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EFV-8 Club Forum / General Ford Discussion / Scrub Radius 1932 - 1948

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Posted By Discussion Topic: Scrub Radius 1932 - 1948     len47merc 03-13-2017 @ 3:55 PM reply profile send p.m. Senior Posts: 1165 Joined: Oct 2013 Tronix - I do appreciate stubbornness so please keep it up. At the risk of being ridiculed personally, I called my old-schooler mentioned earlier and asked again about measuring the scrub radius. The response was he no longer had the equipment nor the need to continue the process. I then called 3 other shops that I know can bridge-the-gap between modern alignment equipment/techniques and vintage cars. Again, a no go with any of them on scrub radius. I have colleagues who race Porsches at Virginia International Raceway and their equipment designed to accommodate and align custom racing wheel offsets does not produce the number. I suspect the silence you are hearing is that many of us have no idea how to ACCURATELY determine scrub radius on any of the cars we have access to. I could measure (with time and distance) a stock '35, '36, '40 and '47 - all relatively local colleagues' or personal cars - but in the absence of CAD level software, using a scribe, (elastic) string, plumb bob, tape measure, protractor, compass and other common measurement tools on 'stock' axles, wheels, tires (hmmm...what pressure/psi per tire/car???) and quality/new front end components one has to question how accurate a figure could possibly be achieved. And who knows if the front axles were ever bent? I had mine aligned after rebuild and had the driver's side of the axle tweaked to get camber perfect. I researched this on the internet for a while today and could find no credible, reputable, accurate methods for determining this, only theoretical using CAD. If you have a process that is fail safe and you are confident that will produce an accurate figure that you will accept and trust please let us know. I am sure there are many of us eager to learn something here that will be glad to help you. Perhaps I am out in left field and if so someone please send me the gospel so I can learn more about this - I've just not found it thus far. Thanks in advance - Steve Tronix 03-13-2017 @ 12:59 AM reply profile send p.m. Member Posts: 5 Joined: Mar 2017 With a drunken man's stubbornness, I continue with my questions about scrub radius. In the years 1932 -1948 was the front axle, camber, caster and KPI virtually equal. The exception is the first three years. Ford changed the size of rims and tires as follows: 1930 - 31 19 x 3 -- 4.75 x 19 Model A 1932 18 x 3 ¼ -- 5.25 x 18 1933 - 34 17 x 3 ¼ -- 5.50 x 17 1935 16 x 4 -- 6.00 x 16 Last spoke wheels 1936 - 39 16 x 4 -- 6.00 x 16 Wide five 1940 - 48 16 x 4 -- 6.00 x 16 These changes should also reasonably have affected the scrub radius during these years? Or was scrub radius unchanged during these years? If you own a EFV8 from those years, or have access to one, it would be very useful if you could measure the scrub radius. Please! Carl len47merc 03-12-2017 @ 2:33 PM reply profile send p.m. Senior Posts: 1165 Joined: Oct 2013 Thanks Tronix & Stoker - I am familiar with what scrub radius is and its impact on handling/drive-ability of a vehicle. What I am struggling with is why this parameter is important beyond the impact of changing to larger or smaller wheels/tires and/or varying offset - on a fixed axle - ? As you recalled Stroker one could bend the steering arm on the spindle (finding someone who knows how to do that could be a challenge) but I am curious why anyone would need to go to such extremes if staying with stock wheels, wheel offset and o/a tire height. Perhaps Tronix is desiring such a change and intends on optimizing the scrub radius for his new configuration - ? After rebuilding the front end on the '47 and taking it to a seasoned old-schooler who still had all the equipment, tools and processes to align it, I asked him about how 'far out' my garage toe-in set-up was and how bad, if at all, the axle was bent/camber out. After a very supportive session of feedback from him I asked him what the scrub radius measured out at (given I'd changed the tire size to 710s) to which he smiled and gave me the 'way too much book sense and not enough common sense' look so I let it go to save face and, until now, had never heard anyone bring this up on our EFV8s. After all the discussions I've read here on changing to different size wheels and tires from stock it just struck me as odd this is the first I've ever seen anyone bring up this issue, particularly given the speeds and the assumed low o/a number of miles most of our EFV8s are driven. Tronix gets the prize! Steve This message was edited by len47merc on 3-12-17 @ 3:05 PM Stroker 03-12-2017 @ 1:12 PM reply profile send p.m. Senior Posts: 1460 Joined: Oct 2009 Folks.. I don't "read Swedish", but if you want a real TREAT, check-out Tronix's website. Lots of FASCINATING photo's from Flathead-powered Japanese submarines, to Ford Aircraft engines... http://www.earlyfordv8.se/ Tronix 03-12-2017 @ 1:08 PM reply profile send p.m. Member Posts: 5 Joined: Mar 2017 Thank you Steve for your answer! Scrub radius affects the car's handling at all speeds and also its behavior during braking and punctures. My assumption was that Ford gradually changed scrub radius to handle higher speeds and also steering of heavier cars at low speed, such as parking. With the KPI (8 degrees kingpin inclination), which is the case in 1932 -1948 is the only possibility to change the scrub radius to change the tire diameter and rim backspace (offset). I prepare for a possible discussion with some traffic and car control authorities. Carl Stroker 03-12-2017 @ 12:32 PM reply profile send p.m. Senior Posts: 1460 Joined: Oct 2009 One can "change" the scrub radius by changing the offset of the rim center-line OR the diameter of the outside diameter of the tire. One could conceivably also make a NEW spindle and "tweak" the axle to bring the kingpin inclination angle and the resulting point of tire/pavement contact patch closer or farther from the axis of the kingpin. Obviously, Ford engineered different scrub radius specifications depending upon the overall front tire diameter which Changed radically in the "early" years with the adoption of smaller rim diameters. Since a positive scrub radius has a tendency to seek a straight ahead position, less positive caster is needed to maintain directional stability. About the only negative "effect" experienced by having an excessive positive scrub radius is "bump-steer". Many years ago, the 1940 edition of "Ford V8 Cars And Trucks" by Victor W. Page contained a comprehensive discussion of this subject. Included was also a discussion on how to change the Ackerman-steering angle by bending the steering arm on the spindle to accommodate different wheelbases on commercial Fords. len47merc 03-12-2017 @ 8:23 AM reply profile send p.m. Senior Posts: 1165 Joined: Oct 2013 First I recall ever seeing this addressed on our Forum. When selecting performance rims for his racing car my son was all over this as it relates to rim offset, this given the adjust-ability of this parameter with his car's front end. Never read or heard it questioned regarding our EFV8s, so as usual something new to learn here. Dovetailing your question Tronix - how is this adjustable, if at all, with our solid front axles (excepting for running non-stock/original rims with varying offsets &/or wheel/tire heights)? And what are your thoughts on the need for this specification beyond non-stock wheel/tire considerations? This morning I hunted through my modest resource library and on-line and could find no scrub-radius specification for '32s - '48s, only general, non-specific references to and explanations of 'spindle pin sideways inclination' and Max/Min range specs in degrees for 'Camber Plus Side Inclination'. Steve Tronix 03-12-2017 @ 12:22 AM reply profile send p.m. Member Posts: 5 Joined: Mar 2017 I would be very happy if someone could give me the dimensions of the scrub radius of the models with rigid front axle from 1932 to 1948 with the original tires and rims. Or if you could recommend a book where I can find these dimensions? I know that the Model A 1931 scrub radius was 1/2 inch. I think the scrub radius of the Model 40 in 1933 was 1 inch. But what is the measure 1935 with 16 x 4 wheels and 6:00 x 16 bias tire? // Carl

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