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Hi all,
I took my car out for it's first drive in about a year, complete with improved suspension setup and sunshine. After 20 miles on a right hander the handling suddenly went very loose but fortunately kept it on the black bit. On inspection I've found the upper left rear wishbone has cracked almost through next to the balljoint (photos below). Needless to say I called out a truck to take the car home again! Lucky my insurance covers tows.
I have looked at the other rear wishbone and there was a crack in the powder coat in the same location. I cleaned the coating off and cannot see a crack in the steel but I don't have any dye pen equipment to verify this. My confidence in it is not high though. The car has covered about 2200 miles since construction and hasn't hit anything! Probably worth anyone with these wishbones checking theirs too. I've emailed Marlin the photos of course and await their comments. I had previously had concern about the strength of the rear wishbone arrangement - try putting a socket on a wheelnut with the car jacked up and the handbrake on and giving it a pull - the rigidty is not too high!
Phew - it's good that when it gave out you weren't going overly quickly, and that it didn't fail completely and pitch you off into the verge / oncoming traffic.
When I dismantled my car after about 1500 miles, I noted the large flat plate on the lower arm, to which the bottom ball joint mounts, had bent on both arms. If your upper arm has done this, it's probably prudent to check the plate on the lower arm, and even if it is still true, weld some stiffening webs on to it.
Does your insurance also cover the cost of new pants?
It's interesting to see that it's the tube that has fractured rather than the the weld itself. I beleive from some limited reading that welding can make the surrounding metal britle?
I assume you guys are aware of the Sporster wishbone failures and recall?
One thought that entered my mind, is the rear upper arm is effectively the same as the front, and the rear looks to have failed from the front backwards. If that's the case, then it suggests the braking force might be the higher load on the rear. The fronts could fail in the same way...as they see an even higher load on the upper arm?
Yes, I remember the sportster recall and it struck me there was potentially a bit of a similarity there!
Not really fair of me to speculate about failure mechanism as I haven't investigated it thoroughly....but what the heck - my best guess would be a fatigue crack growing from the stress raiser at the toe of the weld, probably started from an original fabrication toe crack in the heat affected zone where as you say the ductility may have been reduced. If I'm right, that would mean that arms without an initial defect may be OK for a much longer life but if the stresses at that point are high enough to grow a crack so fast (which must have been initially small enough to pass inspection) then I'd be a bit concerned about the loading at that point anyway. Hmmm.
Oh and John, the insurance company have inspected the pants and sadly written them off
Messages crossed. John, I was thinking exactly the same thing. It's definitely cracked from the braking side, as it were. The optimistic way to look at that would be to say: well it doesn't look like it's the high loads from the k20 engine that did the damage, therefore there are lots of 5exis out there with more miles on them and lots of wishbones subjected to similar loads without failing - hence it looks more likely to be a fabrication defect in this case rather than a design flaw.
No, I've stuck with the original design. Am I right in thinking that the new setup moves the top ball joints a lot further apart? If so I'd have thought that should help a lot with loads in these areas.
If so I'd have thought that should help a lot with loads in these areas.
Looking at the failed arm very simply though, and just considering the longitudinal (relative to the car) force applied to the end of the front-most ball joint, whether it's the new design or old, the front-most tube is under 3 point bending regardless. Surely that load will be the same whether it's the new or old design, as the rear ball joint can't offer fore and aft support.
I agree the bending load can't alter between the two designs, but wouldn't there also be a superimposed lateral load at the joint - i.e. the main wishbone and the track control arm are in complementary compression/tension to resist the torque on the wheel. This load would reduce as the spacing between the joints increased. I guess this should be a lot smaller than the longitudinal load though - magnitude would depend I suppose on the relative positions in space of the upper and lower main ball joints and the tyre contact patch.
I agree the bending load can't alter between the two designs, but wouldn't there also be a superimposed lateral load at the joint - i.e. the main wishbone and the track control arm are in complementary compression/tension to resist the torque on the wheel. This load would reduce as the spacing between the joints increased. I guess this should be a lot smaller than the longitudinal load though - magnitude would depend I suppose on the relative positions in space of the upper and lower main ball joints and the tyre contact patch.
My fag packet calc' for a heavy braking scenario, ignoring dynamic spikes, reckoned that the difference in the resultant load seen by the front balljoint varies by only 10% between old and new design. It was only a fag packet calc, where I guesstimated arm lengths, etc, but I don't think it's too far out in relative terms, so I'd be checking my sus' arms if they were standard wall thickness and seamed.
That said my car runs a mixture of thicker wall seamed tube, and thick-walled CDS, and I've checked it religiously after track use, as ultimately every kit car is in effect a prototype.... failure of some sort or another is almost inevitable? Maybe I should just grow a pair, but there's always a nagging worry in my mind that something on my car will let go horrifically at high speed. It's what puts me off "proper" circuits, as I take comfort in the relative safety of an airfield - the concept of hitting a conveyor belt at speed scares the hell out of me.
Hi all. Bad news is Marlin don't have any wishbones on the shelf at the moment and it's going to be several weeks until they fab a batch and I can get a replacement. Thought it was worth seeing if anyone has a spare wishbone knocking around - I know that some people at least have upgraded to the new rear end design so might have some old parts? I think a spare front wishbone would be fine too as I don't need the track contol arm attachment tab, having fitted the alternative rose joint mod. Suitable reimbursement can of course be provided!
Can't believe it's going to be several weeks to get one made up for you. Nope sorry I can beleive it.
I upgraded to the latest suspension. But one side (drivers side was damaged in the accident I had (spun 180 and hit a kerb). The other side I used as a bracket for the chargecooler about a month ago.
In fairness to marlin it's a combination of holidays and the fact that they don't have the laser-cut parts in stock at the moment as they haven't scheduled in a batch of wishbones. I do expect to get one - just not as quickly as would be nice given we're in the middle of the driving season!
16swg (1.626mm) I believe, and clearly seamed. The threaded insert is welded at the outer end, but I can't see any means of the inner most end being held in place, so I'd imagine it could flex about the end weld slightly (kinda rock about in the tube). Plug welding the inner end of the threaded insert wouldn't do any harm.
What happened to getting a wishbone from John Cooper?
I know what you mean John, the support on it doesn't look great, though I guess this must be fairly typical of threaded insert fitments? I see where you're going with plug welding but I'd be a little cautious about creating more problems than I solved... and after all that wasn't the failure mode! Out if interest how are your current wishbones terminated?
