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Author Topic: Why do car engines shake in the wrong gear?  (Read 1660 times)

Martin [Admin]

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Why do car engines shake in the wrong gear?
« on: May 22, 2012, 06:21:28 PM »


Why do car engines shake in the wrong gear?
Ie. If I miss a gear and put it in 4th instead of 3rd the engine complaints by vibrating like angry alarm clock... why?
What's actually going on?
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boatmadman

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Re: Why do car engines shake in the wrong gear?
« Reply #1 on: May 22, 2012, 06:35:07 PM »

Its the designed in 'plonker' alert!

Seriously though, if you are going up the box and go from 2nd to 4th, the gear ratios are such that the revs of the engine are not matched to the drive shaft revs.

Ie. in 2nd gear at 3000 revs the drive shaft to the wheels might be doing 1500 revs. If you go to 3rd gear, the engine revs drop, say to 2000 revs, but at the point of letting the clutch out, the drive shaft is still doing 1500 revs.

Now, for the same road speed, ie the same drive shaft revs, if you go to 4th gear, the engine revs will drop again, say to 750 revs. At this speed, there isnt enough rotational momentum to keep the engine spinning smoothly, so, you get that 'lumpy' effect.

Ian
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Shipmate60

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Re: Why do car engines shake in the wrong gear?
« Reply #2 on: May 22, 2012, 07:00:21 PM »

The engine wants to stall as it is below its idle speed but the transmission and the weight of the car keeps it spinning.
It will still fire but at too low a speed so the engine and the car judders.

Bob
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Bob K

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Re: Why do car engines shake in the wrong gear?
« Reply #3 on: May 22, 2012, 07:07:55 PM »

It could be a similar device fitted to the control column by aircraft manufacturers, a 'stick shaker' to warn you of an engine stall condition.  Just push the steering coumn forwards to lower the front of car and pick up speed.
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mook

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Re: Why do car engines shake in the wrong gear?
« Reply #4 on: May 22, 2012, 07:16:31 PM »

It is a bit like the crunching sound you get whan you miss a gear it is designed to wake you up :-)
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ACTion

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Re: Why do car engines shake in the wrong gear?
« Reply #5 on: May 22, 2012, 08:07:18 PM »

It's the gremlins, Martin. They're relatives of the ones that creep into Jiffy Bags in the post and kill brand-new speed controllers, then creep out again leaving no trace..............  :o :o
DM
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tobyker

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Re: Why do car engines shake in the wrong gear?
« Reply #6 on: May 22, 2012, 09:42:26 PM »

It is vital to be in the right gear on before you shake or rock  otherwise you will just look dead uncool, man.
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oldiron

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Re: Why do car engines shake in the wrong gear?
« Reply #7 on: May 23, 2012, 12:17:04 AM »

Four stroke internal combustion engine are equipped with cam shafts to open and close the engines valves. Depending on what the egine is expected to do, the camshaft will be designed to open, and close, the valves at a certain point relative to the crankshafts rotation and, hence, piston position in the cylinder.
  Some cams are designed to give the engine high torque at low rpm' s(eg. Tractors). Other cams give higher horsepower at higher rpm's (eg. Race cars). Each type requires certain air flow to be present to deliver the fuel properly to the cylinders with the correct volume of air for combustion. If you run your engine at a low enough rpm that your engine can't deliver enough air to match the fuel present your engine will not fire properly nor give complete combustion. This results in poor firing nd misfiring in extreme cases that results in the bucking effect  you experience.
  Camshafts bring in horsepower and torque peaks on different curves. The torque peak comes in before the horsepower peak. The torque portion is what gets your car moving off the line. The horsepower allows you to maintain and increase speed. Basically, the more horsepower the more speed.

John
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derekwarner

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Re: Why do car engines shake in the wrong gear?
« Reply #8 on: May 23, 2012, 08:00:18 AM »

Simple......just purchase a vehicle with an automatic transmission installed {-) ......Derek
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RAAArtyGunner

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Re: Why do car engines shake in the wrong gear?
« Reply #9 on: May 23, 2012, 10:58:54 AM »

Simple......just purchase a vehicle with an automatic transmission installed {-) ......Derek

Which is the reason why most company fleet cars are automatics, because the engines won't be laboured by incorrect/late gear changing.
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Umi_Ryuzuki

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Re: Why do car engines shake in the wrong gear?
« Reply #10 on: May 23, 2012, 05:59:17 PM »

I always thought it was the clutch slipping, or bouncing, that made the shudder.

 :((

Shipmate60

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Re: Why do car engines shake in the wrong gear?
« Reply #11 on: May 23, 2012, 06:12:53 PM »

Umi,
You slip the clutch every time you pull away or change gear in a stick shift.
The more modern Dual Mass flywheels can cause vibration but only when breaking up.

Bob
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malcolmfrary

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Re: Why do car engines shake in the wrong gear?
« Reply #12 on: May 23, 2012, 09:54:36 PM »

I always thought it was the clutch slipping, or bouncing, that made the shudder.

 :((
Oldiron is technically correct.  I prefer bob k's explanation and cure.  A slipping clutch will be super smooth for the engine.  For a time.  Only time I ever came across a bouncing clutch was when the box it came in opened itself at the wrong end.
How long has our Martin been driving?
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Sandy

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Re: Why do car engines shake in the wrong gear?
« Reply #13 on: May 23, 2012, 10:18:48 PM »

Conventional automatic gearboxes rely on the slippage in the torque convertor to avoid the stall/shudder effect.

Most autos now come with a lockup torque convertor that electrically or hydraulic locks the turbine and the stator together, hence they avoid the slippage and increase efficiency and mileage per gallon. The cars ECM tends to command the convertor to lock up based on speed, which gear it is in, load on the engine. When you brake, they convertor unlocks. You can tell if the convertor is locked or not by driving, say at 50mph and flicking the brake pedal with your left foot. The convertor will unlock, revs will blip upwards and then the convertor will lock back up again, and revs will drop.

When I used to drag race an automatic, 1987 Buick with a four speed auto trans and lockup convertor, you could (knowing where to break in to the circuit) install a switch that forced the convertor to lock. This tended to give you a bit more mph on the run (no slippage). You had to remember to flick the lockup off though as, when you slowed down to manouvre, the torque convertor acted like a conventional clutch and the car shuddered like a manual in the wrong gear.

Manual trans cars are frankly, and literally, a waste of time. There is no manual trans or 99.99% of drivers who can change gear faster than an automatic can, nor miss a gear, or over-rev, or be so consistent.

If you want to change gear fast in a manual trans car, you shouldn't lift off the gas, and you need to kick the clutch in and slide your foot off the side of the clutch, then be prepared to do that again (successfully) two or three seconds later.
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steamboatmodel

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Re: Why do car engines shake in the wrong gear?
« Reply #14 on: May 24, 2012, 06:33:47 PM »

"If you want to change gear fast in a manual trans car, you shouldn't lift off the gas, and you need to kick the clutch in and slide your foot off the side of the clutch, then be prepared to do that again (successfully) two or three seconds later."
If you don't do it successfuly you find you need rubber gears or an automatic trany.
Regards,
Gerald.
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justboatonic

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Re: Why do car engines shake in the wrong gear?
« Reply #15 on: May 25, 2012, 08:25:05 PM »

My wife should be able to give an expert answer on this. She is always in the wrong gear when driving and every time I tell her she tells me to  :-X

So, I bought an automatic which means she never has to bother about this any more.  {-) Trouble is, she says she cant drive an auto   <*<
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