I have a 1986 Lotus Challenger SX that I fixed up and started to ride it a lot this summer. It originally came with stem friction shifters.
I have gotten used to the shifters but I feel like downtube shifters are a lot better.
The frame does not have existing mounting points for the shifters, but I have seen on ebay used downtube shifters that are attached to collar style clamps, and you fit it over the downtube and tighten down.
I had an idea was to cut off the cable housing holders that are welded on to the downtube, sand it down, and then put the downtube shifter collar in its place. Would this be a good idea in practice?

No. Any mod that is not reversible is a questionable idea. Do the casing stops interfere with the levers' placement/travel? If not then reconsider the removal of the stops. The way you position this bike in your life will undoubtedly change over time and you might want to try brifters, bar ends or sell the bike. Andy

just mount the band above or below the stops. You can also use new ones.

https://www.amazon.com/SHIFTER-MOUNT.../dp/B007Q4NKN2

I'd reconsider the downtube shifters and think about barends instead. They will let you keep (indeed require) the downtube housing stops and are more convenient than downtube shifters too.

Or look at your Gevenalle brake/shifter system; you could probably use your shifters: https://www.gevenalle.com/product/audax/

This is the bike?

Challenger SX & Mixte SX

It's worth fixing up. Downtube shifters are the cheapest/easiest assuming that the cable stops don't get in the way of the shifters; otherwise go with bar end shifters. You're best bet for finding vintage shifters are either a bike co-op or the C&V for sale section. Just put up a WTB ad. I just bought a set of suntour ratcheting bar end shifters. Those are great shifters and they'll do the job for you without having to modify the frame.

Some of the modern bar end indexing shifters have a friction option and will work as well but they will generally be more expensive than vintage bar end shifters. Be careful because some modern indexing bar end shifters do not have a friction option.

Friction bar end shifters are still being made. The dia compe bar end shifters are very good but not cheap at $56:

https://www.newegg.com/Product/Product.aspx?Item=9SIA4M57EG4763&ignorebbr=1&nm_mc=KNC-GoogleMKP-PC&cm_mmc=KNC-GoogleMKP-PC-_-pla-_-Cycle+Parts-_-9SIA4M57EG4763&gclid=CjwKCAjwjZjZBRAZEiwAPeLSKy6z_xawqb3zBLyROpIQQ9u3sXJe6Es7DhtNEs9XzudZ9jX2fGZG8ho C_vAQAvD_BwE&gclsrc=aw.ds

They'll do the job. I've used them extensively on a vintage touring bike that I ride and they're very good.

They aren't that close. Buy some Microshift 7 speed brifters. They're cheaper and will work better.


You'll need an older style Shimano or Microshift derailleur if you don't already have one.