Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - how to smoothly shift back into gear from coasting in neutral (with engine running)

Occasionally6 · 03-27-2014, 06:38 AM

While no doubt people do get away with doing it, I'm not a fan of bump starting. The reason is the clutch pressure plate is designed for the engine to drive the car, not the car to drive the engine.

Google up a few images of clutch pressure plates and you will see that in most cases the connection between the clutch cover and the ring that clamps onto the clutch driven plate is made using straps of steel.

With the engine driving the car those straps are in tension - and very strong in that direction. Reverse the direction of drive and those straps are in compression and liable to buckling. Of course the torque required to bump start an engine is much less than the engine is capable of generating in the opposite (correct) direction.

There will always be some loss of speed with bump starting because you are transferring kinetic energy from the rolling car into the rotation/reciprocation of the engine parts. That the energy required to do the latter is small relative to the KE of the car means you can do it smoothly enough to be barely perceptible.

A minor point when rev matching is to match the engine revs to road speed - and hold there - before disengaging the clutch (depressing the clutch pedal). That way the gearbox shaft and gear speeds also rev match.

If you disengage the clutch before rev matching, the shaft and gear speeds aren't rev matched because they are disconnected from the engine. The inertia of the shafts and gears is low so it's not super critical but it is better to do it that way.

(That is why you de-clutch in neutral when double de-clutching. You don't have to come all the way off the clutch pedal when doing that, just enough to drag the driven plate with the pressure plate i.e. right at the clutch take up point.)

You will know when you are doing it right when there is very little resistance through the shift lever as you select the gear.

If you get really good at it, it is possible to select the gear without depressing the clutch pedal at all. Don't practice that on a car you care about though.

03-27-2014, 06:38 AM	#16 (permalink)
Occasionally6 Master EcoModder Join Date: Apr 2013 Location: World Posts: 385 Thanks: 82 Thanked 82 Times in 67 Posts	While no doubt people do get away with doing it, I'm not a fan of bump starting. The reason is the clutch pressure plate is designed for the engine to drive the car, not the car to drive the engine. Google up a few images of clutch pressure plates and you will see that in most cases the connection between the clutch cover and the ring that clamps onto the clutch driven plate is made using straps of steel. With the engine driving the car those straps are in tension - and very strong in that direction. Reverse the direction of drive and those straps are in compression and liable to buckling. Of course the torque required to bump start an engine is much less than the engine is capable of generating in the opposite (correct) direction. There will always be some loss of speed with bump starting because you are transferring kinetic energy from the rolling car into the rotation/reciprocation of the engine parts. That the energy required to do the latter is small relative to the KE of the car means you can do it smoothly enough to be barely perceptible. A minor point when rev matching is to match the engine revs to road speed - and hold there - before disengaging the clutch (depressing the clutch pedal). That way the gearbox shaft and gear speeds also rev match. If you disengage the clutch before rev matching, the shaft and gear speeds aren't rev matched because they are disconnected from the engine. The inertia of the shafts and gears is low so it's not super critical but it is better to do it that way. (That is why you de-clutch in neutral when double de-clutching. You don't have to come all the way off the clutch pedal when doing that, just enough to drag the driven plate with the pressure plate i.e. right at the clutch take up point.) You will know when you are doing it right when there is very little resistance through the shift lever as you select the gear. If you get really good at it, it is possible to select the gear without depressing the clutch pedal at all. Don't practice that on a car you care about though.