Byrepower
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| posted on 14/6/06 at 07:04 AM |
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Castor angles !!
seem to be having trouble getting castor,even with the alloy boss turned fully to the rear the car still won't self centre,I'm able to
turn into a roundabout and let go of the wheel and the car drives round it itself ! anyone else have these problems,and how did you overcome them
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nitram38
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| posted on 14/6/06 at 07:09 AM |
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I had to remake my top bones to get 7 degrees of castor to make mine work (not a 7).
Do a search as this has been covered lots of times.
Draw your top and bottom ball joint centres on paper first and then draw a line at 7 degrees from the bottom ball joint. Measure the distance from
the original top ball joint centre where it intersects with the 7 degree line. This will give you the distance that the top bones need to be
modified.
The top ball joint needs to be further back from the front of the car than the bottom one.
Try to use this method rather than the toe out or springs on rack method.
It will be safer and feel better to drive.
[Edited on 14/6/2006 by nitram38]
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simonk
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| posted on 14/6/06 at 10:18 AM |
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Please ignore if you've already checked, but are the front top wishbones on the right way up ? They're almost symmetrical to the eye but
will make getting the castor right difficult if they're on upside down. 
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James
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| posted on 14/6/06 at 10:30 AM |
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And however much you turn the ali boss it won't change the castor!
Cheers,
James
------------------------------------------------------------------------------------------------------------
"The fight is won or lost far away from witnesses, behind the lines, in the gym and out there on the road, long before I dance under those lights."
- Muhammad Ali
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procomp
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| posted on 14/6/06 at 01:13 PM |
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Hi just out of interest who the hell invented the offset boss for so called castor adjustment.
cheers matt
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simonk
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| posted on 14/6/06 at 02:39 PM |
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Unless I'm seriously mistaken, I believe that the eccentric boss in a Sierra upright gives you the ability to alter pretty much every aspect of
front-end geometry. With the upper ball joint you can obviously adjust camber. By combining this with the eccentric mushroom you can also adjust
effective king-pin inclination and therefore mess with the relationship between the turning centre of the hub and the centre of the tyre contact
patch. This will affect bump and brake steer, which may be bad.........
You can likewise adjust castor by using the mushroom to move the upper pivot forward and backward relative to the body of the upright. I believe that
this will affect the castor, moving the pivot back will decrease castor, forward to increase.
What you cannot do (as far as I can see) is change the castor using this method without also changing the effective king-pin inclination, which as I
explained may be bad.
As I've read a bit more about front end geometry I've come to the conclusion that there should probably be a design orientation for the
mushroom and it shouldn't be used as a tool to adjust castor as it cannot be done without affecting other aspects of the geometry.
Would any of the manufacturers care to comment ? - I'm an amateur builder with schoolboy geometry and have yet to set up the front end of my
Viento, so a properly informed opinion would be very welcome.
Simon
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simonk
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| posted on 14/6/06 at 02:49 PM |
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Aaagh !!
Just spotted my own mistake  James is quite right, you cannot change castor with the mushroom. You can however affect the trail, as moving the
mushroom moves the stub axle forward and back relative to the 'king-pin centreline. You do however, in conjunction with the top ball joint
change the king-pin inclination, so I stand by my comments about there probably being a 'correct' orientation that gives a design top
wishbone length. As before, happy to be enlightened
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Byrepower
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| posted on 14/6/06 at 04:55 PM |
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cheers everyone,will take all this to the workshop tomorrow and see if I can get some answers...will post the results later !!
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Byrepower
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| posted on 17/6/06 at 12:05 AM |
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Took the top wishbone out today and removed the bush and the pipe,completely removed the lip of the back bush in the lathe and shortened the pipe,also
removed the washers and put them to the front,when reassembled the top balljoint is now 7/8 of an inch to the rear of the centre line which I have
been told is just about right for 7-8 degrees of negative castor.....test drive at the start of next week,fingers crossed !!
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DIY Si
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| posted on 18/6/06 at 07:37 PM |
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Indeed, you want to aim for roughly 21mm of offset. Or 7/8" in english.
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