Early steel control arm as a mechanical fuse
03-05-2013, 11:07 PM
#31
Rainman
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Engineering precision is never a bad thing, you're right. But if you're tinkering around for your own purposes, and have a good idea of what you're asking the part to do, you can still come up with something that should work in an array of situations.
03-06-2013, 05:12 AM
#32
RL Community Team
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His main focus is art, but he does things like custom gates, chandeliers, etc to pay the rent. He also does commercial welding for pay, but he considers himself a blacksmith, not a welder.
03-06-2013, 04:41 PM
#33
Burning Brakes
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Provided you copied an existing design and used good materials with strong welds, i don't see the harm in making your own arms. OTOH i remember one of the cheaper arm manufacturers mentioning that they weren't getting a whole lot of markup after materials costs, so it's hard to say.
03-06-2013, 09:11 PM
#34
And... having FEA doesn't mean you know how to use it and interpret the results worth a damn. 
03-10-2013, 01:59 PM
#35
Three Wheelin'
Thread Starter
They wont bend from regular use, but fairly certain there have been known fatique failures of stock steel arms. Which is why the reinforcements are often done. I would expect some 944 Spec/Cup guys to chime in on that.
Eric Steinel (well known shop in Cleveland) tells a good story of when he was on Fred Baker's team running the 944 Turbo prototypes in the Playboy/Escort 24 hr endurance races at Nelson Ledges and Mid Ohio in '83 and '84. One of the cars broke a steel front control arm and the Porsche engineers from Germany referred to the steel arms as "VW Scheisse", which is where the development of the aluminum arms came from.
There are well documented alum arm failures going back to the late 80's. But most are the weaker '86 arms, and most of those were using aftermarket front sway bars (weltmeister), which had the solid mounting bushings on the control arm (they used to be aluminum bushings, not sure if they still are). Problem with that, and with any of the solid sway bar mounting parts, the control arms and the sway bar pivot in different planes. If there is no rubber to allow flex, binding occurs in the suspension range of motion, and the stresses at the mounting points become huge. So most of the aluminum arm fractures were at the swaybar mounting points (this is a separate issue from the balljoint failures).
Late arm failures of the casting itself, are relatively unheard of if using factory swaybars (including the 968 M030 bar) and retaining the rubber mounts.
Eric Steinel (well known shop in Cleveland) tells a good story of when he was on Fred Baker's team running the 944 Turbo prototypes in the Playboy/Escort 24 hr endurance races at Nelson Ledges and Mid Ohio in '83 and '84. One of the cars broke a steel front control arm and the Porsche engineers from Germany referred to the steel arms as "VW Scheisse", which is where the development of the aluminum arms came from.
There are well documented alum arm failures going back to the late 80's. But most are the weaker '86 arms, and most of those were using aftermarket front sway bars (weltmeister), which had the solid mounting bushings on the control arm (they used to be aluminum bushings, not sure if they still are). Problem with that, and with any of the solid sway bar mounting parts, the control arms and the sway bar pivot in different planes. If there is no rubber to allow flex, binding occurs in the suspension range of motion, and the stresses at the mounting points become huge. So most of the aluminum arm fractures were at the swaybar mounting points (this is a separate issue from the balljoint failures).
Late arm failures of the casting itself, are relatively unheard of if using factory swaybars (including the 968 M030 bar) and retaining the rubber mounts.
I will play with reinforcing the arms as I have a welder at the shop and it should be relatively easy to do.
I also plan on replacing the arms and ball joints every season as a preventative measure since both are cheap and easy to install.
03-10-2013, 02:19 PM
#36
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With that said, I think there should be some pics and discussion that show up in threads here on rennlist. I do recall seeing pics of the steel fatiguing at the front mounting bushing. And I also recall seeing pics of additional steel welded around that area and also boxing of the edges.
03-10-2013, 03:25 PM
#37
Burning Brakes
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I've decided that i'm going to replace my wheels with some made out of plastic. That way if i hit a big pot hole they'll just shatter and save the rest of my car!
03-10-2013, 08:21 PM
#38
Burning Brakes
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03-11-2013, 12:53 AM
#39
Three Wheelin'
Thread Starter
I am not suggesting that a car should be loaded with intentionally weak parts that act as mechanical fuses, but in the specific case of a 944 front control arm there is something to be said for that notion.
If I had a factory aluminum arm, it likely would have snapped in half and left me stuck on the track, and I would be faced with a tow home and and expensive replacement.
If I had one of the stronger aftermarket arms, it likely still would have bent (maybe not as much), and I'd be faced with replacing a very expensive part, and I would not have made it home. Even if the arm was incredibly strong and had not bent, the vertical impact could have possibly transferred to something like the spindle or strut and caused more expensive damage.
I always carry a spare steel arm and ball joint, and was able to change them at the track between sessions and keep running.
Now if I had a late offset car or if I had a full race car that was radically lowered and had big sticky tires in front, I would likely use one of the stronger aftermarket geometry correcting arms.
03-11-2013, 09:03 AM
#40
Rennlist Member
If your preventive maintenance plan is to replace the arms every year, I would worry much less about reinforcing them, as fatigue failures would be very unlikely w/ that limited use. And, w/o post weld heat treatment or annealing, there are possible risks of embrittlement or weakening w/in the heat affected zones from welding alloy or heat treated steels (not sure if the steel arms are made from just mild steel).
With that said, I think there should be some pics and discussion that show up in threads here on rennlist. I do recall seeing pics of the steel fatiguing at the front mounting bushing. And I also recall seeing pics of additional steel welded around that area and also boxing of the edges.
With that said, I think there should be some pics and discussion that show up in threads here on rennlist. I do recall seeing pics of the steel fatiguing at the front mounting bushing. And I also recall seeing pics of additional steel welded around that area and also boxing of the edges.
Droops, really glad you started this thread. I've been pondering the same thing and since my control arms are getting a bit long in the tooth it seems like a logical alternative I've always wanted to pursue.
For a point of comparison, what is your suspension/tire package? You mentioned you weren't mega-stiff and running slicks so I'm guessing my setup is similar to yours.
Thanks,
Collin
03-12-2013, 02:29 AM
#41
Three Wheelin'
Thread Starter
Agree with this sentiment; I would probably go every two years. The steel arms can handle it (remember, many spec racers are still running the ORIGINAL arms) and by replacing every year you negate much (if not all) if the cost benefit you get from using the steelies over the aluminum ones.
Droops, really glad you started this thread. I've been pondering the same thing and since my control arms are getting a bit long in the tooth it seems like a logical alternative I've always wanted to pursue.
For a point of comparison, what is your suspension/tire package? You mentioned you weren't mega-stiff and running slicks so I'm guessing my setup is similar to yours.
Thanks,
Collin
Droops, really glad you started this thread. I've been pondering the same thing and since my control arms are getting a bit long in the tooth it seems like a logical alternative I've always wanted to pursue.
For a point of comparison, what is your suspension/tire package? You mentioned you weren't mega-stiff and running slicks so I'm guessing my setup is similar to yours.
Thanks,
Collin
I run 16x8 wheels all around with 225/50-16 tires, either Toyo RA1s or Dunlop Direzza ZIIs or Hankook RS3s.
