http://www.youtube.com/watch?v=gxVkJuidyfg So who is very familiar with 4WD chassis dynos and can technically explain how and why the front wheels are spinning in this video. Bob S.
The how part is a chain in many cases becasue it has the least power losses and easily keeps front and rear speeds sinc'd, but any number of other method could be used such as a drive shaft, cog belt or even electric motor I guess. The why, well, I don't know exactly, but I'll give you my best guess. Many newer cars have traction control systems that limit power when they detect wheel spin. There is always a button to disable the system, but the truth is the system is never completely disabled. The ferrari graphs I've seen chop/flatline the power at about 5k rpm for example. Running a car like that on the 4 wheel dyno will convince the computer that all is well and to let the engine run at full power. You lose a bit of power driving the front tiers, but you don't have a choice becasue the computer won't let the engine make full power if you don't.
Mark, That was my speculation as well. For the guys with 4WD like a Gallardo I guess they just drive all 4 wheels or can they configure the car differently to obtain a higher number via just the rear wheels? Anyone out actually operate one of these things?
Yes, the 4 wheel drive cars drive all 4 roller, but they are still chained together front to back on the machine. I don't think any of the car 4 wheel drive cars out there can disable the front axle at will.
