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Adjusting your chain
By Jason Allen
Requirement of weight/sitting on bike for chain adjustment in road bikes is generally a well-versed myth, along with that of chains stretching. You should really only do this when a chain is brand new, or very close to it. Most of the time when adjusting a chain, it will not be in brand new condition.
The reason that sitting on a bike during chain adjustment is often given is generally to overcome bad techniques in chain adjustment. By loading the tension on the rear springs, you are more closely horizontally aligning the rear sprocket with the front sprocket, and thus diminishing the pivot point or travel of the rear wheel. Often, when people have correctly adjusted a chain (12-25mm) with weight sitting on bike, they have not taken into consideration any tight spots in the chain or worn sprockets, as well as chain travel whilst the bike is not under weight load (e.g. braking into a corner, etc.)
For a non-brand new condition chain, the real ideal chain adjustment method is to put it on a centre/race stand, rotate the rear wheel many times so that the chain does a complete loop several times. On each loop, mark the tightest spot. Keep checking until you mark the same spot a couple of times so that you are sure you have found the tightest spot. If their is minimal travel between tightest spot and slackest spot, (e.g. it's close to new) adjust chain 12mm (at below swing arm half-way between the two sprockets). If their is more than ~10mm of travel in chain between tightest spot and slackest spot, then adjust chain to 25mm. At the rear horizontal point of the rear sprocket, pull chain off sprocket and ensure that it does not come off the sprocket anymore than half the height/distance of the teeth. Do this several times at various points to ensure chain is in good condition. If it comes off the teeth more than half the tooth height at any time, then you have tight spots in the chain and need to replace it. Pick it up early enough and you may not need to replace sprockets too.
Sitting on the bike while adjusting the chain will only give you about 1-4mm of difference in chain travel between when no weight is on the bike. This difference is negligible and will not generally affect proper chain adjustment. By telling people to ensure that someone is sitting on the bike, generally means that the person will neglect the more important parts of finding the above tightest spot when performing the adjustment and checking the the chain does not pull off the rear sprocket teeth more than half the tooth height.
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