Julian Allen

Fabcar arms are 4130 weldments. With Dave's background I have no doubt these are properly heat treated after being TIG'd together.
Check out Fabcar Products and notice that OG Racing is the sole source for new arms.
I think it's great that new pieces are being developed, but on such a highly critical part, engineering input is critical. I couldn't see putting any arm on my car that has not been tested.
Just my thoughts, YMMV.

TurboTim

Just to clarify a little about our arms.After some thought, we will not be making these out of chrmoe-moly.They will be made out of mild steel instead.This is really the only change that we will make to the arms.

As far as the construction of the arms goes......The 3/4" heim joint is a 4130 piece.It uses stainless steel misalignment spacers with a 17mm 4130 machined spindle pin(excuse me for calling it a machined bolt). We can not use a 19mm spindle pin because there would not be enough articulation in the heim.The heim joint itself is teflon lined and yes the teflon will wear down over time and becasue of heat but it is designed to be replaced every so often.It is easy and relatively cheap to change it out.The heim joint itself will not break.It is a super heavy duty unit and is actually a bit overkill for this application.These are the same heims that are used on just about any pre-runner truck you see in the BAJA 1000.They are the same heims used in Indy cars(actually Indy cars only use a 1/2" heim). These heims are used extensively in Nascar, Formula 1, etc.......

The bung for the heim is a 4130 peice that is spot welded to the arm and then TIG welded around the perimeter.This will insure that is does not flex or move under sever load.We will be using 1 1/4" .120 wall ODM tubing instead of the chrome-moly.The reason we are not using the moly is because of the possibility of fracturing and complete failure.It is not becasue of the need for heat treatment since we have a heat treating facility a block away.The mild steel tubing will not break but will actually bend in an impact which seems like a much better alternative to total failure at a weld joint.The way the arms are bent insures that these will be strong pieces.Anybody that knows about building suspension components or roll cages can look at the arms and see that they are top notch.The bend near the castor block is notched into the straight tube for the castor mount which provides a very strong and secure weld joint that will not fail.There is also a straight tube notched into the bend to provide another strong and secure weld joint and to add stifness to the bend.Basically these arms are indestuctable.To prove it we will have them x-rayed and then tested for tensile strength at a facility near us.We were going to have a list member do it but I want to make sure I get all the data asap, to give people a sense of confidence in our product.The swaybar mounts have sleeves welded into them to prevent the tube from crushing when you apply force or tighten the sway bars down. The bushings and spacers we will be using (and supplying) with the arms will proved a secure mouting point for the sway bar when torqued down properly.

I hope I helped people realize that alot of time and engineering went into these arms. They are not just some tubes that were welded together for the sake of having a replacement a-arm to sell.This is the real deal and a much more affordable arm them some of the other alternatives out there.Take care.

MachSchnell

Good to hear Tim, I loved the look of the original protos, but the chromoly was the one X factor that made me nervous...going to mild steel, now I'll definitely be working through you...just gotta figure out when one of these suckers will fail (I actually think one is close, PO or PPO replaced drivers' side and not pass, so it's getting close to end of life)...will there be a price break for mild vs the original chromoly design?

mumzer

ugh.... where to begin....the part is a good example of why motor builders should stay away from chassis parts. The heim joint should not be threaded outside the tube. (it really shouldnt be loaded in bending either, but thats a nit. The arms should have a machined bung at the ball joint end and be fitted with a cicrclipped monoball instead of a heim joint...no threads=no stress rises.

the section changes from the welded tube to the scallopped throat of the heim would be bad enough without the threads extending out there, but this looks like a multiple choice stress riser quiz from a first year ME text.


The load path for the braking forces is not straight....the bend at the caster block is a guaranteed fatigue failure. I dont know when, becuase the tubing is heavy enough for a stadium truck, but the design isnt right. How much do these things weigh compared to stock?


KEEP BRAKE REACTION LINKS IN LINE WITH THE APPLIED FORCE. this tube should be in pure tension and compressiion.


PS....

Is the ball joint pin threaded with a cut or rolled thread?

i have no doubt the welds are adequate...craftsmanship is not the issue....you need to have a chassis engineer look at these....no way a pro signed off on them as is.

TurboTim

[QUOTE]Originally posted by mumzer
[B]ugh.... where to begin....the part is a good example of why motor builders should stay away from chassis parts.



I knew this would be said eventually.I knew you guys wouldnt let me down;^) The fact of the matter is that a chasis and suspension guy designed these. I just did the welding.You can take a look at some of his work on our project car that appeared in Popular Science Magazine in March of 2003.


The heim joint should not be threaded outside the tube. (it really shouldnt be loaded in bending either, but thats a nit. The arms should have a machined bung at the ball joint end and be fitted with a cicrclipped monoball instead of a heim joint...no threads=no stress rises.



Sure we could use a Monoball but that is defeating the purpose.The downside to using a monoball is you will have no camber adjustments like you do with the heim.


the section changes from the welded tube to the scallopped throat of the heim would be bad enough without the threads extending out there, but this looks like a multiple choice stress riser quiz from a first year ME text.


Why dont you fill out the engineering work sheet from Aurora so that you can see that the heim we are using is up to the task and then some.If that still is not enough for you then you can argue with them. Afterall they are one of the foremost bearing makers that supplies bearings to numerous companies in the aerospace industry.


The load path for the braking forces is not straight....the bend at the caster block is a guaranteed fatigue failure. I dont know when, becuase the tubing is heavy enough for a stadium truck, but the design isnt right. How much do these things weigh compared to stock?



I never said the bend wasnt a fatigue point. However, I dont think it will ever be a problem because of the diameter and wall thickness of the tubing.They are similiar in weight to a stock arm.



KEEP BRAKE REACTION LINKS IN LINE WITH THE APPLIED FORCE. this tube should be in pure tension and compressiion.


I guess Porsche should have their aluminum arms re-cast to remove the bend near the caster block too? Good grief!

mumzer

well...you started this...


what is his experience and training?


um...right...first of all, for the adjustment to be of any practical utility at all, the arm needs to be made of two pieces with left and right hand heims connected by a tie rod...as it is now, the only way to adjust camber is to remove the spindle pin from the spindle, adjust the heim, reassemble the car, check camber and repeat. And all this is to be done on the alignment rack or in the race paddock with a trammel bar? Impractical. As a racer, the stock eccentric system is bad enough....this is just a redundantly slow way to make a camber change. Two word solution...camber plate.

If you are after an increase in track width (which surely can't hurt) make the arm longer! Porsche did this on their own in 87-89 cars to reduce camber change.

as a practical matter I think camber adjustments at the heim are a invitation for user induced disasters at your local alignment shop...how are you going to prevent ham fisted doofus alignment guys from backing the heim joint out so far that there is no longer adequate thread engagement?

Even assuming the end users manage this risk properly and keep the shank threaded to an adequate depth, understand as Cervelli points out that the section change at the jam nut is a area of localized stress concentration. The thread roots (which as i pointed out in my prior message shouldnt even be there at all) are loaded in bending both in the vertical axis and in the plane parallel to the ground under braking. (which incidentally is how race car a-arms tend to fail) Allowing the end user to change the location of this stress concentration by lengthening the shank of the heim takes away your ability as the engineer to manage the loads that the parts actually see.

In service, you have dissimlar metals in a threaded joint under a car exposed to all manner of notch creation risks (ie stone impacts and corrosion pits from brake dust) and you have given the end user the ability to increase the loads on the notch sensitive region by this adjustment.


The size of the rod end and the quality of the components is not the point...its application is...rod ends are intended to be just that...rod ends....they are intended to be fitted into the end of tubes loaded in tension and compression to allow angular misalignment around the bore axis and rotation. they should be used in bolted joints in double shear with the race (body) bearing (thats where the name comes from) the loads. DATA sheets from NMB, Aurora fk etc will all give you numbers that support the use of a heim joint in this application in static loadings...but you cant predict the dynamic loadings, and they are not constant...further, the data sheets assume that the rod end is going to be used as a rod end...not axially loaded through the bore to retain a bolted joint.


so...you acknowledge that its a fatigue point. Let me get this straight...you want your customers to rely on "i dont think it will be a problem" as they go into the braking zone for turn one at Fontana after four seasons of racing because you use heavy tubing?

Come on.

um...no...the cast arm doesn't have

1. any welded joints in it, or;
2. the attendant heat affected zones around the aformentioned welded joints.

(your welds are ceratinly pretty...no issues there)

the stock arm also resists loadings in a completely different way....the cast webbing of the stock arm carries the loads the same way a shear plane of a torsionally loaded bulhead does...to flex the stock arm you are not simply bending a piece of steel tubing...you must displace this shear plane.

Further, the webbing spreads the load over the entire chassis-end of the arm and is sculpted in such a way that there are few localized areas of higher stress...where they do exist, Porsche has thoughtfully provided a deeper section or larger radius to cope with it.

Finally...aluminum is a relatively inelastic material...in service, the stock part is designed not to flex (which is a good thing since aluminum does this poorly and tends to fail catastrophically instead) whereas mild steel (all steels actally) are relatively flexible and elastic....this elasticity is what makes mild such a great material for chassis parts...if the part does encounter a load that is too great for the material, it SHOULD yield and bend without rupture or catastrophic failure.

My problem with your part is that by creating areas of localized stress that are coincident with welded joints you have designed in spots where elasticity of the tubing is compromised to begin with.

Making the thing out of heavier tubing because you suspect a fatigue risk just isnt the right answer.

ps...feel free to respond with whatever tech you want...im not interested in pissing in your cornflakes, but this part could use some refinement before you go to market. Never bring a knife to a gunfight.

Chris Cervelli

Well, Mumzer covered the engineering issues better than I could. But I want to clarify one thing engineering-wise before I move on. Hopefully this is in language that everybody can understand.


For going around the skid pad your design is more or less ok. The cornering force pushes on the shank of the rod end in compression. That size shank can hold that force, probably even ten times that force. On the unloaded side of the car, there will be a little tension load, which is nothing to that size rod end.

The problem is when you hit the brakes. Now the shank of the rod end is trying to bend back (the wheel tries to move back in the wheel well). Yes, the rod end is very easily going to handle this load this first time, and the second time....

After a few thousand brake applications (or hard bumps, which push the rod end the same way) a crack is going to appear at the thread root right next to the jam nut. Because the shank of the rod end is huge it won't break right away, but it is only a matter of time. Failure there is GUARANTEED!

It doesn't have anything to do with the quality of the rod end, Tim's fab skills, the kind of steel used, or anything else.

There really isn't any way to do this part right without redesigning the spindle as well.

Now, the reason I came on here and said anything in the first place:

I am in the business of building Porsche race cars. I try to build the fastest and safest cars I can. Some customers can afford to do this my way, and some cannot (including myself, actually).

Whenever somebody comes up with a whizbang solution to a problem, it undermines my ability to sell what is the best solution to the problem. My only defense against this is to try to educate the customer. That is the only reason I ever use Rennlist.

It is really, really tough to outengineer Porsche. After all they are one of the top engineer firms in the world. Sometime we 'improve' on things by taking the cost-effectiveness part out of the engineering equation. Charlie arms are a good example of this.

I am pretty sure that I have never seen anyone come up with a part that did the job better and cheaper than Porsche's part. It seems unlikely that this kind of thing is going to come from the aftermarket. Nobody has the resources to do the R & D to make that happen.


We haven't even discussed the pin yet. That is even more tricky than the rod end issue.

GaryK

I'm not qualified to comment on these arms from an engineering perspective, but as a DE enthusiast who is still running the stock arms, I am not inclined to replace factory components with unproven aftermarket ones in such a critical area. And though I have been searching for a less expensive alternative to Charlie or Fabcar arms, I am not convinced there are inherent problems with the stock arms. I am also concerned about running arms with exposed joints on the street.

What I would really like to know, and maybe this thread is not the right place to ask, is has anyone experienced a catastrophic failure of the stock control arms? If so, could frequent inspections have detected anything? Were the failures limited to the ball joints? Were the cars lowered excessively to the point the ball joint pin was binding? Did other modifications contribute to the failure?

And finally, any negative experiences with rebuilt stock arms used on the track?

MachSchnell

*WHOOOOOOOOOOOOSHHHHHHHH*
[the sound of my enthusiasm being sucked out like so much hot air from a salesman confronted with superior knowledge]

MachSchnell

However, having said that, in regards to the challenge of successfully re-engineering Porsche parts for less money and making them superior, I would have to cite things like the stock afm, the stock rubber piping, the stock hardlines, the stock wastegate, and the entire electrical system as things that can or have been upgraded by many people (including IceShark and Danno to name just a couple) for what amounts to significantly less money than what Porsche replacements cost (even factoring in dealer cost minus a bit for the actual Porsche production costs)...while I suppose the argument could be made for cheaper costs during production being significantly lower than in aftermarket Porsche parts sales, that becomes a moot point when considering the OUTRAGEOUS markups presented by Porsche (small volume notwithstanding, many aftermarket companies are beating the bejesus out of Porsche in pricing and are likely facing smaller overall volume than the big P for comparable if not superior parts)...

TurboTim

What is with all the assumptions? Who said we were going to release an untested product? The fact is that we plan on doing some extensive testing before anything is sold. Ofcourse the best test is the race track and thats where a couple of the prototype sets are going.

As far as the arms go.........You can make an adjustment to the camber before you put the spindle pin in.You should not rely soley on the arm for camber adjustments since it would be very difficult to do. However, it will allow you a little more adjustment then your camber plates will permit. We also intend to plate the center of the arms and add gussets to the castor block mount and the crossmember mount. This will completely eliminate any fatigue point from the bend in the arm.We always want to have the best products out there and we will do whatever it takes to make that happen.We feel that this will be one of the better arms on the market for a reasonable cost and we intend to prove it.

Peckster

I think it's great people are developing new products for our cars.

In the area of vintage bikes, there's an amazing collection of brit shops making parts. The only part you can't buy new for a Norton 850 today is the crankcases, and most of the new parts are as good as or better than the originals.

It takes a bit of development time, but if people are willing to support aftermarket suppliers, that's a good thing for all of us.

Steve Lavigne

Agreed.

Also, I think it is great that we have people like Mumzer and Cervelli who can bring the TECH.

M758

Yep,
As mechanical engineeer myself I agree with Mumzer & Cervelli.

The loading of rod end is completly wrong and will result in failure. I could draw diagam of the loading on these control arms underbraking and that would show this clearly. There is also an agrument about the stiffness resulting from the bend near the caster block. The loading will tend to undo the existing bend. This will also probably cause a crack to develop at weld there. Also there is no buckling support for the center of the angled bar. This will not cause failure (probably) but mean less stiffness underbraking.

Even these Blazek arms don't use the rod ends right.



Here the load is taken ok at the ball joint (still and issue with neckdown to square tube & a welded joint), but the inner fwd rod end will also be loading bending although to a much lesser degree.

The rear rod end is fine since the caster block is not designed to take a thrust load there will be miniumal to no bending there.

Here too how ever there will be bending about both welds for the angled part. They can be strenghtened greatly with additon of large web fillets.

Remember also that a weld joint is typically a weak spot due the basic nature of welds. Always try to put peak stress points away from welds and especially keep fatigue type cyclic loading away from them.

As they stand however I'd still rather stay with stock.

Russ Murphy

"As they stand however I'd still rather stay with stock. "

That being the case, how do the stock units with rebuilt ball joints fare on the track (i'm assuming your cars are significantly lowered from stock ride height). Do you do anything to combat the dropped control arm angle (messed up roll center, ball joint near the end of it's travel,etc.)?