TWSBI Fountain Pen Balance - Center of Mass Analysis

TWSBI Center of Mass Analysis

With an attractive range of well functioning piston filler fountain pens, it's no wonder everyone is talking about TWSBI.  A TWSBI question I often receive at the shop is, "what's the difference between each model"?  Apart from standard metrics such as size, weight, and aesthetics, I thought it would be valuable to provide a method to suggest how your TWSBI will actually feel in your hand. 

Traditionally, a pen review will describe the balance as a largely subjective observation that primarily relies on individual user experience with added references to overall size and weight.  While this information is helpful, it leaves a lot of ambiguity in the decision making process of the discerning writer. To create a more standardized approach to describing fountain pen balance I'm going to begin aggregating center of mass measurements that we can use in conjunction with other pen metrics to better describe the feel of the pen in hand. 

After a bit of experimentation (and one contributing engineer father) we were able to develop a substantially accurate method for measuring the center of mass of a fountain pen.  The following chart displays the center of mass for our current stock of TWSBI fountain pens for both a posted and unposted configuration:     

TWSBI Center of Mass Analysis

As you can see from the results above, the TWSBI Diamond 580's have the most dramatic shift in center of mass between posted and unposted configuration. This is due to the fact that, while they do technically post, the cap only slides down about 1/4" onto the top of the Diamond 580 causing the majority of the cap weight to be a much further distance from the nib. As such, they are shown with an unposted configuration in the chart.  Also noteworthy is the shorter disparity between the center of masses in the posted and unposted configuration for the Diamond 580 AL versus the standard Diamond 580 model.  This is due to the added mass of the metal components in the piston filling mechanism of the AL.

The TWSBI Vac Mini clocked in with the smallest disparity of center of mass between posted and unposted configuration.  This gives the Vac Mini the most consistent mass dispersion between posting and unposting.

The TWSBI Mini had the most dramatic shift of center of mass between inked and dry in the posted position of any model.  This is likely due to the fact that the Mini is the lightest of the fountain pens analyzed, and therefore has the greatest percent of it's total mass supplemented from adding approximately 2 grams of ink.  

See the table below for the results from my analysis:

  Weight Posted Weight Unposted Weight Inked, Posted Weight Inked, Unposted Length Posted Length Unposted Center of Mass, Dry, Posted Center of Mass, Dry, Unposted Center of Mass, Inked, Posted Center of Mass, Inked, Unposted
Diamond 580 28g 14g 29g 16g 17.73cm 12.98cm 10.80cm 7.10cm 10.80cm 7.30cm
Diamond 580 AL 31g 17g 32g 19g 17.73cm 12.98cm 10.55cm 7.40cm 10.50cm 7.40cm
Classic 23g 16g 25g 17g 16.51cm 12.70cm 9.45cm 7.45cm 9.20cm 7.30cm
Eco 21g 12g 22g 14g 16.60cm 13.00cm 9.95cm 7.40cm 9.60cm 7.35cm
Vac Mini 27g 18g 29g 20g 14.73cm 11.38cm 9.00cm 7.10cm 8.40cm 7.10cm
Mini 18g 10g 20g 12g 14.13cm 10.68cm 8.80cm 6.10cm 7.50cm 6.00cm

If you're thinking about adding a TWSBI fountain pen to your collection I hope you'll find this data helpful in your decision making process. Please let me know anything that surprised you as well as any other analyses you'd like to see done in the comments below!