MadBill wrote:David Redszus wrote:MadBill wrote:You would be mistaken. Again: Maximum acceleration in a given gear occurs at peak torque. Max. accel at a given speed occurs at peak power.
I am afraid to say that is not true. Or more correctly, almost true.
At lower speeds where aero is not yet a significant factor, maximum acceleration is obtained at maximum engine torque. The gearing and tires should be sized so that wheel forces are just below the tire traction limit.
As vehicle speed increases, the target engine speed moves toward the horsepower peak. Each upshift will require running the engine above the torque peak, moving toward the power peak. Assuming the road is long enough (and straight enough)the terminal velocity (zero acceleration) will occur at the horsepower peak.
If one is using data collection the issue becomes very easy. The linear acceleration G force, multiplied by the vehicle weight will equal the force being applied by the tire to the ground. Now it is possible to calculate how much power is being transferred to the ground. Even race cars with the same engine power do not always transmit the same power to the ground every time.
If we have the engine torque curve, gearing and tires, aero Cd and frontal area, air density and drive line losses, it is a simple matter to calculate the maximum acceleration in each gear and at which speed. That is how race cars are geared for a specific road course.
I don't have time for the math right now David, but I think you're confusing gearing/ratios for best performance with the (mostly irrelevant) physics of acceleration rate in a
specific gear.
David, I think you're still misreading the question, as it is asking about the acceleration at
any specific RPM in a single gear . Except to a minor degree, and then only if the torque curve is nearly flat, wind resistance doesn't come into it and the answer has no
direct effect on optimizing gearing for a specific track. That's a totally different discussion*
Try this thought sequence:
1. Acceleration of a vehicle (to simplify matters let's say with a non-slip clutch fully engaged, no wheelspin ) is a result of axle torque acting at the loaded tire radius Vs. vehicle weight, rolling resistance and (eventually) air resistance growing as the square of the speed.
2. In lower gears the latter two can be safely ignored, so, with a given transmission and axle overall gear reduction, say 10:1 for simplicity, the only variable affecting acceleration is axle torque.
3. Per 2, axle torque (ignoring drivetrain losses which are usually assumed to be a fixed percentage anyway) will be 10X engine torque and will therefore peak at the maximum engine torque RPM.
4. Therefore acceleration will be a maximum at peak engine/wheel torque.
5. At higher speeds air resistance becomes a major factor in
selecting the right gearing for optimum performance, but still won't impact to any measurable degree the RPM at which max acceleration occurs
in any specific gear unless the torque curve is so flat that the drag rises faster that the torque, thus moving the net maximum thrust RPM
down .
*[and per your post re shift RPM, if you give it some further thought I think you'll see that (given no mechanical constraints) you would never want to shift at anything less than many hundreds of RPM over peak power...]
