Van

You're taking the weight of the ball out of context... if you throw 2 ***** with the same force, the heavier ball will not go as high. I did not mean to imply that gravity would affect it differently.

So, I seem to understand what you guys are saying - but then I was just looking at my track data...

Here's 2 seconds of time from Watkins Glen - right at the shift point from 4th to 5th on the back straight.



I'm accelerating at about .1g. I step on the clutch. Immediately, the engine is disconnected from the wheels - at this point, the primary force is just aerodynamic drag and friction, right? However, the car still accelerates and gains another half mile per hour - UNTIL the acceleration is zero, at which point the car is decelerating (negative acceleration) and the velocity starts to drop off.

Then, after the shift, as the clutch is grabbing, and providing power to the wheels, the rate of deceleration changes, and once g forces have crossed over the x-axis, velocity resumes rising again.

So, what's happening at the beginning of this graph? What is helping the car gain velocity while the acceleration is dropping off from 0.1 to 0.0?

Am I missing something?

333pg333

So in effect. If we floor the throttle and get up to eg 6000rpm in 4th gear, then just push the clutch in, do we instantly begin to decelerate?
Better yet, what happens if we are going 100% wot through the gears and get into 25psi boost in a high gear. Boost is just hitting really hard.
Now we depress the clutch fully. Do we slow down immediately even though we were just accelerating at our highest rate?
I know this is velocity rather than the engine itself accelerating but it's kinda related.

Van

This makes sense to me.

I'm not trying to be a contrarian here... I'm just trying to understand it in my own terms.

Van

Not according to my traqmate data... There is a time lag for the acceleration to drop. Once we're in the "deceleration zone", then the car begins to slow down.

Again, that data is sampled at 20 Hz and the chart shows 2 seconds of time. We're talking about a lag of less than 1/2 a second, but in that time period, the velocity increases by 1/2 a MPH.

Van

Extra points to the person who can explain it to me in haiku...

Deceleration
Does it drop off to zero
Immediately?


Theory and data
Why don't they agree today?
I want an answer

333pg333

That's what I'm thinking. Inertia has to play a part in this discussion. Watch a golf ball stop on the edge of a cup. It isn't going any further but it still topples in. Sure wind and grass etc can have an effect
but that isn't always the case. It still has inertia bubbling around in there somewhere.

Chris White

Try removing the smoothing in the software....
If its like mine (Race Technologies) the accelerometer measures the acceleration and the speed is figured by GPS. GPS is very laggy and also not smooth. The GPS data is smoothed so that there is not a jagged speed graph.

Sort of. If there was not friction you would not pick up any further speed – which is a total loss of acceleration. But due to friction (air and mechanical) you will actually begin to decelerate immediately.
[QUOTE=333pg333;7305211Better yet, what happens if we are going 100% wot through the gears and get into 25psi boost in a high gear. Boost is just hitting really hard.
Now we depress the clutch fully. Do we slow down immediately even though we were just accelerating at our highest rate?
I know this is velocity rather than the engine itself accelerating but it's kinda related.[/QUOTE]
Its the same thing, engines, cars, rockets, baseballs. You need force to accelerate an object. Remove the force and it does not accelerate any more.

For those of you that have been lucky (!) enough to feel the overboost trip on a 944 turbo you know that you usually leave a nose print o the windshield – that is because that is the best example of an immediate removal of force – the change in acceleration rate is what propels your nose into the windshield. If you look a velocity you would not see a very big change – but the force on your body due to the sudden absence of acceleration is fairly dramatic.

You really have to separate velocity change and acceleration change to have it make sense. Another good example is if you are cruising at 30 in second gear and you floor it and then lift quickly several times in a row – very little change in velocity but a serious change in acceleration. You don’t feel velocity, you feel acceleration.

Chris White

Yes and no, it wasn’t out of context, the terminology wasn’t quite right! The quote above now contains some more info – ie – using the same force.

Now if we look at the original quote – there is a conception problem…maybe I can help a bit
First – it is important to understand that velocity has two factors – speed and direction. Acceleration has only one – rate of change.
With the example of the ball going up and then back down the rate of acceleration is constant throughout the entire arc. The peak upward velocity is when the ball leaves your hand and after that the velocity is changing due to the force of gravity. The force of gravity is a constant (in this situation) 9.8 m/s^2. This creates a constant acceleration of the ball towards the earth. On the upward path the acceleration towards the earth appears to be a deceleration of the object…but that is just semantics. Rate of change in velocity is directional neutral.

So to put it in some physics math – the ball is accelerating at 9.8 m’s^2 towards the earth for the very second it is released – regardless of what direction you throw it – up, down or straight. This disregards any interesting aerodynamic property that can be induced with spin – we really don’t have the time or space to get into that!

I am not sure that helps…unless you contemplate it for a while. The main thing si to forget the ‘common’ use of the word acceleration….

BTW – I am trying to restrain myself from a science geek fest on this….my father was a Physics professor at Cornell University…I spent a lot of my youth playing is some really fun science labs (running around underground particle accelerators…Oh! There is that word again…)

Chris White

inertia is the measure of how much resistance matter has to acceleration. the more inertia something has, the less it wants to respond to forces and accelerate. this statement is mathematically stated a = F/m. newtons second law. m is the measure of inertia, called (inertial) mass. the more force, the more acceleration. the more inertia, the less acceleration.

momentum is the product of inerta (m) and velocity (v). p=ma. it turns out that momentum is always conserved in any closed system. momentum is related to inertia, as you can see. the more inertia something has, the more momentum it has, when in motion.

And I don't think it bubbles much....

Chris White

I'll give it a try

yes,yes,yes,yes,yes
yes,yes,yes,yes,yes,yes
yes,yes,yes,yes,yes

The data is bad
remove the smoothing and
you will see the light.

lart951

Now time for the required video illustration

M758

There is a time constant to depressing the clutch. As you push in the clutch you acceleration is reduced until the clutch fully disengages. It is fast, but takes time. If you stopped adding fuel the acceleration will stock instantly.

Remember once the F = MA Force = Mass* Acceleration.

We can rewrite this equation as A = F/M Accel = Force/Mass

So we know Mass is constant in this equaltion as we can forget about the instantanous change in fuel weight for this purpose.

When you cut fuel you cut the Force of the engine provides. The only forces left are gravity, friction and drag. The all act to reduce th acual force level.

So once force goes to 0. Accerleration goes to zero. Really force goes negative once engine power is stopped because of friction, air drag and gravity.

Bottomline is once you remove the force you have no way to have positive acceleration.

Rogue_Ant

Ok, so do any of you "instant decel" types want to explain this? Rev-limit set to 2000rpm (fuel-cut), no smoothing, done with a fairly fast logger:

Duke

Bad engine management system

Either the cut is a soft cut that allows xxx rpm until a hard cut.
Or the EMS processor is just plain slow. There are signals, processing, and reaction required by the EMS before the cut is a reality.

Duke

I can guarantee that the log wouldn't look like that with a modern EMS with hard cut.