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05-28-2006 #1Registered User
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minimizing scrub, high KPI or lots of BS?
Hi guys,
I'm getting ready to tear back into my Datsun 240-Z project. I'll soon be buying a suspension design program to help me figure things out, and then I'll begin the front snout of the spaceframe.
Been reading over material on scrub radius and have a question. I searched the archives and didn't find anything about this.
I may be missing something obvious or my suspension theory may not be 100%, so let me make sure I know what I'm talking about first off... Scrub is the distance at the ground between the wheel centerline and the intersect point of the imaginary line running between the two ball joints yes?
So, most people solve the problem by using a steeper KPI (Herb Adams' book recommends 7-9*) to make the imaginary ball joint line more angular... thereby making the intersect closer to the wheel centerline.
That is fine, but what I don't see people doing is using a spindle with 0* KPI and just moving the steering knuckle further into the wheel. With today's larger diameter (and wider) wheels, it seems like this would accomplish the same thing. If you have a short BJ to mounting surface length, (say 4'') you could use a 10'' rim and have the ball joint centerline directly in line with the wheel centerline.
Maybe this just isn't practical, but is there any other reason not to do it this way (aside from space considerations inside the wheel)?
Thanks for any comments. At present, I'm planning to probably go with C5 front knuckles and will build the control arms around that.
John
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05-28-2006 #2Registered User- Join Date
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Running zero KPI would be fine if it were possible to space the wheel such that you could obtain near zero SR. It is very unlikely on a typical street car to obtain near zero SR with no KPI. This would involve running wheels with as much as 7" back space on an 8" wheel.
05-28-2006 #3wouldn't zero scrub radius provide very little steering feedback or feel?
05-29-2006 #4Registered User- Join Date
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Usually when you run anything less than 1/2" of SR you eliminate the P/S. At true zero the steering would have no feel and you could one finger the wheel without assist. I would not recommend less then 1/2" of SR.
05-29-2006 #5Registered User- Join Date
- Aug 2004
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Thanks guys! I'm sure I'll have more questions as I go.
Speaking of which... I've downloaded the demo of Suspension Analyzer from Performance Trends, and have started modeling some potential designs.
So, here's another question if anyone is reading... how much roll center migration is acceptable? My initial modeling is just trying to get my RCH in the ballpark. Again, Herb Adams' book recommends static height between 2'' above ground and 1'' below. If I start within this range, how far out of it should I safely go between roughly 3.5'' of bump and jounce.
I'm playing with a level lower control arm that is fairly long and a slightly shorter upper arm that is angled downward at the center ever so slightly (giving me an intersection point between the two at between 150'' and 200'' away). RCH still migrates quite a bit. Any suggestions for a beginner?John
05-29-2006 #6Registered User- Join Date
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It will take months of trial and error to get RC migration nailed down and still fit all the suspension under the car. My suspension took about a year to figure out on paper and RC migration both lateral and vertical is less than 1/2" at max roll/dive. Good luck, shot for less than an inch.
05-29-2006 #7