Lower A arm failure

[Bare]

Frankly it's rather hard to tell V much at all from those Photos.. They illustrate the damage but liitle of the offending/accused Bits.

The Front Bush Tube lhowever ooks like it was Pre-split.. for some time actually..judging by the Rust along what looks like a Piece of ERW tubings' separeated Seam... Bad piece of tube perhaps?

It seems the Braking loads initiated the destruction.. and would be consistent with the front bush tube damage/separation causing all the rest.

Likely the thing coulda given way at any time.. but I suspect the Braking was a perhaps a bit more 'ernergetic' than alluded to :-). But that in no way excuses the failure, as one would assume the the metal bits could withstand any forces generated by the machine as a basic structural requirement. .. A bit of dubious Quality Control and Bad Luck ..combined?.

[7sRWild]

Given the state of the potholed, cracked, pitted, heaved and otherwise horrid roads in rural & particularly urban California, the 600 miles on the clock of this car might rightly be considered in the same way one would consider the wear and tear that a WRC car's suspension receives on a single weekend's run. The stress or damage to the member that lead up to the failure may have occured miles and /or weeks before, driving on that S***. I grew up driving on dirt tracks that were better made and maintained than the "paved" byways of California.

 

The roads in California have beautiful vistas to behold, and many highways are nice and curvy and so forth, but the condition of them is on average pathetic, sometimes terrifyingly so. They basically really SUCK.

 

 

Wamba

 

Edited by - 7sRWild on 1 Mar 2005 06:23:36

[moosetestbestanden]

I actually think it was the rear that failed first.

 

Photo 121_2111 shows a very clean failure of the rear arm, while the front appears to be torqued as a result of the subsequent moving of Everything, as Magnus suggests.

 

Note in the photo that a major portion of the break appears to be right by the weld. The weakening of the metal from the heat comes to mind. Once a tear starts...

 

Man, you guys were really lucky that the failure happened in that residential area rather than on the freeway.

 

 

-------

Chris

M5 = 1/7

[Tony C]

I wonder if any jacking or support on axle stands has ever taken place at the front fitting.

 

The front of the front fitting has definite signs of corrosion in what could have been a crack - as previously noted - whereas the rear mounting is bright metal and shows signs of torsional failure.

 

Have a close look at the bottom of the front fitting in the vicinity of the fracture to see if there's any evidence of pre-incident impact.

 

I'll pass the photo's by a couple of experienced engineers I know, without saying anything beforehand, and see what they think.

 

BRG Brooklands SV 😬 It seems that perfection is achieved not when there is nothing more to add, but when there is nothing more to take away. (Antoine de Saint-Exupery)

[EFA]

I concur - problem looks to have started at the front bush. With consideration to ileage etc, this could be a manufacturing problem in terms of dimensions - either of the bush or the internal diameter of the wishbone end.

 

The bushes can be tight to fit in, and maybe this one was just too tight??

 

 

 

Edited by - EFA on 1 Mar 2005 07:55:01

[magnusfeuer]

EFA: The bushes being overtightened, thus "pinched" in place is the only theory apart from a production problem that I can think of.

 

How would a frozen rear joint affect the stresses on the arm? Is this a frozen joint even likely?

 

/Magnus F.

[Tony C]

My understanding is that the metal bush through which the bolt goes is held stationary by the bolt and that the suspension relative movement is possible due to the rubber part flexing. Wrong tree???

 

If I understand what EFA is saying correctly - the bush metal outer is an interference fit in the housing that fractured and if there was too much interference the housing holding the bush would be subjected to excessive stress - a possible cause of the housing failure of the forward fitting.

 

What loads would the lower wishbone be subjected to in normal service?

 

I visualise mostly backward, on the premise that vertical wheel movement is taken care of by the rubber bush flexing torsionally? Anybody know what the loadings would be?

 

BRG Brooklands SV 😬 It seems that perfection is achieved not when there is nothing more to add, but when there is nothing more to take away. (Antoine de Saint-Exupery)

[Luke Beaumont]

> My understanding is that the metal bush through which the bolt goes is

> held stationary by the bolt and that the suspension relative movement

> is possible due to the rubber part flexing. Wrong tree???

 

That's my understanding too, hence the requirement to torque the suspension with the weight of the car on the wheels.

 

> What loads would the lower wishbone be subjected to in normal service?

 

I gather braking does put a fair bit of tension in the front wishbone arm. Freestyle had a strut brace across the front mounts on their demonstrator, apparently Gary had seen race cars with the front bolts bent. Very much doubt you'd get that kind of load on road tyres though.

 

I'd be interested to hear what your engineers say Tony. FWIW I think the corrosion along what is presumably the ERW seam on the front bush housing is telling. I've read that modern ERW technique is so good that the tube is very unlikely to fail at the seam, but if a bit if tube slips through the QA procedure... I think it probably did rip apart in a 'C' shape but probably got twatted into the shape you see when it hit the deck. The braking action would have twisted into the tarmac as soon as it broke free of the bush.

[Tony C]

So - the front bush housing has a seam - are you saying the parting could have been at this seam?

 

My engineer friends did a lot of teeth sucking and tut-tutting, but were both of the opinion that the front does seem to have gone first as the corrosion at the line of fracture does indicate the possibility of it failing over some time period, whereas the rear tube is bright metal - indicating a very recent break, in comparison. Not conclusive of course, but the balance of available evidence does seem to point to that conclusion - you know what engineers are like 😬

 

I haven't had a chance to have a look at mine yet - I keep the front suspension coated with Waxoyl, so it needs a good cleaning and putting on axle stands.

 

BRG Brooklands SV 😬 It seems that perfection is achieved not when there is nothing more to add, but when there is nothing more to take away. (Antoine de Saint-Exupery)

[kslingla]

Hello,

 

The subject Caterham SV with the "lower A arm failure" is...er, was mine. Magnus and I were in the car together this past Saturday when the failure occured. I am the second owner of the car which is registered as a 2003 SV. It is not clear to me exactly when the car was constructed, and it is quite possible that some of the parts are from a 2002...or maybe even a 2001. I only say this because nothing seems to move very fast regarding Caterham construction and teething in California anyway -- for what that's worth.

 

The car does in fact have only approx. 675 miles showing on the odo. I purchased the car in July 2004, but did not take delivery until mid February 2005. I have only had the car in my possession for approx. 2 weeks. The reason for the time delay revolves around an engine upgrade from a 2.0 duratech to a 2.3 duratech, as well as a few other adjustments. I was advised that the 2.0 was difficult to map -- and that the 2.3 would be easier / better. I am now not so sure that is completely true...but that is what I did. I had attempted to retrieve the car a few times prior, but teething / mapping / fuel delivery issues haulted my progress until mid February. Continued mapping and running issues is how/way I found Magnus, as I had met with him this past Saturday to seek advise for what to do next when the....Costa Mesa Caterham Crunch occured.

 

I joke, but I feel fortunate to have found Magnus that day. He has taken great interest in this incident and has done a nice job documenting the event thus far. Without his involvement, it is quite likely the failure would have occured at a much greater speed on the freeway. To that subject, much as been written thus far about front vs. rear failure. While I cannot add anything new at this time, I think the car was little bit like driving a granade with the pin pulled. I will assist anyway I can to gain knowledge, including any external factors that could have also occured. Any findings that help solve this mystery will aid us all. Till then, give your A arms a quick once over -- at the front and the rear. :>) Ken

 

Ken -

the guy with the brittle

A arm.

[Luke Beaumont]

Sympathies, Ken, glad that it happened at such slow speed.

 

I guess I'm probably being presumptuous about the bush eye being ERW, but given that the failure is along an almost rule straight line, it's too coincidental to be seamless. I've got a bushless Freestyle front wishbone that I'll check tonight but that may not be to the same spec as the Caterham 'bones.

[patneale]

Having got a SV with 16" wheels and 205/45 tyres, I was throughly alarmed and went out to the garage and checked my suspension and found all appears OK..... 14,000 miles.

When I downloaded the photos it was interesting to see that the car was basically the same as mine with the same colour and tyres, but with the uprated AP racing brakes.

The photos are so good that it is quite easy to see the train of events.

The tube on the end of the front arm split and therefore released itself from the bearing and the braking forces would have forced the arm downwards and outwards so it hit the asphalt road surface where it dug a groove in it. Then the impact carried away the Bilstein damper top rod and with the full load now on only the rear arm snapped it off at its weakest point just above the weld to the tube bearing carrier.

Why it failed is the worry.

The Front arm end assembly is clearly shown in photo 121 2107(complete with a bit of asphalt inside it!).This shows a little rust on the inside where the bush was pushed in and would be totally normal and of no consequence, but the split along the weld of the tubing shows rust along part of it which can only indicate that part of the failure existed for some time before the incident.

Photos 121 2109 & 2110 show the front mounting bush remaining on the mounting bolt and it is clearly not right as it is bent downwards . The bolt is a square fit in the front of the chassis tube and should be almost hidden up inside the nose cone.

The failure of the arm alone should not have caused this.

So we are faced with 2 possibilities :-

1) Some time in the past the suspension was damaged by impact with something and the tube on the end of the arm was partially split and the bolt and mounting bent downwards and the braking for the dip in the road caused the split to totally fail OR

2) The forces generated by a combination of the large tyres, the wide track ( fitted as standard to SV's) longer arms creating more force on the parts due to greater leverage, the extra weight of the Ford engine and the upgraded brakes create such high forces on braking that the parts are not strong enough for the forces involved.

 

Hello

Mention has been made by others that they have seen bent front mounting bolts on cars that have been raced. But this could have well been the result of contact with kerbs or other cars during a race, rather than a structural weakness.

Having looked over my own car, the whole assembly of mounting points, bolts and the "ring" or tube on the end of the arm are of reassuringly sturdy portions that I would rule out a possible structural weakness as the cause of the incident.

I think that some time in the past the suspension has been kerbed or hit and caused the damage.

If it is proved that it is a structural weakness,then there are hell of a lot of cars needing a recall !!!

Hugh P N Robinson Caterham 140 bhp (Xpower) SV Roadsport 16" wheels.

 

 

[xflow7]

Not sure I agree that the front mounting bolt looks obviously bent. I'm going from memory at the moment, but the fit there between nosecone and mounting bolt looks about right to me compared with my standard car.

 

Also, just by eye the spacing washers are flush up against the bearing surface on the frame, and the gaps between bush and washers look to be pretty even around the circumference. Would expect an obvious variation around the circumference if it was bent significantly.

 

Dave

[Tony C]

Me either Dave - it looks good to me too.

 

BRG Brooklands SV 😬 It seems that perfection is achieved not when there is nothing more to add, but when there is nothing more to take away. (Antoine de Saint-Exupery)

[Bob Simon]

Hugh,

 

Photos 121 2109 & 2110 show the front mounting bush remaining on the mounting bolt and it is clearly not right as it is bent downwards . The bolt is a square fit in the front of the chassis tube and should be almost hidden up inside the nose cone.

---

 

Actually, the bolt is meant to be parallel to the road surface and the centreline of the vehicle, give or take some chassis rake. Given the limited scope of the photo, I'd venture the bolt is pretty much in it's correct position.

 

I would have to concure with the rest of your analisis. I believe the wheel took a hit sometime in it's life that led to fatiguing of the bushing carrier. I would also venture that the seam weld in the bushing carrier's DOM tubing was situated in the worst possible location. It sure looks as though the tube split right down the weld seam.

 

I've personally always assumed that DOM was equally strong with the seam in any position. In normal compression and tension loads this is probably true. When DOM tubing is used as a bushing I think that assumption needs to be reevaluated. Maybe a part turned from the solid would be more appropriate for this particular application?

 

 

-Bob

95 HPC VX

 

[patneale]

Bob

You are right I have checked mine and there is a drop in the angle of the mounting relative to the under side of the body. So angle looks correct.

It would have to be this way as the rear mounting is on top of the chassis rail and thus about 2 inches higher than the front so it has to be at a slight angle. Any distortion would show with cracking of the powder coating and I don't think this has occured.

This all leads to the fact that the weld has failed along the length of the tube, but part of the failure occured earlier due to the presence of rust in part of the split.

Why ?

The welding of the arm to the tube is quite a hefty one and the tube would be heated considerably as a result. Could the extra heating cause the existing weld to become brittle and fail ? Only an experienced Metallurgist would know the answer to this. And this would have happened many times before especially at Brands Hatch and other race meetings if this was so. And it hasn't.

From my experience of working with some product liability claims with a major worldwide Insurance Company there could always be the possibility of the actual tube being made of substandard materials.

Most unlikely however !

But it only needs one batch of metal to have a slightly lower Chrome content for it to have lower strength. So it is always remote possibility.

As I haven't heard of this problem before, the possibility of a former kerbing incident or impact with something cannot be discounted.

And as one length of tube would make quite a lot of these mountings then there would have been numerous failures elsewhere if it was faulty materials.

I feel inclined to think that it is a previous bump that has caused this failure before the new owner bought it.

HUGH

 

 

[xflow7]

IMO, we're starting to tread on somewhat sensitive ground here throwing around insinuation of design/manufacturing defects without comment from Caterham.

 

Dave

[7sRWild]

Looking at the pattern of the rust on the bush, on both sides of the split, it would seem to have been open, at least a little, well prior to the final failure. There's clearly superficial rust and what appears to be even deeper corosion down in the metal.

 

I don't think that the car hit anything but one or several or many of the millions of average, garden variety California potholes thereabouts. That / they easily could have caused enough stress to split that bit. A fiver says that the bolts were torqued w/out the suspension loaded too, and the the joint was all bound up.

 

That said, seamless tubing in this application sounds like a really good idea.

 

Wamba

[Tony C]

I wholeheartedly agree with Dave - let's be very careful with these comments. After all, we're only speculating from photo's as to the possible sequence of failure.

 

From all "evidence" this is an isolated incident and thankfully nobody was hurt - nobody else has had this happen despite lots of 7's being thrown about on track and bumping into all sorts of objects over a fifty year period.

 

I think we're all going to be having a close look at our own cars and will continue to monitor the suspension more closely in future.

 

Unfortunately this valuable "Lesson" has been at the expense of poor Ken - I hope you went straight out and bought yourself a lottery ticket 😬 One doesn't like to think about the consequences of this incident if you'd been driving even a little bit faster - scary.

 

Nice looking 7 by the way and excellent photo's. Thanks for sharing your experience.

 

BRG Brooklands SV 😬 It seems that perfection is achieved not when there is nothing more to add, but when there is nothing more to take away. (Antoine de Saint-Exupery)

[kslingla]

Tony C.

 

Thank you for the comments. No lottery tickets yet...but thats not a bad idea. I had far to little windscreen time in this puppy dog to date...and I miss it not being in my garage. But, I refuse to be discouraged in anyway. Sports Cars of all kinds are my only true escape for day to day "stuff". Its my/our therapy so to speak. So, while disappointing and shocking, the real crying shame is that I am without my brand new toy for coming weekend! Just exactly what does a guy do for temporary sporty wheels --when the greatest value in supercars is in the hospital? Buy a motorcycle? Fly a plane? Bungee jump....none of these sound anywhere near as appealing.

 

We will back on the road someday real soon. Until then - if everyone could just send me a dollar or two $$..... 😬

 

Ken -

the guy with the brittle A arm

[magnusfeuer]

Ken,

 

If you want some Caterham time, give me a call. I'd be happy to let you have my car for a day. If you can live with the 5500 RPM rev limiter, that is.

 

I am working so much right now so that the car is sitting in the garage unused, releasing a drip of oil in sadness every now and then.

 

 

/Magnus F.