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If you want to do a short upper arm, why not do a 3 link?

The upper bar does not have to go forward, nor does it have to be exactly in the center.

This will allow for more roll without binding.

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Honey anyone?

Hi Marcus,

It's not obvious from that table entry what that configuration looks like. One huge benefit to the table is you can select a table entry (row) and pop back out with that configuration set in the program. Here's what it looks like. The right (pass.) side is set with the links slightly divergent (they look parallel, but they're not) at the front and the left side has a more traditional, swing arm point of convergence. I say it's asymmetric, because the right and left sides are not setup in the same way.


We've all seen circle track cars with very wonky looking setups where the car is asymmetric and lots of wheel camber on some wheels, etc. It makes sense for that environment, but you wouldn't want to set up your street car that way. I'll go with a symmetric, or nearly so, setup on mine.

I was re-reading some of my reference texts last night because I felt like I needed to refocus on the practical and not just the theoretical. One of my favorite books has become Fundamentals of Vehicle Dynamics by Thomas D. Gillespie (SAE, Inc., Warrendale, PA, 1992). It is very rigorous and has a lot of mathematics in it, but it is also very practical and reduces many cases down to concise models you can reason things out with. I think of it as "Milliken for Mere Mortals."

I found a simplifying model that I believe is just what I need at this point. I'm going to spend the rest of the day reviewing other build logs and sketching out some ideas that I think will allow me to settle in on a range of adjustments that I can actually implement, but will also provide the post-build adjustability I want for the car's development stage later down the road.

With the setup shown above, there is no front suspension in the model. So, the turning radius is based on what the solid rear axle is doing. It's not turned much, so that' why the turning radius may be so large. It would only be the difference in the numbers between two configurations that would be meaningful, IMHO. That is, is it getting larger or smaller?

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

I looked at a 3-link early on, but felt the 4-link is much easier to package in these cars and was plenty adjustable too. So, I went that way.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

Fixed that for ya Lonnie, I'm sure that's what you meant to say!
Yes, Team Slotus is a unique group, to say the least. Uhhh... You know I include YOU in that group, right?

We had a pretty good weekend here, Bro. I hope you and yours did too!

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JD, father of Quinn, Son of a... Build Log
Quinn the Slotus:Ford 302 Powered, Mallock-Inspired, Tube Frame, Hillclimb Special
"Gonzo and friends: Last night must have been quite a night. Camelot moments, mechanical marvels, Rustoleum launches, flying squirrels, fru-fru tea cuppers, V8 envy, Ensure catch cans -- and it wasn't even a full moon." -- SeattleTom

I'm honored to be included, JD.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

"Just because your car has a heavy-duty, solid axle doesn't mean that the rear wheels don't steer your car", is the way Herb Adams puts in in his book, Chassis Engineering. He built a lot of high-performance race cars with live axles and if you attended SCCA Trans Am races, you do doubt saw some of his cars run. That notion is what motivated me to put so much time into doing the research and information gathering for my live axle design. I didn't want to put all this work into a Locost and end up with a twitchy, tail-happy, marginally performing, 7-clone because I just did what has been done before without examining the reasoning behind it. I feel confident that I've done my home work and developed some objective criteria to use for the design of my particular setup in all the important areas of a 4-link design.

I'm going to be spending a few days getting my findings written up and my design criteria put in tabular fashion and prioritized. Then it's just a matter of testing out the few candidate configurations that are in the ball park and implementing the best choice. Best choice here also mean a design that lends itself to easy tuning after the car is built. My plan is to take some time and develop the car after completion until I'm totally happy with the way it performs.

I'll likely be back within 7-10 days with something specific.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

Work continues. I didn't want you guys to think I'm slacking, so here's a couple of screen shots from my 3D package today.


As you'll see from the screenshot below, the arms are parallel to each other and level with the ground; classic Locost position.


So, given what I've been saying I wanted to do, namely non-parallel trailing arms, what gives here? It's just a way of making sure the structural landing spot for the forward end of the links ends up in a place where my adjustable mounts have enough real estate to work. The 3D model is actually easier to work with than the real chassis for this kind of detail. The current plan is to have enough adjustability to go from understeer through neutral steer to oversteer by manipulating the 4 front mount locations.

I've learned a lot and done a lot of figuring and calculating, but haven't finished my design yet. I did discover that the classic, equal length, parallel and short trailing arms have a lot of positive characteristics save one; probable binding near the end of travel during bump and droop. You all know about that from Cheapy's experiments.

In practice, the lengths of the arms have much less effect than the angle of their inclination and virtual intersection point or "instant trailing arm center" and the inclination of a line from the axle center to that imaginary point. I'm thinking of writing a "Locost 4-Link Implementation Guide" where I lay out all the stuff that I've learned with references to various sources like books and the Locost USA website and build logs. Actually, I started it, but stopped because I was doing a lot of writing and no designing.

My current search is for the best spread of the forces across the structure. I can fix up the geometry to do what I want thereafter. After today's work, I believe I'll shorten the upper link about 2" and run the lower link forward several inches to better tie into the diagonal from the lower rail to the seatback nearer their junction. The longer lower link will effect my pinion angle, so I'll have to be on the watch for that.

After that, I've got to do my adjustable front mounts themselves (I have those pretty well noodled out), the jacking screws for the ride height (have ideas about those) and the Panhard rod mounts. I was thinking I'd get a 5" roll center at rear with a straight Panhard rod running from side to side, but that does not look as practical now. My axle bracket would have to be too long and extend down below the bottom rail of the chassis rail an inch or more. I'm thinkin' 6" roll center now and having the Panhard rod sweep forward at an angle to shorten up the axle bracket rather than run side-to-side.

I'm sorry to see several guys drop out recently and list their partially completed projects for sale. I've been on here quite a while, but I'm going to stick with it until it's complete. The fact that I've done a custom chassis with a novel donor has cost me a sh*tload of time because I have to learn each design topic as I go, then come up with new solutions to fit this unique situation. However, on balance, I think it's been worth it. I'm knowledgeable and competent on several subjects I'd be clueless about otherwise. There's more to go too. I still don't know enough about braking systems or (big gulp) programming/re-programing the ECU. There's always another hill to climb isn't there?

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

Lonnie, check out this interesting Watts-link style setup on the front suspension of the Kurtis 500K.

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The B-3 build log: http://www.locostusa.com/forums/viewtopic.php?f=36&t=13941 unfortunately, all the pictures were lost in the massive server crash

The beginnings of the Jag Special,
https://www.locostusa.com/forums/viewtopic.php?f=36&t=19012
Again, all pictures were lost.

That is interesting and amazingly simple. Thanks.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

OK, I think I'm pretty close to done on my adjustable trailing arm set-up. Below is my V3 design and I'm posting it for constructive criticism by y'all to make sure there are no obvious and/or glaring errors.

It's going to be a challenge to make, but doing a lot of simple right angle stuff just looked too gomer-like and I think this design will be both stiff and strong. My original plan was to use 3/8" bolts, but the real estate needed for the slots, washers and nuts makes the size go up substantially over the 4 X 1/4" bolts I settled on. With good marine grade hardware, they will be more than strong enough to resist any shear force I can envision plus give enough clamping force to make sure the brackets don't move around, which in this case is up and down.

Due to the components used, the axle-bracket rod ends plus a Panhard rod, there will be very low lateral forces (axial as they see it) on the 4 bolts. There should only be small "prying" forces on the rubber bushed ends due to small movement of that end of the trailing arms in response to axle motion, side-to-side. That's my judgement, anyway.


It's not easy to see the details of how it's put together above, so here's a sort of X-ray, wireframe view. It's constructed of 1/8" mild steel plate welded together.


Here's the assembly:


This will explain the context better:


Here's a view looking forward from the rear axle that shows the double-shear arrangement and pocket for the 1/2" nut and washer:


I'm going to move on to other parts of the bracketry for the rear suspension and then come back to this one. Sometimes upon reflection and reviewing, I see simplifications that can be made. The one thing I like best here is that there will be only 4 small holes in the ally skin. It will be non-trivial, but workable, to change the inclination of the trailing arms in the development stage. Changing the angle by 2-3 degrees is all that's needed and changes of 0.5 degrees will effect handling. I also did not feel good about a single shear bolt with a "safety" washer outboard as a solution. I think that's great for a race car where you expect constant inspection and maintenance, but not for a street car that's going to be a crusier.

Cheers all,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

WOW! Fannnn-Ceee!!!
Looks good, Lonnie, as well as functional. Like you, I need to study on it a bit... But at first sight, it looks good!
Well done, Sir!

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JD, father of Quinn, Son of a... Build Log
Quinn the Slotus:Ford 302 Powered, Mallock-Inspired, Tube Frame, Hillclimb Special
"Gonzo and friends: Last night must have been quite a night. Camelot moments, mechanical marvels, Rustoleum launches, flying squirrels, fru-fru tea cuppers, V8 envy, Ensure catch cans -- and it wasn't even a full moon." -- SeattleTom

One issue is that a weldment can have a good bit of distortion, so it may not go together as easily as you like or bolt down flat etc.

I am not sure I am following you on adjusting the handling. You can move the roll center by making the pan hard bar adjustable. The trailing arms will let you play with roll steer, bump steer. squat and lift. Roll steer and bump steer are more like problems than solutions. The issue here is the designer puts something in for instance to produce a little roll understeer - but when the driver performs corrections they can bring the opposite affect into play. So the driver can land up chasing the car.

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Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

@GonzoRacer
Thank you, sir. I just finished reading the latest Slotus episode and I wish you the best with your rear fenders as we close the curtain on oil leaks and low oil pressure.

@horizonjob
Yes, to everything you said, Marcus. It's really about having flexibility and options for me, though. One aspect of building a Locost I'm looking forward to is the development stage. I've had a fascination with suspension design and vehicle dynamics forever and I plan to experiment as scientifically as I can given my limited budget when the car is complete. The Suspension Analyzer software I have also offers an optional data logging and sensor integration package, which I will likely purchase (if I can ever get this baby built ).

I've quietly invested a lot of time into researching 4-links and vehicle dynamics. I believe the small amount of adjustability I'm building in to the design will pay big performance dividends when I get to that stage.

And, yes, I agree, keeping distortion down is going to be a big challenge. There's a lot of heat going into a small area and at least one weld will have to be done fully on the inside pocket before its put on the mounting plate. I expect a failure or two before I get it down right. I wish I could find a shop to do small cutting jobs so I could engineer a few slots and tabs into the component pieces. That might help a lot.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

They aren't failures really, it's just development, your allowed to try things out. You'll know when you go from leading edge to bleeding edge.



Feel free to offer advice and pointers for Car9 soon. One thing that helped a bit for me was to just look at this problem in terms of the motion of the actual axle spindles. As they bump and droop, if they move forward or backwards that produces the anti-squat and anti-lift. It doesn't matter how the linkages do this (instant centers etc. ) the bottom line is if the hub move forward or backward it's as though the rear of the car is on a bearing on an inclined plane, nearly vertical but never the less it causes lift or squat.

Car9 is basically in the same boat as your rear axle because i am not using lateral parallel links.

I do have a couple more comments. The neoprene gasket could be a problem. The parts should clamp tightly and lock together by friction when clamped. For the washer you would need something a bit thicker to handle the open space of the slot. Don't use a split washer anywhere on your car, just nylock and or Loctite etc.

I am thinking you have rejected using a vertical bolt with spacers on something like a U shape bracket? That avoids you having to machining slots, and also getting the clamping surface flat after you weld it. The bolt is still in double shear, it's basically the same strength wise. You can use a rod end on the front of the trailing arm to give more adjustment space.

That's a good think about making this bolt on, you can try some different things and you are not boxed in. I do applaud your effort to do all this, it's a good thing.

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Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

yeah, saw that and also saw the bracket wiggling around on top of that sheet of rubber. suspension attachments need to be solid. I would also swap the stainless bolts for grade 8, unless you're going to use the A286 SS alloy. the usual 300-series stainless alloys aren't all that strong really.

FYI, all the B-3 front suspension rod end pivot bolts are grade 8.

EDIT: just noticed that those SS bracket attach bolts are only 1/4 dia, I'd up them to 5/16 minimum, though 3/8 would make me feel better.

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The B-3 build log: http://www.locostusa.com/forums/viewtopic.php?f=36&t=13941 unfortunately, all the pictures were lost in the massive server crash

The beginnings of the Jag Special,
https://www.locostusa.com/forums/viewtopic.php?f=36&t=19012
Again, all pictures were lost.