Slip angles in the wet

On wet surfaces, or anyway on low friction surfaces, we often see racing cars traveling at much greater yaw angles than on dry tarmac.

It looks like  slip angle for maximum lateral force increases as  maximum friction coefficient decreases.

I have been wondering why.

I see three possibilities.

1) There is something missing in  Pacejka's Magic Formula in this regard

2) There is something I am missing in Pacejka 's Magic Formula in this regard

3) The phenomenon is related to diminishing lateral load transfer, but again I cannot at the moment fathom why.

Opinions?

IIRC the relative slip angle vs friction curves for tires are quite similar in shape (i.e. max relative friction coefficient vs slip angle) for wet / dry... However, it might be that the tires optimised for wet running, which are softer and have cuts, work better at higher slip angles than the usual slicks...

On wet surfaces, or anyway on low friction surfaces, we often see racing cars traveling at much greater yaw angles than on dry tarmac.
It looks like slip angle for maximum lateral force increases as maximum friction coefficient decreases.
I have been wondering why.
I see three possibilities.

1) There is something missing in Pacejka's Magic Formula in this regard
2) There is something I am missing in Pacejka 's Magic Formula in this regard
3) The phenomenon is related to diminishing lateral load transfer, but again I cannot at the moment fathom why.

Opinions?

The pac model for a tire is based on the measuring conditions at the time of test. For instance my tires are tested on dry 3M 80 grit paper. As it stands the pac formula includes no modelling of wet grip, unless you measure in the wet.

@sarbav, the yaw angle is the difference between the vehicle path and the vehicle attitude.

Edited by Greg Locock, 30 June 2018 - 20:05.

On wet surfaces, or anyway on low friction surfaces, we often see racing cars traveling at much greater yaw angles than on dry tarmac.

It looks like  slip angle for maximum lateral force increases as  maximum friction coefficient decreases.

Maybe drivers, cars and tyres are behaving like before we had slicks and wings. Drivers put the car into a "controlled drift" or "controlled oversteer" by applying more or less power to the rear wheels to override neutral steer characteristics. 

In ye olde days, cars had dry setups established during practice and wet setups based on best guess and previous experience. A bit more front wing for the wet, perhaps?

Slip angle is more related to carcass stiffness than tyre-road grip. As a result they are modelled in Pacejka as 2 independent effects.

What you might be seeing could be due to wet tyres having a lower carcass stiffness or simply that the drivers step over the limit more often in the wet.

Wet tyres have tread blocks which move about and effectively make the tyres narrower, I'm sure that this may be a contributory factor. I recall that in F.Ford, a brand new set of Firestone F100s had the car moving about all over the place - The well-off young turks would have their tyres ground down before they used them.

Maybe drivers, cars and tyres are behaving like before we had slicks and wings. Drivers put the car into a "controlled drift" or "controlled oversteer" by applying more or less power to the rear wheels to override neutral steer characteristics. 

However if we are talking about expert drivers, we should assume that the tyres are being operated at somewhere near the optimum slip angle.

The only way to answer that is to get real test data on tire performance with wet tires being tested on wet road.. I am sure Michelin has that..

https://www.nhtsa.go...r2006010559.pdf

That's a nice paper, actually includes real numbers. I have never looked at wet tire performance before, which since I'm doing some split mu modelling at the moment is a strange admission!

Here's is how I picture this.

The slip angle in the wet is not actually increasing vs what it is in the dry.

However, because the road is slippery, you have to induce more of a yaw angle in the car to be able to get the same slip angle (for the rear tires) in the dry.

So, it is not a matter of there being a higher slip angle causing more yaw, but rather that on a slippery surface you have to induce more yaw in order to create the same slip angle (for the rear tires) as on a dry surface.

The same thing holds true of the front tires, though you don't notice this as anything more that that you have to put in a bit more steering lock.

What do you guys think? I'm not an engineer like most other people here.

https://www.nhtsa.go...r2006010559.pdf

The paper was based on tyre test machine results. So it is a bit theoretical and a bit empirical.

Figure 5 caught my eye. The figures are for a single tyre so we have to imagine how weight transfer makes a difference on a four wheeled vehicle. The wet performance curves at 60 or 75 mph are nothing like the 30 mph dry curve.

Load transfers make more of a difference in the wet. A car with big tyres at the back and skinnier ones up front will act more like one without wings or other downforce aids.

The paper was based on tyre test machine results. So it is a bit theoretical and a bit empirical.

 

Figure 5 caught my eye. The figures are for a single tyre so we have to imagine how weight transfer makes a difference on a four wheeled vehicle. The wet performance curves at 60 or 75 mph are nothing like the 30 mph dry curve.

 

Load transfers make more of a difference in the wet. A car with big tyres at the back and skinnier ones up front will act more like one without wings or other downforce aids.

That is quite the dramatic change from dry to wet in Figure 5, but if I'm interpreting it correctly, the graph is plotting linear friction (i.e. traction) vs vertical load.  If you want to look at slip angles, then Figures 14 & 15 is where you need to be.  There we find that in the dry, the tires have a very sharp peak around 5-10 degrees slip angle.  The angle at the peak increases with increasing load.  In the wet, the response is much flatter with increasing slip angle.  And if we compare the Fz=2300 lbs curves, the peak lateral load in the dry is about 11 degrees, but in the wet it is up around 15 degrees.

What I get from this is that in the dry, there is a big penalty for exceeding the optimum slip angle.  However, in the wet, there is no "optimum" slip angle, just a "minimum".  So in the wet, it's better to be several degrees over the minimum because the curve is much flatter there than under the minimum.

The simple model in my head says that as water depth, increases, more of the front of the contact patch is involved with pumping water away from the CP, and so is not available to generate lateral force. Therefore I'd predict that as water depth increases the lateral force for a given vertical force and slip angle will decrease, and the pneumatic trail will increase, because the centre of the lateral force will move back.. That's a simple model, it may ignore subtleties that are important. I have some time in the next couple of weeks to look at that paper in detail and see if the simple model is useful.

The pac model for a tire is based on the measuring conditions at the time of test. For instance my tires are tested on dry 3M 80 grit paper. As it stands the pac formula includes no modelling of wet grip, unless you measure in the wet.

 

@sarbav, the yaw angle is the difference between the vehicle path and the vehicle attitude.

Slip angle will be smaller in wet tyres right? because of not enough friction, or maybe the wet tyre's groove edges will mechanically hold on to tracks surface that are coarse.

Im a noob help me out.

Slip angle will be smaller in wet tyres right? because of not enough friction, or maybe the wet tyre's groove edges will mechanically hold on to tracks surface that are coarse.

Im a noob help me out.

Treaded tyres typically need higher slip angles to generate maximum force. Tread "squirm."

The model in my head thinks in terms of vertical force being shared between water film/wedge and tyre contact. The section in contact with the road has usual value of u and the water film has u = 0.

Yes, that's it. The wet bit of the CP is mostly involved with squeegeeing water away, but it can support vertical force. That's a neat way to think about it.

A 'proper' wet tyre is usually baggier and moves more on the rim. Or so many use the same size tyre on a rim narrower than usual, eg about an inch.

The stiff case stuff often around currently is good for dry conditions but dreadfull in the wet. This includes some so called hi performance road tyres.

A  few years ago at Bathurst it was raining heavily and the cars could not keep up with the pacecar! A purpose built tyre is generally great in dry conditions but often not in the wet. It seems most wets around these days are at best an intermediate.

Yet 20 years ago the radial wets were really very good, whatever the brand or category.

A great way of investigating the simple model would be the brush model as in pacejka's book. Just modify mu for the front part of the contact patch. Since I'm on holiday I don't have the right computer for that, the matlab file is on a backup somewhere.

However what i will do is extract the peak lateral force vs speed vs water depth and see what those trends are.

Well as it happens the first couple of graphs in SAE 2006-01-0559 examine the effect of water depth and speed, and roughly shows a linear relationship between grip and speed.

Here's a plot of peak lat mu vs peak long mu (eyeballed values) showing that the friction circle is roundish whatever the speed and wet or dry.

http://www.mediafire...y2x/wetgrip.png

I don't know why that won't display properly. the link should still work

Edited by Greg Locock, 18 July 2018 - 07:17.