Broken Rear Hub

[ejsprint30]

While greasing my tripods, I noticed 2 of the 6 main hub bolt heads were broken off and floating inside the tripod housing drive hub. The other 4 were a bit loose, but not terrible. I ended up replacing all 6. When I did this, I replaced 1 at a time.

After an SCCA regional, I re-inspected and found one of the new bolts was broken again, and the others were loose. I removed them all, and the drive hub was loose, so I removed it. While cleaning, I noticed what looked like cracks in the bolt section of the rotor hub. The attached picture tells the rest of the story. The broken race is the one the drive hub presses into.

Just curious if this has happened to anyone else, and what the possible causes might be. Don't know if I inherited this from previous owner, or if it happened on my watch. Car is a 2012 Elan DP08

ej

[Rick Kirchner]

Are those mil-spec (probably NAS) bolts? If so, they have a big radius under the head and need a washer with a matching radius. having said that, I can't believe the aluminum would be able to notch those and facilitate a failure.

I can't tell about the one on top, but the one on the bottom is a classic notch failure.

Aren't these usually metric? I know 8mm and 5/16 are pretty close, but I wonder if there's enough play to get things sloshing back and forth - I'm thinking that the bolts went first and the rest is damage from the assembly ratcheting back and forth on the remains.

[David Ferguson]

I've had this happen on my Van Diemen Sports 2000. Had to have Elan make new hubs from drawings. First pass looked beautiful, but it was the front hub. Second attempt got the correct part.

[tlracer]

What tightening sequence was used on the bolts?

[ejsprint30]

The 12 points were 5/16 and had washers on both ends. I replaced with 8mm cap screws supplied by Primus. My gut tells me that the 12 points were original, as the nylon abutment O.D. had to be slightly turned down to fit within the new cap screws.

I have always snuck up on final torque with an opposite pattern, not just one right after the other. I can't speak to the ones that broke.

I have been struggling a bit this year with the new bias tires with the rear end never seeming to feel planted despite adjustments...wondering if this has contributed? Another divisional in a few weeks and I'll know the answer to this.

Appreciate the feedback

ej

[problemchild]

Appreciate the feedback

ej

EJ
Stating the obvious, but with a 15 year old ex-pro race car, it is impossible to do a proper forensic study. How many times was this car bounced off walls? Moving forward with new parts from credible vendors seems like your only option.

[Rick Kirchner]

The 12 points were 5/16 and had washers on both ends.

Rice makes a great point - but check those washers - NAAS bolts have specific washers. The VD hubs have a specific thick washer. If the ID radius doesn't match that of under the head, there's your culprit.

The 8mm bolts and washers are the same for the late model cars vs the mid-year cars, and i have mine apart right now. The washers have a slight champfer on the ID of one side, and very little on the other, and a lot of mine are pretty rough. Unfortunately I don't have a 5/16 NAS to compare the radii, but especially if the washers ever got flipped, I think there's your culprit.

[DaveW]

...I have been struggling a bit this year with the new bias tires with the rear end never seeming to feel planted despite adjustments...wondering if this has contributed? Another divisional in a few weeks and I'll know the answer to this.

Appreciate the feedback

ej

Was the car not planted at any time with the radials? You have said it was pretty good. I would think that if this issue was there then, it would have also made the car uncomfortable with the radials. So I'm guessing that the issue could have started back then, but has quickly escalated to its current state (a few cracks turning into what you have now) since your change to the bias tires.

Incipient failures often go un-noticed until they have progressed to a state that causes issues.

[ejsprint30]

Last year on radials with less experience, I felt more comfortable and was faster. I do suspect that you are right...I think the problem has gotten worse with use.

I will check the original bolts and washers and see if this could have been the issue.

[S Lathrop]

Last year on radials with less experience, I felt more comfortable and was faster. I do suspect that you are right...I think the problem has gotten worse with use.

I will check the original bolts and washers and see if this could have been the issue.

Where on you car are these parts you are having trouble with?

Also have you checked the diameter of the hubs. Because this is an on going problem, my bet is that you are getting a lot of movement of the hub assembly and it is the movement that is braking the bolts.

Bottom line, you may have to replace the hubs and possibly have the upright bored and sleeved. Your issue is not the torque as much as it is that the parts do not mate properly and there is a lot of movement between the parts because the matting surfaces have worn.

[Bob Wright]

This is more common than you think with the late model Van Diemen hubs. There's a reason Primus Parts carries lots of bolt kits. The bolts need to be checked on a regular basis and they will shear if loose. If the hub bolt holes become even a little elongated, you will continue to break bolts and maybe have a catastrophic failure out on the track. We have found cracks in the hubs that will eventually lead to your picture above.

Curbs are not the hub's friend (yea, nor rod ends or suspension or shocks either). At the runoffs last fall we had 4 failures in the space of two days in three cars; go figure. We got really good at rebuilding uprights and Primus made some sales. My guess is it was just circumstance they all failed at the same time, but probably there were slowly developing issues during the season. All three cars saw a lot of action during the year.

A personal note on the radial vs the bias tire- The radial feels more stable and wears better (more consistent heat cycles). I really haven't seen any large performance difference- maybe a little faster on the bias tire but not much. We're still tuning the car with both shocks and camber settings.

[ejsprint30]

I am sure the main bearing and hubs were original to the car (they are all that green color). When I removed the remains of the main bearing from the upright, I had to put it in the oven, as it was still a nice interference fit. The intact bearing race is still tight on the broken rotor hub shaft below the break.

I bought a new hub and bearing from Brad. Bearing (out of the freezer) pressed perfectly into the upright with a press. New rotor hub pressed nicely into the bearing. New bolts drew the tripod drive hub into the bearing race. As a machinist, my opinion is that at least the new parts mated with a perfect interference fit with my existing upright and tripod drive hub.

Rice might be accurate in thinking an off track hit may have done the damage in its past history. If the bearing remains weren't still tight on the broken hub or loose in the upright, I might agree with you. This is a right rear btw.

I have decided to replace the left rear bearing and rotor hub as well...a little shout out to Brad and Jon for the quick service and quality parts.

ej

[Bob Wright]

warning! The bearing should drop into the hub when done properly- freeze the bearing, upright in an oven to ~400 deg. If you used a press, be very careful you didn't crack the upright. How much pressure did you use?

[ejsprint30]

Thanks for the input Bob...this will be on my "check after every race" list.

ej

[ejsprint30]

It dropped in about halfway, and slight pressure with an arbor brought it home

[Bob Wright]

You should be fine. I sometimes have to give the bearing a slight tap to seat it.

I watched someone a few years ago who pressed the bearings in without heat and cold- both rear uprights failed before he got to the end of pit lane...

[ejsprint30]

I think I would have heard it or seen it...