Jesus-Ninja
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| posted on 4/1/08 at 08:49 AM |
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Techy McPherson strut question.
Before anyone flames me for asking about McPherson on here, I have to admit that it's not strictly in respect to locosts, but more that I just
spent the last hour or so driving to work getting my head round this to understand how my current suspension set up behaves, so that what I implement
on my build is close to it, in terms of changes to angle of camber in respect to deflection of the suspension. Perhaps the Techies / Gurus can correct
me if I have this wrong. OK, deep breath.....
McPherson: For the purpose of understanding changes to camber, relative to the vehicle, we can consider this to be made of 3 elements: The lower arm,
the vehicle itself, and the strut, forming a triangle. All elements remain of fixed length, except for the strut, which can change length. the three
elements are all free to pivot on each other. Any compression of the strut will cause the arm to rotate upwards around the point at which it pivots on
the vehicle.
If the arm is horizontal at a state of rest, then as the strut compresses, and the arm rotates, the hub moves inboard. The movement inboard is
therefore coupled with a change in the angle of the strut, effectively decreasing the amount of negative camber, relative to the vehicle. This is also
true if the arm is angled up at state of rest - ie the pivot on the hub is higher than the arm / body pivot.
If the arm is angled down at a state of rest - ie the pivot on the hub is lower than the arm / body pivot, then as the strut compresses and the arm
rotates, the hub moves outboard, increasing the angle of the strut to the body, and so increasing the amount of negative camber. Until of course the
arm passes beyond the horizontal (relative to the vehicle), at which point it will behave as above.
The further the arm deviates from horizontal at a state of rest, either up or down, the more pronounced the effects described above - ie the steeper
the angle of the arm from horizontal, the greater the change in angle for a given vertical movement AND the greater the movement of the hub (inward or
outward) for a given angle of rotation of the arm.
Additionally, the longer the strut, the less compression, proportional to it's length, so the less change in angle from it's position at
rest. And vice versa.
Have I got this right?
[Edited on 4/1/08 by Jesus-Ninja]
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JAG
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| posted on 4/1/08 at 09:02 AM |
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Yes, that's the basic kinematics of the MacPherson strut.
As I'm sure you have guessed though - that is JUST the basics. The individual design of each of the three elements can have an influence on the
behaviour you describe.
Justin
Who is this super hero? Sarge? ...No.
Rosemary, the telephone operator? ...No.
Penry, the mild-mannered janitor? ...Could be!
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matt_claydon
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| posted on 4/1/08 at 09:08 AM |
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Sounds pretty much spot on. Not a hard job to draw out the elements and create an equation for camber in terms of strut compression.
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Howlor
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| posted on 4/1/08 at 09:22 AM |
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So in theory if you have much longer bottom arms the change in camber will be much less?
Steve
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bimbleuk
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| posted on 4/1/08 at 09:26 AM |
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A simple straight forward suspension setup used by many manufacturers. Then Toyota for example go and make it all complicated!
Superstrut suspension
I had this on a late Celica FWD car and it did make a fine handling car but I knew how much of a pain it was to service so I sold it before anything
broke 
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procomp
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| posted on 4/1/08 at 09:38 AM |
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Hi this sort of replacment mounting for the top of struts is often used for tweeking camber settings on strut setups.
Cheers Matt
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Dusty
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| posted on 4/1/08 at 10:06 AM |
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The camber angle is usually dependant on the Mac strut angle and isolated from the lower arm angle by some form of ball joint. They usually have a
very long theoretical swing axle.
[Edited on 4/1/08 by Dusty]
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matt_claydon
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| posted on 4/1/08 at 01:56 PM |
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quote:
Originally posted by Dusty
The camber angle is usually dependant on the Mac strut angle and isolated from the lower arm angle by some form of ball joint. They usually have a
very long theoretical swing axle.
[Edited on 4/1/08 by Dusty]
Surely that's not true? With a swing axle you have zero camber with the axle horizontal, negative camber on bump and positve camber on droop.
With Mac strut you have maximum negative camber with the lower arm horizontal and the camber heads towards zero/positive with movement in either
direction from horizontal.
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Jesus-Ninja
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| posted on 4/1/08 at 02:12 PM |
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quote:
Originally posted by matt_claydon
quote:
Originally posted by Dusty
The camber angle is usually dependant on the Mac strut angle and isolated from the lower arm angle by some form of ball joint. They usually have a
very long theoretical swing axle.
[Edited on 4/1/08 by Dusty]
Surely that's not true? With a swing axle you have zero camber with the axle horizontal, negative camber on bump and positve camber on droop.
With Mac strut you have maximum negative camber with the lower arm horizontal and the camber heads towards zero/positive with movement in either
direction from horizontal.
I would agree with you there. As in my original post, if the arm is horizontal, then the hub is at it's outer
most point, as the arm swings in either direct, the hub moves inward. This motion inwrd causes the angle of the strut to reduce (on the assumption
that it is angled into the body at the top), and so neg camber is reduced.
Hence my point about not having a horizontal arm at rest, but angled down at the outside, such that camber is increased, relative to the car, when the
shck is compresseed
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SkinnyG
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| posted on 4/1/08 at 04:29 PM |
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Now add to the mix the fact that struts need fairly high steering axis inclination to get the strut out of the wheel (since everything cannot be
packaged ~inside~ the wheel).
This also causes a loss of negative camber when the outside wheel is turned. This can be countered to a certain extent by increasing caster, but
increasing caster has its own drawbacks.
Struts are cheap - they are not good. They can often be made to work, by optimizing the (wretched) camber curve as you describe, and by running high
enough springs and bars to stop the struts from moving - thus stopping them from doing anything bad.
G
The Lethal Locost
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