Is a 5" scrub radius going to really suck? Am I totally screwed? If i want to run original 1934 Chevy spoke wheels I have to add 2" wheel spacers to clear the brake caliper. Running different rims (2-1/4" backspace), and no spacers, the lowest scrub radius is about 3".This is freaking me out because I'm going to use manual steering. Does anyone else have an SR that high on a car that drives well?
A 5" scrub is going to mega suck and yes you are. Even power steering will not handle that much well. It's going to jerk the steering wheel with every bump. Steering will be very hard at slow speed and unpredictable at high speed. Try to get it under 3' and the less the better. Not to mention the fact it will be hell on wheel bearings.
My current thought is to run a steering damper to compensate for the wheel jerk. In terms of low speed, its my understanding that the older cars had 3-4" of scrub because it makes turning at low speeds easier since the wheels are tracking around a larger radius circle rather than just spinning in place.
Scrub radius ?? I thought scrub was the line from bottom of rim to bottom of rim Nothing was to hang lower, so when you had a flat nothing would catch the ground and cause a problem.
The scrub he's talking about is the distance between the center of the wheel rim and the king pin projected axis, where both points meet the road. The greater the distance, the greater the steering scrub/harder steering.
This is what he's talking about... (I like pictures) http://www.freepatentsonline.com/6532062-0-large.jpg
If that's what he means, and his center of tire contact is 5" away from the king pin line....then forget that whole idea. It won't work. See you on the next death wobble post begging for help Some or most disc brake rotors move the line out too much as it is, then add 2" and it's just not going to work. Maybe put up some info on what axle, spindle, wheel info, pics etc...someone might have an idea to help.
learn something new every day! So the bigger the scrub radius the more the wheel is being pushed or pulled around the arc of the pivot axis. I can see where bump will be greatly amplified the bigger the S.R. gets, and harder on parts.
That's an understatement. Any mechanical engineer from hamb could list all the things that would be affected.. but just imagine the compounded stresses on king pins as well as the strain on those old wire wheels. Tie rods, steering arms, steering box, etc It's real hard to get a rod correctly done even when using correct parts
I have had "suspension gurus" tell me worrying about scrub radius is much ado about nothing. One theory is the larger the radius the better the on center feel and better tracking with more neutral caster/camber settings. Of course the larger the scrub radius the more pressure on the steering components, namely the tie rod. But that's about it, so long as the rest of your geometry is correct. If this were true you have to wonder why modern suspension geometry is placing the KPI incident point as close to the center of the tire contact patch as possible. That said, 5" seems like a ton but in reality it is about an inch more than most of the Ford beam axle setups I have around here.
SO just for fun I worked out you problem.. I think I assumed the King pin angle was 7 degrees and you were running a 30" dia. tire. I figured that a taller tire would change the scrub radius.. In therory a taller tire would decrease the SR and smaller tire would increase the SR (according to Scooters drawing) In order to get a 3" SR with only a tire change, you would need a tire with a diameter of 65.5" Better Scrub Radius.... Yes Higher stresses... Yes Impracticle....Yes Thats what you get when you think outside the box. Jeff
EP, can you put up some more words on that? It just does not sound possible with a wheel like a 40-48 etc. You are always way ahead of most of us backyarders, I know , but I am lost again.
A picture might help to see what 5" away from the tire center contact would look like. It looks worse that I expected with a skinny tire like most of us use. Sorry the pic angle is off a bit, but I was trying to show most of the king pin bosses.. Here is a 33-36 Ford axle with the commonly used 37-41 spindles and 40-48 16" steelies. It appears that this setup is about 1" to 1-1/4" from the true center contact on this tire diameter. But you can see how wrong it would look to have the line run at 5" true inches off of center. My opinion is that all the steering parts will be max stressed when a wheel drops into a big pothole. I was even thinking about "off shore" cheaply made steering u-joints and weak tie rods ??
I will elaborate in a few, got way too much going on at the moment. From your picture there, looks like 2.5" of scrub. The cars I measured have Lincoln style or Buick brakes and aftermarket wheels.
It's misleading at that angle,but honestly it can't be more than 1.5...anyways...... I'm glad you brought up the finned Buick drums. I ended up not looking for those because it sure looks like it moves the line quite a bit.
While not optimal bearing strength wise it still may be able to be comfortably driven if you add a stabilizer. If you look at just about any older solid axle 4x4 truck the center line of the kingpin or ball joint is quite a distance away from the centerline of the rim and many of these trucks had manual steering prior to the mid/late 70's or so. On my daily driven 4x4 F-1 I have close to 6 inches and it still drives well. I did add a steering stabilizer (I initially did get death wobble ever 5,000 miles or so if I hit a series of pot holes just right...no DW since I put the stabilizer on) AND I did replace the steering box since I have been driving it (the box wasn't the greatest when I put the truck together). I will say that I am a big guy (6'-8" and 245 pounds) and a smaller guy may have a tough time steering my truck BUT the last 40,000 miles have been trouble free.
That's where my mindset is. Then consider a 10" wide wheel with a 2 1/2" back spacing and 44" tires...long story... Personally if there is a way to not run spacers I would do that but if you can't then run it and make it work. You may run into some issues but if determined I feel you could make it drive just fine. BloodyKnuckles
A model t as well as other early auto manufacturers had front spindles with with 0 inclination and approx. 4" or more scrub radius. They wern't going that fast and mostly on dirt roads, they finally saw the engineering light and changed the design. I see many early rides with wire and steel wheels that have buick drums and or spacers with as much, and they see many miles of driving. It is harder on a lot of parts in the steering areas for sure, but it is in the area of form over function, as with alot of hot rod styling cues. Can you redrill the bolt pattern? get thinner adapters?
Yes, the line made by the kingpin down to where the tire rubber contact is with the ground. That line is call the angle of inclination. The line should be hitting the ground inside the tire ground contact zone. I guess the ultimate is to have that contact in the center of that zone, but would almost never happen with solid axles on old cars. Take a look at the front wheels on big trucks, you will note how the tire sits way in and exposes the rim center and brake drum in an attempt to account for this theory. If the contact is out side the zone you should be taking a 2nd look at your set up, but if it is that far out I would be very concerned. How about considering a larger diameter rim to clear stuff. I hate to mention it, but safety 1st, especially since you are aware of the problem, you are 1/2 way there to solving it.
split a steering box in half when i hit a pothole with to big scrubradius and crashed in a tree when i was young and smarter than my dad that told me thats whats going to happen
Big trucks are a different animal than lightweight hot rods. The rods we're talking about generally don't run power steering, which would offset the steering effort on a large scrub radius situation.
Contacted Rally America and bought some new Buffalo style wheels for the car. They will clear the calipers and I can avoid the wheel spacers. Not to mention, the Buffalo style from Rally America has a 3" backspace, so the Scrub Radius is down to 3" or a little less. I knew full well that the 5" was no good. When things go bad you just need someone to confirm your concerns to make you do what you know you should do anyway. I don't replace rusty parts with more rusty parts, so in terms of stresses on components, they can all take it. I machine a lot of my own components and do engineering analysis on most of it. I think it will all work out well now. The front end is 400# lighter than the stock 1934 Chevy Sedan. I'm running 1953 Pontiac Chieftain spindles and 6 bolt C1500 rotors. I built the front axle from 2"x2"x1/4" steel tubing. Turned axle bosses and cut the axle tube ends at the KP angle of 6° before welding them on with an alignment jig. Steering box is a Saginaw 525. Tie rod is 3/4" with a custom drag link connector in line to run cross steer. Now you have the specs and my final solution. Thank for all the posts.
Nice to see he followed the big truck idea and pulled his wheels in and remember a steering principle follows whether it be for a truck or a car.
Try a taller tire, the kingpin inclination will end up being closer to the wheel centerline at the tire footprint. The problem you've got is also very common on macpherson strut front suspension [ with widened or smaller diameter wheels ]
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