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PostPosted: March 7, 2017, 11:52 am 
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I'm not sure what the best subforum was for this but this one at least seemed appropriate. I'm in the process of building an autocross tire trailer. For those unfamiliar with them, it's basically a way to haul your expensive sticky tires to the autocross event or track day when your car can't physically fit a whole set of tires inside. A big focus of my build has been having a soft, smooth riding suspension because I hate watching those dinky harbor freight trailers bounce around on every little bump. To most people, my design is overly complicated, but I'm pretty happy with how it's working. And I also built it to challenge myself. So with that said I'll show some pictures and discuss the problem.

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The main problem I am having is that my swingarms are turning out to be way too flexible. They need much more torsional rigidity to keep the camber under control when the suspension is loaded. Probably at least five times more to be acceptable.... I originally thought the moment arm between the tire and the pushrod would be so small that the lightish build would be ok. Turned out I was wrong. Between full droop and about half travel I get several degrees of camber change from the twist. The swingarms are built out of .080" mild steel square tube with 3/8" rod ends at the two ends. The two ends are not connected. The arms aren't overly heavy but they are heavier than I'd like already. What would be the most efficient way to boost the torsional rigidity of these arms?

Also I will likely add some stiffening mods to the frame, especially around the rocker arms and shocks are, but I can figure that easily enough.


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PostPosted: March 7, 2017, 12:55 pm 
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Since the pivot ends of your swingarms are not constrained in rotation, the only thing resisting a twisiting moment on the outboard end is the bending stiffness of the long legs of the swingarms. There are two changes I would make.

First, make the swingarms out of larger diameter/cross section tubing. You can keep weight from creeping up too much by decreasing the wall thickness while increasing the outside dimensions. A 2" square, 0.050" wall tube has about five times the moment of inertia compared to a 1" square 0.080" wall tube, with only about a 25% increase in mass.

Another thing to consider is a cross brace just ahead of the pivot points. As the outboard end of the swingarm twists it causes the legs to rotate relative to each other. A brace on the inboard end will prevent this relative rotation, so the rotational stiffness of each leg will play a role in addition to the bending stiffness.


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PostPosted: March 7, 2017, 1:45 pm 
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How about just adding a tube connection between the two "stub" axles, laterally under the trailer floor. If you try not to make it too rigid a tube, it might help control the camber while still allowing for some independent action between the two wheels. If necessary, you could then stiffen it up with a rib and then remove some height of the rib until you got the "right" balance between rigidity and flex.

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PostPosted: March 7, 2017, 2:02 pm 
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Where do you find .080" mild steel square tube? I didn't know it was readily available.

This is a great locost building adventure experience. In my life education is a somewhat expensive and painful experience. That feeling you have right now is the jaws of reality with a firm grasp on your hindquarters :rofl:. Same thing happened to Icarus. Sigh.

As Posthumane has described, tubes that are twice the diameter are 8 times stiffer. I think the best use of material though would be to place an additional tube directly between the wheel axles. See how that would balance the torque of one wheel against the torque of the other wheel? It's elegant when you can resolve these forces against one another within one piece of metal, that means that none of those loads make it to the frame of the trailer, or the mounts of the suspension.

Typical stiffness mods involve integrating the fender mounts with an upper frame rail. You need an upper mount for the fender, so then you run a diagonal from the top of that mount to the front and rear of the sides of the frame. That makes you frame much taller and the stiffness again rises by the cube of the difference, just like doubling the size of a tube, but in this case it would likely be at least 8 times larger which would make it 512 times stiffer.

You get extra credit if you can move your coilover arms to the sides you are building to hold up your fenders. Then you get super, extra double credit if you can make plans for taking two of these trailers and by adding some more tubes make it into an actual locost! Oh wait, something's biting my butt - damn that reality :rofl:

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PostPosted: March 7, 2017, 5:43 pm 
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Make a much longer version of you connecting rod and connect it between the 2 "lower control arms" That way the wheels will stay the same distance apart which should dramatically reduce the camber but you will still keep the independent suspension.

I think part of the problem you are facing is that you gave the wheels about a 4:1 motion ratio through your rocker arm. Typically when you make a push rod system you want a 1:1 ratio (or higher in my case). Currently you need a really strong spring to get a reasonable wheel rate which makes the shocks virtually useless.

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PostPosted: March 7, 2017, 6:38 pm 
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I misspoke earlier, the 1" tubes are about .090-.095, 13 or 14 gauge iirc. So to list out some of the ideas presented to stiffen these swingarms up that I like, these are mostly in order of easiest to most difficult to change, and probably in order of increasing effectiveness:

1. Retain the rod ends and weld in another tube to connect the two ends and close the triangle
2. Lose the outer rod end replace with a bronze bushing so the outer tube can resist some of the torsion.
3. Cut out the outer tube and put in a large OD tube. Obviously this would be done in conjunction with options 1 or 2.

As far as attaching anything between the two arms, I'm hesitant about. That would probably be a last resort.

I will think about ways to stiffen the frame around the fenders but I don't think they will help much. I've already bought some nice plastic fenders that should mount pretty easily. In regards to flipping around the rockers to be fore-aft, it's too late for that and the frame doesn't have any way to support loads in that configuration.

I will see how the shocks work out on the road. I think they'll be OK. They are being run on a 3:1 ratio. That gives a ride frequency in the low 1hz when fully loaded.


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PostPosted: March 7, 2017, 8:08 pm 
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freetors wrote:
As far as attaching anything between the two arms, I'm hesitant about. That would probably be a last resort.


Think of it as adding an anti-roll bar into your suspension, then you might not be as hesitant.

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PostPosted: March 8, 2017, 1:42 am 
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I was trying to get Freetors to see the elegance of a solid axle, maybe I was being too subtle. I think connecting the two wheels that way is the easiest thing to do and also the best fix. Less educational though.

I'm not sure you understand what I was trying to say about stiffening the frame, I think you were asking how to do that and it basically requires increasing the height of the sides to make it more truss like. You see this in every landscapers trailer and also many race car haulers.

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PostPosted: March 8, 2017, 12:10 pm 
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I think what I said and what Horizonjob is saying are pretty much the same thing. Tie the 2 independent axles together. Horizonjob's idea about moving the arms and shocks to the side makes sense to me. It clears up the floor for a more usable space.

Pardon the very crude markup.


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PostPosted: March 8, 2017, 1:00 pm 
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horizenjob wrote:
I was trying to get Freetors to see the elegance of a solid axle, maybe I was being too subtle. I think connecting the two wheels that way is the easiest thing to do and also the best fix. Less educational though.

I'm not sure you understand what I was trying to say about stiffening the frame, I think you were asking how to do that and it basically requires increasing the height of the sides to make it more truss like. You see this in every landscapers trailer and also many race car haulers.


Oh believe me, I have no qualms about using solid axles if the application warrants it! I've read a lot of Erik Zapletal's posts on the fsae forum about the virtues of beam axles and he has convinced me to someday build a superlight autocross car using beam axles. But in this case it was one of my goals to to have a truly independent suspension because the roads here are very bumpy. I've always kept it in the back of my mind that an anti-roll bar might end up being a necessity, but the thought of introducing a semi rigid member directly connecting the two sides and the unpredictable binding that produces kind of worries me. Also thinking out loud here, obviously in two wheel bump the axles moves up the same amount on each side so the interconnection beam is flexed down relative to the axle height and providing resistance to the swingarm flexure. What exactly happens during a single wheel bump?

I have thought about replacing both sets of rod ends with bronze bushings but I would end up losing the toe adjustment I currently have with the rod ends. I think this alone would pretty significantly boost rigidity since both tubes could then resist the twist. It would also be pretty easy to cut out most of the outer tube and replace with some 2" square that I have left over from the frame. The cut off part could then be welded between the two ends to close off the triangle. I think all of those things together should provide great rigidity.


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PostPosted: March 8, 2017, 1:22 pm 
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rx7locost wrote:
I think what I said and what Horizonjob is saying are pretty much the same thing. Tie the 2 independent axles together. Horizonjob's idea about moving the arms and shocks to the side makes sense to me. It clears up the floor for a more usable space.

Pardon the very crude markup.


I've been pretty coy about some of the reasons I don't want to move the suspension to the side. The side suspension was actually one of my earliest design variations many months ago before I ever started building. The biggest reason is for easy loading. In the picture with all the stuff loaded on the trailer you'll see that the tires are in that area. The tires are located on the trailer by a pipe that runs through the center hole (this isn't shown) like this:

Image

Since my wheels and tires weigh ~45 pounds each I want the most unobstructed loading possible. Also it would be easier to step up onto the deck without all the rocker arms and stuff in the way. Also as has been said before the structure isn't strong enough to support the weight of the load at the sides of the frame. It's designed to channel the loads into the center of the frame.

I do have some ideas for adding some vertical height into a few places with some lightweight triangulation.


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PostPosted: March 8, 2017, 3:33 pm 
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I got the opportunity to actually put it down on its wheels today. Some observations; the suspension is soft like I wanted, it moves very freely so little binding or friction, the damping seems to be in the ballpark, I also noticed one of the frame tubes on the side is bent down a bit at the rear section of the frame. I'll have to figure out how to straighten it and possibly brace it. Another good point, the swingarms aren't actually flexing as much as I thought they were, but one side still has more camber than I'd like and my camber adjuster is maxed out.

I'm really strongly considering just chopping off the swingarm mounts,, using bushings instead of rod ends, and welding the brackets in a set alignment that I like. I'm really starting to think that simpler is better for some parts...


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PostPosted: March 8, 2017, 10:21 pm 
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What benefit does a rocker arm suspension give you over normal leaf springs on a trailer?

If you want a nicer ride that doesn't bounce around as much unloaded then something like this is going to be your best bet: http://www.northerntool.com/shop/tools/ ... _200649000

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PostPosted: March 8, 2017, 11:59 pm 
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a.moore wrote:
What benefit does a rocker arm suspension give you over normal leaf springs on a trailer?

If you want a nicer ride that doesn't bounce around as much unloaded then something like this is going to be your best bet: http://www.northerntool.com/shop/tools/ ... _200649000


Because there are no commercially available leaf springs that would give as soft as suspension as I was looking for. The rocker arms allow me to get the wheel travel and rate I desired with an off the shelf spring-damper.


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PostPosted: March 9, 2017, 12:28 am 
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Just wanted to note that the spring rate is affected by the square of the rocker arm ratio... That always surprises me a bit.

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