Nisseven
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| posted on 15/12/05 at 11:06 AM |
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Castor Alternate method
Hi Guys,
I've attached a drawing of two ways to achieve castor. The first is the usual, or book method, and the second the way someone, who I respect as a
good engineer and car builder, has sugested to do it. The drawings are from the side looking straight at the stub axle and for clarity the top
wishbone is drawn parallel to the bottom one.
As you can see the front is built level with zero castor then the whole thing tilted back to give the desired castor.
Anyone got any thoughts good or bad on this method.
Bruce Kelly
 
Rescued attachment Suspensiondwg.jpg
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Gav
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| posted on 15/12/05 at 11:27 AM |
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Wouldnt doing it like this also induce more front end dive when on the brakes?.
Im halfway through reading the suspension book by Des Hammil and it looks to me that it would be the opposite of anti dive wishbone pickup points in
the book.
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Bob C
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| posted on 15/12/05 at 11:50 AM |
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Yep 'extra dive' front suspension geometry, (just like I see in the "tiger avon" book. . . . . . !)
Bob
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WIMMERA
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| posted on 15/12/05 at 12:38 PM |
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Yes thats a perfectly legitimate method of building in castor, many older production cars used a similar set up (MG B's from memory had the
front cross member rotated a few degrees) and as mentioned the Avon uses that method.
Wimmera
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britishtrident
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| posted on 15/12/05 at 12:55 PM |
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MGB or any other definitely did NOT use this method, On the MG the caster was achived by canting the (real) king pin. I think the confusion arises
because on some 1950s designed cars small adjustments to caster were on some models by inserting packers between the subframe mounts and the body,
This method wasn't used to set the caster angle merely to adjust it within specs to allow for changes caused by variation in the rideheight at
the rear.
As a method of acheving caster It is not a good method because it introduces dive into the suspension ie softens the suspension in dive and causes
backward movement of the outer track rod end on bump which will cause big changes in toe.
If anything you want the opposite a little ant-dive in the suspension ie both or just the lower wishbone angle a couple of degrees down at the
front.
In any case the real self centering problem is due to lack of king pin inclination more than lack of caster.
[Edited on 15/12/05 by britishtrident]
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Surrey Dave
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| posted on 15/12/05 at 04:39 PM |
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How do you get more KPI?
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Dusty
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| posted on 15/12/05 at 07:24 PM |
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There isn't much you can do about the KPI.
It's a function of the design of the donor uprights and any changes are reflected degree for degree in the camber angle.
Usual fix for self centering is getting the castor right as in your first drawing.
[Edited on 15/12/05 by Dusty]
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britishtrident
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| posted on 15/12/05 at 10:32 PM |
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Ways to get more KPi
Run a lot of negtive camber
or run a Sierra front end with offset mushrooms
or use a Triumph Hearld based front end a Caterham do
[Edited on 15/12/05 by britishtrident]
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Mark Allanson
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| posted on 15/12/05 at 10:57 PM |
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quote:
Originally posted by britishtrident
MGB or any other definitely did NOT use this method, On the MG the caster was achived by canting the (real) king pin. I think the confusion arises
because on some 1950s designed cars small adjustments to caster were on some models by inserting packers between the subframe mounts and the body,
This method wasn't used to set the caster angle merely to adjust it within specs to allow for changes caused by variation in the rideheight at
the rear.
As a method of acheving caster It is not a good method because it introduces dive into the suspension ie softens the suspension in dive and causes
backward movement of the outer track rod end on bump which will cause big changes in toe.
If anything you want the opposite a little ant-dive in the suspension ie both or just the lower wishbone angle a couple of degrees down at the
front.
In any case the real self centering problem is due to lack of king pin inclination more than lack of caster.
[Edited on 15/12/05 by britishtrident]
KPI will only take any effect when you are approaching full lock, castor is affecting the geometry even in the straight ahead position.
If you can keep you head, whilst all others around you are losing theirs, you are not fully aware of the situation
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MikeRJ
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| posted on 15/12/05 at 11:23 PM |
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I'm surprised no-ones spotted it, but the second pic is exactly the same as the first except for the angled wishbones. What I mean is you still
have to move the center of one wishbone in relation to the other, angling the wishbones at the same time buys you nothing at all! (apart from more
dive etc.)
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Mark Allanson
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| posted on 15/12/05 at 11:33 PM |
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It's the dive that will kill you, rapid braking as you approach a roundabout, used up all the movement in the suspension, hit a manhole cover,
ask St Peter to pass throught the gate, eat philadelphia cheese forever more.
If you can keep you head, whilst all others around you are losing theirs, you are not fully aware of the situation
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MikeRJ
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| posted on 16/12/05 at 12:03 AM |
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Right but what I mean is it looks like whoever came up with the second idea didn't realise there are balljoints on the end of the wishbones! If
the suspension system used a kingpin and trunions, I could see why you might come up with this idea (though still a bad idea), but with ball joints
you can simply move the wishbones wherever you want, within the limits of the balljoints articulation.
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MikeP
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| posted on 16/12/05 at 02:00 PM |
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The MGB suspension most definitely DID (does?) use the second method of providing caster - no choice 'cause it uses trunions. If you've
got the shop manual check out the frame picture "Vertical Alignment Check".
Have a read of Smith's "Tune to Win", page 34-35. He doesn't recommend anti dive at all. If you read between the lines he
says it's a really bad idea for the street - if you hit a bump under braking the suspension can lock solid and you will lose control.
So front end compliance instead of anti-dive has benefits when approaching that bumpy round about, both in safety and ride. Most of us use enough
spring to keep off the bump stops under braking.
There's advantages to method 2, but it's a bit trickier to work out how to mount it on the locost front end.
[Edited on 16/12/05 by MikeP]
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britishtrident
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| posted on 16/12/05 at 05:07 PM |
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Ever examined an MG or Triumph king pin really closely off car ? the bottom trunnion is at an angle other than 90 degrees to the king pin axis to
allow for the caster angle.
Fairly small percentages of anti-dive are used on most production cars I can't think of any that uses the opposite.
[Edited on 16/12/05 by britishtrident]
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MikeP
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| posted on 16/12/05 at 11:28 PM |
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quote:
Originally posted by britishtrident
Ever examined an MG or Triumph king pin really closely off car ? the bottom trunnion is at an angle other than 90 degrees to the king pin axis to
allow for the caster angle.
Only the 'B, but more than a few times... they're perpendicular, no angle. The front cross member holds the a-arms and tilts up toward
the front of the car to provide all of the caster, just like in picture 2.
Most modern cars around here are FWD mcpherson strut grocery getters - hard to imagine anything much further than what Smith was writing about  .
I've no experience at all with the design requirements for them.
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WIMMERA
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| posted on 17/12/05 at 12:48 AM |
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The Moss Motors catalogue part number for the king pin is 264-970 2 off required, the trunnion number is 264-450 2 off required, no differentiation
for left and right hand.
Wimmera
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Nisseven
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| posted on 18/12/05 at 10:47 AM |
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Thank you guys for your replies. I suspected that this configuration would promote dive but then as someone said that may not be a problem. The idea
was intended to make the thing easier to make. Basically you make the front as a seperate subframe with wisbones level and no caster, tilt it back,
weld on and Bobs your Uncle. I think I will stick to convention though, at least as far as caster goes. See seperate post.
Oh and I think that the person sugesting it did realise there were ball joints and I can assure you is not as thick as one respondant sugested.
Thanks again.
Bruce.
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