Join Date: Apr 2009
Location: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
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If the shim is roughly 3" long, there's be zero mechanical issues with using one. Many bikes have a built in "shim" in that they use an aluminum reducer in the top few inches of the seat tube, so you won't be any worse off.
There is one issue to deal with if you ride in the wet. The shim's full length slot makes a perfect channel for water thrown up by the rear wheel to enter the frame. Be sure your bike has a weep hole in the BB shell (which IMO all bikes need anyway). You might also try to seal the slot with some silicone caulk to prevent water entry. All my bikes are similarly sealed at the seattube slot even though they don't have shims.
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