Under-Geared

[autogyro]

Power delivery across the usable rpm range is what matters.
It dictates the gear ratios chosen for any particular circuit and establishes the gear ratios most likely to achieve the best lap time.
This can alter depending on many variables not just wind direction and speed.
It is when the choice of gears is incorrect that the specific gear,the diff ratio or the complete gearbox is said to be either 'overgeared' or 'undergeared'.

[alelanza]

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Assuming we had a rev limit at 5500 it would not be advisable to go all the way to that limit in gear ratio because the engine would never get there. It would get hung somewhere between 3300 and 5000 rpm latest.

That is incorrect. You'd have to graph all resistances (air, rolling, etc) and the point at which power curve is intersected by resisting forces curve, that's where the car would theoretically cease to accelerate. This could happen at any point, 2500, 6500 rpms, it's all down to gearing. As you can see, you only need the power curve to determine this, as it already 'includes' torque. Though xpensive already tried to explain this to you, so i see no reason you should understand it this time around.
Don't think about points, don't think about max torque and max hp, that's only for salespeople. Think about the area under the power curve combined with gearing, that's what moves a car.

[riff_raff]

alloush,

The only form of auto racing where wind direction is a serious consideration when selecting gear ratios is oval track racing. As WhiteBlue noted, if there is significant wind blowing down one of the oval's straightaways, it can increase drag in one direction and reduce it in the other. Since drag power losses increase exponentially with speed, wind conditions can have a measurable effect on the straight line speed of a race car on a super speedway.

An oval track car like a Champ Car has an engine with a very narrow torque curve and a rev limiter. The car wants to operate with the engine right at the peak of it's torque curve at all times without shifting if possible, but also so that it does not trigger the rev. limiter. The car will go as fast down the straightaways as engine power and aero drag permit. However, the car must also slow going through the corners. Thus, if the driver does not shift gears, the engine speed must also be reduced while cornering. If the engine speed drops too much in the corners, the engine will be outside of its power band and will not give good throttle and power response to accelerate the car coming out of the corners.

With regards to the disagreement about power and torque, here's how it was explained to me: Torque is the ability of an engine to do work. Power is how fast that work gets done.

Regards,
riff_raff

[xpensive]

Oh please riff, don't start this torque vs power thing again, we just got everyone convinced that for straight-line top speed,
power is the deciding factor as in Power equals Air-resistance times Speed, see aleanza's xplanation above.

In SI-units, "Work"(Wattseconds or Joule)) is Torque (Newtonmeters) times angular length (radians), while "Power" (Watt or Joule per second) is Torque (Nm) times angular speed (radians per second).

[PlatinumZealot]

Power (energy in motion) is king. Power can produce torque (reciprocating engine) or no torque (Rocket engine). Torque is just a geometrical implication.

In the combustion chamber Energy is released in a certain time, the pressure and temperature of the gas rises, the force acts on the piston. No torue yet! Now the torque only arises because there is a crankshaft and that crankshaft rotates.

Power rules, because power is energy in "motion" (time really). Torque has the same units as energy and work, but torque does not have to be in motion. That is it can just be in a stored state, like turning a torque wrench till the bolt tightens and you stop turning. Torque is there, energy is stored in the shaft of the wrench, but no more work is being done. That is not useful to a formula 1 car! I would rather power anyday! With power you can do anything, figuratively and literally. You can go to the moon with power.. you can't do that with torque now can you? lol

[WhiteBlue]

I do not know why there is all this fuss in the first place. In my initial answer to the OP I clearly advised about the way the power determines the drag on a straight. All I did was adding some detail on torque which attracted the superfluous wise cracking.

On a long flat straight in equilibrium (no acceleration or climb of elevation) all your engine power is used to overcome the aerodynamic drag. Drag is a function of relative wind speed to the car. Let us call that aerodynamic speed and assume it is 300 km/h.

alloush,

The only form of auto racing where wind direction is a serious consideration when selecting gear ratios is oval track racing.

Contrary to what riff_raff thinks wind is also very important in F1. Many tracks have just one main straight (compare Montreal) which is very important for passing. So if you get your gearing wrong and you happen to have a strong wind aligned to that straight you are effed.

[xpensive]

May someone please explain to me the term undergeared and how it relates to wind direction on the race track ?


Thank youu.

It relates to gear ratios. Depending of the top gear ratio your top speed can be limited by RPM or by the torque of the engine. If you are too short you run into the rev limit early and never use the full engine power. You want to avoid that. If you are long you are limited by torque.
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Xcuse me WB, but your initial post, see above, is what started the discussion, pardon my highlighting of your xpressions.

[WhiteBlue]

May someone please explain to me the term undergeared and how it relates to wind direction on the race track ?


Thank youu.

It relates to gear ratios. Depending of the top gear ratio your top speed can be limited by RPM or by the torque of the engine. If you are too short you run into the rev limit early and never use the full engine power. You want to avoid that. If you are long you are limited by torque.
...

Xcuse me, but your initial post, see above, is what started the discussion, pardon my highlighting of your xpressions, WB.

Please, stop manipulating my post by cutting it short. I clearly stated the balance of drag and power in my first post as seen below. You cannot discuss that away.

On a long flat straight in equilibrium (no acceleration or climb of elevation) all your engine power is used to overcome the aerodynamic drag. Drag is a function of relative wind speed to the car. Let us call that aerodynamic speed and assume it is 300 km/h.

[xpensive]

Knowing you torque curve for these kind of calculations is relevant IMO. Lets assume the OP was using the above engine with the red power and the blue torque curve.

The torque maximises at 3300 rpm and due to increasing revs the power still rises. The next significant point comes at 5000 rpm. Although the rpm still rises the collapsing torque is now pulling the total power down.

Assuming we had a rev limit at 5500 it would not be advisable to go all the way to that limit in gear ratio because the engine would never get there. It would get hung somewhere between 3300 and 5000 rpm latest.