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Hello all! I've been meaning to get some feedback from the Locost experts for some time but was more interested in just researching and building then asking for opinions each step of the way. I realize it's a bit backwards to build then ask for suggestions afterwards but everything is just tacked in place and nothing is permanent with an angle grinder and ear plugs!







I started a thread a few months back in the suspension design section regarding the uprights I'll be working with:
http://www.locostusa.com/forums/viewtopic.php?f=5&t=14075
The overall concept remains the same but based on the prototype I made a few changes that incorporate more machining.




The rear uprights are designed around the same combination of water jet cutting 6061 AL, minimal machine work, and welding. They are still in progress but the basic layout is set.

The front suspension is the standard SLA w/ push rods. The kinematics are setup for a 1:1 wheel to shock movement and ~1 deg of camber gain per 1" of bump. I'm still working out the bell crank mounting point. Inboard pickups are urethane bushings from a buggy supply shop. Outboard pickups are 5/8" high angular motion heim joints on top, 5/8" spherical bearing on the bottom.



The rear suspension is a McPherson strut setup w/ a tie rod to accommodate the mid-engine arrangement and the corner I designed myself into with the main spars. More on that later. i.e. I would never do that again! The strut top mounts are still in design so please don't get too hung up on them.



Steering is 14" buggy rack w/ the U-joints arranged towards the upper end of their useable range but within spec.

The tank is a AL buggy type set directly between the occupants. The battery is also directly between occupants. Pedals are floor mounted and adjustable fore and aft. The mounting brackets for the seats will also allow for small tilt adjustment and some fore aft adjustment. I have a wife who would like to drive this car but I'm a 95th percentile male and she's a 5th percentile female so I have my doubts any reasonable amount of adjustment will be enough!

The shifter is a MR2 5 speed. The multiple sticks show the extremes of range.

The engine bay will be a bottom mount sub-frame that will complete the "box" of the engine bay. I added in front a rear triangulation points to help strengthen the arrangement. The engine will bolt to the sub-frame in 2 locations with a third being to the main side spars.

The body work is also in design but here's a rough concept of the overall look I'm going for:


Here's where I am with the build:





That's the basics so please shoot away with questions!

You have some big chassis tubes. The loads on a car come from the coil overs and the suspension pickup points. Somehow those things don't seem to connect to each other though…

There isn't any triangulation in the front or rear suspension and coil over mounts You're cantilevering all those loads off those welds on the square tubing. Maybe you can fix this with some small diameter round tubes to carry the actual loads. That's the best I can suggest. It makes sense to do some modeling or math on paper if you're going to build something that doesn't closely follow various accepted and common practices. Don't give up, but you may have a lot of work to do!

If you prefer to work in the real world instead of on paper or computer, maybe think of a way to put a 2-3 thousand pound load on your various mounts for coil overs and suspension pickups. Use dial gauges to see how much things move…

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Marcus Barrow - Car9 an open design community supported sports car for home builders!
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Comments...... ok.
Don't show them spindles to RoosterBooster he'll spend a week of sleepless night wanting to improve them
I like the steering shaft path, a definite benefit in the event of a catastrophic front end collision, you're not likely to get speared by the shaft.
Plus if you center mount the gauges you could go from lhd to rhd really easy
One of our members used a similiar steering rack and had issues with excess play. As I recall, his opinion was that it wasn't made as well as it should have been especially for the price.
Check with your local inspector, some (most I think) jurisdictions will not accept a welded steering arm.
I'm undecided about the center two beams, I'm assuming that you will be adding some more to the chassis (tunnel and such) but to my dilettante eyes it seems that the two boxes should be connected a bit better to reduce flex.
Horizenjob beat me to the rest

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"There are times when a broken tool is better than a sound one, or a twisted personality more useful than a whole one.
For instance, a whole beer bottle isn't half the weapon that half a beer bottle is ..." Randall Garrett

Something was bothering me about the spindles. ..
I'd like to have seen material where I filled in with red. The two yellow lines display the edges of the load supporting area you have, the black line shows the actual amount of cross section.
That's a rather heavily loaded node. .. Spring loads, steering loads, brake load, instantaneous shock loads, etc.

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"There are times when a broken tool is better than a sound one, or a twisted personality more useful than a whole one.
For instance, a whole beer bottle isn't half the weapon that half a beer bottle is ..." Randall Garrett

well it appears you have come to the party a bit late in your build.

you seem to have a nice stiff chassis but chose not to mount the suspension to it but instead chose to build a flimsy box under the chassis to mount the suspension on.

as far as my unskilled eye can see, you need to completely redo the whole thing bearing in mind that what you should be doing is securely mount the suspension points and the chassis is just there to link the front to the back as stiffly as posible.

it apears that you have chosen a design likened to a railroad car with bogies front and back!

its heavy and flimsy at the same time.

it looks like a flimsy ladder chassis with a gerder poorly mounted above it

all those rectangles needing a diagonal when trapisoides could have been used.

try adding a tube from the kink in the lower chassis just behind the rear mounting point of the LCA to the top rail of the chassis, then add another tube from the front of the "subframe" to the lower chassis rail.

after that put some diagonals in the area of the LCA pickup points in a horizontal plane to stop it "walking" around under braking.

steering racks, i have tried dune buggy type racks, they are junk, they wear out and cannot be adjusted, they also have straight cut gears which feel notchy. bite the bullet and get a porsche 914 rack, they are cheap enough on e bay and can be rebuilt for less than $20, they are also adjustable.

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this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.

And now for some contrast, I think it is very good but a different approach than what we are used to seeing.

I hope the "constructive" critque doesn't put you off.

I would have used full size tube transversely above the footwell and at the dash or load spreaders on the ends of the smaller tubing, along with full size tube below the roll hoop with diagonals behind it but that's just me.

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X2 - Some larger fillets all around would dramatically improve the strength of those uprights with very little weight cost.

Either the scale of the seats is off or that of the "passengers" - or are they supposed to be 7' tall? Did you grab those seats off of GRABCAD? I've been looking for some better CAD of those as that is what I have for my project. PM sent.

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-Keith

I like the 8020 Build table; that's pretty cool. As was mentioned before, if your model is correct then you need your roll hoop much higher. It needs to stop the vehicle from pancaking your spine in a roll over. I'd say without more of a cage around the passenger cell you should aim for a minimum of 3 inches from the top of your head to the top of the roll bar to be safe.

First off, thank you all for the helpful critiques! All are valid points when dealing with life and limb! My 19 mo old son thanks you too

Secondly, it seems the general consensus is that more triangulation/support is needed in the front and rear suspension pickups. I was prepared for this concern though and don't disagree that further supports are necessary but my original thought process was that if you follow the load path of a purely horizontal load on the LCA it isn't completely reacted by the box under the lower spar. Much of it is, in fact, carried in the plane of the floor which is triangulated horizontally as well as vertically into the main side spars.

Be that as it may, I did wrestle with the idea of further support similar to Horizon Job's suggestion but decided against due to excessive overbuild. I suppose it's better to err on the side of caution in these matters and easy enough to rectify. I added tubes (red) as suggested, all of which should be carrying the suspension loads into the side spars. I will probably replace the tubes in favor of .125" steel plate as the loads should be tension.



The free body diagram I come up with to describe the pivot point of the bell crank results in minimal deflection of the top support tube as the forces are reacted through the lower support tube. Please let me know if you think otherwise

The rear is a bit more of a cluster admittedly. I'm am seriously considering drastic cutting and redesign if I can't reasonably strengthen the rear suspension pickups in an elegant manner.



I assume by boxes you mean front and rear suspension boxes?? I wasn't planning on running a center tunnel as the thought was to maintain the loads through the 4 main spars but that would of course necessitate further connection between the suspension pickup and the side spars. See above.



I performed a FEA analysis of this simplified assembly with a my estimated front corner loads at a 10g bump and was well within 5+ factors of safety. Adding more radius certainly would help with stress concentrations which is always a good thing! It's 1" thick 7075 AL BTW.


Like this?



Like this (looking from the bottom, car forward to the bottom of the pic)?



Ha, yea. I'm 6'5" so those dummies represent me. I measured all my appendages and resized each joint in the model accordingly so it should be pretty accurate.

8020 is great stuff and it has been a third and fourth hand in many occasions! These are actually Faztek parts which are a cheaper version that interfaces with 8020 just fine.

While I don't disagree with you regarding the function of a roll cage, me being so tall and the car being so low adding an extra 3 inches would make the roll bar look way out of proportion. Besides I see Miatas running around all the time with the drivers head at or above the roll bar/cage so if they can do it so can I! That is, of course, a ridiculous argument I know but I'm sticking to it

As long as you are aware of the situation; beyond that is your sense of fun!

Have you checked how straight your 8020 is? I've had a lot of my longer segments (8 feet and up) show up with some warping. I don't know if it was originally straight and bent during shipping or what but it strongly discouraged me from using it for more precise tasks.

If your head gets squashed in a wreck then you won't be able to see how out of proportion the roll bar is.

A roll bar isn't about looks, it's about function. It looks just right when it's the proper size to do it's job.

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mobilito ergo sum
I drive therefore I am

I can explain it to you,
but I can't understand it for you.

look at the chassis from the front, can you see all those rectangles, if you intend to brace them with diagonals, then thats o.k. but if you were to eliminate as many as posible, then the diagonals would be surplus to requirements and lighten the chassis.

in my opinion there are already far to many tubes, you are fixing a point in space and fabricting tubes to get that point but then having to brace the tubes with additional tubes.

compare your chassis to car 9 or midlana, there tubes are all doing something, not just adding weight.

look at the front of a caterham and then look at yours, then start cutting out stuff that is of no real purpose because its dead weight.

as far as the horizontal diagonals, no not as you have shown in red, put a big "X" in that square in the middle.

in space frame chassis deign, a square or rectangle is your swarn enemy and should be avoided at all cost because it will "rack" unless additional diagonals are used to stop the racking.



in the chassis above, you can see the rectangles, look at the lines of force imposed upon the sides of the chassis along side the engine bay, the two sections form opposing triangles instead of the "parallel" diagonals in a standard chassis due to all the suspension loads being in the bottom of the frame, this gives a stiff side to the engine bay which is a weak spot on a traditional chassis, under load, the apex of the triangle wants to move outward but is contained within the horizontal diagonal bracing in the top of the engine bay, the other weak spot is the driver compartment but it has the benefit of the tunnel for additional support.

now look at the chassis from the front, note the "X" bracing in the inner rectangle, this transmits the loads from the rear of the LCA under braking into the upper frame rail on the opposite side of the chassis, in the foremost rectangle, the bracing is an extended "V" to do more than just brace the rectangle, it also supports the rack mountings and transmits the steering reaction loads into the main chassis.

at the "floor" area rectangle in the front, this is not braced in the conventional manner with diagonals, but has a 16 gauge plate fully welded inside the perimiter tubes not plug welded to the bottom, this does all the bracing and provides a strong place to jack the car from.

look how simple the tubes are, now look at what you have!

_________________
this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.

Dude, what's the wall thickness of those chassis tubes? They're huge!

John, thanks for the breakdown. Very helpful but I have 2 points to note:

1.) With all do respect to Car 9 and Midlana, I don't believe it's completely fair to compare my chassis against theirs. Both Car 9 and Midlana are designed around a 100% function over form design philosophy with a seperate body to give it a pleasing form. All things being equal, this is the best approach but this is not the approach I took as an exo-frame car requires certain design compromises. In the typical function over form approach you set your suspension attach points and build out, as you are suggesting, keeping in mind how the chassis is loaded through the wheels. Unfortunately this is not as easy to accomplish with an exo-framed car, at least in my limited experience. The chassis now has to play double duty by being both functional (primary) as well as following a certain design aesthetic (secondary). This sometimes results in the most obvious chassis design based on the required suspension kinematics and load bearing structure to not align with the intended aesthetic design. In my opinion this is the fundamental flaw with an exo-framed car and the reason I would never go down this road again. But I'm not here to debate the realities of traditional approaches vs. new age exo cars as I'm sure I can guess your opinion but I do want you to understand why I choose certain elements the way I did. Certainly there's room for improvement and admittedly the rear suspension is a "work in progress", all in all this has been a very fun design experience as it required quite a bit of lateral thinking and outside the box approaches, all of which I believe to be the true reason I sit down in front of the computer for hours at a time!

2.) I fail to see anything different from what you show in the chassis above and mine with respect to the LCA mounts. Both are feeding in the wheel loads almost in plane with the floor and both are/will be seam welded with heavier gauge steel plates, not plug welded. Further, the rack is mounted directly to the floor which also reacts the steering loads in the same manner. My thoughts on the diagonals is that by having them on the outside they act similar to a bike spoke in that their real strength is that they act in tension, transfering load into the chassis on the same side as the load is being applied.

Yea, I see my tubes and I like them just fine