Designing Milan GT chain cover

A couple months back I came to the realization that all the projects I had been working on with composites and carbon fiber (CF) were full-on plug – mold – final piece projects, meaning they took lots of time and energy; tons of sanding and polishing, grinding, smoothing and shaping. I needed to try something very simple and quick.

A few weeks after receiving the Milan I went for a riding weekend in mid November, on Cape Cod with my friend David. Dave lives on Martha’s Vineyard, and a Quest is his primary vehicle and he doesn’t own a car, so I met him off the ferry in Woods Hole, MA and we rode off from there; a fun 2 days in cool autumn weather, combination of bike paths and roads. Some rain, but because it was off season the vehicle traffic wasn’t bad.

When packing for this ride, I used a fabric covered foam bag that Howard had given me. It’s a nice hard side foam case, zippered clam-shell design; I used it for holding the chargers and batteries I planned to bring. After the weekend I noticed the mesh side of the bag was scraped up but not gouged fortunately; it was the end of the chainstay that had done the damage, bouncing up and down on it for the weekend.

This is the protective case that got damaged under the chainstay

The chainstay is square metal tubing, about 1.25″/30mm diameter. At the ends there is a metal plate welded on, but it isn’t smoothed down. And the lower end of the shock absorber and it’s mounting bolt are of concern, as well as the quick release for the rear wheel.

So, I decided to make a “down and dirty” piece to cover over the offending moving parts, so that anything in the luggage area left of the rear wheel would be protected. The right side of the rear wheel area has covers already, primarily to cover the chain and front side of the rear derailleur. The rear of the cassette is pretty open to tangling things up though, as it did with the cloth bag that got sucked down into the derailleur and required 20 minutes at the side of the trail to un-tangle. Here’s a view to the left rear inside, and a closer view of the end plates of the chainstay (I really should have ground those edges down and re-painted when I had the chainstay out for the welding)

P4130012
Looking rearward left side; the far end of the yellow chainstay is the primary culprit here.
P4160106
You can see the culprit sharp edge better here; and the gear selector box
(the black box with silver thumb-screw below the chainstay) is also a rubbing concern now that
I have the Rohloff installed.

So I spent a bit of time going back and forth, measuring, checking clearances, what areas did I have to build around, etc. And, in my usual fashion, I can’t just let the piece be of a solitary use, it has to be a multi-use design (it’s a compulsion of mine I suppose….) I also wanted to relocate my simple battery pack holder closer to my seat and make it easier to reach; so I worked that into the design!

I laid it all out using some coroplast (corrugated plastic) sign material. I used blue flash tape to hold the pieces together, and used a glue gun to attach some coroplast reinforcements on the back side where needed to help hold 90 degree corners.

P4130008

Once I had this all done, I laid on 2 layers of carbon fiber along with resin, and put some peel ply fabric over it (peel ply helps to give a smoother more consistent finish, and doesn’t adhere to the resin), and let it all cure overnight. Next day, it removed very easily from the quickie mold; the resin doesn’t stick to the coroplast, and I rubbed it down with 2 or 3 coats of release wax just to be sure!

I cut it down and trimmed off the excess; the 2 tabs above and below my hand in this photo will have velcro glued onto the back side of the tab and will attach to the rear wheel housing just behind my seat.

P4140088

And I notched the piece out to fit my battery holder. The Ankara battery pack has 2 USB outlets; I use this battery to power the Garmin 360 camera up on my camera pylon, and I carry a couple different USB charger cables, for cell phones or to recharge the Fly6 camera.

P4200001
The battery and CF battery holder installed; I added a velcro strap to hold the battery in place

So the design and build was completed within 24 hours. It’s not the prettiest or strongest piece, but it works as intended.

However, as I was finishing with the Rohloff install I realized that my design also meant that I was giving up some valuable luggage space. Looking it over more I recognized that the chainstay, the shock mounting bolt, and the gear selector box for the Rohloff were the things I needed to worry about. And since it’s a ferrous metal, I can finally make a CF piece that incorporates neodymium magnets!

The new design will cover snugly over the chainstay, sliding on from the sides towards the wheel. I ordered better quality magnets, and will embed 3 of them along the length of the piece. Areas will be created to cover over the shock absorber mounting area, and also cover over the quick release skewer as well as the gear selector box. Add a skirt that will be cut just short of rubbing against the rear wheel cover.

I decided to make a negative mold using MDF board, cut and laid up in various sized pieces. I will be building it as a female or negative mold, and instead of using orange tooling gel-coat, I’m going to apply the gray gel-coat that I typically use for coating my plugs. The gray is not as durable as the orange, but is easier to sand , and I don’t think I’ll be making more than a couple pieces from this mold so it will be durable enough for my use. The gray won’t polish to the high gloss that the orange gel-coat will but this is for making a chain cover; it doesn’t need to be super glossy!

I laid up a portion of the mold, but after a while realized I messed up on the design; maybe it was because I was taking measurements in the Milan while it was hanging upside down so the left side is actually the right side, up is down, etc.

I figured I should start with designing the left side first, a bit easier since I wouldn’t be dealing with the chain or tensioner; but the layout got switched around somewhere between my brain and the mold; in the photo below, the mold recess should lead up and off to the left; not up and to the right as I had built it.

Starting to build mold for the left side, but the curve is wrong;
I switched this mold around, and changed to my right side mold!

Glad I caught it at an early stage. Rather than scrapping everything, I moved some pieces, added quite a few more, and built a mold for the right side first!

I felt pretty good about my measurements and decisions, but before continuing work I made a real quick test piece. I laid plastic food wrap into the mold, applied some flash tape to seal the seam. Then I took scrap pieces of CF and placed them on the plastic wrap; I keep most scrap pieces from my cuttings, they often come in handy. Once I laid the CF in I brushed resin on the outside of all the pieces, and I applied a heavy coat so it would soak through. This was only 1 layer of CF, too flimsy to use for anything of real use but just stiff enough to hold it’s shape.

Scrap pieces of CF laid onto plastic wrap, and pressed into the mold. The tensioner would be towards top left side, return line chain would run along the top.

From the test piece I realized that the part of the mold for covering the return chain line (below the chainstay) was extended too long and would be hitting the floor of the velomobile; I changed that. I also used some auto body filler to narrow down the mold out behind the chainstay. I was giving up too much luggage space! πŸ™‚

And this was cool πŸ˜‰ πŸ˜‰ …….. I set the Milan up on some crates, took off the rear wheel fairing and set up the Contour camera under the left rear side. Then I installed my test piece on the chainstay as it’s designed, and used a foam block to wedge against the side of the velomobile and keep the test piece in place. While I crank the pedals and shift the chainrings up and down, The Contour is recording movement of the tensioner and chain movement inside the test piece, showing the movements of chain and tensioner. Here is the video.

I could see as I changed the front chain rings from small to large and back again, it appeared to be moving the test piece up and down a small distance; it could have been rubbing on a bubble of CF fabric on my quickie CF piece, I couldn’t really tell. But I decided to add some additional curve to the mold, to allow for more tensioner movement in that area. And above the chainstay I built the mold so that the final CF piece will give me a simple clean area where I will cut a slot for the shock absorber to pass through and move cleanly.

A little more sanding and this will be ready for gray gel-coat. This will be for the right side chainstay, chain and tensioner. I also cut down the outer edges and ran a belt sander over the corners,
to prevent tears in the vacuum bag.

The mold above is after final changes; a bit more sanding and ready for the gel-coat. I’ve always been applying the gel-coat using a disposable bristle brush, but the recommendation is to use a spray gun for less sanding time and a better coat. I’ve avoided doing that in the past, I really don’t like having to clean the gun after a small job.

So, I gathered up 3 projects I’m working on, and I’m going to spray gray gel-coat on all 3 at the same time; and only clean once πŸ™‚

In the photo below, on the left is my plug for the Milan’s SPAI; I’m filling in the center void, and shaping with some foam to re-create the Milan’s front curve. This will be for making an SPAI cover plate for the cooler weather. In the middle is my chain guard mold, and the one on the right is for foot hole covers on a Quest, I’m making some pieces for a friend.

Will be spraying gray gel-coat on these 3 at the same time

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s