toe in / out - alignment settings

[preludacris]

hrm, well could also be coz my toe in b4 the alignment was 20mm total. 12mm on the left, and 8 mm on the right.

i buggered it up after takn out my upper control arms, and putting them back in.

felt screwed up b4 the alignment.

[JohnL]

And the wheel coming back to center (flicks back) is controlled by the caster settings NOT the toe settings

Sort of. Its not really the caster angle per se, but more specifically its the 'trail' created by the caster angle, which is an indirect effect of caster angle rather than a direct one.

The amount of trail is by how much the centre of the contact patch 'trails' behind the point at which the steering axis intersects the ground. This is obviously greater the more caster angle is designed into a given front end all else being equal, but its quite possible to design with more or less caster with more or less trail depending on whether the hub / wheel centre is placed in front of / behind / or on the steering axis (as veiwed from the side of the car), which can move the contact patch longitudinally closer to or further from the steering axis / ground intersection point regardless of the caster angle that may be used.

Assuming zero scrub radius (for convenience of visualisation), when a front wheel is traveling straight the centre of the contact patch is travelling directly behind the steering axis / ground intersection point, and this is where it 'wants' to be (i.e. its the position of greatest stability where all forces are in balance). To move the contact patch away from this 'preffered' position requires a 'destabilising' force, provided by the driver turning the steering wheel.

When this happens the contact patch 'attempts' to move laterally away from being directly behind the steering axis / ground intersection point (both contact patches attempting to move toward the outside of steered direction), but because of lateral grip the contact patch(es) resists this force and will 'push' against the chassis, literally pushing the front of the car sideways toward the corner entry. This action creates a faster change of direction (i.e. improves steering response), and the affect is stronger the greater the trail is.

Its also why greater trail creates heavier steering, stronger 'spring back' of the steering wheel to the straight ahead and improved on centre feel, i.e. because the greater the trail the more force is required to divert the contact patches from their 'desired' place directly behind the steering axis, even if this is only by a small degree of steering input.

Also, these affects are 'travel direction dependant', meaning that the steered wheels always 'want' to point in the direction in which the car is actually travelling at any moment rather than the direction in which its steered or the angle of the chassis to the actual direction in which its travelling. This creates a rather nice characteristic whereby when you have severe oversteer the front wheels will 'want' to automatically counter-steer (opposite lock) into the slide, and the greater the trail the more the car will tend to do this.

These effects of trail are seperate really to the direct affects of caster, which is mostly desirable for its affect on steered camber gains at both front wheels, i.e. increased steered neg camber at the outside front and increased pos camber at the inside front, which assists in countering adverse camber gains caused by body roll. And, when significant scrub radius exists then caster also contributes to the steered front wheels attempting to change height (relative to the hub), which they can't do due to the road surface being in the way, so a weight 'jacking' effect is created that can be helpful or unhelpful depending on the car specifics.

[JohnL]

Whew.... all that to say changing the caster setting will change the speed and force of come back to center. All nice theory but it is still controlled by the caster setting on the vehicle or changing parts.

Guilty, I'm long winded, so shoot me!

Its nice to know whats actually going on, to say something like "is still controlled by the caster setting on the vehicle or changing parts" isn't the whole truth and in reality somewhat misleading.

Better re-read my post, I've added a bit more, suffer!

[m0nty ITR]

Iced, haven't seen you on msn lately but it'd be great if we go down to WP sometime soon so I can get some setting advice from you. I'm heading down on the 29th for SOTS, but next time there's a CC meet down there give us a heads up.

[iced]

im on appear as offline dude
just message me anyway and i will reply

[JHMDA9]

Guilty, I'm long winded, so shoot me!

Its nice to know whats actually going on, to say something like "is still controlled by the caster setting on the vehicle or changing parts" isn't the whole truth and in reality somewhat misleading.

Better re-read my post, I've added a bit more, suffer!

I always enjoy reading your posts even if it takes me a while!!!

[JohnL]

"to say something like "is still controlled by the caster setting on the vehicle or changing parts" isn't the whole truth and in reality somewhat misleading."

What's misleading and untrue about it? Yours is nice theory but has nothing to do with what I said. On a real car there is still only two ways to change the speed of come back to center. One is changing the caster settings using the parts that came with the car, or changing the control arms (which in reality IS changing the caster settings). Your explaining WHY the changes in caster settings work to control the speed, I'm telling them HOW it's DONE on the car.

I'm not having a go at you. It is nice theory, and it does have everything to do with what you said! What you say isn't untrue in itself, but misleading by ommision, i.e. it doesn't actually explain the dynamic at work. If you say something like; 'increasing caster improves on centre feel, back to centre etc', then no-one can say this is not correct, but all you're doing is describing affect, not explaining it. Its misleading because its not actually the caster change itself that is causing the change in behaviour, rather its the change in trail that occurs as a by-product of changing the caster.

We now understand the dynamic better, and the better we understand the first principles that work for and against us the better we can make decisions with regards to chassis set up. If we were to be say setting up a racing car, then the better our general understanding the more likely we might be to be able to sort any problems we may encounter, or improve on an already good set up. Besides, the detail of the dynamic is interesting on its own account.

[aaronng]

Wow, miss this thread for a few days and there is A LOT of catch up reading to do. I understand what you mean JohnL, but for most people, they want to know what to adjust to get the desired effect and not the details as to why do you get the desired effect when adjusting. Most of the cars here are street cars anyway.

[aaronng]

Aaron(ng),
At the risk of stating the obvious:
Straight line stability is always a desirable feature (even for racing cars and karts) so long as it doesn't come at the cost of acceptable responsiveness, the two not neccesarily being always mutually exclusive (though acheiving both may not be easy).

Yes, and the cheapest way is to alter the front toe (at the sacrifice of tyre life and straight line stability).

Car manufacturers typically use multiple strategies specifically and deliberately intended to achieve copious quantities of understeer (with various geometries, spring rates , damper rates, ARB rates and suspension bush compliances), which we as enthusiast drivers must identify and 'rectify'! Most cars understeer so much (even at low speeds) that very few drivers ever have the pleasure of driving a truly neutral handling car. Even many cars with overt performance pretensions will typically have some understeer designed in, just less so than those with a less focussed intent (market niche).

Not only manufacturer-specified understeer but also the OEM parts even on performance models don't give you the adjustability that you require to return the car to a neutrally-handling one. At the moment, I don't have the ability to adjust the front camber. My car is still a street-driven car, with trackdays being rare, maybe once or twice a year. So there still isn't an urgent need to swap my camber settings front to rear at the moment. That said, when I get my weekend car, I will take the effort (and money) to do so.

[JohnL]

I always enjoy reading your posts even if it takes me a while!!!

I've got a fan, makes me feel all gooey on the inside!

[JohnL]

Yes, and the cheapest way is to alter the front toe (at the sacrifice of tyre life and straight line stability).

And can be affected by tyre selection / inflation, trail, caster angle (if this creates a jacking effect with significant scrub radius also present), KPI (creates a jacking effect on both sides of the chassis that lifts the front of the car with steering input, gravity acting on the unsprung mass will pull it back down until it finds its lowest point which is with the wheels pointed straight ahead), and probably a few other things.

A lot of ingredients in the cake! Not all that easy to play with, so mucking about with the toe is the path of least resistance I suppose!

Not only manufacturer-specified understeer but also the OEM parts even on performance models don't give you the adjustability that you require to return the car to a neutrally-handling one. At the moment, I don't have the ability to adjust the front camber. My car is still a street-driven car, with trackdays being rare, maybe once or twice a year. So there still isn't an urgent need to swap my camber settings front to rear at the moment. That said, when I get my weekend car, I will take the effort (and money) to do so.

Its a pain I know, you'd start to think they didn't want us to mess about with all those settings!

If your rear end is similar to mine (quiet up the back...), then thinking about it, it ought to be possible to shim the the upper arm at the chassis mounting point? (where the bracket is attached to the chassis with two bolts). This will bring the arm outward resulting in a decrease in neg camber. I haven't even thought about trying to actually do this (no need, my rear camber is less than the front), but if its possible it should be pretty easy and straightforward (but then Murphy's Law being what it is...!).