Explanation re: Factory Shackle Location

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^)

Jim Chenoweth
TLC Performance
Lancaster, Ohio, USA
Ph. 740.862.2604
TLCA #1914

Posted November 21, 2000

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