I wonder how good the new Brompton double-wall rims are? I’m certainly familiar with the design type on other bikes & on Ralph’s rear wheel (a Sun rim laced to an Alfine 11speed hub). A number of my blog posts have mentioned the Sun rim issue of a rather flat profile & so perhaps no more for this post?

Double-wall rims have the major benefit of the spoke heads being below the tyre mounting area & thus not contacting the tube. Often the rim will have holes in the tyre mount area, that align with the spoke heads mounted in the lower rim. Some rims will even have no holes (aside from the valve hole) & theoretically not require any rim tape. (Note that last careful choice of words, but for this post I’ll say no more about my experience of going without rim tape on some race bike wheels.)

Unfortunately Ralph & Robinson are still running the older style Brompton rims on 3 out of 4 wheels & so fitment of rim tapes is critical. Recently I’ve become aware of an issue that is possibly self-induced but some long-term testing may be required for my fix? Getting back to the rim design (no photos here; just your visualization), the narrow Brompton rim with its deep well means the spoke heads don’t sit flat on the rim bottom surface, but contact the rim part-way up the curved well area & alternately mount left & right of the well centreline. What this does is leave each spoke head at a slight angle & prevent a “super flush” surface. This is where the quality & accuracy of the rim tape installation comes in, to ensure a smooth, complete coverage of the spoke heads (that are just longing to bite into the rubber tube?). My first sight of a Brompton rim with the plastic rim tape loop, alarmed me that the tape had wandered or moved from a path that fully covered the spoke heads (& was so hardened that I couldn’t relocate it), & so I switched to using adhesive-backed cloth tape on my Brompton rims (with a thinner version for the Sun rim).

As the Brompton brake calipers don’t have a quick release mechanism (I haven’t changed the subject; stay focused), I prefer to deflate the tyres whenever I’m removing any wheels – rather than release the brake cables & have to fiddle around adjusting the brakes once the wheels are back on. In my limited Brompton experience, the longest I’ve had any tyres mounted has been the 18 months that Robinson has been running Schwalbe Marathon Plus models. No punctures but rather a lot of tyre deflations/inflations in that time? (I confess; tinkering, cleaning, what-have-you – & some recent airline travel that we felt obliged to follow the airlines advice & deflate the tyres!?)

During & following our recent UK trip, Robinson suffered 2 unexpected flat tyres; 1 to the front & the other to the rear. Each time I found that the tubes had “abraded” or chafed on the inner surface at a spoke head point. The Zefal cloth rim tapes were prefect & no sharpness was detected at the spoke head. Another tube went in & we continued merrily along (with a growing suspicion about those spoke heads). What I had noticed was regular spoke head impressions all around the inside of the tubes. Hard to see in pics; eyesight or feel are conclusive!

I assume the spoke heads make quite a firm contact with the tube & that deflations/inflations/”air top-ups” cause a lot flexing & chafing at the pressure points? What can be done? Well, this could be my chance to justify new Brompton double-wall rims? (or maybe not just yet!) “Do nothing” is one option where I could ignore it & await another unexpected deflation? Regular inspections aren’t really the answer, although very close tube examination may highlight any advanced chafing? (being hopeful aren’t I?)

One thought I had was that the cloth rim tape too easily follows the exact shape of the spoke head & that plastic rim tape loops may be kinder/smoother? For Ralph’s front wheel I fitted some Brompton plastic rim tape but the difficulty of correctly locating the tape within the well resulted in an overnight loss of confidence & I switched to using some thin 16mm wide BBB cloth tape & then refitted the plastic tape over the top. My thinking with the 16mm tape (wider & thinner than the usual 13mm tape I use on the Brompton rims) was to cover the well a little further than usual & leave the plastic tape to fall into the well entirely (rather than the plastic tape alone, trying to “walk”up the side of the well when being fitted).

Next I turned to Robinson’s front wheel & I tried leaving the Zefal cloth tape in place but also fitting a plastic rim tape loop over the top. Unfortunately, the difficulty in refitting the Marathon Plus tyre showed that the total thickness of tapes was leaving insufficient room in the well. I then tried another layer of Zefal tape over the top of the original, & managed to refit the tyre. At this point I feel I’ve got the ideal basis for a long-term test – & decided to leave Robinson’s alone (or maybe I just got fed up at the time & then later decided to make it a three-way comparison test?).

Current state-of-play (early October, 2013) –
Ralph’s front rim with BBB + plastic tapes
Ralph’s rear rim (double-wall Sun brand) with BBB tape
Robinson’s front rim with 2 layers of Zefal tape
Robinson’s rear rim with 1 layer of Zefal tape.

Which will win out? Will there be any difference? Will a genuine puncture mess up my test? My guess is that something else will change – & my test will be redundant?