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    How to design and build A-Arm suspension
    #1
    J.M. Action Figure flyerrider's Avatar
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    After posting these steps at least twice in the last month, I am making a thread for the discussion of proper (as I know it) A-Arm design. Please feel free to add your comments, pictures, drawings, steps, corrections, definitions, and opinions. If anyone has any questions, feel free to ask however please do your due diligence and have a little research under your belt. A simple google search will provide definitions and pictures to the terms being discussed. CAD is not a necessity and getting creative with some card stalk and a calculator can result in a design that functions just as well as a full SolidWorks based design. There are numerous trucks and buggies still blasting through the desert and winning races that were designed on graph paper during the "pre-CAD" ages.

    The only way to design a proper A-Arm suspension system is to work from the spindle and work inwards. Using these steps will make your life easier:

    1: Select your wheel and tire. Use the ACTUAL tire height instead of the size that's printed on the sidewall. They are almost never the same. For wheels, try to find a wheel with the most backspacing possible. This will make for longer arms, a reduced King Pin Inclination (Referred to as KPI from now on) angle, and a more optimal mounting location for your upper arm inner pivots.

    2: Select your hub and snout setup. I try to always have it on hand to verify dimensions. Ask me why and I will reply with a picture of several discarded spindles sitting in my scrap box.

    3: Finalize your mounting point on the spindle for your lower A-Arm pivot. For front suspension spindles, I try to have the lower pivot in line with the snout centerline. This will reduce your KPI angle.

    4: Draw your KPI line for proper scrub radius (1/2 - 3/4 of an inch inside the center line of the contact patch of the tire at ride height)

    5: Design your spindle and make sure you have proper clearance (at least 1/4") with the wheel and upper A-arm pivot and spindle. I usually ball park my steering horn location at this time with proper Ackerman just to get a rough idea of where my rack/spreader bar will end up. Do this to make sure you will have proper clearance with your chassis.

    6: Figure out the spread of your inner arm pivots. If you don't need foot room between the pivots, center mount the arms. You do this by running the lower chassis tubes right next to each other and then plate between the two tubes. Design your mounting points for the lower A-arm inner pivots and make sure you have at least 3/16" of clearance between the inner bushings and chassis tubes. This will make sure you have clearance for welding (I usually Tig for the smaller weld) radius.

    7: Ballpark your ride height, track width and camber (for ride height). I usually run 2 degrees negative camber at ride height. Remember that you don't need massive width to get a good amount of travel. I kind of think 90" wide minis are tad ridiculous. It's a MINI buggy. You also don't need 24" of travel for a light buggy front end. Proper geometry, spring rate, and shock valving are FAR more important than travel numbers. For me, big travel numbers only work well around the campfire when one upping someone after drinking too many beers and running your mouth. I'll take 12" of PROPER travel geometry over 26" of sloppy travel (unless we're talking about I-beams. Bring on the dirty hooker travel!!!)

    8: Draw a circle or arc with your lower A-arm inner pivot being the center and your lower a-arm outer pivot being the radius.

    7: Set your ground clearance at full bump. 4" is usually safely conservative. I usually use 3" but that's just me. Smacking the chassis does NOT feel good. This is why back injuries are very common in off-road racing.

    8: Set your full compression camber. I do this by splitting two aspects. Wheel scrub (where I place the emphasis) and body roll. Set your camber so that at full compression, the center of your contact patch on the tire is the same track width as the center contact patch at ride height. More negative camber, the wider the track width at the contact patch becomes. I factor in body roll by drawing a line between the center of the contact patch at ride height of one tire and then the center of contact patch on the other tire at full bump. As long as the angle between the tire and the line drawn does not exceed 90 degrees past vertical, you're good to go.

    9: Roughly estimate your full droop (fully extended) travel number. Move the spindle and tire assembly to that location on the lower arm arc and set the camber (just as you did the full compression) to minimize wheel scrub. Once again, more negative camber gain, less wheel scrub.

    The reason for roughly setting your full droop spindle location is because you will be limited by the angle between the upper arm and the KPI. You don't want more than 135 degrees on a moderately heavy vehicle. 140 degrees is acceptable by me on a REALLY light car.

    10: Draw a 3 pointed arc between the spindle's upper a-arm pivots at full bump, ride height, and full droop. The center of the arc is your upper a-arm inner pivot. Draw a line between the upper A-Arm inner pivot and the upper A-Arm outer pivot. Measure the angle between this line and the KPI. If it is less than 135 degrees, you can increase your droop travel (taking wheel scrub into consideration). If it is more than 135 degrees, decrease your droop travel. I usually shoot for a ride height with 60% compression and 40% of droop. This seems to be the standard for almost every truck and buggy. I usually fine tune my ride height in my drawing based on 60/40% travel guidline from here. This usually results in a small change in your camber adjustments and upper arm inner pivot location so start with Step 7 and repeat. I can usually get this done with only one revision to the drawing.

    11: Now factor in roll center and set a "compromise" between a good roll center and good camber gain. I usually put more emphasis on proper camber gain. A less than desirable roll center can be "somewhat" dealt with by using a sway bar unless the roll center is absolutely horrible. If the roll center is that bad, a redesign is in order.

    When I have more time I will add in the steps for Ackerman, Castor and castor gain (anti and pro dive), bumpsteer, and shock geometry.
    Last edited by flyerrider; 09-28-2013 at 08:41 PM.
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    Re: How to design and build A-Arm suspension
    #2
    Seņor Banned Member Chikin's Avatar
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    Reserved for further pictures.
    Excellent work.
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    Re: How to design and build A-Arm suspension
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    Keeper of the Asylum K-fab's Avatar
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    Nice stuff!
    I'll have to study this and then apply to my dream ride.
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    Re: How to design and build A-Arm suspension
    #4
    J.M. Action Figure flyerrider's Avatar
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    Sticky por favor. I doubt anything I wrote is something you didn't already know
    Last edited by flyerrider; 09-23-2013 at 05:59 PM.
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    Re: How to design and build A-Arm suspension
    #5
    Keeper of the Asylum K-fab's Avatar
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    Quote Originally Posted by flyerrider View Post
    Sticky por favor. I doubt anything I wrote is something you didn't already know
    Sticky done.
    Yea, I know it, but it's good to see it.
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    Re: How to design and build A-Arm suspension
    #6
    Super Moderator Wheels's Avatar
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    I wish this would have been up when I built my a-arms. thanks for taking the time to write it Flyer.
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    Re: How to design and build A-Arm suspension
    #7
    The Bob Ross of MBN Bullnerd's Avatar
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    Bajatex post,

    https://www.minibuggy.net/forum/suspe...html#post67866

    Damiens post

    https://www.minibuggy.net/forum/proje...tml#post240155

    Skreikems thread,

    https://www.minibuggy.net/forum/proje...-new-post.html

    Between these three and what Flyerider wrote we should have it covered pretty good. I like Skreikems thread because he shows how to locate the inner tie rod using cardboard, which I think would help a lot of people.

    Thanks for taking the time Flyerider,and thanks for the sticky K-fab-er
    Last edited by Bullnerd; 09-24-2013 at 10:36 AM.
    "Speed is time-time is speed"-Dennis Hopper

    Quote Originally Posted by TALON View Post
    did you use a special bigfoot camera or something ,you know all blurry could be a tree stump kinda thing .
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    Re: How to design and build A-Arm suspension
    #8
    J.M. Action Figure flyerrider's Avatar
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    Awesome! Thanks Bull. I'm going to get some pictures up later this week when I have time to do some basic drawings on AutoCad. I'll probably just use the drawings from the design of my son's buggy. I'll also get some of my buggy once I get a chance to get a jack underneath it and edit my original post with pictures and drawings describing the methods I use. I will also cover Rake in a future post. If there is anything else someone would like me to cover, feel free to ask.
    Last edited by flyerrider; 09-24-2013 at 12:05 PM.
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    Re: How to design and build A-Arm suspension
    #9
    Senior Member deaner's Avatar
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    Flerrider, can you explain what you mean by this?

    8: Draw a circle or arc with your lower A-arm inner pivot being the center and your lower a-arm lower pivot being the radius.

    Also, can you give more detail about the 90 degrees in step 8 about roll center and tire scrub?
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    Re: How to design and build A-Arm suspension
    #10
    Bend-Tech Dragon Master
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    For REFERENCE ONLY! Here's the front suspension I used in a my current build (based on my current car as well). Setup for 14" 2.0 Fox Airs and 2.0 bypasses, latest rage rack, etc.
    IIRC 10 deg. rake.


    ACAD2004 file, OLD SCHOOL cad I know.
    http://www.jgsfab.com/IFS_dim.dwg
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