Part 15: First Article Inspection – Finding the Friction

After 200 miles of test rides, I performed a detailed inspection of the first-article QuickSet seats.

Visual and Structural Condition

The top seating surfaces looked brand new. There were no deformations or visible wear marks. This was expected since the top layer is identical to the prototype design.

The polypropylene base layer also showed no wear, cracks, or chipping around the locking holes where the spring plungers engage. I didn’t mention it earlier, but our prototype bases (which were nylon) developed small chips around those holes. To address that, I had the manufacturer deepen the holes and lengthen the top of each plunger knob for better engagement. That change worked; there were no signs of damage on the first-article parts.

The only cosmetic issue was with the silk-screened lettering. Some letters showed wear after the test rides. Since the silk screening was mainly for identification and branding, I asked around and learned that polypropylene is not well suited to silk printing. Feedback from riders and bike shops was consistent: they didn’t want to “sit on a billboard.” We’ve decided to eliminate the silk screening on our first production run.

Measuring Sliding Effort

Functionally, the main concern was the amount of effort required to slide the seat halves along the arced guide rail. Using a digital force gauge, I measured the force needed to move each seat half from the Sport (narrowest) to the Cruiser (widest) setting and back again.

• Prototype seats: less than 2 lbs of force in either direction.
• First-article seats: between 5 lbs and just over 10 lbs.

That was too much, so I listed possible causes:

1. The spring plunger might not be retracting fully into the arced guide rail.
2. The plunger knob might not be pulling clear of the locking holes.
3. The guide rails could be rubbing against the backs of the seat halves.
4. Dimensional differences could be creating friction between the square mounting holes and the arced guide rails.

Investigation and Disassembly

I disassembled one prototype and one first-article seat for comparison. The plungers functioned correctly; when pulled down, they fully retracted into the arced guide rail.

I measured the width and height of both rails. The first-article guide rail was slightly wider, though the height matched exactly. Measuring an arced part with calipers isn’t perfectly precise, so I couldn’t rely on width alone to explain the difference.

To inspect the interior of the seat halves, I cut away sections using an oscillating saw. The nylon prototypes cut easily. The polypropylene seat halves were harder to cut and tended to melt as they cut, confirming the material’s toughness.

Inside, I verified that the mounting guide rails were not rubbing and did not extend past the arced guide rails. The issue was not there. Measuring the interior square holes showed that the first-article holes were slightly smaller than those on the prototypes.

Identifying the Root Cause

This pointed to a tolerance stack-up issue. Every component has a nominal dimension plus a tolerance range. The arced guide rails from the first articles were near the high end of their tolerance, while the seat-half openings were near the low end. The result was a slight interference fit instead of the intended loose sliding fit.
The difference was only a few hundredths of an inch, but enough to increase friction noticeably.

Reducing tolerances or measuring every part would raise costs, so I looked for another explanation... and found one. Polypropylene and nylon shrink at different rates as they cool after molding. If the same mold dimensions are used for both materials, polypropylene will produce a slightly smaller hole. That explained the interference.

The Fix

The solution was to slightly increase the dimension of the square hole in the polypropylene mold. I verified this using what I call the “drop test.” When the arced guide rail is inserted into the seat half and rotated 90°, it should fall out freely. The prototype passed this easily. The first-article did not - it stuck.

After sharing the test results and video with HugeOaks, we agreed on two verification steps for production:

1. Drop Test: Each seat half must allow the guide rail to fall out freely.
2. No-Go Feeler Gauge: A thin metal gauge is inserted between the guide rail and the seat-half opening. If the gauge fits, the gap is too wide (“no-go”). If it doesn’t fit and the drop test passes, the fit is correct.

Conclusion

The tolerance and material-shrinkage adjustments solved the sliding-force issue. With that confirmed, and the cosmetic change decided, the first-article phase is complete and we’re ready for production.

Even small variances - as small as hundredths of an inch - can make a noticeable difference in how a mechanism feels. This round of testing showed exactly why first-article inspections matter.

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Kickstarter is almost here

Kickstarter launches early November. One frame from the shoot.

My nephew Ben and his wife, Giselle, helped prepare the video for our Kickstarter launch, from scripting to shooting to editing.

Their son, Martin (@martin26photos), has been pitching in with high-quality photo and video as well.