Ducati powered Fiat from California

Introduction

Hello everyone! As the title suggests, I have decided to build a Ducati powered Fiat 500!

I have restored a couple classic American cars, some motorcycles, and have designed and built a handful of Formula SAE cars in college, but I've always wanted to build my own motorcycle powered road car.

After a decade of working at one of the largest Spacecraft companies in California, I decided to take a break and try to make this dream a reality. The car needed to be simple, lightweight, relatively cheap, and older than 1975 (California smog requirements). I have a buddy that did this with an old Honda S600, and another with a Fiat 850 spider, but I wanted to do something a bit less roadster-y. Then, as the wife and I were honeymooning in Italy, I saw the answer: the old Cinquecento.

There seems to be lots of motorcycle powered 500s for hill climbing, and Z-Cars is one of the most popular swaps out there with their Suzuki Hayabusa swapped "Fiabusa" (they also make a Subaru swapped "Fubaru"), but I wanted to keep the motor air cooled (no big radiator up front) and Italian. After a bit of research, I decided to go with my favorite motorcycle engine, a Ducati 1100. The last of the big, air cooled, dry clutch L-Twins. How hilariously adorable would this car be with that noise coming out of it?

I spent a few months looking for the perfect car - something that was driveable, in decent shape, but not too nice, so I could save money up front and put it toward the swap. I ended up with a clean-ish 1974 Fiat 500 R with about 26,000 km on the clock that had been imported into the states a few years prior. The body was in pretty good shape until a strap broke towing it home resulting in a head on collision with the trailer. I minimally repaired the damaged area so I could turn the wheels full lock again, then drove it around a few months until I got the car registered.

Now that the car is legal, it's time to begin the project!

I'm looking forward to sharing the progress and hearing everyone's thoughts on the build. I have reached out to a few people individually and everyone has been extremely nice and very helpful!

Reactions: porta and ben

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Latest updates

Engine and Diff installed!

Mar 31, 2025

After a few slow months I have finally installed the Ducati engine and Elite differential into the Fiat!

The biggest challenge was finalizing the belt drive components and getting everything aligned. I ended up redesigning the part to consist of a laser cut splined piece (connecting to the engine output shaft) that bolts to a machined driveshaft that connects to the belt pulley. This worked much better than any welded assembly I could have made!

Once I knew were the engine side pulley was, I needed to align the differential frame to the engine. Drive belts, unlike chains, can only tolerate about .25 deg of misalignment, and with these pulleys being so close, I needed to get them aligned as close as possible. After what seemed like an eternity of aligning and jigging, I finally welded the two frames together!

Now that the two frames were joined, it was time to install them in the car. After a roughly positioning the frame, I wasn't happy with the location of the diff with respect to the wheel arches. These CV angles are already pretty high, so I decided to redesign the mounts to shift everything a bit further back, reducing the CV angles by about 5 degrees. Since I was committed to redesigning the mount, I also incorporated a polyurethane interface to help reduce some of the engine vibration making its way into the car.

With the new mounts installed and the frame in place, it was time to add the last few tubes and officially get this engine and differential installed!

I'm pretty happy with how it came out. Everything clears the rear panel and engine bonnet, but I can totally see myself running this thing without those items, just like in the picture above, with some type of custom bumper to not hide the engine. What do you guys think?

Next up: Rear Suspension!

Reactions: lazylobster and Goldnrust

Diff Frame Update!

Nov 15, 2024

It's been a couple months since my last update, but I made a lot of progress on the differential frame for this car! The frame is pretty much all welded, minus some gussets. I've got a few different sets of shim stacks to shift the differential within the frame, so that the belt can be tightened/loosened, or so I can fine tune the differential alignment with respect to the motor output shaft.

I also made a jig to hold the motor frame with respect to the differential frame so that I can get the two aligned before joining them together. This will be critical in aligning the two belt pulleys since I don't have a lot of tolerance for belt misalignment.

My original plan was to align these two frames then notch and weld tubes to join them permanently, but after looking at the tight clearances in real life (vs in CAD) I decided In need to redesign that portion of the frame to include a bolted split plane. This will allow me to better service the differential without requiring the engine to be removed. So back to the drawing board!

Next up: Making the custom driveshaft to mount the engine side drive pulley, finish joining the two frames together, and then work on installing the assembly into the car and welding up the final braces!

Reactions: Bounding Bambino, Bevelhead, lazylobster and 3 others

Rear Suspension Design

Sep 9, 2024

Hey Everyone! It's been a little over months since my last update where I finished building out the front suspension and the rear engine frame that holds the Ducati motor. Since then I've been designing the rear suspension, differential frame, and the connections to the vehicle. I wanted to share with you a quick update on the design before I get deep into the build!

Suspension Design
Now that the front is built and I know my front travel, ride frequency, roll center height, and camber curve, I used that information to design the rear suspension. I iterated in a 2D sketch to get points that work with the existing body, placement of differential, and engine output shaft, as well as fine tuning the motion ratio so that it works with my GAZ shocks. Because I'm designing a double wishbone suspension, I also needed a custom upright to attach my control arms and hold the stock Fiat hub. This went through quite a few design iterations, mainly due to integration of the shock into the lower control arm, before I came up with something I was happy with. I used VSUSP.com as a crude but free suspension kinematic software to make everything looked reasonable, then went onto designing the frame!

Component Design
For the frame, I had initially tried to design everything using square tubing, since there's less welding and it would provide a good mounting surface for the suspension and differential, but I ended up switching to a plate based weldment. I can get plates laser cut pretty reasonably (thanks Send Cut Send!) and that allows me to adjust thickness throughout the structure and have locating holes for the different mount points (I hate drilling through tubes, they're never square!). A plate weldment also made the most sense for the uprights and A-Arms, where I could use the parts to locate each other and tie into my spherical and shock mounts. After lots and lots of iteration, I came up with the design below:

Connecting it All Together
Now that the differential frame is designed and the engine frame is designed (and built), it's time to connect the two and anchor them to the car. Unfortunately, the diff frame designer (current me) and the engine frame designer (me from 5 months ago) didn't talk to each other, so I did not have a clean way to connect the two frames. Since I wanted to keep the rear seats, the diff is much lower than the engine, and because CV angle is a big concern, I have the engine and differential very close together. This makes for a tricky envelope to connect the two frames and clear the Engine/Differential drive built. I solved with some blank off plates and and mitered tubes. It's not the prettiest solution, but luckily this portion of the frame will be well hidden. To connect the forward end of the frame, I am running a rectangular tube across the Fiat's lower suspension arm pickup point and using this tube to anchor to tie multiple tubes to the differential frame. See below:

I spent a bit of time running Finite Element Analysis (FEA) through Fusion 360 to get some rough stress and defection numbers for various load cases. It's not the most accurate, so I'm holding pretty healthy safety factors, but the structure is mainly stiffness driven, and this tool gave me some good comparative info on how best to stiffen the structure. I am most worried about deformation between the diff and engine since too much deformation will cause the drive belt between the engine and diff to be very unhappy (misalignment or tension). After I got this number down under a pretty extreme load case (3G bump and 2G side load), I sent all my part files to get laser cut!

Next up: I should get all the parts in the next couple weeks and will spend the following few weeks fabricating the rest of this frame. I want to hold off on the control arms until I have the complete frame installed, so I can make sure the wheels fit correctly (it's easy to change position of the wheel with modified A-Arms). I also need to check those CV angles, it'll be tight!