially, when you push the
connector on, there is quite a rush of air from the tire (this is
point to which I was getting before). However, if you push harder
then the valve enters the connector further (by pushing against the
spring loaded pin) and reaches an area where the rubber grip smothers
the airflow enough that you can tighten the lever. So the trick is to
push it on firmly and quickly.

I find this works well now on my back tire and makes an airtight seal.
On my front tire (which has a different make of inner tube) I find
that even with the lever tightened fully, I still get a bit of air
leakage, but it’s not enough to bother me.

Brian, do you fully unscrew the presta nut at 1?

Thanks again.

Mike

So I tried that, and it worked on the front tyre and well enough on the rear one, though the join was rather leaky.