Positive Camber and easier steering.

[theKbStockpiler]

How exactly does having positive camber make for easier steering? The only info I can dig up on it is it puts the car weight on the inside bearing which does not explain why steering force is less.

Thanks for your expertise!

[Marc]

The dominant factor is the scrub radius. That's the difference between the center of the tire's contact patch and the steering axis (imaginary vertical line through the kingpin or balljoint(s) about which the spindle rotates when you steer). The larger the scrub radius, the higher the steering effort - but it's mostly only felt at very low speeds, like when parallel-parking. Normally the contact patch is somewhat outboard of the steering axis; by tilting the top of the tire outboard (positive camber) the contact patch gets moved closer to the steering axis which results in lower steering effort.

Another consideration is caster. More caster makes the front of the car raise up as the wheels are steered - effectively, the more caster you have the more the car weight causes the steering to be pushed towards straight-ahead. That's a good thing for better stability at highway speeds, but it does increase steering effort.


https://en.wikipedia.org/wiki/Scrub_radius
https://en.wikipedia.org/wiki/Caster_angle

[theKbStockpiler]

Higher scrub radius is more positive?





Camber changes scrub radius and scrub radius allows the tire to more forward and reverse in the tirewell so the tire does not have to pivot in place ,correct? I'm sure that there is a point where the added travel the tire has to move;fore and aft in the wheelwell creates more work for the driver than having the the tire just grind and pivot without moving fore and aft.


What about the track of the car? If the wheelbase is narrower, it can turn in a smaller radius and therefore take less effort to make a turn.A smaller amount of positive scrub makes the car narrower in a sense.

[jhoefer]

Changing scrub radius would be equivalent to moving the wheel side-to-side, not front to back.

Wheelbase is the length between the front and rear wheels. "Track" is the distance between the left and right wheels.

Take the left front wheel. Draw an imaginary line through the ball joints to the point where it intersects the ground. Now mark the center point of the tire's contact patch on the ground. If the left wheel's contact patch point is left of the steering axis point, that's positive scrub. If the contact patch point is right of the steering axis point, that's negative scrub.

If the wheel has positive scrub and you reduce it (like by increasing positive camber), at low speeds you increase steering effort and decrease steering effort at speed. If the wheel has positive scrub and you increase it (like by increasing negative camber), at low speeds you decrease steering effort and increase steering effort at speed. The reason for this is, with low scrub and at low speeds, you are twisting the tire around the contact patch, fighting the tire's grip on the ground. With high scrub at low speeds, rather than twisting the tire, it instead wants to roll in a circle around the steering axis, making the steering effort less. At speed, the scrub radius acts as a lever. Any force that impacts the tire (this includes braking) acts on that lever to try and turn the wheel around the steering axis. The larger the scrub radius, the larger the lever, and the more force gets applied into the steering. This increases steering force and the "road feel" or kickback experienced through the steering wheel. The scrub radius behavior at speed is far more important than at low speed. This might make you think 0 scrub is best, but it's usually not. A wheel with 0 scrub, as the contact patch shifts about while driving can flip flop between positive and negative scrub which can cause unpredictability in the steering.

Having low scrub and increasing tire width will increase low speed steering effort because you have more tire you are trying to twist against the ground. Installing a wheel with the wrong offset that greatly increases scrub can increase steering effort and kickback to potentially dangerous levels.

[theKbStockpiler]

Thanks for the great explanation!It's the best one of scrub on the net.

I read a lot of articles/posts on positive camber. It seems that it gets confused with negative scrub and the two get intermixed. You can see the positive camber from one simple look but you can't see scrub this way. Tractors have a lot of positive camber but I think this is to implement negative scrub. Pivoting the tire in place must be easier than rolling it through the mud with positive scrub.




So I think I understand it know.

Thanks again.

[Ol'fogasaurus]

http://www.racecartuner.com/03/307.html

[jhoefer]

I read a lot of articles/posts on positive camber. It seems that it gets confused with negative scrub and the two get intermixed. You can see the positive camber from one simple look but you can't see scrub this way. Tractors have a lot of positive camber but I think this is to implement negative scrub. Pivoting the tire in place must be easier than rolling it through the mud with positive scrub.

Well, a majority of cars run a negative scrub radius. Primarily because negative scrub is safer. Lets say your right tire hits a puddle or drops off the road or has a blowout. Because of the lever action of the scrub radius, the drag from the puddle or blowout will want to turn the right tire to the right if you have positive scrub. This can make the situation worse by making the car turn even more than the drag alone would and making it harder to correct at the steering wheel. If the car has negative scrub, the drag will want to turn the right tire to the left, away from the source of drag and it sort of self-corrects.

Combine that with the fact a lot of those same cars also run mcpherson struts which don't have much camber gain, so in a turn the tire goes into positive camber. That's be my guess why the two might get confused.

On a tractor they are trying to get 0 scrub. That keeps the load on the steering less, keeps the weight in line with the steering pivot for better strength, and reduces the kickback driving over rough terrain. The tire's rounded so no need to keep it flat and speeds are low so no concern about weight transfer and camber changes.