Chain guide for velomobile w/ hanging bottom bracket (e. g. alpha 7) (FreeCAD)

Some velomobiles (like the alpha 7) have a hanging bottom bracket, where the BB is below the front boom. Shifting to the smaller chainring sometimes results in dropping the chain, because in a velomobile with 2× gears the inner chain ring usually is much smaller than the outer one.

I designed this to fit on the square 50 mm carbon boom of my alpha 7. I've tested the prototype by shifting to the small chainring whilst tilting the velomobile to it' left (a sure drop) and it worked.

Please note: the photos and the first FreeCAD screenshot show my prototype (chain guide 05.FCStd).

After mounting the prototype I noticed that it can be simplified.

Both models are available in the files attached, the prototype in it's subfolder.

Customizing

You can change several dimensions of the design by alterding parameter values in FreeCAD in the dim spreadsheet. Parameters suggested to start with (those which may most possibly need to be altered) are marked by bold text. Changing some of the parameters into arbitrary values may result in the need to re–model some fillets and chamfers…

Printing

Printed in orange Prusament PETG, so traces of the chain should be easier to see (for further improvements). 0.4 mm nozzle, 0.3 mm layer height, 2 perimeters, 20 % cubic infill, organic support. (Screenshot of PrusaSlicer shows the prototype.)

Assembly

All you need is one long or two shorter cable ties (5 mm width) to fix the chain guard on the boom.

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If your velomobile has the BB above the front boom check this Model by Lafritz.