Understeer in open wheeled sims are gonna get better

is it possible to drive in this view?

Actually, as slip angle is increased, the cornering force increases to a peak, and then is approximately constant right out to 90 deg. Once the slip angle is past the peak you can think of the tire as just a slab of *** sliding on the road--changing the slip angle doesn't have much effect on the side force. So, the tread won't spring back as you suggest. If the the car has spun half-way-round and is moving backwards, the tread will re-center when the slip angles are near zero (or 180, if you prefer to think of it that way). -- Doug Doug Milliken Milliken Research Associates Inc.

On Mon, 23 Nov 1998 02:36:26 GMT, Doug Milliken >So, the tread won't spring back as you suggest. If the >the car has spun half-way-round and is moving backwards, the tread >will re-center when the slip angles are near zero (or 180, if you >prefer to think of it that way). You're not THAT Douglas Milliken, are you? (I'm referring to the Milliken who co-wrote "The Big, Intimidating Book of Why Cars Go Fast") Martin Nigel Mansell RIP!

Yes, that is _the_ Doug Milliken :) What he is saying is that the slip angle v. Coefficeint of Friction chart looks something like this: | --- | / \ | / ------------- | / cf | / | / | / | / | / |/ --------------------------- slip angle in degrees When the tire passes the peak slip angle, the friction force drops, but not as drastically as you suggest. There will still be considerable force rolling the tire sidewall, even while skidding at 90 degrees, so the tire will not "snap back" in any visually percievable amount. In any event, the transition is gradual.

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> But I thought that as slip angle increases,so does the tire stretching from > the rim up until the tires contact patch loses total grip,at which point the > tire snaps back since theres no pressure twisting it anymore.I reasoned that > > Actually, as slip angle is increased, the cornering force increases > > to a peak, and then is approximately constant right out to 90 deg. > > Once the slip angle is past the peak you can think of the tire as > > just a slab of *** sliding on the road--changing the slip angle > > doesn't have much effect on the side force. > > So, the tread won't spring back as you suggest. If the > > Doug Milliken

-- Pat Dotson IMPACT Motorsports http://www.impactmotorsports.com/pd.html

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> When the tire passes the peak slip angle, the friction > force drops, but not as drastically as you suggest. > There will still be considerable force rolling the > tire sidewall, even while skidding at 90 degrees, so > the tire will not "snap back" in any visually > percievable amount. In any event, the transition > is gradual.

Just to be thorough - after thinking about this a little more, I don't think the tire sidewall will rebound at all as slip angle increases. In fact, maximum sidewall deflection should occur at 90 degrees slip angle. This is because the lateral component of the friction force vector is 100% of friction force at 90 degrees slip angle. If the peak slip angle were at 9 degrees, the lateral component should be (roughly) only 10% of the friction force. -- Pat Dotson IMPACT Motorsports http://www.impactmotorsports.com/pd.html

The car is sliding completely out of control, therefore nothing aplies. Thank you Steven

> Yes, that is _the_ Doug Milliken :) Right -- Just got back from the latest NASCAR Silicon Motor Speedway (Nyack NY). It should be running today (they were giving out free test rides on Wednesday before Thanksgiving). Shameless plug--it's in the new Palisades Center Mall, near the Tappan Zee Bridge. Pat's plot of lateral force (LF or Fy) vs slip angle (SA) (below) is good, but I've changed the axis labels (to match standard terminology). Sometimes there isn't any drop-off at the peak at all, the peak lateral force stays about constant at the peak value. The lateral force is measured (defined) at the road surface, and lined up with the axle (perpendicular to the center plane of the wheel, the "y" direction.) Now to aligning torque (AT)--(another post in this thread). AT vs slip angle (SA) looks different than lateral force vs SA. The AT rises to a peak and then drops to near zero torque once the tire is at peak lateral force. The tire does get "twisted" at low slip angles because the front of the print is just getting set onto the road, while the back of the print is "stretched" sideways. Once the whole print is sliding (above the peak LF) then there isn't much difference between the sliding velocity (and force) at the front and rear of the print. Crudely like the plot below, where SA is to the same scale as the plot above. AT | /\ lb-ft | / \ | / \ | / \ | / \ |/ --\ ---------------\====\---------====== SA (degrees) \------/ The figures and explanation that we put in Chapter 2, of "Race Car Vehicle Dynamics" are much more complete. Another shameless plug-- Get RCVD now from Classic Motorbooks at the old price of $85 -- SAE just raised all the prices at their in-house bookstore, RCVD is now $89 from SAE. -- Doug Milliken Research Associates Inc.

> Im mixed up here.As the front tires experience sidewall flex,causing the > steering wheel to get tighter in relation to the amount of slip angle(self > aligning torque),then how can the steering go light when the fronts lose > grip?If the tire is sliding heavily,the slip angle is even greater,then the > self aligning torque should be even greater.But instead the self aligning > torque gets lighter.Sidewall flex is lessened??? I think that aligning torque is an effect of the spinning of the tire, and doesn't relate directly to tire deflection. At 90 degree slip angle, the tire is not spinning at all, so there should be zero aligning torque (and maximum tire deflection). Same thing with locking up the brakes, no spinning equals no aligning torque. My question is, does the peak aligning torque occur at the leading edge of the peak lateral force? This would directly define what we should feel in a force feedback wheel. If the two coincide, that would make driving on the limit more intuitive. Even if the two don't coincide in reality, I think they should in the simulation of FF effects :) BTW, I'm just learning this too - so if I get something wrong, somone please correct me! -- Pat Dotson IMPACT Motorsports http://www.impactmotorsports.com/pd.html

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> > | --- > > | / \ > > | / ------------- > > | / > > LF | / > > lbs | / > > | / > > | / > > | / > > |/ > > --------------------------- > > SA (degrees) > at the front and rear of the print. Crudely like the plot below, where SA > is to the same scale as the plot above. > AT | /\ > lb-ft | / \ > | / \ > | / \ > | / \ > |/ --\ > ---------------\====\---------====== > SA (degrees) \------/

Ah, so peak AT does coincide with the onset of peak LF. Cool. That means that FF effects should mirror real life steering forces, doesn't it? Does AT change depending on over/understeer? -- Pat Dotson IMPACT Motorsports http://www.impactmotorsports.com/pd.html

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> > > | --- > > > | / \ > > > | / ------------- > > > | / > > > LF | / > > > lbs | / > > > | / > > > | / > > > | / > > > |/ > > > --------------------------- > > > SA (degrees) > > at the front and rear of the print. Crudely like the plot below, where SA > > is to the same scale as the plot above. > > AT | /\ > > lb-ft | / \ > > | / \ > > | / \ > > | / \ > > |/ --\ > > ---------------\====\---------====== > > SA (degrees) \------/ > Ah, so peak AT does coincide with the onset of > peak LF. Cool. That means that FF effects > should mirror real life steering forces, > doesn't it?

No -- look again. The peak AT happens at something like 1/2 the slip angle for peak LF (but not exactly 1/2). By the time you get to peak LF, the AT is down near zero. Not as such...it's a tire property, so it changes with load, traction/ braking, camber, etc., and of course, slip angle. Of course if your car is wildly oversteer at the limit, you may have to spin out to get the front tires to a high enough slip angle to be at their peak LF...! I think I'm cutting into book sales by now...so I'd better quit this thread. -- Doug Milliken

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> > Im mixed up here.As the front tires experience sidewall flex,causing the > > steering wheel to get tighter in relation to the amount of slip angle(self > > aligning torque),then how can the steering go light when the fronts lose > > grip?If the tire is sliding heavily,the slip angle is even greater,then the > > self aligning torque should be even greater.But instead the self aligning > > torque gets lighter.Sidewall flex is lessened??? > I think that aligning torque is an effect of > the spinning of the tire, and doesn't relate > directly to tire deflection.

It's not related to the spinning per se, but the effect of that spinning on deflections in the tread and suspension geometry. This depends on the castor of the wheel. If the effective center of the (sliding) contact patch is to one side of the axis about which the wheel can pivot, there will be an aligning torque. The following picture illustrates the case. Assuming the forces are balanced in magnitude (they will be in a sliding car tire example), the fact that they are offset will result in an into-the-page (counterclockwise) torque. ^ | ---------------- | V That's a little different. If the wheel is already aligned, then the aligning torque from castor will be zero, and the wheel will not tend to center (it already is). If the wheel is not aligned, it's the same case as above. (assuming a non-zero castor) ---Jim

Since this has gotten pretty technical, it's time for some standard terms...even though I swore off this thread earlier...! Aligning Torque (AT) is the torque about a vertical axis (SAE "Z" axis) through the axle and the wheel centerplane. It is one of the tire forces and moments and is independent of steering and suspension geometry. It's primarily a function of slip angle and load. It's not quite zero when the slip angle is zero (tires don't really run quite straight). Trail results from Castor angle and/or Offset in the steering geometry. Tire lateral force at the ground produces a torque about the kingpin. Steering torque about the kingpin is a combination of the AT and the lateral force x trail torque. Then to get it up to the steering wheel there is a gear ratio, and (usually) quite a bit of friction. -- Doug Milliken Research Associates Inc.