Optimum scrub radius thread (3 Viewers)

[highfructose]

It doesn’t appear that we have a scrub radius specific thread.

My understanding is that if you go taller on the tire, you’re supposed to push them farther out from the chassis in order to maintain a proper “swing” to the independent suspension arc.

practically it seems that you lose some handling (and thus safety) capability if you’re taller tires are too close to the vehicle.

Id appreciate critique to the above statements. I’m not an engineer. I know some of you are.

supposedly the rock warriors have the ideal offset to maintain proper scrub radius/handling.

Stock Lexus rims offset mean 31.6” tires
So rock warriors offset mean 32.6” tires.

what is the proper offset for 33.5”?
34.5”?

 

[tincan45]

This has been discussed a lot in other threads, but you are right, nothing specific. The general opinion is something like this...

Stock - 40-50mm
33 - 30-40mm
34-35 - 20-30mm

 

[lx200inAR]

I made this a while ago when i was making a RCTIP table in excel. The numbers can vary based on what assumptions you make about how Toyota spec'd the scrub radius. I added the "Proposed Tires" column, i assume that you pulled the manufacturer spec on a KO2 for the Rock Warrior size and whatever the factory tire is for a LX., since your numbers don't match up to what the calculated size of a 285/70R17 (32.71") and 285/50R20 (31.22"). A Landcruiser spec'd stock tire (31.46") is slightly different than a LX one as well, so you could throw that in. So here's a very specific "looking" table, but really you can move the numbers around quite a bit based on small changes in assumptions. That's the problem with extrapolation.

Calculated Size	Proposed Tires	Average	Offset	Spacer
31.22​	31.6​	31.41​	60​	0​
31.369​	31.7​	31.5345​	59​	1​
31.518​	31.8​	31.659​	58​	2​
31.667​	31.9​	31.7835​	57​	3​
31.816​	32​	31.908​	56​	4​
31.965​	32.1​	32.0325​	55​	5​
32.114​	32.2​	32.157​	54​	6​
32.263​	32.3​	32.2815​	53​	7​
32.412​	32.4​	32.406​	52​	8​
32.561​	32.5​	32.5305​	51​	9​
32.71​	32.6​	32.655​	50​	10​
32.859​	32.7​	32.7795​	49​	11​
33.008​	32.8​	32.904​	48​	12​
33.157​	32.9​	33.0285​	47​	13​
33.306​	33​	33.153​	46​	14​
33.455​	33.1​	33.2775​	45​	15​
33.604​	33.2​	33.402​	44​	16​
33.753​	33.3​	33.5265​	43​	17​
33.902​	33.4​	33.651​	42​	18​
34.051​	33.5​	33.7755​	41​	19​
34.2​	33.6​	33.9​	40​	20​
34.349​	33.7​	34.0245​	39​	21​
34.498​	33.8​	34.149​	38​	22​
34.647​	33.9​	34.2735​	37​	23​
34.796​	34​	34.398​	36​	24​
34.945​	34.1​	34.5225​	35​	25​
35.094​	34.2​	34.647​	34​	26​
35.243​	34.3​	34.7715​	33​	27​
35.392​	34.4​	34.896​	32​	28​
35.541​	34.5​	35.0205​	31​	29​
35.69​	34.6​	35.145​	30​	30​
35.839​	34.7​	35.2695​	29​	31​
35.988​	34.8​	35.394​	28​	32​
36.137​	34.9​	35.5185​	27​	33​
36.286​	35​	35.643​	26​	34​
36.435​	35.1​	35.7675​	25​	35​
36.584​	35.2​	35.892​	24​	36​
36.733​	35.3​	36.0165​	23​	37​
36.882​	35.4​	36.141​	22​	38​
37.031​	35.5​	36.2655​	21​	39​
37.18​	35.6​	36.39​	20​	40​
37.329​	35.7​	36.5145​	19​	41​
37.478​	35.8​	36.639​	18​	42​
37.627​	35.9​	36.7635​	17​	43​
37.776​	36​	36.888​	16​	44​
37.925​	36.1​	37.0125​	15​	45​
38.074​	36.2​	37.137​	14​	46​
38.223​	36.3​	37.2615​	13​	47​
38.372​	36.4​	37.386​	12​	48​
38.521​	36.5​	37.5105​	11​	49​
38.67​	36.6​	37.635​	10​	50​

 

[timjax]

It doesn’t appear that we have a scrub radius specific thread.

My understanding is that if you go taller on the tire, you’re supposed to push them farther out from the chassis in order to maintain a proper “swing” to the independent suspension arc.

practically it seems that you lose some handling (and thus safety) capability if you’re taller tires are too close to the vehicle.

Id appreciate critique to the above statements. I’m not an engineer. I know some of you are.

supposedly the rock warriors have the ideal offset to maintain proper scrub radius/handling.

Stock Lexus rims offset mean 31.6” tires
So rock warriors offset mean 32.6” tires.

what is the proper offset for 33.5”?
34.5”?

If you want to maintain the exact same scrub radius as stock, decrease offset by approximately 5mm (technically varies between 4-6mm) for every inch increase in height. Decreasing another 5mm or so would still be in the same scrub radius realm and should be fine for even the most cautious buyer.

Stock LC200 setup is 18x8 +56 (+60 on HE) on 285/60R18 (32").

285/65R18 (33"): +51mm (Rock Warrior is +50mm)
285/70R18 (34"): +46
285/75R18 (35"): +41

Truthfully, you can increase these limits by a good margin. For example, nearly everyone running 34's and 35's is running +30mm or lower offset, and for rubbing you might likely have to.

 

[highfructose]

If you want to maintain the exact same scrub radius as stock, decrease offset by approximately 5mm (technically varies between 4-6mm) for every inch increase in height. Decreasing another 5mm or so would still be in the same scrub radius realm and should be fine for even the most cautious buyer.

Stock LC200 setup is 18x8 +56 (+60 on HE) on 285/60R18 (32").

285/65R18 (33"): +51mm (Rock Warrior is +50mm)
285/70R18 (34"): +46
285/75R18 (35"): +41

Truthfully, you can increase these limits by a good margin. For example, nearly everyone running 34's and 35's is running +30mm or lower offset, and for rubbing you might likely have to.

Is it the case that going with a wider than “oem” stance has fewer negatives than going with a narrower?

are there technically fewer safety concerns? If so, what becomes the downside of a wider stance?

 

[gaijin]

Stock LC200 setup is 18x8 +60 on 285/60R18 (31.5").

FYI

 

[TeCKis300]

Is it the case that going with a wider than “oem” stance has fewer negatives than going with a narrower?

are there technically fewer safety concerns? If so, what becomes the downside of a wider stance?

Perhaps. To be sure, the handling impacts will feel different. But there's going to be generally the same symptoms of loss of traction in cornering, steering kickback, tire scrub, etc.

As we're generally lifting the car with larger diameter tires, and suspension lifts (which tend to narrow the track as control arms are at steeper angles), guess you could say that wider track width offset beyond optimal, are going to be safer from a lateral stability and rollover standpoint.

 

[AnyMal]

I made this a while ago when i was making a RCTIP table in excel. The numbers can vary based on what assumptions you make about how Toyota spec'd the scrub radius. I added the "Proposed Tires" column, i assume that you pulled the manufacturer spec on a KO2 for the Rock Warrior size and whatever the factory tire is for a LX., since your numbers don't match up to what the calculated size of a 285/70R17 (32.71") and 285/50R20 (31.22"). A Landcruiser spec'd stock tire (31.46") is slightly different than a LX one as well, so you could throw that in. So here's a very specific "looking" table, but really you can move the numbers around quite a bit based on small changes in assumptions. That's the problem with extrapolation.

Calculated Size	Proposed Tires	Average	Offset	Spacer
31.22​	31.6​	31.41​	60​	0​
31.369​	31.7​	31.5345​	59​	1​
31.518​	31.8​	31.659​	58​	2​
31.667​	31.9​	31.7835​	57​	3​
31.816​	32​	31.908​	56​	4​
31.965​	32.1​	32.0325​	55​	5​
32.114​	32.2​	32.157​	54​	6​
32.263​	32.3​	32.2815​	53​	7​
32.412​	32.4​	32.406​	52​	8​
32.561​	32.5​	32.5305​	51​	9​
32.71​	32.6​	32.655​	50​	10​
32.859​	32.7​	32.7795​	49​	11​
33.008​	32.8​	32.904​	48​	12​
33.157​	32.9​	33.0285​	47​	13​
33.306​	33​	33.153​	46​	14​
33.455​	33.1​	33.2775​	45​	15​
33.604​	33.2​	33.402​	44​	16​
33.753​	33.3​	33.5265​	43​	17​
33.902​	33.4​	33.651​	42​	18​
34.051​	33.5​	33.7755​	41​	19​
34.2​	33.6​	33.9​	40​	20​
34.349​	33.7​	34.0245​	39​	21​
34.498​	33.8​	34.149​	38​	22​
34.647​	33.9​	34.2735​	37​	23​
34.796​	34​	34.398​	36​	24​
34.945​	34.1​	34.5225​	35​	25​
35.094​	34.2​	34.647​	34​	26​
35.243​	34.3​	34.7715​	33​	27​
35.392​	34.4​	34.896​	32​	28​
35.541​	34.5​	35.0205​	31​	29​
35.69​	34.6​	35.145​	30​	30​
35.839​	34.7​	35.2695​	29​	31​
35.988​	34.8​	35.394​	28​	32​
36.137​	34.9​	35.5185​	27​	33​
36.286​	35​	35.643​	26​	34​
36.435​	35.1​	35.7675​	25​	35​
36.584​	35.2​	35.892​	24​	36​
36.733​	35.3​	36.0165​	23​	37​
36.882​	35.4​	36.141​	22​	38​
37.031​	35.5​	36.2655​	21​	39​
37.18​	35.6​	36.39​	20​	40​
37.329​	35.7​	36.5145​	19​	41​
37.478​	35.8​	36.639​	18​	42​
37.627​	35.9​	36.7635​	17​	43​
37.776​	36​	36.888​	16​	44​
37.925​	36.1​	37.0125​	15​	45​
38.074​	36.2​	37.137​	14​	46​
38.223​	36.3​	37.2615​	13​	47​
38.372​	36.4​	37.386​	12​	48​
38.521​	36.5​	37.5105​	11​	49​
38.67​	36.6​	37.635​	10​	50​

I know this is a dumb question, but should you be measuring your actual height loaded down on the vehicle or with the tire off and at the intended pressure?

 

[timjax]

Stock LC200 setup is 18x8 +60 on 285/60R18 (31.5").

FYI

Actually if I'm not mistaken, as I mentioned +60 is correct for the HE and pre-2016 models, but the 2016-present LC200 stock wheel is correctly +56. And the "32" inch tire is about a half inch off just like 35's are closer to 34.5".

 

[lx200inAR]

I know this is a dumb question, but should you be measuring your actual height loaded down on the vehicle or with the tire off and at the intended pressure?

Assuming that the loaded heights are relative to the measured / calculated heights, it only matters to stay consistent. Considering all the assumptions/approximations we are already making with this, I expect it only matters to stay consistent in using all of one or the other measurements. Since you can't really "dial" this in, unless you want to run shim spacers or custom wheels and want to choose your tire based on measured circumference rather than all of the other more important features of a tire, all you can really do is get into the ballpark.

 

[LCStve]

is it dangerous to drive on too much positive scrub radius?

 

[grinchy]

For posterity searching. Lx570 docs indicate king pin angle steering inclination angle of 13 degree

 

[highfructose]

Would anyone happen to know whether it is correct to assume that, in lieu of less offset to counteract a taller tire, one might simply make the tire itself wider?

So if a 32.8” tire requires a 50mm offset, could someone who added a 32.8” tire to a 60mm offset wheel simply ensure the new 32.8” tire was 20mm total wider…thereby making the spot on the ground that the tire rests be basically the same?

 

[AnyMal]

Would anyone happen to know whether it is correct to assume that, in lieu of less offset to counteract a taller tire, one might simply make the tire itself wider?

So if a 32.8” tire requires a 50mm offset, could someone who added a 32.8” tire to a 60mm offset wheel simply ensure the new 32.8” tire was 20mm total wider…thereby making the spot on the ground that the tire rests be basically the same?

No its the pivot point that matters not contact patch.

 

[TeCKis300]

Old post with pictures from this thread may help. Scrub radius is relative to the center of the tire so width doesn't matter.

Do we know what the kingpin angle KPI is?

Kingpin angle, aka steering/kingpin inclination angle, is the black line in this graphic. Critically, note how it intersects the mid-point of the tire at the contact patch. That is the "hinge point" of the tire as it steers. Too far from the hinge point, and forces aren't isolated to the hinge any longer and go into the steering rack and other links.

Here's what scrub radius looks like when KPI doesn't intersect that point. The resulting effect is that the tire will scrub and lose traction as it turns. Potentially causing a loss of traction in low grip slick road scenarios. You'll notice it in tight turns in a parking structure for example with the tires squealing. It will pull on the steering as significant acceleration or braking force is applied. Steering effort will increase.

Sure, all this can be dismissed. It does matter however slight one imagines the impact to be.

This video really helps explain things:

 

[grinchy]

2009 lx570 kingpin angle is 13 deg
<edit - put up full page of alignment and steering inclination angle>

 

[AnyMal]

Old post with pictures from this thread may help. Scrub radius is relative to the center of the tire so width doesn't matter.



Kingpin angle, aka steering/kingpin inclination angle, is the black line in this graphic. Critically, note how it intersects the mid-point of the tire at the contact patch. That is the "hinge point" of the tire as it steers. Too far from the hinge point, and forces aren't isolated to the hinge any longer and go into the steering rack and other links.
View attachment 2914293

Here's what scrub radius looks like when KPI doesn't intersect that point. The resulting effect is that the tire will scrub and lose traction as it turns. Potentially causing a loss of traction in low grip slick road scenarios. You'll notice it in tight turns in a parking structure for example with the tires squealing. It will pull on the steering as significant acceleration or braking force is applied. Steering effort will increase.

View attachment 2914294

Sure, all this can be dismissed. It does matter however slight one imagines the impact to be.

This video really helps explain things:

Its worth chasing scrub in all regards and this is all important information, but i want to add a couple things.

Factory scrub is not zero. Its slightly positive by design. Also every single truck ive been loaned at lexus squeals as do most double wishbones.

The other thing is while all this is 100% factual, wheels do not steer in plane. So don’t confuse negligibility of scrub to be a blank check but there is a lot of other movement involved than just steering angle.

No 2 vehicles or makes are the same. I would advise +35 as the ideal and +25 as the very lower bound for us but this is dependent on fitment. The way i will rectify my scrub is a nominal 34” which i should have done off the bat.

 

[linuxgod]

is it dangerous to drive on too much positive scrub radius?

Yes. The question is just how much is too much though. Assuming stock tires and wheels, you're unlikely to notice any difference even under extreme handling if you've changed from OEM +60 offset to +59 offset and added a fraction of an inch of positive scrub. But if you imagine adding 10' of spacers to either side of the vehicle (making your vehicle track width like 26' wide) then any weight shift during acceleration or braking (and especially movement while turning) will be exacerbated by the ridiculously long lever and will make everything you do feel squirrelly.

 

[TeCKis300]

There's more to this. Now that there's generally a better grasp of how offset impacts scrub radius, and resulting symptoms. To complicate things...

Lift effects the Kingpin Inclination Angle (KPI). Stock is ~13 degrees. When lifted, this can change to something closer to ~10-11 degrees, depending on the amount of lift and UCA adjustment. In a dual wishbone suspension, the upper and lower arms are unequal length. Hence they follow a different arc (green) that changes the KPI through its stroke.

Result - with more lift, generally need to bias towards more offset (not less). For example, take a 33"" spec diameter tire at stock ride height, which wants something like 45mm offset. Lift say 2.5", and ideal may be closer to 50mm.

Another factor can be alignment. Camber has a big impact to the effective centerline of the tire. Particularly a wider tire. Most alignments have the camber just square or positive on the LC. LX has square or just negative. Difference likely to due to amount of brake dive from the suspension, where AHC has an anti-dive mechanism, and the differing alignment may be to maximize braking traction.

Both factors are honestly less impactful, but since we're talking about the tech of it. Though they could be used as tools too. For example, more negative camber could be used to adjust an aggressive offset wheel to handle better from a scrub radius perspective. Better turn in, handling, and minimize the tire sidewall from rolling over as much.

 

[TCM]

Regarding scrub radius, it cannot be assumed that the factory specification for the LC 200 is zero. Nearly all OEM suspensions for RWD and AWD vehicles are designed with positive scrub radius as it contributes to handling stability (FWD cars often have a negative scrub radius to reduce torque steer).

Not knowing the exact number for the LC we should assume it is positive. When making wheel and tires changes it is best to err on the the side of adding scrub radius because subtracting scrub radius could bring the actual figure close to zero which would result in handling instability especially under hard braking.

If anyone knows what the LC 200 specification is, please post it up here. I have been unable to locate even the hint of a number after multiple internet deep dives.