I am finding different opinions on the net regarding how KPI should be measured.
Should KPI angle be measured between king pin axis and vertical wheel axis Or between king pin axis and true vertical
Anyone knows this ?
thanks
Hi,
KPI is taken from the face of the upright (where the stub axle mounts) and a line drawn through the top and bottom ball joint centres (centres of the
balls themselves)!
This measurement is fixed and does not change with suspension movements as it would if you used true vertical.
Hope that helps, if you need more help let me know and I'll scan and post a drawing.
Cheers Si.
[Edited on 11/8/10 by Strontium Dog]
[Edited on 11/8/10 by Strontium Dog]
Exact opposite of a above.
The KPI when setting up is measured in the normal laden condition from the vertical (ie the normal to the road surface).
This implies setting more negative camber increases KPI.
KPI will alter with suspension movement as will caster angle.
My understanding is that KPI is measured between the axis of rotation of the kingpin and the true vertical.
Camber is the angle between the plane of rotation of the wheel and the true vertical.
These 2 angles added together will not change with suspension travel.
I notice however that on american websites KPI seems to be referred to the axis perpendicular to the axle axis as Strontium dog says.
From a design point of view it would seem reasonable that the angles were referred to a fixed datum (the vertical ) irrespective of suspension
movement.
Interesting though !
Always measured from true vertical as far as I'm aware.
I am with what Strontium Dog suggested but as i see it there are also mixed opinions about it.
Looking at designs of uprights from different manufactures, the angle is always present meaning that KPI is within the design of the upright.
If KPI was taken from true verical, an upright can be designed with 0 KPI and KPI (and camber) will be added via shorter/longer wishbones.
If you are designing or calculating suspension geometry or forces then you need fixed references planes to refer to, which is why these angles are
referenced as they are.
If you generate all the KPI in the wishbones then you compromise the wishbone geometry which has also to compensate for body roll.
quote:
Originally posted by 9000rpm
I am with what Strontium Dog suggested but as i see it there are also mixed opinions about it.
. It's a definition, and the SAE says measured from vertical.
KPI is a fixed design point, relative to the wheel rotation axis. NOT a moveable thing measured to the vertical at whatever attitude the car is
sitting at the time.
Best described as the angle between a line drawn at right angles to the wheel rotation axis, and the line drawn between top and bottom ball joint
centres.
Look at an old (or not so, eg Sherpa van or big truck) type front beam axle. The king pin is fixed in relation to the beam, thus the kpi is a fixed
and non moveable piece of geometry.
Cheers,
Nev.
Did I open a can of worms with this subject ??
Although i am mostly after the answer for a macpherson setup (for my tintop) but same principle should apply for a double wishbone setup.
here is an scetch not to scale (with exaggerated camber) of both opinions
Which is the KPI angle, 13.57deg or 21.01Deg
[Edited on 11/8/10 by 9000rpm]
quote:
Originally posted by Neville Jones
KPI is a fixed design point, relative to the wheel rotation axis. NOT a moveable thing measured to the vertical at whatever attitude the car is sitting at the time.
Best described as the angle between a line drawn at right angles to the wheel rotation axis, and the line drawn between top and bottom ball joint centres.
). Do you have a problem with measuring camber and caster from the vertical? And with the fact that most 'fixed design points' of a
vehicle suspension - camber, caster, toe, etc - can alter with suspension travel and/or chassis roll/pitch? Why would KPI be defined any differently?
[Kingpin Inclination - The angle in front elevation between the steering axis and the vertical.] This assumes, and is measured at, 0 camber and is a
fixed geometrical figure in the design.
To put it another way, the angle between the axle and the effective kingpin line doesn't change with wheel movement, or camber changes made
through adjustment, thus kpi is a fixed angle between the effective kingpin line and a line drawn at right angles to the wheel axle centreline.
Otherwise, KPI could never be stated, as it would be a dynamic figure with suspension travel, as you would believe. Not so.
I've got no problem with the SAE terminology, after all, it's what I live by. You've just got to know and understand the conditions
applied to derive the terminology. I do. Do you? KPI is measured with the wheel rotational axis horizontal, and is a fixed geometrical constraint, so
is measured as I put.
Torsen diffs? I've worked with them enough to know how a genuine Gleason works. And a Quaiffe is not a Gleason, by the simple fact that Quaiffe
won't pay the Patent License fee to make a proper Gleason Type Torsen diff, and that's why the quaiffe has very different characteristics to
a proper Torsen. All of the Torsens I've worked with WILL drive only one wheel, if the other is off the ground, or missing altogether.
Cheers,
Nev.
[Edited on 12/8/10 by Neville Jones]
Yep. Neville is right, the reference to measured from the vertical assumes the axle is horizontal for design purposes. It is your interpretation of
the SAE which is incorrect.
KPI is a constant and as to other changeable geometries they are specified at normal loaded hight, as an example etc. and are intended to be taken at
these given points. Otherwise how would you ever set them?
And I've got a Torsen diff in my car so do I win a fridge magnet or what?!
[Edited on 13/8/10 by Strontium Dog]
You could have asked this 2 months ago I could have looked it up in college.
I previously had and from what I understand of it that picture doesn't have one. Iirc it's something to do with a certain type of suspension
mostly found on trucks these days. It's very similar to caster angle.
Again from what I understand it's like the difference between measuring a petrol engine in HP and an electric motor in KW, they both move the
car...
quote:
Originally posted by Ninehigh
I previously had and from what I understand of it that picture doesn't have one. Iirc it's something to do with a certain type of suspension mostly found on trucks these days. It's very similar to caster angle.
Right found my notes:
King Pin Inclination (KPI) is the inward lean of the king-pins (up, towards the centre of the vehicle)
KPI causes some of the self centring action of the steering.
It also modifies the amount of camber change caused by the caster angle when the steering is turned, lessening negative camber gain on the outside
front wheel and increasing positive camber gain on the inside front wheel.
It would be unusual to deliberately alter KPI, but it can be adjusted using the 'camber' adjusters.
And from the look of the picture on the next page it's measured against either true vertical or the camber angle...
I was hoping it would say a bit more..