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It's OK, no slide rule needed!

The frame is not so critical as long as it has adequate torsional rigidity.
There are so many different pieces to the puzzle that have to interact in a lightweight sports car design.
First question is What are you going to use it for?
This will help you determine what donor parts might be best.
The donor parts determine what the frame needs to be.

I believe there is an issue with using a Miata (potentially others) IRS subframe encroaching into the seating area.
Your desire to incorporate IRS is a violation of the KISS principals.
Unless the car will be operated at it's limits it is unlikely that an IRS would provide a tangible improvement worthy of the additional engineering.

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Honey anyone?

It will be just a fun weekend car.
I would prefer something in the 250 ish horsepower range. That most likely won't happen, with the Miata engine. I don't want to do crazy modification to the engine, I would like to keep it as stock as possible, also stock ecu and such.
What issues you foresee at the rear end with irs?
I have no idea which donor to use, I need help there as well. I'm looking at ecoboost engines(2.0-2.3L), Duratec 2.5(needs help in the hp departement, but they are very cheap), maybe BMW 3.0L I6, maybe even a Volvo with rwd.
I'm a first timer building something like this, so I can use all the help that I can get.
I would like to make a good decision, before I start.

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Tibor


'20 Alfa Romeo Stelvio daily
Locost/442E in progress

All in my opinion… unless you’re especially picky/discerning or a have a specific requirement, a live axle is perfectly acceptable in a weekend cruiser. IRS is far more complex and much easier to do ‘wrong’. If you enjoy challenges, go for it, but I guarantee you’ll have substantially more invested in that decision. If you prefer simple, a live axle will get you on the road with less headaches, time and money, and can work really well - arguably better than a poorly designed IRS. Here’s an example of how simplified it can be done to a ‘book’ chassis…

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Peace, Ron

That is really simple. Thanks.
I just think, that the IRS is more modern suspension, and it should handle better.

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Tibor


'20 Alfa Romeo Stelvio daily
Locost/442E in progress

Handle better on the typical uneven surfaces we see on public roads and ride far better. The difference in unsprung weight makes all the difference in ride quality.

For track cars the difference on a fairly smooth track can be pretty small. In the real world, I'd always go with the IRS. There are a couple different versions of IRS using Miata components around based on the Haynes book and if you're capable of building a chassis, you're capable of building one of them. Miata's are far and away the easiest donor to find as well these days unless you want truck parts. The days of cheap/easy to find 1st gen RX7/AE86 Corolla axles are long gone sadly. When I built my first Locost in ~2002-2003 they were still easy to get and the Miata hadn't really been done yet.

I'll be the dissenter and say go IRS, you won't regret it.

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Bill J
442 Locost - in progress
Mid-engine Beetle - In Progess
'12 GTI - Daily
'01 Chevy Cargo Van - Workhorse parts hauler

In the last few days I'm checking out CL, Offerup, Facebook marketplace, and there is not much Miata stuff there. If there is something, it is expensive. Power is not there either.
I see some 2016 suspension parts, that would work, or it should be the NA NB parts? Could I use even the latest parts? It would be nice the latest Miata, with much more power, maybe one from Copart.

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Tibor


'20 Alfa Romeo Stelvio daily
Locost/442E in progress

How much HP do you need... keep in mind these are extremely light cars. For a Miata powered version you're looking at dropping 800lbs or more off the stock Miata. If that's not enough throw a turbo on a stock Miata 1.8 and you're easily at 200+whp with a reliable package assuming you tune it properly. For the chassis plans that are out there you'd want NA/NB parts (1990-2005), not ND (2016-). I'm not sure what area you're in but finding parts is pretty easy around here (Pacific NW). Look for Miata specific buy/sell forums on FB for your area. A complete car from Copart would work as well and it's a great way to get ALL the parts you may need (and make some of the money back selling body/interior parts).

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Bill J
442 Locost - in progress
Mid-engine Beetle - In Progess
'12 GTI - Daily
'01 Chevy Cargo Van - Workhorse parts hauler

I'm in LA. Most of the Miatas are around $5000-8000.

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Tibor


'20 Alfa Romeo Stelvio daily
Locost/442E in progress

Here too, parts are plentiful but whole cars are going up in price. Keep an eye on Copart. I saw one on there in Socal last week and the price was fairly low.

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Bill J
442 Locost - in progress
Mid-engine Beetle - In Progess
'12 GTI - Daily
'01 Chevy Cargo Van - Workhorse parts hauler

Check out CL in LA up to $2500:

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Tibor


'20 Alfa Romeo Stelvio daily
Locost/442E in progress

Upper right corner looks like the IRS assembly. Might be worth looking into that. Then you can couple it to any drive train you want... within reason.

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Cheers,
Logan

That's an ND setup so no specific plans available, but could certainly be made to work.

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Bill J
442 Locost - in progress
Mid-engine Beetle - In Progess
'12 GTI - Daily
'01 Chevy Cargo Van - Workhorse parts hauler

Here's a suggestion. Since you want an IRS, look for a Merkur XR4Ti. I've seen a couple on Craigslist for $750-1200 over the last year. You'd have a full set of plans (the Gibbs/Haynes Roadster book + Haynes website) + a simple turbo I4 that can get you to your HP goal.

There are some nice builds that have been done on this site + nice UK and Australian versions too.

Try one of those crawlers for Craigslist and see what turns up.

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

Motor choice determines the hood height.
Trans width determines tunnel width (footroom)
Chassis width determines which nose you run (or have to cut up)
Someone recently started making some 12" rear fenders in fiberglass.

I originally bought a very nice IRS setup, but I am convinced a 3 link will do the job better.
There are too many interacting adjustments on a IRS, also they are not friendly when it comes to ride height changes.
My goal is an AutoX weapon so I am concerned about roll steer and camber issues along with being able to quickly change the balance at the track.
For a street car a simple 4 link is beyond adequate, few could drive it and tell the difference.

For a town cruiser and a first build employ the KISS method with a heavy hand, that will minimize any hair loss.
Don't overkill, keep an eye on weight, to stay within the spirit of a Locost. A big honkin motor is like an anchor when you turn the steering wheel from straight.
The original Lotus 7 came with a ~1000cc pushrod motor.

Start with selecting a wheel and tire package, remember they will have to fit over your choice in brakes.
13" will not clear many adequate brake packages, and 14" has a poor tire selection.

A Duratec is an excellent choice but on the tall side I think, the GM Ecotec turbo is a choice that will help keep the hood height down.
Power to weight ratio then package size will help you determine which direction to go.
Choose an engine that has a trans you like so you don't have to make stuff fit together.

I think for a lot of reasons a single donor limits the builder and can make the build more difficult and or a less desirable finished product.
This allows you to pick a motor that fits your hood height preference and choose suspension parts that are lighter and perform better, etc.
I use Speedway Motors and other similar suppliers for aftermarket suspension parts.
A single donor makes it easy to begin by eliminating choices, but compromises the final product.

Don't start a build without knowing in advance your state's requirements to register your creation.
You are building a Lotus Replica a pre-engineered and recognized design. You may be able to use that as a workaround to beat a requirement for engineering signoff.

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Honey anyone?

Thank you for all the pointers.
I have read a while ago, how to register this type of car in California. I don't remember reading anything, that would be a problem, right Lonnie?
Usually what wheel sizes you guys are using, 15-17"? There is a preferred size? Bigger than 17" is not good for the weight, right?
I don't know yet which donor car should I go with, any ideas are welcome.
Good idea about the Ecotec engines. I see a lot of Buicks, Chevy HHR, Cobalt SS, some Cadillacs, Saab 9-3 etc. use these engines with very good powers 250-270hp.
The question is, which one has a manual transmission and maybe even rear wheel drive? I don't know anything about these cars.
https://en.wikipedia.org/wiki/GM_Ecotec_engine

There is only one Mercur, and it looks very bad($1000), specially the engine.
https://losangeles.craigslist.org/lac/c ... 52358.html
I think that I'm more exited about the Ecotec cars now.

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Tibor


'20 Alfa Romeo Stelvio daily
Locost/442E in progress