Scrub radius

[AnnDee4444]

I found that the effective scrub radius of a stock JL Rubicon with 6° of caster (Mopar lift arms) is roughly 1". Since I don't have any actual suspension measurements, I rotated my tires lock to lock a few times before backing up 2'. This leaves a circular imprint on the ground, and finding how offset the center of the circle is to the center of the tire is the scrub radius distance. If anyone has a better measurement, please let me know.

Assuming the OEM scrub radius is 1", the minimum offset is calculated below.
Disclaimer: None of this has been tested, all numbers are theoretical.

For 2" scrub radius (max. recommended for AWD):

31.5" tire: 22.6 mm offset
33" tire: 18.6 mm offset
35" tire: 13.2 mm offset
37" tire: 7.8 mm offset
40" tire: -0.3 mm offset
​

For 3" scrub radius (max. recommended for 2WD):

31.5" tire: -2.8 mm offset
33" tire: -6.8 mm offset
35" tire: -12.2 mm offset
37" tire: -17.6 mm offset
40" tire: -25.7 mm offset​

The (US) Rubicon should be fine with 3" of scrub radius, since the only time it sees 4WD is off road. Sahara's & other on-road AWD transfer cases should stick to 2" maximum. It is probably best to minimize scrub though, as I suspect it might be a contributor to death wobble.

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[LLRubylady]

I found that the effective scrub radius of a stock JL Rubicon with 6° of caster (Mopar lift arms) is roughly 1". Since I don't have any actual suspension measurements, I rotated my tires lock to lock a few times before backing up 2'. This leaves a circular imprint on the ground, and finding how offset the center of the circle is to the center of the tire is the scrub radius distance. If anyone has a better measurement, please let me know.

Assuming the OEM scrub radius is 1", the minimum offset is calculated below.
Disclaimer: None of this has been tested, all numbers are theoretical.

For 2" scrub radius (max. recommended for AWD):

31.5" tire: 22.6 mm offset
33" tire: 18.6 mm offset
35" tire: 13.2 mm offset
37" tire: 7.8 mm offset
40" tire: -0.3 mm offset
​

For 3" scrub radius (max. recommended for 2WD):

31.5" tire: -2.8 mm offset
33" tire: -6.8 mm offset
35" tire: -12.2 mm offset
37" tire: -17.6 mm offset
40" tire: -25.7 mm offset​

The (US) Rubicon should be fine with 3" of scrub radius, since the only time it sees 4WD is off road. Sahara's & other on-road AWD transfer cases should stick to 2" maximum. It is probably best to minimize scrub though, as I suspect it might be a contributor to death wobble.

This is really educational. Thanks for figuring it out. So turns out with my big offset wheels I'm ok. Seems to drive fine.
I had no idea there was so much to think about with tire upgrades. Guess there's a reason why people always there's doing it and then there's doing it right.

 

[blnewt]

So those -18 offsets are just fine, just need to find the perfect 44s to go with them

Great work Andy, very useful database you're building!

 

[crash13]

I found that the effective scrub radius of a stock JL Rubicon with 6° of caster (Mopar lift arms) is roughly 1". Since I don't have any actual suspension measurements, I rotated my tires lock to lock a few times before backing up 2'. This leaves a circular imprint on the ground, and finding how offset the center of the circle is to the center of the tire is the scrub radius distance. If anyone has a better measurement, please let me know.

Assuming the OEM scrub radius is 1", the minimum offset is calculated below.
Disclaimer: None of this has been tested, all numbers are theoretical.

For 2" scrub radius (max. recommended for AWD):

31.5" tire: 22.6 mm offset
33" tire: 18.6 mm offset
35" tire: 13.2 mm offset
37" tire: 7.8 mm offset
40" tire: -0.3 mm offset
​

For 3" scrub radius (max. recommended for 2WD):

31.5" tire: -2.8 mm offset
33" tire: -6.8 mm offset
35" tire: -12.2 mm offset
37" tire: -17.6 mm offset
40" tire: -25.7 mm offset​

The (US) Rubicon should be fine with 3" of scrub radius, since the only time it sees 4WD is off road. Sahara's & other on-road AWD transfer cases should stick to 2" maximum. It is probably best to minimize scrub though, as I suspect it might be a contributor to death wobble.

So if I'm understanding all of this correctly, any lift won't affect the scrub radius, stock Rubi has 1" scrub, up to 3" should be fine. So with my 35" tires, if I go with Spidertrax 1.75" wheel spacers, all is good. (I also have a Teraflex 2" spacer lift but this should not affect the radius as previously mentioned)
Thoughts?

 

[blnewt]

So if I'm understanding all of this correctly, any lift won't affect the scrub radius, stock Rubi has 1" scrub, up to 3" should be fine. So with my 35" tires, if I go with Spidertrax 1.75" wheel spacers, all is good. (I also have a Teraflex 2" spacer lift but this should not affect the radius as previously mentioned)
Thoughts?

It would be ideal to keep scrub radius as close to stock as possible, if you can get by w/ 1" spacers and still clear components at full flex I'd go that route.Just saying this as it will be a better driving experience on road as far as steering response/performance.

 

[AnnDee4444]

So if I'm understanding all of this correctly, any lift won't affect the scrub radius, stock Rubi has 1" scrub, up to 3" should be fine. So with my 35" tires, if I go with Spidertrax 1.75" wheel spacers, all is good. (I also have a Teraflex 2" spacer lift but this should not affect the radius as previously mentioned)
Thoughts?

A lift will not change scrub radius (well technically caster angle changes it a negligible amount). Tire size, wheel spacers, wheel width, & wheel offset all influence scrub radius.

I don't know what your acceptable limit is, everyone will have a different preference. If you add spacers and find that you want to go back to OEM scrub, then consider having your wheels widened. Widening the wheel 2" with a 1" wheel spacer effectively cancel each other out.

 

[3GunPreacher]

Forgive me please if this was already answered. I see reference to the default being 1" is that accurate? Does an OEM RUBI have a 1" scrub radius? I had always assumed it would be 0. I'm trying to determine the best offset to go for when putting on 34" tires...

 

[blnewt]

Forgive me please if this was already answered. I see reference to the default being 1" is that accurate? Does an OEM RUBI have a 1" scrub radius? I had always assumed it would be 0. I'm trying to determine the best offset to go for when putting on 34" tires...

It should be close to zero when caster is in the 4 degree range, but having caster at 6 give or take a couple tenths will result in much better on-road manners. With your Rubicon you can get by w/ more positive offset then the Sport/Sahara models since you have wider axles. AEV Pintler wheels 17x8.5 have a +25 offset and are pretty popular w/ the Rubicon folks. This should be a good offset for 34" tires and will keep your scrub radius much closer to stock vs. those wheels in the zero to negative offset wheel sets. If you plan to lift it further and really do heavy off-roading w/ full spring flex then you may want to sacrifice some scrub radius to get those tires a bit further out from the suspension.

I would strongly consider upgrading to the Mopar longer LCAs w/ your stock Rubicon, your current caster should be in the low 4s, getting it close to 6 is SO much better!

 

[3GunPreacher]

It should be close to zero when caster is in the 4 degree range, but having caster at 6 give or take a couple tenths will result in much better on-road manners. With your Rubicon you can get by w/ more positive offset then the Sport/Sahara models since you have wider axles. AEV Pintler wheels 17x8.5 have a +25 offset and are pretty popular w/ the Rubicon folks. This should be a good offset for 34" tires and will keep your scrub radius much closer to stock vs. those wheels in the zero to negative offset wheel sets. If you plan to lift it further and really do heavy off-roading w/ full spring flex then you may want to sacrifice some scrub radius to get those tires a bit further out from the suspension.

I would strongly consider upgrading to the Mopar longer LCAs w/ your stock Rubicon, your current caster should be in the low 4s, getting it close to 6 is SO much better!

This sound dumb I know. I can tell as I type it. I'm an IT Guy and this is my first Jeep (sans a brief fling with a Cherokee in 1999) LCAs are Lower Control Arms? will changing out the LCAs do anything to hurt any other parts of my ride? Can the dealer switch these out?

 

[blnewt]

This sound dumb I know. I can tell as I type it. I'm an IT Guy and this is my first Jeep (sans a brief fling with a Cherokee in 1999) LCAs are Lower Control Arms? will changing out the LCAs do anything to hurt any other parts of my ride? Can the dealer switch these out?

Yes, they are the Lower Control Arms. You may have trouble going through the dealer for the order of the parts & the install since these are meant for the Mopar 2" lift kit.

When I ordered mine through a Tuscon Az dealer I told them that I bought the Mopar 2" lift kit from a friend but he kept the LCAs so I needed to order a pair, so they shipped me out a set.

Maybe some dealers will just ship them out no questions asked, but usually they will want your VIN# or at the very least ask you why the part you want is different than what came with your JL.

They are a pretty easy install for any shop though, so you could order the pair and have a shop (or possibly a member on the site) change them out for you.

And no, there won't be any parts affected by changing out the LCAs, or any driving characteristics adversely affected, it's just a win!

Here's a post I made regarding the part #s and the install
https://www.jlwranglerforums.com/forum/threads/rubicon-suspension-on-2-door-sport.40700/#post-919317

And a huge thread on the LCA swap here
https://www.jlwranglerforums.com/forum/threads/mopar-lca-swap.22474/

 

[AnnDee4444]

Forgive me please if this was already answered. I see reference to the default being 1" is that accurate? Does an OEM RUBI have a 1" scrub radius? I had always assumed it would be 0. I'm trying to determine the best offset to go for when putting on 34" tires...

With the my stock JLR, I did the non-so-scientific test of turning the steering lock to lock. The "center" of the rubber mark swirl it left on the pavement were about 1" from the center of the tire, towards the inside. This can be considered about a 1" effective scrub radius. A Sport with it's shorter tires would have an effective scrub radius 4mm less than the Rubicon. 34" tires would be something like 1.25", which is still well within an acceptable range. The only way to truly find the exact scrub radius would require measuring all the front end components, but the test I did above should be good enough for most scenarios.

To achieve 0" of scrub radius on 33s, you would need a wheel with approximately +70mm offset (OEM is +44mm). This is actually not something that is desirable, as you actually do want some amount of scrub for steering feedback (and a +70mm wheel would be limited on clearance or width and be pushed too far towards the center of the Jeep)

I should also note that caster settings would have very minimal effect on changing scrub radius. Ride height will always have zero effect on scrub radius for a straight front axle.

 

[blnewt]

With the my stock JLR, I did the non-so-scientific test of turning the steering lock to lock. The "center" of the rubber mark swirl it left on the pavement were about 1" from the center of the tire, towards the inside. This can be considered about a 1" effective scrub radius. A Sport with it's shorter tires would have an effective scrub radius 4mm less than the Rubicon. 34" tires would be something like 1.25", which is still well within an acceptable range. The only way to truly find the exact scrub radius would require measuring all the front end components, but the test I did above should be good enough for most scenarios.

To achieve 0" of scrub radius on 33s, you would need a wheel with approximately +70mm offset (OEM is +44mm). This is actually not something that is desirable, as you actually do want some amount of scrub for steering feedback (and a +70mm wheel would be limited on clearance or width and be pushed too far towards the center of the Jeep)

I should also note that caster settings would have very minimal effect on changing scrub radius. Ride height will always have zero effect on scrub radius for a straight front axle.

@AnnDee4444
I was hoping to get your detailed reply, and you didn't disappoint

 

[Kevin32199]

To be honest, it is a tough subject for me also. Here's the important bits I found from the article I posted above:


From the way I understand it: if you consider just the drivers front tire encountering an obstacle at the direct center of the tire, with 0 scrub there will be no left/right turning reaction. With more scrub radius, the same obstacle will cause a lever-arm effect on that tire, which would then want to turn the tire counterclockwise. With a smooth surface or uniform left & right bumps, this is not noticed as the forces would cancel each other out.

Scrub radius is basically a way of measuring the lever arm distance between the center of the tire and the steering axis rotation point. A longer lever arm will have a greater effect. Zero lever arm will have zero effect.

AnnDee
Perhaps, its not just the lever arm distance from the center, but the length of the lever arm itself. Anyone who has ever put on much wider tires and not made any other mods can probably confirm (?). For instance, on an old car like a 1965 Lemans with 200 series (8 in wide) tires and a +3-4 inch scrub radius, most if not all of the tire patch is in the area of positive radius, and little or none is on the negative or inboard side of scrub radius--the length of the lever arm, which is what makes torque, is almost all of the patch's 8 inches. Bear in mind that suspension design including tie rod attachment points and steering box ratios have an effect on driver feel and control. On the JL Sport with stock rims assuming your +1-inch of positive scrub radius, approx 60% of the tire patch is outboard or positive, and the remainder is on the negative side. With stock 33s or 287-70s, it stays about 60% also, the latter being 6.75" outboard, 4.75" inboard. So far, so good...the one inch scrub radius stays consistent with the 60/40 percentage of tire outside/ inside the center of radius. When we get to 315s/ 12.5 wide tires, things begin to change. Here, with the + 1-inch scrub we need to add an additional 1/4" spacer to keep 60% of the tire outboard. It would be interesting to see a chart that also takes this "lever arm length" into consideration (hint, hint). It looks to me that the neg side of the tire balances the pos side, when braking, hitting bumps, etc. This is the end result we want to consider in addition to your matrix numbers that maintain stock scrub radius. I agree that the math is purely theoretical, but its nice to know when planning to spend big bucks. Also, thank you so much for all youve contributed. It has helped me greatly in understanding the geometry of our Jeeps.

 

[Yellow Cake Kid]

In general, if you install a larger tire on the same wheel you will reduce the scrub radius, but it is not often feasible to increase tire size substantially without increasing the effective backspace due to clearance concerns with other parts of the vehicle.

As noted in this thread, there is a potential to maintain the scrub radius by coordinating the effects of increased tire size with a minor change in effective backspace, achieved by either swapping out wheels with a suitable offset, or utilizing wheel spacers, but the ideal change in backspace that maintains the factory scrub radius may not offer enough clearance to address the clearance concerns.

Here is an illustration that approximates what happens when you upgrade a stock Rubicon with a common combo such as a 37x12.5 tire on a 17x9 rim with -12mm offset and resulting 4.5" backspace.

The change in relationship can cause all sorts of novel sensations, especially with regards to how the tires react to side load forces contributed by uneven road surface or crosswind.

It would be nice to maintain the factory scrub radius so that the Jeep retained the factory steer feel, but clearance issues can make it impractical to do so.

 

[AnnDee4444]

AnnDee
Perhaps, its not just the lever arm distance from the center, but the length of the lever arm itself.

I think we're saying the same thing, but with different wording.

It would be nice to maintain the factory scrub radius so that the Jeep retained the factory steer feel, but clearance issues can make it impractical to do so.

If you want to maintain factory scrub radius, see this post and the image below (the image is probably more accurate).

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