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I adopted a thoroughly abandoned Univega Super Strada from somewhere in the '80s or '90s. It has Shimano caliper brakes in good condition with pads that are worn but still have plenty of the indicator groove visible, so I haven't replaced them yet. I replaced the cables and housings because they were in rough shape.

When the bike is upside down and I spin the wheels, squeezing the brake levers stops the wheels instantly regardless of the wheel's speed. When I'm walking with the bike I can make the wheels skid by squeezing the levers.

When I'm riding the bike, the brakes are barely engaging. Squeezing the levers all the way to 100% slows me down very gradually, but there is not nearly enough stopping power to ride safely in traffic. This happens even when I'm riding at very slow speeds - slower even than the walking speed at which I was able to make the tires skid with the brakes.

Why do the brakes perform so much worse when the bike is loaded?

My wild stabs in the dark:

  • Loading the bike changes the wheels' shape slightly, changing the pads' pressure points on the rim
  • Increased momentum (adding more mass m in p = mv?) requires additional pressure from pads
  • Old/soft pads need replacement

Or some combination of all of the above?

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    Brakes are not performing worse. The load on the brake is different. The load is proportional to the mass. – paparazzo Sep 18 '14 at 19:46
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    You are listing symptoms of misadjusted brakes and/or old brake shoes. You mention that you have performed part of the required maintenance, but you must go all the way and change the pads and fine-tune the system. I also recomend washing the rim's braking surface with kitchen soap and dishwashing scrubbing pad. An answer already pointed to an article on adjunsing brakes. Several other questions on this site cover the issue as well. – Jahaziel Sep 18 '14 at 20:01
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    In this answer I cover the basics of brake adjustment: bicycles.stackexchange.com/questions/9889/… – Jahaziel Sep 18 '14 at 20:41
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Increased energy (energy goes up linearly with the mass) is one factor, as well as the grippy-ness of the pads.

The first option (wheel shape changing) is bogus.

However, the true culprit is likely that your your brakes are misadjusted.

To quote Sheldon Brown: "A brake with high mechanical advantage will apply a lot of force to the brake shoe for a small amount of finger pressure on the lever; the other side of the coin is that a system with high mechanical advantage will require the hand lever to move a long way to move the brake shoes a short distance toward the rim. If you have too much mechanical advantage, the brake lever will bump up against the handlebar before the brake shoe has moved far enough to engage the rim. " [See in particular the Feel vs Function section on that page]

See this article on adjusting the brakes. Also, while you're at it, throw on some new pads since the old ones may have dried out.

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