- Shop Owner / Manufacturer
- Posts: 299
- Joined: Mon May 26, 2008 12:21 pm
- Location: UK
It's not an easy question, because although the target g/s tension is in the range 130-150 kgf, with the ngs tension set so that the rear wheel has the correct offset, there is an additional factor to be taken into account.
The rims go through a process during manufacturing that presses "flat" spots between the spoke groups, so that once you have correct and even tension on each group, the rim is pulled "in" in the areas around the spoke groups and is therefore deformed "out" in the inter-group areas.
Hence, the total tension that you are looking for in each group of three is defined as much by by how round the wheel is, as it is by a specific tension range.
Older rims that have been subject to a lot of use and possibly poor spoke replacement practice are often deformed and getting them back to an acceptable standard of true and round can be a real challenge and sometimes just can't be done - rare but we do see cases like that.
Equipment-wise, apart from the tools to check and adjust the bearings and a jig and tensiometer (obviously), you will need a spoke holder and the deeper version Campag 6mm (older Euruses, the Campag 5.5 mm) spanner-type key. Avoid the older, shallow steel keys, long-term expereince has shown that they damage the nipples. You can't use an ordinary open ended wrench, the torques that need to be applied are too high and the wrench will open. We retire the Campag keys for this reason after every 5 wheels, as even these can and do stretch. Hold the spoke as close to the junction between the flat area and the round area as you can and make *sure* the holder is properly engaged - otherwise you can end up with spokes snapping or twisting like wrought-iron work!
The nipples are thread-locked at the factory and will be hard to move - make sure you use the wrench "right way around" so that the jaws are not being spread by the torque applied.
Normally our recommendation whether re-tensioning or simply replacing a spoke is to start from "zero" as it were - so, after checking the smoothness and accurate setting of the hub bearings, progressively and slowly take all of the tension out of the wheel, so that it stays as axially true during this process as possible, and try to assess whether the radially most displaced areas in the rim, when there is *just* tension on the spokes, are evenly placed and over the spoke groups.
If the radius variation in the de-tensioned rim is around 1 - 1.5 mm and each "high" spot is over the middle of the spoke groups (as close as you can judge), leaving the spoke to nipple junction "dry" - so don't lubricate (though lube between the nipple and rim is allowable) - slowly use the gearside tension to bring the wheel round and true - don't worry too much at this stage about offset.
Once you have the wheel to within +/-0.5 mm of axial true and +/-0.125 - 0.25 mm of radial true (it is variable because of the damage that occurs to a rim through wear and tear), then progressively and in small steps (we usually use 1/4 turn) remove tension equally from pairs on the g/s and add tension in the corresponding ng/s spokes (again progressively, around 1/3 turn) to correct the offset so that the total tension in the group does not change.
This avoids over-tensioning of any area(s) of the rim and permanent damage to the rim.
Once you are within +/-0.25 mm of correct offset, fine tune the tensions to bring the wheel as round and true as you can. Always release tension before you add it. Try to keep the global tension the same. Pairs of g/s spokes are effectively working as one spoke and should have as close to equal tension as you can maintain.
This is exactly the process used at the factory and the process we use at the Main UK SC. It is a fairly painstaking process but correctly followed, it works and results in a round, true, accurately tensioned wheel.
Tolerances we ideally try to adhere to are the same as the factory, so for Shamal and Eurus, axial +/-0.125 mm, radial +/- 0.15 mm although with aused rim, with wear and tear on the braking surface and so on, we can't always hit these numbers but we get as close as we can.
Once complete, post-apply Loctite 290 to the spoke-nipple junction and spin the wheel in the jig to help it into the threads and to lock the nipples.
If in the de-tensioned wheel, the "high" and "low" spots are grossly at variance with the description above, the rim may be scrap We do get a number where spoke replacements have been badly carried out that we need to bin.
If you are in any doubt, the best thing is to return the wheel to us at the main SC and we can at least advise the viability of repair and undertake for you if you wish. Errors are expensive with Shamal and Eurus (though not *as bad* as with Bora!).
Fronts use 120-140 kgf and are, by comparison, a simple build like any radial wheel. Again, spoke to nipple junctions should be dry, Loctite 290 post build.
Velotech Cycling Ltd
Main Campagnolo SC UK
Head Tech, Campagnolo main UK ASC
I don't know any other wheel I would ride with only 21 thin spokes in the rear. On one particular occasion I've nailed my Bora in a deep pothole hard. Didn't see it as I was in a two abreast paceline, second position and the lead riders didn't call it out in time for me to miss it, so I hit the hole hard and fully seated with no time to "unweight". Thought it was hard enough to break the wheel but although it did pinch flat, the wheel itself is fine. And I'm 200lbs.
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