Acceleration times

[Gruntguru]

Originally posted by oz_olly:
The most simple way to look at it is if F=ma then a=F/m. So to increase a you can either increase F or decrease m. There is a huge number of different functional ways to modify F and m.

Probably even more important: s = 1/2at^2 and Power = F x V.
Assume a traction limit and weight on the driven wheels to give F for the first (traction limited) phase of the run. When V has increased to the point where F x V = your engine Power, you are no longer "Traction Limited". The rest of the run is "Power Limited". Make a spreadsheet with columns for F, a, v, s and t. When the s column gets to 75 yards, the t column shows the elapsed time. Now you can play with different levels of power, mass, traction etc. The results will surprise you and you will quickly discover what is important. Hint, one of the variables is VERY important and the others decrease in importance very rapidly.

[Gruntguru]

Originally posted by Wesley:
I know shift times are a pretty big killer of accel times.

How big? eg if the 1-2 shift takes an extra 1/2 sec, how much does the et increase?

[VFR750R]

Originally posted by Gruntguru:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Wesley:
I know shift times are a pretty big killer of accel times.

How big? eg if the 1-2 shift takes an extra 1/2 sec, how much does the et increase? </div></BLOCKQUOTE>

Depends when you shift, and how fast you're going when you shift. The faster you are going when you shift, the less time is lost because you're already traveling fast during the lapse in accel. Should be pretty easy to make an excel spreadsheet and see what the time loss would be.

[Mikey Antonakakis]

I believe someone at Auburn wrote a pretty good SAE paper on shift times vs final drive ratio.

[JasperC]

Originally posted by Gruntguru:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Wesley:
I know shift times are a pretty big killer of accel times.

How big? eg if the 1-2 shift takes an extra 1/2 sec, how much does the et increase? </div></BLOCKQUOTE>
Well, you just explained how to make a simple simulation of an acceleration run. All you need to do is link car speed to RPM, determine at which RPM you need to shift and say F=0 while shifting.

This might get a bit tricky to do in an Excel sheet but the Matlab/Java/... code for this is pretty simple. And useful too, since you'll be able to determine your final drive ratio, gear ratios (if you have different sets of gears available) as well as the requirements for shifting time.

Then the next step is to link RPM to fuel consumption and you'll be able to determine shifting points for the endurance, which will be different because you're running less fuel efficient at the higher RPM's (this is the case for our single cylinder anyway).

Best regards,
Jasper Coosemans

Chief Drivetrain 2009-2010
DUT Racing Team
Delft University of Technology

[Gruntguru]

Originally posted by VFR750R:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Gruntguru:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Wesley:
I know shift times are a pretty big killer of accel times.

How big? eg if the 1-2 shift takes an extra 1/2 sec, how much does the et increase? </div></BLOCKQUOTE>

Depends when you shift, and how fast you're going when you shift. The faster you are going when you shift, the less time is lost because you're already traveling fast during the lapse in accel. Should be pretty easy to make an excel spreadsheet and see what the time loss would be. </div></BLOCKQUOTE>Yes. For example, for a typical car capable of 4.1 second acceleration times and shifting from first gear at a velocity of 20 m/s, the difference in elapsed time for a 0.5s shift versus 0s shift is about 0.2 seconds - so less than half of the shift time.

[Gruntguru]

Originally posted by JasperC:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Gruntguru:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Wesley:
I know shift times are a pretty big killer of accel times.

How big? eg if the 1-2 shift takes an extra 1/2 sec, how much does the et increase? </div></BLOCKQUOTE>
Well, you just explained how to make a simple simulation of an acceleration run. All you need to do is link car speed to RPM, determine at which RPM you need to shift and say F=0 while shifting.

This might get a bit tricky to do in an Excel sheet but the Matlab/Java/... code for this is pretty simple. And useful too, since you'll be able to determine your final drive ratio, gear ratios (if you have different sets of gears available) as well as the requirements for shifting time.

Then the next step is to link RPM to fuel consumption and you'll be able to determine shifting points for the endurance, which will be different because you're running less fuel efficient at the higher RPM's (this is the case for our single cylinder anyway).

Best regards,
Jasper Coosemans

Chief Drivetrain 2009-2010
DUT Racing Team
Delft University of Technology </div></BLOCKQUOTE>The simplest way to determine shift points is to graph the "Thrust Curve" (force available at the tyre) for each gear and shift at the intersection of successive curves. This technique neglects rotational inertia of components upstream of the gearbox (mostly engine) but is still pretty accurate.