Scratch Build?

[schukar]

I built a Factory Five Roadster 15 years ago and I'm now looking for my next car project adventure. For decades, I've been intrigued with the idea of moving the engine/suspension of a front-wheel drive car to the back of a vehicle to create a mid-engine car. With the 818, Factory Five has shown the way; plus I own an '06 WRX so it seems like destiny. My plan, however, is to forgo the kit and use Miata parts for the front end to make a vehicle that's a bit more unique. This will give me tons of opportunity to try different things (my favorite part).

My first question is this. What is the minimum modification that I will need to make to the WRX front suspension so it will work for the rear suspension of my build? What attributes does the rear suspension have that the front suspension lacks? I don't have a good understanding of why Factory Five made the changes that they did but I would like to avoid unnecessary expense and work. Any advice would be appreciated.

Thanks

[Old Timer]

I think I would buy a 818 manual and use it as a guide for setting up the engine and transmission.

[schukar]

I think I would buy a 818 manual and use it as a guide for setting up the engine and transmission.

Thanks. I've done that and it has been super informative.

I should add that I'm a decent amateur welder and machinist with 40 years of engineering experience so I feel confident making modifications to any WRX parts I use. It occurs to me that FFR kits are put together so no welding is required - a limitation that I don't have.

[NAZ]

Sounds like a fun project. I suggest you first start by reading and learning everything you can about suspension design. The work you are proposing is going to be massive in $$ and time and you will be engineering out of fitment issues most everyday. If you're going to undertake that amount of effort you should understand suspension design and not simply try to re-purpose an OEM design. I would also suggest you consider purchasing a suspension program like SusProg3D to help in your design. I think I paid ~$300 for it a few years ago and it's well worth the money. Like shop tools, modeling programs help design efforts. https://www.susprog.com/

[Sgt.Gator]

Another good resource is the Midlana build book since it uses Miata parts. The author used to occasionally cruise this forum.
http://midlana.com/stuff/book/

[Bob_n_Cincy]

Another good resource is the Midlana build book since it uses Miata parts. The author used to occasionally cruise this forum.
http://midlana.com/stuff/book/

Here is the latest suspension design from Chevy : https://www.youtube.com/watch?v=H-m_...ature=emb_logo

[Santiago]

My plan, however, is to forgo the kit and use Miata parts for the front end to make a vehicle that's a bit more unique...

My first question is this. What is the minimum modification that I will need to make to the WRX front suspension so it will work for the rear suspension of my build? What attributes does the rear suspension have that the front suspension lacks?

Hey schukar, I'm not entirely sure I am reading you correctly here. You want to use Miata based suspension in the front of the 818 chassis and the WRX front suspension bits in the rear of the 818 chassis. Is that correct? If so, here would be my initial thoughts:

The 818 front chassis doesn't have a decent plane of reference on which to mount the Miata control arms. The Subaru front suspension is strut based, not an SLA layout like the Miata. So FFR only needed to locate one (the lower) OEM control arm. They use a bracket to convert the Subaru strut upright into an upright suitable for an SLA layout. That left them free to put their upper control arm where they wanted it without concern for making an existing upper OEM arm function properly. You're in for extensive reworking of the front chassis bars to properly locate the upper and lower Miata control arms.

In the rear, FFR used a 5-link layout loosely based off of the Subie rear suspension bits. The 5-link is a significant departure from the OEM Subie layout at the rear, which is just a 3-link on a strut-based upright. FFR offers brackets to convert this upright to their 5-link layout. There is no way to readily use the Subaru OEM front suspension at the rear of the 818 chassis. Moreover, FFR's bracket could still be used to convert the strut-upright into an SLA upright, but the upper control arm they provide is unlikely to work with any nearby tube at the rear of the 818 chassis. As others have suggested, this sounds like a HUGE amount of work.

The 818 chassis was designed to convert Subaru rear suspension even though it uses the front drive axles of the Subaru trans. They did a pretty good job; the 5-link layout gains more camber than the OEM strut configuration and the top of the upright is well located. Multi-link rear suspensions can be designed to work extremely well. [My Mazda RX-8 used this sort of layout and it was an absolute dream to drive.] So to directly answer one of your key questions, the real attribute the rear suspension has that the front does not is that it will work very well as they designed it in the 818 chassis - not so with the front (see below).

There's a more serious concern with using the OEM Subaru front lower control arm at the rear - namely that the arm is not an "A" arm, it's an "L" arm. This means that the orientation of the long end of the arm is not correct for rear use. You typically wouldn't want the long end of the "L" to point to the rear when used in a rear suspension layout. Conventional thought is that an offset control arm at the rear has the longer leg point towards the front. There are much better OEM control arm options if you were really set on using something different than what FFR supplies.

Glad you can weld! Quite a few of us have found that we wanted to change what FFR delivered (esp. the 818R builders who want to change the angle of the transmission). You may find you need that skill even if you elect to go with the Subaru suspension as FFR intended. =)

Best,
-j

p.s. - I totally agree with NAS and others that you would do well to muscle up (heavily) on suspension design

[schukar]

Santiago -

Thanks for the info!

I am not planning to use the kit. I'm building from scratch.

What would you think of maintaining the OEM geometry for the rear suspension. I know struts have some issues but I think I can live with them.

[Sgt.Gator]

I think we are confused...you are building a mid engine car using a front wheel drive unit (like the Midlana), and you are not building an 818? Or are you building an 818?

[Santiago]

I think we are confused...you are building a mid engine car using a front wheel drive unit (like the Midlana), and you are not building an 818? Or are you building an 818?

I think we are definitely confused. I'm confused.

I think for me the source of confusion is this phrase: "I am not planning to use the kit." The 818 is a "kit car." The whole thing is a kit. So "not planning to use the kit" could mean you are not planning to build an 818 at all...or it could mean you are not planning to use portions of the kit.

If you are indeed planning to build an 818, but you don't want to use portions of the kit - WELCOME! Plenty of us are in that boat.

If that is correct though, I can't see any reason in the world why you would want to maintain the OEM geometry for the rear suspension. The FFR supplied multi-link is superior. Full stop. End of story. Not only does it maintain a better tire contact patch throughout its full range of motion, but the chassis is already set up for it as supplied by FFR. So it's easier to build. Working harder to go backwards makes no sense to me.

If you are not building an 818 and are just hanging out on the forum, then I can see some appeal of maintaining the rear strut suspension. I wouldn't do it if I was building from scratch, but others have. One of FFR's old "Cobra" kit-car competitors used to offer a strut-based chassis. So it's been done, it works, but why? The only answer that comes to my mind is that you know the geometry is decent, so you do like the hot-rodders of old and replicate that geometry in a new chassis. While I have tremendous respect for hot-rodders of old, I also think the knowledge of suspension design has been taken out of the black box and is more widely accessible now to those minds who wish to learn.

Work smart, not hard. That's my pitch. =)