Home  Simultaneous longitudinal and lateral tire forces (or slip). 

Introduction
Each simulator using wheels (whether car or flying or motorbike) will have to deal with combined slip. This happens when there is both a slip angle (wheel turned with respect to its forward velocity) and a slip ratio.
Internally, Racer contains multiple tire models:
As for combined slip, only Magic Formula 5.2 has builtin properties to handle mixed forces (=combined slip). Since these coefficients are hard to measure and are highly suspect (even when measured), I would advise against using them really.
The combined slip methods (that are more or less independent of the chosen pure longitudinal/lateral tire models) that are present in Racer are described here. The are quite a few of them, and as much as tire modeling is a big part of car modeling, combined slip is a big part of tire modeling (for generic use, which in simulator terms means: large and enduring slip).
Magnus Gäfvert's thesis from 2003  Construction of Novel SemiEmpirical Tire Models for Combined Braking and Cornering
This thesis is very interesting; it presents a mix between Pacejka and the brush model. It divides tire forces up into adhesive (sticky/springy) and sliding forces, based on results obtained from the brush model. It is implemented in Racer (v0.9.0 RC6+) as friction_circle_method 8 in racer.ini.
You can find the paper in the 2003 edition here. I've found another mix of papers (3 papers combined) where this thesis is also more or less included, but it is unclear if either of them is more prevalent, since they're both from 2003.
It calculates a 'psi' value for X and Y directions; this is the amount of slip. The psi value for both X and Y is the slip amount, normalized.
I found a few things that I needed to tweak (perhaps correct?) from the equations in the paper:
(last updated March 11, 2013 )