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Old 12-14-2006, 08:20 AM   #1 (permalink)
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Ride frequency and down travel

Sorry for hijacking that other thread... but it got me thinking

It's pretty accepted to design spring rates by ride frequency.

Some simple algebra will give you static spring compression at ride height.

If your ride frequency is say, 1.4, that would give you static spring compression of 5 inches.

Then suppose you are limited to 6" of up travel, due to clearance issues.

If you are running a 12" or 14" coilover, you would need tenders to keep the springs from unseating. So during the last 1-3 inches of droop, your spring rate is nearly zero.

Is that last 1-3 inches of droop going have any noticeable benefit? If you could easily fit 14" coilovers, is there any advantage over 12" coilovers?

My though is that it would, despite the fact that there is negligible spring force, the shock is still damping the chassis movement.

But what do I know? I'm still running leaf springs front and rear.

If you want to check my math, I attached it, g=386.4 in/s^2 and ride frequency is in Hz.
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Old 12-14-2006, 11:49 AM   #2 (permalink)
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I don't know if this is a hi-jack - its related, I can move it if preferred...but here goes...

Interesting topic, I've thought about this a lot. I'm trying to set mine up based on ride frequency, which left me with about 6" compression at ride height (using a slightly lower frequency)

The amount of up travel I have yet to decide on, but I'm building it at full stuff, as low as is somewhat practical, so uptravel can be maximized for a given chassis height. (stretched the axle forward in front of the engine, chopped of the frame rails just before the engine mounts, and ditched the front fenders)

I picked up some 14" coilovers, which leaves me with 8" of up travel, assuming a 1:1 motion ratio, unless I use a helper coil, which I did not want to since it seemed more or less pointless to run a 14" shock if 2" would be at zero rate. ( I too wondered if the added 2" of damping would be worthwhile or not )

The only other option I'm considering is running a triple rate setup, but with something like 4.5" 100 lb/in coils, which would give me a rate of 50lb/in for the first few inches, then 100lb/in secondary, and 225 primary (175 over 225). I would like to run a lower rate tender, say 50lbs/in, just to get me a little farther from unloading around corners and getting into the super light rate for stability reasons, but have not been able to find one.

That should lower me by the travel of the tender, ~3.5", but still give me the same ride frequency, since the tender would be fully compressed at ride height (block load is a couple hundred pounds less than sprung weight)

This is all for a linked front, by the way. I haven't seen anyone mount front shocks up the control arm, but don't see why it wouldn't work....but my travel would only increase by the amount of the tender coil travel multiplied by my motion ratio, so it seems like a lot of effort for minimal gain (ie 1.5x3.5 - 3.5 = 1.75" gain in droop....assuming same ride frequencies and chassis height)

FWIW, I'm planning a 4 link rear, and leaning towards mounting the shocks at about a 1.5:1 MR, for around 20" travel rear. The only problem is it seems logical to run a bushing at the frame to limit rotation in the control arm where the shock is mounted, but it's tough to find a flex joint for the axle end that can handle the articulation that setup would give, since it all comes from one joint, rather than two if each end of the control arm had a flex joint.

Any other opinions on this setup or topic would be interesting to hear.
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Old 12-14-2006, 12:30 PM   #3 (permalink)
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I dont think it is pointless to have a few inches of travel with the spring basically at 0 rate. A lot of leaf spring systems actually probably have a negative spring rate at full droop. They are being pulled down farther than their natural arch. Your shocks are doing way more work than the springs and the effect of the spring at full droop isnt going to do much unless you are way to stiff.
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Old 12-14-2006, 05:48 PM   #4 (permalink)
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One advantage of having a very low spring rate for the first inches of travel is the shocks can absorb the chop and washboard much better.

"The only problem is it seems logical to run a bushing at the frame to limit rotation in the control arm where the shock is mounted, but it's tough to find a flex joint for the axle end that can handle the articulation that setup would give, since it all comes from one joint, rather than two if each end of the control arm had a flex joint."

If you mount the shocks below the centerline of the trailing arm, it will not "flop" under compression, and you can use heims or uniballs at each end.
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Old 12-14-2006, 07:17 PM   #5 (permalink)
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Depending on your ending ride height I would guess you may end up having to use tenders if you design your suspension using the ride frequency formula. If you did not care so much on ending ride height then perhaps you could do it without tenders and using longer lower rate springs. One advantage of using tenders is that your ending at ride height spring rate is higher so you have better handling for high speed turns in general.

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Old 12-14-2006, 09:50 PM   #6 (permalink)
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Quote:
Originally Posted by Scott F
"The only problem is it seems logical to run a bushing at the frame to limit rotation in the control arm where the shock is mounted, but it's tough to find a flex joint for the axle end that can handle the articulation that setup would give, since it all comes from one joint, rather than two if each end of the control arm had a flex joint."

If you mount the shocks below the centerline of the trailing arm, it will not "flop" under compression, and you can use heims or uniballs at each end.
Check out Evan Evans Corr truck for an idea to control this. He uses some very small links perpindicular to the lowers up near the front pivots. These keep the links upright (or small arc anyway) They run between the frame and a small arm off the link top.
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Old 12-14-2006, 10:02 PM   #7 (permalink)
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Quote:
Originally Posted by Scott F
One advantage of having a very low spring rate for the first inches of travel is the shocks can absorb the chop and washboard much better.



You are generally at ride height when driving wash board road?

I think the extra couple inches helps keep a truck stable when experiencing EXTREME flex. I think it is some what useful.

Get 14's and if you dont want the extra travel you can use a limit strap or put more internal spacers.
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Old 12-14-2006, 11:19 PM   #8 (permalink)
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Quote:
Originally Posted by MOSS2
Check out Evan Evans Corr truck for an idea to control this. He uses some very small links perpindicular to the lowers up near the front pivots. These keep the links upright (or small arc anyway) They run between the frame and a small arm off the link top.

Or you could look at the Static-X Bronco buildup on Pirate.

/forum/general-4x4-discussion/405013-static-x-bronco-build-up-thread.html



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Old 12-17-2006, 08:20 AM   #9 (permalink)
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You are generally at ride height when driving wash board road?
Yeah, I thought about that when I was typing it. Maybe I was thinking about how a front end can get light under hard acceleration. At any 'rate', you don't need much spring rate at full droop. You want the tire to stay in contact with the ground, and easily follow terrain contours.
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Old 12-17-2006, 08:26 AM   #10 (permalink)
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Can someone explain the advantage of using the small perpendicular links with a Uniball? Why use a Uniball if it does not articulate? It seems like a bushing would work as well and be a lot cheaper.

Any why not just mount the shocks below the centerline?
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Old 12-17-2006, 10:16 AM   #11 (permalink)
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A bushing would work, except that the bushing material would need to be steel, rubber or delrin (a standard urethane bushing would not be strong enough) As soon as the shocks and springs are directed through the links the loading of the bushings goes up dramatically. I would imagine that the perpendicular links force the need for some amount of angular deflection in the lower link, so most typical derin bushings (0 articulation) would also be unacceptable. Leaving monoball and spherical bushings to remain. (Or oem style inner and outer canned rubber bushings, which are seemingly unpopular)
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Old 12-17-2006, 04:56 PM   #12 (permalink)
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Can someone explain the advantage of using the small perpendicular links with a Uniball? Why use a Uniball if it does not articulate? It seems like a bushing would work as well and be a lot cheaper.

Any why not just mount the shocks below the centerline?
Different combinations of articulation will move the links closer together in a horizontal plane. If you use completely solid bushings your links will have to flex or bend to achieve articulation. I havent built any link setups but I always wonder what keeps the links from flopping on rebound when the mounts are below centerline. Another thing that seems to be a priority with the rockcrawler set is loosing the ground clearance by lowering mounts.
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Old 12-19-2006, 01:58 PM   #13 (permalink)
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Or you could look at the Static-X Bronco buildup on Pirate.
I remember reading that a few times a while ago, but that still doesn't help the fact that the lower flex joint has to provide the vast majority of the flex when articulating.

Another thing I've considered is running the perpendicular arms part way down the link, so it has to cycle as the link cycles. Depending on it's ride height angle, length, and mounting points, you could set it up to rotate the link in the direction of the axle when articulated, but limit it to not exceed the misalignment capability of the shock's joints.

Maybe I'm overthinking before building, and a good single joint probably has all the flex you'd need... static-x's bronco doesn't seem to be lacking, after all.

Beat95, what spring rate/frequency did you end up with? I remember you set your jeep up by frequency initially when you went to coilovers, but found it too soft on road.
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Old 12-19-2006, 11:47 PM   #14 (permalink)
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Beat95, what spring rate/frequency did you end up with? I remember you set your jeep up by frequency initially when you went to coilovers, but found it too soft on road.
This is where I am now.

/forum/jeep-hardcore-tech/405914-polyperformance-doing-sweet-fab-now.html

I have kept a running log of the changes.

I think that my swapping to the antirock while playing with springs didn't help the onroad feel much. If I kept it on the street I would keep the oem swaybar. I was going down the freeway tonight at 75 thinking it wasn't so bad. If anything I would want more of a digressive shock tune for onroad driving. None of the current coil over makers offer this, so it is what it is. Basically I am happy with my current rates and am playing with valving and stop ring adjustments now.
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Old 12-20-2006, 05:57 PM   #15 (permalink)
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I would imagine that the perpendicular links force the need for some amount of angular deflection in the lower link, so most typical derin bushings (0 articulation) would also be unacceptable. Leaving monoball and spherical bushings to remain.
I totally agree, I just don't get why someone would use the perp link instead of just mounting the shocks below the centerline. Plus, the perp link severely limits the front joint articulation.
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Old 12-20-2006, 05:59 PM   #16 (permalink)
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Can someone elaborate on the ride frequency theory?
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Old 12-20-2006, 06:40 PM   #17 (permalink)
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I totally agree, I just don't get why someone would use the perp link instead of just mounting the shocks below the centerline. Plus, the perp link severely limits the front joint articulation.
Re Read above posts and it is explained. Yes that is the whole point of using the links. No most people dont need that much articulation on one axle in race vehicles. Your shock eyes can only move so much as well. The best thing to do is draw it out or mock it up and then you know, not think.
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Old 12-20-2006, 10:51 PM   #18 (permalink)
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I totally agree, I just don't get why someone would use the perp link instead of just mounting the shocks below the centerline. Plus, the perp link severely limits the front joint articulation.
I think it has more to do with time to fab (weld tabs on link vs. fab sheetmetal links) and ground clearance than anything. If the Bronco or Goat1's rig were strictly desert I am sure it would be different.
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Old 02-26-2007, 03:05 PM   #19 (permalink)
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I'm not an engineer but I believe ride frequency is calculated at cycles per second so in this case the suspension cycles on the spring without damping at 1.4 cycle per second. I believe most suspensions cycle between 1 and 2 cycles per second.

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Old 02-28-2007, 08:23 PM   #20 (permalink)
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Is that an air bump above that wishbone? Can anyone expiain that setup. Looks sweet though. The pic is above ,couldnt get it to post.
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Or you could look at the Static-X Bronco buildup on Pirate.

http://www.pirate4x4.com/forum/showthread.php?t=405013



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Old 02-28-2007, 08:46 PM   #21 (permalink)
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Is that an air bump above that wishbone? Can anyone expiain that setup. Looks sweet though. The pic is above ,couldnt get it to post.
http://www.pirate4x4.com/forum/showp...&postcount=314
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