Rolling Roads and ‘dynos’

by Fuggles
8 years ago

We all know rolling roads or dyno’s or whatever general term you want to call them are used for measuring the power output of a car (or bike or pretty much anything with wheels).  What is not so well understood is how they work and what a difference you can make just by doing things differently.

Brake Horse Power (BHP) is often explained as if it was something that existed, rather than it being a convenient number. Bhp can be defined as “A unit of power in the U.S. Customary System, equal to 745.7 watts or 33,000 foot-pounds per minute” or, to put it another way “A measurement standard used by manufacturers to help consumers compare engines”

Torque is another term often sued and you will often hear people arguing that torque is more important then bhp.  Strange then that magazines, tuners, enthusiasts and just about everyone else eulogises about bhp and quotes the bhp figures, rather than the torque figures.

At 5252 rpm bhp and torque are the same.  That is because hp = torque x 5252rpm.  Why that number I have no idea.  In case you’re thinking this is a worthless piece of information, check where the lines cross on your next dyno printout.  If they don’t cross at 5252 then all may not be well with the figures, the readings, the equipment, the user.

So why BHP?  The brake bit comes from the fact we measure torque on a device used an Engine Brake, or dynamometer to use its proper name.  So, we measure torque at a given rpm and then from that we calculate the bhp.

When an engine is set up and when the same dynamometer is used you would expect to see the same results time and time again.  If only everything was that simple.  This is more likely to happen to an engine dynamometer – where we bolt the dyno straight to the flywheel – but even then there will be different results from one day to another.

If you run the engine up to the test rpm quickly and take a reading, (known as a flash reading) it will be higher than if you let the rpm stabilise and then read the load. What happens is that the longer you run the engine, the more the engine heats up the combustion chambers, the spark plugs and the inlet manifold. The power drops off a little as a result. The traditional method of power testing automatically stabilised the engine before taking a reading because you had to set the load, and hence rpm, manually.  One could argue that to get an accurate reading at Club Dyno days we should let the driver (I guess that’s the right term) hold it on the red line for three seconds or so.  Great idea in theory but I can’t see many people wanting to do this myself.

To explain it better we have cribbed some information from a dyno web site:

“As the operator you open the throttle and the load holds the engine rpm back. You then back off the load and as the rpm climbs the engine suddenly comes on cam – and the revs soar. Quickly you wind the load back on and get the engine down to the target rpm where you want to take a reading. You take a reading off a spring-loaded scale and then measure the engine rpm. Now you close the throttle back to idle, write down the data and think about the next reading that you want to take.”

With modern technology I suspect it is possible today to write a program to do all of the above, but where’s the fun in that?

Whatever approach you take maybe the “flash reading” is more fun and also gets higher (better) figures.  A few years ago at my first dyno, my car had done 98,000 miles and I was very nervous about putting it on a dyno.  Every time I looked round I could see Guy circling with his arms and saying very slowly “B-o-o-o-o-o-o-o-o-m”.  I’m not sure I could have stood three of four seconds of the car screaming away at top rev.  So, flash readings get my vote too.

Rolling Roads

A rolling road takes its power reading directly from the driven wheels of the car. This means involving gearboxes, drive shafts, differentials and tyres. A lot of people talk about “bhp at the wheels” (atw) as being the only meaningful number to quote: “It’s what you race with”

Bhp atw is accurate for all the reasons stated above, but then the gearbox gets in the way of the true picture. We measure the torque at the wheels but the rpm is measured at the road wheel roller. Put the car in a lower gear and the torque at the wheels increases – but the rpm of the roller is reduced. In theory the resulting bhp should be exactly the same – but it never is. The lower the gear that you run the car in, the higher the bhp at the wheels. This is because we have rolling losses (some call them transmission losses) that increase with increased roller (and hence road wheel) rpm. The biggest single rolling loss is the tyre.

Some rolling roads have a single contact patch some have two.  Where there are two the tyre is compressed, increasing the resistance and thereby changing the reading.  Also, the type of tyre, tread, atmosphere and many other factors will also change the readings.  So, the more in the way of variables you can remove the better, for consistency, if not for the “it’s what you race with” argument.

A recent experiment on a rolling road showed just how significant tyres and contact set-up can be.  Unfortunately for us it used a VW Golf, but we all have to start somewhere!  After an initial run with everything ‘normal’ they set about making some changes.  The tuner over-inflated the tyres by 10 psi into the tyres and checked the power at the wheels again. The power went up by 4bhp!  Now imagine what happens to the rolling losses when the tyre is compressed by several bodies sitting on the back of the car trying to find enough grip to prevent wheel-spin? Hmm, I remember a couple of times this has been done to Skylines.

In addition to all of the above we also have to take account of centrifugal force which makes the tyre grow – which alters the gearing slightly, putting the rpm out by a tiny amount. When you take all these “fudge factors” into account, it’s a wonder the rolling road is as accurate as it is. Okay, so maybe now we’re getting overly critical.  But it doesn’t take much to get a few points out n half a dozen areas and suddenly that £10 bet that you had with your mate over who had the most power comes down to “fudge and error” not truth and accuracy.

The way to overcome the excess weight, pinching of tyres and other inconveniences is to add back ‘rolling road losses’.  The rolling road makers decide what these are and who are we to disagree?  However, it has to be said this clearly shows a level of inaccuracy.


Having dealt with tyres now let’s consider gearing.  This affects dyno-packs as well as rolling roads.  Several factors prevent you from getting exactly the same result in every gear. First off a lower gear means more torque at the wheels and hence a little more tyre slippage than when you run in a higher gear. The run also takes less time, so the engine accelerates faster and gives you more of a “flash” reading.  Most dyno days are run in 4th gear (5th for R34s) because it is 1:1 (or close) and there is less transmission loss.  Or at least that’s the general argument.

At the Hubs

Not all dynos measure at the rubbers edge.  There are engine dynos (mentioned earlier) and there are ones which bolt straight on to the hubs.  Whilst these remove the inaccuracies of wheels. Tyres, pinching and so forth it does have the disadvantage of not reflecting accurately the true situation on the road.  This should not be viewed as a bad thing.  In some respects it could be argued to be better as it gives a greater level of accuracy and consistency.  That said, it still suffers from some of the problems caused with any car that you are measuring, inasmuch as it still has to account for transmission loss.

The Hardware, what it is, and what it does!

Rollers or drums that the vehicles wheels sit on.  Or what the wheels bolt on to: 
These either transmit the power to the “brake” in the case of braked dynamometers, or are heavy solid steel drum or drums that absorb and store the power & energy during the run.

Dynamometer Chassis:
Basically just a metal frame that supports roller or drum mountings, and vehicle tie down points, and fans / extractors etc.

Cooling Fans:
These are essential equipment!  They do not only stop your vehicle from “overheating” but should also provide enough air for continual back to back accurate runs without heat soak in the vehicle to begin effecting the recorded power curves.  This mean A LOT OF AIR is required!

Exhaust Extractor
More essential equipment!  It both stops people inside the building from getting poisoned!  and arguably more important, it stops the dyno test room from getting both hotter, or having exhaust gas pollution which reduces an engines power.

Room Ventilation!
Essential, because as the engine and the exhaust extractor remove air from the room it must be replaced!  Open some large windows or doors if nothing else because otherwise the rooms pressure will drop giving lower power.

On a braked dyno system there has to be “something” that balances against the power your engine is making.  On most modern systems this is usually an “electric” EMF brake, located alongside the rollers that your wheels will be driving.  Basically it generates electricity, and then feeds it back to cause a load.  Sometimes water or a big disk brake is used instead.

Load cell
This is what a braked dyno uses to sense how much torque is being applied to the rollers.  It is usually an analogue device that sends this signal to the dynamometer for processing. Thin of it as a small “scales” that measures weight.

Gas analyser
This is an electronic gas sensor.  It uses a small pipe that clamps inside the exhaust tailpipe.  this “sniffs the gasses” so to speak!  takes a few seconds, but it can then tell the operator if the engine is weak, rich, worn out, not atomising the fuel or burning the fuel properly, etc.

Computers and software

Brake Dynamometer Software

You do not need any computer software to calculate simple BHP or KILOWATTS from an engine. For example, a simple braked dynamometer can be made with nothing more than a spring balance!

When an engine is running, it turns the roller/hub mount in the opposite direction, which forms the brake.   This twist is the torque.  If a lever is attached to the mount and is the right length and calibrated correctly with a spring balance you can then measure it and, with a few formulas and knowing the rpm and a few other pieces of data you can work out the power.  Very simple in one respect but incredibly complex if you want to get it right, accurate and consistent.  If the lever is one foot long and the spring balance read 20lbs then the reading would be 2 Pound-foot or torque:  simple!

Dynamometer Software can display a graph of the engine speed & Torque.   Because it can do this, it can also calculate horsepower using a simple formula.  So it can also display RPM & Horsepower on the graph.  But the software also needs to have a method of storing and retrieving all the runs, and customers, and other information.  It also needs to know the air intake temperature, the atmospheric (barometric) pressure, the relative humidity so it can calculate the corrected power curves.   It also needs to be able to control the load (how much “brake” during the run. It has to do this to allow x number of RPM rise per second during the run.  And much more!  The software is the complex part of a dyno, and without it the Dynamometer is an extremely basic device.

Inertia Dynamometer Software

This works a little differently. We have no load cell to give us a torque reading.  Here we simply accelerate a heavy steel drum, of known rotational inertia.  This is usually calculated from the drums dimensions on well designed systems, so this is an extremely precise value!   A high accuracy dual hall effect “gear position” sensor is required (or similar) that reads each 360 degree rotation of this drum.  Ideally this drum is timed by a separate, extremely accurate time base – independent from the computer board.  It also MUST read real time engine RPM during each revolution. All this data is sent to a PC which can then calculate the power / torque / speed / gear ratio / etc and store and display each run / customer for future reference.

A normal PC cannot be used for this drum timing because it simply is not accurate enough, and the PC time base “wobbles” slightly. Meaning that much data averaging has to be done to display a “smooth” graph


For mapping, the rolling road’s acceleration mode is all but useless, apart from full throttle runs. You have to be able to switch to fixed (constant) rpm running in order to map and engine.  If ever you have spent time with someone mapping a car it is far loess exhilarating than a dyno day.  It is also very boring watching someone manipulate pages upon pages of spreadsheets.  I’m sure the people doing it know their stuff, but a spectator sport it aint.

So what are the pros and cons of dynos and rolling roads.  If you want engine mapping consistency then a dyno has the edge.  Having said that if you really want to go the whole hog an engine dyno is even better!  However, a rolling road has the benefit of giving the complete picture.  Regardless of the pros and cons one thing is for sure – they are now a vital part of tuning and mapping a car.  Whichever is the best, they all have good points and bad points and they all help make getting a car just right, that much easier and that bit more accurate.


Comments are now closed.