10 replies to this topic

[Ben]

I'm currently working with some data from a vehicle fitted with a TC system. I don't know (and nor am I likely to be told) what the algorithm uses to calculate what to do to the engine but I have a fairly good idea having looked at how the slip ratio varies with various other parameters.

It's got me thinking though about what you would use as your target for developing a TC system. The main thing at the moment that's bugging me is that friction tends to be related to sliding speed, whereas we tend to look at tyre traction from the point of view of slip ratio which is a relative measure largely independant of speed. The result of this is that TC systems to the best of my knowledge target slip ratio ranges rather than slip speed ranges. What would happen if you did it the other way?

Ben

[Ross Stonefeld]

So you'd want more TC in higher speed corners and less in lower speed corners?

[GregorV]

As you say, slip ratio is basically the same as slip velocity, only that it is normalized with respect to the actual travelling velocity of the wheel. The reason why this is the preferred way of looking at the problem is, if the friction of the tread elements of the tyre (do not confuse this with the total "friction" force of the tyre) is largely speed independent and you neglect the inertial properties of the tyre, the tyre forces should not depend on the velocity of the wheel and would only depend on the slip angle/ratio (provided same load, geometry, temperature and the rest). While friction of tread elements with velocity does depend on the sliding velocity somewhat, this is a relatively gradual change within the normal velocities of tyre operation.

Furthermore, when most of the tread elements are not sliding yet (small enough slip angle and ratio, the linear regime), the tyre forces do not depend all that much on the frictional properties of the tread elements but mostly on the tyre construction. Since you have to build your TC software around a robust and largely valid model, the one where the slip ratio instead of slip velocity is the dominant parameter is the more suitable one.

If you would indeed put the target of your TC system to be the sliding velocity rather than slip ratio, you would quickly find that at high speed the TC would cut in when the tyres still have not even begun sliding, as, with increasing travelling speed and a fixed sliding velocity, sooner or later you would find yourself in the linear regime of the tyre where the frictional properties of the tread aren't even all that important yet.

[Ben]

Thanks Gregor, You sort of mentioned what I'd been thinking about when you said "provided the same... ...temperature...". The point about a race tyre operating in the non-linear regime is that the temp isn't constant and may be a function of sliding speed.

Ben

[GregorV]

Indeed, but nevertheless you would still find that the rough estimate of the slip ratio where the tyre is approaching the limits of traction is given much more by the tyre construction characteristics than the actual friction coefficient, and therefore it will be mostly speed independent.

Another point to mention is that the main goal of TC software is perhaps not so much in providing as much tractive force as possible but for the tractive force not to take too much away from the lateral force. When the tyre is already mostly sliding, the total force points roughly opposite the direction of sliding. If you allow a larger slip ratio for a tyre, this means that to obtain the same direction of the total force (the magnitude may be considered rather constant close to the limit) you will also need to generate proportionally more slip angle so that the slip direction and therefore the direction of the tyre force are kept the same. For various reasons (wear, temperature, aero...) you want the slip angle at the rear to be contained, and indirectly the slip ratio as set by the TC software controls that.

[John M Cannon]

Our racing version took lateral 'G' into consideration as well. The lower the 'G', the more active the system would allowed to be. From there, it was simply establishing thresh-holds based on data and the driver's feedback.

I would think that most F-1 systems would be mapped prior to the event by doing sim work and determining approximate optimum slip thresh-holds in advance. Fine tuning would then take place once the car has done some laps and the data analyzed. Changes would then be applied, through-out the weekend, as the track grips up, conditions change, etc.

[Christiaan]

Oaky guys, please for my benefit go back to basics

what is

- slip speed?
- sliding speed?
- slip rato?
- slip angle?

and hw are they measured

[Ben]

Slip angle is the angle between the lateral and longitudinal velocity of the tyre. i.e. it is tan-1 of V lateral / V longitudinal.

I would use slip speed and sliding speed to mean the same thing, tha tis the relative tangential velocity between the rubber and the road.

Slip ratio is simply slip speed normalised w.r.t speed. This means that a slip ratio of 10% at 200mph is a very different sliding speed than a slip ratio of 10% at 50mph.

Any scientific literature on rubber friction and abrasion uses sliding speed, whereas the forces and moments of a tyre are given as functions of slip ratio and slip angle.

Ben

[Greg Locock]

On the track you measure lateral slip with a correvit, a gyro, and an accelerometer, or two correvits (I think, that would be the bare minimum). You also need rack position.

longitudinal slip is measured wth the correvit and then individual wheel speeds via tone wheels.

[Christiaan]

Tell me your reference planes. I assume lateral is on the plane of the road, but that would mean longitudinal is normal to that , which makes no sense to me

[Greg Locock]

lateral is across the car, longitudinal is along it.
?