Measuring Grip

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aviatorflightsims
0
Joined: 21 Nov 2012, 09:29

Re: Measuring Grip

Post

Thank you for all your input -- fascinating, and highly complicated area. Just wanted to show you the example I was using. This is a trace for Turn 1 at Sebring in the iR Daytona Prototype. Second line from the bottom is lateral G and, I believe, it is a good indication of the grip/no grip function of the tyre. There seems to be a lot of grip and loss of grip on the limit of traction through this turn and it feels to be a good representation of how a real car's tyre would perform.

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Richard
Moderator
Joined: 15 Apr 2009, 14:41
Location: UK

Re: Measuring Grip

Post

Do you really mean "grip"? That'll lead to a deep dive into tyre/surface interaction, and your best resource for that is one of the best old threads on here :arrow: viewtopic.php?f=6&t=9333

I suspect your question is more about how to identify the maximum lateral force on a car from the telemetry data? That is dependant on the aforementioned properties of "grip", along with the longitudinal force (braking) and downforce from the cornering speed.

If so, then this is a discussion about the traction circle? If your telemetry is telling you the lateral and longitudinal forces, plus an indication of lack of grip (wheelspin or locking) then it is possible to define the traction circle .. albeit for that specific car with that particular set up at that particular corner and that particular speed (due to downforce) . :arrow: viewtopic.php?p=301383#p301383

DaveW
239
Joined: 14 Apr 2009, 12:27

Re: Measuring Grip

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For interest, you may interested in my experiences.

I first encountered the instability when developing active suspension (a few years ago). Briefly, the irreversible actuators were controlled by load feedback with a number of additional "loops", including one that fed back lateral acceleration to reduce the rate of change of load with lateral acceleration. We found it best to the reduce roll angle per gn to zero. Whilst playing, we left the vehicle with engine turning & the system powered up. After a time we were surprised (panicked would be a better term) to see the vehicle oscillating madly. We found that the vehicle would always start oscillating with no obvious input (slowly, but building up the amplitude relentlessly until it ran out of oil). If the vehicle was pushed forward (or backwards) by hand, the oscillation quickly died away. Reducing the lateral acceleration loop gain solved the problem, as did the "production" fix of lowering the loop gains overall when the wheels were not rotating. I liked to call the instability "dutch roll", because it shared many of the characteristics of the aircraft lateral/directional mode.

The next interesting discovery occurred when we were playing with hub acceleration feedback (really, feedforward). It turns out that if hub acceleration is integrated (to obtain inertial velocity) and used to drive the actuators, the actuator displacements can be made to follow closely the vertical displacement of the hubs. Get it right, & the sprung mass can be stabilized inertially (road inputs will transmit no change in load to the sprung mass). One of the problems with the idea is that the system will not stabilize the hub mode. Anyway, we were playing with the idea on track & discovered that in a turn the hub mode would happily limit cycle with (in our case) the sprung mass remaining largely undisturbed. We concluded (rightly or wrongly) that the tyre itself destabilized the vertical hub mode as it recovered from its deformed state.

Both of those experiences were highly artificial, of course, but they can provide an insight into what might be happening in a passive vehicle. My Touring Car experience occurred when one team introduced linear bearing struts to reduce friction. This allowed them to reduce spring rates & damper strengths, controlling roll angle/gn by increasing bar stiffness. Ultimately, this caused the vehicle to "limit cycle" in a corner on a flat track. The cure (Greg's approach) was to reverse the trend a little (i.e. to increase spring rates, re-optimize dampers, and reduce bar stiffnesses). It worked well. It would be interesting to see if similar changes will work in iRacing.
Last edited by DaveW on 22 Nov 2012, 15:28, edited 1 time in total.

aviatorflightsims
0
Joined: 21 Nov 2012, 09:29

Re: Measuring Grip

Post

DaveW wrote:For interest, you may interested in my experiences.

I first encountered the instability when developing active suspension (a few years ago). Briefly, the irreversible actuators were controlled by load feedback with a number of additional "loops", including one that fed back lateral acceleration to reduce the rate of change of load with lateral acceleration. We found it best to the reduce roll angle per gn to zero. Whilst playing, we left the vehicle with engine turning & the system powered up. After a time we were surprised (panicked would be a better term) to see the vehicle oscillating madly. We found that the vehicle would always start oscillating with no obvious input (slowly, but building up the amplitude relentlessly until it ran out of oil). If the vehicle was pushed forward (or backwards) by hand, the oscillation quickly died away. Reducing the lateral acceleration loop gain solved the problem, as did the "production" fix of lowering the loop gains overall when the wheels were not rotating. I liked to call the instability "dutch roll", because it shared many of the characteristics of the aircraft lateral/directional mode.

The next interesting discovery occurred when we were playing with hub acceleration feedback. It turns out that if hub acceleration is integrated (to obtain inertial velocity) and used to drive the actuators, the actuator displacements can be made to follow closely the vertical displacement of the hubs. Get it right, & the sprung mass can be stabilized inertially (road inputs will transmit no change in load to the sprung mass). One of the problems with the idea is that the system will not stabilize the hub mode. Anyway, we were playing with the idea on track & discovered that in a turn the hub mode would happily limit cycle with (in our case) the sprung mass remaining largely undisturbed. We concluded (rightly or wrongly) that the tyre itself destabilized the vertical hub mode as it recovered from its deformed state.

Both of those experiences were highly artificial, of course, but they can provide an insight into what might be happening in a passive vehicle. My Touring Car experience occurred when one team introduced linear bearing struts to reduce friction. This allowed them to reduce spring rates & damper strengths, controlling roll angle/gn by increasing bar stiffness. Ultimately, this caused the vehicle to "limit cycle" in a corner on a flat track. The cure (Greg's approach) was to reverse the trend a little (i.e. to increase spring rates, re-optimize dampers, and reduce bar stiffnesses). It worked well. It would be interesting to see if similar changes will work in iRacing.
Hi Dave,

Very interesting. The oscillation you mention at standstill reminds me of something similar in sims where the vehicle at standstill *can* produce violent shaking of the wheel through the Force Feed Back mechanism. I will try those suggestions regarding set up too.
Last edited by aviatorflightsims on 22 Nov 2012, 15:11, edited 1 time in total.

aviatorflightsims
0
Joined: 21 Nov 2012, 09:29

Re: Measuring Grip

Post

richard_leeds wrote:Do you really mean "grip"? That'll lead to a deep dive into tyre/surface interaction, and your best resource for that is one of the best old threads on here :arrow: viewtopic.php?f=6&t=9333

I suspect your question is more about how to identify the maximum lateral force on a car from the telemetry data? That is dependant on the aforementioned properties of "grip", along with the longitudinal force (braking) and downforce from the cornering speed.

If so, then this is a discussion about the traction circle? If your telemetry is telling you the lateral and longitudinal forces, plus an indication of lack of grip (wheelspin or locking) then it is possible to define the traction circle .. albeit for that specific car with that particular set up at that particular corner and that particular speed (due to downforce) . :arrow: viewtopic.php?p=301383#p301383
Hi Richard,

Yes, a much more accurate description of what I am trying to achieve is to determine the maximum lateral force which a tyre can sustain -- as viewed via the telemetry. The traction circle graphic is great. In iRacing, you can plot scattergrams for tyre data in the same way as in RL, although my understanding of what they mean -- and how they can be interpreted -- is limited. Learning lots though here, so thank you.

Lycoming
106
Joined: 25 Aug 2011, 22:58

Re: Measuring Grip

Post

aviatorflightsims wrote: There seems to be a lot of grip and loss of grip on the limit of traction through this turn and it feels to be a good representation of how a real car's tyre would perform.
How do you know its a good representation of real behavior just from that trace?
richard_leeds wrote: I suspect your question is more about how to identify the maximum lateral force on a car from the telemetry data? That is dependant on the aforementioned properties of "grip", along with the longitudinal force (braking) and downforce from the cornering speed.
From the method of looking at lat. G traces, wouldn't he actually be looking at max trimmed lateral acceleration? Relating that to max lateral force from a tire is ... well, a lot more complicated. At minimum you would need to have an idea of the load distribution left/right, ie. the amount of lateral load transfer to isolate out a single tire.

If it's tire data you want, and you can get the tire data directly from iRacing, it makes more sense to do it that way than to try to get it from track data;