What is a dynamometer?

A dynamometer is a device that measures force (torque) and power. 

This measurement can be taken a the crank, the countershaft, or the wheels.  A crank hp number is higher than a wheel hp number, because each time you add another rotating part that the engine has to turn, you lose power! 

Most hp numbers listed by the manufacturers are Crank hp.  Most aftermarket company hp numbers are measured from the rear wheel.

There are many different kinds of dynamometers (including some that test springs and shocks).  For measuring engine horsepower and torque, the most popular style currently is an inertia-type dynamometer.  It measures the force and power that the spinning wheels of a motorcycle,  ATV, or car produce (there's also specially built inertia-type dynos that work for snowmobiles, etc). 

A "brake-type" dyno measures the power that is actively absorbed by a water, oil, or eddy-current brake or by a generator.  These are also called hydraulic dynos.  Most brake-type dynos are run off the countershaft sprocket on the engine, rather than at the wheels.

Both designs have their assets and liabilities.

Powroll's Dynojet Dyno

We'll start with the inertia-type dyno (also called and acceleration or drum dyno).  Two popular brands are Dynojet and Superflow.  Dynojet is the current choice for most ATV/Motorcycle tuners, while the Superflow is geared more towards automotive applications.

Basically, an inertia-type chassis dyno consists of two great big heavy drums hooked up to a computer. An inertia dyno uses the power of the engine to accelerate the drums to speed.  The computer measures the time it takes to get up to speed, and the horsepower is determined from these numbers (for a more detailed description of this process, see the info below).

Inertia dynos measure in horsepower and then extrapolate the torque numbers from those readings.

Powroll's Hydraulic Dyno

Many tuners enjoy using brake-type hydraulic dynos.  This design is not as popular currently, simply because of the time it takes to set up a run on a hydraulic dyno compared to the drum dyno.

There are many companies that sell hydraulic dynos Go-Power, Taylor, Stuska are a few of them.

This dynamometer uses a load cell (filled with fluid) to increase the load on an engine, creating a very realistic map of what an engine does under most riding conditions.

The Hydraulic dyno measures torque, then you can extrapolate horsepower from this data.

Powroll initially used Hydraulic Dynos exclusively because they felt the Hydraulic dyno gave tuners a better idea of how the engine would behave under regular operating conditions. 

The hydraulic dyno allows you to increase the load on the engine during a run.  This can show the technician where a camshaft or carburetor is working, and where it isn't! 

For a good example of the difference between what you can do with an inertia dyno and a hydraulic dyno, take a CR 80 with an aftermarket exhaust system.

We'll say some 220 lb logger buys a brand-spankin' new 2004 CR80 with an FMF pipe to use as his new woods weapon.

The big fella takes the machine for a test ride in the shop parking lot, and the thing rips!  The following weekend, he throws on his gear and goes out in the woods.  After an hour, he's smoked the clutch and is ready to throw the thing at the next spotted owl he sees.

He takes the bike back to the shop and says, 'there's something wrong with this bike, Honda says it makes 20 hp, but I can't get it to go up Snot Hill'.  The shop puts the bike on their Inertia dyno and gets a result that looks something like this:

From these results, it looks like our logger buddy should be able to run the thing right up Snot Hill without ever slipping the clutch.

Now, put that machine on a Hydraulic Dyno.  With a light load, you'd see a similar curve.  But put a heavier load (maybe something that would equate to a 220lb guy riding up a 20% grade?) and you might see something like this:

No wonder our logger buddy spent more time pushing than riding.  Time for the big fella to step up to a CRF450X.

With the Hydraulic Dyno, most anything you feel a bike or quad do when you're riding it (midrange hesitation when climbing a hill, etc), you can replicate on the dyno.

So, why doesn't everyone use a Hydraulic Dyno?  A few reasons:

Okay, we've all seen them.  Claims of massive horsepower gains by using everything from fuel additives to lighter weight riding gear -- all backed up by DYNO RESULTS!!!

So, if the dyno is such a great diagnostic tool, all the dyno charts and numbers you see MUST be correct, right?

Well...sometimes.  And only for some things.  And not really anytime if you're comparing results from one dyno to another.  And only if the dyno operator is honest.

Now, for those of you who really want to understand how that Dynojet Dyno works, here's your chance!

You know the stuff you were supposed to remember from high school physics class about Force?  No, not John Force.  Newton's Second law of motion.  On an inertia dyno, the computer calculates the acceleration of the dyno drums by continuous measurements of their speed and the time.  If the surface of the drums spin from a speed of zero to a speed of 10 feet per second in one second, then their surface acceleration is 10 feet per second per second, or 10 ft/s².  Sound familiar?

Force = Mass x Acceleration

That's Newton's 2nd Law of Motion.  Force is one of the things that we're looking for.  Force in the automotive world is called - you're supposed to shout it out..... torque.  Torque is rotational force, and its most common unit for us is foot-pounds.  Plain and simple, if you have a 12-inch wrench and you lean on the very end of the handle with 10 pounds, you're applying a force of 10 ft-lb. to the nut you're trying to turn.

The mass in our case gets a bit complicated.  Mass in most cases is easy - how much does the object weigh that you're accelerating.  In the case of the dyno drums, however, it's not that simple because we are not "moving" the drum, we're spinning it.  We are not creating a "translational" motion on the drum, we are creating a "rotational" motion. 

To understand the difference, think of the actual dyno drums.  Say each one weighs 2700 pounds.  It would take a pretty impressive force to push a 2700 lb. object across the floor.  Now imagine just spinning those drums.  The shaft going through the center of the drums rests on two hugely expensive bearings.  The drums spin with the slightest touch.  To calculate away this difference, physicists came up with the "mass equivalent" of a rotating body, which is very similar to the "moment of inertia." 

We haven't measured the actual numbers (the computer program knows them), but let's just pretend the mass equivalent of the dyno drum is 50 pounds.  That means that spinning the 2700 lb. drum is like pushing a 50 lb. weight across the floor (yes, there's friction, but lets not go there for this demonstration, ok?) 

Some people say that this is not the correct way for calculating horsepower, but in this case - it is.  Many of you are familiar with the hp = rpm x torque / 5252 formula.  In many situations, that formula is the way horsepower is calculated.  In the case of the Dynojet dynamometer, it is not.  The Dynojet calculates horsepower even if there is no torque reading, and it does this in the manner shown.  Those of you that have had your vehicles tested at a shop and have watched the computer screen closely have seen that the computer plots out only the horsepower after each run.  It does this because sometimes there is no torque reading, or the torque reading is faulty.  This can happen because inductive and optical rpm pickups may not be perfect, and/or your ignition system may make things go a little haywire.  If the rpm readings are not perfect, and the computer used the rpm x torque / 5252 formula, the hp reading would be inaccurate.

Work = Force x Distance

Now we get to the good stuff.  Calculating the work is pretty simple for the computer.  It just figured out the force, and it can easily figure out the distance because it knows the circumference of the drum and how many times it has rotated.

Power = Work / Time

We have an answer.  Somewhere the computer factored in the bearing drag, and it throws some constants into those formulas to get the numbers to come out into the right units, but you get the idea.

In the real world -

What's the difference between horsepower and torque?

Basically, Horsepower is the ability to do work in a given amount of time, and Torque is strength. 

The analogy that most people seem to understand is that of a human weightlifter to an automobile engine.  An engine in a typical 4 cylinder commuter car might have 100 hp and 100 ft-lb. of torque.  How about the weightlifter?  Let's hook him up to a handwheel that has a 1 ft. radius.  A big strong guy should be able to push and pull on that wheel with 100 lb. of force, therefore generating 100 ft-lb. of torque, easily equaling the engine.  When it comes to horsepower however, our guy is going to fall short.  One horsepower is defined as lifting 550 lb. up one foot in one second.  Two horsepower could be 1100 lb. up one foot in one second, or 550 lb. up two feet in one second or 550 lb. up one foot in half a second, (you get the idea.)  Let's attach a rope to the handwheel and tie that to a 550 lb. weight.  How is our weightlifter going to do now?  You can imagine - not very well.  Attach the handwheel to a gearbox and then to a wheel with the rope and he might be able to perform some work. Unfortunately, he'll be lucky to generate a few horsepower before petering out.  Bottom line - both the engine and the weightlifter can be strong, but the engine can perform a lot more work.

Which one do I want for my vehicle?

Torque is what gets you started.  Torque breaks your wheels loose when you launch, and torque allows trucks to tow heavy loads.  Torque accelerates your vehicle.  Combine some measure of torque with speed and time, and the result is horsepower.  Horsepower is what gets you through the quarter-mile.  As far as which one is best, the answer is - it's best to have both.  High torque numbers allow you to leave the gate quickly and power out of turns.  High horsepower numbers keep the vehicle accelerating and give it a higher top end.  Just as important as the peak numbers, though, is what your curves look like, and where those peaks are.  An engine might have a high horsepower peak at a high rpm, and a graph that looks like one side of a steep mountain.  This means that in order to take advantage of the high horsepower, the rider must keep the engine at that high rpm.  This requires more shifting, which can result in lost time.  Flat curves mean that the rider can take advantage of the engine's power without as much shifting, but in order to obtain a flatter curve, an engine tuner must sacrifice some power. 

To determine the type of curve that will work best for you, take into consideration the following:

• Riding Style
• Reliability (peaky engines which require lots of clutch work and sustained high rpm don't last as long)
• Skill (smoother power makes beginning and intermediate riders faster, while experts can benefit from a peakier curve)
• Type of riding - Especially if you ride widely varying terrain.  Say you race MX once a year, but ride tight trails every weekend.  You'd be tempted to build an engine that would rip on the MX track, but would that be the best option for the majority of your saddle time?
• The engine's original design and intended use.  Yes, you can make a Banshee into a Tractor Pull contender, or race a Grizzly on the local TT track, but completely changing an engine's power curve is expensive and normally not very effective.

Newton's Law of Universal Gravity was and perhaps still is one of the greatest discoveries in the science of physics (yup, even greater than the Pastrana rule of 360). Sir Isaac Newton came up with this theory by comparing the acceleration of the Moon with the acceleration of objects on the Earth. The law of universal gravity depends on the amount of matter in the objects being attracted to each other, and the distance between the objects being attracted to each other.

Third Law of Motion: For every action, there is an equal and opposite reaction. For every action force, there is an equal in size and opposite in direction, a reaction force. Forces always come in pairs, known as "action-reaction force pairs". Identifying and describing action-reaction force pairs is a simple matter of determining the two interacting objects and making two statements describing ‘who is pushing or pulling on whom', and in what direction.