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Old 01-03-2006, 06:33 PM
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Critique My Frame......The Sequel......Question About Trail Arm Placement....

I want to do some measuring for a 4 link and was wondering, where does someone who has never done this sort of thing before begin? I need some advice on where and how the trail arms should be placed. Should the uppers and lowers be equal length? If I use a triangulated, do I still need a panhard bar? Should the uppers and lowers be ahead of the axle? In my pics you can see where I would like to attach the uppers (I used pvc pipe for mock up) but was wondering if it would work to have the lowers behind the axle. Should the uppers be further out on the axle? Would the air bags be O.K. to be right under the frame (see pics). I really don't know much about this stuff but I want it done right the first time. Any other advice would be great....I don't understand all the technical stuff so please,,,small words only
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Old 01-03-2006, 09:05 PM
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Its a tough read, but get Race Car Vehicle Dynamics by Milliken. There are tons of things to consider like what the main use of the car will be. That book will take you from confusion to master (if you can get past the calculus). Equal or unequal, panhard or no, triangulated or not, ladder bar or multi-link, etc. Its all there including how to set it up right.

In general, you can put arms behind the axle. You'll have to be careful that they're long enough. What happens with one set in front and one set in back, the pinion angle will stay OK for most of the travel, but then it will start going way out of whack the more the suspension goes up and down. In general its wiser to put them up front.

Almost all passenger cars use a triangulated four link. It provides a good positive placement of the axle with easily adjustable geometry. The upper arms are short and steeply angled. Sometimes 45 degrees and they are close together on the axle end and angle outward toward the front where they attach to the car. Then the lowers are long, and angle slightly in as they go forward to where they attach. This geometry does several things. 1) it eliminates the need for a Panhard rod with all the sharp angles, 2) if done right it keeps the pinion pointed in the same direction, and 3) the way it articulates helps handling and weight transfer. Roll center and Instant center are words you'll hear during this whole thing and its tough to explain but important to understand.

For now what you need to know is that the right length and angle will maximize handling and ride quality. The wrong lengths and angles can make it a stiff ride that doesn't handle very well.
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Old 01-04-2006, 08:52 AM
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Thanks for the advice Curtis, it's a good place to start. I'll see if I can find that book. I can do body/paint, engine, and tranny work but i've never done any suspension (not of this caliber anyway). It's hard to understand some of the terms when you don't know them but i'll keep asking on here, seems to be some important people here
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Old 01-04-2006, 10:07 AM
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To get up to speed on some of the terminology this is a good paper to read...
This one will probably be of the most use to you...
Here is another site with a LOT of info.
If you want to try to design your 4-link check out this program...
Well worth the money in my book to ensure you build something that will work for you...
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