A Quick View of Chassis Dynamometer

Testing )@HAROLD BETTES 7/6;6:)@ROB KELLY Chassis dynamometers are very popular provides. The difficulty here is the units you can enjoy the scenery. Or if your to run some quick tests for installed cannot be calibrated easily to establish cooling system is overloaded from traffic and check out the chassis and their inertia values which are typically being slow it might cause the engine to drivetrain. They are quick to use but in units of ft-lbs/sec/sec. So it is easy overheat. At high power levels the heat have some problems that should be to understand that if you can’t verify load increases hence the requirement for made clear before you start down that the calibration, you might get nicely a very large fan or maybe more. That direction. If you plan on testing a few repeatable data but perhaps more or less is why most popular chassis dyno tests sets of exhaust components or any than another test facility. Normally in are just quick spurts that make it easier bolt-on parts that might take more time those circumstances the place that gives on the whole operation. By the way a than if the testing was done on an engine bigger numbers is the most popular test normal expression for a heat load is in dyno, so do some planning and think location. BTUs (British Thermal Units) per time. about what you want to accomplish. – Torque is a twisting motion In order to put this in perspective, if Chassis dynamometers come in all and is typically expressed in lbs-ft. Notice you wanted to test a vehicle that might sorts of designs and configurations but this is not ft-lbs! Although everyone produce 500Hp at the drive , there are some things that are common commonly uses incorrect units for to all. Most designs test a vehicle that description of this very important item, that would easily be a total heat load powers a roll or rolls for all drive the proper reference is indeed pounds- of approximately 1500Hp (3.8 million types. Most can provide some common feet (lbs-ft). BTUs per hour!) that must be dissipated numbers for but not all Horsepower – 1 horsepower is into the atmosphere from the cooling measure torque to do so. Basically there equivalent to 2546 BTU/hr or 550 ft-lbs system of the vehicle and the drivetrain, are two general types of chassis dynos of work per second. The most interesting exhaust system and the tire patches and – inertia only and those that measure is from the calculation of Hp= (T x the dyno itself. Of course that varies torque with either an electric load or RPM) / 5252 and in that equation the somewhat by how much you allow the even a mechanical brake. All have to get torque value is in lbs-ft as described temperature across the room to rise. rid of the heat from the vehicle and the previously. Perhaps this stuff is a little more complex dyno as well. Speed – Most common references than you thought. in the US for speed is miles per hour Speed Capacity – Often a mechanical Terms of Engagement (MPH). Speed can also be in feet per limit set by the manufacturer such as Such terms as torque, horsepower, speed, second such as 88 ft/sec = 60 MPH. 150MPH or some other number that roll speed, dyno inertia, heat load, power Roll Speed – Refers to the speed of should not be exceeded for safety’s sake. capacity, speed capacity, and many others the roll(s) on the chassis dynamometer Power Capacity – Also a number set can easily roll off of the tongue or rattle and can be directly related to the vehicle by the manufacturer that is fundamental from a keyboard. It takes a bit more to speed or simply given as roll RPM. to the capability of the drive tires. This understand what all those things mean. Because of the friendly relationship of capacity number is quite often higher Lest this short article end up being round things to Ÿ or 3.1416, it is easy to than most vehicles can even contest. The something from a text book, let’s stay calculate the circumference of the roll by term is also normally associated with a with the quick and easy method. measuring the diameter and multiplying speed such as 500 Hp at 120 MPH or Dyno Inertia – Can be all over the that by Ÿ. That gives us the opportunity something similar. map for values and most are poorly to verify some dyno basics. defined and too many are listed in Heat Load – Is not the number of The Salesman Said – Is often confused pounds. Although that is not technically cops per city block. The term has to do by what the specifications for a given correct it refers to a weight that should with the heat that the test vehicle and the unit state. When in doubt, look at the equate to a vehicle weight. The inertia dyno must dissipate to the atmosphere specifications and ask questions if the only types measure the time that a or the room the dyno and vehicle are information is not clear. Normally the supposedly known mass is accelerated in. In short it takes a lot of moving air operator is a good source for quality by the test vehicle in order to calculate to keep the overall packages cooled information because of his or her horsepower and because there is a speed down. Normally you never consider experience running vehicles across their signal the torque value is back calculated that as you drive along at various speeds chassis dyno every day or at least several from the horsepower number the dyno the moving air carries heat away and times a week.

34 APR-JUN 2010 engine professional Typical testing for improving the tune up on grocery getters is a normal application for chassis dynos. This photo shows an AWD Subaru wagon undergoing a power test.

Planning a Test operator stabilizes the speed and at the What is best for an engine builder chassis or You need to be very practical when you signal to go the throttle pedal is pushed engine dyno testing? do planning for any kind of dyno test to maximum until the test is terminated. Most engine builders that don’t already but particularly when you are going for That type of test yields the power at have a dyno would prefer working out a chassis dyno session. Not only does the whatever engine RPM is equal to the set engine configurations and tuning on vehicle need to be up to safety standards, speed (60mph in this example). Those an engine dyno. The engine is more but many other things need to be type tests can take up to about 10- accessible and it is generally easier to addressed if you are to get the most from 15seconds per data point (such as 60, work with an engine dyno facility to the time and money spent. 70, 80, 90, 100, 110, 120mph) in order accomplish the tests that an engine to get a completed power curve vs speed builder would want to do. How many tests can you get done before you or RPM. Some dyno manufacturers do Some engine builders do both tests need to stop and cool off the vehicle? not have reliable engine RPM triggering (engine dyno and chassis dyno) if a Depends on how the test facility is and if that is the case, Vehicle speed vs customer has the money and patience. equipped for airflow across the vehicle Power is a test that can be done. With Those tests can provide some impressive and other things, including how the some simple arithmetic after the test the data and allow focusing on how much exhaust gases are handled which can customer can calculate the engine RPM power the chassis loses so more effort drastically influence the test results. and graph the results. can be put to resolving some of those losses. How long does each test take? How much do chassis dyno tests cost? E85 – E15 – pump gas – race gas Depends on the type of test and the capability of the dyno and the facility. Depends on the facility and the services and air –fuel ratio data? Most engine Sometimes, it’s only a few minutes offered. Charges are all over the map and dyno facilities are better equipped for per run. A simple acceleration test ranges from 3 runs for a $100 to $1000 changing fuel and supplying data for typically takes the least amount of time. per day. Work this all out before you go A/F ratios however many chassis dyno Acceleration tests going from about to a facility to test. test sites can simply do the test without 40mph to 120mph are the most common supplying data on fuel tune-ups unless and each run will only take about 15 Who does the work or changes on a vehicle they are equipped with an up the tail pipe seconds (not including the coast down between tests? sampler for Lambda or A/F numbers. time to stop the rolls). It is common to Better figure that one out before you go get the rolls up to speed and the vehicle to the test site because many facilities What data is more reliable for testing in high gear before a test begins. Another have problems with insurance rules and components – chassis or engine dyno? common test is at a fixed speed (using might not even allow the vehicle owner Each can be reliable but generally the a dyno that has variable load capacity) in the car or in the test room much less to conditions in the engine dyno test cell such as 60mph in high gear and the work on the car or truck. are easier to control so the results are

engine professional APR-JUN 2010 35 CHASSIS DYNAMOMETER TESTING )@HAROLD BETTES 7/6;6:)@ROB KELLY

This muscle Mustang is on a large roll dyno about to be tested. The Mustang is on a lift so the dyno can be mounted above floor line. The dyno uses an eddy current power absorber in between the drive rolls. Large rolls are better for either high power or for endurance testing. The fan shown is for short tests only. Longer tests require lots of airflow.

likewise. However for that installed sensible to plan on some set percentage repair technicians on hand can be very condition the chassis dyno is quicker as a loss in the drivetrain even though helpful in solving vehicle problems and easier for a completed assembly. The some chassis dyno manufacturers do without driving in the traffic to attempt best case would be to test on an engine that anyway. It does make the arithmetic to analyze a particular problem. Probably dyno and then on a chassis dyno so that easier but it does not make it correct. works well for anything except for road the user could evaluate the losses in the There are many ways to get a good rattles. chassis (nothing is for free) and verify estimate of the power losses but it is best other details about the engine installation to have good data from an engine test such as fuel supply and cooling system and then find out what is left at the drive How about safety issues on chassis dynos? and exhaust system influences. Exhaust tire(s). The power losses in the chassis Testing on a chassis dyno is normally gas feedback might be more of a problem are not the same across the board. The fairly safe if the vehicle is restrained in a chassis dyno test if the airflow power losses vary with both speed and properly. That problem is best addressed across the dyno is not adequate. That temperature of the lubrication fluids. by the facility and the instructions would change test results without much provided by the manufacturers. There indication. How about finding problems on a chassis needs to be fire extinguishing equipment dyno to keep from driving the vehicle available and care should be taken for How much power is lost in the chassis? around? the potential of a breaking How to calculate the engine power if a That is one of the better things about or a tire shredding and those sorts of chassis dyno gives power at the drive tire chassis dyno testing because it provides things but that should also be discussed patches? Chassis power losses depend an opportunity to evaluate a vehicle with the test facility before you show up on many things but it is assured that without the attendant problems of for a test. Safety on the chassis dyno – there are losses through the drivetrain liability to the mechanic or repair Properly restraining the vehicle should and the tire patches themselves. Good technician because “test driving” is be addressed by the manufacturers, but manual chassis setups done in a more controlled condition and might use over 60Hp to get to the drive there is no traffic to cause problems. it is also important to mention that the tires at a given speed and temperature No traffic that is unless one considers process of tying the vehicle down with (of the oil in the trans and drive axle(s). getting the vehicle to and from the test straps or chains could also affect the Automatic transmissions with drag race facility. It is also a plus to test on the results of the tests accomplished on the type high stall converters might lose over chassis dyno even if the weather outside rolls. Tire inflation variations can even 100 Hp at the same data point. It is not is nasty. So facilities that have test and cause differences in test results.

36 APR-JUN 2010 engine professional CHASSIS DYNAMOMETER TESTING )@HAROLD BETTES 7/6;6:)@ROB KELLY

This turbocharged Mustang engine is easier to access because the hood is removed on this race car. Simple changes can be made but if something more serious is necessary the vehicle will have to be removed from the dyno. Chassis dynos allow quick testing but engine folks normally prefer engine dynos.

This radial drag racing tire is about to be one of the tire patches One of the radial drag tires with distortion showing at over 1000Hp at 120mph that powers the drive rolls. This dyno is about to get a surprise from on the dyno. This clearly shows one of many reasons why the tie-down process this turbocharged Ford Mustang. is important. This dyno uses “cradle style” rolls. The tire is one of many loss points in the chassis dyno test making it difficult to accurately calculate how much horsepower was at the flywheel of the engine.

38 APR-JUN 2010 engine professional LEFT: Hot rod Mazda with twin turbos is on the chassis dyno to verify tune-up work without the liability of test driving on the streets. Technician on the right has command of the dyno and communicates with the driver during the test.

BELOW: The handheld controller on this AWD chassis dyno allows the operator to command all sorts of things including changing the wheelbase. The dyno can be commanded inside or outside the vehicle being tested.

LEFT: This motorcycle tire is distorted at the roll contact point (black arrow) and the tie-downs (white arrows) help to distort the tire as a result of how much tension is in the tie-down straps. Even tire inflation pressure affects the tire and roll interface. This is an inertia only dyno. Read the text for definitions.

Conclusions and The power numbers that are the Comments most reliable from chassis dyno are Engine combinations tested on a chassis those that are referenced to the drive tire dyno can be done rapidly and data can patches. Trying to generate estimated be evaluated quickly as well. However numbers that relate to the flywheel power reliable test data is dependent on many is more often than not a boondoggle things that any engine builder or vehicle and result in inflated claims. When folks owner should acknowledge and be start tossing around percentages such as willing to accept so that they each can “everybody knows the drivetrain takes learn from the experiences. 15%” in order to say what the engine Harold Bettes is co-author of Dyno Testing Regardless of the assumptions that produced at the flywheel be careful. That and Tuning and author of The Engine Airflow Handbook. Harold has been a mechanical en- one might make, testing will help the is much like the politician that states, “I learning process if the variables that have gineer for over 40 years and has been involved am going to watch out for you and your an effect on testing are also evaluated. in motorsports for more than half a century. money…” and we all know how that Engine people tend to concentrate Harold Bettes is a recipient of many awards on the engine results obtained from stuff works out. for his contributions in furthering mechanical engineering, the motorsports industry and the testing on an engine dyno but much can Read all you can get your hands on aftermarket. Bettes is widely acknowledged for be gained by evaluating the chassis dyno and study how dynamometers work and strive to understand where the numbers his expertise concerning dynamometer and flow test results. After all the engine normally bench applications. Harold is a consultant on is installed in a chassis in order to use come from. test facilities, equipment and high performance the power unless the engine is applied Remember that everyone has a vehicle engine designs and configurations and his per- to a generator or an aircraft or boat or that makes at least 500Hp until it is sonal contact information is in the black book of something of that sort. accurately tested that is!Q many engine developers, and team owners.

engine professional APR-JUN 2010 39