Can One Use Too-Small — 700x18-23c and 700x18-25c Road Tubes in Larger 700x26c and 700x28c Clincher Bike Tires — Mounted on 29er Mountain Bike Multi-Purpose Wheel Rims? — Yes, but Not Safely

© 2012 Peter Free


12 August 2012



Introduction — this article covers two “dumb guy” cycling tire and tube experiments


My experimental questions were:


(1) Can one mount and safely ride 700x26c or 700x28c road tires on Alexrims DH-19 “cross trekking” 29er rims?


(2) If this tire to rim experiment works — can one then put 700x18-23c or 700x18-25c tubes inside the 700x26c or 700x28c tires, without having an excessive number of blowouts?



Disclaimer — if you are going to try this, it helps to be an experienced bike handler and psychologically comfortable with death or injury by stupidity


Given that two of these clincher tubes blew explosively during this experiment, killing yourself by repeating it is a possibility.


A worst case scenario might include speeding around a downhill curve (with a cliff on one side) at 60 miles an hour (97 kilometers per hour).



Why would one try to put narrower road tires on wheel rims that were designed for larger volume tires?


Because that’s all you have lying around.



Why would one try putting too-small tubes inside larger clincher tires?


Because you ran out of the correct size tubes and you had the itch to ride today.



Specifics — of these two experiments


I tried these experiments after getting rid of my road racing bikes (due to severe arthritis) and converting a rigid-forked Redline d440 29er into a utility road bike (reviewed here).


The bike has an Alexrims DH-19 wheelset, which the company designates as a “cross trekking” design.


According to the manufacturer, the rims are 18mm across, inside to inside.  There is a cross section diagram, here.


When I bought the bike used, it had 29x2.1 inch (52/47) WTB Nano Raptor cross country mountain tires on the wheelset.


Since I wanted to convert the bike to fenders and road use, the question was whether I could fit a pair of Specialized Turbo 700x26c (which I had left over from another bike) onto the DH-19 wheels.



Experiment One — 700x26c and 700x28c tires (with appropriate sized tubes) on Alexrims DH-19 rims


I used an old pair of Specialized Turbo 700x26c tires for this experiment.


I noticed that, even when filled to the maximum recommended 130 pounds per square inch pressures, the distance between the tires road contact surface and the plane of the rim edges was narrower than on the road rims the Specialized tires were designed for.


Consequently, when I hit significant road cracks, potholes at speeds of 30 to 40 miles per hour — or jumped low curbs — the tires temporarily bottomed out.  This caused a couple of flats.  As well as made me worry about the long term integrity of the rims.


As a result, I went up a size to 700x28c Continental Grand Prix 4 Season tires.  These slightly higher volume tires eliminated the bottoming out problems that I had had with the 26c tires on the same DH-19 rims.



Experiment Two — 700x18-25c and 700x18-23c tubes in 26c and 28c tires


This experiment started when I ran out of proper sized tubes for these 26c and 28c tires (mounted on the DH-19 wheelset).


First, I experimented with 700x18-25c tubes in the 26c Specialized Turbo tires.  These worked fine.  Or if they did not, the problems were masked by the bottoming out problems that I mentioned above (when the experiment was done with proper sized tubes).


After changing out the Turbos for 28c Continentals, a series of thorn flats eventually forced me deeper into my left-over reserve of 700x18-25c and 18-23c tubes.


Here, the too-small 25c and 23c tubes did cause problems.  Two of the 23c tubes exploded shortly after pumping them up.  Examination showed a large tear in each.  Fortunately, they blew in the first 5 minutes, before I had started riding.


Those that failed more gracefully seemed to do so most often at or near the valve stem.  In three cases, the valve stems were sliced by the unchamfered edges of the rims’ valve stem holes.


Curiously, even though the slits later proved to be sizable, air initially leaked dramatically, only when I wiggled the stems to attach a pump.  I managed to ride on on one of these sliced valve stems for at least 30 miles before it finally quit holding air.



The moral? — summarizing the results


None of my results are statistically significant.  But they are good enough for me.


The conclusions that I drew include:


Rim and tire sizes


700x28c tires seem to be the reasonable minimum for Alexrims DH-19 (18mm internal width)


If all one does is road ride comparatively smooth surfaces, 26c tires will work


Tube and tire sizes


700x18-23c and 25c tubes will not work reliably for long term use in 28c tires — although they may last surprisingly well for up to 50 miles or so


700x18-25c tubes seem to work in 26c tires


If you repeat these experiments, be careful.  I did not ride above 40 miles an hour during these tests, and I stayed away from mountain downhills and heavy traffic.