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RTAnalysisTechnical / MeasuredPower

Differences between power at the wheel as measured by our systems, and power at the flywheel.

Our systems in common with all chassis dynamometers measure the power produced "at the wheel". Some dynamometer systems allow you to see an "at the engine" or "flywheel" BHP figure. There is ALWAYS more power available at the engine, than is available at the wheels! Gearboxes, and axles, along with tyres all absorb some power. They absorb more power as the shaft speeds increase due to oil drag for example..

Tyres absorb more power as the road speed increases, so there will be greater losses here in top gear than in say 2nd or 3rd gears...
Now remember, on ALL chassis dynamometers that this "power at the engine" BHP figure can never be accurate! It's is at best a "guide" but to be honest its not really worth bothering with. There are several ways of "calculating, or "measuring" this loss depending on the manufacturer...
Most do it this way, and it is the closest you can get to being repeatable and accurate - but its still wrong!!!

Dyno software can sometimes do this, purely for measuring the drum losses alone (with no vehicle sat on it) as it runs freely down from 200mph to 0mph. This is so that we can allow for the drums own frictional/aerodynamic losses at all speeds. In fact we HAVE to be able to do this because the readings the dyno give us need to be measuring only what is applied to the drum! This "loss" is actually very small, but is "added" to every single dyno run automatically by the software.

This is not a "user" feature however and is only for internal use by myself. The same feature can also measure the losses that a car / bike transmission & tyres cause. It does this the same way, in that after a run the vehicles clutch is disengaged, and then the drum is logged as it slows back down to a standstill. This allows us to create a "negative" BHP curve showing the road speed v the transmission and tyre losses. This curves data can be inverted and added to the measured power curve to give a "flywheel or engine power curve!

It can never be truly accurate though because there are actually MORE losses than this under load because one component of the losses depends on the actual torque being transferred. Gear teeth under high load "slide" against each other giving increased friction (although only minor due to the oil film) and the same applies to thrust bearings etc inside gearboxes and diffs. This method cannot know how much this happens as it measures the power required to turn tyres, gears, chains, etc whilst not under load conditions.

So SOME software adds a percentage! as well... Which is simply wrong! It is a kind of "fudge factor" that hopefully arrives at a figure that is approximately correct! Sometimes this fudge factor is increased and decreased according to the torque / speed etc.

Some (really dodgy dyno systems) just "guess" at a percentage!!! (You know who you are!) which is so far from being correct its simply criminal!

Remember that it is NEVER a percentage.

With an average transmission, (gearbox/differential, tyre roller interface) you are looking at roughly the following variables:

a) Tyre / roller interface - for any particular tyre / drum the following is true... To begin with there is quite a high value from 0 to say 1 mph. This stays as speed increases but the power needed slowly increases with the road speed, (not engine RPM!). So this loss is greater in taller gears and at higher speeds. Again it almost completely unrelated to the unrelated to the power being applied. So is not a percentage of power! UNLESS they spin...
b) Rear (or front) diff / axle (car) losses - This is both load (torque) related as well as Road speed related. Both components need to be considered here. Oil drag, and grease in CV joints and wheel bearings and prop shafts, as well as internal windage losses and external windage losses on prop shafts etc, all increase as road speed rises. They are NOT engine RPM related or Power related. These all increase with road speed only, and are independent of the gear the vehicle is in. These are the largest cause of the total losses in differential, prop shaft, drive shaft, or wheel bearings.
However some other losses also occur in diffs and axles that are load (torque) related... Gear teeth, side loads caused by helical cut gears on thrust bearings etc do experience greater losses as the torque going through them increases. This effect is pretty minor compared to the above, but does occur. There is obviously an oil film present, or the components would destroy themselves, but the increase is measurable.
Every car is different so its impossible to say how much is determined by speed and how much by load... And of course the gear you are testing in matters too! In top, there would be far less torque, but far more speed! So you get more of one type of loss and less of the other! But since the majority of the losses are the first type you will get more power in third than in top gear actually reaching the rollers.** Theoretically, but since 4th gear is 1 to 1 ratio, or "straight through" in most cars there is less gearbox losses in 4th! So in this case you will now read best power at the wheels in 4th gear!
c) Gearboxes - losses in gearboxes are similar to those in an axle or a differential. Most of the losses are simply, bearing, and oil drag. These are engine RPM dependant, not power or torque dependant. Some of the losses ARE dependent on the power and increase as the power transmitted goes up. Marginally. So again a simple percentage cannot be used, and measuring the losses by letting the dynamometer "push" the vehicle with the clutch disengaged also does not allow for these load dependent values either...

So NO "engine figures" can be accurate! Only go by the real recorded "at the wheel", corrected figures!

Page last modified on August 30, 2012, at 02:26 PM