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PostPosted: July 2, 2015, 1:07 am 
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Location: Kamloops, BC, Canada
I cross posted this here from my build log, thinking more people might see it here and it may help someone. I had a minor front suspension issue on my car today, the right lower arm broke just outboard of the lower shock mount. I've auto crossed this car with these control arms on it for 4 or 5 seasons now, so even though it's been good this long doesn't mean issues won't happen at some time. These are the only CMC supplied suspension parts I actually used beyond a mockup stage, and they finally let go. The car and I are fine, aside from the obvious, and it couldn't have happened in a better spot on course. There was a bit of a dip for water runoff, so we had that as a bit of a slow spot, and that's what broke the arm. I didn't even hit any cones when it broke.
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I managed to back it onto my trailer and dragged it off into my shop with some help from my wife and kids and my engine hoist. I've got it up in the air to tear apart, but I looks like all I need to do is build new lower front control arms and either straighten out my tie rod end or buy a new one. Ill post more pictures later, but the carnage is pretty minimal.
Kristian

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PostPosted: July 2, 2015, 1:11 am 
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Here's some more pictures of the carnage.

The bead and tire still runs true, so I should be able to straighten it out pretty easy. It's really only cosmetic anyway.
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The broken control arm.
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The flat plate that the ball joint bolts to broke right at the end of the tubes.
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There's a bit of rust under the edge of the weld bead, so it started moving and fatiguing a while ago.
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The not quite so broken one. I'm glad they didn't both go.
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The plate for the ball joint is supposed to be straight with the tube. I'm not sure why they built these with the tubes flattened like that and ending right before the ball joint. The part of the a arm with the most stress on it is also the weakest part.
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This one isn't far behind the other one. It's already started cracking at the one side of the plate, right at the end of the tube again.
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I think I'll be keeping a closer eye on things like this for cracks and fatigue in the future. I've autocroswed this car for 4 or 5 summers with these suspension arms without any issues until today. I figured they didn't look real sturdy when I got them, but they were from CMC when I bought the frame. They are the only suspension parts (other than the shocks) that came from them that I actually used. I'll build some new arms from either 1" x .065 or 3/4" x .065 tube and run the tube past the sides of the ball joints. I might also put a reinforcement under the shock mount.
Kristian

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PostPosted: July 2, 2015, 2:31 am 
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That's scary!...Thanks for sharing.

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PostPosted: July 2, 2015, 5:31 am 
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I have always disliked the idea of a thin plate like that hanging in the wind with no support under it. Something like the Ford (uk) cortina/granada lower joint comes inboard a fair way which addresses some of the issues.

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PostPosted: July 3, 2015, 4:00 am 
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I like the look of this lower wishbone. It would need to be reduced in size around the balljoint mount but the concept is good.

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PostPosted: July 3, 2015, 9:29 am 
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That CMC design really sucks! Glad you didn't have any personal injury. I am amazed at how many front suspension failures have shown up this year.

I'd recommend staying away from the 3/4" 16 gauge tubing. There are MANY documented failures using that, usually in bending rater than stress cracking.

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PostPosted: July 3, 2015, 9:31 am 
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I'm still not 100% sure how I'm going to build the new arms, but I definitely want the control arm tubing to extend past the center of the ball joint this time. I've got a couple ideas rattling around in my head that I will try this weekend. One of which would look pretty similar to what you just posted Bob. I think the 3/4" tube should be OK as long as the shock mount is close to the lower ball joint and the tubes are straight, not curved. I've put a lot of braking forces on those arms with the slicks without any signs of bending at the tubing, plus my car only weighs about 1300 lbs.
Kristian

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PostPosted: July 3, 2015, 10:56 am 
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I ran numbers years ago and even if you get the spring near the balljoint, 3/4"x0.065" tubing is marginal. I went with 0.083" wall thickness for a minimal weight increase.

The CMC design should have had a 1/8" thick cap strip welded around the balljoint plate's perimeter. I think that would have been enough to stiffen things up and prevent cracking.

Regardless I'd paint the next ones something other than black to give you a better chance of finding something when you're looking over the car between events.

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PostPosted: July 3, 2015, 7:31 pm 
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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.
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Aka "Rudy 14 "

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PostPosted: July 3, 2015, 8:09 pm 
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July must be a bad month for Locosts. This was my ride home on Sunday after the upright (which I modified) failed
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The front right wheel along with hub, brakes etc are in the back of the van.

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PostPosted: July 3, 2015, 8:19 pm 
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wrightcomputing wrote:
July must be a bad month for Locosts. This was my ride home on Sunday after the upright (which I modified) failed
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IMAG2005.jpg

The front right wheel along with hub, brakes etc are in the back of the van.

I saw your incident and I'm having a closer look at the steering arms on my car that I modified for better ackermen and quicker steering. At least mine should be an easy fix, I don't have to reinvent the wheel for anything.
Kristian

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PostPosted: July 3, 2015, 10:29 pm 
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Why not repair the CMC arms, narrow up the front & rear sides of the plate as far as the bolt pattern will allow, then wrap a strap around the end with the tails overlapping the tubes and TIG weld?

This will gusset the plate and control flexing at the point it broke.

I would also pay close attention to the suspension, for binding of joints, bottoming or topping out that might contribute to the problem.

Fix them and then keep a close eye on them, (I doubt they will fail at all) if they did fail they would not fail instantly, you would see bending prior to terminal failure.

Since the originals lased as long as they did, I would have a fair amount of confidence in the repair I am suggesting, they will be much stronger that original even with welded cracks.

Dirt track guys will have tools to fix the wheel.

Check upper joint on the side that failed, and it would not be cost prohibitive to replace the tie rod (straightening it will probably compromise it)

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PostPosted: July 4, 2015, 5:09 pm 
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The biggest issue is that these cars have no service manual...

If your running slicks, suspension arms become disposable items... I would not keep them on the car for 5 seasons, but that's me. 2 season at the most. When you consider the cost of building them it's trivial... We build a jig and 5 set of arms when we first built the cars... Yes we are building a new batch of 5 this year, heck we're actually building a new frame plus a spare :wink:

You guys have to get to see the Radical service manual...
Component Hours Action
Engine  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ 30  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐  Rebuild
Gearbox  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ 30  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐  Inspect/rebuild
Chain  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐-‐‐ 10  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐  Replace
Suspension Bushes  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐---‐‐ 50  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐  Replace
Suspension Rose Joint  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐----- 30  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐  Replace
Front Upright Including Hub  ‐‐‐‐‐‐‐‐‐‐‐‐---------‐‐‐‐‐‐‐ 90  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐  Replace
Front Wishbones  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐----‐‐ 90  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐  Inspect/replace
Rear Upright  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐--‐ 90  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐  Replace
Rear Hub  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐-‐‐‐‐ 90  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐  Replace
Rear Wishbone ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐----‐‐‐ 90  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐  Inspect/replace
Drive Shafts  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐--‐‐‐‐‐‐ 60  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐  Replace
Calipers ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ 60  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐  Rebuild
Brake Discs  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐-‐‐‐‐‐ 10  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐  Inspect/replace
Shock Absorbers ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐----‐‐ 60  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐  Rebuild
Steering Rack ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐---‐‐‐‐ 60  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐  Inspect/rebuild or replace
Brake Master Cylinder  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐------‐‐‐ 60  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐  Inspect/replace
Wing Support Stays  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐----‐‐‐‐‐ 60  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐  Inspect/replace
Fuel Tank  ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ remove & inspect annually

just my 2 cents,

Cheers,

Fred


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PostPosted: July 4, 2015, 5:57 pm 
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turbo_bird wrote:
. . . I had a minor front suspension issue on my car today, the right lower arm broke just outboard of the lower shock mount.
Kristian


Kristian,

Glad you are okay and the car is repairable. But I wouldn't call that minor......could have been much worse.


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PostPosted: July 4, 2015, 10:10 pm 
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xcarguy wrote:
turbo_bird wrote:
. . . I had a minor front suspension issue on my car today, the right lower arm broke just outboard of the lower shock mount.
Kristian


Kristian,

Glad you are okay and the car is repairable. But I wouldn't call that minor......could have been much worse.

Yes it could have been much worse, but it was minor compared to yours, even though the arm coming apart did pretty much the same thing for car control as your mount breaking. I'm pretty glad it happened where it did. I'm planning on going to a track day next month at a decent sized kart track, with lots of elevation changes and a slough on the outside of one of the corners, that would be a bad place for a suspension failure. I'll be doing lots of inspecting of the new ones during the event, and hopefully get to another autocross first too. As far as repairing the existing arms, not going to happen. I'm reusing the bushing tubes at the inboard end because I wouldn't be able to get anything to match locally, but that's all.
Kristian

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