These instructions are adapted from the advice provided in the Thorn Bicycle and Tandem Owners’ Guide.
When and how to adjust the eccentric bottom bracket
According to Thorn, slack train will not do any damage whatsoever, if it is allowed to become very slack the only danger is that it may fall off. By contrast, a chain which is too tight wears prematurely, causes premature wear to the bottom bracket assembly (possibly to the hub too) and causes unnecessary expenditure of energy in use. It’s not a good idea therefore to adjust too often or to over adjust: better to err on the side of, “if it’s not broke, don’t fix it.”
According to Thorn, here is always a “tight spot” on all chain drive systems. When setting chain tensions there must always be some slack in the chain at this “tight spot”. To find the tight point, measure to the mid point between the centre of the sprocket and the chainring. Measure how far you can pull up the chain at this point and work round the chain until you find the point where you can lift the chain the smallest amount. This will be the tight spot.
We will call the gap between the highest and lowest point the chain can reach when pulled up and down, measured at the mid-point, T1 and the gap when pulled together, T2. See here:
Do not readjust the chain tension until T1 minus T2 equals 40mm. The “crude but simple, reliable and effective” system Thorn use for locking the eccentric depends upon the points of the screws being able to bite into the alloy eccentric. If unnecessary chain adjustments are made, the holes created by the points will be too close together and the eccentric will simply slip back into its previous position.
The change should be adjusted before T1 minus T2 reaches 60 MM. Failure to do this makes it possible for the chain to become slack enough to fall off the chain ring (or sprocket).
With a new chain (and especially a whole new transmission), it may not take very long for the 40mm of slack to become 60mm of slack but this situation will improve dramatically with the passing of the miles.
To readjust the transmission chain tension on Rohloff Thorn cycles
- Unscrew the eccentric screws, using a 15mm spanner or large screwdriver.
- Rotate the eccentric to achieve the desired chain tension using the adapted spanner provided by Thorn. Depending on the model of eccentric shell fitted to your cycle you will need to use either an eccentric adjustment spanner or by inserting a metal rod. Using the spanner set simply rotate the eccentric to achieve the correct chain tension.
- Finally re-tighten the eccentric screws and re-check the tension of the chain.
Thorn provide these eccentric screw torque settings for Rohloff equipped cycles:
10 to 17 Newton-meter (torque)
7.38 to 12.5 for foot pound force.