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PostPosted: November 21, 2010, 9:47 pm 
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Any of you FSAE people got a vehicle dynamics text nearby?

I am playing with a string computer for front-end geometry. I'm getting good results at 1 inch of bump and 2 degrees of roll (i.e., zero to 2 degrees of negative camber on the outside wheel for a range of geometries) but for the simple case of 2 degrees of roll alone I am seeing one to two degrees of positive camber. I understand that this positive camber will be reduced (maybe even eliminated?) by steering lock into the turn, and since I know caster and king pin angle I can work out caster and Kpi-induced camber for a range of steering angles - if I derive the formula.

To save me that little challenge, is the formula for camber and Kpi induced camber readily available to anyone on the forum?

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Isuzu Pickup/SR20DE, +401 COLD frame
Build Log: viewtopic.php?f=35&t=11601


Last edited by Warren Nethercote on November 24, 2010, 5:17 pm, edited 2 times in total.

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PostPosted: November 22, 2010, 6:43 pm 
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I haven't been sitting on my hands since my first post, and through reading I see that only castor helps me by reducing camber, whereas kingpin inclination always results in increased camber. It appears that generally castor is 'more effective' than KPi so that steering lock generally results in reduced camber (i.e., more towards negative) on the outside wheel. But still no formula. Maybe I have to think about doing it myself. :wink:

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Build Log: viewtopic.php?f=35&t=11601


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PostPosted: November 22, 2010, 6:58 pm 
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Use "Wishbone" - input bump, roll and steering for camber changes. Dos program but works well, IMHO.

JMR

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PostPosted: November 22, 2010, 7:53 pm 
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I was afraid someone might say that ....

I've been trying to avoid figuring out how to make a DOS program work. Plugging a formula into my spreadsheet seems a lot easier, although that might suggest that I'm not a total Luddite. :)

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Warren
Isuzu Pickup/SR20DE, +401 COLD frame
Build Log: viewtopic.php?f=35&t=11601


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PostPosted: November 22, 2010, 10:04 pm 
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It's me again ....

According to "Build a Faster car - helping drivers engineer" at

http://www.fromsteve.net/tech/Dynamic-C ... nclination

camber induced by Kingpin inclination is:

KPi - (KPi*COS[steering angle])

and camber induced by caster is:

caster angle * SIN[steering angle]

I will work up camber curves for my 'best' geometry so far and apply these formulae to see what happens. It still would be nice to have confirmation of these formulae. Unless the results are disastrous I will post them here, because I'd like opinions on what I have.

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Isuzu Pickup/SR20DE, +401 COLD frame
Build Log: viewtopic.php?f=35&t=11601


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PostPosted: November 24, 2010, 10:35 am 
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Following are the results of my string computer exercise, including the effects of steering lock, using the formulae for camber induced by caster and kingpin inclination according to my preceeding post. The calculations are based on 7 degrees of caster and 10.9 degrees of kingpin inclination. These values seem to be in the ballpark for Miata and Corvette according to ‘Steve’ in my earlier post where I reported his formulae.

So, the question is: for those of you who have done this before, do my resulting cambers for the outer wheel look reasonable? The inner wheel seems to be a dead loss, but that seems consistent with everything I’ve read. Being a Naval Architect by profession, the only value in this table for which I have an intuitive appreciation is roll angle! :lol:

(My apologies for the unformatted Table, but I wanted to put it in the message body. The first column of each table is steering lock: 0, 5, 10, 15 and 20 degrees. The columns are cambers at various roll angles: 0,1,2, ..., 6 for zero bump, and 0,2,4,6 for 1 inch bump.)

Suspension in Roll, No Bump

Roll, deg 0 1 2 3 4 5 6

Lock, deg. Outer Wheel Camber, deg
0 0 0.5 1 1.4 1.75 2.1 2.5
5 -0.57 -0.07 0.43 0.83 1.18 1.53 1.93
10 -1.05 -0.55 -0.05 0.35 0.70 1.05 1.45
15 -1.44 -0.94 -0.44 -0.04 0.31 0.66 1.06
20 -1.74 -1.24 -0.74 -0.34 0.01 0.36 0.76

Lock, deg. Inner Wheel Camber, deg
0 0.00 0.70 1.50 2.30 3.00 3.80 4.50
5 0.65 1.35 2.15 2.95 3.65 4.45 5.15
10 1.38 2.08 2.88 3.68 4.38 5.18 5.88
15 2.18 2.88 3.68 4.48 5.18 5.98 6.68
20 3.05 3.75 4.55 5.35 6.05 6.85 7.55

Suspension in Roll, 1 in. Bump
Roll, deg 0 2 4 6

Lock, deg. Outer Wheel Camber, deg.
0 -1.00 -0.50 0.00 0.50
5 -1.57 -1.07 -0.57 -0.07
10 -2.05 -1.55 -1.05 -0.55
15 -2.44 -1.94 -1.44 -0.94
20 -2.74 -2.24 -1.74 -1.24

Lock, deg. Inner Wheel Camber, deg.
0 0.80 2.10 3.50 4.80
5 1.45 2.75 4.15 5.45
10 2.18 3.48 4.88 6.18
15 2.98 4.28 5.68 6.98
20 3.85 5.15 6.55 7.85

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Isuzu Pickup/SR20DE, +401 COLD frame
Build Log: viewtopic.php?f=35&t=11601


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PostPosted: November 24, 2010, 5:09 pm 
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Here is a plot of the horrible table in my previous post. What do you think?


You do not have the required permissions to view the files attached to this post.

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Build Log: viewtopic.php?f=35&t=11601


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PostPosted: March 11, 2012, 2:17 pm 
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Warren thanks for posting these formulas. I've been trying to firm up choices of spindles on my project. The Mustang II spindles are available in 7, 10 and 11 degrees of KPI. I think the Miata is around 10 or 11. I have also seen oval track spindles with multiple holes for the top ball joint that can be run as 5.5 or 10 degrees of KPI.

What I'm thinking currently is that mostly what's important is pretty small steering angles, perhaps 5 degrees for a round number. This would be for road racing and general sporty driving cases. If you race in parking lots and have small diameter turns in mind, then more steering lock becomes important. When road racing and in general use the turn diameters are larger. So my thought is that if you have 10 degrees of steering angle on a large radius turn that would mean the back tires are unable to generate much force because they would be running at 10 degrees less slip angle.

So just using those formula with a 5 degree steering angle -

Camber from 7 degree KPI .027, 11 degree KPI .04
Camber from 5 degrees Castor .43, 7 degrees Caster .61

So for us it seems pretty safe to ignore KPI in thoughs about camber anyway. It would be important for scrub radius issues and self centering though.

I'm wondering if the self centering is also related to the cos of the steering angle, meaning that the slope of it's value doesn't change very much over small steering angles.

This all seems to point to using the spindles with higher values of KPI. The spindles with lower KPI seem to be targeted to folks who are racing on dirt with *very* large reverse steering angles...

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