Date:         Sat, 14 Sep 2013 16:41:13 -0700
Reply-To:     mark drillock <mdrillock@COX.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         mark drillock <mdrillock@COX.NET>
Subject:      Re: Why Bigger Brakes Was Cause of brake rotor warping
In-Reply-To:  <R3eK1m02v08X5Fr013eMFb>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

I have had many diesel Vanagons and have some now. Yes, the engine has a
braking effect when you let off the pedal, but not as much as a gas van.
I won't go into the technical reasons why that is.

The warning at the top of a hill to "Use Lower Gears" is directed mainly
at drivers of heavy vehicles who are supposed to know what it means for
them. Large diesel trucks often have manual transmissions without any
syncromesh. That means the driver must match engine rpm to transmission
input shaft speed in order to change gears or the gears will simply
grind instead of meshing. Drivers of big trucks with this type of setup
usually shift between the gears without using the clutch. They do it by
ear and by feel, made simpler by the fact that with so many gears in the
tranny they don't have to rpm match over any large gap to switch from
one gear to the next. The problem is that they can't safely downshift
once they have started down a major grade. If they take it out of gear
and then can't get it right back into a lower gear they loose the engine
braking entirely and stand a big chance of overheating the brakes while
they struggle to grab a gear. The engine rpm range of ta big diesel
truck is very narrow and that is why there are so many gears in the
tranny. If you miss catching the next gear when you shift you might not
be able to find any gear at all before it is too late if the downhill is
steep. The driver soon finds he must UPSHIFT to some gear, any gear, to
get it back into a gear at all. Then he either hopes the brakes can save
his bacon or he has to try downshifting again, perhaps repeatedly, to
work his way back to where the engine brake is helping him enough to
avoid runaway. This is the main reason why runaway truck escape ramps
exist. Driver error.

As for the "jake brake" retrofit engine brakes, those are very useful
because the fully loaded weight of a big truck vastly exceeds the
braking power of the engine without that feature. The truck engine is on
the large size but not nearly in proportion to the loaded weight of the
vehicle. That is why they are slow uphill as well as downhill. Like
Vanagons, really. Some skill is needed to drive a loaded Vanagon on a
major pass, both up and down.

Mark



Don Hanson wrote:
> What you would do, is to shift down to a very low gear, a gear that will
> keep you at a speed that doesn't make it necessary to keep the brakes on
> very often...  That is why we encounter very slow trucks on the downhills
> as well as the climbs....They've shifted to a low enough to hold their
> trucks from gaining speed...and thereby they can control the heat they
> create with their brakes.....
>
>     When you crest many of our western US passes there are signs warning of
> downhills, and notifying you..."Use Lower Gears."......the problem is that
> many don't pay much attention to those...until they start to feel their
> brakes losing power due to overheating...and THEN they try to fix the
> situation....
>