Torque Steer
Torque Steer
(OP)
I've been looking high and low and haven't been able to find a satisfactory explanation of torque steer in front wheel drive vehicles. I understand that it's caused by unequal length shafts from the differential to the drive wheels, but I don't understand how unequal torques come out of an open differential.
If anyone knows where I can find an in-depth explanation, I could stop pulling my hair out. Thanks,
Dan
If anyone knows where I can find an in-depth explanation, I could stop pulling my hair out. Thanks,
Dan





RE: Torque Steer
As far as I know (I'm not an engineer) the torque is transmitted to the wheel that grips, not both equally.
best definition I can find is at http:/
Russell Giuliano
Unique Technologies Associates - Cobra Solid Lubricant
www.uniquetechnologies.com
RE: Torque Steer
My understanding is that torque steer effect mainly has to do with the caster and scrub radius built into the front suspension rather than having different drive shaft lengths.
A certain amount of caster will cause the steering to self centre and provide some tactile steering "feel" and driver feedback. These forces increase during braking, improving straight line stability. Unfortunately during acceleration with FWD it all works backwards, the more wheel torque you have the more unstable it can become.
Scrub radius will generate a steering torque on each side of the car. Provided wheel torque and weight is the same on each front wheel the forces should balance, with no net steering effect. During braking, caster should be sufficient to ensure the whole mess does not become unstable. But during acceleration it may start to steer, and lateral weight transfer can exaggerate the effect, so it may then become very difficult to hold a straight line.
I know my turbocharged FWD would gently rock from side to side and weave with ever increasing amplitude under high acceleration. Rather disconcerting, and it had equal length front driveshafts too.
RE: Torque Steer
Best regards,
Matthew Ian Loew
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Torque Steer
1. The first reason could be different drive shaft angles. On early designs the drive shafts usually went directly from the transmission to the spindles and the differential was not centered. So depending on the ride height the drive shafts would operate at different angles. I know that CV joints shouldn't cause this, but maybe the different angles create different loads on the spindles? Do CV joints have significant differences in friction with a slight change in drive angle? 3 ball vs. 5 ball? I supose that this would also be valid on a car with equal length shafts if the car was not sitting level side-to-side.
2. Before I heard the different angle idea, my thought was that the unequal length drive shafts would have different amounts of wind-up on the left vs right. ie different shaft diameters, lengths, etc. And even if the drive shafts are equal, unless the differential is in the center of the car there is still a jack shaft on the right side which would cause a difference in stiffness.
I had a 92 Old Achieva with the 190hp Quad four, Getrag torque biased differential, and equal length drive shafts. It could spin the tires in 2nd gear and it would yank the wheel for a split second until the diff kicked in. It always pulled to the same side, right as I recall. This would lead me to believe it has something to do with the diff, but I can't fathom a reason.
Happy hunting, ISZ
RE: Torque Steer
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Torque Steer
I have a very good understanding. Statics gets you most of the way there. I really wish people would stop guessing and offering opinions and instead frame the question in terms of engineering principles. Sad to see the FBD so under-appreciated.
Best regards,
Matthew Ian Loew
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Torque Steer
Secondly whatever the reason for it,torque steer effect felt by the driver is very dependent on the road surface, camber and undulations as with torque steer the car tends to follow the down side. Opinion based on driving a number of cars on a variety of cambers.
Regards
Sandy Cormack
RE: Torque Steer
However, thinking about it this morning I came up with a list of assymetries that reliably exist in the real world.
I'm going to be a bit hazy about this as I want to see if the numbers stack up.
Incidentally it is probably worth pointing out that classic torque steer is a FWD problem, and to some extent is related to the steering wheel torque and steering system compliance, that is, I think that if the driver held the road wheels straight then the car would not steer any more than a RWD.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Torque Steer
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Torque Steer
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Torque Steer
Norm
RE: Torque Steer
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Torque Steer
Is this phenomenon perhaps more prevalent in models for which NVH isolation and ride comfort carry relatively higher priority?
FWIW, and not being a drag racer, I've noticed this only a couple of times in my '01 Nissan Maxima and never in the '95 Mazda 626 (which does experience quite enthusiastic starts at autocross). Both have manual shift transmissions, though the Maxima also has a viscous limited slip diff.
Norm
RE: Torque Steer
Yup. Well, that's another one to put on the list.
"Is this phenomenon perhaps more prevalent in models for which NVH isolation and ride comfort carry relatively higher priority?"
And/Or maybe in those manufacturers who don't understand how to mount FWD engines properly?
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Torque Steer
Rod
RE: Torque Steer
RE: Torque Steer
1) outer CV joints do create a 2nd turning moment due to angle. This was indicated with the FBD discussion but not expanded on. When at different angles (unequal length) this creates a difference in turning moment accross the axle which is proportional to torque, so you get torque steer
you still get this on equal length shafts due to differences in angle from roll, bump and steering (akerman etc), so there is no such thing as a FWD with no torque steer
2) there is vary rarely such a thing as 'equal length' driveshafts in FWD cars, as even though the barshafts between CV joints can be equal, and is aimed for for the reasons above, the total length isnt, as the differential is rarely central, so a linkshaft is used on one side. This creates a difference in torsional stifness betwwen L and R shafts, so during torque dynamics this results in differences in wheel end torques due to the shaft windup, and therefore torque steer. This can be reduced by trying to equalise the torsional stifness also, but is often difficult to achieve to <10% difference
3) You also have to look at suspension geometry and things like variation in bush rates and deflection, which also have an influence
the aim is to achieve a suspension and steering system that is largeley insensitive to torque steer, as well as minimise the driveline effect on it, as there is no real way to ensure equal wheel torque across the axle
If you want a good example of a FWD suspension system that is insensitive to torque steer, look at Hondas 5th gen Prelude fitted with ATTS (active torque transfer) diff. This had a unique unequal length independant suspension at the front end with double control arm links.
If you want some bad examples look ad the Ford Focus RS and Rover 220 turbo (Euro models). Both of these had McPhereson front suspension (wich is far from ideal) with Torsen differentials and high torque FWD powertrains, and both had major issues with torque steer sensitivity!