How hub dyno measures power? How does the selected gear affect on the power measurement? What is ratio?
When we measure power in a hub dyno we have two basic values that we measure. They are torque and speed of hub (RPM). With those values we can calculate power with simple formula:
Power = Torque x Speed.
So we have load cells and RPM sensors installed inside the brake hubs, our load cell gives the torque and RPM sensor gives the hub speed (RPM). And that is how we have our “hub” torque and power.
So what is “hub” torque and power?
Let’s take an easy example; we have a car that has a two-speed gearbox and engine that produces 200 hp and 300 Nm at the engine. Differential ratio is 3,0 : 1. First gear ratio is 2:1 and second gear ratio is 1:1.
We drive that car in the dyno with the first gear at full load, we will see about 200 hp and 1800 Nm, how the torque can be so high?
It is because we have a gearbox and differential that multiplies our torque and divide our speed (RPM). In this case it goes like this: gear 1 ratio x diff ratio = total ratio (2×3=6). So 300 Nm x 6 = 1800 Nm.
Then we drive the same car with the second gear and the dyno shows 200 hp and 900 Nm. That is because our total ratio is now 3; gear 2 ratio x diff ratio = total ratio (1×3=3). This time we multiply our torque by 3 (300 Nm x 3 = 900 Nm).
With this same formula we calculate torque at engine when we know our ratio and hub power / torque. Because of this we get same engine torque and power regardless what gear is selected.
How the power can be the same although we see a huge deviation in measured torque?
Let’s go back to our basic power formula. Power = torque x speed so when we set higher torque but less speed or lower torque but higher speed the power is the same. A good example to understand this is why you get wheelspin more easily with first gear but max speed of your car is low, it is because you have high torque but wheel (or hub) speed is low. With 5th gear you reach your max speed but won’t get wheelspin, and that is because wheel (or hub) torque is low but wheel speed is high.
That was a very basic example of how a hub dyno works and how your car’s transmission effects on power / torque. There is many many more factors that effect also on power / torque measurement in dyno, but let’s talk about those in the future articles.