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Old 05-07-2004, 09:56 PM   #1 (permalink)
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shackle angles....

I am wondering what are the recommended shackle angles, if any?

My fj40 has extended shackles in the rear which are practically vertical...Is this a good thing?

I believe 55 springs are slightly longer and may add some additional angle...

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manny 72fj40
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Old 05-07-2004, 10:00 PM   #2 (permalink)
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45* rake is nice, btu it really depends on what springs you are running.
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Old 05-07-2004, 10:05 PM   #3 (permalink)
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you're just gonna have to drive it down to whitey's place one wednesday evening so we can pick it apart, woops, check it out!
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Old 05-08-2004, 08:20 AM   #4 (permalink)
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tech links
left column, other suspension tweaks
see my writeup on shackle relocation

also see my writeup on the 40-55 pack swap.
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Old 05-08-2004, 08:54 AM   #5 (permalink)
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Nice write-up, exactly what I was looking for.

Did you have to lengthen your rear drive shaft w/55 springs?

thanks for the links,
manny 72fj40
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Old 05-08-2004, 09:23 AM   #6 (permalink)
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You can't just set them at 45* if you have arched springs or they may hit the frame on compression and limit travel and/or bend your springs. Ideally you want them around 45* when the springs are flat or against the bumpstops. With the shackles near vertical, you will usually get a harsher ride and the spring has to flex more before the shackle will actually begin to swing.


Here is my shackle angle when the spring is almost flat and the axle is hitting the bump stop.

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Old 05-08-2004, 12:21 PM   #7 (permalink)
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The effect of shackle angle on the effective spring rate is quite complicated because it depends on spring shape, the spring arch and the length of the spring, which determine how the eye to eye length varies with compression. The greater the arch, the bigger the effect of shackle angle.

Because a flat spring actually shortens during compression, a backward facing 45 degree shackle actually increases the effective rate (as compared to a verticle shackle), but this negative effect is minimal because the eye to eye distance decreases relatively little in response to the vertical deflection.

For the 4 inch lift spring that I measured, the eye to eye length increases by about 20% of the vertical deflection, so having the shackle angle at 45 degrees would decrease the effective spring rate by approximately 14% compared to a vertical shackle.

The over riding concern is that the shackle have enough movement to allow full compression of the spring. If the shackle movement limits the eye to eye lengthening of the spring during compression, it is like having no spring.

Edit: "effective spring rate" is just a fancy term for ride quality. If you decrease the rate, you "soften" the ride.

Last edited by Pin Head; 05-08-2004 at 04:34 PM.
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Old 05-08-2004, 04:14 PM   #8 (permalink)
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if you put in FJ55 rear packs or reverse your FJ40 packs, it relocates your rear axle back 3.5" and does require driveshaft work.

Pin Head is spot on with his notes on angle....I ran stock shackles for a bit when I was running SUA 4" Softrides, and they would compress to the frame, even at the stock location. Reworking to a 45 degree angle required a ~3" overstock shackle to maintain the angle AND allow it to not hit the frame on full tweak.

Took some experimentation, but worth it for the ride and flex.
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