On Mon, Apr 13, 2015 at 1:14 PM, David Hillman
> On Sun, 12 Apr 2015, Dave Cavanaugh wrote:
> and Eric wrote...
>> FWIW, a truck with a heavy load on a trailer with properly functioning
>> brakes can normally stop in LESS distance than just the truck alone.
> I do not believe this, and it is contrary to my experience, and that of
> basically everyone I can find. It is also the opposite of what you are
> taught when you get your CDL.
The issue, historically, has been that brake proportioning wasn't
adjusted for changing trailer weight. So brakes got set up so they'd
work right when the trailer was loaded. When it was empty, they were
too aggressive, and you couldn't brake the tractor as hard before the
trailer brakes locked up. Better proportioning valves, and for the
last 20 years, ABS, have made this less of a problem for heavy duty
trucks. It's still an issue on smaller trailers; if you have an
electric brake, you can make adjustments, but it's still bad.
In the wet, a loaded tire can have somewhat less bad traction than it
would when unloaded, which can improve braking performance.