OEM scrub radius and front axle WMS

[ksagis]

Anybody know what the factory scrub radius was or what the front axle drum WMS (wheel mounting surface distance) was on early Broncos?

I did a little measuring on my truck a bit today to try and back out scrub radius.

I measured about 2.75 inches of difference on projected line between to the ball joints to ground, backing out an angle of about 8 degrees. The value from my alignment was 10 degrees of SAI, so that compares reasonably well.

Going from that datum to the wheel mounting surface (WMS) was about 3.75". For a zero offset wheel, that would result in a scrub radius of 3.75".

But, my rig has been converted to Chevy front disks and I measured a WMS distance on the front axle of 59.75" I'm probably putting was too much credence on the accuracy of a 1972 Ford sales brochure, but it says common wheel track for the front and back axles and the picture shows 57.4" inches for the back wheel track. My memory of working a full service gas station in high school vaguely recalls that factory steel wheel didn't have much offset. If that is right, backing half of the 2.35" out, would give a factory scrub radius of approx 2.5" for a zero offset wheel.

Why am thinking about this? Well, I recently changed wheels and tires on my rig that resulted in much better driving. The offset and wheel width on the new wheels was quite a bit less that the old wheels (resulting in a big different in scrub radius) and the tires were old. The scrub radius on the new wheels was reduced almost 1.5 inches - if my math is about right, that would be a bit a big percentage of the OEM value so maybe the scrub radius was a big player in the driving improvement I (was mostly focused on improving freeway handling)

Base case, keep an eye on the wheel/tire combo you're putting on and how it affects scrub radius. It helped me out, might help others.

Per my question above, if anybody knows what the OEM scrub radius was, or what the OEM front axle wheel mounting spacing was, I be interested in the numbers.

If any of my math is wrong, feel free to correct it.

 

[Broncobowsher]

I've thought about this over the years, never actually measuered any of it. But a factor is also height. Since you are drawing a line through the ball joints that goes through the contact patch of the tire, and that line is angled, the further the axle is above the ground the further out that pivot point is.

Then again I also think about how doing a tie rod over on the knuckles changes the Akreman angle in turns.

 

[toddz69]

Then again I also think about how doing a tie rod over on the knuckles changes the Akreman angle in turns.

I thought Ackerman was defined by the angles (as viewed from above) of the steering arms on the knuckles. How would a TRO affect that?

Todd Z.

 

[Yeller]

TRO Improves Ackerman for an EB, it raises that pivot point vertically in the plane of the slope of the ball joint axis, increasing the amount of pull the tire inside the turn pulls by turning slightly more than it did before. IMO Ackerman changes have very little effect on steering angles under 10 degrees, which is most steering done over 30mph. At slower speeds is with more steering angle it is a much bigger deal, even off road, you want that inside tire pulling into the turn.

Todd you are correct but in addition the angle changes with the changes in elevation within the ball joint inclination. If you've ever played with high steer, it becomes very evident quickly, the steering angles get really weird fast and do things that you do not expect as far as one tire turning farther than the other, often you wind up with less total steering angle because one side will steer much further than the other, it all depends on the location of the pivot on the steering arm. It is all in the relationship of multiple arcs coming together and how nice they play. I've spent way too much time trying different locations for the tierod pivots trying to optimize that and have found it is very difficult to make radical changes, like high steer that do not make overall negative effects on Ackerman, small changes like going TRO generally are an overall improvement. I believe this is part of the reason Heeps were spring under through the mid 90's, it allowed the tierod location to be higher and optimize Ackerman for the short wrangler/CJ, if you look at them the tie rod is very close to the same pivot point as a bronco/f150 (excluding 76/77) as far as its relationship with the wheel, but vertically the bottom of the tie rod is slightly higher than the top of the axle tube, which in my opinion is the sweet spot for Ackerman, and places it higher than anything that came on a bronco, there are some F150's that were quite close, if not the same.

 

[toddz69]

TRO Improves Ackerman for an EB, it raises that pivot point vertically in the plane of the slope of the ball joint axis, increasing the amount of pull the tire inside the turn pulls by turning slightly more than it did before. IMO Ackerman changes have very little effect on steering angles under 10 degrees, which is most steering done over 30mph. At slower speeds is with more steering angle it is a much bigger deal, even off road, you want that inside tire pulling into the turn.

Todd you are correct but in addition the angle changes with the changes in elevation within the ball joint inclination. If you've ever played with high steer, it becomes very evident quickly, the steering angles get really weird fast and do things that you do not expect as far as one tire turning farther than the other, often you wind up with less total steering angle because one side will steer much further than the other, it all depends on the location of the pivot on the steering arm. It is all in the relationship of multiple arcs coming together and how nice they play. I've spent way too much time trying different locations for the tierod pivots trying to optimize that and have found it is very difficult to make radical changes, like high steer that do not make overall negative effects on Ackerman, small changes like going TRO generally are an overall improvement. I believe this is part of the reason Heeps were spring under through the mid 90's, it allowed the tierod location to be higher and optimize Ackerman for the short wrangler/CJ, if you look at them the tie rod is very close to the same pivot point as a bronco/f150 (excluding 76/77) as far as its relationship with the wheel, but vertically the bottom of the tie rod is slightly higher than the top of the axle tube, which in my opinion is the sweet spot for Ackerman, and places it higher than anything that came on a bronco, there are some F150's that were quite close, if not the same.

Gotcha - I didn't consider the changes based on the ball joint inclination. Always learning!

Todd Z.

 

[ksagis]

I've thought about this over the years, never actually measuered any of it. But a factor is also height. Since you are drawing a line through the ball joints that goes through the contact patch of the tire, and that line is angled, the further the axle is above the ground the further out that pivot point is.

Then again I also think about how doing a tie rod over on the knuckles changes the Akreman angle in turns.

Good point on my 35" tires compares to stock tire diameter! Correcting for half of the diameter difference (assuming OEM was about 30 inch diameter) and the SAI adds another 0.4 inches to scrub radius. My current best estimate is about 2.9 inches of OEM scrub radius, if the factory steely is zero offset.

 

[lars]

Gotcha - I didn't consider the changes based on the ball joint inclination. Always learning!

Todd Z.

I was looking at this when I swapped knuckles a month or three ago. Imagine looking the the knuckle from the side. Draw a line through the center of the balls in the ball joints (the steering axis). Now draw another line perpendicular to that line, through the ball in the tie rod end. The second line is effectively the length of the steering arm. If you go from tie rod under to tie rod over, you just made that line longer (thus the effective steering arm length grew), because the bore of the tapered hole isn't parallel to the steering axis. At least that's how it is on my knuckles.

 

[ksagis]

Rolling up the various numbers in one area in case they are helpful.

-Best guess of 1972 OEM scrub radius: 2.9 inch
-Old tires and wheels scrub radius: 5.6 inch: drove like crap
-New tires and wheels scrub radius: 4.3 inch: 70 mph and drives great

My takeaway is to keep an eye on scrub radius (and yes, the age of tires)

 

[Yeller]

I don't want to be a downer, I've have enjoyed this thread tremendously, I can promise the tires played more into drivability than scrub radius. I have no doubt that it played a factor, but it was not the be all. As an individual with more Krawler experience than most, it is a low production race tire that had DOT approval for race series that required it, they were fantastic dirt/rock tires, terrible road tires, better than many of the bias counter parts but still not a modern radial. This is a great discussion, and some great tech. Remember a machine is the sum of its parts that add up to make it great, but an individual Item can make it terrible.

 

[ksagis]

Good input on Krawlers, hadn’t heard that before, thanks for sharing it.

 

[Broncobowsher]

Yeller, good job describing it. I could mentally figure it, maybe show someone on a white board.