It’s easy to stay warm while cycling in very cold weather. Ride a slow bike and try to make it go fast. It works for the same reason that riding up a hill is hotter work than on the flat; the ratio between energy expenditure and cooling airflow is altered – in favour of energy expenditure. And as the body expends energy, it generates waste heat. If more is generated than airflow can carry away, the result is that the body feels warmer.
So the challenge is to find a “slow” bike. A bike, that is, that goes less fast that expected for a given power output. The problem for many keen road cyclists – when it comes to staying warm in winter, at any rate – is that their mounts are amazingly efficient at converting power into motion. It’s not too hard to find something a little less eager to fly along under minimal propulsive force.
Something like the editorial Fuquay fully-rigid mtb will do the job nicely. It’s not actually that slow, even fitted with dreadnought 50mm section Continental Topcontact world-touring tyres; just a little slower than a regular lightweight road bike on narrow rubber. Those tyres, by the way, are astonishing. At a hefty 740g and £44.99 each, they must be good for 20k miles. There are small commuter motorcycles fitted with weedier rubber. Despite the deep tread, they roll nicely once up to speed although getting there feels like getting a road roller under way.
But that’s the point; make yourself work harder for the same speed or go slower for the same effort and you’ll feel warmer. Alternatively, you could always wear an extra layer of clothing – but eventually, when the conditions get cold enough like right now, you’ll reach some kind of limit to the number of layers you can wear and still pedal. I know I have; riding the mtb is the answer, for now.