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ideal scrub radius

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

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