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Learning how to build Lotus Seven replicas...together!
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PostPosted: July 7, 2015, 11:09 am 
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Don't forget I snapped a CMC lower control arm last year, I attributed it to a twisting for applied by the CMC installed roll bar.

Graham


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PostPosted: July 7, 2015, 10:10 pm 
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Always Moore!
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Rudy14 wrote:
Andrew, Good to hear your LCAs were thicker walled steel. Since they are painted grey I will look more closely before autocrossing again. What general added strentgh would you say this provides ?
No hard bottoming out of the suspension has taken place, as that seems when a stress crack could develop.
Dave
Aka "Rudy 14 "


Hey Dave. I don't remember the exact percentage but it was significant (I want to say 25-30% off the top of my head; 0.083" is 132% of 0.063" so close enough). Braking is really what kills the arms since the forward leg goes into tension and weight transfers from the rear wheels onto the front which further increases the tensile load in the forward leg of each lower arm.

Getting the springs and bars as close to the ball joint as possible also helps to minimize the bending load on the tubes. Graham's illustration is exactly what happens. The "book" seems to encourage this design and it was probably okay for the 1,200-1,300 lbs cars with <100 hp it was intended for.

Nice looking avatar BTW...I feel like I've seen that car somewhere.... ;)

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PostPosted: July 8, 2015, 1:28 am 
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Here's what I came up with for my new lower arms. I have to make a couple adjustments for steering clearance, but I think they'll work out better. I used 3/4"x.065 tube and 10 gauge plate. My car only weighs 1300 lbs and has a rear weight bias, so I think it should be OK. The original arms were made of 3/4"x.065 tube and the tubes aren't bent or cracked at all, even after 4 seasons of autocross on sticky tires.

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Kristian

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PostPosted: July 8, 2015, 6:25 pm 
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Looks good. Did you get the shock mounting points as close to the wheel as possible. That is my main regret on my front control arms. Mine are close but I could have moved them an inch or more closer.

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PostPosted: July 9, 2015, 12:24 am 
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wrightcomputing wrote:
Looks good. Did you get the shock mounting points as close to the wheel as possible. That is my main regret on my front control arms. Mine are close but I could have moved them an inch or more closer.

I could have gotten the shock a bit further out, but I figured that would put the shock on a pretty steep angle and my ride height would be too low. It will already a bit lower at the front than it was before. I should really have longer shocks up front, but that's probably not going to happen any time soon. I'll see what my front ride height looks like first, should be around 1.75" with 20" slicks.

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I ended up with a bit of interference between the rear ball joint bolt and one of the bolts that holds my steering arm on that both wanted to occupy the same space when turning, so I had a bit of reworking to do there. I just trimmed the tube with a zip disc and welded a 3/8 bolt in so the head sits slightly below the a-arm tube. Hopefully I never have to replace that one bolt. As long as I keep the caster under 11 or 12 degrees, there's no interference with the tube either. I need to make new upper arms if I want to get the camber under -2 though.

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Kristian

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PostPosted: July 9, 2015, 2:54 am 
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This is a huge improvement over the original set up. If and when a MK3 version are made it may be an idea to raise the bracket for the shock mount so you dont need to worry about loss of ride height and perhaps keep options open by including two extra holes where the black arrows are. Also wondering if the plate under the tubes should not have extended inboard a touch towards the red arrow as per the original design to avoid a break/possible fracture point where the blue arrow is as per fastg`s lower control arm.

Bob

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PostPosted: July 9, 2015, 7:35 am 
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They look forward all good from here. I'd just make sure the balljoint has enough travel since its sitting at an angle at the normal ride height now.

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PostPosted: July 9, 2015, 9:31 am 
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The ball joint angle should be fine. The pictures are taken at full droop and the angle only gets better. The previous lower arms started out straight with the same angle on the ball joint too, not that I should do anything based on how CMC did it. I had thought of having a couple sets of holes for the lower shock mount, and that might happen on another set of arms in the future. I'd like to use heims on the inner pivots too. And extending the lower plate inward probably isn't a bad idea either. Thanks for the ideas.
Kristian

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