Jersey Tom wrote:machin wrote:and yet "generally" toe-out causes better turn in...
Does it? Says who?
As I said in my post; I don't like general rules, however it is interesting to note that a lot of people agree with it: HampusA for one. Raymondu99's friend for another, Mark Ortiz for a third, the poster of the original question in Racecar Engineering for another.....
For that matter, define "better."
By "better" in this case most people mean "more responsive", i.e. more yaw moment for a given amount of steering angle. On a "twitchy" car "better" might be considered less responsive, but that's by the by.
So I made a
very simple little excel spreadsheet calculating yaw moments against slip angle, and low and behold the answer came back that adding toe-out
does improve the yaw moment for a given steering input... however I will caveat that slightly by saying
it is heavily influenced by the slip angle vs. side thrust curve for the tyre.
Explanation below:-
For a simple model lets assume that the thrust generated by the tyre is proportional to the slip angle, so 1 degree equals say 10 units of force thrust, and 2 degrees equals 20 units of force thrust. Looking at some tyre curves this is approximaltey true at low slip angles, so is a fairly good assumption here. Also I've assumed that the vertical load on the tyres is equal at all times (not that accurate, but lets assume we have a really low c.g)
case 1: If both front tyres point straight ahead with zero steering angle applied (i.e. zero-toe) and that adding 1 degree of steering angle causes both the front tyres to have 1 degree of slip angle then they both generate the same thrust, but due to the effects described in earlier posts, the outside tyre contributes a little less to the yaw moment due to its drag moment being out of the turn, agree?
case 2: Now if we add 1 degree of toe out to both wheels and then add one degree of steering angle (causing 2 degrees of slip angle on the inside tyre and the outside tyre to be straight ahead) then the thrust from the outside tyre will be zero, and the thrust from the inside tyre is twice that of the inside tyre in the first case. Since the lateral force and the drag force on an inside tyre both cause a positive yaw moment into the corner then in this case the total yaw moment is higher than the first case, and the car turns in faster...
Agree? I know, its a bit simple, but it would explain what a lot of people are experiencing.... But I will add another caveat to say that there are of course other factors that will influence what the "ideal" toe angle is for a particular car....