Subframe Swap Ideas

Hypothetically you have an Opel Kadett that is going to be repowered and you also have most of a Miata sitting in the back yard. You want the rear Miata suspension and differential to be under the Opel instead of continuing to sit in the back yard. 

I know that the start is to measure a lot of things and stare at it for awhile but I am thinking about it and wondering what the best way forward would be. 

Option 1: Unbolt subframe from Miata, remove everything under Opel, roll Miata subframe under and fabricate mounting points to bolt the Miata parts onto the Opel. 

Option 2. Cut the rear body structure off the Miata, keeping the subframe mounted. Cut large hole in Opel, insert chunk of Miata and stitch it together with the welder. 

Option 3?

I will be going back to read the Datsaniti and Molvo threads again. 

Option 1

Option #1 as well.

You can do #2 but #2 stinks much worse then #1.

Did you see what I just did there?

I tried the same stunt (option 1) with my Corolla. The Corolla structure was in all the wrong places to utilize the Miata subframes. Basically, the Miata frame structure is further from the ground than the Corolla, by roughly 4-5" It all needed to be cut out (option 2) to work. I'm just going to build a new frame (option 3) and cut the entire floor out of the Corolla instead. I suspect the Kadett body shell will have similar issues.

Edit: Any pictures of the engine bay for a Kadett show a fairly low main frame rail, just like the Corolla. The Miata subframe and Kadett rails occupy the same space. The Miata upper A-arm pivot point will be right where the frame rail is. Measure carefully.

Option 1.

Watched a co worker do this with a '55 Chevy pickup and a MN10 Thunderbird subframe.

Your progress will be eased by the fact that the Opel was solid axle, but you still may need to use a hammer, and a grinder.

Pay attention to subframe height relative to ground level at the ride height you want to finish at.  That ill be the key to making everything work.

If you read the molvo thread, you will see where I explored both options. 

The Miata rear sub-frame is pretty compact and not to hard to integrate into pretty much any structure. How good it looks is up to your fab skills.

Think  ahead about what adopting the entire rear sub-chassis brings to the party:

A factory designed mounting point for that rear suspension

A fuel tank 

Spot for the muffler to nestle under the car

Spot for the battery in the trunk

The only problem with this line of thinking is that it also applied to the front suspension integration, and hence how I ended up with a Miata under a Volvo.

Pete

I would A.  I've done it both ways.  If the recipient is solid there is no reason to chop it all out.  
 

i did an mn12 subframe in a w body grand prix and it was simple compared to swapping chunks of floor.  

Do some preliminary measuring first. Will the subframe generally fit within the space available. Then, if the measurements are compatable Option 1.

Option 1 was my first thought as well. I'll need to do some measuring and think about how to mount the shocks. I only considered option 2 after finding this video (https://www.youtube.com/watch?v=w-t3AQ4ro50) and noticing how much higher above the original floor the Miata chuck appeared to be located. I would prefer to go with option 1. 

With the nature of the powertrain going into this the Miata fuel tank, space for a muffler, battery etc would not be a benefit. 

In reply to EvanB :

Look up Project Suprang.  Gentleman chopped out the floor and front clip from a MkII Supra and shoved them in a '65 Mustang.  There is, or at least was, some photo-heavy documentation of the swap.

Aw heck, I'm feeling lazy,  here's the link. 

jimbbski said:

Option #1 as well.

You can do #2 but #2 stinks much worse then #1.

Did you see what I just did there?

I'm quoting this because I feel like this post didn't get the attention it deserved.

I did Option 1 on my truck with C4 subframes.  It went smoother than I originally told myself it would be.

Knurled. said:

Option 1.

 

Watched a co worker do this with a '55 Chevy pickup and a MN10 Thunderbird subframe.

 

Your progress will be eased by the fact that the Opel was solid axle, but you still may need to use a hammer, and a grinder.

 

Pay attention to subframe height relative to ground level at the ride height you want to finish at.  That ill be the key to making everything work.

So you just measure the height while it is on the Miata, and then just match that under new body? Is that what your saying ?  Just trying to picture how this would all work. 

In reply to Somebeach :

Yes. Ideally the mounting points are a height from the road that ends up in free space under the Kadett. You weld in new mounts and bolt in the subframe. That's the easiest outcome. If those points are not in free space, but end up inside the existing Opel structure then cutting and welding begins. You could modify the car's floor to accept an intact subframe, or modify the subframe mounts to match the floor contours. Careful measuring is involved. I measured stuff from a straight edge laid along the rocker panel pinch weld and compared dimensions to an NA Miata chassis sheet. You also have to know what ride height you want the finished car at because that will dictate how far up into the body the subframe needs to sit.

In reply to EvanB :

Also, while you're having a beer and staring at the Opel ( do a lot of this) , consider the width implications. Will you be narrowing the Miata rear to match the Kadett front track width. Will the car run wider rear than front track widths? Are you thinking of making the front wider?

In reply to Somebeach :

It's what I didn't do on the RX-7 when I put an FC suspension in it, which is why it handled like riding a toilet (biking term) until I relocated the pickup points in the subframe to where they should have been had I sectioned the chassis to take the subframe where it should have been.

You need to have the wheels and tires you plan to use. You can cheat by making a stand that bolts to the hub and represents the radius of the wheel tire combo.

You can play with the ride height a bit after the subframe is  installed, but ideally you want to get it mounted so that you achieve your ride height with the lower a-arms angle downwards from middle of the car. Miata suspension geometry is not at its best once the a-arms angle upwards. This is all true regardless of what way you decide to go with.

Another reason I like the rear section swap is because you can take advantage of work done by Mazda. The rear bulkhead that is circled has a lot of rigidity designed into the structure. Taking it along for the ride is only going to aid your cause when it comes to the rigidity of the final structure.

How feasible either approach is going to be can only be determined after a lot of time on the tape measure.

Depending on how it would mess your your interior packaging, dont sell the Miata fuel tank location short. You will be scarmbling latter on to find room for mufflers and fuel tank. Or maybe mufflers are not going to be a thing?

One of the reasons for the subframe swap that has not been divulged above is the engine. The plan is a mid-mounted motorcycle engine bolted directly to the diff. 

Not going to keep the stock tank location inside the car right behind the taillight?

Thanks for the info. For what it's worth on my Miata the top on the rear subframe is about 17" from the ground. And the "ear" that sticks off is 5.5" from the ground. 1990 with unknown HR springs. 

In reply to EvanB :

Minor details like that might have been worthwhile knowing up front.

Cradle for sure or create a new cradle that integrates the suspension points and engine mount all in one structure.Kinda like a trike without the front wheel.

Pete

DeadSkunk (Warren) said:

In reply to EvanB :

Also, while you're having a beer and staring at the Opel ( do a lot of this) , consider the width implications. Will you be narrowing the Miata rear to match the Kadett front track width. Will the car runner wider rear than front track widths? Are you thinking of making the front wider?

A Kadette is a pretty narrow little car, so making any suspension or subframe swap work will first require some careful measuring.

Option 1 - following the general process of Project Binky.

Ian F said:

Option 1 - following the general process of Project Binky.

So, a well lit and meticulously clean workspace, matching onesies for everyone who works on it, and lots of tea, biscuits, and references to British television programmes?

Knurled. said:

Not going to keep the stock tank location inside the car right behind the taillight?

I'm planning on keeping the stock tank location. The gas tank in the Miata is where the engine will go.