Vehicle Dynamics 2
Discussion
Modern cars are designed so that the front tyres have less influence(greater slip angle)on direction under most circumstances than the rear. This means that the car will understeer and so if we wish the car to turn more to the left while already turning left we need to steer more to the left. This is intuitive and safe. It also cuts down on litigation.
Many years ago however, before car makers knew about vehicle dynamics quite often cars would inherently oversteer. This meant that as soon as they steered in one direction the turn would naturally get tighter, requiring steering in the opposite direction to return to the desired course. Aha! you say, so that's why all drivers in older films are constantly sawing slightly at the steering wheel, because as soon as the car goes slightly one way it has to be corrected.(This was also true of many pre war race cars and why those drivers are constantly twitching the wheel. Even more respect due, it just wasnt possible for them to relax) This isn't necessary in a modern car but you may still see it in modern films as that is seen as the 'correct' way to drive. (If it's on TV it must be right! and the car will be on a trailer). These corrections are less intuitive and not so safe which is why cars aren't designed to handle like that any more.
These differences in influence of the front and rear tyres, or slip angles are not fixed but vary, a subject for my next post
Many years ago however, before car makers knew about vehicle dynamics quite often cars would inherently oversteer. This meant that as soon as they steered in one direction the turn would naturally get tighter, requiring steering in the opposite direction to return to the desired course. Aha! you say, so that's why all drivers in older films are constantly sawing slightly at the steering wheel, because as soon as the car goes slightly one way it has to be corrected.(This was also true of many pre war race cars and why those drivers are constantly twitching the wheel. Even more respect due, it just wasnt possible for them to relax) This isn't necessary in a modern car but you may still see it in modern films as that is seen as the 'correct' way to drive. (If it's on TV it must be right! and the car will be on a trailer). These corrections are less intuitive and not so safe which is why cars aren't designed to handle like that any more.
These differences in influence of the front and rear tyres, or slip angles are not fixed but vary, a subject for my next post
martin a said:
This means that the car will understeer and so if we wish the car to turn more to the left while already turning left we need to steer more to the left.
Eh?
I prefer the definition that to achieve a turn of given radius in an U/S characteristic car, the steering angle is more than the Ackermann angle (given by the wheelbase over the turn radius). In other words, the car does not respond to steering inputs as much as you might expect from neutral steering car.
For an oversteer characteristic car the opposite is true - where steering is applied the turn executed by the car is tighter than expected of a neutral steer car.
A number of factors can influence the extent to which a car will oversteer or understeer. In particular, relative front / rear grip (and effect on it of power application through front/rear wheels), position of longitudinal centre of gravity, relative spring stiffness of front/rear suspension etc etc.
For oversteer characteristic vehicles the extent to which the car steers more than the steering input would suggest is dependent on speed. There is a speed (the "characteristic speed"
at which the vehicle turning rate become independent of steering input (in fact tends towards infinite responsiveness). At this speed the vehicle will rapidly, unpredictably and irrevocably lose control. This is often bad news for its occupants. It is for this reason that all modern manufactured cars are built with designed-in understeer.
7db said:
For oversteer characteristic vehicles the extent to which the car steers more than the steering input would suggest is dependent on speed. There is a speed (the "characteristic speed"
It is for this reason that all modern manufactured cars are built with designed-in understeer.
Actually dB, you mean the Critical Speed - the characteristic speed is for understeer cars and is the speed at which the steering angle required to navigate the bend is twice that at very low speed.
Oh, and it's only one of the reasons why you have steady-state understeer engineered into the cars.
I agree there are many factors affecting and ways of decribing handling. I was just trying to explain things in the simple terms that I use to teach learner drivers or those with little technical knowledge. Sorry if I've confused anyone.
>> Edited by martin a on Wednesday 26th April 16:26
>> Edited by martin a on Wednesday 26th April 16:26
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