Re: Replace leaf spring shackles with AFCO sliders?
[Re: rftroy]
#1581519
03/09/14 01:00 PM
03/09/14 01:00 PM
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Joined: Jan 2003
Posts: 5,394
Pikes Peak Country
TC@HP2
master
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master
Joined: Jan 2003
Posts: 5,394
Pikes Peak Country
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Quote:
That AFCO article is WRONG!
Worse, it is quoted all over the web and taken as dogma, just like "If your coolant is moving too fast through your radiator, it doesn't have time to cool off"; also completely wrong.
Those Summit sliders are for a race car that requires movement of the leaf spring for different track rules or conditions.
Changing shackle angle does not change spring rate. Period.
Further, you never want to have your shackle angling inward toward the spring, i.e. toward the front of the car. This will put the leaves in compression, which is bad.
Evidently the AFCO people do this on cars, but that is in racing. And we know how long some things last in race cars. Longevity is sacrificed for some particular element of performance.
I don't know that I would say the are absolutely wrong, but then again, in the limited space they have for their technical articles, I wouldn't say they are putting all their information out there either. Suffice it to say, there is A LOT more tech going on with a leaf spring suspension than most would know, despite its simple appearance and layout.
As far as shackle angle changing rate, yes and no. I agree that spring rate is spring rate and does not change based on shackle angle. However, I would say that shackle angle will impact a leaf springs ability to actually utilize that rate effectively.
Think about this, if your shackle is angled forward, you have basically put a hinge in a section of your spring. The loading on the spring will find the soft point to move first, so the shackle will pivot before the spring loads so you're not aplyng the full rate of the spring due to the pivot.
Conversely, angling the shackle back, as recommened in the Mopar manual, reduces wasted travel before rate is aplied and actually utilizes the sprng rate more effectively.
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Re: Replace leaf spring shackles with AFCO sliders?
[Re: rftroy]
#1581520
03/12/14 05:14 PM
03/12/14 05:14 PM
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Joined: Mar 2014
Posts: 40
upper So. CA
ntsqd
member
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member
Joined: Mar 2014
Posts: 40
upper So. CA
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Quote:
snip....
Changing shackle angle does not change spring rate. Period.
Oh dear....
"Kerrynzl" is a little mixed up as best as I can tell. He/she really needs to review trig as applied to pivots traveling in an arc. The spring rate itself doesn't change with shackle angle, but the wheel rate does. Depending on the starting and ending angles involved it can be a quite dramatic difference or it can be a tiny difference. Altering shackle angle is a tuning tactic commonly used off road to change the wheel rate.
People always say/type "spring rate" when what they're really talking about is Wheel Rate. Wheel rate is what is important, spring rate means very little to the car. Changing the shackle angle is like changing the motion ratio on a coil-over front suspension. With a near vertical shackle it's all spring rate. There is very, very little upward motion of the shackle end due to changing length of the leaf spring. With the shackle at an angle there is some upward movement of the spring eye as the spring compresses, and that reduces the wheel rate because the wheel is getting movement for "free".
Let's say that the rate of the spring is 100 lbs/in. and let's say that the spring's arch is such that each 1" of compression result in the spring "lengthening" by .25" That is to say that flattening out the arch .5" from ride height moves the spring pin back .125" and the rear spring eye back .25" (spring pin is truly at the center of the leaf spring).
With the shackle vertical a .5" upward movement of the wheel moves the shackle eye back .125", but sin 90 = 1 so the force needed to compress the spring that .5" was 50 lbs. (100lbs/in * .5")
With the shackle at 45° a .5" upward movement of the wheel moves the shackle eye back .125", but sin45 = .707 and .125*sin 45 = .088, so .088"/2 = .044" (centered pin moves 1/2 the distance of the shackle eye both horizontally & vertically) of that .5" of movement was due to the rotation of the shackle. Now the spring has only had to compress .5" - .044" = .456" The force required to compress the spring enough for the wheel to move .5" was 100lbs/in *.456 = 46 lbs. Which is a ~9% reduction in wheel rate just due to changing the shackle angle.
Granted, very few will run a shackle laid over that far and these spring numbers are totally arbitrary with no relation to real parts, but its illustrative of how shackle angle plays a role in wheel rate.
With the sliders it will be just like the shackles were vertical, only it will always be like that. With shackles you loose some wheel rate as the spring compresses due to the increasing angle of the shackle.
I used to swerve around my hallucinations, now I drive right thru them.
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