Steering Caster affected by taller tires (and no other changes)?

[ksdon]

Do taller tires and no other changes affect Caster?

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[freetors]

Taller tires do not affect caster. They do however increase the trail distance, the measurement from the center of the contact patch to the steering axis. This increases stability but steering effort increases a proportional amount too. In practice though, taller tires tend to get wider, requiring more steering torque, but they also tend to tramline more.

One more positive effect that a taller tire gives (with NO other changes) is moving the scrub radius point inboard. Assuming your starting with a negative scrub radius like most RWD and 4WD cars have, this will make your tires pivot point closer to its centerline. This has a huge effect on steering effort when a front diff lock is being used.

 

[ksdon]

thank you for the response.

full disclosure: i'm going to have to look up a few terms but I do appreciate the help.

 

[goatman2]

thank you for the response.

full disclosure: i'm going to have to look up a few terms but I do appreciate the help.

Castor, Camber, and toe in adjustment won't change with bigger tires, since nothing in the suspension and knuckle has changed. The scrub radius will change based on wheel offset. The scrub radius is the arc of movement when the tire/wheel/knuckle turns relative to the ball joint. Ball joint is the center or pivot point, the arc of movement is at the center of the tire. As the wheel offset changes, the actual position of the center of the tire changes relative to the ball joint. If the wheel/tire center is more outward, the distance of the arc of movement of the center of the tire is moved further away from the ball joint, and this is an increased scrub radius. Increased scrub radius will put more strain on the steering, and can/will take more steering effort to turn the tires.

BTW, when folks report that they have broken a tie rod, it would be helpful to know what wheel offset and tire size they are running.

 

[Beach_Bum]

Castor, Camber, and toe in adjustment won't change with bigger tires, since nothing in the suspension and knuckle has changed. The scrub radius will change based on wheel offset. The scrub radius is the arc of movement when the tire/wheel/knuckle turns relative to the ball joint. Ball joint is the center or pivot point, the arc of movement is at the center of the tire. As the wheel offset changes, the actual position of the center of the tire changes relative to the ball joint. If the wheel/tire center is more outward, the distance of the arc of movement of the center of the tire is moved further away from the ball joint, and this is an increased scrub radius. Increased scrub radius will put more strain on the steering, and can/will take more steering effort to turn the tires.

BTW, when folks report that they have broken a tie rod, it would be helpful to know what wheel offset and tire size they are running.

Last statement can't be emphasized enough. The majority of aftermarket wheels start with a zero offset or negative offset. Many buyers are only looking at the design unaware of the stresses that are applied when opting for a wider/taller stance.

 

[JohnnyBronco]

Castor, Camber, and toe in adjustment won't change with bigger tires, since nothing in the suspension and knuckle has changed. The scrub radius will change based on wheel offset. The scrub radius is the arc of movement when the tire/wheel/knuckle turns relative to the ball joint. Ball joint is the center or pivot point, the arc of movement is at the center of the tire. As the wheel offset changes, the actual position of the center of the tire changes relative to the ball joint. If the wheel/tire center is more outward, the distance of the arc of movement of the center of the tire is moved further away from the ball joint, and this is an increased scrub radius. Increased scrub radius will put more strain on the steering, and can/will take more steering effort to turn the tires.

BTW, when folks report that they have broken a tie rod, it would be helpful to know what wheel offset and tire size they are running.

Also would be extremely helpful to know if, fo example, vast majority are driver side or if breaks are evenly divided. It would to me indicate something inherently wrong with suspension geometry on that side.

I still think that if there were a physical steering stop prior to full rack extension that the tie rods would not be in danger of touch force applied by the wheel/tire.

 

[Dingbat]

Roll center will be affected. Where the center of the tire contact patch meets the surface it is sitting on is one of the points that are used to determine roll center. The other points are all generated off suspension pivots. The distance between the two increases with taller tires which results in a different roll center.

 

[ksdon]

Thanks for the good inputs. Steering really is science.

 

[Silver-Bolt]

Thanks for the good inputs. Steering really is science.

Geometry.

 

[goatman2]

Roll center will be affected. Where the center of the tire contact patch meets the surface it is sitting on is one of the points that are used to determine roll center. The other points are all generated off suspension pivots. The distance between the two increases with taller tires which results in a different roll center.

And center of gravity. Both are raised slightly relative to the ground. Besides a higher center of gravity, the driver will also feel more give in the taller tires. Taller tires are for off road ground clearance, any effect on street handling will be negative.

Lift kits also raise the roll center and center of gravity. Most will do lifts and larger tires for looks, which is fine, but it will have a negative effect on street handling. Nothing new here.

 

[flip]

Most manufacturers have taken away caster and camber to some degree. Hell, we still bend axles like they did 80 years ago to make camber adjustments on some vehicles.

Front Camber and Caster Adjustment
Special Tool(s) / General Equipment

Wheel Alignment System

Adjustment

NOTICE: Suspension fasteners are critical parts that affect the performance of vital components and systems. Failure of these fasteners may result in major service expense. Use the same or equivalent parts if replacement is necessary. Do not use a replacement part of lesser quality or substitute design. Tighten fasteners as specified.

1. Using alignment equipment and the manufacturers instructions, measure the front caster and camber.
   • For optimal alignment settings,
     Refer to: Specifications (204-00 Suspension System - General Information, Specifications).
   • If the caster and camber values are not within specification, go to the next step.
     Use the General Equipment: Wheel Alignment System

1. NOTE: Only tighten the nuts and bolts finger tight at this stage.
   On both sides.
   Loosen the lower arm bolt(s) and adjust the caster and camber settings according to manufacturer's instructions.

1. To adjust the caster and camber, refer to the following chart.
   • Adjustments that require moving the front and the rear of the lower arm should be made equally.
     
     
     
     

     Adjustment	Front of Lower Arm	Rear of Lower Arm
     Increase caster​	In	Out
     Decrease caster​	Out	In
     Increase camber​	In	In
     Decrease camber​	Out	Out
     Increase caster and camber simultaneously​	In	—
     Decrease caster and camber simultaneously​	Out	—

1. On both sides.
   With the weight of the vehicle resting on the wheel and tire assemblies, hold the cam bolts and tighten the nuts.
   Torque: 195 lb.ft (265 Nm)

1. Check and if necessary, adjust front toe.
   Refer to: Front Toe Adjustment (204-00 Suspension System - General Information, General Procedures).

 

[Dingbat]

And center of gravity. Both are raised slightly relative to the ground. Besides a higher center of gravity, the driver will also feel more give in the taller tires. Taller tires are for off road ground clearance, any effect on street handling will be negative.

Lift kits also raise the roll center and center of gravity. Most will do lifts and larger tires for looks, which is fine, but it will have a negative effect on street handling. Nothing new here.

If I’m not mistaken, in the rear of the Bronco, with the panhard rod, both CoG and RC will be raised together. In the front with the IFS, it’s not unlikely the RC would lower* while the CoG raises. Would have to have specifics on the suspension geometry for definitive answer.

You’re right though, CoG would go up, which rarely affects handling in a positive way. Good news is it seems there are plenty of folks running taller tires on these vehicles without any tremendous downsides where handling is involved.

*Eta: Front roll center would go up but would lower in relation to the CoG.

 

[goatman2]

If I’m not mistaken, in the rear of the Bronco, with the panhard rod, both CoG and RC will be raised together. In the front with the IFS, it’s not unlikely the RC would lower* while the CoG raises. Would have to have specifics on the suspension geometry for definitive answer.

You’re right though, CoG would go up, which rarely affects handling in a positive way. Good news is it seems there are plenty of folks running taller tires on these vehicles without any tremendous downsides where handling is involved.

*Eta: Front roll center would go up but would lower in relation to the CoG.

Two elements here with just a tire change. Roll center won't change relative to the chassis/body, will only raise relative to the ground since the whole car sits higher. Nearly identical with a change in center of gravity. In the rear the track bar/panhard only affects the roll center, has nothing to do with CoG. CoG is based on weight, not suspension geometry. Roll center with a track bar is basically at the height of the bolt/pivot at the frame side mount. Have no idea where the CoG is on a Bronco.

In an off road car, a higher roll center resists body roll in off camber situations, but it needs to be high to matter much. However, a high roll center reduces driver feel, so you can get closer to the edge without feeling it so much. In road race cars the roll center is put very low to increase driver feel, and stability is done with spring rates, sway bars and other geometry.

Roll center relative to CoG will affect handling and feel. I'm not up on IFS/A arm geometry, I race Jeeps and buggies. Would have to get my books out again.

 

[Dingbat]

Two elements here with just a tire change. Roll center won't change relative to the chassis/body, will only raise relative to the ground since the whole car sits higher. Nearly identical with a change in center of gravity. In the rear the track bar/panhard only affects the roll center, has nothing to do with CoG. CoG is based on weight, not suspension geometry. Roll center with a track bar is basically at the height of the bolt/pivot at the frame side mount. Have no idea where the CoG is on a Bronco.

In an off road car, a higher roll center resists body roll in off camber situations, but it needs to be high to matter much. However, a high roll center reduces driver feel, so you can get closer to the edge without feeling it so much. In road race cars the roll center is put very low to increase driver feel, and stability is done with spring rates, sway bars and other geometry.

Roll center relative to CoG will affect handling and feel. I'm not up on IFS/A arm geometry, I race Jeeps and buggies. Would have to get my books out again.

It’s been a long time for me as well and I used to race Mac strut/torsion beam stuff which is why I was completely forgetting to consider the back end in my original comment. Started to wonder if I even still had my old papers but then remembered we live in the magical age of the Interwebs now.

Behold… https://suspensionsecrets.co.uk/roll-centre-and-roll-moment/

So on the front end, what the article is calling instant center remains constant in relation to the chassis but the roll center is determined by the intersection of the lines from the instant centers to the center of the contact patches. As the radius of the tires increases the instant centers will rise with the chassis but where those lines intersect will not rise at the same rate. Assuming the CoG remains in about the same position relative to the chassis the roll center will likely drop in relation to it, though it would rise in relation to the surface the tires are on.

We we’re doing everything we could to raise roll center in our racing cars but they were front wheel drive, econo-box, production cars so we weren’t starting from a great place to begin with.

 

[acetdeucy]

Castor, Camber, and toe in adjustment won't change with bigger tires, since nothing in the suspension and knuckle has changed. The scrub radius will change based on wheel offset. The scrub radius is the arc of movement when the tire/wheel/knuckle turns relative to the ball joint. Ball joint is the center or pivot point, the arc of movement is at the center of the tire. As the wheel offset changes, the actual position of the center of the tire changes relative to the ball joint. If the wheel/tire center is more outward, the distance of the arc of movement of the center of the tire is moved further away from the ball joint, and this is an increased scrub radius. Increased scrub radius will put more strain on the steering, and can/will take more steering effort to turn the tires.

BTW, when folks report that they have broken a tie rod, it would be helpful to know what wheel offset and tire size they are running.

Since it looks like you're in the business of alignment, I ask you to PLEASE take a minute to read and comment on the included Post (kinda related) that I created a couple days ago... It didn't get many views, so I didn't get feedback about my issues with Castor... or the solution by my aligner.

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