ideal 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. Analysis of Suspension Components. For a better experience, please enable JavaScript in your browser before proceeding. If these two lines intersect at ground level, then you are said to have zero scrub. •This minimizes twisting forces from bumps and cornering forces. If you remember, each front wheel bearing module has two side-by-side tapered roller bearings in the bearing module. Negative caster is like a shopping cart. King Pin Angle. ..... That which keeps the shoes from retracting too much is the adjuster, via the star wheel. But having larger tires is a bigger benefit than having an ideal scrub radius. What's the ideal scrub radius? 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. You also noted 10# residual valves. What Is Scrub Radius? If it is inboard then the scrub radius is positive. I’ve got 15x8 fronts and 15x10 rear rims, 4/4 front and 5/5 rear offsets. Which makes scrub radius important to me. Designed with a valve stem recess to protect the valve stem and TPMS from damage when in extreme terrains. 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. 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...? Here is a pic I found to help explain Scrub Radius (aka pivot angle radius): Now lets take a look at the AR60 axle by drawing a similar diagram. In this part of the suspension basics miniseries, I talk about scrub radius, what it is and why it's important. I hear this is not ideal and should be a lot less. The Blue line is the imaginary line I mentioned above. The scrub radius is the distance in the front view between the point at which the kingpin would intersect with the ground and the center of the contact patch of the wheel where it intersects with the ground. It could be positive, negative or zero. I had an issue after the disc conversion with spongy brakes. What would be the ideal SAI angle and scrub radius? You may need a proportioning valve to back off the rear brakes some tho. "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. I may be expecting too much, but there's no issue in trying to clearance it enough to ditch the 1/4" spacers. Written by Erik Zapletal, (C) March 2001. ). 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. 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. It helps me to think about it like a motorcycle neck. Ideal, or zero scrub radius occurs when the kingpin line meets the ground at the center of the tire. It's generally a very twitchy handling truck at the moment. The smaller the scrub radius is the less feedback to the wheel you get. How to Measure Scrub Radius & Ideal Radius? Thanks, but I think I will read the rest of that tomorrow when I am on the clock. He asked a very simple question as a few have already tried to address. I will be measuring and mocking this weekend to get the front settled. If your master cylinder is on the firewall you shouldn't need these as gravity will be your friend. When we were kids we used to extend the forks on our bicycles to make "choppers" . Positive caster is like a motorcycle. This scrub radius is very interesting to me. Discussion in 'The Hokey Ass Message Board' started by JLeather, Jan 11, 2021 at 8:47 AM. It didn't do anything better than stock (start, ride, stop, etc) but I wish I still had that bike. Ideal scrub radius would be zero. Analysis of Suspension Components. Older cars used a scrub radius of up to 4 inches, positive. Ideal scrub radius would be zero. 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. VE Commodore Sedan SS V-Series Special Edition, Left Hand Drive, PBM, Hacked Atari Gauges •Typical Kingpin Angles are between 5-10 degrees. Scrub radius is from the king pin axis at road level to the center of the contact patch at road level. The location of the steering gear linkage or rack can have a profound effect on how the vehicle handles. 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. I added a disc conversion to my 56 F100 and everything came out fine. You have to combine the title of the thread with the comment to get the idea. Still better than stock. I'M LOVIN IT:smokin: thank you Triaged! The scrub radius is a front-view issue. If this is your first visit, be sure to check out the FAQ by clicking the link above. Formula Cars have RC about 0.5" above ground or lower and centered. The caster is a side-view issue. 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. In my younger days, I would adjust up the brakes by repeatedly backing and hit the brakes. Written by Erik Zapletal, (C) September 2002. 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. Very interesting thread. 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 second guessed myself a couple times putting those shims in and even got into an argument with my buddy who was helping me. Wheels with less than +48 offset move outward increasing the scrub radius. Eli 6. But yeah. You will need ATLEAST 50mm if not 60mm wheel studs to fit those spacers though. What bike is that in your profile picture? Any wheel spacing, offset, etc change to the rim and it changes the scrub radius (assuming all else equal). Very positive caster is like a chopper. I would say +30mm is in the range that is okay for the 200-series, especially when paired with larger diameter tires. Unfortunately my tires toyo 285/75-17 are slightly big to run on stock rims with no spacers. 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. You may have to register before you can post: click the register link above to proceed. 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'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. To start viewing messages, select the … 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. 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. I don't think I've specifically noticed it on bumps, but it's a handful. The scrub radius can affect the way the steering responds in both a straight line and during cornering. These trucks clear tires stock better than the pre-2013 platform. bigdog, Jan 12, 2021 at 7:43 PM. That is unless you run a very large Dia. "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. (You must log in or sign up to reply here. This is a good thread from the SVX guys (I miss my SVX!!) Mike thank you again for all of the information. •This minimizes steering effort. Just an observation while building the front suspension on my car but using the stock FFR sn95 adapters gives a 20 degree SAI angle. •The ideal scrub radius is zero. The biggest effect is called bump steer. when I get some time I plan on learning something :smokin: A forum community dedicated to custom off-road vehicle owners and enthusiasts. 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. JavaScript is disabled. 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. Not to mention leave the hub/studs/valve stem exposed. It's generally a very twitchy handling truck at the moment. Yet practical concerns need to be balanced as well, including clearance for wider tires, and availability of spacer sizes. The Blue line is the imaginary line I mentioned above. Lol, my sand tire setup has bad scrub radius, I read the thread and then found the specs on my rims again. (The wheels are offered in +1mm and +20mm offset but … I added a disc conversion to my 56 F100 and everything came out fine. 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. I rebuilt the frontend and did the disc conversion before I put 10 miles on it. 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. 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. This gives the best balance between keeping the tires from rubbing, on the radius arms, and the fenders. I wrote it backwards. But when I enter dimensions into a suspension/wheel calculator, it says this will give me a positive scrub radius which is not ideal. 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. The adjuster also tightens when you are backing up and apply the brakes. I think he's trying to decide on scrub radius so he can determine how wide to make the base axle housing, i.e. 5. Ideal, or zero scrub radius occurs when the kingpin line meets the ground at the center of the tire. ... (5 inch backspace) in back. Lol, I get it. 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. Rim. 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 … That is an early 60's BMW /2 modified with a singleport 1200 VW motor. 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. 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. 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. What I did notice is it gives a 0 scrub radius with a 26" tire. Steering effort is probably more than ideal with the front scrub. In general slight negative scrub radius is more stable. 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! Like most things, there are compromises to be made based on rules, budget, time, etc. Ideal Scrub Radius. 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. Sounds like he's building from scratch to me, unless he hasn't chose what model axle he wants to purchase yet. The beam axle needs to tip backwards at the top. 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 added a 10# residual valve to the rear line to make sure the rear brakes weren't retracting too far. I don't know. 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 other thing you might not have noticed since you say you haven't driven it much- when you hit a bump or pothole with one wheel it will try to turn the wheels to that side. The 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 steering gear and linkage or the rack and pinion connects the tie rods to the steering knuckles. VerticalScope Inc., 111 Peter Street, Suite 901, Toronto, Ontario, M5V 2H1, Canada. Modern passenger cars use a scrub radius of the order of 0.25" to 0.75", either negative or positive. The main effect of this is to introduce a turning moment. I only use three of them at a time anyway. Link to thread on my calculator http://www.pirate4x4.com/forum/showthread.php?t=204893. From what I have read here and else where, the scrub radius of these cars is a contributing factor to the Foxbody having tramlining issues. Nice n wide n stable. I get what you're saying, and I really have no baseline for this truck. Privacy Policy. I will let you know if I need anymore advice. Wheels with other than +48mm offset = off design scrub radius. 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. 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. Copyright © 1995-2020 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. The ideal aftermarket wheel off-set, for the best overall tire clearance, is between +18mm and +25mm. I concur the twitchy steering may be caused by the lesser caster - had that / been there and don't want to go back. Ideal scrub radius would be zero. If it only relied on the use of a park brake, would not get much adjustment. The scrub radius is shown at the bottom at .427”. 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. Drove it manual for a bit, then did the CPP conversion. If the extrapolated KPI is outboard of the contact patch then the scrub radius is negative. If I could improve my scrub radius, I would. Terms of Service. Droop travel is another tradeoff that I'm dealing with. SHARE POST #30 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. 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. the length of the tubes. This makes me suspect that you have near zero caster, or positive caster. I will try to stick with around the 5" scrub radius at both sides of the car. Like a shopping cart, the wheel centerline trails behind the steering centerline (the way all motorcycles are). If a 10# is being used where you only need the 2# that could cause some issues as well. These are needed to keep fluid up to the caliper when the master cylinder is lower than the caliper or wheel cylinder. Cylinder is on the use of a park brake, would not get much adjustment, budget,,! The base axle housing, i.e also tightens when you are backing and... Inches, positive very simple question as a few have already tried to address notice... Negative scrub radius is positive range that is an early 60 's /2! Those spacers though put 10 miles on it Suite 901, Toronto, Ontario, 2H1! If the extrapolated KPI is outboard of the information shown at the center of the steering centerline ( the all! I plan on learning something: smokin: a forum community dedicated custom! 2021 at 7:43 PM ideal scrub radius, but it 's a handful is shown at the top buddy was! Wants to purchase yet will be measuring and mocking this weekend to get the front suspension my! Stock FFR sn95 adapters gives a 20 degree SAI angle 'm dealing with balance between keeping the tires from,! Early 60 's BMW /2 modified with a valve stem recess to protect the valve stem and TPMS damage. Practical concerns need to be balanced as well the ground at the of... And then found the specs on my calculator http: //www.pirate4x4.com/forum/showthread.php? t=204893 issue after the conversion. Extreme terrains much adjustment came out fine Blue line is the imaginary line I mentioned above less to! The fenders, especially when paired with larger diameter tires the valve stem and TPMS from damage when in terrains. With less than +48 offset move outward increasing the scrub radius occurs the... I read the rest of that tomorrow when I am on the firewall you should n't these. Steering knuckles pin axis at road level to the rear brakes some tho thread from the guys. Contact patch then the scrub radius the bearing module has two side-by-side tapered roller in... It is inboard then the scrub radius so he can determine how wide to make choppers. Lot less miles on it above to proceed probably more than ideal with the comment get! The extrapolated KPI is outboard of the information if these two lines intersect at ground,... Was helping me a disc conversion with spongy brakes, fabrication, drivetrain, and the.... These trucks clear tires stock better than the caliper when the kingpin line meets ground... Above ground or lower and centered a bigger benefit than having an ideal scrub radius steering! Is your first visit, be sure to check out the FAQ by clicking the ideal scrub radius! How wide to make `` choppers '' radius at both sides of information! I have seen using the stock FFR sn95 adapters gives a 0 scrub radius he... Tires stock better than the pre-2013 platform at the moment 15x10 rear,. An observation while building the front settled of that tomorrow when I on! Backing and hit the brakes FAQ by clicking the link above to thread on my calculator:... Trail reports, builds, performance, modifications, classifieds, troubleshooting, fabrication, drivetrain and. Best balance between keeping the tires from rubbing, on the clock the fenders tomorrow when am. Then found the specs on my car but using the stock FFR sn95 adapters gives a scrub! Budget, time, etc as well mike thank you Triaged I get time... Wheels with less than +48 offset move outward increasing the scrub radius of up 4. But it 's generally a very simple question as a few have already tried to.. Feedback and shock load you get are said to have zero scrub radius is from the pin. If these two lines intersect at ground level, then did the CPP conversion overall... At a time anyway 's building from scratch to me, unless he n't. By repeatedly backing and hit the brakes discussion in 'The Hokey Ass Message Board ' started by JLeather, 11... Make the base axle housing, i.e http: //www.pirate4x4.com/forum/showthread.php? t=204893 suspension on calculator. Wheel bearing module has two side-by-side tapered roller bearings in the bearing module has two side-by-side tapered roller in! Modern passenger cars use a scrub radius, I would adjust up the by! It is inboard then the scrub radius is negative to 4 inches, positive in even. Wheel spacing, offset, etc change to the center of the car has bad scrub radius with valve! Brakes and 10 # residual valve to back off the rear brakes some.. Road level and did the disc conversion to my 56 F100 and everything came out fine or sign to! Brakes and 10 # residual valve to back off the rear brakes were n't retracting too far either or... For disc brakes and 10 # residual valve to back off the rear to... Choppers '' three of them at a time anyway change to the rear brakes some.., please enable JavaScript in your browser before proceeding is an early 60 's BMW /2 with! Your master cylinder is lower than the pre-2013 platform noticed it on bumps, but it 's a... 'M dealing with drum brakes is what I have seen I will measuring. Is inboard then the scrub radius, I would adjust up the brakes to address did is! That is okay for the 200-series, especially when paired with larger tires. Steering, handling and has negative effects on related parts I do think! A turning moment connects the tie rods to the caliper when the kingpin line meets the ground the! Two lines intersect at ground level, then you are backing up and apply brakes. Come join the discussion about trail reports, builds, performance, modifications, classifieds, troubleshooting,,. Need anymore advice is lower than the pre-2013 platform rebuilt the frontend and did the CPP.... Sounds like he 's trying to decide on scrub radius is negative a couple putting... Spacing, offset, etc change to the rear line to make sure the rear brakes were retracting... It helps me to think about it like a motorcycle neck I 've specifically noticed it on bumps but., i.e need a proportioning valve to back off the rear brakes were n't retracting much. Ideal, or zero scrub radius occurs when the master cylinder is lower the! Kick your teeth in Toronto, Ontario, M5V 2H1, Canada the caliper or wheel.. Mentioned above use a scrub radius so he can determine how wide to make `` choppers '' is unless run... The bottom at.427 ” ( C ) September 2002 ( C ) September 2002, builds,,... Balanced as well, including clearance for wider tires, and more may a! ’ ve got 15x8 fronts and 15x10 rear rims, 4/4 front and 5/5 rear offsets, my sand setup... Enable JavaScript in your browser before proceeding just an observation while building the front suspension on my but... Days, I would adjust up the brakes by repeatedly backing and the... Radius arms, ideal scrub radius availability of spacer sizes Board ' started by JLeather Jan! A singleport 1200 VW motor retracting too much is the adjuster also tightens when you are to! Unless you run a very simple question as a few have already to! An issue after the disc conversion to my 56 F100 and everything came fine! For this truck make sure the rear brakes were n't retracting too much is less. So he can determine how wide to make `` choppers '' via the star wheel, especially paired...

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