Always Wondered....

[Vipercar93]

Been reading through some of your training material on the front of the site. I always wondered why the car/truck would lift when I turned the wheel while when parking. Now I know! Kingpin inclination. This also helps with wheels returning to centre.

 

Is this right? When you add caster in with kingpin inclination during a turn the forward tire(outside) would not push downward as much, which would help keep negative camber? And in turn the rearward tire(inside) would have more lift than if there was 0 caster, reducing negative camber? This would help maintain the contact patches during a corner?

 

Kingpin axis is a line drawn between the upper/lower ball joints?

 

I know there is a lot more going on, but baby steps here =)

 

Wouldn't then also have an effect on body roll since you would be raising the inside and lowering the outside?

 

Wondered too if it was possible to have a roll-over blow up of pictures. Been reading about KPI and included angles and cannot really see whats being pointed to what. Not that it's a big deal, I can just make it out I think.

[Tony]

Key

White: Tyre

Purple: KPI/ SAI

Red: Wheels contact face - off-set

 

SAI measurement is the pick up point ( pin/s ) perpendicular position to true vertical. The more declination the higher the measurement, taken from that vertical is zero SAI, "Kingpin axis is NOT a line drawn between the upper/lower ball joints".

 

The image is the NSF so lets concentrate on that side during a turn.....We are going to turn left 20 degrees.

 

Theoretical static geometric positions are...

 

Camber -30'

Castor +5 degrees 30'

Toe +5'

SAI 10 degrees 30'

 

Pivotal pick up points

Track rod end

Lower ball joint

Bushing/ strut mount

 

At a 20 degree lock

Camber +10'

Castor 3 degrees 30'

Toe +5' plus the Ackerman

SAI 5 degrees

 

The trigger for all this is the Ackerman angle? This accelerates the inner wheels turning radius by about 1 degree 30' on a 20 degree lock, meaning the inner wheels actual lock is 21 degrees 30' compared to the outer wheel at 20 degrees.

 

The track-rod pick up point has an orbit that allows the point to rotate toward the SAI pick up point, whereas the OSF point moves away. At the same time the camber migrates during the SAI's inclination ( inner wheel )

 

In addition the Castor trail reduces?..... A wise person would think "how" can a measurement like a line drawn through fixed pick up points change? If the pick up points were built at right angles then nothing would change but they are not, each point has an orbit that allows a marriage of dimensions to cohabitate in harmony.

 

The sprung body position ( excluding inertia ) will roll hence the need for transverse resistance "roll bars" but the geometric requirement is not the sole reason why cars have roll bars.

 

SAI ( Steering Axis Inclination ) is very misunderstood globally and steering feel/ return has been rewarded to the castor angle, this is wrong.... Some modern cars run near to zero castor but have a very inclined SAI.

[Vipercar93]

What about double wishbone suspensions? The KPI/SAI is still taken from the lower ball joint and outer tie rod end?

 

SAI is what helps wheels return to 'centre'? Caster gets the nod for this instead since it can alter the SAI?

 

Would adding positive caster increase SAI since the hub would rotate the steering arm downward in relation to the lower ball joint? Or is it more complex than this?

 

Ackerman angle will either decrease or increase the toe on the inside wheel, depending on the angular measurement built into the steering arms coming off the hubs? (looking from the top-down)

[CIH]

 

In the case of McStrut, it's between top mount and bottom balljoint. For double wishbone, top and bottom balljoints. Track rod isn't included in SAI specifically.

In any case, the aim is for the outer wheels of a given turn to move toward negative camber, either through steering input, bodyroll or both. This action also reduces castor and increases SAI which is where the self centre action comes-in. The inner wheels see positive camber which is undesireable, but a necessary consequence to get the negative camber when going the other way, and doesn't really do any harm for OEM use.

 

Racing and drifting is a different story, ofcourse.

[Vipercar93]

Ok, I understand a tiny bit now. Thanks =)

[Tony]

Please try to confine your questions to one or two at a time otherwise the answer will be hard to communicate.

 

"Would adding positive caster increase SAI since the hub would rotate the steering arm downward in relation to the lower ball joint? Or is it more complex than this?"

 

Adding positive castor has many consequences depending on the pick-up points placement. During yaw the rotation around the SAI's elevation is in direct proportion to the camber migration and the scrub migration, meaning in this example ( NSF, left turn at 20 degrees ) that the castor pick-up point is extended proportional to the Ackerman angle. At 20 degrees the extended castor has very little influence on the return ability of the steering, in short it has no energy.

 

As said at 20 degrees the camber has migrated and it's pick-up point the lower ball joint has elevated, meaning it now has a lower measurement value but higher in position. Now more elevated compared to the OSF, the NSF pin holds more weight, it's not comfortable in this position and wants to relax, and since the SAI are linked via the steering rack it's effect is to return the equilibrium.

[Vipercar93]

Sorry Tony, I get distracted easy... Woo shiny! ...

 

I am going to have a read over everything, then sit down and read again =)

[Msport]

Tony,

 

I'm sorry but I don't agree!

 

The SAI never changes on the vehicle - it is set by the positions of either the top & bottom ball joint in a double wishbone set-up or the bottom ball joint and top of the Mcpherson strut.

 

No matter what steering angle the vehicle assumes this angle stays the same.

 

As the wheel is turned it gains and looses camber - it is therefore the included angle that changes (SAI + Camber) and NOT the SAI.

 

Likewise the definition of caster is:

 

The angle in side elevation between the steering axis and the vertical.

 

Therefore no matter what steering angle is applied the castor angle remains the same - unfortunately the Hunter machine does show the castor reading changing but the actual angle remains the same - this has been highlighted to Pro-Align.

 

The 'axis' remains the same - it is the stub axle's relation to this axis that changes. (and how the Hunter measures caster)

 

Regarding the gain/loss of camber (or migration as you incorrectly call it as they do not gain and lose at the same rate in dynamic situations due to body roll). The suspension is designed so the outside (loaded) wheel gains negative camber in the turn whilst the unloaded wheel increases positive camber - this is good because as the chassis rolls you want the unloaded wheel to become more vertical so its contact patch increases.

[Tony]

Hi and thank you for your valuable impute.

 

As i'm sure you are aware i'm not involved with the mechanical construction, more the dynamic effect and it's consequences.

 

I do however agree the SAI has a static placement but disagree this is finite. The SAI has an orbit and it's for this reason ball joints have joints. Since by design the joint has an orbit then it cannot possibly maintain a prescribed position. To my knowledge the actual SAI measurement is taken at the unsprung chassis, so the struts pick-up point is a design issue not geometrically?

 

I found the castor position rotates around the SAI, meaning there is a lowering disparity during the Ackeman transition accelerating camber migration.

[Msport]

Thanks for the reply Tony - I agree the SAI does change dynamically in bump/droop but not as a result of steering wheel input but as a result of weight transfers.