TonyG

Anybody race with the SFR front control arms?

Any problems? I'd like to hear from anybody that has put some serious track time on them.

Any thoughts on the design?


Thanks,



TonyG

MAGK944

I would also like to know, they look very well made and the ball joints seem easy as pie to replace. The one thing that made me hesitate in buying them was the front bushing which doesn't look like a stock size to me.

adrial

The obvious (to any engineer) design thought is that the rod end is put into some serious bending when you jump on the brakes.

The stress concentration factor from the threads won't be an issue statically, but for fatigue it will be VERY significant.

TonyG

I was more concerned with the use of the rod end as a ball joint.

The guys at Kokeln are doing it the same way, which doesn't make it necessarily right, which based on their fabrication & development experience, would suggest that it's acceptable at a minimum.

I read the big thread on these arms where those of us here with mechanical engineering backgrounds chimed in. It all made sense to me. But why the heck would Vision/Koleln do it that way?


TonyG

adrial

The use of a rod end as a balljoint is OK IMHO, thats the way that Fabcar does it and those have been on the market forever with few issues. I have been meaning to build an FEM model w/full contact to show that, but I haven't had a chance. I think the idea is that the joint converts bending in the pin (smaller diameter) into reactions between the pin and the mounting face of the spherical bearing (bigger diameter). Torquing up the joint correctly is definitely important.

Going back to the rod end... A rod end in bending isn't the devil, it just has to be designed big enough to take the loads.

Using a press fit spherical bearing instead of a rod end is a lighter design, but both can work. The question is, is the rod end really big enough if you include the Kt (stress concentration) and do a full fatigue analysis.

Also the balljoints may be easy as pie to replace, but you are going to have to make bigger alignment adjustments after replacing one as compared to the Fabcar design because of the thread in rod end. If you're at a race and you just smashed up the car and you need to fix it ASAP and don't have time to do a good alignment ... you're be better off with the Fabcars or similar.

MAGK944

I must agree now that I look at it, those ball joints in both the SPR and the Kokeln arms do not look like they would take a consistent side load for very long.

Kokeln:


Charlie, everything else is just compromise:


Mike

2bridges

without a doubt - rod ends ARE NOT designed for use in this manner. I too see many do it, Lindsey, SFR, kokeln etc. I suppose if the strength of the rod end is high enough even improper use, is acceptable by most.

I don't care for it, and for my money I would go racer's edge design

jasonlp

+1 on racer's edge
+1 on Charlie arm's

Does Charlie still make arms anymore?

MAGK944

Charlie arms are still available in both early and late offset from Stable Enargies

Racers Edge just come in late offset:



I prefer the Charlie myself and not wanting to change the offset, I have used them on my 86. Why do you prefer the Racers Edge, any advantages?

Kool

How can you tell by looking? I wonder if any of these manufacturers have the data available to show exactly what their arms will take. And for how long. Is there a recommended interval to replace the spherical ball joints? How often do racers replace them.

jasonlp

Racers Edge do come in a early offset I just picked up a pair from paragon. As for preference. I couldn't find any Charlie arm's so i bought Racers Edge they are both fantastic products from what i hear of other people's experience's. Lindsey ones failed for fellow Montrealer Chris Green during a race, also i hear fitment issues with oem sway

MAGK944

The Kokeln & SPR arms have spherical bearing rods at the end . These types of rod ends are designed to support a swivelling motion at the bearing and not a bending force, so they are suseptible to shearing along the threaded portion of the rod during hard braking. Now, that is what you can see as a weak point, but it may not be so. You are right, we need to know how strong these areas are and what stress testing was done to be certain.

Techno Duck

The other advantage of the Charlie Arms is from the looks of it, it is machined out of a solid piece of billet. Most other arms are a few different pieces welded together, which add failure points to the structure. Assuming everything was welded correctly, thats not really a big deal. But it is a point one should consider.

TonyG

Adrial,

>>>The use of a rod end as a balljoint is OK IMHO, thats the way that Fabcar does it and those have been on the market forever with few issues.<<<

That's not what it looks like to me. Here's a picture of the Fabcar arm below:



TonyG

adrial

Sorry, meant to say "the use of a spherical bearing" as a balljoint is OK IMHO.

The rod end in bending is definitely a concern and given two a-arm designs of the same price, I would chose the spherical bearing version. The rod end should immediately make the a-arm cheaper than a similar version with a spherical bearing.