Scrub radius

[Yellow Cake Kid]

I have been looking forward to an opportunity to make some measurements to fill in the blanks

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[Kevin32199]

I also ran across >>this post #7 dated 5/7/17<< on another forum. Here we have a JKU owner who installed wheels giving him the theoretical maximum scrub radius recommended here, about 3 inches. He goes on to describe how the handling of his jeep is basically ruined. Looking at what he did and doing the math, we have the equivalent of a negligible (requiring maybe 1-2 mm spacer) tire diameter requirement that basically maintains stock scrub, but his aftermarket wheels are equal to a 2-inch spacer resulting in 8.8 inches of a 11.6-inch wide tire being outboard of the SAI (presumed +1" scrub radius line) --effectively 75% of the tire outboard. If you see my post above, stock or OEM scrub radius on our Jeeps results in only about 65% being outboard. Also my gut tells me (it just does, anyone else?) that more than 8 inches outboard of the SAI line is asking for trouble. I wish I had more of these negative "I screwed my Jeep up" forum experiences to read, as it helps everyone, me included.

 

[Yellow Cake Kid]

I have read on the internet that you can fix this by increasing the caster.

 

[Kevin32199]

I cant help but notice the difference in rubber that extends to the right of the red line. All of that acts like a lever. The stock one on the left has 60% or about 6.6 inches of the tire to the right of the red line, the 37 on the right has 70% or around 8.3 inches. The scrub radius here should be increased only about 1.3 inches due to the spacer requirements of the larger diameter tire. How would they drive compared to one another? Id love to try them.

 

[AnnDee4444]

I have been looking forward to an opportunity to make some measurements to fill in the blanks

View attachment 486521

h = Hard to measure. But I can tell you on a stock Rubicon, h + o - v (a.k.a. scrub axis) ≈ 1". Actual measurements might not line up with my findings, but the effective scrub axis is something that anyone can easily check by turning the tires lock-to-lock a few times and looking at the marks left on the pavement/dirt. The center of the circle will be scrub axis.

o = Stock = 44.45mm (except Mojave = 37mm, not sure about 392/Rubicon Recon Xtreme)

a = 12 degrees (at least it was on the JK. I haven't been able to confirm on the JL, but have no reason to suspect it's different)

r = Not actually 1/2 tire diameter. Tire sag will have an effect, and it may be better to do it off of the tire's revolutions/mile. See this somewhat related post.

 

[Kevin32199]

I have read on the internet that you can fix this by increasing the caster.

He says the Jeep was new. Changed tires and wheels only. No lift is mentioned. Caster should be the same. But yes, its the internet. (sigh).

 

[AnnDee4444]

effectively 75% of the tire outboard. If you see my post above, stock or OEM scrub radius on our Jeeps results in only about 65% being outboard.

I like how you've put scrub radius as a percentage of inboard/outboard. This is a good way to get people to understand that it's a ratio problem.

I wish I had more of these negative "I screwed my Jeep up" forum experiences to read, as it helps everyone, me included.

Unfortunately there's too much stubbornness and pride around Jeep modifications. That and people blaming it on being a "Jeep Thing"... no it isn't, it's a "Shitty Modification Thing".

 

[Yellow Cake Kid]

I was being snarky about the caster.

 

[Yellow Cake Kid]

r = Not actually 1/2 tire diameter. Tire sag will have an effect...

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[Yellow Cake Kid]

Here is an attempt to illustrate scrub radius movement from a perspective that may make it easier to appreciate how changes in this factor can affect the sensation you feel in the steering system.

It is a crudely generalized illustration that simply compares differences in scrub radius. It does not account for how or why the changes occur. The intent is to simplify the idea so it is easier to focus on the effect of making a change.

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[Kevin32199]

Here is an attempt to illustrate scrub radius movement from a perspective that may make it easier to appreciate how changes in this factor can affect the sensation you feel in the steering system.

It is a crudely generalized illustration that simply compares differences in scrub radius. It does not account for how or why the changes occur. The intent is to simplify the idea so it is easier to focus on the effect of making a change.

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You could also do another one alongside those with the same scrub radii and narrower tires, where any radius changes have less consequences. For instance, a 7-inch wide tire patch with +3-inches radius has 6 1/2-inches wide of rubber outboard and 1/2-inch inboard. There's not nearly as much force at work with narrower tires, so you can get away with murder in comparison. Not that any of us want narrow tires, just using it as a thought exercise. You cant get away with as much positive scrub radius on wider tires because it throws more rubber outboard creating a "Jeep thing", like trying to surf a straight course with uneven-sized sails on either side of a boat. On a foot-wide tire patch with the same +3-inches of radius, you've got 9-inches of tire patch outboard and 3 inboard. Thats a lot more leverage and friction for dips, bumps, grooves and other very common road imperfections to steer your Jeep around for you; and the faster you go the more friction there is multiplying the bad. IMO 9-inches would be bad and over 8-inches is concerning. If we were to make a spreadsheet, it would have tire diameter and spacer thickness at two borders and total scrub radius at the intersection--total radius meaning the final number considers (subtracts) spacer requirements of each tire diameter to maintain OEM radius. From accounts I have read, I would also visualize a red shade diagonally across the chart, probably (?) someplace in the +2-inch range of scrub radius (not spacer thickness). AEV wheels (+25 with ? tires) seem to have good feedback, so maybe that's a starting point. The thing is, we have to know what the scrub radius actually is. Someone looking at this chart could choose their tire diameter, the spacer they are considering, and see what the final scrub radius is and how close to ruining their Jeep they are. I realize everyone's different. My Jeep is a daily driver and the only car I have, so it has to be relatively normal (for a Jeep) on the highway and turning. If I cant run 80 mph its a deal breaker. Someone else who never uses their Jeep on the highway might not care. Someone else who digs trenches with 44s on the weekends isn't going to give a F. Then theres the difference is driver ability and perspective. What might make grandma white-knuckle the 1/2 mile to church bingo, someone else could steer for miles on end with 2 fingers while smoking a doob.

 

[Kevin32199]

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​

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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.

I got about +7/16" on a 2018 Sport S with Rubicon 33s (calculated including oem wheel offset) with a laser line. Ill need to remove the brake caliper or a tie rod end to be more accurate since they were in the way depending on projecting from front or back of vehicle to ball joints and to project the SIA on the ground.

 

[AnnDee4444]

I got about +7/16" on a 2018 Sport S with Rubicon 33s (calculated including oem wheel offset) with a laser line. Ill need to remove the brake caliper or a tie rod end to be more accurate since they were in the way depending on projecting from front or back of vehicle to ball joints and to project the SIA on the ground.

Nice. Good to see my guess wasn't too far off. Realistically though, I'm not sure if you would ever notice a +/- 0.5" difference with the huge sidewall.

 

[Midding]

If I am understanding this correctly going from my AEV rims +25mm to a freshly ordered rim -18mm I’ve potential created a handling issue in my 2021 Rubicon with 315/70/17r tires?

 

[Yellow Cake Kid]

Knowing why you feel the difference in handling will go a long way towards not manifesting the experience into a problematic issue.

If I interpret your post correctly, you seem to be indicating that you are swapping wheels while using the same size tires. Changing the offset in the negative tends to increase scrub radius, while increasing tire diameter tends to decrease scrub radius. So often times the changes made tend to negate, or at least mollify each other, but in your case, it seems as if you are going to increase your scrub radius without without any counteracting decrease due to an increase in tire diameter.

Andy's charts and tables describe this relationship of two factors. I imagine if you are asking this type of question you may already be aware of the info he shared.

If you increase your scrub radius a bit, you will probably notice the change in road feel, but it only has to become a problematic issue if you then go buy every sort of upgrade to mask the effect of the change in scrub radius and never quite get rid of the symptom.

Enjoy the new wheels, and enjoy the chance to get used to the new road feel.

Good luck!



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