So, either it was a coincidence or something to be remembered in the future. Just before Christmas I washed Aston 2209 and put it in the garage where it stayed for nearly 3 weeks over the Christmas / New Year period. In the New Year we decided to go to Anglesey for a day trip and a walk along Newborough Beach to Llanddwyn Island followed by an early evening dinner at The Oyster Catcher in Rhosneigr.
I got in the Aston and selected “R” and didn’t move …… strange I thought, so I gave it a little more right foot and with a loud bang/crack we were moving. I put it down to the brake pads and discs/rotors being stuck and thought no more about it. I guessed that the wheels having just been washed and rinsed off meant that there would still be water between the pads and the discs/rotors. As you will have probably seen when you leave a car for a few days following rain or washing a light rust discolouration can take place on the surface of the discs/rotors which disappears as soon as you start driving. Aston 2209 had been put away with wet discs/rotors and stood for about 3 weeks so I suspect that there was a bond created between the pads and the discs/rotors.
So off we went on our day out but it was soon apparent that something wasn’t quite right. As we pulled out onto the main road there was quite a loud knock that sounded like something banging against the underbody of the car. You start to think of all the worst-case scenarios …. Is it gearbox? Is it the differential? What is it?. I drove on carefully with no other noises and in fact kicked the Aston down and accelerated along the country lane and nothing, no noise at all. I then had to stop when a set of traffic lights further along the road changed to red, as I started to brake we heard the knock again. When we set off again “knock” ….. I was now convinced that we most probably had a bush that had worn and quickly found that as long as I didn’t accelerate hard from rest or brake hard the Aston drove perfectly so we continued on and had a fantastic day.
A few days later I got Aston 2209 up on the lift/ramps to see what the problem was. Out came a couple of levers to start levering all the ball joints and bushes on both sides of the rear end with a bias towards the LHS as that was where the sound seemed to be coming from. I missed the offending bush the first time around and wrongly came to the conclusion that it must be something else.
I have been wanting to change the differential oil for a while and as I had everything already bought and available I thought I would do it now to rule out any damage that might be evident in the differential. My thinking was that if the knock was coming from the differential then when I dropped the oil there might be some metal bits on the magnetic drain plug. After the oil had fully drained I checked the filter which was relatively clean and there was no evidence of anything metallic on the drain plug.
|A warning when undoing the filling plug|
take care you don’t do as I did and damage
the oil cooler fins as the filling plug comes
loose. I had the socket on a breaker bar and
the filler plug being quite tight caused me to
swing the breaker bar into the cooling fins,
fortunately no real damage done.
On the second pass of playing with the levers I found the offending bush. What I found was not actually a damaged bush but in fact a fractured outer bush mounting in the lower wish bone suspension arm on the LHS. The bottom outer bush mounting in the arm was completely cracked across its width as you can see in the photo indicated by the arrow.
The 3D drawing extracted from my workshop manual shows a good view of the rear subframe and the various suspension item locations. I have highlighted the Lower Wishbone.
Once the mounting bolts had been removed from the lower wishbone it was a simple manoeuvre to lever it downwards and out.
With the wishbone out it was easier to see the extent of the fracture that extended the full width of the bush mounting area.
The bush is an interference press fit into the wishbone arm at the outer end where the wheel hub mounts. Having been cracked the required interference fit is compromised and the bush is able to move backwards and forwards when mounted in the car and as it moves from one extent of travel to the other it “knocks” against the body of the wheel hub. The bush was very easily tap out.
The new wishbone comes complete with factory pressed fit bushes already in place. Installation of the new wishbone arm is a direct reversal of the removal sequence however there is one aspect that must be adhered to. Rubber bushes have a predetermined operating range from the as installed position. Bushes are not a rotating joint, the bush is designed to allow say a defection / part rotation of +/- 10-20 deg dependant on the characteristics of the rubber and the application.
Aston require that on assembly none of the bolts are torqued up until the ride height is set and to do this requires movement of the suspension to simulate the weight of the car + passengers + Fuel + luggage.
|How do we do this? In my case I |
have the advantage of a car lift so
what I can do is lower the car so
that the disc/hub rests on wooden
blocks on the floor thus
compressing the suspension as
though the wheel was on the
If you don’t have the benefit of a car lift and you are on axle stands you can use a jack from the floor under the disc/rotor but take care that the disc does not rotate. Apply the handbrake particularly is your Aston is a manual drive as you will not have the benefit of the gearbox being locked in park.
So the car has to be lowered onto my wooden blocks until we reach a normal ride height at a hub centre to underside of rear bodywork dimension of between say 360 – 390mm, but these dimensions are based on the following according to my manual.
“2 x 68kg people in the car + 14kg luggage in the boot (trunk) + a full tank of fuel” ….. really !!!!
I’d better start bringing people in off the street and weighing them as I weigh about 85kg so that would give an uneven weight distribution, besides I can’t be in the car and torqueing up the bolts at the same time and I’ve got less than half a tank of fuel in anyway !!!!!!!!!!
Since the height is to be set without being fully loaded I decided that if the dimension was a little higher than the target 276.5mm then it wouldn’t matter too much as there is quite a wide tolerance of +/- 15mm. This dimension is only to set a neutral position for the rubber bushes so they are in their optimum position for least stress during normal running.
With the lower wishbone bolts in place (snug) but not torqued up the bushes will rotate on the bolts to the correct neutral position as the Aston compresses the suspension to the correct ride height as it sits down on the wooden blocks. With the lower wishbone positioned to the ride height the fixing bolts can then be torqued up. If you simply torque up the bolts with the wishbone in the lower position you are effectively fixing the bushes in a very low position. When the rubber of the bush in the wishbone rotates as the car sits down on the suspension it may, as a result, already be at the end of its working limit with just the weight of the car. The reality is that as we drive the bush has to accommodate +/- movements from the norm and if the bush is already at its extreme position it will then be over stressed and lead to premature failure due to the rubber failing.
With everything back together and checked all that remains is to refit the inner wing liner. As it’s still winter and damp, I have decided to defer the surface rust treatment of the subframe until the summer when everything will be a lot drier and more conducive to doing a good job and getting a good result.
Even though I removed the inner wing liner you could do this job leaving it in place. Unfortunately at the time I didn’t know that the nut for the front inner wishbone was captive as it was hidden behind the inner liner. You now know so you can remove the bolt without having to get a spanner on the nut.
A video of the search for the problem is below and includes the steps to do the repair. If you ever have this problem don’t be afraid to handle it yourself as it’s quite straight forward.
Mike (Aston 2209)
**********************For additional interest **********************
I have cut out a section of the Fracture to photograph for analysis by anyone who has the knowledge to do so, feel free to leave comments in the blog.
Mike (Aston 2209)
12 thoughts on “Loud knock from the rear – on acceleration and braking”
This “bang/crack” happens whenever I wash my Ferrari 360 and leave it in the garage for a few days then back it out. Has not happened on the DB9. Maybe because it’s more a daily driver and thus doesn’t get washed as often, nor sit afterwards. Excellent write up and pix/diagrams!
If you can open the crack and examine the fracture surface you determine what caused the crack/fracture. To open the crack apply tension to open up the crack – don’t stick something in the crack to open it up. Doing that will mar the fracture surface and obliterate the details. If you photograph and post it, I can determine the mechanism – at least determine if overload, fatigue or other.
I’m already in front of you having cut out half of the fracture to be able to look at the surface of the failure. Although I am a retired Chartered Mechanical Engineer and a Fellow of the institution of Mechanical Engineers I am not a Materials Scientist or a Metallurgist so I respect the view of anyone who can analyse what loading case caused the wishbone to fail. It would be interesting to hear your opinion. To me it looks like brittle failure but without a microscope I can’t see if there has been any local ductile failure and I don’t see any fatigue patterns.
Over to you if you can spread any light on things, I will edit the pictures I’ve taken into the article above.
Mike (Aston 2209)
First, I just want to say how wonderfully helpful your articles are. I’ve referred to them many times while tackling maintenance and repair issues with my 2007 V8 Vantage.
Regarding your knocking problem, I was trying to figure out why the broken wishbone mount was making your car resist backing up. After a while, I concluded that it wasn’t. I think your brake pads must have been stuck to the rotors, as you expected. Then, when you gave it more gas, the car tried to move, but the wheels and tires kept it in place, putting a significant strain on the wishbones. So I think that’s what caused an existing weakness in the wishbone to give way.
I’m glad you got it figured out and that the replacement wasn’t too bad (other than the cost of a new wishbone). And now we have another useful Aston1936 repair guide!
PS: Sometime, please advise us on how you removed the rotors on your car. I got the retaining screws out without difficulty, but I couldn’t coerce the rotor off of the hub. Fortunately, it didn’t actually need replacing, so I stopped short of trying to “heat and beat” it into submission.
It’s Mike (Aston 2209) and it was me who posted on Steve’s “Aston 1936” Blog. Steve kindly invited me to contribute to the Aston 1936 website as a DB9 owner who does all the repairs and servicing myself. I post articles and videos on occasions when I do jobs on my DB9 that I think might be useful and of interest to other owners.
Scott offered to comment on the failure if I posted the photos I had taken of the fracture but he has never come back to us with any observations.
I completely agree with your assessment of the build-up and background to the failure. I guess with so much power available under your right foot something had to give !!! As an additional thought it also struck me that even though the pads/rotors were “fused” together the shock resulting from the wishbone fracturing must have given the jolt (or shock load) that was need to brake the seal as the car rolled immediately.
With regards to your question about disc rotors I had one that had a mind of it’s own but unlike you mine were in need of changing being very close to the minimum thickness. I have to admit that brute force was the answer in the end using a wooden block and a heavy lump hammer. Just remember when using this highly technical method to put a couple of wheel nuts on loosely so when the rotor finally succumbs to your gentle persuasion it doesn’t fly off and kill someone !!!!
Steve has a number of articles on brakes, rotors and pads on the website and I’m sure you will have been reviewing them.
Keep us informed of any projects you undertake you can send them to Steve through the Contact page.
Mike (Aston 2209)
Thanks much, now I understand who’s who!
I recently spent 4 half-days working on my V8 Vantage. I replaced the brake pads at all four corners, replaced the air filters, replaced the hydraulic door struts (which wasn’t as hard to do as I was expecting), replaced the brake and clutch pedal pads, flushed the brake and clutch fluid, and replaced the “Bluetooth Switcher Module” with the updated unit (so now my radio works properly).
It all went smoothly and was generally a lot of fun. I was planning to replace the front rotors, but they were only 0.25 mm into their 2.0 mm wear tolerance, so I’m saving that for the future.
If you, Steve, or anyone else would like to see how I use my Aston, check out rsftripreporter.net. I’ve become a “historical tourist,” and there’s no better way to visit scenic, historic, or otherwise-interesting places!
I’m pleased to hear that you have enjoyed doing a few maintenance jobs on your Aston, it certainly can be enjoyable.
Just a note of warning when you come to removing your sticking rotors, try to avoid using heat as there will be heat transfer into the bearing hubs resulting in melting of the grease and possible damage to rubber seals.
Mike (Aston 2209)
I’ll ask you this since you are a retired mechanical engineer. Do you think this could be a casting failure where the knit line (weld line) was the location of the failure? I think, if the processing or mold design is inadequate, the material fronts may allow an air gap to exist within the casting and like what Rick mentioned, the casting, being weak, cracked because of additional force.
If you still have the old part, you could check the parting line and see if it translates to the same height as the crack.
Many thanks for your thoughts on the wish bone failure. I can follow your thinking and as you will see from one of the photos in the main article the actual crack runs exactly along the “knit line” as you describe it. Also in the same photo you can see where the knit line has been machined off as part of the post casting machining process.
It’s difficult to come to a final conclusion on the mechanics of the failure other than to say that the line of the crack is at the thinnest (outer) part of the bush housing but then it would be expected to fail at what was effectively the weakest (thinnest) point. As this is a cast part there has to be, of course, a knit line where the two halves of the mould come together and yes there is the potential for “issues” (such as air inclusions) to be experienced in this region. I’m sure I’m not the first to experience this type of failure however I would have thought that were it to be of concern ( a common event) then AM would have made changes to design out any issues by now, my DB9 (Aston 2209) is after all from 2005 so its 15 years old.
Interestingly however the new wish bone casting is different but I don’t know when the change was made. With the improvements in designs and materials the new wish bone certainly seems more “slender” than the old one and is no doubt lighter, again this can be seen in the main article photos where the “New” and “Old” are side by side.
On inspection of the additional photos I posted there is no evidence that I can see with the naked eye of any air inclusions that would suggest a casting problem and from my experience of a few cast part failures over the years this looks like a total brittle fracture and I can’t see any ductile of fatigue failure evidence.
For the benefit of readers with a non-engineering background or knowledge of materials science ……..
Castings can fail due to fatigue however they tend to be more catastrophic failures due to sudden loading, they are either ok or they break instantly due to their brittle nature, there is no in between state such as bending as with a ductile material. Cast items exhibit negligible ductility (bending/deformation) or energy absorption ability due to impact for example.
Cast items are good in compression but have poorer tension or bending characteristics therefore they are prone to catastrophic failure i.e. they fail with out warning and in a devastating way.
Ductile metals (such as steel) deform or bend before failure, Brittle (cast) materials show little or no distortion.
As with anything as long as the loading case is within the design limits of the materials then everything will be OK, it’s generally when something unexpected happens that failures occur.
Thanks for the post Tracy and Season’s Greetings for Christmas.
I wasn’t expecting such a quick reply.
I agree, the new part does look a little different. Also, I don’t see any gate on the bore for the bushing (maybe on the bottom). I wonder if these were molded separately and welded on to alleviate the problem.
On the pictures it looks like there are a few darker regions. I agree with you on the brittleness of the cast aluminum it seems to cleave like glass where there is no fatigue leading up to the failure. If there was a very fine gap (maybe a few nanometers) moisture might wick into the area and freeze. Not sure if your car is ever exposed to wet, freezing conditions. That’s all speculation, I believe you know more about this than I do.
You have a wonderful Christmas as well,
There is no “gate” by which I assume you are meaning a slot to allow the bush to be fitted and then tightened in place by compression using a bolt. ……… sorry if I misunderstand your meaning of a “gate”.
The bushes (3 in total) are a press fit into the new wish bone by the manufacturer so there will be an interference fit between the bush outer sleeve and the housing at each location. Of course the design of the wish bone (interference tolerances) will be tried and tested over the years so I don’t suspect there would be any issue resulting from the bush fitting unless the tolerances were machined too tight and I think that very unlikely as production/machining would be computerised.
I do have a 5T bench press in my workshop/garage so I could replace damaged/worn out bushes but repairing a fracture in a cast item is way above my welding pay grade.
I am in the UK (located about midway between Manchester and Liverpool) so we do get freezing conditions but nothing particularly extreme, usually only about -5C, if we see -10C that classes as an extreme event !!!!! So it is possible to get wet and freezing conditions but the fracture happened in mid summer, however, that’s not to say there hadn’t been any “weakening” during the previous winter not to mention the last 15 years.
The actual mechanics of the failure remain a mystery, I guess along with any potential pre-event contributory factors. In the end you just have to bite the bullet and throw money at it !!!! Anyway I enjoyed doing to job and being able to bring it to the Blog for the entertainment of all.
Happy driving and stay safe,
I was using the term “gate” for the location on the mold where material enters the part cavity. If this was changed, it might indicate that the part was weak in that area and they changed it for the better. I just have a curiosity for things like that. I, too, enjoy working on my car, maybe a little more if I had a lift like yours. My next project is to replace the power steering fluid which I look forward to.
Have a happy new year,