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What causes the RR axles to shear off?
On more than one occassion during my studies on wingless sprints I've seen horrific crashes resulting from the RR axle shearing off in the turns w/o contact or seemingly rough conditions. Why does this happen? Is it a birdcage issue and possibly the bearing froze up and cut thru the axle? Or is it a lack of inspection? What is the rule of thumb on RR axle life span? 2 of the worst I've seen were both at PAS for the 360 Oval Nationals during qualifying. Like Markus Niemelä's crash... I talked with someone familiar with the crew that said it was all new equipment.
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That's a good question, that is hard to answer. It could be a combination of things, the car may not make contact just before the wreck, but something could have happened multiple nights ago that could have damaged the axle and it just finally gave out. I would say lightweight axles, excessive re-straightening, and not noticing cracks would be the main cause. And many times all three. Personally I don't believe lightweight axles provide much advantage. Not sure how much weight is saved, but I couldn't believe it is much. There is always the argument that lw axles have less rotating weight, but the radius of which that weight is rotating at is less than an inch so that advantage is slim to none. I believe the risk-reward of LW axles is not worth it, especially when you see how hard sprints and midgets are run.
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Not all axles are built the same and there are manufactures recommendations for the amount of laps that they should be run. Many don't follow those guidelines.
Other times a birdcage bearing may have an issue and it cuts the axle causing it to break. Sometimes it is damage from contact. |
Re: What causes the RR axles to shear off?
How often did the steel ones break "back in the day"? (like 70s & 80s?). If the answer is "not too often" then they should be mandatory. & if the thought against that is they didn't have the load put on them "back them"; then restrict the motors SOME & have smaller RR tires...
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Preventive maintenance can reduce the chances of this. Removing,cleaning and servicing birdcages nightly only takes a few minutes. Having rearends checked and replacing bearings and seals periodically is fairly inexpensive considering the alternative. Same with front hubs and spindles, a front bearing freezing up can cause big issues.
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Re: What causes the RR axles to shear off?
Thankyou, I appreciate everything I can learn as I work towards my move into sprints. That poor guy I mentioned has permanent brain damage.
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Re: What causes the RR axles to shear off?
Folks,
Any root cause analysis of this problem must start with the location of the break. These axles have several shoulders machined onto them to allow for bearing locations, etc. As anyone familiar with stress concentrations can tell us, square corners at these shoulder locations are stress risers which invite crack development and further propagation (growth). I would assume the axle manufacturers have a designed radius at these locations but maybe some aren't using a generous-enough radius as to minimize the concentrated stress. Another possibility is that the axle radius is proper but the steel spacer used between the bearing and shoulder may not have a radius in the interface location that is more generous than the axle. This would result in a cutting action into the softest member of the interface, the axle, which would further result in crack initiation. Compounding this, the cantilever forces acting on the axle at the birdcage interfaces along with any heating/cooling cycle embrittlement just add to the possibility of a crack. And, just to help things along here, we reduce the axle wall thickness so that the critical flaw size necessary for crack initiation becomes smaller. Any additional high forces resulting from contact pretty much guarantee issues. Inspection and proper birdcage maintenance along with insuring that stress riser points are minimized are a few of the things I do in an effort to maximize axle integrity. I am also not a fan of the ultralight axles for the reasons mentioned above but I understand the physics of keeping rotating weight as low as possible. I just have not been convinced that the reward is worth the risk. Tim Simmons |
Re: What causes the RR axles to shear off?
I would say the amount of races on the axle is one cause as things do not last forever and the other is a lot of people striaghen bent axles which also leads to failure.
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Re: What causes the RR axles to shear off?
It doesn't take much, catch a rock between the axle and a chassis tube, even a tommy will get the axle into a chassis rail, and put a ring on it, and that's all it takes I'd say any axle with the anidization removal ring on it, has little time left, the rear axle goes thru a lot of gyrations, torque, bending, flexing, twisting, probably more than any piece on the car, so it's a sure hi maint item. Bob
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Over the years of watching dad take car of race cars and servicing parts for others througj the 70s and 80s and listening to him talk..... There were several cars that broke the old tapered steel axles and the splined ones.
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And most of them either broke in the keyway, or at the end of the taper. A high stress area. Bob
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Yep
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I think it has more to do with heat. I've never broke one on rr always the lr . The heat from the header and brake will turn the axle to a copper color. I think its to much for the axle and pulls some of the strength out of it. We run turn outs on the headers to keep the heat to a minimum.
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Re: What causes the RR axles to shear off?
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Re: What causes the RR axles to shear off?
I cut a lot of sheet aluminum and since I can't afford a $1200 stomp shear I clamp the sheet in my break, drag a box cutter blade across it a couple times and bend it. Some of the cheaper softer alloys need to be bent back and forth 2-3 times to separate. The more expensive harder alloys such as 6061 and 7075 always snap clean on the first bend...usually at 45 degrees or less. It amazes me how easy the stuff separates once its continuity has been interrupted by a bit of a scratch.
I have always felt like one of the best indicators of a potential axle problem is an interruption in the anodize coloring. The reddish color on a winters, the purple on a Frankland (I think??) and the black on DMI axles are awesome because you can easily see the contrast of the raw aluminum color underneath. I can't remember which brand made gold anodize axles but you had to look a bit harder. I don't worry so much when the color starts wearing off the splines where the wheels typically live as it is a very smooth, gradual change...almost like it's being polished off from constantly removing and moving wheels. When I see any sort of lateral scratch it gets frequent attention. When you see a scratch/groove running around the circumference, usually close to a birdcage, especially when they are an eighth inch wide and you can feel how deep they are with your finger, it's a good idea to find a replacement, at least for long enough to have yours inspected. |
Driver riding the brake really puts the heat in the axle, that is the first thing I ck when my car is back in the pit, not by feel but with the temperature meter, it radiates out to the wheel and tire, sometimes really hot, but on a non wing, the RR gets the most bending action. Bob
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If you go to any welding or a good body shop supply company they sell a spray that looks for cracks in aluminum I have it in my body shops to check all aluminum reinforcements you would be surprised how many we find cracked that you can't see any damage to
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Overfilling the case with grease, no room for expansion thus seal pressure on axle surface , cutting into axle creating the shear points, Kreitz sells axle saver sleeves for this very easy fix
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Re: What causes the RR axles to shear off?
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Re: What causes the RR axles to shear off?
In my 35 plus years of spectating sprint car races, 25 plus races a year in the midwest, I think I can count on one hand the number of RR axle failures that I have seen. Is this a bigger problem elsewhere in the country? Or is my mind just gone? :10:
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No, I don't consider it a game changer, it is not something to worry about, the guy who started this thread sounded like it was for him, plenty of other things that can happen before you need to worry about an axle breaking off, it happens, but a lot of other things do also, and more often. Bob
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We rebuild between 150 to 200 Midget and sprint rear ends a year. We see it in our shop about 8 times a year. Of course there are others that happen in Indiana and elsewhere that we don't know about.
A solid inspection and maintenance program and respecting the manufactures recommendations goes a long way. |
Re: What causes the RR axles to shear off?
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Tim Simmons |
One possible cause can be stagger, especially on pavement, the poor old axle winds up and unwinds a lot every lap, just a thought. Bob
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Re: What causes the RR axles to shear off?
I apologize if this is a non-issue. I'm new to sprints and saw that horrific video I mentioned and was concerned it was more common, but sounds like just being vigilant with maintenance will take care of it. Thanks. :6:
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Re: What causes the RR axles to shear off?
Keep asking questions; it's good to hear from someone who's new to open wheel racing and excited to learn...
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Follow up question, how often should the rear end be rebuilt?
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