TripleZee Cycles

What is a Dyno??

A Dynamometer or Dyno is a device used to present a "load" to an engine, or a horse or a person for that matter.

Rowing machines and stationary bicycles are examples of dynos that give readouts of calories, power etc.

But as far as Motorcycles are concerned a modern dyno is used to test engines and bikes under load so tuning and diagnostics can be carried out.

Basically there are two types of Dyno, Engine and Chassis. An engine dyno is normally connected to the crankshaft of the engine under test. Primary, gearbox, wheels will have no effect because they arent there. So the main advantage is that the engine is the only thing affecting the output. The disadvantage is you would have to take your engine out of your bike to test it.

A motorcycle chassis dyno is one where the whole bike is parked on the Dyno, the rear wheel drives a roller and data is collected. Using a chassis dyno the transmission, wheels etc will all have an effect on the dyno output. The advantage being you dont have to take the engine out, the disadvantage is that the whole powertrain (gearbox, wheels etc) affect the output.

So we will look at chassis dynos because 99% of the time that is what you will see at your local motorcycle shop if they have a dyno.

Two main types of chassis dynos in current use, Inertia and Brake.

Inertia, basically the roller is a big heavy roller (around 1000 lbs) and as is fairly obvious it takes a lot of energy to spin that roller up. The bike drives the roller and accelerates from say 2000-10,000 rpm, depends on bike and setup, the electronics takes speed readings from the roller and using that data, the known inertia of the roller, the computer is then able to figure out how much torque has been supplied by the rear wheel during the run to accelerate the roller. Roller diameter, and engine rpm are used to figure out the reduction ratio and then HP and Torque at the engine are calculated. Depending on the manufacturer various "corrections" are made to account for the various losses in the system and then the final results are what you get on your bit of paper. The power the bike puts into the system is used to accelerate the drum and it is dissapated when you slow the drum down. Either by heating up your brakes if you use those to slow everything down, or heating up your engine if you use that to slow it down. Many Inertia dynos have brakes connected to the drum. If you used a 100 hp to spin the drum up in 10 seconds you will need a 100 hp to slow it down in 10 seconds.

Advantages The big advantage Inertia dynos have is consistency due to the fact the drum remains consistent and never needs recalibrating, as long as you grease the bearings. Simple concept, you could even use it with a stop watch and a rev counter if the power went off.

Disadvantages The disadvantage is this consistency only applies to back to back testing of an individual bike. bring the same bike back next week with new tyres and you will likely get a different reading. Change the gearing and the reading changes. This is all to do with the inertia effects of the individual bike, you have no idea what they are without some way of calibrating each bike. Usually this isnt a big deal, sometimes it is, but it is something to be aware of when people start throwing numbers around.

Brake The difference here is the roller is normally much lighter, 2-300 lb, and it has some sort of "absorber" or load device, or brake, on the end of it. The idea being the bike spins the roller up and the brake loads the roller so the that it spins at a constant rpm. ie like driving down the road and applying the brakes. If you give the vehicle more throttle and at the same time push harder on the brake you wont actually go any faster but the engine will be working hard because you are giving it plenty of throttle. The 100hp your engine is putting out trying to accelerate your car will be dissapated in the brakes as heat and sooner or later they will start smoking and give up.

Back to the Brake Dyno, you set a certain rpm, the bike accelerates up to that rpm and the brake comes on and holds the engine at that rpm If too much brake is applied the engine starts slowing down and the computer backs the brake off a bit. If not enough brake the engine starts to speed up, the computer applies a bit more brake to bring it back. The computer is doing this 100's of times per second, in the olden days we used to manually do this with a big knob. Basically the brake absorber assy dissapates the 100hp that your bike is putting out as heat. The computer is reading drum rpm, plus the roller diameter, get a rpm reading, coupled with engine rpm the computer can work out HP and torque and print it out for you.

Advantages A modern brake dyno is far more versatile than even the best inertia dyno. Loading at a particular rpm is particularly useful for both tuning and diagnostics. eg testing for adequate fuel flow at full power, can be done using a brake, cant be done using Inertia. A brake dyno can do everything an Inertia Dyno does plus all brake functions, whereas an Inertia dyno can only do Inertia testing

Disadvantages the only disadvantage is the load cell needs to be calibrated every so often. If the power goes off you may as well go home, although I have used to use an old waterbrake and an old hydraulic dyno neither of which needed electricity. Thats about it

Testing

Two main types of test, Sweep Testing, and Step Testing

Step Testing, can only be done using a brake, basically program in your steps, say every 1000 rpm, and the dyno will hold the bike at every 1000 rpm as you move through the rpm range. ie 2000, 3000, 4000, 5000, etc. Hold it at each rpm for a few seconds to let the engine settle, take you readings and move to the next one.

Sweep Testing, basically the bike starts at a low rpm, you give it throttle and it accelerates through its rev range until you shut the throttle. This is what an inertia dyno does.

A few different types of sweep test,

Uncontrolled Sweep this is what an inertia dyno does, the load is the drum and the acceleration varies as the power of your bike varies. Ie normally slow to start off with and as the power comes on the acceleration rises. Basically there is no control over any aspect of the test.

Constant Acceleration, the drum accelerates at a constant rate. You need a brake to do this, As the power comes on the brake has to come on to compensate and stop the drums acceleration from changing.

Variable Acceleration Similar to taking your bike down the strip. The initial acceleration is high and as the test goes on the acceleration rate drops. Also need a brake to do this.

An Inertia dyno can only do uncontrolled sweep testing. If any of the other tests need to be done then a Brake is required.

A big disadvantage of an Inertia dyno is you cannot target particular rpm for testing. eg you have a problem at 7000 rpm you can only run through 7000 during an uncontrolled sweep test whereas using a brake you can test at 7000 rpm

Using a brake you can even program a sweep test for 6000 to 7000 rpm. While you can program the same test using an inertia dyno, the bike still has to do an Inertia run starting at 0 before it can get to 6000 rpm.

A Dynojet 250i is a hybrid dyno, ie it is an Inertia dyno with a brake attached. This setup is designed for auto tuning Power Commanders to an AFR, not recomended by Dynojet for use as a Brake dyno because of overheating issues. OK for power Commanders but limited functionality as a Brake Dyno.

more to follow

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