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Longitude Tire Force: low-mu to high-mu road

Hi Everyone.

I am a bit stuck.

I am running a straight line ADAMS/Car full vehicle model. The vehicle is travelling on low-mu=0.1 and then enters high-mu=0.9. I am tracking the longitude tire force. While on the low-mu the tires are slipping.

I run two load cases (LC#1 , LC#2). LC#2 is the same as LC#1 except the vehicle enters the high-mu with a higher vehicle speed, higher wheel speed and more tire slip. Roughly same tire normal force.

The results predict that the longitude tire force magnitude is roughly the same between LC#1 and LC#2 shortly after entering the high-mu road. I would expect that LC#2 would have a larger tire longitudinal force since it enters the high-mu road with a higher vehicle speed, higher wheel speed and higher wheel slip.

I also reran using PAC89 and see the same trend.

Would the ADAMS/Tire be able to differentiate these two situations and generate larger longitudinal force if the vehicle hits a high-mu road with greater speed...what am I missing?

joe.

0 jsph_inc over 1 year ago

Forgot to say..I ran using PAC2002 and PAC89 tire models.
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0 jsph_inc over 1 year ago in reply to jsph_inc

Update,

I suspect the tire longitude slip is very high such that it has "saturated" the long tire force. So even more tire slip wouldn't generate additional long tire force...I think.
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0 Jesper Slattengren over 1 year ago

Hi Joe, did you run at constant speed, braking or accelerating at the point you hit the high-mu border?
What is "shortly after"? A second, a tenth of a second, a hundredth of a second? Does your tires have lag?
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0 jsph_inc over 1 year ago in reply to Jesper Slattengren

Hi Jesper,

No braking at any point. Throttle demand is 100% while on low-mu. once the front wheels hit the high-mu road, throttle demand goes to zero.

LC#2 has a higher wheel speed and wheel slip at the start of the high-mu road. The resulting long tire force magnitude is essentially the same in the high-mu road as LC#1 (which has a lower wheel speed and wheel slip at the start of the high-mu road).

Apologies, by "shortly after" I mean from when the front wheels enter high-mu to the when their wheel slip goes to zero slip.

Could this be because for the tires long force vs slip relationship, the wheels are slipping so much that they are in the flat portion of the curve...and so a difference in wheel slip at that point won't generate a different long tire force? (see below)
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0 Jesper Slattengren over 1 year ago in reply to jsph_inc

What is the slip? You should be able to plot that and understand what is happening.

The slip should drop quickly when you drop the throttle. How fast depends on the inertia of the driveline, seen from the wheel end. The engine (flywheel) is very "heavy" seen from the wheels, with I_eff = I_eng*r^2 where r is the gear ratio. So it might take some time for the wheels to slow down. All should be visible from plots of wheel speed, tire slip and vehicle speed.
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