Problem with axle shaft / rear wheel bearings 308

All very true. I went to the point of doing a dye penetrant crack test on the 3 suject caps when returned to my shop. I left the crayon marks on the caps put there by the aircraft shop so to pin point the effected areas. Absolutly nothing showed up with the dye using a 10 power scope for inspection. This is an area of expertise that is truely left to the real pro's and not the self taught.

Regarding the stub axels, I tried some mods to the old English cars I used to race and found some interesting results over the years. In a TR3 I cut wedges out of the top of the axel tubes and bent both sides and welded them back together. This gave me about 1.6 degrees of rear negitive camber on a solid tube type rear end. It also required the axel to be constantly bent and over time the failure prediction of 2 race weekends proved quite accurate over a number of years. When the supply of axels got dangerously low I mounted a pair in the lathe and greatly undercut the transition area from the axel to the flange (same area we have been seeing fail on the Ferrari stubs). The new, greatly increased radius was then polished out and a spiral polish pattern put on for a final finish. With this mod the failure rate of the axels in this race car went from 2 race weekends to 5 years running before someone else had to have the car worse than myself. Last I heard he never had a failure as well.

With this knowledge in hand, any axel or stub I remove to re-bearing gets the same treatment. If you look closely at the Ferrari parts they have a very small radius in this area which leads to an obvious stress riser point. There are many areas on the Ferrari's that have far too small a radius used in their machining of the parts. Dont just wash the parts and reinstall, inspect them closely and question if the design is proper.

Dave

 

Great info.

Please clarify . . would a surface magnaflux performed by an aircarft inspection person tell you anything? Again, the one aircraft inspection fellow I chatted with gave me the impression that most cracks can be detected on the surface . . . isn't this where the crack would form on this radius that is being referred to? . . on the surface?

Sean

edit:

from the other thread . .. quote from ricrain . .. still like to know what you guys think about an aircraft guy doing surface inspection

At a track event, back when the 348 challenge series just started, I spent a good 20 minutes talking to Umberto Masoni, the (then) Technical Liaison for FNA about this problem. He told me that this problem originated back in the days of the early 308's. In the case of 308's, it was mostly, according to him (and the official FNA line), due to the fact that owners were buying aftermarket rims with incorrect offsets. This put undue stress on the 'T' intersection of the axle and hat, and resulted in "premature" failures. It was his contention that anyone racing 3X8's, especially with track or race tires, have the stub axles inspected annually. He said that magnafluxing would not typically reveal the problem, but X-raying would.

YMMV

 

David,

Would you see any cracks in the shafts on the British cars when you swapped 'em out after a couple weekends?

Thanks,

Sean

 

Somone . . Scott? . . . should model it on Solidworks or something and see where she yields . . .play with the radius and weaker materials, etc.

Like I said before, the P car parts I think are smaller and I also thought they shoulder on the flange the same way but different kind of bearing . . really want to tear that down now to compare cross sections, radius, etc.

Sean

 

I'd start with a cornering force of 1.5g's with a 3500 lb load . .. maybe worst case the entire 3500 lb applied laterally at the edge of the one rear tire . .. the radius of the tire is the moment arm.

You could add a engine tq moment in there also . .. worst case flywheel tq.

Be a starting point . . . keep increasing the load and seen when she fails.

Sean

 

Looked like a spider web with the iron dust from the Mag check. Nothing visually detectable without. Remember there is a consumer acceptable factor in play here. It involves a number of additional operations to undercut, radius and polish these areas and that equates to additional costs. Considering the abuse these stubs take I think they have a great track record but we started checking these anytime they were in hand about 10 years ago.

Dave

 

Not only do they have the glide path of an anvil, they seem to be pretty good at disintegrating to pieces on the way down. The only saving grace is that they will probably spin you up fast enough on the way down to make you pass out. That way you wont feel it when you splat.

 

Thanks . . so what are the benefits of Xray on a part like this . .. is the fracture not going to begin on the skin?

 

From my meager technical background and constant re-readings of Carroll Smith's writings, ..."All cracks start on the surface", (unless there is a major internal inclusion/flaw). from Engineering To Win.

With respect to the engineering profession,
chris

 

Knew I'd read that somewhere in the last few year Love his books . .. referred to them above in another post . . .one of them has the proper bearing layout for the upright . .. somewhat irrelevant to this problem though.

I think the reason for the Xray is to know it was a good part from the factory . .. that one that broke in the other threads had a void.

Sean

 

Martin, please clarify about what they should be warned about doing/not doing.

 

I have a set of Mondial uprights and shafts on the shelf. I will pull the stubs this AM and measure to see if we can identify the differances and if parts might be used in one anothers applications.

Martin, I am not sure I would let the scoring on your shaft keep you from using the car if it passes Mag testing and the effected area is properly polished out. I am far more concerned about the transition area at the flange as that has such a small radius and is the area of the highest stress. A standard drive train simply doesnt have enough torque available to concern me with the damage you have. Had the scoring been on the outer bearing seat it would be a different story as this is the area of concern to me.

Obviously photos only tell part of the story and you have the parts in hand and can see the damage far better than any of us. Sticky tires, non stock offsets, tracking the car..... all change the story considerably.

Dave

 

As told to me by the pro's in the field, yes almost all cracks start on the surface. The pro Mag guys can identify forging and casting flaws before they even become cracks and depending on the current levels they use in testing can identify these flaws subsurface.

The last time I used XRay was to inspect 40 year old Magnesium race wheels to see if they were OK for Parade laps / photo shoots at the track. These tests were backed up with Sonic checks and we were able to see that Jim Clarks life was in danger when the car was quite new! Voids everywhere and a miracle they lasted this long in one piece. Most of the XRay and Sonic checking I have had done is on non-ferrous material and is wonderful to see what the first serial number was on some Vintage Ferrari blocks before that was filed off and restamped. I have not used either methods on steel parts.

Dave

 

Looks like they might of bought a copy of Carrol Smith's "Prepare to Win" and looked at the illustrations/discussions on p 93-95 . .. an important feature is the use of a spacer between the bearing and flange so you can machine a bigger radius in there . . just like Ferrari did in their revision. This is a poor design . .. now I'm looking at this more my mentor would have fired me for designing a part like this when there's desktop references available . .. would expect this from some dune buggy outfit . . . looking forward to the next guy that says "they're not dummies" . . . this is the kind of part that should be bullet proof if you build F1 cars . . . the electircs/slow windows I can excuse . . . they should be embarassed . .. "fundamental design error".

After sleeping on it at least I have a little confidence in the fact that if the part is properly inspected, I "might" not be putting my life on the line . If I stay the course looks like I'll need a bunch of the axle nuts for the periodic inspections.


Sean

 

I think your being a bit harsh Sean. I would tend to side more with Dave here a bit, in over 30 years the cars have held up remarkably well as long as they are serviced reasonably. The tires and rims being used on these cars today were not available back when the cars were designed, and certainly the use of spacers and larger offsets have always been frowned upon by every manufacturer, not only Ferrari. While its certainly a bummer to have one break, I think the odds of it happening if the car is assembled correctly and the wheels and tires are reasonable, are probably about nil. The Mondials that broke stubs, and the one 348 I know of, were all, or had been run on race tracks and all had had offset rims and or spacers. And that all adds up to three cars total, two Mondials and one 348. I suppose there are others but I dont believe the sky is falling. Not yet anyway

 

This discussion has - obviously - revolved around the 308/328/Mondial. What about cars like the 288 GTO and especially the F40, which have seen profesional race action and in that environment, have been subjected to nearly twice the power and way over 1g cornering forces? I've never heard of one of their stub-axles failing; what design/layout do they use? I'm sure these teams test these components, but did they use different parts that corrected the design faults of the 308 series?

Keep in mind, the 288 GTO/F40 is a closer relative to the 308 series (oval steel tubular space-frames, steel A-Arm suspensions, cast-aluminum uprights, etc...), than the bespoke, high-tech F50/Enzo cars (carbon-fibre monocoques, I'm guessing forged arms/uprights and other exotic materials...).

 

I have 12" wide Hoosiers on the back of my '73 911 . . that car came with tiny tires from the factory . .. guess the engineers at Porsche expected their customers to track their cars . .. Ferrari engineers figured the 308 was a slow POS so there was no need to over-engineer the component . . .

Now that's harsh .

I thought Rifledriver saw more than just a couple of failures . . . . rifledriver, rifledriver, rifldriver . . . (trying the Beetlejuice thing to get him to appear gain )


Sean

P.S. I'm still completely intoxicated by the red mist but this is sobering

Edit/Addition: Had an aircraft inspection guy over yesterday . . if it wasn't for the Ortega drive tomorrow I'd be pulling 'em right now for him to check . . . be pulling tomorrow after the drive.