Thursday, 24 October 2013

F109 to 1/12 Conversion

This all came about because of a slightly unexpected desire to run 1/12 at the QLD Titles. It was more a case of timing than anything else as I could only attend one day and with the current Qualifying format excluding anyone that cannot attend every day, I had to pick one of the demo classes being offered, F1 or 1/12. While I have more experience with F1, it was being run on Saturday and I couldn't organise to attend on the Saturday, so 1/12 became "it". There was only 1 problem - I didn't own a 1/12 car, or any 1s batteries, or any 1/12 tyres etc etc.

However I DID have a lot of F1 foam tyres and a spare F109 car sitting around not doing anything. A bit of research online, downloading the appropriate rules and a bit of measuring and I became convinced I could turn the F109 into a 1/12. This would allow me to use the majority of the F109 rear suspension which would make things infinitely easier. Of course the practical was far more involved than the theory..

First thing was getting the car to 170mm. On the rear this was a 2 part approach. Standard F1 tyres are 45mm wide, but 1/12 has a max tyre width of 38mm. The obvious answer here was to cut down the F1 wheels and tyres as this would take 14mm out of the width straight away. I decided to cut 10mm off each tyre which would get the rear to 180mm. This did mean a little bit less tyre on the track but I felt this was acceptable.

The 2nd part was to narrow some of the rear axle components. On the non-diff side this was rather easy as there is a 16mm spacer on the original setup. At first I thought of cutting this down but in the final build I used a bucket load of shims which allowed me to adjust the width to perfectly match the diff side.

The diff side took a bit more modification. This is where the power tools came into play, mainly my dremel. The original diff part has a hub which includes the grub screw which allows it to be tightened onto the axle. As luck would have it, this hub was about 5mm thick, so grinding this away would net me the extra space I needed to get to 170mm. Some careful work with the dremel and then with sandpaper and a flat surface gave me this result - 
OK, I am not going to win any prizes for beauty, but it was flat and straight and that was the important part. Of course the next puzzle was how to attach the axle? Gluing was the obvious answer so after little reading on the different epoxies and I purchased a high strength epoxy that was also readily available. Even the common 5 minute epoxies have a breaking strength of around 200lbs/in2. It was also interesting to read about the different way's of gluing aluminium. Like a lot of things on the net, it can be hard to find a consensus, but overall, gluing aluminium with epoxy is, usually, quite successful. After all, they do glue airplanes together with it.

The main issue I encountered was with air not being able to escape easily from the joint. I had  thought it could be an issue and had put some groves in the carbon shaft but it turned out to be only just adequate, but still required being held firmly together until all the air had escaped and the epoxy set to stop the axle from popping back out from the trapped air. So far, with the limited track time I've been able to get, it has stood up well to 10.5T, single cell.

In the end I didn't quite get the rear width to 170mm, ending up at 171mm but this is still within the rules.


Rear axle complete it was time to modify the rear pod. The original F109 pod/bottom plate has the link mounts a long way back near the wheels. With the wheels moved in 5mm these mounting points no longer work. I had to move the mounts forward to clear the tyres. It ended up being a move of 25mm. I tossed up between buying some pre-made carbon fiber sheet or making my own fiberglass sheet. Since the rest of the chassis was fiberglass I chose the latter, though if I make another version I will go the CF option. I again did some research, finding the most useful info on a model helicopter site. I went the simple route of woven mat and polyester resin (if I did it again I'd use epoxy resin). I did get the number of pieces of mat wrong and ended up 2.5mm thick rather than 2mm. This just meant I had a chance to try out my new drill press with a milling bit from my dremel in it.

Next time, maybe some black colouring?

Obviously, the link mounting location on the main chassis also needed to be moved the same amount to maintain the rear suspension geometry, but this was very easy.

The front was a far more complicated job. While F1 and 1/12 tyres share a common set of dimensions they have different offsets. This allows a 1/12 car to have a wider front track which should, in turn, mean it has better rear grip. I wanted to retain the use of F1 tyres and this led me to investigate several different front end designs from very simple to very complex. Originally, I used pieces from the standard F109 front suspension, cut and then attached to pieces of CF. This was attached to the chassis using threaded steel spacers, not unlike the TRG F1 cars. Camber and caster was adjusted using a layout similar to this, also attached to the chassis using threaded steel spacers. However, this proved heavy and impossible to adjust equally, despite, literally, hours of measuring showing the suspension was square and equal from side to side. My suspicion is that there were too many joints in the mounting system and it led to an error rate that I couldn't overcome. Fortunately no pictures of this front end survive.

In the end I followed a path traveled by another scratch builder I read about and purchased the Tamiya RM01 suspension. Not without its faults (soft plastic, no adjust-ability), this old school style front suspension is easy to mount and takes all standard F1 pieces. The only extra parts I needed were the RM01 suspension shafts to mount everything correctly. I mounted it to another piece of CF, ensuring a 170mm width, and mounted that to the chassis. Its not perfect but its straight, square and on the track the car goes in a straight line - when the servo-saver lets it. It's also about 50grams lighter than my first attempt. There is now a stiffer version of the suspension available from Tamiya if it proves necessary, but I also hope to re-develop my own front suspension in the future.

The servo is mounted using double sided tape. The F109 has a lot of holes in the chassis and this prevented me from drilling any holes for servo mounts. I use a very good quality tape used for installing aluminum windows so there haven't been any issues there. The servo saver is another issue altogether. I've tried using the F109 servo horn on a Tamiya servo saver but I don't get a consistent center. I am assuming this is a fault in the servo saver and I've purchased a Kimborough to replace it. I've also wondered if there is a possibility the servo is misbehaving using 3.7v and a booster, but one thing at a time.

The front bumper came with the RM01 suspension. Some quick work with a drill and bits out of my spare parts box had body mounts done for the front - the rear uses the standard F109 body mounts. Mounting the body turned into a total farce however. I was under the impression that all 1/12 cars were built around the 170mm most advertise and that fits to the rules. It turns out I was wrong and bodies are made in both "narrow" and "regular" widths. I made the mistake of buying a narrow body which is designed for a car running less than 170mm. I have no idea which car/s that might be as none advertise themselves as less than 170mm. It was suggested that I try some heat to modify the shape around the wheel arches but this also turned into a massive fail. I ended up with something I could drive around the track but I wouldn't put much faith in it's performance.

With the cancelling of 1/12 at the QLD Titles, I had to wait a few weeks for a regular club meet for the car's first proper shake down. There was no class to race it in so I used some breaks in normal racing to do some runs. Unfortunately, the tyre choice was next to useless on this particular surface, but I could test steering and tweak etc. Except for the steering servo and the lack of grip it was very good. I made a small change to the springs all 'round and I found some improvement but the tennis court surface we race on doesn't offer much grip to foam tyres. 

This means I am moving on to version 2 of the car much more quickly than first anticipated. I've ordered some Jaco tyres and some bits and pieces to allow me to fit them. It means making a new rear axle and I've already started work on that. I am hoping to have it ready before the next club day and I am hoping to take it down to Brisbane the next time I go, but its next year I am hoping to be more serious about it.

F109 next to F109-12


Unknown said...

Awesome! This is the true spirit of RC car racing. Taking a kit and modifying it to fit racing rules. I enjoyed following what you've done so far, and hope to read more in future.

Adrian said...

Thanks... and there i more to come... some boxes arrived in the mail today