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Tech information furnished by Jim Chenoweth
Date: Fri, 8 Feb 2002 13:30:44 EST
From: FJ40Jim@aol.com
Subject: Re: 40 series trivia
From: post to the LCML:
<< Everyone agrees that the stock front shackle setup isn't optimal on the 40
series. My question is, are the Toyota engineers idiots or is there a reason
for the shackles to be up front and not in the rear of the springs where they
logically should be.>>
Bzzzt! Wrong assumption. The stock front shackle location is the
optimal compromise for several reasons.
First, let me explain how this seemingly simple leaf spring suspension is
mechanically modeled. There is a multi-leaf spring that is relatively flat
in its stock configuration. It is mounted with a fixed eye in the rear and a
free eye at the front. The spring has a full 2 wraps at the rear fixed eye,
making the rear of the spring stiffer. The subleaves are also shifted
slightly to the rear. The front axle housing (hereinafter known as "FA") is
mounted above the spring at about the 55% location. The rearward offset of
the FA, combined w/ the extra leaf positioning toward the rear, tend to
create a leading link effect.
As this nearly flat spring is compressed upwards, it goes totally flat
and extends forward, but only slightly, due to it's long, low-camber
configuration. If upward travel continues, it develops reverse camber and
begins to move the FA rearward slightly. Because the FA is mounted closer to
the fixed end of the spring, it's horizontal displacement is less than half
of the change seen at the free eye. This is a good system because small
displacement causes minimal bump steer. With extreme up travel, the type
that would occur over a big bump, the axle is displaced slightly rearward,
lessening the perceived impact and reducing stress on the vehicle & contents.
The leading link effect mentioned earlier makes for good braking &
acceleration performance. When the brakes are applied, a compression load is
placed on the rear half of the spring, into the fixed eye. The stiff rear
half of the spring acts as a leading link and is able to handle this
compression load with minimal deflection. Because the tire contact patch is
below and forward of the fixed eye, braking tends to drive the FA back under
the vehicle, thereby jacking the nose up. This is what is meant by
"anti-dive geometry". The harder the brakes are applied, the stronger the
jacking force is. The reverse happens when accelerating with a powered FA.
The FA will try to go forward, away from the leading link, and pull the nose
down. Coincidentally, a similar but reversed braking & accelerating effect
is at work on the rear axle, except it is trying to pull the rear axle back
and up, thereby keeping the rear of the truck from raising too much during a
stop.
Final consideration in placing the fixed eyes at the rear is strength.
These vehicles are sturdy workhorses. If you look under the truck, you will
see the rear spring hanger is attached in same area where the frame goes to
fully boxed, the rear torque arresting motor mounts apply their loads, most
of the body weight rests, and most of the engine&tranny weight. All the
braking, road shock and acceleration loads go into this straight, boxed, main
section of the frame. If the spring hanger was at the front, then the FA
loads would be remote from all these other loads, increasing stress on the
frame. Additionally, the frame would have to be beefed up considerably to
handle the FA loading that would travel over the curved (weak) front frame
rails to the vehicle center.
There is also a practical reason for having the fixed eyes at the center
of the vehicle. It puts the front & rear driveshaft arcs roughly parallel
with the axle travel arcs. This translates to less slip yoke travel on the
D-shafts and less slip-yoke wear.
That's the basics of the J40 front supension design, as seen by me.
Discuss amongst yourselves {8^)
Thanks,
Jim Chenoweth
TLC Performance
Lancaster, Ohio, USA
Ph. 740.862.2604
TLCA #1914
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