26 replies to this topic

[siztenboots]

courtesy of Lotustalk

It should "fail" exactly the way that it did.

In the first diagram below, the force (in red) is exerted along the toe-link to the ball joint. The ball joint resists this force by holding laterally in single shear with the mounting bolt (the blue lines). But part of the force exerted on the ball joint acts as a rotational moment around the ball joint. That is resisted as a torque (bending) of the ball joint mounting stud. The premature failure of the inner ball joint happens when the ball joint mounting stud is not tight - this allows the mall joint to move around and causes the mounting bolt to dynamically bend back and forth causing the bolt to fatigue and fail.

But the designed in "failure mode" is shown in the second diagram below. In this case, the force on the toe-link is greater than it should be (due to impact). The bending moment about the ball joint is greater than the strength of the threaded connection between the ball joint and the toe-link rod, causing that threaded section to yield (bend). When the force exceeds the ultimate strength of that section, it breaks, stopping the force from being transfered to the chassis by the ball joint. The ball joint fails, and the chassis is undamaged.

In the third diagram, the double shear joint is shown. Again, the force is in red, exerted along the toe-link rod. But there is not bending moment about the Heim joint at the chassis mount. The force is exerted laterally by the rod and resisted latterly by mounting bolts and brackets. There is no bending moment. The result is that if the force along the toe-link rod is too great (as from an impact), that force will be completely transfered to the chassis. There is no part that will bend and fail before transferring that load to the chassis. There is no sacrificial part. The joint is stronger, but the failure mode transfers the damage to the chassis.

fig 1 -
fig 2 -
fig 3 -

[Crazyfrog (Fab)]

thanks steve for the pm

[VIX]

I can see why (like hub bolts) you would want them to fail during an impact but not when you're driving along the motorway!

[Muncher]

Yeah, it's designed to protect the subframe in the event of you hitting something. However I'd rather be paying the £800 or so to replace a subframe after I twatted something rather than picking myself out of a ditch/armco when it failed without warning at speed.

[Exmantaa]

Ehhhmmmm...
How can a sperical joint transfer a rotational moment and so causing a bending force on the tie rod fixing???

[siztenboots]

Yeah, it's designed to protect the subframe in the event of you hitting something.

However I'd rather be paying the £800 or so to replace a subframe after I twatted something rather than picking myself out of a ditch/armco when it failed without warning at speed.

link arm would go next , if the inner joint was in double shear

this kit uses , QA1 XMR8 (1/2" x 1/2") rose joints

[_VX220_]

The condition of our roads must put serious stress on this.

[Crazyfrog (Fab)]

when it failed without warning at speed.

no warning that is a pure fact and was pretty stressful and glad it happen the way it did without accident
i am still alive to tell the tale my moral is on the up and next week need to have no fear and carry on driving the way i have been for seven year and try to put it in the back of my mind
the new set of spitfire toes link should be with me next so no vx for bank holiday instead i will have half a case of wine i never work on bank holiday

[Duncan VXR]

Yeah, it's designed to protect the subframe in the event of you hitting something.

However I'd rather be paying the £800 or so to replace a subframe after I twatted something rather than picking myself out of a ditch/armco when it failed without warning at speed.

link arm would go next , if the inner joint was in double shear

this kit uses , QA1 XMR8 (1/2" x 1/2") rose joints

Steve, what kit is that?

DG

[cheeky_chops]

Yeah, it's designed to protect the subframe in the event of you hitting something.

However I'd rather be paying the £800 or so to replace a subframe after I twatted something rather than picking myself out of a ditch/armco when it failed without warning at speed.

surely it would just pull a hole in the subframe and the end result would be the same - spin? And a new subframe needed?

[Muncher]

Not quite sure I see you point cheeky?

[siztenboots]

something like this ?

[theolodian]

Not quite sure I see you point cheeky?

I think that he means if you hit a kerb or something hard enough.

You guys have me all worried about mine, but I have to say they did a good job on my car. It went over the kerb with a direct hit on the back wheel. Bent the end, but stayed together. Exactly as it should have done.

[Crazyfrog (Fab)]

so the moral of the story if you fit a upgrade dont hit the curb hard but i prefer to see some damage in the rear chassis than to be in a cardboard box it happen once to me to have a road failure but wont happen again i hope my life is too precious

[cheeky_chops]

Not quite sure I see you point cheeky?

OK, It seems you are advocating the 2nd option here :

std tie rod failure so bolt breaks, spin, hopefully dont crash = New tie rod + geo

Change tie rod bolt so bolt doesnt break but rips hole in subframe, spin, hopefully dont crash = Probably new tie rod, geo AND subframe + fitting

Or is it not really possible to rip a hole in the subframe?

[Muncher]

Not quite sure I see you point cheeky?

OK, It seems you are advocating the 2nd option here :

std tie rod failure so bolt breaks, spin, hopefully dont crash = New tie rod + geo

Change tie rod bolt so bolt doesnt break but rips hole in subframe, spin, hopefully dont crash = Probably new tie rod, geo AND subframe + fitting

Or is it not really possible to rip a hole in the subframe?

If you change to an uprated joint it doesn't fail, you don't spin and you don't crash...

[cheeky_chops]

Not quite sure I see you point cheeky?

OK, It seems you are advocating the 2nd option here :

std tie rod failure so bolt breaks, spin, hopefully dont crash = New tie rod + geo

Change tie rod bolt so bolt doesnt break but rips hole in subframe, spin, hopefully dont crash = Probably new tie rod, geo AND subframe + fitting

Or is it not really possible to rip a hole in the subframe?

If you change to an uprated joint it doesn't fail, you don't spin and you don't crash...

sorry if i am being thick, what happens if you whack it? Something has to give/break/weaken/bend? I pic would be useful

[Spitfire Engineering]

Not quite sure I see you point cheeky?

OK, It seems you are advocating the 2nd option here :

std tie rod failure so bolt breaks, spin, hopefully dont crash = New tie rod + geo

Change tie rod bolt so bolt doesnt break but rips hole in subframe, spin, hopefully dont crash = Probably new tie rod, geo AND subframe + fitting

Or is it not really possible to rip a hole in the subframe?

If you change to an uprated joint it doesn't fail, you don't spin and you don't crash...

Good thread
Try not to forget that there are two ends to a toe-link!
The O.E ball joints are cheap, formed components and not very strong.
The inner is weaker as the forces act to load the bolt as well as bend it, and it has a longer lever arm so it can clear the chassis though the biggest issue is stiff/worn joints which then resist housing rotation and so load the threaded spigot instead, considering the high mechanical advantage, over time stress cycling weakens the spigot close to where it meets the housing and this is where they invariably part company! There are hundreds of examples of this failure it is not just scaremongering.

If you replace the standard setup with an uprated kit then you remove the weak inner joint with a double shear setup, and also improve the bearing quality (which does not have a 1mm plastic insert to keep everything tight). however the outer joint onto the upright is only uprated by the bearing and is therefore not as strong as the inner joint.
This will mean when excessive forces are applied the link will break at the outer end and unlikely to damage the chassis.

Though you need to be careful, in the picture above the kit has the outer link replaced with a high tensile tapered component, although ultimately much stronger you lose any fuse properties offered by the 10mm bolt and with a major kerbing clearly the energy will be conducted further down the component chain to the subframe.

Edited by Spitfire Engineering, 11 January 2010 - 08:36 PM.

[FLD]

Well my subframe has creases in it just inboard of the tie rod mount point as if loaded excessively through th tie rods. It measures as it should and the geo is spot on (guess this was done to sort it). I suspect the subframe tower parts aren't the strongest and prone to bending over time especially if you give your car a hard life. I've just bought a new subframe and I'm considering tie-rod options such as the spitfire kit, the kit that mounts them to the wishbone and perhaps making my own.

[Spitfire Engineering]

Well my subframe has creases in it just inboard of the tie rod mount point as if loaded excessively through th tie rods. It measures as it should and the geo is spot on (guess this was done to sort it). I suspect the subframe tower parts aren't the strongest and prone to bending over time especially if you give your car a hard life. I've just bought a new subframe and I'm considering tie-rod options such as the spitfire kit, the kit that mounts them to the wishbone and perhaps making my own.

Any pictures of the damage??



Gaz