For anyone following along with a '50-'60s car with IFS, an anecdotal story: I flared my C2 'Vette in the '80s, and somewhere along the line decided to put 10" Americans in the front, as they fit the flares great and I could rotate with the rears. I don't know how I managed to drive it that way, any bump in the road and it was all I could do to keep it on the road. Went back to 8" rims, with the 2" gone from the outside. Made all the difference.
2" scrub either positive or negative is the optimal target. So it's proportional - Less and you loose feel as well as grip with a change in suspension loading. and when more critical things and parts begin to fight each more than necessary. Toe change in turns is exactly what the Ackerman is all about
If someone still can't grasp why large scrub radius is bad, this childrens toy is a nice and exaggerated example. The entire axle turns when you turn it, meaning it has the biggest scrub radius possible. On nice flat ground w/o front brakes it will steer fine, but if one wheel hits a bump ur front brake force is just slightly uneven the axle will turn violently, quite possibly resulting in a crash. Kids don't understand why, adult car builders should.
Actually this is not a very good example as related to vehicles with caster and some king pin inclination as the steering dynamics are totally different even if the final result is somewhat the same. Roo
On the subject of big scrub radius it would be interesting to see the end result of the latest trend in running huge offset wheels on pick up trucks (at least in this end of the world). I guess that the power steering damps a lot of the kick back when they hit chuck holes etc but it must be hell on the tie rod ends/ steering linkages/wheel bearings. Roo
I'm not bashing disc brakes, I have them on my project car. But some aftermarket kits push the WMS out way farther than they should be IMO. Had a '34 hiboy in the shop set up that way.....everything chrome and beautiful. Two strong guys could not push that car ten feet with the wheels turned slightly. Wish I had a pic...
Take a vehicle that runs positive offset wheels (FWD type) and switch the wheels to zero or negative offset (RWD type) and it's an advertisement- better than any "idiot on board" sticker.
See a lot of it around here and it's hell on wheel bearings and on Chevy with upper/lower control arms it will tear upper ball joints up with ease. Add a 4" or more lift kit and the wear factor goes up even more. Try to tell the kids what it's doing and they don't care but bitch because the new upper ball joints only last a year.
With a larger tire (taller/wider) a deeper offset wheel will help keep the scrub radius closer to original design. Although, as has been mentioned, it's a disaster for suspension and steering component wear and tear.
A lot of the trucks around here are not running much bigger diameter (overall) so the scrub offset is huge--a bit like the Buick brake drum example earlier in this thread) Roo
Ackermann was actually the recipient of the principle, Elliott was the discoverer. Elliott was a bicycle builder/mechanic, wont (good word) to solve a 'scrub' problem encountered on a 4 wheel vehicle he was designing. He was in his bathroom, which had two doors, opposite each other, swung from North, so they were 'parallel'. He tied them together, made some marks, and voila. Observing that when adjusted 'progressively', they were no longer parallel, so an imaginary 'outer' wheel would make a linked arc that was controllable via 'spindle arms' that connected in a line between the 'hinge' line (king pin center) and the center of the rear axle. Ackermann 'borrowed' the writings, and patented them. (From 'off the bathroom wall'..., ergo, "off the wall").
Hmmm...to be honest, when it comes to scrub radius, tire diameter is part of the geometry I had not considered. Good to keep in mind.
Okay I’ll weigh in here, since Frank has chosen to basically drag me through the mud claiming that I’m ignorant and don’t know what I’m doing etc., etc. I misread his post on my other thread when he mentioned scrub radius (my first thought when I saw the word scrub was scrub line,things dragging etc) so right away he jumped up and said I didn’t know what it was. https://www.jalopyjournal.com/forum/threads/transverse-front-suspension-travel.1132416/ Anyway he ranted there and here how I don’t know what I’m doing and I’m building something sketchy etc. It’s easy to look at pictures of something someone is doing and then say it’s terrible without actually knowing what you’re looking at. Supposedly my scrub radius is way off. So today when I came home from work I spent some time in the shop and did some measuring of what I actually have for scrub radius. Some careful measuring shows that my scrub radius is + 2”. So much for being way off. Unless I’m wrong I think that’s acceptable.
Do you have a clear plastic ruler? Just lay its edge down the center line of that king pin. 'Camera distortion?' No. The spacer inboard of the inner bearing is the first 'member', as the kit herein utilizes a 'flat rotor hub', and, spaced accordingly (OUT???) makes for a simple, non-stepped bracket for the caliper. We used a deeper disc hub, machined the 'stub' of the Ford spindle, and got the caliper 'inside' by milling a 3/4" blanchard ground plate, water cut. Everything nice and 'Ford-narrow'. There was some flim flam about the Volvo disc setup used in '72...Tex Smith penned the story in R&C. The Volvo was adequate in stopping a 2800 lb. street roadster, AND 'tucked-in' nicely. The addition of stacks and stacks to achieve a w-i-d-e tread is fruitless. I'd rather run drum brakes and F100 steering!
Not really, it looks like about a couple of inches of scrub radius. But Rich B is right, Blue did change to a drum brake system with backing plates and drums fro Boling Bros.
I'm LMAO. We just knew we'd never use that stupid geometry. Now look at us...ARGUING about it. I'm pretty sure that a lot of us here didn't pay real close attention in geometry class. I bet Blue One and F&J sat in the back row and shot spit balls at Gertrude McGillicuddy...that chick with the big...ummm...eyes. Small wonder we might have misunderstandings when we try to discuss the shit...
Yes, drums a complete setup all new parts. Boling Bros Lincoln brakes , they supplied the Pete and Jakes hubs with the complete brake kit. The wheels I’m using are 16x4 WV 14 gennies with 5:00 x16 Excelcior Stahl sport radials. And yes like I said I measured 2 inches of positive scrub radius.
Just to add to the toe-in adjustment deal, my Napco has a tie rod with a nice bend in it to clear the front differential, so it has to wind up in a pretty specific orientation, plus or minus a few degrees. However, it has an 18 tpi tie rod end on one side, and a 16 tpi on the other, allowing you to add to one side, and take away from the other as needed. Now back to scrub radii.
You DON'T want a zero scrub radius as it makes the front end unstable as you drive down the road. Back in the 70's when power steering on most heavy trucks was of the assist cylinder type and integral assist boxes were just coming on the market (and a lot of trucks still had manual steering), Rockwell came out with what they called a Center Point Steer axle. The design used hubs that projected noticeably outboard of the front wheels in order to allow the projection of the king pin to intersect the center of the tire's contact patch. IOW, a zero scrub radius. The steering effort was surprisingly low for manual steering, especially considering each front tire was carrying weight about equal to the weight of a mid size 4 door sedan. Once the truck was rolling at even walking speeds, the steering effort was about the same as power steering. Unfortunately, at highway speeds you found yourself doing small steering inputs continuously, much the same as is necessary with worn out tie rod ends or a steering box with too much clearance. IOW, you had to sorta herd it down the road rather than just holding the wheel and going along for the ride as it rolled straight down the highway. The concept would've worked well for trucks doing in-town delivery where the biggest part of the driving is slow speeds and turns, but it was aimed at the highway truck market and it never caught on there. A front end will inevitably have some slack in it, even when new. The scrub radius works to keep things in either tension or compression at all times when going straight down the road. Setting the scrub to zero allows the components to cycle between tension and compression as the forces on the steering tires vary within normal limits, and the result is a tendency to wander as the vehicle travels down the road. Its not nearly as dangerous as one of those arm breakers with a ton of offset, but its extremely irritating nonetheless. Re those pickups with wheels that put 2/3 of the tire outboard of the fender, a fellow I know who runs a front end shop says they're like a money machine for his business. Funny thing is, he says when he tries to explain why ball joints and tie rods on those setups often won't last as long as a set of tires, he just gets a blank stare in return about 90% of the time. So, he's found it easier to just hand 'em a bill and tell 'em he'll be there to take care of 'em next time. I guess most everyone here knows what Ron White says about fixing stupid
Is there a "standard" for an old Ford style straight axle with steering behind the axle, on either bias ply or radial tires (assuming all newish tight parts)? How much offset are you talking? Are you saying a 1/4 inch or a whole inch? Or is it too variable to make a determination? I'm guessing that the caster, tire diameter, tire width, tire pressure, hubcap shine, etc all make a difference, huh?
OK. There has been a lot of theory discussion here. Let's bring this down to a real world deal. I, along with probably thousands of others, have a Speedway axle with their disc brake setup in my 26 T coupe. The added scrub radius has bothered me since I put it in there 20K miles ago. I have a slight vibration or oscillation in the front end about 48-52 mph. I have chased it for 20K miles. Have had a dial indicator on everything that spins on the car. Wheel stud concentricity. Driveshaft @ front, middle and rear. Rear wheel flanges. Anything I could think of. Two different sets of tires and wheels. Different balance jobs. Beads. You name it, I checked it. Added a steering stabilizer on the tie rod. I even tried balancing the rotor hubs. Very precise alignment. Nothing. Nada. Still there. I just avoid that speed range. This thread brought it back to mind. I just went and checked my scrub radius, taking great pains to be accurate. With the speedway axle and disc brake conversion, Ford spindles, 5" wheel with 2.75 backspace, and a 22" tire, I have a scrub radius of right at +2". According to several posts above, 0 scrub radius is not desirable. I see the reasoning-- preloading of all components. 31 Vicky stated above that 2" is optimal. I had never heard a specific number as being optimal, however, if 2" is the optimal number, then the Speedway setup put the scrub right on the money. So, is it worth the effort to try and reduce this amount of scrub. In my case, it would be easy to do by changing the backspace of the wheel to 3.75". Giving 1" of scrub. I don't know if this is the cause or a contributor to my problem or not, but it could be something else to cross off the list. Whaddaya think?
LOL I always thought you were ignorant. That's a good trait in my book, not smart enough to know it can't be done. Everything to do with suspension geometry has an effect on everything else to do with suspension geometry. There are general guidelines that we want to follow but they are not really carved in stone and some experimentation or fine tuning can greatly change the outcome if someone is of a mind to make it perfect and some things work by accident for no apparent reason. There that'll get 'em off your back for a minute.
Not accurately measured but scrub on my coupe is estimated at about 2” positive with a 3 1/2” backspace wheel and it always handled great, no issues. The brakes are PSI aluminum hubs and 3/8”rotors from Gene Scott BITD with Airheart calipers. The brake mock up on the bench is slated to go on my Fordor this Spring. I found NOS 1960’s 122S/P1800 Volvo rotors and the calipers are 4 piston Wilwoods with the 5 1/4” mount spacing. There is no spacer for the inner bearing, but the spindle does need machining to move the outer in 1/4”. Checking wheel mounting surface from hub to inside of spindle (surface where backing plate was mounted) is 3 1/16” compared to a ‘40 front brake assembly still on the spindle that I measured at 3 5/16”. Poor photo of the coupe brake/wheel but it’s the best I have right now. Disregard the odd fasteners on the mock-up, they will be replaced at final assembly. I have no qualms with the solid rotors, they have always worked fine on the 5 window.
You don't have to live with it. I certainly wouldn't, as those speeds are where my 32 daily is driven at a lot. The above is a loaded question..meaning wondering if scrub radius error causes it? No, but... the scrub error will "amplify" any forces to one or both front tire/wheels. That amplification comes from physics, meaning "more leverage" induced to the steering linkages from a scrub error. You have to first find out where the vibration is coming from, and BTW there is a difference between ossilation and vibration, If it's possible, I would jack one front tire up at a time, and spin it at those higher speeds. An old Hunter brand "on the car balancer" motor would be perfect, or just make one if that's what it takes to track it down, or finally be able to eliminate the tires/wheel/rotor/& lug centricity altogether. If the car is fenderless, can you & a passenger see "oscillation" in either tire??. In these high tech times, do you have a Gopro to mount under it, if it's a full fendered car? Because you included a long detailed list of what you tried up front, we have to wonder if it's not coming from the front end of the car, so that is why I feel it's best to spin the fronts on the car. As you all know, finding any odd issue, includes the process of elimination, and spinning the fronts would greatly help at this point for "over the internet help", (which is far more difficult than people there in person to help find the problem). Do you feel this in the steering wheel as far as slight right to left motion?, or do you feel a "vibration" in it? .
Actually on a light car like we drive engine balance can cause a slight vibration that is felt though the wheel even as far as death wobble. No one will ever believe that so take it with a grain of salt. On our old heaps with all the mix matched parts I think that one car wheel balancing is the only way to go by the way.
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