Been thinking again about suspensions

Basically what I was thinking about was tall front uprights for camber control. Basically the taller the upright the less the suspension arms change the camber of the wheel because the side to side motion induced by the arms rotates the upright in the camber plane in inverse proportion to the distance between the ball joints.

GLOSSARY

Camber – The angle between the tire and the road surface. Positive is the top of the tire tilted away from the center of the car, negative tilts toward the center of the car. This is backwards because positive camber is almost universally bad for tire grip and is therefore a negative, but the terms were defined prior to the invention of the pneumatic tire and now we are stuck with them.

Kingpin inclination – Another term that is archaic because kingpins are from solid axles except for a very tiny number of early independent front suspensions. It refers to the angle of the axis of steering rotation for front wheels between the upper and lower ball joints.

Scrub radius – The distance between the intersection of the steering axis with the ground and the center of the wheel. Basically this is the moment arm of the force generated by the brakes through the steering system. The greater the absolute value of this number the larger the force felt at the steering wheel under braking. The direction of this force depends on the positive or negative value of the radius. If positive the steering wheel will turn in the direction of the greatest braking force, and opposite if negative. Now there are schools of thought that if the system is designed to turn away from the stronger braking force it will be self-correcting in the case of brake failure. There are other schools of thought that drivers will instinctively turn away from forces trying to steer the car so positive scrub radius is better, with the debate coming down to do you want your driver to be an active part of the car.

Upright height – The distance between the upper and lower ball joints of an independent front suspension

This is a hypothetical discussion because the front suspension for the Sprint-T has been frozen as a straight axle, and because my fabrication tools are not up to building suspension uprights. So theory only. No labs for this lesson (shoutout to my teacher friends who “get” this joke). Theory for this starts with the statement from the opening paragraph camber change is inversely proportional to the distance between the ball joints. Second is that the position of the bottom joint is constrained by the dimensions of the wheel used, because kingpin inclination and reducing scrub radius dictate the location somewhere inside the wheel. Excessive positive scrub radius has been proven bad experimentally by excessively increasing the force required to steer under braking.

And it just dawned on me this post needs a glossary to keep track of all the technical terms. We already have camber, kingpin inclination, and scrub radius besides upright height. Basically I’m assuming my reader has a working knowledge of terms used in suspension design, which is probably not the case. So I’m inserting one following the first paragraph. It’s there now but wasn’t before I started this paragraph.

Anywho, the design parameters basically dictate the lower ball joint be as far inside the wheel as possible constricted by the brake disk because of scrub radius minimization. It doesn’t matter if you want positive or negative scrub radius, you still want it to be small or variations in braking force caused by changes in road surface can tear the steering wheel out of the driver’s hands which is bad no matter how you look at it.

Also, the amount of camber change caused by body roll is proportional to the ratio of the distance between pivot points on the frame and the distance between the pivot points on the upright. Desirable geometry of defining the instant center dictate this is going to be close to unity and usually less than 1:1. There are some old GM cars that had this greater than 1:1 and they understeered excessively with the outside front wheel riding on the sidewall of the tire. Look for images of the late ’60s Chevelles making hard turns and you can see this in action, but not many of those pictures made it to the internet and the only ones I have are in paper books on suspension and my cell phone camera did not get a good picture of the picture. One of the things about having the ratio between the distance of the suspension pivots greater than unity is the front roll center is underground, which causes the body to roll excessively and further causes the car to ride on the sidewall of the outside front tire. This meant that racers using this car as a base for stock cars had to crank in ridiculous amounts of static camber and excessive amounts of roll stiffness to keep the car driving on the tire treads instead of the sidewalls. They made it work but it would have been easier if the front suspension was designed right in the first place.

Getting back on track for this essay without any more digression (squirrel!), we want to basically define a height range for the front upright. Too tall and either we get “funny” geometry in the suspension or we get the pivots for the upper suspension arm somewhere in the boonies, which technically is “funny” geometry. Too short and we get poorly controlled camber as the car goes through bump and/or roll. My personal take is I design about the same as the inside diameter of the front wheel as my max, and about 8″ as my minimum except for suspension karts. No matter what you do those are “funny” because no room.

And I seem to have lost the point of this post except that I woke up in the middle of the night thinking about front suspension uprights. So since we hit over 1000 words this seems like a good place to stop and hope I get my train of thought back on the rails for a future post.

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