Caster Provides Stability and Better Feel for Supermodified Drivers
By Group R Motorsports owner Bob Bogwicz
Caster provides straight line stability to the racecar and prevents it from wandering while going up the straightaways. A wander at 130 MPH never ends well, so we dial in some caster.
I know, I’m a little late getting this ground breaking article to Bobby G. this month. But only because I’ve been slaving away at my day job so I can keep the lights on AND build a new supermodified! Hopefully, this new car will be the source of upcoming articles and pictures (I can’t show you any pictures of my nitrous system or traction control-sorry!), so stayed tuned.
Today’s pit-side chat is about a topic that every vehicle on the road, including shopping carts needs. It’s called caster.
Caster is the angle of the steering axis of inclination in reference to a vertical line. As shown in Figure 1, the outside view of a RF tire shows a caster angle of 20 degrees. The steering axis of inclination on a supermodified with a solid front axle is the angle of the spindle kingpin. The kingpin is the rugged steel pin that connects the spindle to the axle and allows the spindle to rotate and steer the car.
Note how the steering axis of inclination intersects the ground AHEAD of the vertical line. This is designated as “positive caster”
Caster angle can be established in different ways, depending on the type of front suspension used.
For a racecar that has an independent front suspension, caster angle is set by the relative position of the upper and lower ball joints. If one were to draw an imaginary line through the upper and lower ball joints and that line intersects the ground forward of the vertical line, it is positive caster just like Figure 1.
With a straight axle supermodified, caster is set by the angle of the king pin that attaches the spindle to the axle. This is accomplished by rotating the axle assembly to achieve the desired angle. If the top of the king pin is towards the rear of the car, positive caster is established, again as in Figure 1.
So now you’re saying “jeez, that’s swell Bogwan, but why do we need caster, anyways?” Quite simply, for stability.
Caster creates a force in the steering that wants to “self-center” the front wheels. This is illustrated in Figure 2, which is the top view of a RF tire turning left. The vertical line which intersection the ground is the dot in the center of the tire. This is effectively the geometric center of the contact patch. The steering axis of inclination intersection is the dot forward of the vertical axis dot (remember, positive caster!). Note the distance between the two. We’ll call this distance a “Lever Arm”.
When the wheel is turned to the left, the centrifugal force due to cornering act on both points equally. However, because of the “lever arm”, the force on the steering axis point creates a torque (torque is a force applied to a lever arm that creates rotation.) This torque wants to force the tire back to the “straight ahead” position.
Caster provides straight line stability to the racecar and prevents it from wandering while going up the straightaways. A wander at 130 MPH never ends well, so we dial in some caster. In addition to the self centering force, caster also provides feedback through the steering to the driver. An experienced driver will use this feel to his or her advantage.
Now, a little disclaimer: The 20° of caster show in the figures is an exaggeration for clarity. In real life the caster on my supermodified is set to +7° on the RF and +2° on the LF. There is a difference in caster between the LF and RF because the car wants to turn in the direction of lesser caster. So, less LF caster helps the car to turn to the left. There is no such thing as rear caster because those tires don’t steer the car.
Just like camber, caster is initially adjusted in the garage according to a race teams notes or the chassis builders recommendation but instead of using tire temperatures to fine tune the adjustment, caster is more of a “driver feel” type of variable.
So, next time you’re in your local supermarket, grab a shopping cart and watch how the front wheels are always stable when going straight ahead.
This is caster in action! Now, turn left at the dairy section and go fast.
Next months installment of Supermodified Tech Talk will be about tire temperatures. Reading tire temperatures is an art AND a science so you won’t want to miss this one!
Thank you for your questions about supermodified technology. If you have something you’d like to see covered in a future Supermodified Tech Knowledge column, you can contact me at motosports@groupRtech.com. Follow me @groupRmtrsports and submit questions via Twitter as well.
The Bogwan enjoys testing his shopping cart caster in the less than 8 items check out lane. If you yearn for more supermodified tech knowledge leave a comment below or Hit the Wailbag and The Bogwan will answer them from the beer aisle of a supermarket near you.