meb as in the old meb that used to post?

Adding caster can correct SAI too right?

- Andrew

 

Si!!! I couldn't remember my old password so I created a new account...back to Nuetral status, boo hoo How are you andyroo?

SAI or king pin is a front view elevation geometry, caster is side view but both project leverage points or fulcrums to the contact patch (SAI) and ahead of the tire (positive caster) Both affect steering feel and steering wheel returnability. It has been written that adding a little caster after adding more neg camber can help introduce a little more feel into the steering but I suspect that's an over-simplification; more positive caster will strengthen on center tracking and to some this means feel. Since adding neg camber can remove some feel - the tire is perched more onto the inside portion of the tread potentially affecting the self aligning torques in straight ahead - we use a little caster to bring some 'weight' into the steering. But it's artificial steering weight in my opinion...doesn't tell us about tire saturation or slip in a turn, just helps with straight ahead...caster can infact over-ride those tingly messages if there is too much straight line tracking. A lot of positive caster will actually reduce the mechanical leverage in the steering slowing turn-in while requiring much more steering effort for the same degree of steering wheel angle.

andyroo, you are cprrect about RC and lowering; a lower RC relative to CofG will increase roll moment. RC and track width are the two most findamental aspects of tuning we have at our disposal...then spring rates etc. I wrote a longish reply but it was somehow under review and still isn't here. The coupling affect Cristo wrote about defines what happens when the steering wheel is turned. The RC acts on the CofG - center of gravity, not theother way around. This is fundamental. So there is a virtual lever arm connecting the the two - coupling. The longer this lever arm, the greater the roll moment and vise versa. A longer lever arm will not load up the tires quickly, say in transient but it will in steady state. One of the reasons I liked a litte rake in my set-up; higher rear RC loads up rear tires a little quicker helping rotation especially in quick transients. ...sort of like turning a big boat as compared to a little boat only different.

Obviously where RC moves in transient is important too...and conversely, we want some weight transfer so having the RC at the center of gravity probably isn't a good idea...

 

Good to have you back. What are you driving now? I've finally got some suspension on my car.

Adding caster is the all the rage on Subarus and EVOs, and it does make a pretty nice difference. Subarus didn't have much to begin with until the 2008+ version and have very weak/lame steering feel anyway. The SAI on those cars is not great, so adding caster (to a point) has a nice positive impact on the car. The Mini obviously is a bit different and doesn't need as much.

Roger that on the RC.

- Andrew

 

andyroo - Audi A4 3.2...not lots of fun at all. Comapny car...can't play with it. It does have some long legs though but the M&S all-seasons don't like to be hurried along through turns

One of the chief popular reasons for adding caster is a slight gain in neg camber in roll or as the steering wheel is turned. But it has to be understood that adding positive caster becomes a very bad thing at some point and at some degree for every car design and individual setup.

The second popular reason for adding more positive caster is to increase steering feel.

I'll comment about the second popular reason first.

Caster adds to straight line stability there by increasing steering effort, most notably just off center. It does this, funny enough, by increasing the castering affect of the wheel. The farther out in front of the tire positive caster projects, the stronger the castering affect, and the harder it becomes to turn the steering wheel. The problem here is that at some point the castering affect ruins or corupts real steering feel because the self centering affect of caster is so strong. Steering feel is fundamentally a factor of SAI angle, scrub radius/Wheel offset and tire type. A poorly matched tire can also ruin goemetry setup for great feel - big block tread patterns can do that.

Steering feel...to you the driver. As your car enters a turn and begins to build cornering forces, steering effort increases. This is so because the tire's contact patch begins to distort. As you can imagine, when we bend anything that is made from rubber it wants to snap back to its original shape. So distortion creates self aligning torques at the contact patch - the tire wants to return to its non-driven shape. The more the tire distorts, the larger the self alighing torques...up to a point. As the self slighning torque increases, so does steering wheel effort. Now, when a tire begins to reach saturation - it is corner near its limit, self aligning torques begin to return the tire to its original shape. Wen this happens steering wheel effort begins to lighten telling you the driver that you are approaching the tire's limit of adhesion.

Back to caster and SAI. On most front drivers the SAI angle projects a point onto the ground outside the tire's center line. If you imagine that the tire literally has to circle around this fulcrum you are following along nicely. The problem here is that the leverage point is typically outside the tire's actual contact patch centroid axis - the meat of the contact patch. So, if you will, we have the shape of the contact patch dictated by tire type, air priessure, geometry and that corner weight. Outside of this region, seemingly in conflict, is the point where SAI intersects the ground. If we were to move the SAI point closer to the contact patch centroid axis, the fidelity of those self aligning torques would increase...sending much clearer messages about tire slip while cornering. Lets also keep in mind that a tire rolls over the rim at the limit, suspension bushings deflect and uni-bodys bend. So what appears to work on paper doesn't always transfer to the road.

In the above scenario, if the caster affect on the tire is too strong, you will never feel those tingly little self aligning torques at the contact patch and those are more important in my opinion.

A note about center point steering - SAI intersects the ground at the center of the tire. This is worse than negative offset - SAI intersects the ground outaside the tire's centerline. This is so because the leverage point as the potential to cancel out self aligning torque...or reduce to such a low level it cannot be felt...the steering wheel feel very light and floaty at times especially on center. You may, through trial and error with your own setups find that this is the best way to go if you can determine the through all of the tire and bushing flexing SAI actaully falls someplace else. Last note about center point steering...because the self aligning torques are reduced, so is fristion and the power required to overcome this distortion. I know a few gravity racers - purely non-motor down hill race cars - who began to employ this because the affect was sobering, very positive. Good for gas mileage aswell I imagine...

In the end Caster also raises one side (inside steered wheel) of the front and and lowers the other, essentially canceling out the Affects of SAI - both sides raise when the wheel is turned and gravity and driving forces help to force the steering wheel back to center. Because the inside steered wheel raises the body of the car at the corner, it also throws weight diagonally onto the outside rear tire. Not a comforting thing if you happen to be in a dangerous opposite -lock condition.

I increased my front track width by 22mm per side or a total of 44mm and the rear 12mm perside or a total of 24mm. Increasing front track increases roll resistance up front adding in a bit of understeer. But because a wider track also decrease roll moment, the tires can perform more work (counter intuitive)...and this allowed the use of somewhat more aggressive rear swaybar settings.

Sorry for the long post!

 

I wrote a very lengthy post again today - it is being reviewed by the moderators?...the previous one frm yesterday still isn't viewable...hope this doesn't happen often.

 

Maybe it's because you're "new?"

 

I am sure, but the one from yesterday isn't viewable yet...the one from today broke down SAI, Caster and steering feel...

Driving an A4 3.2...not lots of fun but comfy. I feel like I need a stogy! A better set of shocks and replacing the M&S all-season tires might make this car a lot more fun. But it's a company car, I cannot play with it.

 

Ah, there it is, post #29. Thank you Moderators!!!

 

Great discussion, guys. I'm learning a boatload of stuff!

 

I made a mistake, caster raises outside steered wheel and lowers insdie steered wheel...I'm old