Here is a post I put on the speadfreaks site about building an Interia type dynamometer

Build you own rolling road

The biggest problem for us speed guys is getting a nice flat place that is long enough to run the car up near its full speed. It takes me ages to get to the track and when Iím there it usually rains. Itís more of a problem if you have an IC engine as you need to make a number of runs to tune the car properly so a decent speed run is a hit and miss affair

To help resolve this I am going to build an inertial dynamometer for the speed car. This is the rolling road type dynamometer. The principle is that it takes a certain amount of HP to accelerate a mass at a given rate. So the HP of the engine is directly linked to how fast and how quickly the back wheels can spin a heavy drum.

Whatís needed is drum like roller that you can put the wheels onto and then rev the car and gather data on how fast the drum is rotating and how quickly it changes revs. The problem is ďHow big do you need the drum?Ē

This is not an easy question to answer. I want to try to get the car to accelerate like it would in the real world so it should take about 10 seconds to spin the drum up to full speed from start to full throttle. If Iím using a single engine in the car and want to get about 115mph I can calculate the drum to be 350mm wide by 60mm dia and made of steel. Now if I use this roller on a car with 3 engines it only takes 2.5 seconds to get to full speed. I need a roller thatís 80mm dia to get a 10 second run. It will still measure the HP ok but I need to see how the car reacts with the engines at full revs for the whole 10 seconds so need a different roller for a 3 engine car

If the roller is mounted on bearings and the cars back wheels placed on the roller and secured so it doesnít take off down the road you can then jam the throttle and measure how long it takes to accelerate the roller. What you need is to take hundreds of measurements a second and this can be done with a simple car logging device like the Eagle Tree system. You can then plot a graph to show the HP and any given time and itís a very useful tool to help tune the engines and adjust the gear change points.

If anybody wants to make an inertia dynamometer I have all the calcs on a spreadsheet so you can work out the roller size for your car