THE CONSULTANT
FIRST PRINCIPLES
Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. Here Mark answers your chassis set-up and handling queries. If you have a question to put to him Email: markortizauto@
windstream.net Tel: +1 704-933-8876 Write: Mark Ortiz 155 Wankel Drive , Kannapolis NC 28083-8200, USA
It’s all about maximising the yaw moment, you see THIS MONTH:
Q1 Is popular opinion that toe out makes for faster turn in actually true?
A1 Yes, with toe out, in a corner both tyres produce lateral and drag force, creating a yaw moment and greater yaw acceleration
Toe out for turn in Q
Popular wisdom suggests that for a faster turn in, toe out is the way to go. However, I’ve never read a
satisfactory explanation why, and it seems counter-intuitive to me. Thinking about what happens at the tyres, if you have toe out on turn in the outside tyre has to pass ‘over centre’ before it begins building grip in the direction of the turn. Thinking that the outside tyre is carrying most of the load, and therefore creating the majority of the grip, I would think that toe in would provide a faster turn in as you would be building grip faster as the more loaded tyre would already have a slight slip angle before you even turn the wheel.
that this does not have to do with the lateral (y axis, per SAE convention) forces from the front tyres, but rather the longitudinal (x axis) forces, which can also produce yaw moments. When the car is running
A
straight, and the front tyres have either toe in or toe out, the tyres are both running at a slight slip angle, and accordingly generating both some lateral forces and some drag forces.
Cars do generally exhibit quicker initial turn in with static toe out. My analysis is
I have a theory that maybe the cause of this
prevalent opinion could be the change in relative wheel heights as the scrub radius / caster kingpin / trail cause the inside front wheel to move down in relation to the chassis and the outside to move up. It would seem that with a fast enough turn in
that the inside was temporarily the heavier loaded tyre, its steered angle would create more grip than the outside until the point that the load transfers to the outside tyre. If this is the case, it would seem that in softly
sprung vehicles with lots of steering inclination, toe out would be the way to go, at least on tight courses requiring fast turn in. Your thoughts?
The drag forces are roughly equal, and additive. The lateral forces are roughly equal, and opposite in direction, so they approximately cancel. When the steering wheel is
turned just a tiny bit, one front wheel will be running straight, and the other will be turned into the corner, generating a bit of drag, and some lateral force into the corner. If the car has toe in, it will
be the inside front wheel that’s running straight, and the outside one that has some slip angle. In this condition, the lateral
force creates a yaw moment into the turn, but the drag force creates a yaw moment out of the turn. The two yaw moment components are subtractive. If the car has toe out, it will
be the outside front wheel that’s running straight, and the inside one that’s making lateral force and drag force. Now the lateral and drag force both create yaw moments into the corner, and are additive. Consequently, the net yaw moment is greater, and the car experiences a greater yaw acceleration. That is it turns in quicker.
September 2011 •
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