# Why Tachometer shows idle RPM range instantly when closing the throttle and clutching

I have studied that, RPM is the measure of how fast the Crank Shaft is rotating.

Lets consider the following case.

1. You are riding in some speed which shows the engine at 5000 RPM.
2. Now you are closing the throttle and pull in the clutch.
3. Now the tachometer shows the RPM reading as close to idle (like 1.5k to <2K)
4. When you again engaged the clutch (Still throttle is closed) the RPM will regain again close to 5000 RPM (since we have dropped some RPM by closing the throttle)

But the above case clearly says that, still the crankshaft is rotating. By why in this case, it shows/have to show a different RPM range.

I am having Apache RTR 180. I have tested this in some same models and all are reproducing this same scenario.

May I know, Is there any kind of advantage/use in doing this one.

The following clarification/understanding is added in the question itself, since unable to add big text in the comment.

*PS: So, What my understanding is:- Lets consider "X" amount of fuel/air is passing in idle RPM range and "y" be at some 5K RPM. So, rear Wheel is connected to the crank shaft. A movement in crankshaft will have effect in the rear wheel and vice versa too. So, when the clutch is pulled in (and throttle is closed completly), since of the limited air/fuel supply (x), it maintains the idle RPM range. When the clutch is engaged again (still throttle is closed), that same x is maintained since, we are not giving throttle.

Q1. Only the rear wheel plays a vital role in making the crankshaft to match its speed at this entire case right?
Q2. I believe for the FI, it will supply the same "x", since the TPS will say "no throttle is given at the moment" right? *

Thanks.

It's a bit hard to understand your question, but I'm assuming you're asking about the RPM jumping between idle and back toward 5,000 when you disengage/engage the clutch.

When you start the throttle roll off, the engine and drive wheels are connected at a set ratio (the gear you're in) through the transmission. Neither are spinning independently.

When you disengage the clutch, the engine and drive wheels disconnect, and can spin freely. This is what allows you to have the engine running without the vehicle moving. With no throttle, and nothing else forcing the engine to spin faster than idle, it returns to idle speed.

When you again engage the clutch, the weight and momentum of the vehicle are strong enough to spin up the engine. The engine, since it is not making power with the throttle closed, "wants" to return to idle speed. The amount of energy that is required to force it to spin will be witnessed as a loss of speed and a lower RPM than you originally started with. This is commonly referred to as engine braking.

• Sorry, Missed something to add in the question. Now Updated. Mar 22, 2017 at 10:19
• Updated my answer Mar 22, 2017 at 10:25
• And, it has been common in some cars, to change from 4th to 5th at somewhere around 70mph but select 3rd in error - result the engine has a catastrophic failure bending the valves etc. The evidence one customer with an xr3i did the same thing 3 times!!! Mar 22, 2017 at 11:23
• @raydowe: Thx for the ans. But have a question. U r going in some 5K RPM and now pulling clutch instantly. Yes at this point nothing forces the engine to spin since throttle is also closed. But the engine will lose its RPM slowly only right? Why pulling the clutch instantly makes the RPM to drop to idle instantly? Could u explain more on this one... Mar 22, 2017 at 12:44
• @NANDAKUMAR Why doesn't it return to idle instantly? Because there is a huge amount of angular momentum. The crankshaft, camshafts, camchain, and all the other moving parts of the engine have a larged combined mass and are spinning very fast. As Newton's first law says, "an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force." The only thing that will slow the engine down is friction (which oil minimizes), compressing the air fuel mixture, and secondary functions like pumping oil and water. Mar 22, 2017 at 12:59

From what I can understand, assuming the OP is curious as to why instantly pulling on the clutch the RPM goes down in a swift moment and not gradually?

Simple: Inertia

The reason for this is that the piston is not happy trying to convert reciprocating motion to rotational motion(it is losing momentum) so as soon as you pull the clutch the rpm drops down rapidly but it is not in a single swift moment as you suggest, it is still linear but rapid.

We should also take into account the inherently mass of the piston and in some engines the counter weights which causes the slow down of rpm quickly.

• I believe this is the correct interpretation. When clutched out it drops at a more-or-less freefall rate. When the clutched back in at speed, the engine is forced to accelerate much faster than it decelerates when clutched out. Also, the RPMs drop at a rate that is also partly determined by ECU programming. That is, as the RPMs approach idle, valves will open and injectors will start firing again to not 'overshoot' the idle range and stall.
– Nick
Mar 22, 2017 at 14:51
• Also, had a case where the person wanted to "bump start" the car engine. They told us they let the clutch up very fast and heard a big bang and it didn't start. Turned out they had managed to snap the crankshaft.... Mar 22, 2017 at 15:15
• @SolarMike That is new rofl Mar 22, 2017 at 15:36
• @ShobinP we thought so, but they didn't, especially when they saw the bill!! Mar 22, 2017 at 15:37
• @ShobinP: I have updated the question with my understanding after all comments/answer. Could u suggest on my understanding in the PS section. I am unable to add big text in the comments that's why I have added in question itself. Thx. Mar 23, 2017 at 19:01