Thread: Shock Geometry

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Its time to weld the shock tabs onto my leaf spring straight front axle. The upper shock mount will utilize the original 55 Chevy location. I have two choices for the lower shock mounts. I can place them slightly outboard on the upward turn of the axle, or directly below the upper mounts next to the leafs. With the shocks mounted diagonally outbound it seems I would have better articulation to allow the axle and shocks to move differentially as in one end going up, while the opposite end could drop. This is how I've always done it for off-road truck applications.

With a vertical mount, the shocks would move up and down together, and this car is being built for the drags. It seems to me that the shocks would work well together even if angled outboard at the bottom. And there is always the possibility that this car will become a street rod in the future. Thoughts?

The shock guys will tell you you get maximum efficiency with the straight up and down configuration. There are charts I've seen that tell you how much "degradation" you get per angle of inclination from vertical. That being said your visualization of the axle swinging on an arc to one side or the other is accurate, and speaks to why an angle mount works well enough. If you were going drag only I'd say vertical because you'd be unlikely to have uneven axle deflection when needed most (or hopefully). If you think it will go street at some time and don't want to change it then, some inclination now wouldn't hurt that much.

Thanks for your input Bob. I've seen lots of pictures of rods here where the shocks angle outboard at the bottom. It seems like the transverse spring guys usually have them inclined. I also thought that the offset could help to control body roll. That could come in handy even on a flat strip if the car tries to swap ends!

A bit of angle, maybe around 10 degrees or so should help with stability. As the angle increase, the shocks become more stiff, too...

I was just in the garage checking the shock angle on my 56 Chevy. It looks like the angle on the straight axle will be about the same as original GM. I know its apples and oranges. As for stiffness, I plan to go with Chris Alston VariShocks. They are double adjustable and advertised to really help with weight transfer initially, then control rebound as the car planes out. If I ever get this thing to actually run, I'll let you know how it works out!

I'm with Dave on this one.....10 degrees shock angle.... any axle weather it be straight or on control arms travels an arc on compression & rebound

Had a little more time this morning to do some searching. I couldn't find the original chart I recalled, but found this that is similar in it's discussion; http://www.4x4review.com/feature/shock-genius.asp
About a quarter of the way down the page is the discussion on geometry.

Originally Posted by Bob Parmenter

Had a little more time this morning to do some searching. I couldn't find the original chart I recalled, but found this that is similar in it's discussion; http://www.4x4review.com/feature/shock-genius.asp
About a quarter of the way down the page is the discussion on geometry.

For the sake of discussion, That article is good, but seems to be aimed at 4wd vehicles... Would you want the shock on the front of the straight axle to behave in the same manner as the shock on a 4wd axle??? Geometry is geometry, but wouldn't you be looking for the shock to do different things on the two vehicles? The part of the article that I consider important for any vehicle is to have the shock as far outboard as possible....I would imagine something to do with getting the energy created by tire when it encounters irregularities in the road surface(or track) transferred into the shock and dampened before it travels any great length through the suspension components? Seems I recall the old gassers having the shocks on the rear of the axle and angled back to the top mount?????

According to what I read, would the correct placement of the shocks being on the shackle side (forward in this case) of the axle????

In the matter of a "solid" axle I'd say yes they are very similar in that the entire axle will move to some degree with deflection at one end, as opposed to say IRS where the side deflected will move independently from the other. Though I used that article mainly for the geometry discussion, not matters specific to 4x4's, since it was a quick find just to put a known number to loss of effectiveness. And yes, I would agree that the shock should be mounted to the front of the axle here since the axle will move slightly forward because the shackles are at that end (as opposed to the other end being fixed mounted I presume). Which would in turn argue for the top mount of the shock to be somewhat forward of the lower shock mount, though there's probably enough movement in the shock bushings to deal with the slight movement of the axle (unless those springs are really soft).

And just to continue for the sake of discussion, the needs of a drag car will differ from a street machine in that the front axle, once the entire suspension is dialed in for traction purposes, pretty much only goes up and down in unison (left to right, unless there's a lot of chassis flex), so accounting for the arc of either end as opposed to it's opposite should be a non-factor (well, except when turning onto the return road or unintentionally going off road ) All that being said, 10 degrees of inclination only costs 2% efficiency.

Thanks for the informative discussion guys. With only a slight loss of efficiency, and the added benefit of greater stability, I have decided to go with the angled mount. And I hope nobody else has good reasons against it, because, I already took it into the welder. My Lincoln does not have enough juice for for that heavy Iron bar!

Angling outboard is fine. It just reduces the damping and rebound action of the shock by Cosine (a), where (a) is the angle between the shock and a vertical line from the top shock eye to the ground.

For example:
cos(10) = .985, so shock action is reduced by 1.5%
cos (30) = .866, so shock action is reduced by 13.4%
cos (45) = .707, so shock action is reduced by 29.3%

Just make sure that the bolts holding the shock to the frame and axle are parallel to the centerline of the car (or nearly so). If they aren't, you will break the shocks.

This discussion really brings up lots of interesting thoughts. I really love the formulas, but they exist in a static laboratory world. As a Commercial Airline Pilot, I work daily with performance data that can cross your eyes. But once you exceed 130Kts and leave the pavement, it sort of boils down to just do what it takes! Perhaps it is best to regard shock performance in the same way. For instance, if we live in a static world where all pavement is perfectly flat then all shocks must be absolutely straight. But don't move the steering wheel, because then the center of gravity will shift and body roll will occur. Now, even though the tires remain on the pavement (hopefully), the relationship between the suspension becomes different, or articulated. I guess we are trying to achieve the best possible average performance for a particular situation. It sure is fun to think about, but don't most of you guys operate in a world of common sense and experience? Now, if I could only get this thing to a point where I can drive it.....

Originally Posted by drofdar

It sure is fun to think about, but don't most of you guys operate in a world of common sense and experience?

Is that still legal?

Sure, here's a front shock arrangement I did last week.

Well, if we relied totally on common sense and experience a lot of the fun of experimentation and pushing the limits wouldn't exist....But I sure hope you don't do that on MY commercial flight!!!!

The one thing I have noticed with shocks, rather then depend strictly on charts and graphs, is giving up a bit of the shock's efficiency by increasing the angle will help the stability.... Valving changes in the shock seem to have the biggest effect on this. Keeping the shocks travel in the "sweet spot" of the shock really helps too. On a typical drag race, street performance shock, the actual movement of the shock in normal driving is probably less the 1" to 1 1/2" either way of center..... The long shocks are essential for 4wd with huge amounts of suspension movement, not so on the street----well unless you have roads like some of them her in SoDak!!!!

I really love the formulas, but they exist in a static laboratory world.

Agreed. However, if we don't get the static right, the dynamic will eat our lunch. The charts and graphs don't tell the whole story, but they give us good starting points. With hot rods, geometery is our friend. Without it, we die.