Maths on brtkwr.com

Is there an optimal cycling speed in the rain?

Sun, 03 May 2026 08:00:00 +0000

TL;DR: I thought there was. There isn’t, unless there’s a tailwind. In vertical rain, faster is always drier.

Motivation Link to heading

I was cycling home yesterday in heavy rain and started wondering about the right speed. Slower means more time getting drizzled on. Faster means more rain per second on my front. It felt like there had to be a sweet spot.

The setup Link to heading

Picture rain falling at speed $v_r$, with $\rho$ drops per cubic metre. I cycle at speed $v$ over distance $D$. Let $A_t$ be the area of my head and shoulders (top), and $A_f$ the area of my chest (front).

Trustless pseudo-random number generation

Wed, 31 Jan 2018 00:00:00 +0000

I have been working on an Ethereum lottery Dapp with folks attending a meetup hosted by Strange Labs (Gloucester Road, Bristol - NOW CLOSED) and we faced a problem where we realised that Solidity does not have a built in random number generator and therefore picking a winner turned out to be more difficult that anyone imagined.

In order to make the system truly trustless, one of the ideas involved each participant generating their own random number and submitting it to the contract when they buy their lottery tickets. However I was concerned that taking a sum of of these numbers would result in a normal distribution allowing the early and late participants to have a built in disadvantage.

How well does population distribution predict evacuation time?

Tue, 10 Mar 2015 12:50:00 +0000

Previously, we tried to predict 90th percentile evacuation time $T{90}$ determined from ABM simulation using 90th percentile free flow clearance time $T90f$. Bigger the city, greater we can expect $T90f$ to be since it would take longer to traverse. Plotting $T90f$ vs $T90$ produces the figure below. $T90f$ under predicts $T90$ with a poor fit ($R=0.31$) but it is clear that $T90$ is never less than $T90f$!