mark kibort

We were talking about the forces that break short shafts and 5th gear, and there was not an agreement with a few on the topic. I think Richard still was fighting me on the idea. certainly GB was too.
Here is something i wrote on the racing list, when the top had come up.
Just a simple analogy to show why downshifting with out matching gear to RPM speeds can put loads much greater than what the engine can produce under acceleration or even a speed shift.



Originally Posted by brendon View Post

The source of the "FORCE" is unimportant. In both cases you are doing the same thing, which is changing the rotational speed of the engine. In both cases the thing that is on the other side insisting that the engine change speed is the car/driveline. Here is a thought experiment: would the there be a significant change in engine acceleration if you had a truck pushing on your bumper as you drop the clutch? If so, why would the inertia of the engine change because a truck was on your bumper?


I just thought of a great analogy for you, if what i wrote wasnt clear enough.

Yes, the source of the KE is a MAJOR factor, and here is an explanation and thought example for you.

My response to the above:

First off , we are doing the same thing (accelerating or decelerating, the engine RPM, or it's inertia, in both cases, but the rate at which we are doing it is grossly different. (i.e. rate of KE change)

Picture a car that is tied to the track via gear and toothed belt. tires are the gear, and the belt is the road, so no slip.

you accelerate in 1st, push the clutch at 6kRPM to shift , floor it, take the RPM to 7000rpm and then dump the clutch.... from the moment you release the clutch , it takes 1/10th of a second. the engine inertia, and and power all dump into the driveline and the car lunges forward the power is always there (and torque), but the 1/10th second of decelerating the engine is some KE value comprised of the rotating mass of the engine and flywheel, as the RPM drops from 7000 to 4500rpm.

maybe thats worth, 100ft-lbs.

NOW, take a car going 100mph, and you shift from 4th to 3rd and dump the clutch. 4500rpm to 7000rpm the engine revs up twice as fast 1/20th of a second. why? because the vehicle speed is so much faster (about double) the force acting on the driveline is now double through the gear box, and the wheels are tied to the road due to the "belt and gear" coupling.

The source NOW IS IMPORTANT, as the engine is being driven by the car's KE, and not the car being driven by the engine as it was in the first example. This is because of the velocity of the vehicle, the gearing forces the engine to accelerate at 2x the time, greatly raising the torque transferred to the gear box.

does that make it any clearer?

SeanR

How many threads do you need to start that contains the same discussion?

mark kibort

How many discussons here are regarding the same thing? I lose track.
there was some disagreement, so i thought of posting another way of looking at it. If it helps someone match RPM on a downshift and saves one transmission life... its worth it. transmissions are an endangered species for 928!

kurt M

Shock. /thread

Matt Romanowski

QED

multi21

+/- 15 pages

Ray S

Meh, I think folks are getting sick of the self-serving drama.

I don't think it'll break 10......

SeanR

I doubt this one will go two pages.

mklaskin

Where's VR?

ProCoach

Coming... Soon, I hope!

danielyonker

Karen England ...... Where are you?

sbelles

Obviously it's torque that brakes things on downshifts, not horse power.

mark kibort

Yep, you're half right.

2BWise

Inertia

KaiB

None of this matters as we should all be freewheeling....why upset the car to begin with...