How a rolling road (chassis Dynamometer) works

The car is driven onto a rig so that the driving tyres are resting between two steel rollers. The torque is measured at different speeds in exactly the same way as the engine dyno works except that it is torque at the rollers rather than torque at the flywheel. The braking load is applied to the rollers by either a hydraulic (water brake) or electrical system again in just the same way as the engine dyno would apply a torque to the cranksharft of the engine.

The same universal equation at the top of the page can then be used to calculate bhp at the rollers by knowing the torque and the rpm of the rollers (NOT the rpm of the engine at this stage) - but if the engine rpm is measured simultaneously then we can know roller bhp at a particular engine rpm.

The BIG problem with all this is if any tyre slip is taking place. Remember most of these other rolling road tuners use smooth steel rollers, which over time get quite polished. How much grip do you think you would get if roads were made of polished steel rather than tarmac? The effect of tyre sleep are complex but what we do know is that you can get some really strange bhp figures from highly tuned engines on narrow tyres and the readings are invariably too high not too low.

http://www.g-force-motorsport.co.uk/...surement2.html