Can anyone with a Sasquatch Bronco measure their scrub radius?

[North7]

Between the Sasq and Non-Sas there is also a slight lift difference (0.5" or so?)

The difference is 0.67", my analysis and data are here:

The Definitive Guide to 2021+ Bronco Ride & Lift Heights with Wheel & Tire Sizes

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[MillerAndCheeto]

The difference is 0.67", my analysis and data are here:

The Definitive Guide to 2021+ Bronco Ride & Lift Heights with Wheel & Tire Sizes

This post you did was amazing, by the way. I bet probably every person on this forum has read it at least once. As you can tell with my Q on scrub radius I'm a details guy, and I am super appreciative of all the work you and others have done.

For the 0.5" reference I was referring to the Sasquatch vs Badlands specifically, I just realized I never put that anywhere in my post!

 

[webspoke]

The electric steering rack may also have some calibrations to mask scrub radius differences, if not different settings for base, badlands. sas. When I switched from badlands to sas wheels (55->30 ET), there was no seat of the pants change in the steering effort, feel, kickback etc.

 

[MillerAndCheeto]

The electric steering rack may also have some calibrations to mask scrub radius differences, if not different settings for base, badlands. sas. When I switched from badlands to sas wheels (55->30 ET), there was no seat of the pants change in the steering effort, feel, kickback etc.

I appreciate your input, thank you very much. What's interesting is if you compare a stock Badlands with 285/70R17 +55 with a 315/70R17 +30, the additional height coupled with the pushing out of the wheel means that the SR is not increased by 25 mm it's only increased by 19 mm (theoretically, as this is assuming tire squish is the same even though going from an 8" rim to an 8.5" rim).

 

[Bradley Thornton]

I have Factory Sasquatch wheels and KM3 37x12.5s with 1/4in spacers for the tire markings BUT I didn't use the 1/4in spacer when making my marks in the ground for the rotor only the tires. So shift tire 1/4in back for your #s)

(The 37X12.50R17 BFGoodrich Mud-Terrain T/A KM3 has a diameter of 36.5", a width of 12.5", mounts on a 17" rim and has 569 revolutions per mile.)

So I checked the rock rails at 18in then I took a measurement of the lower controller arm at 11.5in that was the lowest point the bolt that points down. After this I marked the tire then took it off. let the truck down to 18in then jacked the lower controller arm up check all measurements to make sure they were the same as with tire on then made the other marks.

I will be running the 1/4in spacer (How does my scrub Radius look?)

 

[MillerAndCheeto]

I have Factory Sasquatch wheels and KM3 37x12.5s with 1/4in spacers for the tire markings BUT I didn't use the 1/4in spacer when making my marks in the ground for the rotor only the tires. So shift tire 1/4in back for your #s)

(The 37X12.50R17 BFGoodrich Mud-Terrain T/A KM3 has a diameter of 36.5", a width of 12.5", mounts on a 17" rim and has 569 revolutions per mile.)

So I checked the rock rails at 18in then I took a measurement of the lower controller arm at 11.5in that was the lowest point the bolt that points down. After this I marked the tire then took it off. let the truck down to 18in then jacked the lower controller arm up check all measurements to make sure they were the same as with tire on then made the other marks.

I will be running the 1/4in spacer (How does my scrub Radius look?)

Dude you rock!! Thanks so much for this! Being able to check the ride height and control arm height and controlling where it all lines up for getting the measurements is pretty awesome.

So it looks like the intersection of the steering axis is at approximately 5 15/16" and the center of your tire at 6.25" gives your current ride - with 0.25" spacer - a scrub radius of -5/16" or -8mm. From everything I've read online, anywhere between +15 mm to -15 mm is the sweet spot that engineers design their vehicles, so this seems like it's dead-on for your current ride setup.

If I take the spacer out, the ball steering axis mark stays the same but the tire would be pushed towards your truck, or the "ball" marker pushed away from the truck relative to the wheel.

This gives your +30mm offset rim on those tyres approximately a -9/16" or -14mm scrub radius without a spacer.

Using the online tool for comparing wheels/offsets etc. of a 315/70R17 to your 37x12.5 (approximately 36.5" height) then the stock Sas should have an approximate offset of -6mm (-0.25") scrub radius. So in my thoughts of changing from the +30mm offset tyres to a 0mm offset my overall scrub radius would be approximately +25mm. Changing from slightly negative to the positive side has its own implications but at only ~1" scrub I'm MUCH less concerned than if I started at +10mm and was moving to +40mm scrub. I'm sure there's a solid 5-7 mm +/- accuracy from looking at marks on the ground and using an online calculator, but this is great information.

@North7 you seem to know your shit. Can you double check my math?

 

[North7]

Dude you rock!! Thanks so much for this! Being able to check the ride height and control arm height and controlling where it all lines up for getting the measurements is pretty awesome.

So it looks like the intersection of the steering axis is at approximately 5 15/16" and the center of your tire at 6.25" gives your current ride - with 0.25" spacer - a scrub radius of -5/16" or -8mm. From everything I've read online, anywhere between +15 mm to -15 mm is the sweet spot that engineers design their vehicles, so this seems like it's dead-on for your current ride setup.

If I take the spacer out, the ball steering axis mark stays the same but the tire would be pushed towards your truck, or the "ball" marker pushed away from the truck relative to the wheel.

This gives your +30mm offset rim on those tyres approximately a -9/16" or -14mm scrub radius without a spacer.

Using the online tool for comparing wheels/offsets etc. of a 315/70R17 to your 37x12.5 (approximately 36.5" height) then the stock Sas should have an approximate offset of -6mm (-0.25") scrub radius. So in my thoughts of changing from the +30mm offset tyres to a 0mm offset my overall scrub radius would be approximately +25mm. Changing from slightly negative to the positive side has its own implications but at only ~1" scrub I'm MUCH less concerned than if I started at +10mm and was moving to +40mm scrub. I'm sure there's a solid 5-7 mm +/- accuracy from looking at marks on the ground and using an online calculator, but this is great information.

@North7 you seem to know your shit. Can you double check my math?

Your math seems on the right track, here are a few thoughts. While your getting your scrub radius back to the sweet spot, the old rule of thumb that lifting your truck will be harder on all drive train components may mean your still going to get wear somewhere, as you said " might not be a great idea for longevity of the components".

Going with a 0mm offset wheel will give you a lot of poke, slinging mud and gravel at your paint, unless you get wider fender flares &/or mud guards, or you may not care about this.

Lets see if we can have a Ford suspension engineer give us his thoughts, @TeocaliMG can you give your insight please?

 

[North7]

@MillerAndCheeto I would also keep your eye on @tmason10 steering rack fix he is going to market soon.

Details & update for 2021 Bronco Badlands with steering rack failure on King of the Hill K2 Trail

Yes we broke the first day! I then prototyped a new part that solves the 2021 steering rack problem. Then went out three days later and was a huge success

Plan is still to eventually go Hydro. But currently patenting fix on the electric rack that will allow people to safely install lift and tires.

Thank you it’s been fun! I’ll fave prototype steering fix real soon to market.

 

[TeocaliMG]

Your math seems on the right track, here are a few thoughts. While your getting your scrub radius back to the sweet spot, the old rule of thumb that lifting your truck will be harder on all drive train components may mean your still going to get wear somewhere, as you said " might not be a great idea for longevity of the components".

Going with a 0mm offset wheel will give you a lot of poke, slinging mud and gravel at your paint, unless you get wider fender flares &/or mud guards, or you may not care about this.

Lets see if we can have a Ford suspension engineer give us his thoughts, @TeocaliMG can you give your insight please?

Hey North7, I am obligated to mention that I anything I say is not in any official capacity, but as a fellow Bronco owner and off-road enthusiast and general fan of vehicle dynamics.

First thing I would point out is that the online tool can only do so much in approximating scrub. You really need to know the king pin angle and the hub face location with respect to the ball joints making the king pin axis. So I would be very careful to assume that you are starting with a negative scrub radius and moving to a positive scrub radius. You could be moving from slightly positive to very positive. Marking/scrubbing the ground is a crude but creative approach to check this though.

Obviously the lower the scrub radius the better. Also, generally speaking the further out your wheels are from the hub/chassis, the greater the load on the wheel bearings and ball joints, thats another reason to keep the poke to a minimum.

As mentioned above, increasing the tire diameter will move the intersection of the king pin axis through the contact patch outward from the vehicle. Adding spacers has the opposite effect. I may do some scrub test on my non sasquatch badlands to see how it compares.

 

[johndeerefarmer]

I asked months ago what the scrub angle was and I got the same answer " why do you want to know"?
It appears that a majority of the new Bronco owners must not be gearheads and will be paying the dealer to change their oil. LOL

 

[MillerAndCheeto]

Hey North7, I am obligated to mention that I anything I say is not in any official capacity, but as a fellow Bronco owner and off-road enthusiast and general fan of vehicle dynamics.

First thing I would point out is that the online tool can only do so much in approximating scrub. You really need to know the king pin angle and the hub face location with respect to the ball joints making the king pin axis. So I would be very careful to assume that you are starting with a negative scrub radius and moving to a positive scrub radius. You could be moving from slightly positive to very positive. Marking/scrubbing the ground is a crude but creative approach to check this though.

Obviously the lower the scrub radius the better. Also, generally speaking the further out your wheels are from the hub/chassis, the greater the load on the wheel bearings and ball joints, thats another reason to keep the poke to a minimum.

As mentioned above, increasing the tire diameter will move the intersection of the king pin axis through the contact patch outward from the vehicle. Adding spacers has the opposite effect. I may do some scrub test on my non sasquatch badlands to see how it compares.

One quick clarification that may have gotten lost in translation. @Bradley Thornton and I came up with an idea that you could measure the truck's rock rail from ground, and the lower control arm from the ground, and mark the tire width on the concrete. Proceed to lift the truck (straight up with a 2 post) and remove a tire, then lower until the rock rail is at the starting height, stick a jack under the lower control arm and position it in it's starting position so you have essentially what is a "Bronco x-Ray" of the suspension, imitating all the angles as would be observed sitting idle with all 4 tires on the ground. He then used a straight line between the upper and lower steering axis and followed to the ground for the "ball" marker on his pics. While I admit not not perfect, I think it's far more accurate than just moving the tires left/right and recording scrub.


Thoughts?

 

[MillerAndCheeto]

I asked months ago what the scrub angle was and I got the same answer " why do you want to know"?

Information is always good. "The more you know" as they say rings true - never a bad idea to make informed decisions.

 

[TeocaliMG]

One quick clarification that may have gotten lost in translation. @Bradley Thornton and I came up with an idea that you could measure the truck's rock rail from ground, and the lower control arm from the ground, and mark the tire width on the concrete. Proceed to lift the truck (straight up with a 2 post) and remove a tire, then lower until the rock rail is at the starting height, stick a jack under the lower control arm and position it in it's starting position so you have essentially what is a "Bronco x-Ray" of the suspension, imitating all the angles as would be observed sitting idle with all 4 tires on the ground. He then used a straight line between the upper and lower steering axis and followed to the ground for the "ball" marker on his pics. While I admit not not perfect, I think it's far more accurate than just moving the tires left/right and recording scrub.


Thoughts?

That would be more accurate but if you use a jack under the control arm the ride height/geometry will not be retained as the load is further inboard than before. You could add weight to get the rocker back down to where it was but that sounds difficult and dangerous. Maybe you can juggle jacks to get a jack right under the control arm ball joint (not possible with the wheel on as far as I know) but even then the geometry/loading might not be exactly the same.

Just brainstorming here, but after marking the contact patch of the tire on the ground (inside and out) you steer lock to lock. At each lock you hold a tire sized section of plywood flat against the tire and trace a straight line along the base while marking the exact center of the wheel contact at that line. Then use a square to trace from the line and center at each full lock and the intersection of each line is the turning axis. There is a lot of room for human error but I think it could be reasonably accurate if done very carefully.

Edit: As a check, the intersection should occur on the same line as the "center" when steered straight ahead. So you could take multiple intersections and check them against each other or average them out.

 

[MillerAndCheeto]

That would be more accurate but if you use a jack under the control arm the ride height/geometry will not be retained as the load is further inboard than before. You could add weight to get the rocker back down to where it was but that sounds difficult and dangerous. Maybe you can juggle jacks to get a jack right under the control arm ball joint (not possible with the wheel on as far as I know) but even then the geometry/loading might not be exactly the same.

Just brainstorming here, but after marking the contact patch of the tire on the ground (inside and out) you steer lock to lock. At each lock you hold a tire sized section of plywood flat against the tire and trace a straight line along the base while marking the exact center of the wheel contact at that line. Then use a square to trace from the line and center at each full lock and the intersection of each line is the turning axis. There is a lot of room for human error but I think it could be reasonably accurate if done very carefully.

Edit: As a check, the intersection should occur on the same line as the "center" when steered straight ahead. So you could take multiple intersections and check them against each other or average them out.

Oh dang, that is a great idea.

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