over and under steer 101
08-10-2012, 11:05 PM
#31
Nordschleife Master
It's not just you Mdrums, I think that way the chart is worded, unless you read it very carefully, it can confuse the reader.
08-10-2012, 11:07 PM
#32
Nordschleife Master
I still stand by my statement that the first 2 are backwards - I only used 40psi as an example to make the point. I have never, ever, raised tire pressure to get more grip - ever. I lower tire pressure to get more grip at the end that needs more grip - obviously up to a point - these are small adjustments as you say.
And, this also makes sense with all the other methods mentioned as well:
- increased tire pressure = more spring = more ride height = more push (at that end)
- decreased tire pressure = less spring = less ride height = more grip (at that end)
I am removing the conversation of street tires that may need more psi because of overly soft sidewalls that basically collapse with track side loads - in that case, and from very low psi, increasing psi may in fact increase grip
And, this also makes sense with all the other methods mentioned as well:
- increased tire pressure = more spring = more ride height = more push (at that end)
- decreased tire pressure = less spring = less ride height = more grip (at that end)
I am removing the conversation of street tires that may need more psi because of overly soft sidewalls that basically collapse with track side loads - in that case, and from very low psi, increasing psi may in fact increase grip
08-10-2012, 11:29 PM
#33
Nordschleife Master
08-11-2012, 08:30 PM
#35
Instructor
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I think the dangerous thing about 'lists' like the one under discussion is the reverse engineering that people do to try and arrive at a cause for a given effect.
Oversteer and understeer are terms used to describe sub-limit, linear range tire operation (the increase in lateral force per degree slip angle is constant). Terminal spin or plow are better terms to describe limit handling stuff. The important thing to keep in mind is that ALL of this handling stuff is about managing tire characteristics, and the cornering stiffness (derivative, not spring rate) is probably the most important concept to look to. This is simply the rate of change of lateral force with slip angle, or the slope of the lateral force vs. slip angle curve. Note; the important thing is in fact the rate of change of cornering force (front axle to rear), not so much the absolute values. At the peak of this curve, the lateral force no longer responds to slip angle (obviously), and small changes in steer angle no longer have an effect. Changes in vertical load, however, still has an effect, as does camber, on lateral force production.
So look at your lists, and see what makes sense to you, and remember that there are NO hard and fast rules. All of this depends on tire construction, steering system and suspension compliance (a lack of rear toe control will almost always be reported as excessive roll by a driver). Just a couple of things to consider.
Oversteer and understeer are terms used to describe sub-limit, linear range tire operation (the increase in lateral force per degree slip angle is constant). Terminal spin or plow are better terms to describe limit handling stuff. The important thing to keep in mind is that ALL of this handling stuff is about managing tire characteristics, and the cornering stiffness (derivative, not spring rate) is probably the most important concept to look to. This is simply the rate of change of lateral force with slip angle, or the slope of the lateral force vs. slip angle curve. Note; the important thing is in fact the rate of change of cornering force (front axle to rear), not so much the absolute values. At the peak of this curve, the lateral force no longer responds to slip angle (obviously), and small changes in steer angle no longer have an effect. Changes in vertical load, however, still has an effect, as does camber, on lateral force production.
So look at your lists, and see what makes sense to you, and remember that there are NO hard and fast rules. All of this depends on tire construction, steering system and suspension compliance (a lack of rear toe control will almost always be reported as excessive roll by a driver). Just a couple of things to consider.
08-15-2012, 02:37 PM
#36
I think the dangerous thing about 'lists' like the one under discussion is the reverse engineering that people do to try and arrive at a cause for a given effect.
Oversteer and understeer are terms used to describe sub-limit, linear range tire operation (the increase in lateral force per degree slip angle is constant). Terminal spin or plow are better terms to describe limit handling stuff. The important thing to keep in mind is that ALL of this handling stuff is about managing tire characteristics, and the cornering stiffness (derivative, not spring rate) is probably the most important concept to look to. This is simply the rate of change of lateral force with slip angle, or the slope of the lateral force vs. slip angle curve. Note; the important thing is in fact the rate of change of cornering force (front axle to rear), not so much the absolute values. At the peak of this curve, the lateral force no longer responds to slip angle (obviously), and small changes in steer angle no longer have an effect. Changes in vertical load, however, still has an effect, as does camber, on lateral force production.
So look at your lists, and see what makes sense to you, and remember that there are NO hard and fast rules. All of this depends on tire construction, steering system and suspension compliance (a lack of rear toe control will almost always be reported as excessive roll by a driver). Just a couple of things to consider.
Oversteer and understeer are terms used to describe sub-limit, linear range tire operation (the increase in lateral force per degree slip angle is constant). Terminal spin or plow are better terms to describe limit handling stuff. The important thing to keep in mind is that ALL of this handling stuff is about managing tire characteristics, and the cornering stiffness (derivative, not spring rate) is probably the most important concept to look to. This is simply the rate of change of lateral force with slip angle, or the slope of the lateral force vs. slip angle curve. Note; the important thing is in fact the rate of change of cornering force (front axle to rear), not so much the absolute values. At the peak of this curve, the lateral force no longer responds to slip angle (obviously), and small changes in steer angle no longer have an effect. Changes in vertical load, however, still has an effect, as does camber, on lateral force production.
So look at your lists, and see what makes sense to you, and remember that there are NO hard and fast rules. All of this depends on tire construction, steering system and suspension compliance (a lack of rear toe control will almost always be reported as excessive roll by a driver). Just a couple of things to consider.
I think you already lost me in the 3rd phrase 
And that is exactly the reason why I don't start messing around with my suspension / tyre pressures. I take a "general" setup done by somebody who knows his job and I try to fix the rest by adjusting the nut between the seat and the steering wheel.
