8smokingbarrels
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| posted on 12/7/03 at 01:19 PM |
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Too much KPI?
Took kb58's advice and stopped 'dickin' around and got started! 
Chopped off the top of my strut (who didnt realise it was full of oil then! oops!). Thing is - when I measured the kpi between the centre of the
lower balljoint and where I was intending putting the upper balljoint- the KPI angle is 27 degrees.
This seems quite alot (Hammill's book mentions about 10-12 max). Can anyone think of any problems I might face with this amount of kpi?
I can move the balljoint taper whereever I like really. But for ease of measurement was going to position it exactly in the centre of the hole left
by the strut tube which i cut off. Hope u can follow what im blethering about! lol
Oh yeah, the distance between the up and lower balljoints is working out at 240mm- this is a bit more than usual as well- anyone see problems with
this?
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kb58
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| posted on 12/7/03 at 06:02 PM |
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Yes, 27 deg is a lot, so I'm guessing this hub is from a FWD car. FWDs need negative scrub radius so there's equalizing force in the left
and right sides during braking and acceleration... but you aren't using it that way.
The Milliken book, Race Car Vehicle Dynamics (pgs 710-711) has a good discussion about it, but never says exactly how much is "bad."
You can change the inclination by moving where the pivot point is, but like you said it is convenient to use the center of the tube. Still, I think
I'd weld a plate on the top, and offset the upper pickup point outward...or move the lower one inboard.
I saw something interesting in a design book about how to solve this, how to move the pivot point to somewhere you can't physically reach. That
is to use two separate upper tubes, that have two separate outboard joints. The "virtual pivot point" is where the two cross. I
didn't do it because I was able to get "close enough" without, but found it very interesting nonetheless.
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Rorty
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| posted on 13/7/03 at 05:06 AM |
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8smokingbarrels, 27 degrees sounds extraordinaraly high, even for a FWD car. Not being familliar with the front hub/upright in question, I'm
wondering if you're measuring the item correctly. Did you measure the angle of the strut tube in the hub to arrive at your figure of 27 degrees,
or did you measure it by drawing an imaginary line through the bottom joint and the postition where you would like the centre of the top joint to be?
If it's the former, then that's not the KPI, just the strut to hub angle. You need to measure the KPI through the top and bottom joints.
It shouldn't be too dificult to move the top joint around to where you want it anyway. Work it out with the wheels and tyres you will be using.
Cheers, Rorty.
"Faster than a speeding Pullet".
PLEASE DON'T U2U ME IF YOU WANT A QUICK RESPONSE. TRY EMAILING ME INSTEAD!
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8smokingbarrels
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| posted on 14/7/03 at 06:32 PM |
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Thx guys
kb58- yes I also havent read anything about too much kpi. Hammill's book talks more about the instability caused by running wheels with too much
offset which are outside the kpi axis line:
"What happens is that when the wheel comes into contact with a bump in the road, it tends to want to turn towards the bump. This is caused by
the leverage factor of the relative position of the wheel and its turning axis.
The closer the axis (kpi) is to the centre of the tyre tread contact patch, the less the adverse effect and conversely, the further the patch centre
is away from the axis, the more pronounced the instability will be"
Judging by this the main effect of a larger kpi is that the axis line is going to end towards the outside edge of the tyre (or outside it
altogether). In which case I will probably have to fit wheels with a bit more offset to counter this. In the mean time, as u suggest I will move the
upper pivot point outwards as much as poss to reduce the kpi.
One other thing KB- what sort of angles are u setting ur steering arms to get full ack angle? Im thinking of using the another set of struts at the
front -tho frankly im having enough bother with them at the back! lol
Rorty- yeah I think I measured the right angle - used a bevel and protector (Hi-tech eh? ) to measure the line between the centre of the top and
bottom ball joints. But Ill borrow a mate's digicam and get a foto up, in case Im making a mistake somewhere
chis
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kb58
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| posted on 15/7/03 at 01:55 AM |
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As much Ackerman as I could fit. Since my setup is "front steer," that means moving the steering arms outward as far as possible, about
1/8" away from the brake rotor. That amount results in the "typical" amount, mostly because I have such a short wheelbase of
80".
Show here, http://members.cox.net/kimini22/car/diary3/inside%20front%20hub.jpg
is how it was before I shifted them outward, note the tack welds.
One consequence of front steer is that the steering arm has a tough time staying out of the way of both the tie-rod tube and the lower A-arm.
[Edited on 15/7/03 by kb58]
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