Dyno Testing – What Gear is Best? (Copyright Torque Developments International)
This is a question often asked of Torque Developments International Technical Director – Sam Borgman.
When testing the output of a vehicle with a chassis dyno system we are testing the vehicles entire driveline all together at once.
This might at first glance seem like a rather obvious statement but it is important to keep this fact in mind when discussing chassis dyno’s as the tendency is for people to use chassis dyno’s to gauge just the engines performance, so variable loses attached to the rest of our driveline (which are inherently included in any measurement) do have the ability to interfere with the test results we’re looking for if they are not properly monitored and controlled.
When thinking about where driveline variables may arise (or “transmission losses”) most people will think about the gearbox as the major source of troublesome variables but in reality it is only one a part of the whole system, driveshaft joints and wheel bearings are all equally able to produce variable losses. The good news is that as long as a gearbox is consistently tested in the same gear across any bout of testing then most other variables remain relatively stable on a test by test basis. The main remaining area of concern being to try to avoid large shifts in the gearbox’s operating temperature.
Can we just test in any gear and still get accurate results? well technically yes, but it’s a whole lot easier to do it if the gear selcted has a ratio close to 1:1.
Here’s why… Working with a gearbox whilst a high input/output ratio gear is selected, 1st gear for instance, causes the effects of gearbox temperature variation to become amplified. This is due to running with greater relative bearing surface speeds along the path taken inside the gearbox by our driveline energy, these higher surface speeds increase our exposure to things like oil windage and metal on metal friction.
Conversely all of these temperature related variables will have the least effect upon our driveline energy when the gearbox is operating with little or no input/output ratio, the ideal ratio logically being that of one to one, where the relative speeds along the driveline energy path are minimised.
In order to help the gearbox behave in the most consistent way possible throughout all of our testing we should;
• Ensure we take all measurements with the driveline in one specific gear
• Try to use a ratio as close to 1:1 as possible
• Make effort to ensure that the gearbox temperature is as consistent as possible throughout all of our testing.
Those who have read my previous writings on the subject of chassis dyno’s will already be aware that when a constant acceleration (sweep test) is performed the engine speed rate of change (or “ramp rate”) is a critical variable that must be kept in check across an
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