Originally Posted by
_dylan
I'm looking for some inspiration (so show me pictures if you've done this) and advice!
I want to take my old '75 Sekine and turn it into a monster. I'd like to take the handlebars down to some bull horns,
That smacks of hipster fixed gear butchery. Don't.
Keep the drops, or at least hang onto them if you have to switch bars.
Was there something wrong with the current one?
and get some bigger mountain bike tires put in place to do some more off roading.
Your Sekine is not likely to be a good mountain bike. The gearing is too high, for one thing, and the geometry is going to be wrong (too steep for comfort). However, if it is a 27" and you are interested in gaining some clearance around the stays and the forks, you might consider converting to 700c. This should allow you to run 700x32 cyclocross tires for additional flexibility, without trashing the geometry.
I've learned that after getting my Sports, that's all I really want to ride around, which is why I want to change it up on the Sekine--it'll give me more places to go ride (i.e. trails, etc).
Has anyone done anything like this to one of their old steel road bikes?
Yes. The current conversions I've run personally have been 700c to 650b (which I was unhappy with) and 27" to 700c (which was much better). This does not mean that you would not be happy with a 650b conversion - but it's likely that a 700c would be easier to pull off for your intended purposes.
I've also converted a 700c hybrid to cyclecross/gravel. That particular conversion is actually easy, since there are no tire clearance or brake reach issues to deal with.
Anyone know what kind of components I'd need?
For 27' to 700c - you'd need new wheels or new rims and spokes to match. It goes without saying that you would need the cyclocross tires (Michelin Mud2 works well, as do some of Kenda's current offerings). You would also want to consider swapping out the chainrings for smaller ones, and also the freewheel cluster for something with a few more gears and a 28-34 tooth low gear. Plan to get a new chain if you change the gearing. The brakes might need to be swapped out for longer reach ones, though it's less of a problem if you have centerpulls already, as they already have sufficient reach in a lot of cases.
27' to 650b conversion is a much more involved process. You would definitely need long-reach brakes, in addition to the new rims, spokes and (IMO, somewhat rare) tires. A 650b does allow one to run large carcass tires (32mm and larger), thought the selection of knobby types is limited at present (unless you can make 27.5 tires work somehow). The end result is a bike that rides really well, has clearance for fenders, racks, panniers, etc. without being too bulky or slow. Drawbacks include slightly reduced ground clearance at the crank, and reduced availability of tires/tubes (Wal-Mart would be a no-go).
Don't ruin the appearance. Convert with kid gloves. Keep the colors the same if possible. Hang onto the original parts, because you never know when you're going to want to convert back.
One upgrade worth the cash (IMO) is a swap from stem-mounted or downtube-mounted shifters to bar-ends. This adds instant value to the bike, and makes it easy to use in the dirt and gravel. It's a great cyclocross setup and what I use on all my bikes (except the Vista).
For the Sekine, Suntour bar-cons, Dura-ace or Campagnolo friction-shift barends should be the most appropriate / period correct. Indexed bar-ends are okay, too, as long as they have a friction option.