I'm looking for some advice on building a solid, stable commuter. I typically ride what I consider fairly relaxed road bike setups: 28mm tires, randonneur bars, older frames with a reasonably long wheelbase. Recently, I've started riding with my boys on their tandem trail-a-bike, which is incredibly fun. Comparing the experience on my road bike vs my wife's (frankly crummy) rigid mtn bike, the latter is much more stable despite the otherwise lousy ride, whereas my road bike can get wobbly enough that I worry about staying upright, and it's especially tough to handle at low speeds with serious resistance, e.g. uphill from a stop sign

I've got it in my head to configure a bike designed to ride with the boys or with moderate loads when the boys aren't on-board. What should I be shooting for here in terms of frame, handlebars, tire size (diameter or width), and whatever else I'm not considering? I have a currently unemployed, fairly low-trail 700c frame with plenty of tire clearance, but I'm not averse to finding a different frame. I'm hoping to get some input at the outset so I don't set off on the wrong foot.

Thanks in advance for any suggestions.

I'd start with a good quality, early 90s rigid MTB. They are sturdy and stable plus will handle a wide variety of tire sizes.

Have an 80's stumpjumper, the lower angles of the headtube and the Big contact patch of the tire all add to the stability.
Schwalbe Big Apples are smooth rolling big and cushy .

Is 700c out? Would a touring frame provide a decent base? I like the idea of being able to change up the bike for longer distance riding down the line, but maybe I'm just setting myself up for too much of a compromise and I should suck it up and go with a mtn bike?

I think a touring frae would be perfect for what your wanting....Maybe one from Nashbar or Performance? Some decent wheels with the 28mm tires you like and build it up with 105 of some kind (dosent have to be new). You could get a decent starting point from pricepoint, then modify to your liking as $$ allows!

Good Luck!

This really appeals to me, but I guess this also highlights my main question, i.e. what should I shoot for if I need a bike like this to be particularly stable? Does bumping up tire width beyond 28mm make a big difference? Or should I be more worried about handlebar width, for example? Again, I'm not hell-bent on a road bike, but I'll gladly stick with one if I can get the ride good and stable with the right setup.

I'd poke at a couple things. Measure how high the bottom brackets are, and how long the chainstays are on both bikes. If you can find out what kind of tubing each bike uses, that might also be informative.

Stability comes from a bunch of factors, some of which can be hard to measure. Some kinds of tubing are very stiff and difficult to bend. This can result in a harsh feeling ride (which tends to feel unstable) but the bike will also never feel like it is bending or wriggling under load (which also tends to feel unstable). Other kinds of tubing are a bit flexible, which means you get a pretty compliant ride, but can get wriggly feeling under a heavy load. The fork and seat tube angles can affect how stable a bike feels. It's customary for those angles to basically match and be around 72 degrees. Going up to 75 degrees can make a bike feel very responsive/twitchy. Going down to 70 degrees can make a bike feel sluggish/stable. If the angles don't match, some riders might feel like the bike is mysteriously not riding like a proper bike. The chainstay length and bottom bracket height are pretty closely related, and they tend to have a lot to do with how stable a rear load feels. If you have a 42cm chainstay, that will almost force a particular bottom bracket height. Go up to 44cm or 46cm, and the bottom bracket pretty much has to be lower to the ground.

I'm sure there are a lot of other factors. I think you'll end up happier with the changes you make if you get concrete about what is going on with the bikes you have .

kyselad, what type of old road bike frame are we talking about here? I ride an old Fuji road bike with 1 1/4inch wide tires and I can't say it is any more twitchy at slow speed than my long wheelbase, fat-tired hybrid.

The available frame is a hi-ten Bridgestone "Kabuki". Best headbadge ever if not the greatest design (aluminum lugs make for a very bizarre seatpost setup!). Wheelbase is 104 cm, which seems not particularly long, chainstays are 45 cm. Trail is fairly low thanks to a lot more rake than I see on most modern forks. It's been a while since I've been on this bike, but I rode it geared and then fixed for a while, and though it's obviously a low-end frame, it handled quite well even with a moderate load on the rear.

If that seems like a good candidate with the right tweaks, great. If not, I'm happy to source a better frame -- I wouldn't mind better tubing, cantis, rack bosses, etc. Long Haul Trucker is especially appealing as a potential touring rig down the line, and the VO Polyvalent would be a great all-arounder. If the LHT had horizontal dropouts (I like the option of internal gears) or the Polyvalent was designed for something other than 650B tires, I'd probably have picked up one of these by now!