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Old 04-10-2009, 11:33 AM
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Rear Shock Position and Movement

I am using a Ford explorer axle for my build and have some questions regarding mounting this on my chassis. It is the usual Hotchkiss design.

First I have noticed that most people position the shocks so they slant in from bottom to top but are usually aligned with the crossmember, i.e. if you look down on the chassis they are perpindicular to the centerline of the chassis. The explorer has them at about a 45* angle pointing towards the rear. (Hope you're following this!) Are there any advantages/disadvantages to doing this?

Secondly there seems to be no provision to allow for the movement of the bottom or top of the shock to move back and fore with the axle as the spring straightens. Will bushing compliance allow for this movement (about 1 inch)? It would seem to me that bolting the top of the shock solidly to the crossmember will put stress on the bolt/shock eye.

Thanks for any help.

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Old 04-10-2009, 04:10 PM
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I had the same thought on my build, but I see a lot of cars with that style of mount. I'm going to go with a stem mount on top just in case because it seems to me that would allow more flex, or the other thought I had was to put the mount sideways at the top, since the bottom is mounted in line with the frame. That would also allow more movement.
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Old 04-11-2009, 02:16 PM
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hi there


well the only thing i can say right now is that they started mounting shocks at an angle and one in front of the axle and the other behind the axle in order to help prevent wheel hop axle wrap.
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Old 04-11-2009, 05:05 PM
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The rubber bushing will be compliant enough to permit the movement you need. I have seen urethane bushings tear up a mount if mounted perpendicular to the natural compression/rebound motion of the axle, but unless you are going to do some rock crawling, you won't lean that car enough to do any damage withe even the urethane bushings mounted parallel to the axle..
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I would bet ,(and this is a guess) that the forward leaning shock angle you see on many truck and suv shocks is to do a little more weight transfer control on braking than the sideways angle would....

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Old 04-11-2009, 05:30 PM
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The more angle away from the arc of suspension movement, the less effective the shock will be. In other words, if the suspension travels vertically, and the shock is mounted at an angle of say 20 degrees either inboard at the top or rearward as you describe, as the suspension compresses the shock angle increases and for every inch of suspension travel you get less and less shock travel. This results in the shock being less effective over a large bump. If you ever noticed after lowering a Hotchkiss suspension without relocating the shocks, their angle at the new ride height lessens their effectiveness and the suspension gets bouncy.

I suspect the reason they are mounted at an angle in the Explorer is due to packaging. They need to get a long shock in place without getting into the passenger compartment, or to clear exhaust. And I suspect the valving was adjusted to compensate for the increased angle.

The same rules apply to mounting coil-overs, if they are installed at an angle that doesn't match the arc of the suspension, you can get into a situation where the spring rate decreases or at least doesn't increase at the same rate the more compressed the coil over unit gets. That is called a falling-rate installation.

Picture in your mind compressing a shock straight down 2 inches, then put the shock at 45 degrees and move it 2 inches, it will compress an inch and rotate an inch. If you go to the extreme and lay it flat, it will rotate 2" and not compress at all.

A long explanation for a short question, but as with every system on a car, there are pros and cons to every modification, and everything is a compromise somewhere.

Andy
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Old 04-12-2009, 06:14 AM
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Appreciate all replies. I feel confident now to modify the shock angle and bring it in line with the crossmember, it will be far easier to fabricate the top mount. Thanks again.
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Old 04-12-2009, 08:52 AM
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I think aosborn has it right on the packaging concept. You can use a longer shock at an angle and since the suspension has a limited range of movement it works ok.

The front and back location on trucks is to help preven spring wind up. It works for the changing ride heights due to loads. Cheap fix that works more or less. It does act as a condition indicator....when you start getting shudder from a stop lite when you let the clutch out it's time to replace rear shocks on the truck. Mine worked fine up to about 100k miles ago. Still haven't replaced them. haha
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