Self centering

[robo]

If you have a dig about on here someone did an explanation of all this. The mushrooms dont add caster they give you trail. Its this trail that helps with the self centering, a bodge is a better word for it. There is not enough caster in the front suspension set up and no way to alter it.Thats why I am making my upper bones with adjusters on the inner joints. When they are on the car I will take some measurements of them in the corrected adjusted position then perhaps someone can do an ammended top wishbone drawing so they are built to include the caster without the need for adjusters. Caster is an important must have for straight line stability not just self centering. I am no expert on trig but i bet its only a few mm added to the front top wishbone tubes to sort it.

================... KPI ......... caster

Lotus Elise........................ 12° ......... 3.8° caster
Corvette C5, C6................. 8.8° ...... 6.5° caster
Pinto/MustangII/Wilwood... 11°
Ferrari 512BB............... 9 or 13°
ATS AFX Tall..................... 8°
Triumph Spitfire................ 7° ..........4.7° caster
Lotus Seven..................... 9° .......... 5° caster
Mazda Miata................... 11.3° ........ 5° caster
Ferrari F355................... 13.16°
McLaren F1...................... 9° ........... 6° caster ...... 16.25mm scrub

Bob

[TSM Locost]

I would suggest that you lower the tyre pressures to 1.1 bar as a starting point,

[AshG]

set the toe to 0deg and try it. if that fails then 0.5deg toe out will get it self centering but it will feel twitchy at high speed.

[alga]

Originally Posted by TSM Locost

I would suggest that you lower the tyre pressures to 1.1 bar as a starting point,

I thought so. But now I have 1.5 bar in the rear tyres, zero camber, and the marks left on the asphalt when spinning wheels are just narrow stripes along the edges.

[alga]

Originally Posted by robo

If you have a dig about on here someone did an explanation of all this. The mushrooms dont add caster they give you trail.

One is as good as the other:



Fixing the Sierra's trail inceases positive caster distance, too. It's the distance between the kingpin axis' projection intersection with ground and the centre of the contact patch, and it is precisely the lever that does self-centering.

[robo]

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. 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. That in turn makes the steering feel vague, thats the reason all decent suspesion systems have caster built in. The mushrooms help but its only a patch up fix for an inherent problem, they do infact make the set up act as scrub which most would regard as undesirable.

Bob

[alga]

Originally Posted by robo

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.

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.

[robo]

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

[DRCorsa]

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.

[robo]

.
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