BKL

hey guys,

I have a question about the car backlashing. I will try to explain to the best of my abilities.


I think i understand the concept behind rev-matching. It is done in order for one to match the rpm of the transmission to that of the engine, and the poor matching of these two elements can either result in the car to jerk foward (engine pulls on the tranny), or the car to feel "drawn back" (the tranny is pulling on the engine; engine "gives"). However, I do not understand one thing.

Say right now i am in 3rd gear at 5000 rpm, and i want to drop to 2nd to "launch the car". So i clutch in, and gear to 2nd, however, i dont keep on the throttle but drop clutch. The car backlashes as a result. I am wondering, in that case, what is really happening?

Is the transmission pulling on the engine after the clutch "grabs" ? (since the engine rev drops due to no more throttle) , and when the clutch grabs, why do I get the backlash? I am trying to examine this at the mechanical level. I dont understand which part of the car actually "gives", is it the engine, clutch, or tranny?

The same backlash happens say when i go from first to 2nd gear and drop clutch with insufficient rev up.

** and finally, is this backlash harmful to the car in any way? (say put unecessary stress on the tranny, clutch, or engine?)

I hope that I have described the situation with enough detail.

Help me out guys, i am stumped on this one!

Thanks lots in advance

adickerson0

An engine and clutch give. As do tires and sadly differentals in our case. In our cars I tend to think smooth is defiantly better for the car.

A engine gives by raising or lowering RPMs. A Clutch gives by slipping. Tires give by spinning with no traction. Differentals give by breaking.

I will give you a good example. Lets say you are at red line in first gear and you switch to second really quick. You have 3 options. First you can let the RPMs drop so you make a smooth shift (not all that great for drag racing type runs). The second option is that you drop the clutch fast with the engine RPMs too high, this will result in a jerk of the drive line and possibly a chirp (small spin) from the tires. The third option is to let off the clutch slow, this will burn the clutch some and it will smell like smoking dead fish (since I use this method on a AP2 I plan to get a stronger clutch).

The option is up to you...I chose to replace clutches and that is the reason why AP2s have the clutch delay valve thingy from Honda.

However if you are slowing down things get messy. Lets say you are in 3rd and decide to shift down to second. This means that the speed of the engine will be controlled by the trany. This is good if you do it right as it keeps you in gear and lets you react. However if you mess up your tranny will try to spin your engine faster then it can go. If you drop the clutch on a down shift there is no buffer (again I chose to replace clutches) and your engine may decide to spin at 10500 rpm. This is called a mechanical over rev and it is bad.

So...on a downshift you can blow your engine or burn clutch if you happen to not be a perfect driver 100% of the time.

Does that make sense?

BKL

Thanks for the reply.

I am wondering though...

as to when you have a big difference in the tranny rpm and the engine rpm, and you drop clutch, then there is a big shock to the car.

What part of the car is actually absorbing this shock? Right now, my best guess is that when the two things (engine and tranny) come into contact, there is a big shock in which the clutch slips and then grabs eventually. And that the backlash is from the shock created during contact.

Can anyone clarify this further?

Eluded

The car will indicate poor shifting with bucking and/or clunking. If a shift isn

SIIK2NR

I think it's called "gas".... the car needs it to run...

Both senarios your choking out your car...

Why don't you just roll to a stop without putting in the clutch... there's your "backlash"...

Jasonoff

[QUOTE=Eluded,Aug 29 2005, 03:25 PM] The car will indicate poor shifting with bucking and/or clunking. If a shift isn

BKL

Does anyone have a mechanical explanation for the shock? I understand most of the the concepts behind rev matching.

The thing i am really pondering upon is the relationship between the engine and clutch and tranny as a whole. In that when there is a clunk, what is really happening and which part is absorbing the energy.

Thanks all!

Jasonoff

Inertia and driveline loading.

http://auto.howstuffworks.com/engine.htm

http://auto.howstuffworks.com/clutch.htm

http://auto.howstuffworks.com/transmission.htm

http://auto.howstuffworks.com/differential.htm

Read all 4 of those and your questions will be answered

Eluded

Once the syncronizers are destoyed due to bad driving you will be REQUIRED to double clutch. Nothing like clutch in, rev match, shift to lower gear.... ** CRUNCH ***

Jasonoff

I drove an old jeep without syncro's before. It's tough to get used to.

I think i managed a couple non crunch shifts