I am trying to convert my Schwinn Mountain bike into a road bike. I bought a set of Shimano SL-SY20A 3 (2) x 7 Speed Down-Tube Shifters. they have a clamp attached but my downtube is very wide and the clamp won't fit. how can I mount them to the downtube. I'm thinking of drilling holes on each side and screwing them in, mounting them that way. Does anyone know how I go about doing this? thanksfor your time!
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1You do risk weakening the downtube if you drill through it. How serious this risk is depends on the material used in the downtube, plus your skill at drilling a clean hole, and how much stress the actual mounts generate. It would not be a serious risk for steel frame that is not lightweight, but would be more of a risk for aluminum, and very iffy for carbon.– Daniel R HicksCommented May 6, 2017 at 3:35
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If its a steel bike you might get away with brazing some mounts onto the frame. But if you're going to put drop bars on it, consider brifters (integrated brake/shifters)– Criggie ♦Commented May 6, 2017 at 10:50
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2 Answers
Without any additional knowledge on the bike, my advice would be don't drill into the down tube. You run the risk of making it structurally unsound. That being said, some bikes with downtube shifters have a hole drilled in the downtube with a rod going between the mounts and maybe some epoxy using something like this:
Some other options are:
Find a different clamp (the Problem Solvers downtube shifter mount is bigger than most, has a 31.8 mm diameter)
Try making your own clamp (e.g. with a hose clamp; this may be necessary if you have oval/other shaped rather than round tubes)
Braze on some downtube shifter mounts (Shouldn't be too hard for steel frames)
Switch to using bar end shifters/brifters (Shimano makes a Tourney 7 speed brifter)/Retroshift levers (which mount a downtube shifter on the brake lever)
An aside:
As noted in various other questions on this site (do a simple search), its often not advisable to convert a mountain bike to a road bike or vice versa due to cost and geometry reasons.
I have drilled my Helliot's steel downtube to install downtube shifters and it's OK. I made 3 videos for Youtube with the whole process.
Now I'm thinking to switch to an alloy frame and I'm going to avoid to drill it. Vintage alloy frames had downtube bosses but probably the tube was reenforced on that zone or the bosses were weldded (no drills), but I'm not sure. As an engineer, I think it's not sound to drill alloy, specially if the hole is 8 mm wide.
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But "steel" bikes are almost always steel alloy, aren't they? (Except at the lowest end.) Are you sure there's a meaningful distinction?– DavidWCommented Nov 5, 2022 at 14:58