Kingpin length affects

[Bookhout]

I am currently designing a front suspension for a hypothetical formula car, since my school didn't provide enough money to fund a team to design and build the entire car.

I came across a statement in various texts (including RCVD) regarding kingpin length and how it affects the forces on the suspension components (such as the a-arms).

But what I can't find is an explanation of why this is true. Does anyone know why an increased kingpin length would decrease the forces on the suspension components?

[Bookhout]

I am currently designing a front suspension for a hypothetical formula car, since my school didn't provide enough money to fund a team to design and build the entire car.

I came across a statement in various texts (including RCVD) regarding kingpin length and how it affects the forces on the suspension components (such as the a-arms).

But what I can't find is an explanation of why this is true. Does anyone know why an increased kingpin length would decrease the forces on the suspension components?

[TG]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Bookhout:
I am currently designing a front suspension for a hypothetical formula car, since my school didn't provide enough money to fund a team to design and build the entire car.

I came across a statement in various texts (including RCVD) regarding kingpin length and how it affects the forces on the suspension components (such as the a-arms).

But what I can't find is an explanation of why this is true. Does anyone know why an increased kingpin length would decrease the forces on the suspension components? </div></BLOCKQUOTE>

I'm not quite sure what you mean by kingpin length. Do you mean kingpin offset? Kingpin offset is the distance between the center of pressure of the tire contact patch and where the axis that the upright/tire rotates about intercepts with the ground, looking at the suspension from the front. Looking from the side, this distance is know as the caster offset. Many people combine these distances (or mistakenly just use the kingpin offset) and call it the scrub radius (even though it's more of an ellipse than a circle). A lot has been talked about scrub radius on here and elsewhere; so you can use the search feature and find many references on here. You'll find that it has a lot to do with steering effort.

[Bookhout]

By kingpin length I mean the distance between the upper and lower ball joints on the upright.

[TG]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Bookhout:
By kingpin length I mean the distance between the upper and lower ball joints on the upright. </div></BLOCKQUOTE>

Oh, I'm not exactly sure what that is call... upright height?? Anyways, I wouldn't be concerned with the actual height of the upright. I would be more concerned about the actual geometry of the suspension, like where the instant axis and roll centers are and their transient behavior. Just keep in mind packaging when you are figuring this out.

[rjwoods77]

Yes. The furthur your balljoints are spread the smaller the force input into the wishbones through the balljoints. TG is right about getting the geometry to do what you want it to but for general start points try to get the balljoints spread as far as you can. Think of this and everything in these terms. What if theoretically you could get the lower ball joint to be at the contact patch which is possible but very stupid. Then the forces at the tire contact patch would be the forces at the lower balljoint since there is no moment arm. Now take it to the opposite extreme. What it the lower balljoint was at the tire centerpoint or even worse. The larger the torque arm,the larger the forces. This applies to anything on suspension. For example: What does camber do to suspension forces? Look at camer at 0 degree and look at it at 90 degrees. That will give you an understanding of where the forces could go. Always look at the extremes for understanding. Edward K. taught me this and it really helps.

One thing to note. The furthur you spread your balljoints out in the rim the less steering angle you get because the closer you get to the top and bottom of the rim the less of a rim "arc" you get in the side view. Basically if you push it to the extreme and get good suspension geometry you might find you only have 10 degrees of steering angle which is no good. You cant just look at any one thing at a time when designing suspension. Before you even start with suspension you have to start with brakes. All of suspension design starts with packaging the brake, rotor and wheel center,wheel width and wheel diameter(have to know what tire you want to run even before all of this). This lets you know what kind of scrub radius you can get without guessing. Once you do that then look at steering angle clearance for a certain scrub radius and ball joint center to center. Another part that plays into that is what kind of ackermann you are going to run but that is limited by steering angle and also in some cases you find where you want you steering joint to be at the upright happens to be where the brake rotor is. You'll find in alot of cases that brake packaging severely limits you on what you want to do. On a bigger note you have to design all of these things concurrently. If you dont youll end up with hodge podge solutions you see on alot of fsae cars.

[Alan]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Rob Woods:
Always look at the extremes for understanding. Edward K. taught me this and it really helps.
</div></BLOCKQUOTE>

It's funny you mention that. That is a technique of understanding that I discovered on my own through all the dumb engineering mistakes I have made. On a similar note, when I think about compliances I like to ask myself two questions. What would happen if this were infinitely stiff and what would happen if it were a wet noodle?