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Rod ends in bending

Posted by Thomas Kimsey 
Morison
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Re: Rod ends in bending
September 19, 2014 11:47PM
Could the weak points be intentional? Essentially a fuse for suspension damage so they know INSTANTLY where to look, what to change and how to fix it rather than spending precious minutes figuring out what is bent/broken?
Remember that these suspensions will get full tear-downs between events, if not between days of these events.



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Re: Rod ends in bending
September 19, 2014 11:54PM
Billy, to answer your first question:
The drive and braking forces (in reference to the picture) are -> and <- for an upright/knuckle. In a live axle rear end the drive and braking forces are for and aft usually. So in the picture above the braking and drive forces are trying to push the ball out of the Heim joint. In a live axle setup, the forces are going into the ring of the Heim (much better).
Another thing to note is that the strut mounts to the top, and take all of the vertical suspension loads. So the orientation of the heims is only for suspension articulation. This can be solved in other ways though.

Is this design going to fail? Probably not, those rod ends are huge and way overkill. They do also offer a quick way to adjust camber and changes are easy for when things get bumped and bent. So it's probably just fine and works great, but in the Motorsports engineering world, that's is a big design no-no.
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Re: Rod ends in bending
September 20, 2014 12:31AM
Ah, I see, I never considered the ball's location in the joint. (insert ball jokes here)

Also, I'm Timm lol grinning smiley



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Re: Rod ends in bending
September 20, 2014 07:03AM
Quote
Morison
Could the weak points be intentional?

I think so, as this would appear to be the failure point (multiple times over the years) with one of the guys hitting something or someone hitting them, and then having the rear wheel flopping around.

By far the most common failure I've seen for srtusa while racing in grc. I assume it is intentional so they aren't ripping out important stuff.

I also haven't seen it be an issue at any of the events lately? But I might not be remembering correctly.
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Re: Rod ends in bending
September 20, 2014 04:17PM
Yes, it's that is a compromised design to use rod ends like that. But, everything in a car, particularly in a competition car, is a compromise. To allow them other benefits, like ease of repair, compact design, or reduction of the number of parts, they used a rod end. But to make up for it, they used a massive rod end that should have enough capacity even when used the "wrong" way. My guess is that if you start bending parts in that design, other parts will fail sooner, or at a similar time than the rod end.

I faced the same nay-sayers when I competed in my Lotus7. I gave up some sunsprung weight to make it easy. Yet, it was one of the most successful and easy to adjust front ends on a car and despite being abused for years, it never failed, or even wore out or bent a rod end.





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Re: Rod ends in bending
September 20, 2014 04:35PM
cool push-rod suspension! gots more pics?
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Re: Rod ends in bending
September 20, 2014 06:22PM
Take a look at some Midgets sometime... you will see suspension links bolted to the frame in single shear with a 7/16" bolt...

I figure, they figure that it is strong enough but if they get into a tussle that will damage it, then they are already out of the race, but there's probably another race that night so at least it is simple and fast to repair.



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Re: Rod ends in bending
September 20, 2014 07:35PM
Goddamn, I hate the whole "rod end in bending" bitching. I especially hear this from people who did Formula SAE in college and got judges bitching at them about it. It's not theoretically ideal, everyone knows it's not theoretically ideal, but you know what? Looking through my photos directory I've seen it on F-1 cars (from when they were still running steel suspension arms) Formula Atlantic cars, CanAm cars, IMSA GTP cars, LeMans Prototypes... and that is just stuff that was purpose built from the ground up as a professional level race car. So I'm really inclined to say "welp, just size the things properly, check 'em, and move on with your life" you know?
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Re: Rod ends in bending
September 21, 2014 01:35PM
Quote
mekilljoydammit
Looking through my photos directory I've seen it on F-1 cars (from when they were still running steel suspension arms) Formula Atlantic cars, CanAm cars, IMSA GTP cars, LeMans Prototypes... and that is just stuff that was purpose built from the ground up as a professional level race car. So I'm really inclined to say "welp, just size the things properly, check 'em, and move on with your life" you know?
Then look better at those fotos, my old 94 VanDieman Formula Ford car had similar setup to that, but at least it had 2 lower control arms vs one top and 1 bottom. It puts completely different forces on the rods. Also those cars have no power and are not meant to be bumping ugly's with one another. Even the lotus7 picture above has the rods on the kingpin. And its mounted horizontally, which it wont force the ball out of the cage.

A very nice example and still simple that they could have done is what Ford did when they upgraded the GrA Cosworth to WRC. 2 Lateral links and a trailing arm.

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Re: Rod ends in bending
September 21, 2014 01:51PM
Looks like there is a lot more hight adjustability on the rear link vs. the front link. Playing with roll steer?



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Re: Rod ends in bending
September 21, 2014 07:16PM
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Cosworth
Then look better at those fotos, my old 94 VanDieman Formula Ford car had similar setup to that, but at least it had 2 lower control arms vs one top and 1 bottom. It puts completely different forces on the rods. Also those cars have no power and are not meant to be bumping ugly's with one another. Even the lotus7 picture above has the rods on the kingpin. And its mounted horizontally, which it wont force the ball out of the cage.

A very nice example and still simple that they could have done is what Ford did when they upgraded the GrA Cosworth to WRC. 2 Lateral links and a trailing arm.


I have. For that matter, I have a VanDieman kicking around too. I'm not saying it's universal, just that I've seen a lot of examples of stuff setup with rod ends in bending at the end of control arms. Shadow and McLaren CanAm cars, Nissan GTP cars on some links, Riley and Scott MkIII... and when the bottom rod end is horizontal, how is hitting a bump not going to try to force the ball out of the cage?

Again. Not ideal. Never claimed it. But do you truly, honestly think it's worth all the hand-wringing people do over it?
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Re: Rod ends in bending
September 21, 2014 08:04PM
I don't see any green paint on the SRTC car, so clearly they just listened to what everyone says, followed the blind, didn't actually think about anything, and didn't bother reaching out to any relatively local suspension ex-spurts.



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Re: Rod ends in bending
September 21, 2014 08:12PM
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mekilljoydammit
...and when the bottom rod end is horizontal, how is hitting a bump not going to try to force the ball out of the cage?
Because there's no resistance forces... unless that specific arm has a shock attached to it, but even then its not the end of the world. And in my comment I didnt say it was all terrible.

Care to show those pics of the Riley MK3, I dont remember all GrandAm's but have worked with them for a long time, and even though DP's are a pile of garbage, I still think the Riley's had A arms, not the single arms like the Subaru.

Quote

Again. Not ideal. Never claimed it. But do you truly, honestly think it's worth all the hand-wringing people do over it?
No of course not, but its what this forum is all about, and some people do learn some stuff from this blabbering.
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Thomas Kimsey
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Re: Rod ends in bending
September 21, 2014 09:54PM
Anyone have a picture of current suspension on Higgin's car? Seems to make sense to add a failure point in a rallycross car where there is service 100 meters away and speed of repair is critical. Do they incorporate the same "failure" point in the stage cars?
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Re: Rod ends in bending
September 21, 2014 11:15PM
Quote
alkun
cool push-rod suspension! gots more pics?

Lots o' pics: http://s683.photobucket.com/user/aaa-111/library/lotus7?sort=2&page=1

More rod ends! OMG, the world is coming to an end!





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