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Control arm failure, which way do they flex?

2turbotoys

2turbotoys

Salaminizer
Joined
Nov 13, 2007
Location
Reading pa
I got a precut set of plates to box my 242 control arms, the metal seems a bit thin for preventing the arm from bending in on itself, so I'm curious about how the arm fails. Does the "u channel" bend open or shut? The plates seem thick enough to prevent the top of the channel from spreading, but not collapsing.
 
Are you preparing one of our valued old Volvo's for a demolition derby?

Despite owning, maintaining and loving these cars for 40 years I have never heard this type of question come up.
 
https://yoshifab.com/store/volvo-240-control-arm-box-plates.html

Front control arms. This really isn't a rare mod, it's been suggested to me several times for a high horsepower build that will see track time and hard street use. I've had a few Porsches that the previous owners had done this to.
Definitely not a demolition derby car, building a serious street car that can handle whatever it needs to. The control arms will be one of the most stock suspension components even after boxing. l aready bought JRZpro remote reservoir coil overs along with a ton of other stuff.
 
I believe Dirty Rick was referring to the trailing arms, which should be boxed if you use stock ones, I am not using stock ones.
https://www.bneshop.com/collections/240/products/240-adjustable-trailing-arms
These are what I'm using.

Is this really an odd question? Why would this mod have been suggested a bunch of times? Why is it even available in our small world of RWD Volvos? Testpoint I believe your claim of 40+ years of not hearing it, but my baseline hp BEFORE turbocharging is 450 hp. With much larger rims and tires.
 
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Think of it as a simple beam with fixed ends, and a downward load (the spring seat) in the middle. They will flex with a bow convex downward, meaning the channel will want to spread. It won't take much material to keep this from happening.
 
What are ya'll doing to reinforce the unibody construction. How much HP and torque can spot welds take?
 
Ours failed like this. I don't know how/when it failed....before the start of a race Saturday morning I was noticing the tire clearance was lacking on the left front. I couldn't figure out why the tire was so close to the fender, looked at the other side, then back to the drivers side....something's not right here. Got underneath and found the control arm split open. I would guess that it was about to fail catastrophically, but loading/unloading from the trailer and turning the wheel sharply while rolling slowly may have yanked it just right to break BEFORE we started a 14.5 hr endurance race. :omg:

161732864.jpg


The arms were boxed, but some metal cut away from the reinforcing plate near the end because of the spacers we used between the control arm & ball joint. These arms were road raced for a long time and subjected to even more load with the wider front track hanging off the end of the control arm.

160231672.jpg


Here's another picture of a split control arm. On Pat D's 600k mile 240
http://forums.turbobricks.com/showthread.php?p=5744949#post5744949
 
2turbotoys said:
I got a precut set of plates to box my 242 control arms, the metal seems a bit thin for preventing the arm from bending in on itself, so I'm curious about how the arm fails. Does the "u channel" bend open or shut? The plates seem thick enough to prevent the top of the channel from spreading, but not collapsing.
Click to expand...
Think about how that control arm manages to survive with the bottom side open (not boxxed). It's fairly thick (compared to the reinforcing plates), and it doesn't fold up in normal road use or high-grip track use from what I've seen. Welding the plate across the open side makes it a box that would be much more resistant to crushing inward, like when slamming into a curb and folding the wheel back into the chassis. This is likely why the Volvo version didn't completely cover the bottom side of the control arm. It reinforced the weakest area, but still left an area to crush & twist.

However, I do think there is room for improvement. A hole saw and a dimple die could be used to make the reinforcing plate lighter and stronger.

Here's the volvo version of the reinforced arms.
p/n 1229923 & 1229925
$204.77 each at Tasca

93291311.jpg
 
I’ve never bent a control arm on the track from braking or cornering forces, but instead from smacking curbing or dropping a wheel off.
The rally car has reinforced control arms, and they’ve never bent. Except for when a strut broke in half.
The drift car has KL Racing arms, and I bent them while dropping a wheel off track while drifting. Now they’re almost fully boxed.

The main benefit to the KL arms are the added caster you get.
 
Looks like the KL arms are 25mm longer and give you +5 deg caster? Am I interpreting the description right? Anyone know what +25mm will add to the camber?

Edit: I took a guess on the length of a ride height strut at 80cm (32"), and arctan(25/800) = 1.8 deg more negative camber. I don't feel like walking outside to measure strut length...
 
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tjanson said:
Looks like the KL arms are 25mm longer and give you +5 deg caster? Am I interpreting the description right? Anyone know what +25mm will add to the camber?

Edit: I took a guess on the length of a ride height strut at 80cm (32"), and arctan(25/800) = 1.8 deg more negative camber. I don't feel like walking outside to measure strut length...
Click to expand...

With BNE top mounts and KL arms, I'm at 2.5deg camber and 6.5deg caster. I could increase camber a bit more, probably to 3deg.

https://www.bneshop.com/products/240-spherical-offset-strut-mounts
 
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