ideal scrub radius

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

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