Perhaps it's time to discuss scrub radius. The little I know about the subject is; an imaginary line drawn thru the kingpin to the ground should contact the center line of the tire resulting in zero scrub. I had a 54 F100 with a beam axle. I added 8" front wheels with a lot of offset and the result was terrible handling. I switched to 7" wheels with std. offset and the handling problem went away. Those of you with knowledge on this subject please help us understand the tech.
Pretty simple. If the scrub radius is correct, (as you stated) the tire rotates left and right directly on it's contact area with the ground. It turns freely and doesn't fight the geometry of the steering or affect handling adversely. If the scrub radius is not correct, (as with your first setup) the tire is being moved through an arc across the ground. It's harder to steer and very much fights the steering geometry.
Related, thru their influence on handling, steering, are king pin inclination (above) and caster angle. The proper settings will offer to 'return to center' in steering, Without excess effort needed to turn in either direction. Camber is easier to recall, as tire foot print, meeting either a crowned, or flat road.
Also, the greater the scrub radius, the more the bumps, pot holes and other pavement variations cause the wheel to be deflected from it’s desired path. The steering wheel will be rotated by the wheel deflection....kind of “the tail wagging the dog”..... Also tends to increase loading on suspension components and wheel bearings, especially the distribution of weight between the large inner bearing and the smaller outer bearing. But king pins/ball joints suffer too. All in all, no good comes from too much scrub radius
The more scrub radius is more leverage on that imaginary line through the king pin, forcing the tires to tow out when going straight and brakeing
Other than a few spelling errors, most all that has been said is correct. Keeping the center of the tire as close as possible to the center of the king pin/ball joint center line, the happier the suspension will be (the least amount of scrubbing). One thing to remember, this doesn't help the "cool" look of the front wheels-tires with deep offset wheels. While cool looking, it doesn't normally fit the above, "center line" requirement. Mike
That being said, how do we determine how far off we might be on a modified car? Or, how could we calculate getting it right during the very first stages of the chassis' build? For example, if we knew what all the stock vehicle's specs were for the OEM front end, say for a Ford Model A (overall tire diameter, kingpin inclination (KPI), the hub to hub distance (track?), wheel widths and offsets, how would we compensate for wheels and tires any different than stock? If someone could come up good way to work all that out, like a drawing or animation, that would be great. I've tried with graph paper and pencil, but without accurate numbers (KPI seems the hardest to determine) the best I can do is a good guess. Thanx, Gary
Plotting where the KPI (approximately) intersects the ground is not too difficult. The more difficult, as you suggest, is the variations of hub flange location (compared to OEM) and wheel offset. A view of the front suspension from head on at ride height and the KPI drawn as a line thru the KP centerline to the ground is good starting point. With spindles in hand, better still mounted on the axle or A arms, the hubs would be next to install. A line or plumb bob straight down from the hub flange will meet the floor/ground and the scrub radius will be evident. Next will be wheel selection (choosing rim width and offset, if any) to retain, or correct, for the desired minimal scrub radius. At least that seems to me to be one way to approach it.....hope it makes sense when read by other persons.
Toe-in will help compensate for how far out you are. More toe-in would have helped Blake 27 but as a crutch. Track width will not make any difference. Use one of those tire fitment tools with the spindle at ride height or strip it down, put a big piece of cardboard behind it and draw your lines out on the cardboard.
Positive scrub radius (KPI line or SAI line hits the ground inside the center of the contact patch) imparts a toe out force on the tires. This is typical of RWD cars. Negative scrub radius (KPI/SAI line hits the ground outside of the center of the contact patch) imparts a toe in force on the tires. This is typical of FWD cars. The selection of tire diameter and rim offset can radically change scrub radius. Sometimes changing it from + to - or - to +. I was told all cars have a little bit of scrub radius designed in and it helps with road feel. I need to do a little more research on that.
Like the videos! Let me ask... when you buy a re-pop Ford axle for your hot rod from any of the reputable dealers, how much camber is built in? Do you have zero, negative or positive numbers? How does that re-pop axle compare to an OEM part mfg by Ford in, say, 1932? Do all the re-pop axle mfgs use the same spec as OEM, or something else? How would you know before purchase? Buyer beware? Further, what do you get if you get a complete rolling chassis to slide under your project, complete with front and rear end. Do you get the camber specs? What's the KPI? You can build in the caster at the front cross member unless you get a store bought chassis and decide to raise the butt with larger diameter tires. Does the axle or chassis come with a spec sheet of recommended tire diameters and wheel offsets for the best steering? I think you know the answer, eh? Instead, we trust the builder or don't even know enough to be interested. Then we throw on wheels and tires that have the right "look" and call it a hot rod and wonder why it handles so funny.
^^^ This is important to understand, as factory toe setting spec was determined by the car manufacturer to work properly with the design of that cars OEM front suspension. If a rod builder has no clue about keeping the original scrub as designed, then the other alignment settings certainly will come into play. More on that below ^^^ this is what must be understood fully when building the traditional styled solid axle cars. You might think that you are a strong guy and can deal with the steering wheel being yanked as you hit a hole, but think of how many front end components are involved in that chain reaction to get to the steering wheel. This gets deep. If your build has heavy caster, that caster "tries harder" to keep that tire/wheel from "steering outwards" as it hits a pothole. However, you must then think about that when that tire/wheel does steer outwards, despite that heavy caster, it rams the tie rod in compression, against the opposite side which with it's heavy caster, tries to stay straight. More below on those forces: Think about the typical OEM Ford spindle arm design. They certainly do flex a bit if the force is great enough. That flexing will increase on both steering arms if the caster is heavy, because that opposite tire/wheel is trying to stay straight due to the heavy caster. Then, you also must think about the tie rod itself, as if it is a thinner wall tubing, it too can flex. This was proven by a hamber here some years ago. He was chasing death wobble. and he was big enough/strong enough to lay in front of his beam car on stands and grab both tires to spread them, and pull them, and he saw the tie rod bowing. That bowing, along with other things flexing with heavy caster can cause death wobble. Death wobble is best understood as a chain reaction, and "any" flexing will cause that reaction to amplify. That is why a steering damper will prevent it, by dampening that chain reaction...and flexing. The main reason I am running long here, is that I seem to have ruffled a few feathers on Blue One 's recent thread. He and a few followers there are just not understanding why I got involed in that thread. His scrub is way off, but he did not even know it...he saw my term scrub radius, but then replied that his scrub "line" was fine. Then, as his "above the frame rails tie rod" was in the way of the fan & front pulley, he decides to put a bend in the tie rod. He still refuses to understand that he then cannot propery set the toe by screwing just one toe rod end. Then somebody advised him to put an aftermarket rod adjuster in the center of that bent tie rod. Any engineer following along here, understands that the center of that bent rod will take some flexing...right at that adjuster. The internal threading in the tie rod tube for that center adjuster, has in effect, reduced the strength of that tube because threads ARE cut deeper into each half of that tube and are not supported by the threaded part of that new adjuster. Will it fail if the car ends up with wobble? Will the adjusters jamb nuts loosen?.. Or will the car even have wobble at all???..we don't know yet, but "why build something that sketchy to begin with" was my point... I bailed out, as some people just are unaware of what I was getting at, or think I am just a shit starter. I was trying to help, but some just don't understand front end dynamics....and some likely never will. .
I cringe every time I see an early Ford with that disc brake setup that pushes the wheels out 2-3 inches per side. Always wondered how they steer and how much stress that is putting on parts. Often used with small diameter tires which makes it even worse. Edit, Frank explained it pretty well as I was typing, thanks.
Not only disc swaps, but gets much worse with added Ford wire wheels with spacers...and also Buick finned drum swaps not done correctly. a little more on toe settings.... Many people don't know that a car with proper toe "in" at dead center, as designed properly, changes to an increasing amount of toe "out" as soon as you come off center as you steer from dead straight. There are specs in Motors or Chilton manuals, and the spec is called "Toe out at turns". Those specs are given in the manuals to determine if the steering arms are bent, or still as designed. I mention that term to have you think of what the wheels/tires are doing during the start of death wobble, they are rapidly going from toe in to toe out, over and over. That amount of "increasing toe out" is affected greatly by correct or incorrect Ackerman. And if a car has the tie rod moved to the front, but uses relocated steering arms that are meant for rear steer tie rod, that build will go from proper toe "in" at dead center, to increasing amounts of toe "in", and then have both tires steering in the wrong directions. They are fighting each other.
Praise to Mr. Ackerman! F&J, your mention of adj. both ends of tie rod is correct. (edit) The entire system flexes when loaded, death wobble is best addressed immediately so failure is avoided.
I see it far worse with finned Buick drums. I have disc brakes on the front end of my A, and the scrub radius is not bad at all. Yeah, but let's bash disc brakes.....
LOL I think that is also bump steer. Where it really becomes noticeably harder to drive is when the tires get wider, or when the wheel diameter gets smaller.
No one ever mentions it on those how to avoid bump steer treads. Of course I always forget and you are just too shy.
I was not bashing discs...it's just that I have yet to see any particular kit that did not throw the scrub radius off "with" the typical Ford steel wheels that most rods use. On the subject of discs sometimes touted as must haves...that is simply not true in so many types of terrains and driving habits. Lightweight rods driven as most typically are driven, >>in "average" terrains, simply do not "require" discs. Come drive mine as hard as you dare, and you will see. All I am trying to get across in this post is to dispel more web hype that certain things are absolutely needed, or that some errors don't matter, when in fact a person should instead tailor a build to suit what you plan to do with it....before just adding random kit discs, then having the scrub radius be incorrect which will cause other issues. I can't quite follow your meaning here for sure. But what I said about "not being able to set toe correctly to specs" by using just one tie rod end?...that is because you need to go one full revolution of that rod end to have the stud go back in it's hole. That is what Blue One is so stubborn on thinking/ not understanding. If you go one full turn, it moves the tires too far if it only needed a few degrees of a turn. What is so hard to understand about that?...unless a person never aligned cars before and does not know how little a tie rod can turn to go from zero toe to 1/8". I got shit to do today on a 48... so I'm out for now. I damn near quit the hamb "again" over that jackass thread, taking insults from a totally clueless person... but no...I will re-up my Alliance membership when it expires in a few days. Why let the hamb decay if we feel common sense and advice is not wanted or understood here by "some". Some may think that I feel as I know all there is about front ends...but a can assure you all, that after almost 50 years of studying/reading/testing...I simply do NOT. What I do know for fact, is that I really miss Dick Spadaro's teachings on beam cars on here... .
I know you weren't Frank. Oh man, if I wasn't 3000 miles away I'd already be out the door. I'd love to put it through it's paces.
Much of what's been said is great. In the 50s n 60s some of us made our own offset in the rims,by pulling center out an rewelding were we needed it. Too be simple about it an why it can drive crappy,drawing line down through the center of king pin all the way to ground=point"A",it needs to hit tire patch/foot print{were tire tread is flat on ground} at center,but you can get away *with up too 4in. max.outside of that spot.. Do not try an run tires on front wider then about 6in. across tread{ when over about 6in. wide,it dose two bad things;one is tire n rim will be too heavy for shock control,other is that it adds even more bad leverage ratio to scrub/offset. As offset/scrub gets bigger,every bump has bigger movemint in put on steering ,same with braking pull left or right. The photo above has little to do with type of drum,and every thing to do with very bad choice of rim offset/inset. Yes that's a mess,like too many rods,builder did what they saw,he thought he liked ,not knowing it was, too often repeted mistake of bad engineering. There is always some hard head,that will say his drives fine that way. He may even believe it. Dose not know better,has not driven well set up rod.
The reason for 'toe out on turns' is because the inner front wheel turns a tighter radius than the outer front during cornering. Without it the inner wheel will be dragged slightly sideways. If you draw a perpendicular line from the center each wheel of a during a turn, the lines should intersect at a common point. (The point about which the car is rotating during a turn.) Because of slip angle, dynamically that point is slightly ahead of its theoretical location. Slip angle is how far you have to turn the wheels compared to how far you would have to theoretically turn them to make a corner. The heavier the car, the faster you are going and the tighter the corner, the greater the slip angle.
That seems like an awful lot, but I'm glad to hear from someone with experience who can give us a practical parameter.
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