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Learning how to build Lotus Seven replicas...together!
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PostPosted: Sun Aug 25, 2013 3:31 am 
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I've searched high and low, but can't seem to find out how everyone is calculating their scrub radius. :BH: I think I have all the needed values:

Miata uprights (11* KPI), 15x7 wheels, with a +30mm offset, 24.69" tire height (205/60/15)

Does .75" sound right?

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PostPosted: Sun Aug 25, 2013 7:34 am 
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Tire height is reduced below the axle by deformation. How much depends on the tire pressure and corner weight.


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PostPosted: Sun Aug 25, 2013 7:50 am 
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From a front view of the car, draw a line between the ball joints and make it intersect the ground. The distance between the centerline of the tire and where this line meets the ground is your scrub radius.

I've never found an equation - its always been a ruler or a drawing.

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PostPosted: Sun Aug 25, 2013 8:04 am 
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It could be calculated but the missing dimension in your list is the distance between where the KPI crosses the wheel mounting surface relative to the to the stub axle center. It is much easier to measure from a picture.

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PostPosted: Sun Aug 25, 2013 10:20 am 
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google is your friend ;)
lots of pics and articles pop up if you google "scrub radius"....

Image

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PostPosted: Sun Aug 25, 2013 4:16 pm 
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a.moore wrote:
From a front view of the car, draw a line between the ball joints and make it intersect the ground. The distance between the centerline of the tire and where this line meets the ground is your scrub radius.

I've never found an equation - its always been a ruler or a drawing.


I was planning on doing that, as I am going to make a Staniforth string calculator... Just wondering if anyone knew an actual equation that would give a value to compare against the results from the straight line/ruler method. Thanks

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PostPosted: Sun Aug 25, 2013 6:04 pm 
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Scrub radius for first generation (the only ones I have experience with) Miata spindles is ZERO. That is with the stock 14" rim and stock size tire. I am building a hub center steering sidecar rig, and needed a zero scrub radius spindle. The Miata was the only one I found, although I believe there are others.


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PostPosted: Sun Aug 25, 2013 9:37 pm 
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Sketched up the wheel/upright on a piece of poster board and ended up with a scrub radius of 5/8", so that's good... Next up is making a 1/2 scale string computer out of some thin wood, then playing around with roll centers and pick up points...

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PostPosted: Tue Sep 03, 2013 12:28 pm 
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Hi Lotus19 -

I just plugged your values into VSusp on a whim but I got a scrub radius of 2.4". I adjusted the default spindle dimensions until it had 11 deg KPI. And the wheel offset and tire size are what you gave earlier. If you load this project: scrub radius test and mouse over the "scrub radius" boxes, it should highlight those dimensions.

I'm definitely not saying that your calculations are wrong. I'm just trying to figure out why there's a pretty good disparity here. Am I missing something obvious?

Also, I might be able to come up with a formula and simple plug-in-numbers-and-go tool for this. But I'll definitely want to make sure that it will be returning valid numbers.
-Rob


edit: Just realized I was missing something obvious (must be losing it). I should have added the missing dimension, exactly what rx7locost mentioned. I'll update the project using the dimensions from this post in Spindle/Upright pics and measurements and repost the link.

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PostPosted: Tue Sep 03, 2013 3:39 pm 
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Yep, I was mistaken. I just added in the correct Miata knuckle measurements (from an NA Miata, I believe that's what you're using?). I now get a scrub radius of .65 inches. Seems very close to what you're getting. In real life, since the tire is compressed at the bottom due to weight, this will increase the scrub radius a bit.

NA Miata front spindle test

Vsusp has an output called "spindle length" which is similar to what rx7locost was referring to. But spindle length is the distance between the KPI line and the tire center. I'm getting a value of 2.97 inches. Add 30mm for your wheel offset and that'll be the value rx7locost was referring to.

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