|Replacing a Mavic CXP23 Rim||Ron AKA|
Sep 14, 2002 4:24 PM
|How hard is it to replace a rim with basic tools? I was unable to get a CXP23 rim as it appears to be sold to OEM's only. Have ordered a CXP33 which appears to be dimensionaly the same.|
|It's not difficult||Jofa|
Sep 16, 2002 3:21 AM
|You will need to have a decent spoke wrench, and ideally a truing stand, though it is possible to make use of the bike for this. If you are replacing the rim because the sidewalls have worn out, then simply read on: If you have damaged the rim in a crash, check to see if any of the spokes are severely bent, from having been caught on debris: if there are any, replace them. Any spokes that are not obviously damaged will be fine.
Line the new rim up to the old one, matching up the spoke holes, noting their handedness, and Sellotape the two together. Note the average amount that the spokes protrude above, or recede below, their nipples: then undo a spoke and move it across to the new rim, one at a time, working around the wheel. Clean the threads with a rag and oil them, and the nipple bed, with a little midweight oil as you go. Car engine oil is fine. Tighten the nipple to about 2mm LESS than the amount you noted: ie, so that each spoke protrudes 2mm less.
Eventually you will have moved all the spokes across, and you can remove the old rim. Your wheel will be loosely but evenly tensioned. The process of bringing the spokes up to tension is a repetition of small steps. It is not difficult but you should allow yourself time to do it carefully.
Working around the wheel from an identifiable beginning - the valve hole is ideal - tighten the spokes one full turn at a time. As the spokes start to get appreciably tighter, reduce this amount to half a turn. Only work on all the spokes at once: go all around the wheel, and if waves start to appear in the rim (it goes out of true), ignore it until you have been around the wheel one full turn. The exception to this is if you are working on a rear wheel. In this case, the driveside spokes must be tighter than the others, in order to centralise the rim. To effect this, go around the rim lossening the non-drive spokes half a turn, then again, tightening the driveside half a turn.
As waves begin to appear- side to side and up and down - deal with them by determining whether it is a 'high spot' or a 'low spot'. This is where a proper truing stand is useful. If a high spot, tighten the spoke at its centre a half-turn, and those toward its edges a quarter turn (if a low spot, then do the reverse). Tackle the worst areas first, and don't try to perfect them: move onto the next worst areas instead. Keep working around the wheel, and pluck spokes near the nipple with your fingernail as you go, to ensure that their tensions are similar. If you find yourself in a situation where some spokes are taut and others are loose, back all the spokes off to a consistent level and start again.
As the tensions rise, the friction between spoke and nipple will become appreciable, and your manipulations will start to twist the spokes. This needs to be dealt with as you go, or the spokes will detension themselves in use. At each turn, you will 'feel' the spoke twisting: overturn the nipple by a quarter turn or so, and then back it off the same amount. You will quickly get a feel for it. To begin with, you can try putting a post-it note on a spoke- in the plane of the wheel- before you turn its nipple: the note will give you a visual indicator of the twist. (You can also use this method to prove that holding the spoke with pliers doesn't work).
|It's not difficult...||Jofa|
Sep 16, 2002 3:22 AM
|Arguably, you can get away with stopping when the tensions are similar to other wheels you might have, of similar design and spoke number. However, ideal - maximum - tension is reached when the rim is nearly at failure point (it would deform into a saddle shape). The proper way to find this is by stress-relieving the spokes. It would be nice to think that this critical process had be done to the spokes already: but I doubt it, as hardly anybody seems to bother anymore. The reasons for stress-relieving are subtle and not obvious: they are elucidated in "The Bicycle Wheel". To do it, grasp opposite pairs of spokes in your hands- perhaps wrapped in cloth- and squeeze them as hard as you can. This overtensioning will stress the rim enough that it should fall into a saddle shape if the spokes were too tight, in which case they should be backed off a halfturn or so. If it didn't, then they should be tightened.
This might all sound complex, but it is easy to find a rhythm, as long as you remember not to make any single, drastic, changes. And your wheel will last for a long time.
|It's not difficult...||Ron AKA|
Sep 21, 2002 2:07 PM
Thank you for the detailed response to my question. I will try it just as you suggest. The rim is quite bent and crimped so I cannot see that it could be salvaged. While some spokes are loose, none are broken or bent. I managed to have the wheel (rear) slip through the space between planks on a bridge across a river. I couldn't get my foot out of the binding fast enough and over I went.
I'm still waiting for the new rim to be delivered. I picked up a few tools today and removed the cassette. Any suggestions on how to prevent the nipples from dropping into the rim cavity? Does it help to put an anti-seize compound on the spoke threads to prevent the spokes from twisting?
|It's not difficult...||Jofa|
Sep 23, 2002 3:27 PM
|Ordinarily- with shallow rims- it is not a problem to retrieve the nipples, though it can be fiddly. I've not built a wheel with your rims, but if it proves to be impossible, then you should just buy them new. Make sure however that you get the right size (probably 1.8 or 2.0 mm). 2.0 mm nipples will thread onto 1.8mm spokes, but will strip as tensions rise, so take an example into a shop and check them thoroughly. (Be sure to keep your old spokes, however)
Anti-sieze is unnecessary: and spoke-prep, or any other glue, should not be used (I suspect this is what you meant). A carefully built wheel will not loosen its spokes, and a badly built one will be propped up by spoke-prep only for so long. Ordinary mid-weight oil is ideal, but the spokes will start to twist as tensions rise during building, inevitably. This should be dealt with as I described.
|It's not difficult...||Ron AKA|
Sep 24, 2002 7:18 PM
|Thanks again Jofa,
I was actually thinking of an antiseize containing moly and copper to reduce the friction in the threads to try and minimize the spoke turning effect. Normally you can get about a 30% reduction in thread friction with these compounds compared to plain oil. They are commonly used in industrial applications where uniform torque is critical.
On not losing the nipples, one suggestion I've been given is to thread an extra spoke in from the top to hang on to it. Still waiting for rim, so still more time to keep thinking about it.