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BGTV8 · 04-02-2012, 02:46 AM

Folks - you are confusing torque, total tractive effort availble and power.

The amount of torque produced by an engine at a given engine-speed will be constant (near enough).

Power is the amount of "actual work" that can be done and is actually torque multiplied by time (usally expressed in engine rpm).

Total tractive effort is the torque available at the road and is influenced by overall final drive ratio (including OD of the wheel/tyre assembly)

The weblink to dyno charts graphs the same "power" but it is achieved over slight differing time periods and IMHO misrepresents the true facts .. I beleive the graph is showing "tractive effort" on the vertical axis. It is the slower time to acheive the same output (tractive effort) that you can perceive as "less power".

What is not clear is the relative diameters of the wheel/tyre combination as this will cause the time for a given torque (available tractive effort at the tyre/dyno roller interface) to differ as it influences the gearing. Bear in mind that a gearbox and fnal drive ratio are effectively a torque multiplication device and will influence the time it takes for a given amount of torque to achieve a given rotational speed (and hence tractive effort measure).

The seat of your pants will tell you that a 20" wheel with 35 profile tyre will accelerate more slowly that a 19" wheel wth the same profile tyre and this wil accelerate more slowly than a 18" wheel fitted with a 35 profile tyre - this is due to the realtive overall gearing ratio diferences.

In fact, the same torque and engine rpm mean the same "power" figure at the flywheel, it is just that by changing the effctive gear ratio, you are altering the relative "total tractive effort" and this is what manifests as acceleration which you feel thru the seat.

Differing weights of wheel/tyre assemlbies will not affect the torque (and therefore power) generated by the engine at the flywheel, but it will affect the ability of the engine to generate "tractive effort" as a consequence of gearing changes.

To determine the differences in tractive effort due to rotational inertia alone (differing wheel assembly weights), you would need to repeat the tests with wheel/tyre combo having exactly the same rolling diameter.

That said, lightweight wheels will reduce unspung weight which will improve mechanical grip as the damper has less weight to control and this makes the suspension more predictable in its behaviour - they will contribute to lower overall vehicle weight which improves the ability of a given amount of torque to accelerate the vehicle, as well as a reduction in torque losses attributed to accelerating the wheel and tyre itself.

The discussion thread is about "power" but I really think the peception is about "time to accelerate the vehcile" and these are not the same thing.

RB

04-02-2012, 02:46 AM	#20 (permalink)
BGTV8 A True Z Fanatic Join Date: Dec 2009 Location: 03350 Australia Posts: 1,503 Drives: 09 Nissan 370Z M6 Rep Power: 29907	Folks - you are confusing torque, total tractive effort availble and power. The amount of torque produced by an engine at a given engine-speed will be constant (near enough). Power is the amount of "actual work" that can be done and is actually torque multiplied by time (usally expressed in engine rpm). Total tractive effort is the torque available at the road and is influenced by overall final drive ratio (including OD of the wheel/tyre assembly) The weblink to dyno charts graphs the same "power" but it is achieved over slight differing time periods and IMHO misrepresents the true facts .. I beleive the graph is showing "tractive effort" on the vertical axis. It is the slower time to acheive the same output (tractive effort) that you can perceive as "less power". What is not clear is the relative diameters of the wheel/tyre combination as this will cause the time for a given torque (available tractive effort at the tyre/dyno roller interface) to differ as it influences the gearing. Bear in mind that a gearbox and fnal drive ratio are effectively a torque multiplication device and will influence the time it takes for a given amount of torque to achieve a given rotational speed (and hence tractive effort measure). The seat of your pants will tell you that a 20" wheel with 35 profile tyre will accelerate more slowly that a 19" wheel wth the same profile tyre and this wil accelerate more slowly than a 18" wheel fitted with a 35 profile tyre - this is due to the realtive overall gearing ratio diferences. In fact, the same torque and engine rpm mean the same "power" figure at the flywheel, it is just that by changing the effctive gear ratio, you are altering the relative "total tractive effort" and this is what manifests as acceleration which you feel thru the seat. Differing weights of wheel/tyre assemlbies will not affect the torque (and therefore power) generated by the engine at the flywheel, but it will affect the ability of the engine to generate "tractive effort" as a consequence of gearing changes. To determine the differences in tractive effort due to rotational inertia alone (differing wheel assembly weights), you would need to repeat the tests with wheel/tyre combo having exactly the same rolling diameter. That said, lightweight wheels will reduce unspung weight which will improve mechanical grip as the damper has less weight to control and this makes the suspension more predictable in its behaviour - they will contribute to lower overall vehicle weight which improves the ability of a given amount of torque to accelerate the vehicle, as well as a reduction in torque losses attributed to accelerating the wheel and tyre itself. The discussion thread is about "power" but I really think the peception is about "time to accelerate the vehcile" and these are not the same thing. RB Last edited by BGTV8; 04-02-2012 at 02:50 AM.