Hi All
I don't get to drive many cars other than my own these days - I know, I should get out more. We use hire cars at work when out on business and it's always interesting, though often disappointing, to see what gets delivered. The other day I was quite pleased to see a 2.0 turbo Astra SRi sat outside - that's a pertol engine BTW. I thought hmmm, perhaps I can some fun in that.
And so, to the point .....
Under "enthusiastic" acceleration, the steering seemed to twitch to the right. I found this really disconcerting. It felt like some sort of torque effect. I was wondering if any of you automotive experts out there could provide an explanation as to the cause. Yes, it was my heavy right foot, I know ;-)
Cheers
Les
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I'm not that familiar with the design of this car, but torque steer in FW cars is worst when the driveshafts are of unequal length. They transfer the torque form the gearbox to the wheels and any driveshaft will twist slightly when torque is applied. For this reason, when the shafts are different lengths the longer one is made thicker so that they have similar ammounts of twist in them. In reality this doesn't quite work and there's a considerebale kick in the steering when accellerating hard.
Its made worse when cornering or when there's different levels of grip available to each of the driven tyres.
Doubtless Number Cruncher will arrive shortly with a far better explanation than this.
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I read often, only post occasionally
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>>Doubtless Number Cruncher will arrive shortly...
Well, a good technical question, and a nice Vauxhall too! How could I possibly keep away?
with a far better explanation than this.
Hmm - I'm not so sure now. I'll let you be the judge of that.
I think that spikeyhead is right to point to unequal length driveshafts, but I'm not sure that its the twist in them that's important. Any difference in twist will simply be handled by the diff - the torque in the two shafts will be the same despite their differing twist angles.
The problem with unequal length driveshafts is that it guarantees unequal angles (when viewed from the front of the car) between the driveshafts and the steering axis on each side of the car. This angle difference becomes even more extreme if the front of the car lifts during acceleration.
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It's probably easier to explain the system of torques via an equivalent system of forces.
Imagine you have three pieces of rope, each fastend to each other at one end only (like a Mercedes badge).
If two people pick up ends of the rope and pull, the rope forms a straight line between them.
It is only when a third person picks up the remaining rope end and pulls that the force system can be anything other than a straight line. This is the important point!
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In a front wheel drive system, there is torque in the driveshaft, and there is torque in the hub. To allow this change in direction, there *must* be a third torque. This is the guilty party, it tries to steer the wheel.
But, if the driveeshaft angles are equal, the steering torque at one roadwheel balances that at the opposite road wheel, and the driver feels nothing at the steering wheel.
Some cars use a short jack shaft, with a bearing mounted on the lower crankcase to equalise driveshaft to hub angles.
The Astra in this example is effectively betraying its origins as a family car rather than a performance car. In the lesser engined variants, torque steer isn't noticable.
Number_Cruncher
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OK, thanks guys - I think I see what you mean. Torque steer - If nothing else it's nice to put a name to it.
I know for sure I couldn't live with it.
Les
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NC's explanation I guess outlines why some cars might pull more to one side than another however the characteristics of the differential are a primary issue here. Torque steer is most apparent when under hard acceleration in low gears where the tyres are momentarily at the limits of adhesion, i.e running over white lines, manholes covers, patches of grit etc. in such circumstances the differential will transfer the torque from the wheel that loses grip to the other driven wheel and then back again as the wheel regains grip hence the steering wheel will often pull from side to side.
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Now every high power FWD car I have had has always torque steered to the right.
Question - has anyone had a FWD torque steer to the left, if not why not?
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TourVanMan TM < Ex RF >
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Even my diesel will torque steer to the right if you give it some welly in second gear.
Answer: Pass.
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Now every high power FWD car I have had has always torque steered to the right.
Yes, that's a consequence of the passenger side driveshaft usually being the shorter of the two. This means that the angle on this side is always steeper, so the passenger side "wins" in the steering torque balance I was describing above, and so, the passenger side wheel steers in, loading up the steering system in that direction, resisted only by the driver's input.
In the interim, I have thought of another way of describing the physical basis of troque steer - which is not at all the same as the limit behaviour when one wheel skids.
If you imagine using a socket set with the set up - socket - extension bar - universal joint - extension bar - tee bar. With some angle in the universal joint, as you apply torque to the tee bar, unless you can steady the universal joint, it moves sideways, and the socket probably slips off. This is exactly analogous to the mechanism which produces the steering torque in a FWD system.
Number_Cruncher
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.. Torque steer is most apparent when under hard acceleration in low gears where the tyres are momentarily at the limits of adhesion, i.e running over white lines, manholes covers, patches of grit etc. in such circumstances the differential will transfer the torque from the wheel that loses grip to the other driven wheel and then back again as the wheel regains grip hence the steering wheel will often pull from side to side.
The diff will apply its rotary force to both sides. When one side loses grip then there's no transfer of grip to the other side unless the diff has a limited slip mechanism in it.
Just think of a car stuck in mud, usually only one wheel will spin, the other goes nowhere. Alternatively, jack up one side of the car, turn the engine on, put it into gear and tell me how much torque is being applied to the wheel still on the ground.
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I read often, only post occasionally
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A limited slip diff will start to lock up when an undesirable difference in wheel speed occurs, so I assume this would transfer up to 50% (but not more) of the torque to the wheel with better grip?
A Torsen diff will transfer all the torque to the wheel with grip, or as much torque as that wheel/ tyre can handle, up to a pre-determined limit.
My first car was a Mk2 Cavalier 1.8i, which was only 115bhp but was an 8 valve engine so had good low down torque, and with no power steering. With a heavy right foot when going round low speed bends, the normally heavy steering would lighten up dramatically and if really giving it some welly, would try to wind on full lock itself. I think it did this both directions and can't remember whether it was worse going right rather than left.
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Lee,
Get yourself a nice E46 BMW none of that nasty front wheel drive torque steer. Front wheels are for steering !
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And rear wheels for oversteering
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TourVanMan TM < Ex RF >
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Lee, Get yourself a nice E46 BMW none of that nasty front wheel drive torque steer. Front wheels are for steering !
I think PU might mean me.
Yes, I agrtee - that's why I normally drive an Omega ...but then TVM's right too.
Les
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I remember driving a modified Lancia Delta HF turbo once. I'd never experienced torque steer before. I hit the gas and the thing nearly leapt into the hedge! A real brown trousers moment that!!
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but then TVM's right too.
He always is........;-)
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