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 Post subject: Help with shock mount.
PostPosted: Mon Feb 06, 2012 4:08 pm 
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I'm almost ready to mount my front shocks and springs. Please help me decide the best angle.Here are three different photos depicting different options. Please note that the bracket at the top is just a temporary bracket.
Option 1 positions the lower bracket next to the ball joint and the upper bracket positions the shock pointing directly at the frame tube. I would think that this would be almost ideal, however, this results in the shock angle of about 50 degrees.(not so good)
Attachment:
option-1.jpg
option-1.jpg [ 148.57 KiB | Viewed 807 times ]

By moving the upper bracket outward, the shock angle improves to about 55 degrees, I think this is better, but now the upper mount is less desirable.
Attachment:
option-2.jpg
option-2.jpg [ 154.17 KiB | Viewed 807 times ]

In option 3, I have moved the lower bracket away from the ball joint slightly. Not ideal, but this improves the angle to about 60 degrees.
Attachment:
option-3.jpg
option-3.jpg [ 147.56 KiB | Viewed 807 times ]

The closeup shows the amount I have moved the bracket. With option 3, I could add a vertical rib under the plate to stiffen it if needed. This would add about 4-5oz to the weight of the lower arm.
All feedback is welcome.


Attachments:
option-3-closeup.jpg
option-3-closeup.jpg [ 143.39 KiB | Viewed 807 times ]
option-3-closeup.jpg
option-3-closeup.jpg [ 143.39 KiB | Viewed 807 times ]
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PostPosted: Mon Feb 06, 2012 4:25 pm 
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The further outward you can move the lower mount will be best. There will be less stress in the control arm. We have seen some failures here due to poor designs in that area. As far as angle of the shock, 45 is not too much. Whatever fits best with the shocks and springs you choose. My car is running abut 48 degrees static.

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Last edited by rx7locost on Mon Feb 06, 2012 4:34 pm, edited 1 time in total.

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PostPosted: Mon Feb 06, 2012 4:31 pm 
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technically, the spring should be in rising rate, that means that as the suspension is loaded, the spring angle moves to
wards the vertical, if you can arrange for this to happen it will allow you to use softer springs, as the spring increases its effective rate, over and above that built in to the spring itself.

this is also a benifit to the shocks as damping in shocks is per linier inch of travel.

all the old guys would locate their suspension bracket in the line of force through the a arms, whilst this is good, this is sometimes impractical, but do the best you can and if necesary add some bracing.

don't wory too much about adding weight to the hubs end, in this application it is better to have it work and be strong than be super light.

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PostPosted: Mon Feb 06, 2012 4:34 pm 
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I think you could argue for any of these setups. You just have to take into consideration the potential drawbacks of each and adjust your design to handle them. That said, I think you're always better off with a more outboard lower mount, and shock angle is probably a less critical factor than structural integrity.


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PostPosted: Mon Feb 06, 2012 5:03 pm 
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How in-depth do you want to go in determining which option to use? As there is very limited information on your suspension design here, are you just looking for a 'gut feeling from eyeballing it' or are you looking for specific reasoning? If I'm interpreting your pics correctly though, you're dropping an inch from the shock length (14, 13, 12) with each positional change at this shown ride height. Have you measured the change in motion ratio that occurs as the suspension moves through its entire intended range of motion for each of these options, and not just at ride height? Are you certain that your selected shock will have enough travel in compression as you decrease the shock length at ride height? The shortest shock I see in the Bilstein catalog is the 4" stroke ASN, which has a ~10.5" compressed length, and they only get longer from there. That would leave only ~1.5" of available compression for a shock length of 12" at ride height, not counting any type of bumpstop. Would that be enough?

So I guess if I had to throw something out there *for educational purposes only* it would seem that based on what you've shown, the longer shown solution might be at an overall functional advantage to the shorter shown solution.

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PostPosted: Mon Feb 06, 2012 5:29 pm 
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Location: Tennessee
rx7locost wrote:
The further outward you can move the lower mount will be best. There will be less stress in the control arm. We have seen some failures here due to poor designs in that area. As far as angle of the shock, 45 is not too much. Whatever fits best with the shocks and springs you choose. My car is running abut 48 degrees static.

Thanks for the input. If being closer to the ball joint is more important than shock angle(which I suspected), the option one would be the choice.


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PostPosted: Mon Feb 06, 2012 5:30 pm 
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john hennessy wrote:
technically, the spring should be in rising rate, that means that as the suspension is loaded, the spring angle moves to
wards the vertical, if you can arrange for this to happen it will allow you to use softer springs, as the spring increases its effective rate, over and above that built in to the spring itself.

this is also a benifit to the shocks as damping in shocks is per linier inch of travel.

all the old guys would locate their suspension bracket in the line of force through the a arms, whilst this is good, this is sometimes impractical, but do the best you can and if necesary add some bracing.

don't wory too much about adding weight to the hubs end, in this application it is better to have it work and be strong than be super light.

Then, I think number three is your choice?


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PostPosted: Mon Feb 06, 2012 5:35 pm 
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I would say not to worry about the 50 degree angle and worry more about bending loads from the shock on the upper bracket or the wishbone. It seems simpler and produces less undesirable loads.

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PostPosted: Mon Feb 06, 2012 5:37 pm 
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Driven5 wrote:
How in-depth do you want to go in determining which option to use? As there is very limited information on your suspension design here, are you just looking for a 'gut feeling from eyeballing it' or are you looking for specific reasoning? If I'm interpreting your pics correctly though, you're dropping an inch from the shock length (14, 13, 12) with each positional change at this shown ride height. Have you measured the change in motion ratio that occurs as the suspension moves through its entire intended range of motion for each of these options, and not just at ride height? Are you certain that your selected shock will have enough travel in compression as you decrease the shock length at ride height? The shortest shock I see in the Bilstein catalog is the 4" stroke ASN, which has a ~10.5" compressed length, and they only get longer from there. That would leave only ~1.5" of available compression for a shock length of 12" at ride height, not counting any type of bumpstop. Would that be enough?

So I guess if I had to throw something out there *for educational purposes only* it would seem that based on what you've shown, the longer shown solution might be at an overall functional advantage to the shorter shown solution.

Actually, the different length sticks are what I had at hand. With real shocks, I can use any of the three set ups and still have plenty of travel in bump and droop. With option one, I can use a Bilstein shock with six inches of travel (11-17) and set the installed length at 15. this would give me four inches of bump and two in droop, not even considering the shock moves only about 60percent of wheel movement. Also, Afco has a similar shock. Thanks.


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PostPosted: Mon Feb 06, 2012 5:41 pm 
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horizenjob wrote:
I would say not to worry about the 50 degree angle and worry more about bending loads from the shock on the upper bracket or the wishbone. It seems simpler and produces less undesirable loads.

Well, this was my first option. Bottom mount right next to the ball and top mount at the frame rail. A Bilstein shock with 11-17 inches of travel set at 15 inches at ride height would work, giving 4 inches of bump and two at droop. Actually more since the shock moves less than the wheel, so I think I am safe there.


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PostPosted: Mon Feb 06, 2012 5:46 pm 
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Concerning bending loads on the lower wishbone, at first I tried mounting the lower shock bracket directly over the ball joint. Actually,this is what the Mazda does with the Miata. Unfortunately, that arrangement didn't allow clearance with the upper arm, so I moved the mount just inward which allowed plenty of clearance.


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PostPosted: Mon Feb 06, 2012 8:03 pm 
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The voice of reason
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Sorry, I didn't have time to write much of a reply earlier.

There is a compromise for the top mount that can work better then building an ear on the top rail to cantilever the shock mount. I have seen a couple of builders just weld a 1" square tube across the top rails and extend it out a few inches. It makes for a solid headlight mount and avoids as much of a bending/twisting load on the top rail. It also raise the mount an inch which may help or hurt depending on your coilover length...

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PostPosted: Mon Feb 06, 2012 8:24 pm 
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That cross piece for me was under the top rail not across the top . Will deoend on your shocks where it is best located. If you are streeting your car, this extension can do double service as the headlight mount. Most of us use it for that.

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PostPosted: Mon Feb 06, 2012 8:51 pm 
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I plan to place a tube under the top frame rail all the way across. Probably will bolt it rather than weld it. I think I will use a 1.25 x1.25 tube since the inside of the shock bracket is 1.25.


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PostPosted: Mon Feb 06, 2012 8:52 pm 
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rx7locost wrote:
That cross piece for me was under the top rail not across the top . Will deoend on your shocks where it is best located. If you are streeting your car, this extension can do double service as the headlight mount. Most of us use it for that.

This is what I plan to do.


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