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Old 10-29-2019, 07:20 AM   #1 (permalink)
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Shock Angle Questions for Go fast/rocks (ultra4)

So I've been mulling over this question for a while now, and I still can't figure out what the best option is. We have a 1986 Jeep Comanche that we're building into a 4600 race truck for the 2020 race season, and I'm trying to figure out the best shock angle versus shock length. The basic question that i'm trying to figure out is: with a sprung under leaf spring, and assuming the shocks are mounted to the rear of the axle tubes, wouldn't I be able to lay the shocks back a fair amount at the frame side, meaning further to the rear of the truck since the axle will swing back in an arc as the leafs flatten and the shackles swing back? I made this little picture to kind of demonstrate what my thoughts are. We're limited to a 14" shock, and I doubt we can even get that much length without cutting the bed, which we aren't allowed to do. I want to heavily reinforce how the shock mounts to the frame, so i plan on cutting off the stock mount anyway and building a double shear mount. If I lean the shocks back, I can gain some more shock length, but I'm not sure what will happen in the rocks as the axle twists and flexes. I do plan on running sway bars for better stability in the desert.
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Old 10-29-2019, 09:52 AM   #2 (permalink)
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So I've been mulling over this question for a while now, and I still can't figure out what the best option is. We have a 1986 Jeep Comanche that we're building into a 4600 race truck for the 2020 race season, and I'm trying to figure out the best shock angle versus shock length. The basic question that i'm trying to figure out is: with a sprung under leaf spring, and assuming the shocks are mounted to the rear of the axle tubes, wouldn't I be able to lay the shocks back a fair amount at the frame side, meaning further to the rear of the truck since the axle will swing back in an arc as the leafs flatten and the shackles swing back? I made this little picture to kind of demonstrate what my thoughts are. We're limited to a 14" shock, and I doubt we can even get that much length without cutting the bed, which we aren't allowed to do. I want to heavily reinforce how the shock mounts to the frame, so i plan on cutting off the stock mount anyway and building a double shear mount. If I lean the shocks back, I can gain some more shock length, but I'm not sure what will happen in the rocks as the axle twists and flexes. I do plan on running sway bars for better stability in the desert.
Assuming your leaf springs never go into negative arch, the axle will move back sightly as the leaf flattens out.

Running the shocks at any angle will result in them being less effective at damping, so you'll need to run stiffer shocks. The damping ratio will change (becoming even less effective) as the axle compresses because the shocks will deviate even further from the direction of suspension travel.

As long as you're running heim joints, articulation will be nothing to worry about. The suspension might start to behave funny as one shock is fully compressed/at more of an angle/now has less effective damping while the other shock is extended/at less of an angle/has more effective damping.
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Old 10-29-2019, 11:29 AM   #3 (permalink)
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just took a look at the build in the jeep section for this truck and looks like the leafs are under the frame and not outside. if you keep it like that and are running full width diffs are you not able to mount the shock outside the frame? that would let you move the upper mount into the wheel well and be taller vs being under the box?
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Old 10-29-2019, 12:18 PM   #4 (permalink)
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That's a good thought but from what I understand I cannot modify the wheel wells at all, so I couldn't French in some shock mounts. There might be some gray area there, especially for the other 4600 guys that have been racing a while, but I expect to have tech give me a hard time at KOH. So I want to make sure they have nothing at all they could ding me for.
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Old 10-29-2019, 01:02 PM   #5 (permalink)
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For best shock performance, you want the shock to be straight up and down (or dang close to it) to the travel of the suspension when at full bump.

Getting the right length shock to fit in your application is a whole different can of worms.

Any changes in angle, forward, back, inboard or any combination of, will lessen effectiveness of the shock.
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Old 10-29-2019, 01:41 PM   #6 (permalink)
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i'd also add: don't be afraid to mount the shock below the axle housing.

as far outboard and as low as you can get on the axle side, as far outboard and as high on the frame side, do some quick math to figure out how much the axle will swing on articulation for the inboard lean, set it square to the flat leaf for the fore/aft lean.

anything you can't cut, go around and obviously tune it up as good as you can from there. being able to run air bumps in 4600 helps a bunch, add limit straps to protect the shock and enjoy

edit: same rules apply from links to leaf springs, your fixed hanger side of the leaf become the link dimension you want to work with though
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Old 10-29-2019, 02:21 PM   #7 (permalink)
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i'd also add: don't be afraid to mount the shock below the axle housing.

as far outboard and as low as you can get on the axle side, as far outboard and as high on the frame side, do some quick math to figure out how much the axle will swing on articulation for the inboard lean, set it square to the flat leaf for the fore/aft lean.

anything you can't cut, go around and obviously tune it up as good as you can from there. being able to run air bumps in 4600 helps a bunch, add limit straps to protect the shock and enjoy

edit: same rules apply from links to leaf springs, your fixed hanger side of the leaf become the link dimension you want to work with though
This has actually been something i've also considered, running some mounts below the axle housing like a stock XJ or JK would have, they'd be rock anchors but so will the Sterling 10.25 I imagine Thanks for the insight guys.
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Old 10-29-2019, 02:29 PM   #8 (permalink)
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That's a good thought but from what I understand I cannot modify the wheel wells at all, so I couldn't French in some shock mounts. There might be some gray area there, especially for the other 4600 guys that have been racing a while, but I expect to have tech give me a hard time at KOH. So I want to make sure they have nothing at all they could ding me for.
should be able to fit something like this in the wheel well and not have to french it in. then do like others said and make the lower mount lower on the axle you should be able to fit a 14" shock no problem

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Old 10-29-2019, 03:10 PM   #9 (permalink)
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This has actually been something i've also considered, running some mounts below the axle housing like a stock XJ or JK would have, they'd be rock anchors but so will the Sterling 10.25 I imagine Thanks for the insight guys.
being very close to the tire makes it much less of a rock anchor fwiw
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Old 10-29-2019, 06:00 PM   #10 (permalink)
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If the lower mount is out near the brake and rear wheel - it's well protected from the vast majority of objects you'll be going over.

What you don't want is a mount like this. This isn't close enough to the wheel:


This is how close they should be:


Keep that lower mount as far outboard as you can. This is literally a major reason the Tundra won the Baja 1000 stock class while the Raptor wasn't competitive. The last Raptor team lost a shock within a few miles of the starting line due to poor design choice in how they were mounted in an exposed place.

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Old 10-30-2019, 08:14 AM   #11 (permalink)
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Running the shocks at any angle will result in them being less effective at damping, s
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For best shock performance, you want the shock to be straight up and down
This is the old way of thinking. 2 years ago they thought so but have since learned in testing the difference is negligible.

I learned that in '18 at a KOH shock tuning session. Fox told me the new data and testing has proven that old "the shocks should be vertical" assumption false.

Angles are not a big deal.
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Old 10-30-2019, 09:57 AM   #12 (permalink)
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be straight up and down (or dang close to it) to the travel of the suspension when at full bump.
.
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Originally Posted by Pokeman View Post
This is the old way of thinking. 2 years ago they thought so but have since learned in testing the difference is negligible.

I learned that in '18 at a KOH shock tuning session. Fox told me the new data and testing has proven that old "the shocks should be vertical" assumption false.

Angles are not a big deal.
I think you cut off the important part

while for tuning it may not be a substantial impact if it is a few degrees one way or the other, it is tough to argue with the fundamental math.

either way will need to be adjusted to the chassis. if there is enough ability in the shock to get a tune that works, then right on and it doesn't really matter.

from a function and math standpoint though, there is no reason for the shock to be beyond perpendicular at full bump, outside of physical space constraints. the axle cannot move any further back and it prevents the shock from changing direction.

so, shooting for as close to good enough and then tuning what you get that actually works will always win the day.
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Old 10-30-2019, 10:07 AM   #13 (permalink)
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This is the old way of thinking. 2 years ago they thought so but have since learned in testing the difference is negligible.

I learned that in '18 at a KOH shock tuning session. Fox told me the new data and testing has proven that old "the shocks should be vertical" assumption false.

Angles are not a big deal.


If you are going to quote me, make sure you quote the whole comment.

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For best shock performance, you want the shock to be straight up and down (or dang close to it) to the travel of the suspension when at full bump.
Probably not the most accurate way to describe it, sorry if it wasn't clear enough.

Let's try it this way: If you want progressive travel of your shock, you want the shock to end up perpendicular (or a hair less) at full bump through your suspension's range of travel.

It's the geometry of the movement and how the shock fits in to control that movement.

Though it is true that most shocks don't care what angle they are ran at from a mechanical standpoint, part of how they well do their job depends on where you have them running through your range of travel.
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Old 10-30-2019, 10:15 AM   #14 (permalink)
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This is the old way of thinking. 2 years ago they thought so but have since learned in testing the difference is negligible.

I learned that in '18 at a KOH shock tuning session. Fox told me the new data and testing has proven that old "the shocks should be vertical" assumption false.

Angles are not a big deal.
This isn't an "old way of thinking" as shock technology in the offroad world really hasn't changed much in the last 10-15 years.

The difference may be negligible for small angles, but the further from direction of travel the shock is angled the less effective it is. This is simple physics and trigonometry.

Small shock angles are not a big deal. At a 20 angle your shock is 94% effective. Good enough.

Large shock angles are a big deal. At 40 your shock is 77% effective. That's not good.
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Old 10-30-2019, 11:59 AM   #15 (permalink)
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There's really two primary considerations.

The first is the question of how far the piston travels for a relative movement of the wheel. And even a 2:1 ratio is no problem at all - so long as the shock is sized and valved appropriately. The front of almost every IFS is typically in the range of 2:1 travel of the wheel vs the shock. So as a result you need dampening in the shock twice that of what you'd use at a 1:1 leverage ratio. The same result happens when you put the shock at an angle. The piston travel is reduced for relative to the travel of the wheel. As a result - more angle = larger piston diameter.

The second factor to consider is the rate of change of that relationship. The greater the angle of the shock off of a vertical position, the greater it will tend to provide a digressive dampening curve through the range of compression and a progressive dampening curve in rebound. Those aren't really a proper use of the terms, but they will tend to act similarly. Progressive and digressive are relative to shock speed. This is a change relative to shock position that will have effects somewhat similar. As the axle travels up - the angle will increase and the relative piston motion will decrease resulting in less dampening force (much like a digressive compression shock). So as you hit a big bump and the suspension travels up it will get softer the further it travels. And the reverse is true on droop - further down it drops on rebound, the firmer the effective dampening will be.

The angle can ultimately be used by proper engineering to act as a variable dampening adjustment that is position sensitive. In short - with the proper angle it can act somewhat like a bypass only with a continuously dynamic dampening range. However - you only get more dampening at the bottom, and less at the top - so you'd better plan on using the bump stops a lot and/or having a secondary bump shock if you take this to the extreme. The point being - this behavior isn't inherently bad. It's just different. And you can plan around it for some potential benefits if that's what you're looking for. With bypass shocks - you should be able to effectively eliminate this issue by valving each stage appropriately as long as the angles are not extreme. There is a point where the angle is so extreme you cannot compensate with bypass valving, but that would be pretty unusual. For example if the shock went entirely horizontal at full compression. That would be bad. Don't do that.

It's not inherently bad. And I'd bet a 15* angle would make so little difference that you couldn't tell either way. When you get down to 45* - it's probably not going to work very well unless you're doing a lot of planning for the effects of the angle.

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Old 10-30-2019, 01:19 PM   #16 (permalink)
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Quote:
Originally Posted by Pokeman View Post
This is the old way of thinking. 2 years ago they thought so but have since learned in testing the difference is negligible.

I learned that in '18 at a KOH shock tuning session. Fox told me the new data and testing has proven that old "the shocks should be vertical" assumption false.

Angles are not a big deal.
That's not new either. We learned that back doing shock testing with Mini-Baja teams in 2001-02. Exceed the 40-45 degree range and you have issues with performance. Which then confirmed from the shock manufacturer we were using at the time (they already knew that we just hadn't asked).
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Old 10-30-2019, 02:41 PM   #17 (permalink)
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Quote:
Originally Posted by Pokeman View Post
This is the old way of thinking. 2 years ago they thought so but have since learned in testing the difference is negligible.

I learned that in '18 at a KOH shock tuning session. Fox told me the new data and testing has proven that old "the shocks should be vertical" assumption false.

Angles are not a big deal.
Lets be more precise. First, the damping provided by a shock absolutely depends on the angle. The damping force depends on the relative velocity between the two ends of the shock and for any give suspension movement changing the angle changes the relative velocity. Whether or not that change is significant enough to impact overall performance is a separate question. My guess is the "data and testing" demonstrated that for the range of angles in common use the performance impact was indeed minimal. Even if there were performance impact you could tune the shock to the mounting angle so from the perspective of damping there is no rule that vertical is best.

Damping aside, aligning the shock stroke with the direction of suspension travel (i.e. vertical) allows for maximum suspension travel. For example, mount a 14" shock vertical and you end up with 14" of usable suspension travel (assuming something else does not limit travel). Mount that shock at 60 degrees up and you now have 12" of vertical travel before the shock bottoms out. Mount the shock at 45 degrees and you end up with less than 10" of vertical travel.
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Old 10-30-2019, 05:09 PM   #18 (permalink)
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Just want to throw this out there I used these motobilt shock mounts very happy with them.

https://www.motobilt.com/product/low...ount_side_axle

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