Failed rosejoint caused disaster

[DamperMan]

This thread shows exactly why we believe (and many others in motorsport) that rod ends should NOT be used on front wishbone mountings! Rod ends are weak in shear and the forces on the front wishbones are far greater than the rear due to the 'twist' from braking forces.

Disagree if the parts are good quality and correctly spec'd .. Regularly checked they are ideal for Motorsport. But the converse is absolutely true.

[ibbo]

wow looking at some of the pics glad ive got old school live axle rear end

[stephenh]

Rosejoints are ideal for motorsport, but that isn't Blinks point, his point ios that they are ideal in the correct application (ie in engineering terms). They are designed to take (in layman''s terms) push and pull loads. Of course you could use them for an application where they are put into sheer load, but only by "over engineering", ie by using a massive joint.

[Dave Eastwood (Gadgetman) - Club Chairman]

...and should really be mounted in "double-shear" as a best practice.

[phelpsa]

Rosejoints are ideal for motorsport, but that isn't Blinks point, his point ios that they are ideal in the correct application (ie in engineering terms). They are designed to take (in layman''s terms) push and pull loads. Of course you could use them for an application where they are put into sheer load, but only by "over engineering", ie by using a massive joint.

It's not over engineering, if anything it's under engineering. Over engineering would be over complicating the design to solve a problem that isn't necessarily there. And if it works just fine, causes no problems and has a few real benefits over the original design then who is Mr Blink to say that it should not be done?

[Martyn Vann - Warwickshire AO]

It's not over engineering, if anything it's under engineering. Over engineering would be over complicating the design to solve a problem that isn't necessarily there. And if it works just fine, causes no problems and has a few real benefits over the original design then who is Mr Blink to say that it should not be done?

 

I think Fraser is suggesting that there are many engineers that feel the rose joint is used inappropriately at times. I would add that my son is a senior stress analyst with a highly respected motorsport/automotive engineering company with a great deal of experience of suspension design and failure modes.  He is not happy with rose joints in some positions on a race car, though designers do spec them, but it is a definite no no when applied to a road car as the maintenance is generally insufficient to spot early signs of problems.

 

Ultimately I think we need to agree to disagree as there are as many engineers for as against using rose joints where excessive shear and/or bending forces may be encountered or inspection and maintenance may be a little sparse.

[Dave Eastwood (Gadgetman) - Club Chairman]

I think the other part of Frasers point is that IN SOME POSITIONS, rose joints aren't accepted in the regs of some race series. Not sure if I remember correctly, but I've a feeling Formula Student might be one of them; where in some parts of the suspension, a properly engineered rose joint is acceptable, but in other spots, they're not allowed.

[phelpsa]

I think the other part of Frasers point is that IN SOME POSITIONS, rose joints aren't accepted in the regs of some race series. Not sure if I remember correctly, but I've a feeling Formula Student might be one of them; where in some parts of the suspension, a properly engineered rose joint is acceptable, but in other spots, they're not allowed.

First of all that's not what he wrote, and secondly your formula student point is incorrect. Old Pat Clarke might take issue with it in the design competition if you don't have a good justification for it, but it's certainly not dangerous.

As I said initially, it's not an ideal engineering solution, load paths are poor so not very structurally efficient, but as long as the the rod end is specced to survive the load case required then it is a sound solution. The same applies to bolts in single shear.

[Mighty Mart]

Interesting post this...

It would appear that some of the failures are at the thread end 'runout'.

This is normally a weak point, but typically in a thread rolled application, the thread is a lot stronger than a cut thread. Thread rolled correctly, a very strong thread is the result, but thread rolled incorrectly, ie over rolled, causes stress risers that can be substantially more liable to failure than the perceived weaker cut threads...

There can be bad batches of material as well thrown into the mix.

At least this is my experience of high end rod ends used in Helicoptor applications, where we do our own cut up and microstructure examinations on our thread rolled components as a matter of course.

Mart.

[BLiNK Motorsport]

It's not over engineering, if anything it's under engineering. Over engineering would be over complicating the design to solve a problem that isn't necessarily there. And if it works just fine, causes no problems and has a few real benefits over the original design then who is Mr Blink to say that it should not be done?

 

 

Reading my post you will see that it is 'opinion based' - I think it is poor design that offers no advantages over a simple nylon bushed lower front wishbone - maybe someone can suggest some?

 

I specified the Location on the car I don't like to see rod ends being used so Dave's post is accurate and is what I said.

 

This thread is about failure of a rose joint on a rear axle (i think there are several people who have experienced this?) which is horrendous - try controlling the car with the failure on the front lower wishbone whilst braking - you probably think that was dangerous?

 

Please read the following - I think it is quite informative:

 

https://www.formulastudent.de/academy/pats-corner/advice-details/article/pats-column-rod-ends-in-bending/

 

 

I would be interested to see the calculations performed to show the braking forces through the lower front wishbones whilst using different tyre compounds/brake materials that the people who are using rod ends in this position have performed to ensure their joints are 'specced correctly'

[phelpsa]

Are you talking front lower outboard? I'm still not sure what position you're talking about? That would be a poor position to use them in due to damper bending loads on the wishbone, but I haven't come across a Westfield with then in that position. Shear loads still aren't the culprit though.

If you're talking inboard then there are very little shear or bending forces on the rod ends even with high braking loads. They are actually very well suited to that application. Edit: Should probably note that I'm assuming that the damper is mounted in a reasonable point on the ob end of the wishbone.

I'll dig out my old spreadsheets on the weekend if I have time and plug so numbers in for different types of car.

[Paul Aspden]

wow looking at some of the pics glad ive got old school live axle rear end

Well that's ok as long as the trailing arm mounts are in good shape........these do need checking regularly

[BLiNK Motorsport]

Are you talking front lower outboard? I'm still not sure what position you're talking about? That would be a poor position to use them in due to damper bending loads on the wishbone, but I haven't come across a Westfield with then in that position. Shear loads still aren't the culprit though.

If you're talking inboard then there are very little shear or bending forces on the rod ends even with high braking loads. They are actually very well suited to that application. Edit: Should probably note that I'm assuming that the damper is mounted in a reasonable point on the ob end of the wishbone.

I'll dig out my old spreadsheets on the weekend if I have time and plug so numbers in for different types of car.

 

 

No desire to argue.

 

 

Still trying to think of any advantages though?

[phelpsa]

No arguing required, just qualification of your generalisation just a couple of questions for you.

1) Which position of the rod end is the one that you are saying experiences excessive shear loading for the spec of rod end?

2) What examples do you have on a Westfield or similar of the design being ineffective? I've not come across a failure on the front end of a Westfield at all (not claiming that there havent been!).

I would suggest from the reports in the thread (and others) that the application of the rod ends in the outboard rear could be considered marginal.

It should be noted that I'm not in any way suggesting that you should choose to design a car suspension system from scratch using rod ends in bending, that would be a wasted opportunity, but given the set of constraints laid out by most kit car chassis (particularly tolerance wise) and upright options (lack of built in camber adjustment), the design certainly has its pros. Hence its popularity throughout amateur motorsport.

[DamperMan]

No desire to argue.

Still trying to think of any advantages though?

Mmm. nothing other ultra low friction, no unwanted flex with the added benefit of multiaxis Artication!!!! That surely beats much higher friction+ stiction of poly or nylon bushes. Also unwanted Locking under extreme load of High stiction surfaces . . That's my impartial un blinkered view I sell neither!