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Rear A-arm

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Minesh View Drop Down
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    Posted: Jul/03/2013 at 6:42am
Guys i need your help regarding the Double wishbone suspension mounting in the rear. As we all know, in double wishbone the best suspension characteristics are obtained when the suspension is mounted on the lower A-arm. But, in our BAJA ATVs we are unable to do so in the rear due to the drive transmission shafts. Inspite of this many teams use the double wishbone type suspension with the suspension mounted on the upper A-arm. My query is does it really give a satisfactory result and what are it's disadvantages or advantages over lower A-arm wishbone mounting..?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pedro UFPBaja Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2013 at 12:07pm
Lowers the CG and allows the upper arm becomes extremely light, since it will only have to control camber.

To the suspension geometry and constants makes no difference.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pedro UFPBaja Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2013 at 12:19pm
It is NOT impossible to place the shocks in lower arm even though the drive shaft passing by












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Post Options Post Options   Thanks (0) Thanks(0)   Quote Minesh Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2013 at 11:43pm
ok ok. Thanx a lot.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote joey.m.gonzalez Quote  Post ReplyReply Direct Link To This Post Posted: Jul/04/2013 at 3:37am
Hah, that last pic is our car from two years ago... that setup worked like crap but I blame the designer, lol
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ahmed Quote  Post ReplyReply Direct Link To This Post Posted: Aug/24/2013 at 12:40pm
hello guys,

I've a little problem with understanding the REAR double wishbone mounting, in Race Car Vehicle Dynamics there is a form consisting of two A-arms and a toe link (or sometimes called track rod).

what's the use of this link? can i ignore it especially i've see many rear double wishbone systems with only the A-arms and the shock absorber !!

Thanks
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Richie_Dagger Quote  Post ReplyReply Direct Link To This Post Posted: Aug/25/2013 at 4:08am
It adjusts toe. And depending on how you design your suspension, you can get toe change throughout suspension travel.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ahmed Quote  Post ReplyReply Direct Link To This Post Posted: Aug/25/2013 at 12:22pm
so why i see teams using just 2 A-arms without this link ( like this in the picture below) , how can they adjust the toe angle. you know that much space is needed in the rear to mount the half axles.


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Post Options Post Options   Thanks (1) Thanks(1)   Quote Soccerdan7 Quote  Post ReplyReply Direct Link To This Post Posted: Aug/25/2013 at 1:02pm
Originally posted by ahmed ahmed wrote:

so why i see teams using just 2 A-arms without this link ( like this in the picture below) , how can they adjust the toe angle. you know that much space is needed in the rear to mount the half axles.

That picture shows H arms, not A arms. A arms have a single outboard pivot. The upper and lower a arm pivots create a steer axis about which the wheel can turn (just like the front suspension). That is why you need a rear toe link. With H arms, they constrain any pivoting about a steer axis which means you can not independently set toe and you don't need that added link.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ahmed Quote  Post ReplyReply Direct Link To This Post Posted: Aug/28/2013 at 8:48am
Originally posted by Soccerdan7 Soccerdan7 wrote:


Originally posted by ahmed ahmed wrote:

so why i see teams using just 2 A-arms without this link ( like this in the picture below) , how can they adjust the toe angle. you know that much space is needed in the rear to mount the half axles.

That picture shows H arms, not A arms. A arms have a single outboard pivot. The upper and lower a arm pivots create a steer axis about which the wheel can turn (just like the front suspension). That is why you need a rear toe link. With H arms, they constrain any pivoting about a steer axis which means you can not independently set toe and you don't need that added link.


so what's the difference between H-arms and A-arms with twin outer ball joints like this in the image ?
because in this type we need a fifth link too (the toe link)
i think i need to understand the difference between the outer joint type in A-arms setup and H-arms setup ?!!

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Post Options Post Options   Thanks (0) Thanks(0)   Quote mikem Quote  Post ReplyReply Direct Link To This Post Posted: Aug/28/2013 at 9:35am
An H-arm has a bushing for its outboard point. It thus constrains two additional degrees of freedom then a pure A-arm, which has a ball joint instead.

Think about the motion an a-arm would restrict vs an H-arm and it should make sense to you. Think about how the tire can move with the different constraints.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ahmed Quote  Post ReplyReply Direct Link To This Post Posted: Aug/28/2013 at 3:18pm
Originally posted by mikem mikem wrote:

An H-arm has a bushing for its outboard point. It thus constrains two additional degrees of freedom then a pure A-arm, which has a ball joint instead.

Think about the motion an a-arm would restrict vs an H-arm and it should make sense to you. Think about how the tire can move with the different constraints.


thanks

Edited by ahmed - Aug/29/2013 at 3:33pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ahmed Quote  Post ReplyReply Direct Link To This Post Posted: Aug/29/2013 at 3:32pm
hello guys,

I'm designing the geometry of the REAR suspension, and i can't see the difference between using a normal A-arm and an A-arm with twin outer ball joints

if somebody can help, please do. Thank you ^^


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Post Options Post Options   Thanks (0) Thanks(0)   Quote zmpeck Quote  Post ReplyReply Direct Link To This Post Posted: Aug/29/2013 at 3:50pm

What more of an answer do you are you looking for?  You posted a picture that clearly describes the differences.  Both will work, one uses less parts. If you're seeking to implement some anti/pro's or dynamic toe characteristics, you will have more freedom with the A-arm/toe link configuration.  If you're just trying to make the suspension go up & down, i'd stick with the H-arm.



Edited by zmpeck - Aug/29/2013 at 3:53pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ahmed Quote  Post ReplyReply Direct Link To This Post Posted: Sep/16/2013 at 8:11am
i need to know the name of this rear suspension setup to search for its advantages,disadvantages, joints, installation ...

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Post Options Post Options   Thanks (1) Thanks(1)   Quote cujdubs Quote  Post ReplyReply Direct Link To This Post Posted: Sep/16/2013 at 9:15am
That is a H-Arm. You notice that the upright has no ability to rotate about the king pin axis and there are no toe links.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ahmed Quote  Post ReplyReply Direct Link To This Post Posted: Sep/16/2013 at 9:32am
do you know a suspension software to model this setup and simulate it , i need to check how camber,caster,toe, kingpin, ... change with wheel travel?
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Red_Beard Quote  Post ReplyReply Direct Link To This Post Posted: Sep/16/2013 at 9:57am
Originally posted by ahmed ahmed wrote:

do you know a suspension software to model this setup and simulate it , i need to check how camber,caster,toe, kingpin, ... change with wheel travel?


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Post Options Post Options   Thanks (0) Thanks(0)   Quote ahmed Quote  Post ReplyReply Direct Link To This Post Posted: Sep/16/2013 at 11:57am
but this suspension mechanism is only kinematic if your chassis attachment points are perfectly parallel and your upright axis are also perfectly aligned.
Any angular error particularly with pivot joints all-round will cause the suspension to lock-up under bump/rebound travel as it is over constrained.

don't you think so ?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote zglazer Quote  Post ReplyReply Direct Link To This Post Posted: Sep/16/2013 at 1:38pm
You can account for that with a good jigging setup and by using spherical joints or rod ends at some of the pivot points to allow for misalignment.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote cujdubs Quote  Post ReplyReply Direct Link To This Post Posted: Sep/16/2013 at 3:03pm
Some teams and some production vehicles use hard rubber or something similar for the bushing material so that there is alittle give to compensate for things not being perfect.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ahmed Quote  Post ReplyReply Direct Link To This Post Posted: Sep/16/2013 at 5:27pm
so what's the advantage of this setup over using a H-arm and single upper link, why would someone take the risk of having his suspension locked up especially that i think the setup below provides a similar behavior like the two H-arms and can replace it, am i right ?

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Soccerdan7 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/16/2013 at 5:58pm
A setup with 2 H arms is better at reacting moments from the wheel loads and makes it easier to mount the shock to the upper link. The picture you attached with the single upper link is essentially the same anyway since the pivots are bushings, and those bushings will probably wear out quickly.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ahmed Quote  Post ReplyReply Direct Link To This Post Posted: Sep/16/2013 at 6:10pm
Originally posted by Soccerdan7 Soccerdan7 wrote:

A setup with 2 H arms is better at reacting moments from the wheel loads and makes it easier to mount the shock to the upper link. The picture you attached with the single upper link is essentially the same anyway since the pivots are bushings, and those bushings will probably wear out quickly.

i thinks it's much better to mount the shock on the lower arm !,
i guess i will have the bushings problem in both setups :(
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Post Options Post Options   Thanks (1) Thanks(1)   Quote OffTrac Quote  Post ReplyReply Direct Link To This Post Posted: Sep/16/2013 at 6:57pm
If you're looking for software to simulate a geometry you've already got drawn out, Lotus is useful. But to be honest, it's just to validate a design and show it's characteristics throughout the range of motion...you have to have the majority of the things you listed above (king pin, caster, etc changing with wheel rate) to get a good simulation output.

Least that's what I've found it useful for - It's definitely not a "my top arm is X and bottom is Y, I want Z caster and...." and it spits out a finished geometry you can cut and weld together.

It's a lot of work. But it's worth it in the end.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ahmed Quote  Post ReplyReply Direct Link To This Post Posted: Sep/17/2013 at 1:42pm
Does anybody know the advantages of the H-arm & single upper link suspension rather than the few number of links and the easy packaging ?
I'm puzzled to use this setup or use a three link suspension like this in Polaris rzr
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jeiB Quote  Post ReplyReply Direct Link To This Post Posted: Sep/17/2013 at 4:21pm
Originally posted by ahmed ahmed wrote:

Does anybody know the advantages of the H-arm & single upper link suspension rather than the few number of links and the easy packaging ?
I'm puzzled to use this setup or use a three link suspension like this in Polaris rzr
 
Its the same difference.
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