Question about double-wishbone suspensions

[Biggy72]

Looks like you've got a start, confusing, but it's a start. You will need to account for scrub radius (I think this is what you're refering to when you say scrub but your description doesn't make sense to me) also. The scrub radius is the distance between the center line of the wheel, and a line drawn from your upper ball joint to your lower ball joint and then to the ground. This can be adjusted by wheel offset and by king pin angle.

Your roll center can be found in the front view by taking a line, running it down the a arm toward the center of the car. Make another line for the other a arm, and then extend them out until they touch. This point where the two lines intersect is called the moment center. Then Take a line from your moment center and run it back to the centerline of the wheel. Do the same for the other side, and where both of the lines from the moment centers to the wheel centers intersect is your roll center. Try not to get the roll center to cross the ground plane as you go through your motion of travel as this will cause inconsistent handling characteristics.

If you were to put your roll center way below ground this will cause you to have a fairly forgiving set up that has little need for a sway bar. If you put your roll center above ground this will cause your car to be much quicker acting and a sway bar will help with handling quite a bit.

On the suspension geometry I designed last November I tried to keep my front roll center just above ground and it almost touches the ground at full bump, while my rear starts out just below ground and goes farther below ground as the car goes into bump. This allowed for a fairly level line between the front and rear roll centers which should keep the car fairly easy to balance front to rear as far as being loose or tight go.

I'm not totally sure how all of this will work, but it's what I've gathered from going through milliken and milliken and countless other sources I've read through. I will be able to test all of this hopefully about this time next year. Right now we're testing a car with two under ground roll centers, and it does have quite a bit of roll, but that may have something to do with a not very rigid chassis as well as many other things.

Oh and solid works is worth every penny. That's how I design everything from suspension geometry to complex parts I will cut out later on a cnc mill. And you can do FEA analysis on cosmos with it too.

[nine_one_4]

Thanks. I think I'll probably pick up this book next week and then the Milliken book a little later. I have to learn some of the fundamentals first before I jump into Race Car Dynamics. I also have to figure out if Solidworks will run on my computer. Their web site has a list of approved video cards and the one in my laptop is not listed.

[byndbad914]

Originally posted by Biggy72
If you were to put your roll center way below ground this will cause you to have a fairly forgiving set up that has little need for a sway bar. If you put your roll center above ground this will cause your car to be much quicker acting and a sway bar will help with handling quite a bit.

where did you find that info? I have M&M and couldn't seem to find info on "why" to place a roll center above or below ground or any rules of thumb. I am not suggesting you are incorrect by any means - would just like to be able to read over that section I somehow missed.

On the suspension geometry I designed last November I tried to keep my front roll center just above ground and it almost touches the ground at full bump, while my rear starts out just below ground and goes farther below ground as the car goes into bump. This allowed for a fairly level line between the front and rear roll centers which should keep the car fairly easy to balance front to rear as far as being loose or tight go.

Isn't it the case then that the line between your front to rear roll centers is always sloping down, therefore wanting to create oversteer? Neutral IIRC is when they are equal height.

[Biggy72]

There's more to having a neutral car then equal height roll centers. You also must look at weight distribution, which tires are getting power, roll characteristics and I'm sure there's more. But we will have about a 50/50 weight distribution and the overall height between the two roll centers is about two inches at all times.

I designed the geometry quite awhile ago so I'm not sure where exactly everything came from but here are a couple pretty good sites I found really quick.

http://www.teamassociated.com/racerhub/techhelp/marc/Handling.5.html


http://www.miracerros.com/mustang/t_roll_center.htm

http://web.umr.edu/~formula/library/sae_paper/saepaper.pdf

[nine_one_4]

I read an article on suspensions recently and it said that before modern computers and software, designing suspensions was more of an art than a science. That seems to be true because multilink suspensions with their complex geometries didn't come out until around the late eighties. Here's the equation I came up with to describe the plane of the wheel of a simple double wishbone suspension where the ends of the wishbones move along the arcs in the first pic.

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