Under-Geared

[alloush]

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


Thank youu.

[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. That can compromise your speed as well. So you want to go just short enough that you reach the rev limit at max speed.

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. Lets assume the wind blows exactly along the straight at 30 km/h.

If you drive with the wind your total absolute speed or track speed is the aerodynamic max speed plus the wind speed (330 km/h).

If you drive against the wind your track speed is the aerodynamic max speed minus the wind (270 km/h).

The difference is significant because we assumed a relative strong wind and it is completely lined up with the straight. If the wind was blowing at an angle you would have to take only the component of the wind that falls into the direction of the straight.

[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.
...

That would be limited by engine-power at the given rpm, torque is irrelevant.

[alloush]

Cheers for the replies.

[Lurk]

That would be limited by engine-power at the given rpm

which is related to torque

[xpensive]

That would be limited by engine-power at the given rpm

which is related to torque

And torque is related to length, what's your point? Power is Force times Speed which is all you need to know.

[Lurk]

"torque is irrelevant" makes me wince. We are talking about vmax, so on revs between 17500-18000.
If you know torque at these revs, you have engine power (at these revs).
Neither power nor torque are irrelevant.

[xpensive]

Who said we were talking F1?

Power is Force times Speed, stick to that and it doesn't matter where your rpm or torque is, when power is universal.

[madtown77]

"Horsepower sells engines, torque wins races"

Nuff said. Neither is irreverent, but torque is often the more useful quantity especially when related to a race cars. In terms of straight line speed horsepower is of greater interest, but torque gives you the answer as well. Draw a free body diagram and see how the force is applied to the ground. It is from the torque at the wheel divided by the wheel radius.

Power may be universal, but is not always useful. It does you no good to make 800 horsepower at 50,000 RPM is you can't build a sensible transmission to get that power to the wheels.

[xpensive]

Perhaps you should have paid more attention in the physics class?

[WhiteBlue]

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.

[xpensive]

Again, Power will always be Force (as in air-resistance) times Speed. No torque curve will ever change that.

Where your terminal speed is, where your power is, the trick with gearing is to match said terminal speed against the engine's max power.

[madtown77]

Again, Power will always be Force (as in air-resistance) times Speed. No torque curve will ever change that.

Where your terminal speed is, where your power is, the trick with gearing is to match said terminal speed against the engine's max power.

So like I said horsepower is useful for top speed, but not terribly useful anywhere else. For an F1 car it determines what the ratio will be for seventh, but not what the other 6 gears will be doing.

Torque and horsepower are related, but functionally, you look at a torque curve to determine most of your parameters, not the HP curve.

[xpensive]

I believe this thread started with discussing gearing for top-speed, at least that seemed to be WB's interpretation.

Perhaps it's wise to stay that way?

[Lurk]

Well, I didn't catch you were talking about max power and max torque in a general case.
So yeah, I agree: on an sufficient straight line, max power fix max speed and torque has nothing to do with that.

To stay on the subject: in term of force, drive with the wind is the same as drive behind another car. Global pressure is low so you need less power than usual to reach the speed.
The only difference is the way the pressure get lower (relative molecule speed and number of molecules).