bigdog, Jan 12, 2021 at 7:43 PM. That is unless you run a very large Dia. This is a good thread from the SVX guys (I miss my SVX!!) I haven't put the gauge on it in a couple years since I installed the shims but I'll double check what I've got. Thanks, but I think I will read the rest of that tomorrow when I am on the clock. If these two lines intersect at ground level, then you are said to have zero scrub. If I could improve my scrub radius, I would. I think he's trying to decide on scrub radius so he can determine how wide to make the base axle housing, i.e. This makes me suspect that you have near zero caster, or positive caster. In general slight negative scrub radius is more stable. For a better experience, please enable JavaScript in your browser before proceeding. You have to combine the title of the thread with the comment to get the idea. You will need ATLEAST 50mm if not 60mm wheel studs to fit those spacers though. The larger the scrub radius is the more feedback and shock load you get. Lol, I get it. on a dana 60 you would need a lot of back spacing to get them back in line with one another because the wheel mount to the kingpin axis is a long distance and it isnt on enough of an angle to allow for a neutral backspaced wheel. Drove it manual for a bit, then did the CPP conversion. This scrub radius is very interesting to me. •This minimizes steering effort. Written by Erik Zapletal, (C) March 2001. when I get some time I plan on learning something :smokin: A forum community dedicated to custom off-road vehicle owners and enthusiasts. Eli 6. While the 3-D nature of the geometry involved means that (effectively) rotating the knuckle in side-view means the steering axis in front-view will change because the cosine of the caster angle changes, the angles involved are (or should be) small, which means the cosine of that angle is a pretty good approximation of 1. Yet practical concerns need to be balanced as well, including clearance for wider tires, and availability of spacer sizes. The upper and lower control arms should be placed so that the instant center is 1 to 2 inches inside the opposite lower ball joint. If you have done this and ride the "chopper" you will understand what positive caster does- very stable at speed( easy to ride no handed) but steers harder when going slow. (The wheels are offered in +1mm and +20mm offset but … I'M LOVIN IT:smokin: thank you Triaged! The caster is a side-view issue. I made an "adjustment" to the spindle steering arm to correct that but also made my own adjustable drag link that would allow me to get the pitman arm in a vertical position when the truck was pointed straight. When we were kids we used to extend the forks on our bicycles to make "choppers" . If it only relied on the use of a park brake, would not get much adjustment. I guess I never knew how lucky I was... For the "twitchy" condition, I had something similar on mine and after re-taking measurements I found my drag link was angled just enough to give me bump steer. ... (5 inch backspace) in back. When viewing a wheel from front view, the scrub radius is the distance between where the extrapolated line of kingpin axis touches the road and the tyre contact patch centre point. Wheels with other than +48mm offset = off design scrub radius. Lol, my sand tire setup has bad scrub radius, I read the thread and then found the specs on my rims again. Formula Cars have RC about 0.5" above ground or lower and centered. 5. The ideal aftermarket wheel off-set, for the best overall tire clearance, is between +18mm and +25mm. You may need a proportioning valve to back off the rear brakes some tho. I will let you know if I need anymore advice. Here is a pic I found to help explain Scrub Radius (aka pivot angle radius): Now lets take a look at the AR60 axle by drawing a similar diagram. Terms of Service. You also noted 10# residual valves. The Blue line is the imaginary line I mentioned above. •Typical Kingpin Angles are between 5-10 degrees. I'm not confident I have the rear prop valve set 100% correctly but I've got it adjusted a little light on the rear brakes right now if anything, definitely not too heavy on the rears. I added a disc conversion to my 56 F100 and everything came out fine. Designed with a valve stem recess to protect the valve stem and TPMS from damage when in extreme terrains. I’ve got 15x8 fronts and 15x10 rear rims, 4/4 front and 5/5 rear offsets. Come join the discussion about trail reports, builds, performance, modifications, classifieds, troubleshooting, fabrication, drivetrain, and more! What's the ideal scrub radius? The scrub radius can affect the way the steering responds in both a straight line and during cornering. Like a shopping cart, the wheel centerline trails behind the steering centerline (the way all motorcycles are). What Is Scrub Radius? So let's suppose the stock scrub radius is -25mm and you bring it down to zero, you will now have squirm from the one inch spacer. The location of the steering gear linkage or rack can have a profound effect on how the vehicle handles. This gives the best balance between keeping the tires from rubbing, on the radius arms, and the fenders. Copyright © 1995-2020 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. its not a lot by the numbers, but for a track car, it does effect it a bit when driving really really hard such as track use. You may have to register before you can post: click the register link above to proceed. It could be positive, negative or zero. Since the truck is spring-over-axle putting the thick end of the shim at the rear gave me positive caster (per the posted diagram) which I thought is what I wanted? That is an early 60's BMW /2 modified with a singleport 1200 VW motor. He asked a very simple question as a few have already tried to address. the length of the tubes. Modern passenger cars use a scrub radius of the order of 0.25" to 0.75", either negative or positive. Ideal scrub radius would be zero. How to Measure Scrub Radius & Ideal Radius? In this part of the suspension basics miniseries, I talk about scrub radius, what it is and why it's important. (You must log in or sign up to reply here. Analysis of Suspension Components. The beam axle needs to tip backwards at the top. The scrub radius is the distance in the front view between the point at which the kingpin would intersect with the ground and the center of the contact patch of the wheel where it intersects with the ground. It's generally a very twitchy handling truck at the moment. "ideal" back spacing has to do with the steering knuckle geometry and where the bearings are at relative to the contact patch of the tire. Ideal scrub radius would be zero. •The ideal scrub radius is zero. Positive scrub radius effects steering, handling and has negative effects on related parts. If your master cylinder is on the firewall you shouldn't need these as gravity will be your friend. IF You are running say a 7" wide wheel, then you will be moving your scrub radius off by about 1" total, or about 14-15mm. To start viewing messages, select the … I will try to stick with around the 5" scrub radius at both sides of the car. It helps me to think about it like a motorcycle neck. typical ft RC on paved track stock car is 1.5 to 2.5 inch above the ground and offset 3 inch to the right of the car centerline. Ideal scrub radius would be zero. The Blue line is the imaginary line I mentioned above. I second guessed myself a couple times putting those shims in and even got into an argument with my buddy who was helping me. The scrub radius is the distance on the ground between the centerline of the tire contact patch and the point at which the SAI intersects the ground. Privacy Policy. The SKF engineer told me this was the ideal offset that Toyota established to center the load of the vehicle over the wheel bearing module. Steering effort is probably more than ideal with the front scrub. Sounds like he's building from scratch to me, unless he hasn't chose what model axle he wants to purchase yet. Droop travel is another tradeoff that I'm dealing with. I added a 10# residual valve to the rear line to make sure the rear brakes weren't retracting too far. I will be measuring and mocking this weekend to get the front settled. AEV’s JK wheels feature an ideal offset specific to the JK’s suspension geometry, accommodating larger tires while maintaining ideal scrub radius and smooth handling on or off-road. With the scrub radius known, and the track width chosen, he can adjust the length of the housing accordingly, or he may wish just to extend the track width for turning clearance. •This minimizes twisting forces from bumps and cornering forces. From what I have read here and else where, the scrub radius of these cars is a contributing factor to the Foxbody having tramlining issues. Nice n wide n stable. The adjuster also tightens when you are backing up and apply the brakes. ideal scub radius Hi all ok the question , if we forget about front upright design what scrub radius would be best I have some ab performance uprights and can adjust the wheel off set to get 12.5 to 32.5 mm scrub radius as I have not set the suspension lengths yet. I've made many changes to the brakes since then including trying several different m/c's (currently running an early 70's Ford F250 1 1/4" bore), changing from a GM-style PV2 prop valve to a Wilwood adjustable, etc. King Pin Angle. What I did notice is it gives a 0 scrub radius with a 26" tire. ..... That which keeps the shoes from retracting too much is the adjuster, via the star wheel. This means there is less than half an inch between the king pin inclination line and the tire center line where the tire contacts the road. Scrub radius is from the king pin axis at road level to the center of the contact patch at road level. Analysis of Suspension Components. I do not have any other tires to try out it, although it needs a new set any time now... Hmnn, I always thought the opposite, tilting axle back being negative degrees. I rebuilt the frontend and did the disc conversion before I put 10 miles on it. Positive caster is like a motorcycle. Written by Erik Zapletal, (C) September 2002. The other thing you might not have noticed since you say you haven't driven it much- when you hit a bump or pothole with one wheel it will try to turn the wheels to that side. The smaller the scrub radius is the less feedback to the wheel you get. I added a disc conversion to my 56 F100 and everything came out fine. Mike thank you again for all of the information. It didn't do anything better than stock (start, ride, stop, etc) but I wish I still had that bike. I don't think I've specifically noticed it on bumps, but it's a handful. But when I enter dimensions into a suspension/wheel calculator, it says this will give me a positive scrub radius which is not ideal. Very positive caster is like a chopper. I would say +30mm is in the range that is okay for the 200-series, especially when paired with larger diameter tires. Ideal, or zero scrub radius occurs when the kingpin line meets the ground at the center of the tire. The other thing you might not have noticed since you say you haven't driven it much- when you hit a bump or pothole with one wheel it will try to turn the wheels to that side. What would be the ideal SAI angle and scrub radius? Link to thread on my calculator http://www.pirate4x4.com/forum/showthread.php?t=204893. Also we have all seen the "death wobble" a shopping card gets when pushed fast across the parking lot- but very easy to steer in the tight store. Any wheel spacing, offset, etc change to the rim and it changes the scrub radius (assuming all else equal). Here is a pic I found to help explain Scrub Radius (aka pivot angle radius): Now lets take a look at the AR60 axle by drawing a similar diagram. In my younger days, I would adjust up the brakes by repeatedly backing and hit the brakes. The main effect of this is to introduce a turning moment. Wheels with less than +48 offset move outward increasing the scrub radius. I had an issue after the disc conversion with spongy brakes. I get what you're saying, and I really have no baseline for this truck. The biggest effect is called bump steer. If the extrapolated KPI is outboard of the contact patch then the scrub radius is negative. 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