Re: Tie Rod Protector

[Tom Harais <tjhemh@domain.elided>]

Allan:

Like a lot of things, it's not that sprung weight is bad and unsprung is
good or something. It is the ratio of sprung weight to unsprung weight
that affects ride and handling (to some extent). And it really isn't
weight per se, it's mass, but we measure mass by weight generally in our
imperfect world.

It all has to do with Newton's law of motion - you know the one where an
object at rest tends to stay at rest and one in motion tends to stay in
motion. Well, the more mass an object has, the more force that must be
exerted to get that mass moving - right?

"Sprung weight" is everything "supported" or "sprung" by the springs in
the suspension. It includes the frame and body, the driver, about half
the spring itself as well. "Unsprung weight" is everything that is not
actually supported by the springs in a suspension. The differential on
our Scouts, the wheels, tires, brakes the tie rods, etc. It gets a
little complicated on independent suspensions because some items fall
into both categories, like the A-arms.

The idea is that the less mass the unsprung parts of the suspension have
in relation to the unsprung parts, the more those parts will move (as
they are supposed to) when acted on by a force (bumps in the road)
instead of the sprung parts moving, like the body. Again, this goes back
to that physics law I quoted above. You want the wheels and tires to
absorb the bump but the body and frame to move as little as possible
(not counting the forward motion of the vehicle here). That means the
suspension does the work and the vehicle rides smoothly. That's why a
5,000 lb Caddy rides so much smoother than a 3,000 lb Chevy on virtually
the same suspension. All the extra weight is in the sprung side on the
Caddy, but the unsprung weight is pretty much the same as the Chevy.
Hence, that ratio is higher and the Caddy's wheels will do a lot more up
and down movement without moving the body up and down than the Chevy's
will. It's also why your pickup rides smoother when you put 1,000 lbs in
the bed than when it is unloaded.

Now obviously our IHs, at least the 4X4s, by their very design, aren't
the best as far as unsprung weight ratio to sprung weight ratio. Solid
axle differentials mounted to the springs are much heavier than say, a
standard auto A-arm or MacPherson Strut type of setup. An A-arm is
supported on one end by the frame and the other by the spring. That
makes it about half sprung and half unsprung. But a differential hung by
the springs is much heavier and 100% unsprung weight. So, I wouldn't
worry about the added unsprung weight of those tie rod protectors. Your
1210 isn't going to hold ride like a Lincoln no matter what suspension
improvements you try and make.

If you want to see the idea of lightening the unsprung weight taken to
an extreme, look at CART racers or at a Jag coupe. The Jag has the fully
independent rear end. The differential is mounted to the frame, making
it part of the sprung weight. They even attached the disk brakes
inboard, so that their weight is sprung rather than unsprung.

I hope this gives you a real clear idea of how this all works.

Tom H.