Kimini vs Midlana rear suspension

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Vudu
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Kimini vs Midlana rear suspension

Post by Vudu » 20 Feb 2018, 18:49

These are probably questions mostly for Kurt but I appreciate anyone’s informative input.

I have a 1979 Mini that I am planning to build up as a tube chassis, Honda K24 mid-engine little beastie. I’ve been working on the chassis design for weeks on end (using Adobe Illustrator for now but teaching myself freeCAD as well) while reading and re-reading the Kimini and Midlana books with a particular focus on the chassis and rear suspension design. I’ve also downloaded every photo available of Kimini and Midlana.

Needless to say I am dealing with the same limited space issues of Kimini - a.k.a “how to fit 10lbs of s**t in a 5lb bag”, as my father would say. What’s compounding that issue are some aesthetic concerns - I don’t want track width to be as wide as other mid engine Minis I’ve seen and I would like to run 15” wheels with ideally 245/40/15’s in the rear.

All of that brings me to the first question: what are the performance benefits of the Midlana rear suspension design versus the Kimini? I understand that space and packaging factor into some of the decision-making but my chassis layout work so far has me believing I can fabricate a modified version of the Midlana rear suspension that still fits within the very limited space available. I would be able to say that with more confidence after more measuring and drawing.

Second question: the Kimini rear uprights were custom fabrications. Considering what was involved and the end result, does a Miata unit still make more sense?

I should elaborate on the wheel/tire sizing; I’m favoring 15” wheels for bigger brakes and also because I’m considering reproduction Shelby Cobra style Halibrands. The tire diameters front and rear will be 22.5”. I’ve done some work in Photoshop and a 15” wheel with low profile rubber is about as big as one can use before it looks stupid IMHO. With custom flares and some sheetmetal tailoring, the larger diameter is easily accommodated however tucking in greater width is the big challenge. Complicating things further is the fact that in the rear I’d really like to run a wheel with zero or slightly negative offset. To do that, the engine/trans, suspension and chassis are going to have to be super cozy.

This will not be a track car. It’s intended for “spirited’ driving sessions in the mountain twisties and maybe even an auto-x or two but unlike a more track specific car, it does need to have “the look”. I say this only as a disclaimer and to let anyone know that i’m aware of the compromises i’m making in the name of style but if I was after pure performance and no hassle, I probably wouldn’t be building a Mini... I would just buy a new Corvette.


Thanks in advance for the advice and info.

Midlana1
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Re: Kimini vs Midlana rear suspension

Post by Midlana1 » 21 Feb 2018, 07:00

Draw out the drivetrain, suspension, and tires, and see how they stack up - that'll give you the minimum required track width. You may find that the real problem is CV angularity. Huge flares were added to Kimini to make track width work with the chosen drivetrain, and even then, the axles ended up really short, and go through more angular change in bump and droop, which risks hitting the edge of the CV cups. Additionally, because the axles move in a circular arc, the ends move in and out of the CV cups more that standard-length axles. In bump, they can bottom out, and in droop, they may possibly pull out of the CV cup - and that's in addition to hitting the edges.

Once you've drawn that out to scale and confirmed the axles will work, next is seeing how much space is available for the springs - they may well have to be mounted remotely and activated via push rods.

Suspension design is all a compromise. Designing the "best" suspension doesn't work if suspension pivot points end up being where the crankshaft or transmission are - you have to design around the drivetrain. A Macpherson strut arrangement can work very well and is simple, but requires space for the strut tubes and springs.

Only you can decide whether fabricating your own uprights is worth it - if I was doing it over again I'd either use existing uprights or compromise and use bolt-on hub/spindle assemblies.

I've sort of avoiding saying what's "best" because it's subjective. The main point of such a car is that it literally weighs half that of what the engine came out of - that covers a multitude of sins as far as suspension goes. Said another way, you don't need suspension perfection to have a faster car than someone else because the light weight tips the scales heavily in your favor.

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Re: Kimini vs Midlana rear suspension

Post by Vudu » 21 Feb 2018, 17:07

Thanks for the tips regarding the axles and the potential CV joint problems. I’m going to work on the overhead view to get a handle on the unknowns.

Regarding the rear suspension design differences between your two builds; was the design of Kimini’s rear suspension driven by knowledge at the time or space considerations?

Thanks again for the info.

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Re: Kimini vs Midlana rear suspension

Post by Midlana1 » 22 Feb 2018, 07:03

was the design of Kimini’s rear suspension driven by knowledge at the time or space considerations?
Yes :)

Given that I was starting with the shell, everything had to fit, so after the tires and drivetrain were put in position, whatever left over space was available for suspension. Doing Kimini over again I'd probably find some FWD struts and just use those, or do a de Dion (see below) but it's all a compromise. As said elsewhere, our claim to fame is that the resulting car weighs roughly half that of the donor, so it's going to perform better without even trying.

If you're looking for that last 5% of performance, a full four-link A-arm setup will do the best, but takes up the most room and leaves the greatest chance of setting it up wrong. At the other extreme is a four-bar link to a straight axle, stupid-simple yet does surprisingly well on-track (though on the street, the unsprung weight will make the ride painful). Another variation is a de Dion setup; I've never used one but it's a pretty attractive compromise.

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