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  #11  
Old 12th September 2012, 06:18 PM
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alga alga is offline
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Originally Posted by robo View Post
Not quite the same thing because the trail wont become effictive untill you move where as the caster will be effiective at all times. Hence the lack of self centering when stationary.
I have no problem with lack of self-centering when stationary. I would say it's a feature of most cars. Turned wheels stay turned as long as the car is not moving.

Now, the kingpin inclination angle, which is cast into the upright geometry, should provide some self-centering effect, as turning the wheels results in slightly raising the car, but, apparently, with the Roadster's weight, this effect is not enough.

Quote:
That castor like on a shopping trolley could be kicked arround 360 degrees and it will only straighten if you move forward. Also the inside wheel on turn in on the proper set up is forced downwards whereas the trail set up remains static.
You're missing the bit that Sierra's upright has negative trail, and offset mushrooms set it to zero, making the wheel axle intersect the kingping axis.
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  #12  
Old 12th September 2012, 06:43 PM
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I have not gone for the sierra set up so I cant comment on it but dig about with the search and its was made clear that the offset mushrooms were used to give the suspension trail. That might or might not be correct so moving on to the basics and there is not enough caster in the system to give it self centering end of. i have gone for the mazda rx8 uprights and will be making fully adjustable top wishbones to address the problem. Caster is what is needed. If the mushrooms set the trail to zero that makes the whole thing worse!! Zero or close to zero caster and zero trail "no thanks"

Bob
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Last edited by robo : 12th September 2012 at 06:58 PM.
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  #13  
Old 12th September 2012, 07:52 PM
DRCorsa DRCorsa is offline
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You don't merely need caster angle. What you need is the benefit that caster angle gives you. This "benefit" is actually called "mechanical trail" and this along with the "pneumatic trail" gives the "total trail".
Caster is one way to obtain mechanical trail. Another way is moving the kinpin forward of the wheel center.

Mechanical trail is not an "angel". You dont need "much" of that, because if pneumatic trail is only a small percentage of the total trail, then when you are at the limit of traction (where pneumatic trail tends to disappear) you will not get a good feel of it.

Last edited by DRCorsa : 12th September 2012 at 07:55 PM.
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  #14  
Old 12th September 2012, 08:05 PM
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.
Caster is one way to obtain mechanical trail. Another way is moving the kinpin forward of the wheel center.
[/quote]

Which we wont be doing as we are using ready made uprights so caster it will have to be. Unless of course we fabricate our own uprights! I have checked the mazda uprights the best I can and the king pin line runs through the center of the stub axle so there is no trail built into those particular uprights. All valid stuff this and needs looking at.

Bob
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  #15  
Old 12th September 2012, 08:18 PM
DRCorsa DRCorsa is offline
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Offset mushrooms is another way to obtain some mechanical trail and you don't need any special uprights to do this.
What i want to tell is that caster is not what you actually need. What you need is trail and there are a number of ways to get trail including caster angle.
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  #16  
Old 12th September 2012, 08:30 PM
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You're missing the bit that Sierra's upright has negative trail, and offset mushrooms set it to zero, making the wheel axle intersect the kingping axis.
Quote.

Alga has already pointed out that the mushrooms set the trail to zero<which i was told gave a positive trail> so where do the sierra upright boys go now. More caster.?????? We have what the donor of our choice gave us and the only practical way to rectify the problem is to frig the top wishbone or pivot points. Any other method would involve purpose made uprights which no one is going to do.

Bob
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Last edited by robo : 12th September 2012 at 08:33 PM.
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  #17  
Old 12th September 2012, 11:44 PM
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Robo, here's a piccie of the Sierra upright. The negative trail, while exaggerated, is fairly obvious.



The book design has the caster angle of approximately 5.5, which, while not a lot, is far from zero.
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Last edited by alga : 12th September 2012 at 11:47 PM.
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  #18  
Old 12th September 2012, 11:53 PM
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Hm, moving the upright back by 2.5 mm will increase the caster angle to about 6.15. That's a pretty small difference, I doubt it's worth it.

Actually, making another set of top wishbones with larger offset might be the neatest way to increase the caster. The question is, by how much?
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  #19  
Old 13th September 2012, 06:12 AM
DRCorsa DRCorsa is offline
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Alga, have you actually measured your car's caster angles in an alignment shop?
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  #20  
Old 13th September 2012, 08:53 AM
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Albert help might be at hand here. The extract below was from a drift forum. Where I cant comment on the 2wd sierra stuff I can check out the 4wd stuff because I have two sets of 4wd hubs here. If the below is true that might be the best option. I will check them out for trail today and let you know. Its the negative trail that is wiping out the effect of the caster and causing the problems. Its a shame it was not noted when the cars were being developed because they could have mounted the rack behind the front axle and just reversed the hubs giving positive trail, but thats not where we are.

For drifting Sierras lack caster and lock, so I want to use 4WD front hubs as they give more caster angle than the 2WD hubs. I had them left over from when I broke the XR4x4 I had. I've already removed the CV joint innards.........

[image]

.......and to increase lock I want to get the hubs redrilled to change the location of the track rod end. Like this.......

[image]

.......although I want a tapered hole so I can use the OE track rod ends, instead of the bolt and rose jointed affair I'm the photo.

Using 4WD hubs means I must use the pinch bolt type track control arms (which I've also got) and the larger diameter 4WD dampers too (which I don't have).

Bob
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