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Trailing arms and toe links

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traveller View Drop Down
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    Posted: Mar/11/2018 at 9:41am
Okay, I have a lot of questions...

Before I get to it, I must say that I'm an FS participant and never worked on a Baja car. In my time, I've worked on straightforward double wishbone suspension setups, and only recently begun thinking about trailing arm setups (just out of curiosity). It seems to be a somewhat popular setup in Baja (at least in comparison to FS), so as went through some images on the internet, the designs seems to leave me puzzled...

1) Are trailing arms mounted on the frame with a revolute joint (bolt and bushing), or a spherical joint (rod end)?

2) Is the wheel upright rigid with the trailing arm, or is it allowed to pivot in one or all axes?

3) I came across a setup (image attached) with revolute frame mounting, and a rigid upright with he trailing arm. In such a situation, I would say that the shock absorber will HAVE to be mounted such that its eyelets have their axes parallel to the revolute mounting axis, or it cannot be actuated as the shock mounts will not have the necessary rotational degree of freedom. But this as not the case in the in this particular setup. the shock mounts were mounted quite randomly in regard to their axis orientation. Am I missing something here?

4) This question bothers me the most. In the same setup, I noticed that they used two toe links, from the upright to point on the frame. Rod-ends were used on both ends of these links, and they were oriented quite well to bear lateral loads. I'd say that in theory they are not necessary for a trailing arm (with revolute frame mounting), but they definitely are needed to counter the compliance effects due to lateral loads. But in my opinion, the kinematics of this setup are just wrong! The trailing arm would force the motion of the wheel to follow a circular arc in the side view. Let's call this "Locus A". The toe link (with spherical rod-ends) would allow the wheel to move over the surface of a sphere, or with two toe links, it would be something more complex than a sphere. Let's call this "Surface B".
My issue is that Locus A and Surface B can only intersect at one point. Which means the suspension cannot move, and this linkage system has zero degrees of freedom.

So what am I missing here? Am I correct in saying this design would not work?
uploads/4688/1.jpg

Edited by traveller - Mar/11/2018 at 9:48am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bracer Quote  Post ReplyReply Direct Link To This Post Posted: Mar/11/2018 at 10:07pm
First off, I am not a suspension person, and the systems amaze me at how much goes into making sure the tires move in a correct manner.

Since our team runs a trailing arm system, I can answer some of your questions. The main trailing arm is connected to our frame with spherical joints. Our upright is bolted to the trailing arm. with 2 control links with spherical bearings to control the camber of the tire.

Without the picture, i am unable to give inputs on your third and fourth question.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote traveller Quote  Post ReplyReply Direct Link To This Post Posted: Mar/12/2018 at 1:06am
Thank for the response!

Spherical joints for the trailing arm at the frame makes sense. It would provide the necessary extra degree of freedom to the trailing arm. But is simply bolting the arm on to the frame also practiced?

Sorry, here are the images that I hadn't posted initially. You can see the trailing arm bolted with a bushing to the frame, the non-parallel shock eyelet mounting addressed in question 3, and the two control links I mentioned in question 4, in the second image. I'd say the trailing arm is bolted on to the upright, based on what I can see trough the wheel...

http://forums.bajasae.net/forum/uploads/4688/Image.jpg

http://forums.bajasae.net/forum/uploads/4688/Image2.png


Edited by traveller - Mar/12/2018 at 1:14am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RLM Quote  Post ReplyReply Direct Link To This Post Posted: Mar/12/2018 at 7:13am
to answer some of your questions, 

1: depends on the trailing arm setup, the most common in baja sae are the linked trailing arm which use the camber links in the rear end and a singular pivot point on the actual trailing arm. this style is usually connected to the chassis with either a spherical end, or a very compliant bushing (due to cost and rigidity, 99% use the spherical bushing). the other style is either a pure trailing arm or a semi trailing arm which usually uses 2 pivot points that are part of the trailing link. 

2: could be done either way, for weight and simplicity our team has always just made the bearing carrier part of the trailing link, though by making is seperate, you can control the camber and toe change a bit better.

3: this one is a bit tricky, with a single trailing arm and no camber links in the rear, yes, you want the shock to be mounted in the revolute axis of the arm, once you add those rear links though, the trailing arm rotates in 3 axis' during suspension movement. this is where shock style choice and placement becomes important and very geometry dependant. when using a linked trailing arm ( commonly called a 3 link around these parts) choosing a shock with spherical bushes in the eyelets is critical as it will allow the shock to also pivot in multiple axis'. then its a matter of positioning such that you minimize the axial rotation of the shock through travel, though we have compromised a little in the past to get the chassis mounting right on a strong node. 

4: you are correct, the kinematics of such a setup make no sense, unless you have a compliant bushing in the trailing link to chassis connection. our team has run both a semi trailing arm with no rear links and a semi trailing arm with rear links. Both are very light and strong setups, however for the lateral loads encountered in baja from jumps and ruts, the linked setup proved better for our team. 

hope this has answered some of your questions. if you want to look more in depth at real world uses and see some proven designs for those rear links, take a look at the rear suspension in 2002-2013 MINI Coopers, they use the linked setup with a bushing in the trailing arm, as well as bushings in the camber links and an eccentric bolt on one link for camber adjustment. pretty decent design overall. 

cheers, 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote traveller Quote  Post ReplyReply Direct Link To This Post Posted: Mar/14/2018 at 6:52am
Thank you so much for the detailed response. It's made things clear for me.

The Mini Cooper design was very interesting. It is a pretty neat design. I noticed that the trailing arm and the camber links have axes that seemed parallel so rod ends can be avoided. 

I beginning to wonder if such a design could be feasible on an FS car. Maybe make the frame more compact and let the rear wheels extend beyond the frame of the car.

Thanks again!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MYSTERIO Quote  Post ReplyReply Direct Link To This Post Posted: Jun/24/2018 at 2:32pm
WHICH HAS MORE AVANTAGES :
1-SEMITRAILING WITH CAMBER LINK
2-TRAILING WITH CAMBER LINK
3-MULTI-LINK WITH SPHERICAL BEARING TO JOIN THE TRAILINGN AND THEHUB 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sujandinesh Quote  Post ReplyReply Direct Link To This Post Posted: Jun/24/2018 at 3:29pm
4. A BAJA GUY WHO CAN RESEARCH AND WORK RATHER THAN ASKING TO BE SPOON FED ON THE FORUM. And also the guy who can use caps only when necessary. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MYSTERIO Quote  Post ReplyReply Direct Link To This Post Posted: Jun/25/2018 at 7:24am
Can some one tell me how much camber should i introduce to the rear wheels? I can find the camber inclination needed for the front wheels ,but can't find camber for the rear.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote MYSTERIO Quote  Post ReplyReply Direct Link To This Post Posted: Jun/25/2018 at 7:25am
how much camber should i introduce to the rear wheels? I can find the camber inclination needed for the front wheels ,but can't find camber for the rear.
AND USING CAPS IS MY STYLE ,SO DON'T INTERFERE!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Nick914 Quote  Post ReplyReply Direct Link To This Post Posted: Jun/25/2018 at 10:24am
the one that better suits your desires
reading a vehicle dynamics book will help you figure it out
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RLM Quote  Post ReplyReply Direct Link To This Post Posted: Jun/26/2018 at 8:03am
since you have posted this basic question multiple places and don't seem to be willing to research it yourself, I will help you out. as a general rule of thumb, you want to have enough adjustability to set it dependent on the maximum temperature swing available in the race area (in ºC). on race day you will want to set it based on the temperature that day (ºC) since the ambient temperature directly correlates to the coefficient of friction the tries can create with the ground. 

Best of luck!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sujandinesh Quote  Post ReplyReply Direct Link To This Post Posted: Jun/26/2018 at 8:41am
Originally posted by RLM RLM wrote:

since you have posted this basic question multiple places and don't seem to be willing to research it yourself, I will help you out. as a general rule of thumb, you want to have enough adjustability to set it dependent on the maximum temperature swing available in the race area (in ºC). on race day you will want to set it based on the temperature that day (ºC) since the ambient temperature directly correlates to the coefficient of friction the tries can create with the ground. 

Best of luck!
Of course this is when you want to use tyres made of rubber. Tongue
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RLM Quote  Post ReplyReply Direct Link To This Post Posted: Jun/26/2018 at 10:32am
Originally posted by sujandinesh sujandinesh wrote:

Originally posted by RLM RLM wrote:

since you have posted this basic question multiple places and don't seem to be willing to research it yourself, I will help you out. as a general rule of thumb, you want to have enough adjustability to set it dependent on the maximum temperature swing available in the race area (in ºC). on race day you will want to set it based on the temperature that day (ºC) since the ambient temperature directly correlates to the coefficient of friction the tries can create with the ground. 

Best of luck!
Of course this is when you want to use tyres made of rubber. Tongue

Yes of course, silly me, I forgot to state my assumptions. rule of thumb is different for steel running surfaces with wooden spokes.
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