Blog-off: Why the best first bikes are secondhand ones

But how? And what’s with all these equines?

Watt wanted to measure his horses’ power

With 85.9 lb.ft and 205bhp, BMW’s S1000RR has recordbreaking output. But this doesn’t necessarily mean it would hold off 205 horses in a tug-o-war fight.

In an engine, combustion pressure pushes a piston down a cylinder. The piston is attached to a conrod, the other end of which attaches to a crankpin journal sticking out from the crankshaft on a web. As the piston descends it uses the web as a lever to turn the crank. This is the work done by the engine – its torque. Torque is an engine’s punch, determined by the force from the combustion process multiplied by the crank’s throw – the distance from crank centreline to crankpin centreline. For more torque, you need a bigger bang (more force), larger piston (bigger bore, so more surface to push against) or increased throw (longer stroke, bigger lever size).

What about the horses?

If torque is punch, horsepower is how often this punch is delivered – its rate of work.

A clever Scottish fella called James Watt was using horses to pull coal out a mine in the 18th century and wanted to measure their power. He decided the average horse could drag 330 pounds of coal a distance of 100 feet in a minute, so did 33,000 pound-feet of work per minute, or 550 lb.ft per second (which is why torque is measured in lb.ft – but MCN usually puts ftlb instead so the figures are easier to read). Watt declared that this was one horsepower. It’s the measurement still used. We find it by measuring an engine’s torque on a dyno (the equivalent of Watt’s mine), multiplying by engine revs and dividing by 5252.

Hang on, why 5252?

Torque is pound-feet but horsepower is pound-feetper-second, so we need the ‘per second’ bit. Dividing revs-per-minute by 60 gives seconds, but it needs to be in radians, a dimensionless unit for revolutions (equivalent to the angle between two radii enclosing a section of a circle’s circumference, if you must know…). Each crank rev is 360 degrees of a circle, and a circle’s circumference is 2 x Pi x radius, so there are 2-Pi radians in each rev. Revs-per-min to radiansper-sec is rpm x (2-Pi/60) giving 0.10472 radians-persecond. So for horsepower, we divide 550 lb.ft per second (from Watt’s nag) by 0.10472 to get the 5252 correction factor. It’s why power and torque curves on a dyno graph cross at 5252rpm.

Back to horses…

All of which tells us yes, the BMW really has the power of 205 horses… as long as they’re as strong and eager as the ones James Watt used in his mine.

Powerful, but can it lug coal like a horse?

As the piston descends it uses a web to turn the crank and creates what we know as torque Units of power We use bhp (brakehorsepower) to the SAE (Society of Automotive Engineers) standard, based on Watt’s measurements. Horsepower is simply a way of stating an engine’s rate of work; so that’s basically how often it can deliver its punch. Torque is pound-feet (lb.ft); if it’s in metric Newton-metres (Nm), divide by 1.356 to get back to an imperial measurement. Torque is combustion force x crank throw, and is the work done by an engine – what pushes you down the road. The power Units of torque Welcome to the punch