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Cracked Front Wishbone


Guest mdav1970

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Guest mdav1970

Hi,

 

Anyone ever suffered a cracked front wishbone front joint, the tube where the nylon bush sits and it mounts to the body?

 

I did yesterday after a couple of hours driving across the Staffordshire Moors and Peak District, lucky enough i had stopped just a few miles from home, went to move and the steering was funny. Lucky it never happened driving across the hills.

 

Parked it up, rang the misses to come out to pick me up. Jacked the car up and dismantled the wishbone. Took it home, reshaped it back circular with a hammer, welded the crack, knocked the bush back in, back to car and back on. Only a 3hr job.

 

Now the reason it failed was due to the bush having metal washers either side to give it toe in / out, 3 on left, 4 on right, and i guess they dried out and ceased up. Probably also the bolt was done up too tight when i had the cars geometry set up. So a good greasing between each washer and around the bush should prevent this, however i will change these for nylon washers / spacers, as i dont think they should be metal on metal anyway.

 

post-13087-0-11371200-1442863352_thumb.jpg

post-13087-0-98546600-1442863363_thumb.jpg

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Can I refer you to this recent thread on locostbuilders. The failure there was almost certainly due to seamed tube used for the bush housing. Yes there may be other contributory factors. Lubrication, carefull shimming and use a crush tube in the bush but still basically a weak HAZ in the seamed tube.

 

http://www.locostbuilders.co.uk/forum/3/viewthread.php?tid=200355

 

Nigel

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I tried to resist commenting, but sometimes I can't help myself :wub:

 

I saw that thread, but saw no evidence for a HAZ related issue. Possible mechanisms for a HAZ related failure could be "cold cracking" where the HAZ gets heat treated by the welding process and forms very hard martensite. This can lead to cracking, generally quite quickly after welding, so i'd suspect a failure soon after it was made. Only happens with certain types of alloyed and carbon steels, not the steel i'd expect it was made from.

Alternatively, the tube could be heavily cold drawn (cold worked), and the welding process softens it around the weld. If the stresses were high enough in normal use, it could then fail by fatigue, but I feel it unlikely as the actual strength loss by annealing in the HAZ would be small in a mild steel.

There has obviously not been any testing to prove this, and I don't suspect many people would let me chop up their wishbones!

 

The wishbone has a huge stress concentration factor at the weld, due to the shape of it. The failure will almost always occur at the weld, but that does not mean the weld is at fault per se.

I'd go along with fatigue failure due to excessive stiffness / misalignment of the bush / crush tube too short etc, unless there's any evidence to show otherwise.

 

Andy

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Guest 2b cruising

I like the thread that grim has made and hope the manufacturers pick up on this.

In my mind it could be an easy fix without any cost involved.

Just weld the tube with the seam against the centre line of the V bars.

This would take away the weak spot by adding extra strength with the seam being trapped in the weld.

Prevention always better than the cure.

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Guest mdav1970

The only thing is that i don't have a crush tube in the middle of the bush. Both the front and rear bushes are solid white nylon type bushes, the bolt just goes through and the nylon bush pivots on the bolt as the bush doesn't flex or move. Is this common or perhaps just a 3A thing?

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Mono Robinhoods were not conventional. These types of solid hard 'nylon' bushes were standard. Usually fitted flush with the metal wishbone and no crush tube. They were supposed to rotate on the unthreaded part of the bolt and be shimmed for fore and aft positioning of the wishbone and a nylock or splitpinned nut used so they weren't clamped up solid.

 

Nigel

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Guest mdav1970

Thanks Longboarder that's exactly as is fitted. Im guessing its just the lock nut had been tightened up too much eventually causing it to lockup and snap.

 

I'm guessing i could change them for a more conventional polly bush with crush tube?

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Guest mower man

Non of the stuff Tricky designed was any way near conventional , when I found the bushes on mine worn I simply bored them out to 14mm and used long allen cap heads and a grease nipple on the wish bone real hoodie engineering! :rofl: :crazy: mick

Edited by mower man
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As above, drilled and tapped the tube and fitted a grease nipple,. also a mod that locked the shaft and ensured that the bushes moved on the shaft and not the shaft in the stainless mount holes. You can just see this in the pic below.

Edited by HAWKNORTH
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I altered mine on the 3A to increase the bolt size (to get rid of elongated holes in the stainless mounting points) and to take a crush tube. Had to turn up new bushes to suit. You will probably find that the seamed tube is oval due to the welding and the seam also causes problems when fitting the bushes. I hadn't got a drill or reamer big enough so I ended up carefully filing the bore and used a go/no go gauge to check the bush fit. Grease nipple also fitted. The large shim washers are for adjusting the castor angle.

 

I have a photo somewhere of the parts I used. I'll post it if I can find it.

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