February 13, 2011


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This bike was supplied new with Dunlop K70 series tires.  I'd replaced them many years ago with K81 tires, and though they didn't have many miles on them, and they had plenty of tread left, they were at least 30 years old and were noticably harder than new tires.  It seemed that traction could be a problem, especially in wet conditions.

I found to my happy surprize that Dunlop still supplies the K70 tires, apparently mostly for vintage bikes.  Another happy surprize was that they didn't cost as much as I feared.  It looks like the K70 tires are a little higher in profile than the K81s.  Also, the K81s had markings on the sidewall indicating the correct direction of rotation.  The K70s apparently have no preference regarding which way to spin.

When I had the tires off, I decided to also check the wheels for true running.  I made a temporary axle out of some threaded rod and some nuts with one side turned to just fit in the inner race of the wheel bearings.  Also made a jig to hold the wheels while truing and balancing.  

I was prepared to learn to lace the wheels, but both wheels turned out to be pretty true--within 0.030 both in concentricity and runout.  I was a little disappointed, actually.

The back rim was pretty clean inside, but the front one had some rust on the tire contact areas.  I wire brushed the rust off, and applied two coats of a rust conversion coating.

Here is all the stuff that needs to go into the rear wheel.  There is the inner tube, a rubber rim strip that covers  the spoke nipples so they won't chafe the tube,  and two "security bolts", sometimes called rim locks. These bolts clamp the tire to the wheel so that on hard acceleration, the tire can't move in relation to the wheel, possibly causing the tube to shift,  which would stress the valve stem.  The front wheel does not have the security bolts.  

The yellow mark is put on each tire by Dunlop to mark the lightest side.  Since the only known imbalance in the wheel is the valve stem, the yellow mark is normally aligned with the stem.

This was not the original rear wheel for this bike.  The original was a "QD" (Quick Detach, I think) type in which the sprocket and brake drum was a single piece.  I believe this was a difference between the US models and the "UK and General Export" models.  When I went to replace the worn sprocket back in the States in the late 70s, I couldn't find a QD part, so swapped my rear wheel for a used US style replacement.  I was sad to find when I took the tire off that some past genius mechanic had used security bolts for a much narrower wheel.  They not only didn't clamp properly, but got deformed by the tire bead.  

After the wheels were cleaned up and checked for true, I started to assemble them.  It is common to rub talc on the inner tube so it will inflate smoothly and not get twisted or bind. Some baby powder today is actually corn starch.  I don't know if it works as well on tubes, but actual talc can still be found.  There apparently exists actual "tire talc".

First put on the rim strips, aligning the hole with the valve stem hole in the rim.  On the back wheel, it's necessary to cut holes for the rim locks.

I won't detail the mechanics of convincing a stiff rubber tire with an 18-inch inner diameter to go over a rim with a 19-inch outer diameter, but it can be done.  A couple of tire irons, a little soapy water, and a determined attitude is all it takes.  Put one bead on, stuff in the inner tube and the rim locks, then try to get the other bead on without pinching (and thus puncturing) the inner tube.  The rear wheel went without a hitch, but I ruined the first tube on the front.

With the tires on and inflated, I statically balanced them on the same jig used for checking true.  Set the wheel slowly in motion and mark the bottom of the rim when it comes to rest.  Repeat this a dozen or more times, and it will become obvious where the heavy side is.  A "false indication " can sometimes happen where the heavy side actually stops at the top, since there is no torque arm to pull the wheel around.  Once located, the heavy side can be checked by placing it at the nine o'clock or  three o'clock position and releasing.  The marked point should consistently fall to the bottom.

These self-stick motorcycle wheel weights, are 1/4-ounce (about 7 grams) each.  I taped them on opposite the heavy side one at a time at first just to see how many it would take.  When balanced, the wheel will stay in any position it is released, and show no preference for any particular point being on the bottom.  In the case of my rear wheel, it took seven weights--almost two ounces.

On my rear wheel, the light side happened to be right at one of the rim lock bolts, so I was able to add some of the weight with extra nuts on the bolt.  This let me get the number of 1/4-ounce weights to an even number .  I split them up to both sides of the rim for better symmetry.  With any self-stick item, adhesion is improved by cleaning the surface with alcohol first.

My front wheel already had this balance weight that fits on a spoke nipple.  I had to add anpther 1/4 ounce to make it perfect.  That yellow spot will come off with soap and water.

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