Drivechain shunt

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Tue 27 May 2008 19:02
Drivechain shunt - Statistical outlier
I've been wondering - what exactly is drivechain shunt? I don't mean what is it in the sense that I know it's the 'clonk' as the drivechain takes up the slack as you move from no throttle to part throttle when moving and in gear.

What I want to know is what parts are likely to be taking up the slack, and why my Accord has suddenly developed quite noticeable drivechain shunt in all 5 gears when previously it had none at all, even in 1st. I'm curious, and also wondering if this is nothing, or another problem episode in my ongoing experience of a 'statistical outlier' Accord: 2 months off the road and counting in the 29 months of ownership from new.
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Tue 27 May 2008 19:06
Drivechain shunt - Avant
Isn't 'shunt' commoner the other way - i.e. when you lift off the accelerator while in gear? If you get this too, perhaps it could be the engine mountings.
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Tue 27 May 2008 19:33
Drivechain shunt - 659FBE
It's drivetrain shunt. On most cars other than odballs like the old SAAB 900 there are no chains involved.

Shunt arises from the normal reaction on the engine and transmission mounting system to the engine torque turning the load - ie propelling the car. As you accelerate, by Newton's third law there is a reactive force on the power unit which will reverse on the overrun. If there is slop (free movement) or excessive compliance in the mountings, the movement of the power unit between drive and overrun will be felt as "shunt". If you really mess up the design, shunt can become oscillatory and "kangarooing" is the result.

BL transverse power units were always prone to shunt - until on the Allegro they stiffened it up so much that it just felt rough all the time.

659.
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Tue 27 May 2008 19:46
Drivechain shunt - Lud
Not just compliance in the rubbers 659 but also free play in the transmission, usually these days the final drive pinion and the drive shaft universals, but of course in a rwd car also in the propshaft universals and differential. Violent shunting can result from the combination of this wear and a temporary or permanent engine fuelling or ignition problem, exacerbated usually by clumsy driving.

It is a commonplace experience. Until it and a lot of other nasty things have happened to you you can't claim to be an experienced driver. Not that I recommend it of course. It is unpleasant and damages the car still further.
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Tue 27 May 2008 19:53
Drivechain shunt - Alby Back
I used to have a Mk2 diesel Mondeo estate ( Bought at about 80k miles ) which "clunked" every time you set off from rest. It felt as if there was play in the drivetrain somewhere. Always intended to have it seen to but never got around to it in the additional 100k miles I put on it. Can't have been anything too vital I guess. After a while you just sort of got used to it.
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