wearwell wrote: Brucey wrote:
wearwell wrote: It's not about "perfection" it's about normal practical tolerances and quality control....
yes, and you are assuming that those tolerances are not
'normal' whereas IME they are. ........
The acid test is whether your seat pin moves in normal service or not. ...s
Er - isn't that what the OP was complaining about?...
AFAICT the OP has stated that
a) an undersize seat pin needs the clamp to be 'significantly tightened' to grip the seat pin and
b) that there is still some detectable wobble with a seat pin that is closer to the correct nominal size and
c) the next size up seat pin (+0.2mm) won't fit
Maybe I've misread it but this is not the same as
- there is a measurement of the (well lubricated) bolt torque required to clamp a seat pin of the correct size or
- the seat pin moved in service
- that the fit has been compared with seat pins that do and don't move around in service for the OP (or other folk)
- that the permitted tolerances of the frame are known or
- that the frame has been carefully measured and is known to exceed said tolerances
The usual course of events with an OTP frameset is that
a) a (nominally correctly sized) seat pin is fitted
b) it is seen to move in service and
c) possible issues with the clamp and the actual size of the seat pin are eliminated
In this situation if
the seat pin is also seen to be a bit of a wobbly fit this is almost certainly part of the problem. However if a little movement is evident, but the frame has not been ridden or compared with other similar frames, to then assume that this is definitely abnormal or a problem of some kind is jumping the gun a bit.
Note that the stresses that are seen around this joint and factors which might influence slippage can vary enormously, with rider weight, seat pin length outside the frame, seat pin length inside the frame, terrain ridden, riding style, materials used, surface finish of the parts, bolt torques employed etc all making a difference. Also note that there is a always a stiffness mismatch between the frame and seat pin, so that, under load, the parts may deform enough to move a little even when they are nominally a good fit. Even that the seat pin has moved (even if it moved because the bolt wasn't tight enough having been OK for years beforehand) this can change the finish inside the frame so that the seat pin will move ever after.
I have encountered situations (which BTW were not with heavy riders, but were most often on MTBs) where no commercially available seat pin could be made to stay put in the frame with a normal clamp. A seat pin that was less than + 0.1mm larger did stay put and this was a good solution. However with the best will in the world, no-one could have predicted whether the original seat pin would definitely move or not beforehand, only that it conceivably might
do. IME if you try the seat pin fit of (say) most OTP bikes in a typical dealers then they are more likely to wobble than not, but most of them won't cause problems in service.
Much of the discussion in this thread has been concerning practical solutions to seat pins that are found to persistently move in service; if you can (by whatever means) make the fit practically wobble-free, it is almost certain that you can solve the problem this way. However that is not
the same thing as if you have a little wobble evident, that this will definitely be a problem.