ideal scrub radius

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

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