Okay, maths is not my strong suit... but could I get your opinions on where I am going wrong.

This is for a raceleda sierra upright, I wish to prove my measurements are correct, so to do this I thought I could try to find the KPI based on angles and lengths (bear with me on this)

So if I take the upright as vertical i.e. no castor and measure from the middle of the top and bottom balljoint I get 240.6mm
With the upright still vertical I work out that
the top balljoint is 41.5mm closer to the center of the car than the bottom.
With this in mind I can now rotate the upright 7 degrees to create some castor.

Using sin(83) * hypotenuse and sin(7) * hypotenuse I can work out the distance further toward the rear of the car the top balljoint has moved (29.32mm) and the height of the balljoints from each other (they haven't got further apart, this is the distance apart they look when looking from the front) = 238.81

Now to get my KPI I should do tan-1(41.5/238.81) = 9.86 degrees

I should be getting a kpi of 13 to 13.5
Any-one here got any ideas of what I have done wrong. Is it

a) my maths is as ropey as a ropey thing
b) my measurements are just as bad as my maths
c) raceleda uprights have a different KPI to Sierra's (not saying this is a problem)

If some-one could take a gander at this problem I would be most grateful.
(We must have a maths genius here surely ;-) )

If you are wondering why I am doing this, I'm doing a suspension program for my pc, and would like the figures accurate before I get it going... doesn't look like it.

If any-one got to this point Hello

[Edited on 16/3/08 by clockwork]

suspension program...?

you can download autocad from the web for free, you can then model it properly

I'm a software engineer by trade, this is as much about learning what is going on as the results are.
I've also never used autocad so it would take me longer in that than the maths I have done so far.
Cheers for the thought though.

im not entirely sure what the problem is, you seem to have maths covered better than most would?!

Is it just that you've calculated KPI at 9.7-9.8 degrees and it should be 13 ish to match a sierra? If thats the case, i guess its a raceleda thing.

i just did invtan of 41/240, not much point in including camber or castor as KPI is between the hub face and pivot angle?

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Is it just that you've calculated KPI at 9.7-9.8 degrees and it should be 13 ish to match a sierra?

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KPI is between the hub face and pivot angle?

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Originally posted by clockwork

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KPI is between the hub face and pivot angle?

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Are you saying KPI includes the castor?
Camber is assumed at 0

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As far as I can see as you rotate the upright the KPI does change, the further it gets from upright the greater the angle.

If I use your figures i get 17 degrees

my figures?

what about trying this download (page 2)

http://locostbuilders.co.uk/viewthread.php?tid=81376&page=1

JoelP by your figures I meant "invtan of 41/240"
Gives me 17.
Aboardman, haven't seen that one before, but it doesn't really help in this instance I am afraid, as is it the KPI in degrees I am trying to prove, which isn't shown in that spreadsheet.

inv tan is what you did, it gets me 9.69

Sorry JoelP, don't know what I did there. You are correct in your result on inv tan (41/240).
That said, I still think the caster does make a (small) difference. I think I'll just have to put it down to the difference in the raceleda set up. I can't be that far wrong.
Cheers.