LocostUSA.com

Hey folks, long-time lurker here.
I've daydreamed this build for years and have spent a good part of this year designing a frame and dismantling a donor GL 1500 for it's engine, wiring, radiators etc. The ideal outcome is an 8/10 scale replica of either a 917PA (1969 can-am entry) or a 908/03. Square space frame, tie rod suspension, fiberglass clamshell and a cockpit as close as possible to period racecars.
I am having a difficult time realizing my frame because I am not confident in my suspension geometry. (I want the frame to triangulate at all of the pickup points). I am trying to learn Sus Prog 3d but I can't really figure out where to start after the tutorial. What have you guys done to get past this? Can I rough it out based on basic geometric principals or should I have simulated my suspension before starting on my frame.
I'm sure many of you will have experience with a front suspension set up similar to what I'm using (2 A arms and a coilover shock), but my rear suspension seems to be out of convention with even a lot of mid-engine builds here.
Lower reverse A-Arm, Upper Lateral Link, outboard coilover, twin trailing links. Used on gt40, 917, 908, formula ford, etc.
Really just looking to get in touch with a few people that would be knowledgeable of problems I may encounter on a project of this nature.
Thanks all!
[img]https://ibb.co/WsD7GnX[img]

I don't understand the issue. Computers aside, a good start is to be able to draw and triangulate each plane of the chassis in the overhead view on paper, then side views. You will probably need to shift nodes as needed to make it work in each dimension. Fine tune in 3d with the program.

You might also consider initially designing the basic irs and ifs individually as modules to tie into the passenger compartment.

I assume you are converting to an automotive diff for practical reasons.

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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12

Given that my intention is to use NA miata uprights, I'm now thinking it would be ideal to copy the pickup geometry from one as nearly as possible except for the lower rear control arms and their trailing links.
Is that methodology lunacy?

I have the front differential from an AWD Porsche 911 (964), which rotates the way I need it to in its correct orientation. An issue with using it (or any differential) is that the Goldwing's output flange is about 5 inches to the right of center. In my most up-to date plans, my rear bulkhead allows for the differential to be placed 5in from center to be inline with that; but I can't find a lot of info about whether or not it would cause any issues to have significantly unequal length axles.

The Car9 design uses the same rear suspension as you, so you might want to read through that. There might even be some geometry discussion about it in the main thread or elsewhere around here.

https://www.locostusa.com/forums/viewtopic.php?f=39&t=13810

There is nothing wrong with building a frame, then design the suspension and figuring out where and how to build the mounting points. That's basically how most 'book' Locosts are pretty much all built. However, there are also certainly many advantages to knowing where the suspension mounts will go before building at least that part of the chassis. As noted, you can rough the design, then move the designed nodes as necessary once the final locations are determined. It may also be possible to even start building sections of the chassis that are not directly supporting the suspension box(es), before the suspension mounting points are finalized.

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-Justin

"Orville Wright did not have a pilots license." - Gordon MacKenzie

That's how I designed Kimini and Midlana. It avoids placing the inboard suspension pivots in just the right place, and only then realizing they're nowhere near chassis nodes.

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Midlana book: Build this mid-engine Locost!, http://midlana.com/stuff/book/
Kimini book: Designing mid-engine cars using FWD drivetrains
Both available from https://www.lulu.com/

That rear suspension design was fairly typical of formula cars of that era. Horizonjob's use of it on the Car 9 design was based on his Formula Ford experience. The Car 9 chassis is built around three strong bulkheads. The front bulkhead carrying most of the loading from the front suspension. The dash bulkhead fixing the front of the long rear trailing arms. And the rear bulkhead carrying the inverted-Y and lateral links you described. Everything in between is mostly just tubes and triangles.

The inverted-Y rear suspension works for front engine cars like my Car 9 build or mid-engine cars designs like you are doing. I used Wishbone and VSUSP for both front and rear suspension analysis. For analysis purposes, the rear suspension you described can be thought of as upper and lower A-arms with three pivot points each.

Keep up the good work!

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Cheers, Tom

My Car9 build: viewtopic.php?f=35&t=14613
"It's the construction of the car-the sheer lunacy and joy of making diverse parts come together and work as one-that counts."
Ultima Spyder, Northstar 4.0, Porsche G50/52