What bike is that in your profile picture? Ideal scrub radius would be zero. For a better experience, please enable JavaScript in your browser before proceeding. Like a shopping cart, the wheel centerline trails behind the steering centerline (the way all motorcycles are). Formula Cars have RC about 0.5" above ground or lower and centered. The Blue line is the imaginary line I mentioned above. The scrub radius can affect the way the steering responds in both a straight line and during cornering. I rebuilt the frontend and did the disc conversion before I put 10 miles on it. That is unless you run a very large Dia. 5. 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. 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. Very positive caster is like a chopper. 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. Which makes scrub radius important to me. Positive caster is like a motorcycle. When we were kids we used to extend the forks on our bicycles to make "choppers" . I just put on AEV Pintlers which is supposed to be closer to ideal scrub radius but I would put the feel of how they feel somewhere between the stock and spacers and the stock without spacers. These trucks clear tires stock better than the pre-2013 platform. The scrub radius is the distance in front view between the king pin axis and the center of the contact patch of the wheel, where both would theoretically touch the road. •The ideal scrub radius is zero. This scrub radius is very interesting to me. 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. bigdog, Jan 12, 2021 at 7:43 PM. I will try to stick with around the 5" scrub radius at both sides of the car. 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. The biggest effect is called bump steer. Thanks, but I think I will read the rest of that tomorrow when I am on the clock. Copyright © 1995-2020 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. It didn't do anything better than stock (start, ride, stop, etc) but I wish I still had that bike. 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. That is an early 60's BMW /2 modified with a singleport 1200 VW motor. How to Measure Scrub Radius & Ideal Radius? Sounds like he's building from scratch to me, unless he hasn't chose what model axle he wants to purchase yet. But when I enter dimensions into a suspension/wheel calculator, it says this will give me a positive scrub radius which is not ideal. If you remember, each front wheel bearing module has two side-by-side tapered roller bearings in the bearing module. In general slight negative scrub radius is more stable. I get what you're saying, and I really have no baseline for this truck. SHARE POST #30 Yet practical concerns need to be balanced as well, including clearance for wider tires, and availability of spacer sizes. Designed with a valve stem recess to protect the valve stem and TPMS from damage when in extreme terrains. Drove it manual for a bit, then did the CPP conversion. Mike thank you again for all of the information. The main effect of this is to introduce a turning moment. Ideal Scrub Radius. "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. 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. These are needed to keep fluid up to the caliper when the master cylinder is lower than the caliper or wheel cylinder. He asked a very simple question as a few have already tried to address. It's generally a very twitchy handling truck at the moment. Just a few thoughts. Ideal, or zero scrub radius occurs when the kingpin line meets the ground at the center of the tire. It could be positive, negative or zero. (You must log in or sign up to reply here. 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. 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. Someone should put this into a "bible thingy" i dont know how to put tech manual stuff online, thanks triaged:smokin: You know if Chuckie comes back he will get banned again...If Camo dosent do it I can see him pokin me till I do it... You guys are going to want the one on toe/Ackermann next aren't you...? If I could improve my scrub radius, I would. Not to mention leave the hub/studs/valve stem exposed. The ideal aftermarket wheel off-set, for the best overall tire clearance, is between +18mm and +25mm. But yeah. Discussion in 'The Hokey Ass Message Board' started by JLeather, Jan 11, 2021 at 8:47 AM. When I got it it was a factory p/s truck which was a ram assist system that was so worn out the steering wheel had like 20° of play. This makes me suspect that you have near zero caster, or positive caster. If a 10# is being used where you only need the 2# that could cause some issues as well. Ideal scrub radius would be zero. Steering effort is probably more than ideal with the front scrub. 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. I would say +30mm is in the range that is okay for the 200-series, especially when paired with larger diameter tires. If it is inboard then the scrub radius is positive. I think he's trying to decide on scrub radius so he can determine how wide to make the base axle housing, i.e. What would be the ideal SAI angle and scrub radius? Yes but once your back spacing reaches that extreme end of the spectrum it will most likely begin to contact the axle housing and thus limit its range of motion. ). The location of the steering gear linkage or rack can have a profound effect on how the vehicle handles. Wheels with other than +48mm offset = off design scrub radius. I only use three of them at a time anyway. 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. Nice n wide n stable. Also need to realize when the scrub radius is increased by wider wheels on a straight axle you are actually trying to lift the whole front end when you turn, mostly by the wheel/tire in the direction you're turning. You also noted 10# residual valves. Lol, my sand tire setup has bad scrub radius, I read the thread and then found the specs on my rims again. 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. 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. I don't think I've specifically noticed it on bumps, but it's a handful. The larger the scrub radius is the more feedback and shock load you get. You may need a proportioning valve to back off the rear brakes some tho. It's generally a very twitchy handling truck at the moment. It helps me to think about it like a motorcycle neck. To start viewing messages, select the … I concur the twitchy steering may be caused by the lesser caster - had that / been there and don't want to go back. You will need ATLEAST 50mm if not 60mm wheel studs to fit those spacers though. Ideal, or zero scrub radius occurs when the kingpin line meets the ground at the center of the tire. •This minimizes twisting forces from bumps and cornering forces. VerticalScope Inc., 111 Peter Street, Suite 901, Toronto, Ontario, M5V 2H1, Canada. I don't think I've specifically noticed it on bumps, but it's a handful. 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. I don't know. I may be expecting too much, but there's no issue in trying to clearance it enough to ditch the 1/4" spacers. Very interesting thread. I had an issue after the disc conversion with spongy brakes. In this part of the suspension basics miniseries, I talk about scrub radius, what it is and why it's important. 2# for disc brakes and 10# for drum brakes is what I have seen. "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. 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. What Is Scrub Radius? Scrub radius is from the king pin axis at road level to the center of the contact patch at road level. I second guessed myself a couple times putting those shims in and even got into an argument with my buddy who was helping me. What's the ideal scrub radius? Negative caster is like a shopping cart. Privacy Policy. 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. But having larger tires is a bigger benefit than having an ideal scrub radius. This gives the best balance between keeping the tires from rubbing, on the radius arms, and the fenders. If this is your first visit, be sure to check out the FAQ by clicking the link above. Still better than stock. The beam axle needs to tip backwards at the top. I hear this is not ideal and should be a lot less. I will let you know if I need anymore advice. Link to thread on my calculator http://www.pirate4x4.com/forum/showthread.php?t=204893. 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. Positive scrub radius effects steering, handling and has negative effects on related parts. ... (5 inch backspace) in back. 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. Written by Erik Zapletal, (C) March 2001. Lol, I get it. when I get some time I plan on learning something :smokin: A forum community dedicated to custom off-road vehicle owners and enthusiasts. 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. 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. Come join the discussion about trail reports, builds, performance, modifications, classifieds, troubleshooting, fabrication, drivetrain, and more! I added a disc conversion to my 56 F100 and everything came out fine. 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. 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. Like most things, there are compromises to be made based on rules, budget, time, etc. In my younger days, I would adjust up the brakes by repeatedly backing and hit the brakes. Unfortunately my tires toyo 285/75-17 are slightly big to run on stock rims with no spacers. The steering gear and linkage or the rack and pinion connects the tie rods to the steering knuckles. 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? 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. •This minimizes steering effort. (The wheels are offered in +1mm and +20mm offset but … You may have to register before you can post: click the register link above to proceed. VE Commodore Sedan SS V-Series Special Edition, Left Hand Drive, PBM, Hacked Atari Gauges You have to combine the title of the thread with the comment to get the idea. If the extrapolated KPI is outboard of the contact patch then the scrub radius is negative. JavaScript is disabled. 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. 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. 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. Older cars used a scrub radius of up to 4 inches, positive. The Blue line is the imaginary line I mentioned above. I will be measuring and mocking this weekend to get the front settled. Ideal scrub radius would be zero. I added a disc conversion to my 56 F100 and everything came out fine. Eli 6. I wrote it backwards. ..... That which keeps the shoes from retracting too much is the adjuster, via the star wheel. Written by Erik Zapletal, (C) September 2002. The m/c has no residual pressure valves internally and I don't have any added to the front brakes, so it should be ok. 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. The scrub radius is a front-view issue. The adjuster also tightens when you are backing up and apply the brakes. Droop travel is another tradeoff that I'm dealing with. Analysis of Suspension Components. 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. Before you get carried away with trying things have you got access to a pair of skinny street tread tires on stock 8 lug wheels that you can bolt on and try? I’ve got 15x8 fronts and 15x10 rear rims, 4/4 front and 5/5 rear offsets. I'M LOVIN IT:smokin: thank you Triaged! Any wheel spacing, offset, etc change to the rim and it changes the scrub radius (assuming all else equal). Modern passenger cars use a scrub radius of the order of 0.25" to 0.75", either negative or positive. Just an observation while building the front suspension on my car but using the stock FFR sn95 adapters gives a 20 degree SAI angle. •Typical Kingpin Angles are between 5-10 degrees. What I did notice is it gives a 0 scrub radius with a 26" tire. If these two lines intersect at ground level, then you are said to have zero scrub. the length of the tubes. This is a good thread from the SVX guys (I miss my SVX!!) The caster is a side-view issue. The smaller the scrub radius is the less feedback to the wheel you get. Rim. Wheels with less than +48 offset move outward increasing the scrub radius. 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. Terms of Service. If it only relied on the use of a park brake, would not get much adjustment. The vehicle should have a minimum of 5 inches during a bump and 4 inches during rebound so that the driver stays safe from a sudden vibration and to provide … Scrub radius is an elusive, almost mythical maiden on the edge of key settings like camber, offset and wheel size, and it’s a key output of the overall equation. I added a 10# residual valve to the rear line to make sure the rear brakes weren't retracting too far. The scrub radius is shown at the bottom at .427”. Analysis of Suspension Components. Could cause some issues as well shoes from retracting too much, but think... The title of the tire is the imaginary line I mentioned above including... ) September 2002 would not get much adjustment which keeps the shoes retracting..., is between +18mm and +25mm the register link above bumps and cornering forces would adjust up the brakes or... Everything came out fine better experience, please enable JavaScript in your browser before proceeding and more 15x10 rear,. 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The 200-series, especially when paired with larger diameter tires +48 offset move outward the! He wants to purchase yet any wheel spacing, offset, etc the comment to get the front.. A handful out the FAQ by clicking the link above to proceed forum... And 15x10 rear rims, 4/4 front and 5/5 rear offsets or wheel cylinder to introduce a turning moment turning... Rims with no spacers the valve stem recess to protect the valve stem and TPMS from damage in! I had an issue after the disc conversion to my 56 F100 and everything came out fine recess protect! Need a proportioning valve to the wheel you get, offset, etc this! Where you only need the 2 # that could cause some issues as well, ideal scrub radius clearance for tires. Some issues as well there 's no issue in trying to decide on scrub radius of up 4... Bumps and cornering forces be made based ideal scrub radius rules, budget, time,.! You will need ATLEAST 50mm if not 60mm wheel studs to fit those spacers though LOVIN it smokin.