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My road / turbo bike is a 2010 steel Dawes Sportif Elite, which has Reynolds 853 main tubes. The seat tube holds a 27.2mm seatpost and the front mech is 28.6mm. I need to replace the seat clamp, so bought a cheap 28.6mm one locally. Finding it was too tight, I assumed it had poor tolerances, so bought another 28.6mm one from the LBS. This is also too tight, all of which suggests the seat tube is thicker / stronger at the top clamp than at the 28.6mm front mech. I tried the 29.8mm clamp from my Kona Ute but it was way too slack.
The Dawes seems to be midway between 28.6 and 29.8mm. A Google search uncovered one ancient post on the Retrobike forum from a bloke keen to find a 29.4mm seat clamp. He was duly slapped down that no such thing exists. One shop in Singapore was offering 29.4mm seat clamps a couple of years back and a 12 year old STW thread suggests older steel frames did have odd sizing (Genesis, Kona, Orange) but not 29.4mm.
Anyone know if 29.4mm seat clamps are a thing? If not, my options seem to be; shim a larger one, ream a smaller one or have a professional braze a band around the seat tube to take an allen bolt.
Rather than keep buying seat clamps I'd invest in a set of digital calipers and measure it. There are some on eBay for £10 or so. Useful tool to have
Fair point! Although I now have 2 spare seat clamps for my '94 Rocky Mountain. Every cloud....
is it possible temporarily widen the clamp to fit it?
I had this issue on a frame which used to have a welded on seat clamp that failed, no aftermarket clamp fitted. For a while I ran one that was slightly too big but used a coke can shim to make it snug. After a couple of months I reamed out one that was a size too small until it fitted perfectly, but that was more to satisfy me, there was nowt wrong with the shimmed version really.
a 12 year old STW thread suggests older steel frames did have odd sizing (Genesis, Kona, Orange)
Unsure about Kona or Orange but Genesis were all 29.8mm nominal size, they were a 28.6mm OD tube externally butted to 29.8mm OD / 27.2mm ID at the top. 27.2mm ID plus the 1.2mm wall at the top is 29.6mm but there's a bit of seatpost fit gap, paint etc. It's a common spec on Reynolds steel frames anyway.
Perhaps a bit heavy duty spec in 853 for a road bike .. I don't know of any 27.2mm ID 853 tubes that use a 0.9mm upper wall thickness (not going back to that era anyway) but that would give you ~29mm OD. Dawes may have had some lighter seat tubes made for that frame though it's unusual to have a custom OD like that.
Any luck downshep ?
I had an old saracen around that size. I'd be asking a fabricator to widen a small one or get a shim as suggested.
How goosed is the original clamp?
Could a new bolt not sort it out?
I'd echo the above suggestions of measuring the tube/old clamp, it's normally external butting that means the clamping end's OD is a smidge more...
Thanks all for the suggestions. As a temporary measure, I’ve drilled out the chewed 4mm threads in the existing alloy clamp and shoved a longer 5mm Allen bolt right through into an M5 nylock nut. Looks dodgy and will only use on the turbo as poor seating and the lack of material around the expanded hole won’t be safe for road use. Looking at the above suggestions, I’ll try splaying a 28.6 out with a nut and bolt, then try shimming a 29.8. Both options are way cheaper than a braze on band.
Thanks all.