That's assuming that all failures on the bar have been from people running two bolts. As a fact, that is not true. Try again. Or just ignore the advice, your choice.

-Andy

 

I've had jzr's old bar for a few months now, and I pulled it apart to re-lube a couple weeks ago. jzr had been using two bolts, so I left them in place when I first installed the bar. I found that the second bolt (non-comptech) in the #1 position was broken. The threaded portion of the bolt had broken off, so about 3/4 of the bolt remained. I believe this was the cause of a pretty significant clunk that developed shortly after I installed the bar. The comptech bolt in the #5 position (full-stiff) remained unbroken, but the welds on the inner bar at both ends of the guide tube are completely cracked. The tube wiggles slightly inside the bar, but it's still tight enough that it won't fall out. I reinstalled the bar with a stronger second bolt, and ran an event this past weekend. Everything seems to be fine, but do you think that the crack will lead to an enlargment of the #5 hole in the inner bar or some other type of failure?

 

The bolt that comes with the Comptech bar is a pretty high grade that can take quite a bit of shear. The second bolt that jzr used was probably not as strong.

Yes, I think the cracks on the #5 hole welds are an indicator of future problems. When I looked at my bar 2 months ago it was pretty much the same as yours. Now I have a 3-piece bar.

-Andy

 

If the bar is marginal on 1 bolt, it will be even more marginal on 2. Rather than being so quick to be critical , try to think it through.

What happens structurally when you put 2 bolts in the bar?

Think it through.

Y2kMyWay

 

Ack, I'm sorry John! Guess that Home Depot special wasn't up to the task. When I took the bar apart after removing it from my car the bolt looked okay...at least a replacement should be cheap.

Mr. Y2kMyWay, we have thought this bar's design through quite a bit, which is why we used two bolts. Since you seem to be convinced of what the second bolt will do, why don't you enlighten us?
Using any part in a race environment is going to cause a failure sooner or later, this bar is no exception. Invincibility is not a trait of the Mugen bar, our factory axles, or even our A-arm brackets for that matter.

Doug Hayashi had an interesting experience with his bar - if one is interested try contacting him for his story...

 

I'm not trying to be Mr. Smart@ss. I'm just looking at this in simple structural terms. A swaybar will not just twist indefinitely. It will twist up until the point that the torsional load exceeds the shear strength of the material, at which point it will break. The longer the length of bar, the more twisting load it will carry before it shears. In otherwords, a short piece of bar is going to shear at a smaller rotational load than a long piece of bar.

If two bolts are used, the length of the center tube between the bolts becomes a large diameter thick-wall tube. For a given load, that section between the bolts will have considerably less torsional flexure (rotational twist) than the tube sections to the outside of the bolts. That means the tube sections outside the bolts must now carry a larger torsional flexure (twist more over a shorter length), i.e., the shear rate applied to the tube wall thickness increases. Unless the tube wall thickness of the inner small diameter tube is considerably thicker than the outer large diameter tube, it will shear first. That seems to be the case.

Does that mean using 2 bolts will shear the bar and 1 bolt will not? No, it all depends on what holes the bolt(s) is(are) in and how much shear load the tubes are carrying. Torsionial cycle fatigue also enters into the equation. It's entirely possible to use 1 bolt and have the bar shear, but it's more likely to happen at a stiff position then a soft one, or in a bar that sees considerably more torsional load cycles than one that is rarely driven on, or one that sees very extreme rotational loads than one that does not.

The bottom line is it sounds like Comptech needs to reassess the diameter and wall thicknesses on the bars to increase the overall shear strength at some sacrifice to weight, though I'm confident that when the bar was originally designed it was intended that only one bolt be used. In their defense, they do state that this is an all-out racing bar, which generally means that long term durability is sacrificed for maximum lightness. That also means that it was never designed for quiet operation; the orginal starting point of this thread, just maximum performance.

Y2kMyWay

 

I'm having trouble understanding why this is the case. If you isolate that short section of the bar, why would it twist more when attached to a stiffer section, when the load is the same?

 

As an example, it's the difference between twisting 10 degrees over a 20" long bar section, or twisting 10 degrees over a 15" long bar section. The amount of twist does not change, the shear load becomes more concentrated over a shorter length.

The actual bar rate will not change when using two bolts (assuming you put the 2nd bolt to the inside). The bolt located on the farthest end of the outer large diameter tube sets the rate. The shear rate will equalize across the bar length. The second bolt to the inside effectively shortens the length of the outer tube over which twist can occur. This causes the shear rate in the inner small-diameter tube to increase considerably over using just one bolt.

S2kMyWay

 

[QUOTE]Originally posted by S2KMyWay
[B]The actual bar rate will not change when using two bolts (assuming you put the 2nd bolt to the inside).

 

oops, you caught me. That was a mistake that I didn't think through

hey, I never claimed to be perfect , especially while I'm trying to do paying work.

S2kMyWay