MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Torque steer


The torque steer has already been described in the motor road tests back in 1958. "Totally forgetting a tendency for the car to pull slightly to the right during acceleration or left on the over-run, which at first acquaintance was fairly evident". It was a brand new ¼ elliptic AH frogeye sprite where it was noticed first. My car still had it 50 years later. What is the cause; the rear or the front axle geometrie?
Flip Brühl

Torque-steer is a label that does not apply to rear-wheel drive cars with live axles. The term refers to front wheel drive cars with unequal length front drive-shafts. As one shaft twists under torque more than another, the delivery to the wheels is unequal and thus 'steers' the car under heavy load.

Pulling to the left or right can be due to uneven brakes on front or back, uneven bushes or swivels on front or back. But the most common deign problem with 'our' cars that happens regardless of good condition is Bump-steer. Bump-steer is caused by suspension bump and droop changing the fore-aft location of the axle unevenly due to the uneven flexing of the leaf springs.

Acceleration and braking will have a similar effect if the axle is not set on its springs evenly with evenly fitted bushings and dampers but the problem can persist even if all is well simply because the car is usually driven one-up; an average driver adding 10% or more to the weight of the car and all on one side.

The addition of a Panhard rod or Watts linkage to improve lateral location along with radius arms or 'anti-tramp bars' to improve fore-aft location solves the problem. Another route is to ditch the leaf-springs altogether and fit a 4-link system.
Nick Nakorn

Nick,

I agree with you about the bump steer effect. I disagree about that torque steer does not affect rear wheel driven cars. The torque on the differential makes the car lean to one side. The length of the leaf spring changes and so the rear axle starts to steer the car. Mark Ortiz in the latest number of Race Car Engineering describes torque steer 4 and 3 link suspensions on RWD cars.
A Pannard rod lowers the roll centre so there is less roll and torque steer. In the quarter elliptic sprite in corners the rear axe changes at least 2 inches in position from left to right. A Pannard rod stops this. You are right the load of the driver is important (I am 90 kg, my sprite is 650 kg). The interesting thing is that left and right hand drive sprites both behave the same wit acceleration and power off. Is it the rear or the front suspension geometry? Bump steer or the self-steering of the live axle?


Flip
Flip Brühl

I suspect it will be the poor location of the rear axle. There was a video a year or so ago where a camera had been mounted under the back of an AH 3000. The movement of the axle was quite amazing.

The frog eye has quarter elliptic springs. Are these better or worse than half elliptics?
Mike Howlett

Mike,

Quarter elliptics are much worse. In rallies and on the circuit, I can see the cars in front of me behave like a crab. Back axle left or right depending on the direction of the last corner. This has nothing to do wit the steering effect of power on and off.
Flip Brühl

Austin Healey rear axle

http://www.youtube.com/watch?v=tN-4LLAKlpw
Geoff_MGB

Flip,
I agree that a poorly located rear axle will have the effect you describe, but - in common parlance - we don't call that 'torque-steer'. We call it 'axle-steer'. In other words, if a fwd car with unequal shafts had no suspension at all (solid mounted)the effect would still be there. By comparison, a solidly mounted rwd live axle would not display the effect. The problem with the live axle is primarily one of location not of uneven torque. It helps to keep the nomenclatures even if they don't fully describe what's happening, that way communication is easier :-)

It's amazing how terminology changes so I might well be being old-fashioned. In the 1990s I once asked a young lad in a shop for a brake slave cylinder and he didn't know what it was until an older bloke said 'wheel cylinder, mate!' I have no idea when the nomenclature changed - late 1980s?

In other words I'm happy to stand corrected just as long as we all know what we mean.
Nick Nakorn

"brake slave cylinder" vs "wheel cylinder".

Daft isn't it. One makes sense, and the other doesn't. There's a master, so obviously there's a slave. I prefer old fashioned in this case, and in many cases.
Lawrence Slater

That video shows motion of the rear axle that you don't normally think is there. At least if you have good dampers. Thanks Geoff.
Martin Washington

Nick, Laurence, Geoff

Thank you for mentioning axle steer. I am a native Dutch speaker and dyslectic (like my father and grandfather). With Microsoft Word I do come where I want but it will never be perfect. I looked in the archive under Axle steer and found what I was looking for, Lawrence contributed to that thread: If the rear springs are saggy on a quarter elliptic the wheel goes forward if the spring is compressed; it introduces understeer. My springs are saggy and I want that; it gave a better road-holding.
Second: there can be bump steer although unpredictable and dependent on many variables: I have to measure that again.
Geoff, your YouTube illustrated clearly that an axle does not only moves up and down.

Flip
Flip Brühl

This thread was discussed between 15/08/2015 and 17/08/2015

MG Midget and Sprite Technical index

This thread is from the archives. Join the live MG Midget and Sprite Technical BBS now