Slightly bent rear axle beam

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halfbytecode
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Slightly bent rear axle beam

Post by halfbytecode »

Hi,

The rear axle beam is slightly bent in my car. It is affecting whatever coil spring I put at the rear right. The ride height is normal, but the car feels drooped down at the rear right.

The rear camber and toe are slightly affected. Camber values: -01°55" and -1°09", with the spec being -1°30" +/- 0°10". Toe values: +0°17" and +0°37", with the spec being 0°10" +/- 0°10".

The issue has been there for quite some time, and I have replaced bad shocks, front control arms, lower ball joints, sway links, sway bar bushings, and also the rear axle beam bushings, but the issue remains.

I would like to attempt to have the rear axle beam straightened from an automotive frame repair shop.

Please let me know how should I ensure the issue with the rear camber and toe, goes away.

Hopefully, the car feeling drooped issue would go away after that.
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Re: Slightly bent rear axle beam

Post by alexperkins »

If there was any concern the axle beam is deformed, it must be replaced and under no circumstances repaired or straightened as this will modify/compromise its structural integrity.
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Re: Slightly bent rear axle beam

Post by Polonut »

Sorry that the OP has spent all that money and time unnecessarily.
As Alexperkins says... IF the frame IS bent, do not attempt a forced applied correction - replace it. It would be quite rare for this to happen. Note that disparities in camber angles is quite normal.
As a matter of interest - have you owned this vehicle from new?
You say the frame is bent - has this clearly been professionally established and confirmed or are you making a guess here?

Some of us are a little confused that 'the ride height' is correct / car feels drooped at the o/s/r.
Excuse me asking a very simple basic starting question.. are you sure that the rear wheels have the same size tyres fitted - in particular the same profile measurement.

PS - have you by any chance fitted a kit anti-roll bar to the rear (or the previous owner -if there was one?)
halfbytecode
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Re: Slightly bent rear axle beam

Post by halfbytecode »

alexperkins wrote: Tue Jan 02, 2018 10:14 am If there was any concern the axle beam is deformed, it must be replaced and under no circumstances repaired or straightened as this will modify/compromise its structural integrity.
Alright, thanks
Polonut wrote: Tue Jan 02, 2018 12:23 pm Sorry that the OP has spent all that money and time unnecessarily.
As Alexperkins says... IF the frame IS bent, do not attempt a forced applied correction - replace it. It would be quite rare for this to happen. Note that disparities in castor angles is quite normal.
As a matter of interest - have you owned this vehicle from new?
You say the frame is bent - has this clearly been professionally established and confirmed or are you making a guess here?

Some of us are a little confused that 'the ride height' is correct / car feels drooped at the o/s/r.
Excuse me asking a very simple basic starting question.. are you sure that the rear wheels have the same size tyres fitted - in particular the same profile measurement.

PS - have you by any chance fitted a kit anti-roll bar to the rear (or the previous owner -if there was one?)
Thanks for the detailed reply.

The parts replaced were all worn. Sorry, I did not say that clearly.

I am the first owner of this car.

The frame is bent very slightly, like 3 mm. It was professionally established. I even saw it myself when the axle beam was mounted on the professional equipment.

I meant the ride height is as per the manufacturer specs, if we measure it from the center point of each wheel hub right up to the fender/quarterpanel above.

While driving the car, it squats at the rear right. Swapping rear coil springs does not help for too long. No matter the coil spring, the rear right eventually starts squatting.

All tyres are of the same profile, and were bought together.

There was never any rear anti roll bar kit fitted.
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Re: Slightly bent rear axle beam

Post by Polonut »

What happens if you go into reverse? - do you experience the O/S/R lifting?
Find a quiet non busy place and try it - then report back. I ask this because a badly binding brake will lower the cars rear - in forward motion. If this is the case, then a lifting of the O/S/R would take place in reverse.

Further questions..... If the ride height is correct at rest, it follows that the 'frames trailing arms' are at a correct starting level on both sides in the vertical plane. However as the wheels themselves will be at the same level/point, IF there is a frame bend this disparity can sometimes be identified by one tyre appearing flatter than the other, as positional/vertical compensation is made. IE the cars weight not being shared equally by both sides at the same time.

The only other factor is downforce that will be from the drivers weight (which would be biased on the O/S) and would not normally make any difference.

You say that you have altered springs - are you able to tell us the make and part number of the springs used? Also (very important) have you attempted to lower the cars suspension?

We really need to know far greater detail as to where you feel 'the bend' has taken place.
We also need to know if the frame was completely removed from the car when 'the bend' was noted.
Also are the rear tyres wearing identically on both sides? Are they directional tyres - fitted with the correct direction markings.
Last edited by Polonut on Wed Jan 03, 2018 9:09 am, edited 1 time in total.
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Re: Slightly bent rear axle beam

Post by halfbytecode »

Polonut wrote: Wed Jan 03, 2018 1:10 am What happens if you go into reverse? - do you experience the O/S/R lifting?
Find a quiet non busy place and try it - then report back. I ask this because a badly binding brake will lower the cars rear - in forward motion. If this is the case, then a lifting of the O/S/R would take place in reverse.

Further questions..... If the ride height is correct at rest, it follows that the 'frames trailing arms' are at a correct starting level on both sides in the vertical plane. Howeverr as the wheels themselves will be at the same level/point, IF there is a frame bend this disparity can sometimes be identified by one tyre appearing flatter than the other, as positional/vertical compensation is made. IE the cars weight not being shared equally by both sides at the same time.

The only other factor is downforce that will be from the drivers weight (which would be biased on the O/S) and would not normally make any difference.

You say that you have altered springs - are you able to tell us the make and part number of the springs used? Also (very important) have you attempted to lower the cars suspension?

We really need to know far greater detail as to where you feel 'the bend' has taken place.
We also need to know if the frame was completely removed from the car when 'the bend' was noted.
Also are the rear tyres wearing identically on both sides? Are they directional tyres - fitted with the correct direction markings.
The car squats while in reverse as well.

I have not alterered the springs. I have merely swapped the OEM coil springs that came with car. Only the rear ones for figuring this out.

The rear axle beam was completely removed from the car. I have the version with rear drum brakes. The brake hardware was removed, with only the spindles left attached to the frame. The frame was then mounted on the equipment, from both ends such that the frame could be rotated, and any differences or bends could be seen.

It is a bit hard to explain, but fixed reference points were kept on both sides near the spindles, and the beam was rotated. While rotating, the difference between both sides was unequal. I say unequal, considering the slight negative camber the car is supposed to have as per the spec.

The tyres do have unequal wear at the rear. They are asymmetrical tyres and are correctly mounted.

I should say that the car feels like the rear right coil spring is weak. However, even if the rear springs are swapped, the issue again pops up at the rear right. This makes me think the coil springs may not be the issue, but the slightly bent beam is affecting whatever coil spring is installed at rear right.
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Re: Slightly bent rear axle beam

Post by veteran »

halfbytecode,

Been meaning to give you my own pennyworth on this matter for some time.

I strongly suspect that the bent rear axle beam has come about because someone has, against published VW advice, used the rear axle as a means for lifting the rear section of the vehicle. VW do warn that if you lift the vehicle on either the front subframe, sump, rear axle beam, or even most of the floorpan you will cause distortion of those parts, with negative consequences. Normally, on VW's there are specific built-in, strengthened positions on the floorpan for lifting, though on some vehicles the front two positions aren't always obvious.

You might wonder about the sills also. As for using the sills as overall bearers for the entire weight - or even half the weight - of the car, do bear in mind that the designated lifting points on the sills are for the purpose of wheel-changing only; owners and those who perform work for owners should not use the sills as lifting points for the entire vehicle, as what will happen is that the seam of the sill will crush under the weight and will eventually split, causing moisture and rust to take hold.

Currently, I'm trying to find out where the four official lifting points (as I say, not the wheel-changing sill points) are, on my 2017 Polo. I'm pretty sure that the two small inboard quadrants, just forward of the rear wheel arches, are the rear lifting points, but I don't recall seeing any obvious points at similar positions at the front end of the floorpan. Usually, they're raised bumps on the floorpan.

I could well imagine that if the rear axle got bent by being wrongly used as a lifting point (perhaps repeatedly over some years), the camber of the vehicle would change, with not only consequential sag on one or both sides but also increased tyre wear at the shoulders.

On the beam, in your case, are there any tell-tale signs of it having been misused in that way, like areas of paint having come off or gouge marks in the lower edges of the beam?

On my former VW, a Golf, I used to have to specifically instruct the MOT tester to use the designated lifting points for that part of the Test, as otherwise he'd most definitely use the wrong ones, usually on the seams of the sills and/or on the axle beam.
halfbytecode
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Re: Slightly bent rear axle beam

Post by halfbytecode »

@veteran

Thank you for the informative post.

There are some signs below the coil spring seat, I think on the same side as sagging. It is as if it got hit and scratched on some surface below, perhaps while going through a pot hole or something similar. The spring seat seems bent upwards a bit, reducing the space for the spring, and exerting higher compression on it.

I am the only person that drives this car, but I cannot recall something like this happening though.

Regarding raising the car from the axle beam, I don't allow it to happen, just like you said. It may have been raised like thay once by a tech while I was away.
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Re: Slightly bent rear axle beam

Post by TWR »

Did you check the previous history of this car? no matter what car & the intension is to raise it, the weight should be spread across the beam that way no harm will occur, for the work, money that you have spent & time you would have been quicker/ easier to replace the beam with a 2nd hand one, you are just wasting your time trying to rectify something that you will not achieve. TWR
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Re: Slightly bent rear axle beam

Post by RUM4MO »

He/she says that heshe owned it from new!

Edit:- though that might not answer your question?
halfbytecode
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Re: Slightly bent rear axle beam

Post by halfbytecode »

Okay, this seems to have brought about some good discussion. The car has been with me, and I have driven it carefully. This is just something that seems to have unfortunately happened, due to the roads being rife with potholes, around here. Having said that, it seems to be something quite minor.

We have to remove the axle beam again at the dealership, to correct any error they made. I plan to measure at different points of the axle beam of car that's new-ish, and work from there.

Salvage yard stuff is not really a good idea, around here. Most of the stuff there is pretty much worthless, with most cars being abused, either by the drivers, bad roads, or both.
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