Automotive Relationship Between Tire/Wheel Size and Alignment

[BaconTaco]

As the title says, how will installing a larger tire/wheel combination or altering wheel offset change the alignment with respect to caster, camber, and toe?

I know that the parameters are affected, but I cannot verbalize or visualize these changes. Feel free to use big words and jargon.

TIA!

 

[jerromyjon]

Hi and welcome to PF!
Changing wheel size/offset will not affect the settings as they are, the wheels are aligned to the vehicle dependent upon the suspension design, which is in turn designed usually for a specific size and offset tire/rim combination and handling characteristics. Alignment adjustments may or may not compensate for the changes depending on the design.

 

[J Mc]

By larger, do you mean width or diameter?

 

[Simas]

By changing the wheel offset, you change the tire "scrub radius", and this will affect the handling.
http://www.auto-repair-new-port-richey.com/image/scrub_radius.jpg

If you change the tire diameter but keep the the same tire width and same rim, the tire sidewall height will change which will affect the tire stiffness and this will also affect the handling (and moreover of course the transfer of transmission torque to road-contact force).
If you change the tire width, this will again also change the handling because it also affects the tire stiffness.

Thus, as jerromyjon said above, changing the wheel offset or tire size will not change the alignment. However, it will change the handling (handling depends among others of the aligment and the tire). Consequently, if you want to keep the same handling with different wheel offset or different tire size, you will have to change the alignment such as caster, toe, and camber.

 

[Ranger Mike]

Any time you modify or deviate from the factory specs you run the risk of screwing things up big time. Millions of design dollars have been spent to engineer the automobile to be a balance of performance and comfort for the driver. And the driver varies between the little old lady driving to the market to the hot rod teenager thinking the grocery getter is an Indy car. That being said, the front suspension is a huge compromise. The A-Arm or control arm on McPherson strut suspensions is made to move a certain amount to yield optimum camber range. It is a central part of the entire front end geometry calculation. Note the attached diagram on front end calculations. Lot of intersect points!
Caster is dialed in for best turning vs. steering effort. Toe is dialed into minimize steering feel and best fuel mileage/tire wear. When you switch to a taller tire you change the intersection points of the suspension. When you add a wider tire you change the scrub radius and effectively soften the spring rate ( wheel rate) miniscule as it is. You also impact wheel bearing wear with wider offset wheels. The whole point of this is to warn you that you will change the handling , tire life and wheel bearing life. Who wants a wheel bearing seizing up at 50 mph?

It does happen. Going too wild on 20” rims because they look cool means the tire will hit the strut and you are only able to crank the steering wheel 15 Degrees and it takes a corn field to turn the darn thing 90 degrees..

 

[BaconTaco]

By larger, do you mean width or diameter?

Both.

Specifically, this question was sparked by a "debate" (read: argument) over whether or not installing over sized tires required a new alignment.

In this particular case I am referring to a wider and larger diameter tire mounted to a monobeam axle; that is a mud tire on aftermarket wheels on a 4x4 3/4 ton pick up, or bro dozer. A flat bill hat and white Oakleys may or may not be a part of the equation.

So, we are talking about a steering knuckle w/ upper and lower ball joints, two outer tie rod ends, and a unitized hub bearing.