How is the static toe setting determined

[TMAPV]

Anyone got any experience in what governs the toe setting? Looking online, different OEMs maybe have even different signed (toe in vs out) especially at the front with sometimes going as low as 0.05 deg. Since the overall balance front to rear axle is made up of many other factors, static toe may be miniscule among them, so my thought is that somehow tire transient properties come into play?

 

[GregLocock]

Static toe is set by tire wear, understeer and braking stability, and initial turn in (and no doubt other things). At the front it also provides fake ackermann, which probably leads back to tire wear, and turn in.

It also has to be set to account for the various errors designers have made in the roll and compliance steer characteristics in an effort to sort the worst of them out.

Most of my work is on truck-like-objects these days, so we tend to worry more about tire wear, hence run very little toe or camber at the design load. Static toe and camber are vey strange settings to me, in general something that is good for the front suspension must be reverse for the rear suspension, yet toe in and negative camber is generally a good thing for both ends. Oh well, if it was easy it wouldn't be fun.




Cheers

Greg Locock


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[cibachrome]

Used to be that on-center steering feel played a roll in this setting, but these days, the goal is minimum road load tire rolling resistance AT SPEED. This is measured during a dynamic toe test in which precise measurements are made of the individual toe values for constant speeds, and under various acceleration and braking conditions. I point out that the minimum Fx or Iy moment is not necessarily at zero toe. Because of ply steer, conicity and residual aligning moment, the minimum Fy or Fx will be non-zero. This is all set by tire specifications given to tire makers. The vehicle build settings are coordinated with these specs.

 

[TMAPV]

Is the on centre feeling linked to cars in racing series having toe out almost 100% of the time? There must be some objective reason for it?

 

[GregLocock]

I doubt on-centre feel is of much interest to racers.

Cheers

Greg Locock


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[cibachrome]

The use of a toe-out static setting on many 'race' cars is often based on the belief that this increases the maximum lateral acceleration capability of the PAIR of tires on that axle. It's a cheap form of Ackermann steering in that at some steered angle the pair of tires works better together. However, as is often the case, this concept is very dependent on the properties of the tire construction, rim, and pressure. Tire tests can easily reveal whether this tactic is effective. A plot of lateral force vs slip angle at multiple vertical loads will indicate where the peak force levels occur. If the peaks line up on a vertical line, there is no point in running other than parallel steer. Each tire operating at drastically different vertical loads will peak out at the same slip angle. Yes there are some other factors involved, such as tire aligning moments on each tire and their camber positions. And this can be addressed by steering linkage geometry too, but not easily adjusted at a track on race day. But the tire data tells the tale.

The drawback to all this theory is that 'optimizing' one end of the car (front in this case), tightens up the grip on this axle. Without an equivalent 'improvement' in the rear grip level, there is a risk of making the car 'worse'. So, the front steer change makes the car more 'loose'. Yes, if the car is a disaster to start with, a driver will beg for a fix and this might help, but only if the tire responds as imagined.

Yes, this could work on the rear, too, but tractive force compliance steer may be the benefactor in this case. Same deal for FWD sleds.

Unfortunately, my library of 'race' and high performance tires, shows that there would be little benefit to differential inner and outer steer angles. This is because the load capacity of these tires is VERY high (I.E. the tire's load rating is much higher than the corner weights. Keep in mind that we're not talking 20 degrees of upright/knuckle steer angles, but more likely 4 to 5 degrees at most. And the curvature values on the track come into play, too.

But it makes for getting some really good bullshit points by hucksters selling books, articles, and bootcamps. In God we trust, everybody else we're gunna need some data...

 

[GregLocock]

A race engineer friend of mine occasionally has time on the track and runs a swing of toe on each end, and then considers which are tight corners (I think) and feeds that back to the team when considering ackerman for the build for next season. Another sets his car up for parallel steer, ie no ackerman, and if he has time at the track does a swing on toe for that track. Since he always races the same car on the same circuits he has a library of settings.

Cheers

Greg Locock


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