Can an aluminum Trailing Arm get bent? Yes/No.

[octanemaestro]

Some people have written that aluminum Trailing Arms can only crack or break, and simply do not bend (for example when a car's rear wheel hits a curb during a misjudged spin out / drift / fishtail).

Can some tell me conclusively:

Can aluminum Trailing Arms (such as in the 911SC) get bent? Yes/No.

[group911@aol.co]

I guess if they couldn't be bent, the factory wouldn't make a tool specifically to measure for bends.

[chris_seven]

Yes, they will bend and can sustain enough damage to prevent the rear geometry from being correctly adjusted.

It is virtually impossible to straignten them when they bend as there isn't too much ductility to start with.

Uisng heat is not really an option as most cast Aluminium Alloys tend to be 'hot short' and can crack.

[DRACO A5OG]

Yes they can, +1 can not be bent back without weakening or breaking it.

[GT-2]

Anyone have some pics of some bent aluminum arms?

[juanbenae]

Originally Posted by GT-2

Anyone have some pics of some bent aluminum arms?

I might suspect that any damage enough to bend them so it can picked up by naked eye would likely cause it to break. ive seen a couple at race weekends that took huge impact and they looked as if they exploded. they tend to shatter almost.

[octanemaestro]

Originally Posted by DRACO A5OG

Yes they can, +1 can not be bent back without weakening or breaking it.

On slide out wheel/tire impact with a curb does the Trailing Arm bend without causing the mounting point to bend? In other words, which bends first or more readily?

[juanbenae]

once the TA gives way at side impact your next biggest concern is the axle damaging the transmission. imagine a pool Q if you will.

bending where they tie to the body would be much easier to discern by eye id think. with the adjacent mount any distortion would likely be evident... maybe?

[juanbenae]

if the car has monoballs the body would be more susceptible you'd think. rubber mounts could absorb a ton more distortion at side impact.

[octanemaestro]

Originally Posted by chris_seven

Yes, they will bend and can sustain enough damage to prevent the rear geometry from being correctly adjusted.

It is virtually impossible to straignten them when they bend as there isn't too much ductility to start with.

Uisng heat is not really an option as most cast Aluminium Alloys tend to be 'hot short' and can crack.

Would it be possible it could bend as much 1/2"-1" or only a few millimeters would be possible without a crack or break?

[octanemaestro]

Originally Posted by juanbenae

once the TA gives way at side impact your next biggest concern is the axle damaging the transmission. imagine a pool Q if you will.

bending where they tie to the body would be much easier to discern by eye id think. with the adjacent mount any distortion would likely be evident... maybe?

Do you think you could discern whether there is a potential bend where the trailing arm attaches to the body by way of examining a photograph like a radiologist would dowith an x-ray?

[Fixer]

I was hit in my drivers rear wheel which bent my control arm pretty good

[octanemaestro]

Originally Posted by Fixer

I was hit in my drivers rear wheel which bent my control arm pretty good

If you look at the left trailing arm in the photo above, it doesn't have the same red clay dirt tint that everything else has in the undercarriage photo. I suspect the left trailing arm was already replaced in an attempt to solve the problem and yet the problem is not solved. The previous owner said it was replaced with the wrong trailing arm but then said he wasn't sure if the mechanic replaced it. I'm convinced it was replaced. The left rear wheel sits too far in and there's too much inward tension on the left axle, making it seem that there's a unibody tweak rather than a bent control arm currently. How this can be fixed, I have absolutely no idea. Where would they attach the frame machine? I have no idea. Would they be willing to straighten under there? I don't know.

[Cobalt]

Originally Posted by octanemaestro

Do you think you could discern whether there is a potential bend where the trailing arm attaches to the body by way of examining a photograph like a radiologist would dowith an x-ray?

They can bend but it is odd to have happen. If in a huge impact odds are they will crack more than bend or both.

You can straighten them but you would need to anneal them say 550+ degrees F for at least 3 hours then you need to straighten them right away. They would need to be solution treated and aged afterwords to strengthen them. Problem is they could warp again if not quenched properly after the solution heat.

X-ray or Penetrant inspection on a part like this will only show cracks and not bends.

[octanemaestro]

Originally Posted by Cobalt

They can bend but it is odd to have happen. If in a huge impact odds are they will crack more than bend or both.

You can straighten them but you would need to anneal them say 550+ degrees F for at least 3 hours then you need to straighten them right away. They would need to be solution treated and aged afterwords to strengthen them. Problem is they could warp again if not quenched properly after the solution heat.

X-ray or Penetrant inspection on a part like this will only show cracks and not bends.

Cobalt, you must referring to the aluminum trailing arms when you refer to "they" and the process of unbending them, but actually in my question above I was referring to the mount points' propensity to bend (or be catastrophically "re-positioned"). What is the likelihood of a curb impact so devastating that it not only bends the banana arm (trailing arm) but also dislocates the banana arm mount point? Perhaps this dislocation I am referring to is the feared torsion tube bend or kink? I can replace the banana arm (trailing arm) but much more difficult to fix would be a torsion tube replacement (bad job with OK result) or body tweak fix on frame machine (worse job and never perfect results). For some reason not yet ascertained my wheel is a good 4cm-5cm too far inward, yet parallel to the other wheels (note, it's not parallel when car is on a lift...it hangs down somewhat more than the other wheel, like a wasp foot....perhaps that's a clue?).

[Cobalt]

Originally Posted by octanemaestro

Cobalt, you must referring to the aluminum trailing arms when you refer to "they" and the process of unbending them, but actually in my question above I was referring to the mount points' propensity to bend (or be catastrophically "re-positioned"). What is the likelihood of a curb impact so devastating that it not only bends the banana arm (trailing arm) but also dislocates the banana arm mount point? Perhaps this dislocation I am referring to is the feared torsion tube bend or kink? I can replace the banana arm (trailing arm) but much more difficult to fix would be a torsion tube replacement (bad job with OK result) or body tweak fix on frame machine (worse job and never perfect results). For some reason not yet ascertained my wheel is a good 4cm-5cm too far inward, yet parallel to the other wheels (note, it's not parallel when car is on a lift...it hangs down somewhat more than the other wheel, like a wasp foot....perhaps that's a clue?).

Ok that makes sense. I was on my first cup of coffee when I responded this morning.

The X-ray comment threw me off. Optical comparator is what he was commenting about.

Measurements are in the shop manuals. If they are out a good shop with a Celette can fix them if their willing to take the time and it is a lot of work. Torsion tubes are a royal PITA to get right i have done a couple and had good results.

Have you checked the torsion bar or the mounts?

[porsche930dude]

how do you know its too far in? Measuring off the body is not a good indicator

[chris_seven]

The 1" of bend you suggest is most likely due to a 'body' rotation and the region where the local bend has occurred may only have been subjected to a relatively small amount of strain but this probably uses up most of the available ductility depending on the alloy.

We have seen several bent arms when they are used on gravel rally cars on very rough surfaces and we have also seen them break with a sufficiently large impact.

I am not sure that there is too much merit in trying to use X-Ray inspection methods as if they have been bent I would be inclined to just replace them.

Annealing Aluminium Castings is really only practical if the alloy is of the Non-Heat Treatable family.

I think that 550degC is a bit on the cool side. There is also the potential that if there is any there is any porosity present in the casting then blistering could occur and this could help produce fatigue initiation sites.

As the alloy is unlikely to be very ductile then the action of straightening the arm will also damage the metallurgical structure and result in a breakage the next time the arm suffers an impact.

If the alloy used is a heat-treatable type then annealing is not really practical as it will tend to overage the alloy and if you manage to create a significantly over-aged structure the arm will lose strength and become brittle due to the precipitates becoming too large.

Over-aging tends to produce 'equilibrium precipitates' that will not respond to solution treatment and will never regain their strengthening effect.

For the costs involved in replacing a bent arm I am not sure that the risk and cost of heat treatment would be worthwhile.

I have never really seen torsion bar brackets bend. I have seen tubes crack and I have seen them bend at the ends where the spring plate covers bolts to the bodyshell.

It would be difficult to rectify this without some form of accurate jig.

[chris_seven]

Originally Posted by octanemaestro

Perhaps this dislocation I am referring to is the feared torsion tube bend or kink? I can replace the banana arm (trailing arm) but much more difficult to fix would be a torsion tube replacement (bad job with OK result) or body tweak fix on frame machine (worse job and never perfect results). For some reason not yet ascertained my wheel is a good 4cm-5cm too far inward, yet parallel to the other wheels (note, it's not parallel when car is on a lift...it hangs down somewhat more than the other wheel, like a wasp foot....perhaps that's a clue?).

If you look at the torsion bar with the trailing arm and the spring plates removed they should be central in the tube to within 1mm.

We have seen them at last 15mm off centre when the end of the tube has kinked due to a kerb impact while the car was sliding.

The should also 'stick out' of the end of the tube by the same amount both sides.

If these two approximate measurements are OK it is likely that the tube is UK.

We snapped an aluminium arm a few years ago when we hit a large tree on a forestry rally and wiped out the rear fender, oil tank and arm - the tube survived.

We hit a kerb on a closed road rally in Ireland in a SWB car and bent a steel arm and kinked the tube.

The kink was about 10mm from the end of the tube and reasonably easy to repair.

[juanbenae]

if I'm following along, which is often not the case you are trying determine is the trailing arm mount flanges have been bent? consider on of these;;;



you don't have to get one of the fancy digital ones, simple old school units can be had at a hardware store for < $10. check the angle off the torsion tube to the outer mounting flange on the perceived good side vs the perceived bad side. you will have to hold it level in the t-tube, capture the angle of one side, torque down the stop and check it vs the other. for at least the outer flange you won't even have to pull the trailing arms. id have to think on it some, but I bet you could measure to the inner mounting flange location by pulling off your new tool at 90* and measuring.

id think if it was impact from the side pushing the wheel inward would cause more damage to the outboard mounting flange at it's base, and the inner flange more towards the end. right?