Author Topic: A-Arm design  (Read 475 times)

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Offline deranged

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A-Arm design
« on: February 23, 2018, 05:23:05 AM »
Hey fellas! 

Well, I am getting the itch to build another car...  yeah another 2 seat car.  This time I want to take on the challenge and build from no plans and using what I have learned in my last 2 cars.  Well on the front ends, a lot of the desert guys run the standard if you will lower a arm and a form of an upper arm.  On some of the sand cars the arms are a little different.  The front arm pivot is on the front of the car if you will and the rear pivot is back on the chassis.  From an engineering/function standpoint what are the main differences in these 2 designs?  Below are a couple pictures of what i'm talking about.  My old 2 seat a arm car and a very similar car.  I believe it's an X-18.

I live nowhere near a desert....

Offline dsrace

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Re: A-Arm design
« Reply #1 on: February 23, 2018, 07:23:20 AM »
i cannot remember the correct name for the 2nd rails front end design. i call them J arms but i know that isn`t it. but until someone corrects me i will just call them J arms. i am not an engineer so i cannot speak on the question. i will say they were a design to gain leg room w/o increasing wheelbase. some time you`ll see an a arm on the lower and the j arm on the top. rare to see that but i have which allow extra room for pedal set hung from the top tube. that design does have to be built stronger and has 1 downside imo.........has the wheel strokes or shock enters a bump moment ( however you choose to look at it) the wheels move forward. as a trailing arm pushes the wheel back through the stroke of the shock, in the front that J arm design pushes them forward. this adds a certain extra load to them and may effect the amount of feed back through the wheel. i`m sure yummi will correct me on the name  :D and others will respond on what they know or experienced with that design on that cyclone black rail.
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Offline deranged

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Re: A-Arm design
« Reply #2 on: February 23, 2018, 09:15:43 AM »
Good explanation!  It could almost be considered a radius arm then.  Good point on the arc the wheel will see during the movement.

I do know that my old car (the first pic) I should have deviated from the plans and added in rake.

Justin
I live nowhere near a desert....

Online fabr

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Re: A-Arm design
« Reply #3 on: February 23, 2018, 09:19:52 AM »
ds is right about the wheel motion with the j arm. Personally,I hate the design.  A arms will have either straight up/down or up and back wheel motion depending on whether the arms are angled/raked. I think the best is a double a arm with rake built into the design to give the wheel a rearward motion on compression/bump.Dual a arms are ,IMO,much easier to design desirable camber curves into as well as controlling bumpsteer/toe change as well.
« Last Edit: February 23, 2018, 09:22:56 AM by fabr »
"There can be no divided allegiance here.  Any man who says he is an American,
but something else also, isn't an American at all.  We have room for but one
flag, the American flag... We have room for but one language here, and that is
the English language... and we have room for but one sole loyalty and that is a
loyalty to the American people."
Theodore Roosevelt 1907

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"I fear the day that technology will surpass our human interaction. The world will have a generation of idiots" Albert Einstein  (IT'S OFFICIAL THAT DAY IS HERE NOW!)

Online fabr

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Re: A-Arm design
« Reply #4 on: February 23, 2018, 09:22:11 AM »
I believe your first car was a rorty design? I did the same on car 2 and also wish I had added in rake.
"There can be no divided allegiance here.  Any man who says he is an American,
but something else also, isn't an American at all.  We have room for but one
flag, the American flag... We have room for but one language here, and that is
the English language... and we have room for but one sole loyalty and that is a
loyalty to the American people."
Theodore Roosevelt 1907

-----------------------------------------------------------
"I fear the day that technology will surpass our human interaction. The world will have a generation of idiots" Albert Einstein  (IT'S OFFICIAL THAT DAY IS HERE NOW!)

Offline dsrace

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Re: A-Arm design
« Reply #5 on: February 23, 2018, 09:26:34 AM »
def add rake since your building the chassis. make sure you have clearence for the upper a arms to the upper frame tube. i dont build my a arm front kits like that. they are designed to sit level because they are primarily used for beam to a arm conversions and on most of the old beam frames they are very low sides. upper a arm clearence is an issue and 10* pitch would put the a arm into the upper tube depending on how the end user attached to the frame. most dont consider it.    i will say for the added room inside the J arm served a purpose but the downside is the forward movement of the wheel which is counter productive. in sand only for most people it wouldn`t be an issue but dont take that down a hard pack road or pavement and run it hard.
Don't never argue with an Idiot!
Because he will drag you down to his level and beat you with experience

Offline dsrace

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Re: A-Arm design
« Reply #6 on: February 23, 2018, 09:30:26 AM »
if you blow up the pic of that predator rail ( says cyclone on roof) that front end has no rake built into the cradle. its flat level and caster is set from ofset of lower to upper arm placement on those bulkhead style tabs they use that the tab zone sells as kits.               i only meantion this as i would think it would even more important to install rake in the front cradle to help offset the forward motion and added steering input from that J arm design,???
« Last Edit: February 23, 2018, 09:32:32 AM by dsrace »
Don't never argue with an Idiot!
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Online fabr

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Re: A-Arm design
« Reply #7 on: February 23, 2018, 11:20:55 AM »
Rake in a j arm design is a bit weird. You can build it in and still have no rearward wheel motion on compression/bump since it has that forward arc/ motion naturally that will kill at least most of the benefits of using rake. I probably didn't state that well but I think you see what I'm getting at. Adding rake to the cradle itself wouldn't solve much other than a bit higher front clearance of the frame approaching an obstacle.
"There can be no divided allegiance here.  Any man who says he is an American,
but something else also, isn't an American at all.  We have room for but one
flag, the American flag... We have room for but one language here, and that is
the English language... and we have room for but one sole loyalty and that is a
loyalty to the American people."
Theodore Roosevelt 1907

-----------------------------------------------------------
"I fear the day that technology will surpass our human interaction. The world will have a generation of idiots" Albert Einstein  (IT'S OFFICIAL THAT DAY IS HERE NOW!)

Online fabr

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Re: A-Arm design
« Reply #8 on: February 23, 2018, 11:25:46 AM »
I do not like the j arm design in general for other reasons as well. A good a arm setup is just superior and you can add as much rake as you wish. There is a builder I won't mention that used to get really radical with the rake but has toned it down since too much of a good thing is welllll,just bad..
"There can be no divided allegiance here.  Any man who says he is an American,
but something else also, isn't an American at all.  We have room for but one
flag, the American flag... We have room for but one language here, and that is
the English language... and we have room for but one sole loyalty and that is a
loyalty to the American people."
Theodore Roosevelt 1907

-----------------------------------------------------------
"I fear the day that technology will surpass our human interaction. The world will have a generation of idiots" Albert Einstein  (IT'S OFFICIAL THAT DAY IS HERE NOW!)

Offline dsrace

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Re: A-Arm design
« Reply #9 on: February 23, 2018, 11:44:50 AM »
i have to agree.....i do not like the j arm design either. i do understand there would never possibly be any rearward/deflection, movement of the wheel due to the nature of the design alway pushing forward. the thought was just simply to point that foward movement up instead of dead forward. either way doesnt matter as it will never be capable of  obtaining the true goal of the front end suspension should. it is a space saver design only. many have it and dont know the diff. so i dont know the builder you speak of that had too much rake in his front ends but have seen a few rails with 14* rake in the cradle and center load rack configs.
Don't never argue with an Idiot!
Because he will drag you down to his level and beat you with experience

Online fabr

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Re: A-Arm design
« Reply #10 on: February 23, 2018, 01:18:48 PM »
 :m you know who.
"There can be no divided allegiance here.  Any man who says he is an American,
but something else also, isn't an American at all.  We have room for but one
flag, the American flag... We have room for but one language here, and that is
the English language... and we have room for but one sole loyalty and that is a
loyalty to the American people."
Theodore Roosevelt 1907

-----------------------------------------------------------
"I fear the day that technology will surpass our human interaction. The world will have a generation of idiots" Albert Einstein  (IT'S OFFICIAL THAT DAY IS HERE NOW!)

Offline deranged

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Re: A-Arm design
« Reply #11 on: February 23, 2018, 05:52:37 PM »
GREAT Information!  I have been leaning toward the a arm design as you mentioned that it is easier to design in camber gain etc.  You are correct fabber, that was a Rorty car.  My new car will have rake built into the chassis and that is a very good point to make sure there is enough up travel clearance as that "hood" part of the chassis will be as low as I can make it as well as short as possible for visibility.  The old rorty car has piss poor front vision.

While we are talking car design, I will do some splain-nin' 
Rorty based chassis layout. 2 seat
Lengthen the wheelbase to 118 to 120 ish (the rorty was 102) too short IMO
Width 86" or so.
Plate control arms in the front with 7*ish of rake 2.5 IBP shocks 12" ish and tabbed for bypasses
33" tall tires
2.4 Ecotec
CVT to an RPM  A transaxle is out of the question LOL  I know how I drive and it's just not in the budget.
Entertaining the possibility of a arms in the rear but leaning more to trailing arms
2.5 coilovers/2.5 bypasses
20" travel in the front
18-20" travel in the rear
Kind of a cool radius roof line

So basically a class 10 car by design..lol
I live nowhere near a desert....

Offline dsrace

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Re: A-Arm design
« Reply #12 on: February 23, 2018, 06:12:54 PM »
i cannot speak on trailing arms as i have never run them as i just don't like them. i built a lot of rails with 5 and 6 link ( multi link ) rear ends. i built a dual a arm style rear end for this new power plant i have now. i can say now that the a arm rear end is now my favorite. i have tried 5* and 10* rake on the front cradle. i could tell no diff between the two but 5* still is an advantage and allows for more clearance to upper frame. just my opinion based on my experiences. the a arm rear is more involved to align than a multi link but stronger as well as drives the rail differently in a very good way. better hole shot, better control over body roll with appropriate shock valving and wheel base doesn't grow through the stroke plus the ability to add a camber curve to name a few.
« Last Edit: February 23, 2018, 06:14:59 PM by dsrace »
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Because he will drag you down to his level and beat you with experience

Offline dsrace

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Re: A-Arm design
« Reply #13 on: February 23, 2018, 06:17:54 PM »
this is my first and needs to be refined a bit  ;D but over kill none the less.








Don't never argue with an Idiot!
Because he will drag you down to his level and beat you with experience

Offline deranged

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Re: A-Arm design
« Reply #14 on: February 23, 2018, 06:54:23 PM »
That is super nice!!  My single seat car I have now is a arm rear and feels a lot more stable than my old TA car.  It seems as is if is easier to design in the camber and toe gain throughout the movement.  My car now can't adjust the toe but it has about 7* negative camber at full bump.

Justin
I live nowhere near a desert....