Rear axle sideslip stiffness 3

[sierra4000]

Hello,
how is defined "rear axle sideslip stiffness"?
how does this value affect handling and performance?

Thank you for explanation,

 

[sierra4000]

Thank You Cibachrome,
although I have AWD Car 34/66 % F/R torque distribution, i think not depends, because lateral acceleration effects pays more or less for all cars (if main load is lateral)

what is specific case when front toe-out decrease understeer?

 

[BUGGAR]

Watching the neighbor kid riding his skateboard and watching his trucks steer/countersteer with roll. He distributes his weight for or aft on the board, depending on his desired feel for the turn. He could install front and rear trucks with different pivot axis angles for over/understeer. Although his wheels are not powered, is this what you mean?

 

[cibachrome]

What I was suggesting is the use of driveline torque to 1) apply a controlling yaw moment to the car by means of difference in side to side tire mu (on 1 series I'm familiar with that's lefts turns only). The inside tires have about 2.4 mu and the outsides have 1.4 mu. So, under a driver controlled amount of power, a rear induced yaw moment can be applied. Now recall what the aim of most rear steer algorithms for a 4WS vehicle are: Zero sideslip. 2) Additionally, a clever mechanism in the back end can also be used to induce actual rear axle steer (Tractive Force Steer. EXR would be the symbolic name for it from a K&C test. In fact, some clever teams have produced variants of this to the point where engine rotation direction has been sneaky peeky hidden under the hood to gain the best use of this phenom. Same deal though: a throttle steer effect is created with the goal of producing the zero sideslip motion (front and rear tires on the path at a tangent speed that is very high).

So here is the technical dilemma for the propeller heads: Since the rear induced yaw moment is driver controlled in both scenarios, is it counted as part of the vehicles Understeer/Oversteer recipe ? For a true 4WS vehicle it is not (just like a front steer angle is not counted as part of the slip angle family). In both cases, the contribution is NOT side-force induced (as prescribed by conventional vehicle dynamics theory, terminology and practice).

And Bugger: Yeah, that's sort of what I mean except he is using a roll steer effect to get the job done.

 

[sierra4000]

I've been thinking about it for a while, and I think more or less I understand
Thank you Cibachrome,

How front toe-out can reduce understeer?

 

[cibachrome]

What car and what brand and size of front tires ? Very tire dependent. I'm guessing here, but it may be related to the amount of tire reserve (Load rating vs. actual wheel loads). If you had some tire data, there is a cool way to get the value, but put on your seat-belt, it can often be more than 1 --> 2 degrees (per wheel). This produces the sum of the peak Ay forces for an inside and outside tire at some slip angle (hence some Ay level). Depends on a few other determinants and could really ruin your day if it goes too far. For some production cars, it can be toe-in. Change tire size and pressure then you have to recalculate. Makes for a great student exercise related to "What should my Ackermann function be ?". Then all the negative effects come forward: air drag in the straights, rolling resistance, tire wear, difficulty going straight, single wheel bumps, turn in, transient response, etc. Like I said, if you overdo it and not change the rear settings, a heap of hurt can result.

If you have a serious amount of understeer, fix the rear first. Maybe I should dig up an example (typical FSAE question, BTW).

 

[sierra4000]

"If you have a serious amount of understeer, fix the rear first??"
Dig up an example!!

 

[cibachrome]

OK, so make up a table as an array of Front Cornering Compliance (DF), Rear Cornering Compliance (DR) plus Lateral Response Time and Yaw Velocity Overshoot as key metrics for your issues. Since understeer (K) is simply the scalar difference between DR and DR, you can talk it all you want but your crew chief will have to guess which end of the car is THE problem if all you can say is that "Its understeering".

so:

DF DR K T_Ay and R_po is your score card. T_Ay is Ay response time (time to reach 90% of steady state) and R_po is the yaw velocity overshoot during a quasi step steer input. Wheelbase is typical of a mid-sized car with a slight forward weight distribution. The model is linear, which is best to educate the masses.

DF DR K T_Ay R_po
3 2 1 .27 4.2
6 3 3 .30 18.1
4 3 1 .38 5.9
4 2 2 .24 8.7
3 3 0 .50 0
2 2 0 .33 0

So if you have a heavy understeering car (6,3), response time is good but the overshoot is NFG. So, you 'fix' the front by tightening it up (4,3) and the overshoot backs down but the response times (think bandwidth) are horrible (think fully loaded P/U truck. So now you 'fix' the rear down to 2 (tires, wheels, air pressure, compliance, axle weight) and drop the front to 4 and you have a maybe decent car (the 8% overshoot may take getting use to).

If all you can do is play instead of design, then all your magazine buddies will call for that famous 'NEUTRAL' car (K=0.0), Yes the overshoot is zero, but the response times are worse than a fully loaded armor plated hearse (which may be convenient for you at a track). Sure, getting the neutral steer really low front (and rear) cornering compliance car could be a player for you but there's no forgiveness by the car as your tires go on vacation, fuel tank is filled and some damage occurs to your aero package.

Meanwhile, the steering gain of you car is so high that you are going to need a really high steer ratio gear to be a player. When diving into the pits at a low speed, make sure you can handle the huge amount of steer angles required to park the thing (get a chest friendly steering wheel knob).

Also (as you mentioned), just kick up the front static toe to +2 degrees in your BMW to experience that low front cornering compliance at a large Ay level. See attached graph from a PFG simulation. Yep, you fixed it all right. But it's transient response and steering gain makes for a lot bigger wish list. And make sure you have LOTS of tires on hand because I can smell the burning Dunlops from here. (Conti's would still be my preferred skin's).

Enjoy.

 

[sierra4000]

Thank you for explanation,

and what you meant:
A rear bar often is used to fix a compliance problem rather than change a roll trait. And in a race car, roll is not one of those 'features' you want to entertain. ?

 

[sierra4000]

How can ARB affect compliance?
Is bar attached with link direct to upright and due to roll create force for adding toe change?
What are benefits? before "classic" roll steer or compliance steer?
Is independent on wheel travel and lateral load but dependent on pure roll?

 

[GregLocock]

Using the ARB to create toe in roll is not my favorite thing. Your torque idea is good if it mounts to the spindle, the way I think of it is to angle the droplink so that it pushes whatever arm it is attached to, on the bush compliances. It reduces the efficiency of your ARB, and means that your roll steer is now affected by ARB diameter and changes in the bushes.


Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[sierra4000]

Thank You Greg

Radek

 

[NormPeterson]

A rear bar often is used to fix a compliance problem rather than change a roll trait. And in a race car, roll is not one of those 'features' you want to entertain. ?

Cornering compliance, probably. A rear sta-bar is likely to change LLTD faster than it changes the amount of roll.


Norm