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View Full Version : STRAIGHT AXLE STEERING GEOMETRY


SS100
04-08-2004, 12:20 PM
I am building a complete SS 100 Jaguar. I am using a straingt axle similar to the origional but the steering arms that come off the spindle are the wrong angle. I understand that you pull a string from the center of the king pin to the center of the rear axle and the king pin and the tire rod end are supposed to be centered on the string. Now my question: where on the king pin center do you place the string , as you know the king pin angles and there is a vast difference from top to bottom? Would that point possibly be on the same plain as tire rod end ? In other words the same distance from the floor to tire rod hole and the king pin? Any help would be Appreciated very much .
Thanks
Jack

Hemirambler
04-08-2004, 02:35 PM
Hi Jack, I don't know squat about SS 100 Jaguar's, but I have put together a couple Straight Axle street/strip cars so hope fully this will help.

What you are doing is correcting the "Ackerman" based on wheel base (among other things). The string method is an easy way to get this very close.

What I think most guys do is to simply use the VERY TOP center of the king pin. However theoretically speaking I think the "proper" place is to project straight up from the CENTER of the king pin (all this is looking straight down from above). So as an EXAMPLE let's say you've got King Pins approximately 5 inches long and your Kingpin Inclination (Caster) is in the 5 degree range - as viewed from the TOP the inclination at that amount is .218" - effectively offsetting your String STarting point REARWARD 7/32" (.21875).

Being a relatively small number is why I think most guys don't worry about it (but I would).

Keep in mind - "proper" Akerman is at best a compromise between "IDEAL" steering geometry and "Acheiveable" (read - "close enough") geometry. It's ALL about minimizing your compromises - glad to see you're looking to do that!!!


I hope I understood your question correctly and answered it in an understandable way.



Good Luck!!!!!



Jacin in Ohio


SOMB



I just RE-read your question several times - SO I'd like to add that I believe the "up/down" relationship of the actual steering arm is another one of those area's where most guys simple ignore it. And I can't help but think thats' OK to do as I believe the steering geometry will be effectively the same regardless (it's all about ratio's - and I can't see how that changes) UNLESS you have the left and right at different heights - then of course there's problems.


I will anxiously await to see what the Professional's have to say about this.

SS100
04-08-2004, 02:44 PM
Thanks Hemirambler, but i'm not talking about the caster in the king pin I'm talking about the fact that it tilts inward . If I place the string on the bottom of the king pin and the tire rod end centered on the string thats one thing, but if i place the string on the top of the king pin I would have to bend the arm another 3/4 " to get it to center there . should i place the string on top, on the bottom or the center?

Hemirambler
04-08-2004, 02:48 PM
Hi Jack, I see I misread it BOTH times!!! - Again I would go with the CENTER or better stated the point there the SPINDLE intersects because if you think about it the king pins could be 10 inches apart or 100 - but if the angles were the same it would pivot identically - so the "constant" is the center where the spindle pivots from.


Makes sense to my simpleton's noodle.




Jacin in Ohio


SOMB

SS100
04-08-2004, 08:16 PM
Thanks Hemirambler, I understand now , I think you are right.

Boogiemanz1
04-08-2004, 09:11 PM
I've always checked it at the level of the steering arm, slightly above or below...0.100" won't make a discernable difference in the handling............john