Date: Mon, 22 Jun 1998 13:16:19 -0300
Reply-To: Tim Smith <smitht@UNB.CA>
Sender: Vanagon Mailing List <Vanagon@vanagon.com>
From: Tim Smith <smitht@UNB.CA>
Subject: <syncro> spring pre-loads
Content-Type: text/plain; charset="us-ascii"
Hi Syncronauts et al,
If you jack the van up, the springs will push the lower A-arm down, less
weight on the springs. Eventually the wheel leaves the ground. Now measure
the distance from say the axle centre to fender edge. Lower back to ground,
bounce it and remeasure height. The difference between unloaded and load
height is the preload on the spring, and if you know how many pounds on
that corner you MAY be able to get the spring rate, in lbs/in. If the
suspension, even when fully extended, is still limiting spring extension
then there is some preload due to geometry/shock travel, which is why I said
_may_. Either way you have say 5" of travel available before the shocks/arms
limit it.
Now, back to the resting Syncro, you have about 5" of travel in spring
before front tire lifts, so in theory if you stuff in a 5" spacer top or
bottom of the spring you will RAISE the van by 5" and end up with same
preload on spring, but with full extension of shock. This hasn't changed the
springs properties, nor added any extra compression, so the spring rate and
preload stay the same. Put in a 6" spacer and the rate still stays the
same, but the preload increase.
So you can raise a Syncro 5"? Well, you'll pay for it twice. Airborne and
the lower bump stops of the shocks will get hit, more often than when you
had say 5" of 'reserve' travel downwards. "Topping out" instead of bottoming
out, maybe not a problem, likely shortens shock life. And you won't likely
bottom out, since the shocks are operating 5" further out. BUT a good bump
and that 5" travel may get eaten up, along with a bit more travel and the
shock's bump stops don't even get used. What happens most likely is that to
coils in the spring all come together (coil bound), and with the 5" spacer
the whole 'spring' is rigid, the impact of the bump goes right into the
upper spring miount and things get really loaded hard, breaking eventually.
Having said all this, a mild lift by spacering the springs would appear
good/feasible, I'd go for as much as 2" myself, but as to how, I dunno!
Really well made spacer in/above the lower perch is best guess. A 2" long
bit of 4" diam. steel pipe, thick enough to allow a spring seat to be cut
into the top, and bottom contoured to locate well in the perch. Still
risking becoming coil bound!! The idea of Crown Victoria springs is good, go
a little longer to get the lift, and a little fatter wire to get the
stiffness FYI the Zeikel springs have wire 3mm thicker (1/8") than stock.
Too stiff and the regular shocks won't contain the rebound and you'll get
some interesting bouncing depending on speed/bumps.
This weekend I blew by a J#@p Cherokee at 50 mph in a potholed highway
contruction site, van was tracking smoothly, wheels/springs/shocks doing
their thing together. This is what _all_ vanagons do well. Handle rough
stuff at speed due to good suspension design/tuning at the factory. Jeep was
doing 35mph, and the occupants didn't look relaxed. He passed me after the
site, at about 70mph, I was still doing 50, it was Sunday.
One final note, springs/shocks are matched up, with the dead (sprung) weight
that the wheel/tire/suspension provides. Makes up a system that has it's own
natural frequency/dynamic response. Change any one of the 3 components and
you change the systems behaviour. The much heavier 235/15 MTs on 15" rims
have done that for me. Tires/grip feels 'looser', tend to float more off
bumps at high speed because the springs aren't stiff enough to push them
back down fast. I suspect this is in part why the Syncro-16 has stiffer
shocks, to contain heavier wheels/tires at higher speeds.
So suspension mods need to figure in both crawling/static and
high-speed/dynamic implications. Don't want a tall Syncro that bobs all over
the place and handles badly on the highway. HTH, bye, Tim
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