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Old 03-25-2011, 12:47 AM   #1 (permalink)

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Default A-arm design/setup

Very well written
Quote:
Originally Posted by KrunchWorkz View Post
To properly design a front A-Arm suspension system for an off-road vehicle, there are several things that need to happen, and happen in the correct order so that the geometry comes out correctly. It is recommended that AutoCAD or a similar program be used. In order to start there are several things that have to either be on hand, or the dimensions known.
The first of these is the tire size and width. The next is the wheel and the offset, and the last is the hub dimensions. This information is needed because A-Arm front ends are designed from the tire inwards.
The first step is to figure out the track width, or how far apart the tires are as measured from the outer edge of each front tire. Once track width has been set, the spindle needs to be designed.
Here is where attention to detail becomes so important. It is time to design the spindle. The spindle needs to be designed with some scrub radius built in. Scrub radius is determined by drawing a line through the upper and lower pivot points until that line intersects the tire contact patch. If it intersects in the dead center, then that is a zero scrub radius. For a larger tire, you want about inch scrub to the inside of the tire. With a smaller tire, about inch to the inside. Here is where the hub measurements become important. The center of the bottom pivot must be parallel with the center of the spindle on a horizontal line. The offset of the lower pivot from the hub will help to determine what caster angle and where the upper pivot will be in relation to the lower pivot. Once this has been determined, then the spindle is designed.
Once the spindle is designed, the length of the lower A-Arm is decided upon and designed. This will determine where the lower pivot points will be on the car.
Here comes the tricky part. It is time to decide how the spindle will behave through the total travel. At this time, it has to be decided what percentage of travel you want for droop and what percentage you want for compression. Usually it is somewhere around 40% droop and 60% compression. This determines your ride height. Set the spindle with how much camber you want at right height. This is the middle. Then re-draw the spindle at full compression with the correct camber, which is typically about 1 degrees negative camber. You always want negative camber on compression so the car does not lose control. Once that is done, this is the top position. Do the same for full droop. Set the spindle with how much camber is desired. This is the third position.
Once the three positions are set, it is time to determine the inner pivot points for the A-Arm. To do this, draw a 3 point arc that goes through all three lower pivot points. The center of this arc is where the upper A-Arm inboard pivot point will be. Take the time to make sure the calculations are correct.
Once the calculations are correct and the arms are moving as desired, repeat the process to find the inner pivot point of the tie rod. This will get you 90% there to eliminate bump steer. Play with the measurements until there is between .90 and .100 bump steer. Make sure that the bump steer causes the wheels to toe in, not out, or the car will be unstable.
Need to set caster to 7 degrees. Either angle the spindle back pivoting on the lower pivot or bend the lower frame rails at 7 degrees upwards and lay the entire front end back. Either way works.
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Old 03-25-2011, 11:42 PM   #2 (permalink)

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Default Re: A-arm design/setup

Thank you. This process was taught to me by a mechanical engineer that use to build A-Arm buggies and race cars. This is the same person who designed my A-Arm setup for my Manx buggy that I have a build thread on.
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Old 03-26-2011, 11:27 AM   #3 (permalink)

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Default Re: A-arm design/setup

Quote:
draw a 3 point arc that goes through all three lower pivot points
Shouldn't this be "all three upper pivot points" or upper ball joint points?
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Old 03-26-2011, 03:46 PM   #4 (permalink)

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Default Re: A-arm design/setup

You are correct. I mis-typed it originally. Here is my original document.

To properly design a front A-Arm suspension system for an off-road vehicle, there are several things that need to happen, and happen in the correct order so that the geometry comes out correctly. It is recommended that AutoCAD or a similar program be used. In order to start there are several things that have to either be on hand, or the dimensions known.
The first of these is the tire size and width. The next is the wheel and the offset, and the last is the hub dimensions. This information is needed because A-Arm front ends are designed from the tire inwards.
The first step is to figure out the track width, or how far apart the tires are as measured from the outer edge of each front tire. Once track width has been set, the spindle needs to be designed.
Here is where attention to detail becomes so important. It is time to design the spindle. The spindle needs to be designed with some scrub radius built in. Scrub radius is determined by drawing a line through the upper and lower pivot points until that line intersects the tire contact patch. If it intersects in the dead center, then that is a zero scrub radius. For a larger tire, you want about inch scrub to the inside of the tire. With a smaller tire, about inch to the inside. Here is where the hub measurements become important. The center of the bottom pivot must be parallel with the center of the spindle on a horizontal line. The offset of the lower pivot from the hub will help to determine what caster angle and where the upper pivot will be in relation to the lower pivot. Once this has been determined, then the spindle is designed.
Once the spindle is designed, the length of the lower A-Arm is decided upon and designed. This will determine where the lower pivot points will be on the car.
Here comes the tricky part. It is time to decide how the spindle will behave through the total travel. At this time, it has to be decided what percentage of travel you want for droop and what percentage you want for compression. Usually it is somewhere around 40% droop and 60% compression. This determines your ride height. Set the spindle with how much camber you want at right height. This is the middle. Then re-draw the spindle at full compression with the correct camber, which is typically about 1 degrees negative camber. You always want negative camber on compression so the car does not lose control. Once that is done, this is the top position. Do the same for full droop. Set the spindle with how much camber is desired. This is the third position.
Once the three positions are set, it is time to determine the inner pivot points for the A-Arm. To do this, draw a 3 point arc that goes through all three upper pivot points. The center of this arc is where the upper A-Arm inboard pivot point will be. Take the time to make sure the calculations are correct.
Once the calculations are correct and the arms are moving as desired, repeat the process to find the inner pivot point of the tie rod. This will get you 90% there to eliminate bump steer. Play with the measurements until there is between .90” and .100” bump steer. Make sure that the bump steer causes the wheels to toe in, not out, or the car will be unstable.
Need to set caster to 7 degrees. Either angle the spindle back pivoting on the lower pivot or bend the lower frame rails at 7 degrees upwards and lay the entire front end back. Either way works.
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Old 03-26-2011, 04:46 PM   #5 (permalink)

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Default Re: A-arm design/setup

Its a great write-up, exactly how I do my designs in plain old autocad. Amazing what some 2d cad can accomplish, the 3 point circle tool is the key to getting the locations and lengths right.
You work with existing frame locations to re-design a spindle but its A LOT harder (trust me I know from my last project).
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Old 03-27-2011, 08:09 PM   #6 (permalink)

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Default Re: A-arm design/setup

Good info, but it's ony the tip of the iceberg. Figure in Ackerman and a design that's rake angle is different than the caster and it's not so cut and dried. End load racks present their own problems too.
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Old 03-27-2011, 09:28 PM   #7 (permalink)

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Default Re: A-arm design/setup

Quote:
rake angle is different than the caster
Is there a real difference between caster from rake and just caster?
Ackerman is set by the spindle and wheel locations (width AND length). As long as your wheel base is the same ratio (width vs length) as the wheel base they where designed for Ackerman will be the same location....
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Old 03-28-2011, 01:57 PM   #8 (permalink)

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Default Re: A-arm design/setup

What??? You've lost me with the Ackerman statement. Ackerman can be achieved regardless of wheel base width to length ratio.
And yes rake and caster are seperate....... But, some designs have rake set the same as caster. This makes the a arms easier for some to build.
In the example KrunchWorkz gave rake and caster can be related.
"Need to set caster to 7 degrees. Either angle the spindle back pivoting on the lower pivot or bend the lower frame rails at 7 degrees upwards and lay the entire front end back. Either way works."
If you are setting your caster up using the later of these two examples rake and caster will be related.
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Old 03-28-2011, 02:41 PM   #9 (permalink)

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Default Re: A-arm design/setup

Ackerman angle is a constant figure dictated by the spindle.
If your starting with a prebuilt spindle you have no way of adjusting Ackerman angle unless you change the wheel base width ( front ) and length.
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Old 03-28-2011, 05:11 PM   #10 (permalink)

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Default Re: A-arm design/setup

I disagree. It can be adjusted somewhat with a prebuilt spindle. Although I wasn't talking about a prebuilt spindle. I'd never let a spindle determine my wheel base. Is this what some of us are doing? I think most of us are building our own. Fore aft rack position effects Ackerman as does raising or lowering the tie rod's height at the spindle.

JGS, Is your car designed with Ackerman?

Last edited by RickS.; 03-28-2011 at 05:18 PM.. Reason: Question for JGS
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