SLC 24 Howard Jones

Howard Jones

Another bit of drama.

Saturday. Cold at 8am, 50f. Track very cold and covered with grit and dust. COTA ground the track nice and perfectly flat for the Moto GP the following weekend a few days before we got there. I never can understand how profession racers can't race on a perfectly fine track with even the slightest bumps. In any case they tried to clean the track but basically failed before we used the track for the first time on Saturday morning warmup session.

The grinding machine made nice little uniform pebbles about a 1/16" in diameter........billions of them! The back straight was so bad you couldn't see through the dust and particles as car ahead of you went into the cloud. Fuck! I have been in motocross races that were worse but not many!

Anyway, got off the track after a couple of laps. They went back out with the jet blowers and tried again and by the time I got back out on track in the first session two other groups had run and it was a hour latter so things were better, Not good but better. Fairly clean line but the off line was still bad.

So......setup changes. Still same shock settings and springs. added another 1/2 degree of camber, so now I have -1.5 F and -2 R. Added another degree of AOA to rear wing, now at +5. Tire pressure cold 28 all. I like that setting and the car seams to like it.

Had a hard time putting heat in the car, tires and brake, because of the procession nature of the track conditions, but all went well. Water temp still rock solid at programed temp 181F, oil temp at 210-120F, GRBX temp at 180-190F. Tires only pumped up 1-2 PSI.

2nd session, air temp up to about 65F, all setting same as 1st session, brakes began to warm up and tires gained 3psi. Car ran good. Carb better with reset floats about 1/16" above center of float window in Proform carb. That's just a bit high. Changed power valve from 4.5 to 7.0. I have been tuning carb and now have 12-13 "s of vac @ 1100Rpm idle.

Engine now better and runs clean down to about 1500rpms. Much less stumble off corners. If kept above 2500 runs pretty good. I can pull new ZO6 in straight line on back stretch. He asked me later in the day what I had in it and "old SBC with a carb" put a frown in his face.

3rd session, Gave wife ride............she did good lasted whole session. I ran at about 90% of my comfortable pace and she said she had fun. No data too slow.

4th session, Ran it hard, Could almost stay with GT3 Porsche drive pretty well. We had a bit of fun together lapping traffic. Car was perfect on temps still, tires went up from 28 hot to 33 hot. Brakes are good but will want to try a better pad with higher friction value. Still on BP10 wilwoods.

Sunday, 1st session good fun like 4th on Saturday. I'm starting to think that it may be time to move up a run group. Really big speed difference at end of straights with some cars. Morning cool, track cleaner and line is quite a bit wider. I think they worked on it over night. All data like yesterday, temps still good, tires warm up and brakes good when heated.

Drama session....................second lap spun hard between 15 and 16. Tank slapped twice and then went around a couple of times. Brake peddle on floor with little to no brakes. That was because the pads on the L rear were being knocked back by the upright/caliper waving around on the rotor/wheel. Got it stopped all good no other damage. Track guys were very careful and asked if I could steer it after report from corner worker said I had a suspension failure. Slow tow and back at trailer found problem.

The post mortem. The broken one is the Drivers side. The complete one is the other side but if you look closely something is going on in between the bearing barrel and the wheel flange. Race coming out? The last one is me and the co piolet.


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Howard, did the driveshaft actually break where it pass's thru the splined hub or lose the nut off end, do you have pic of it, just trying to work out order of failure here. FWIW we run the same hub on TVR ( 15 years and counting ). Only 8" rubber though.
Let me see if I have this right from discussion with Howard.
The stub axle did not break, the wheel mount flange broke off where it necks down to pass through the upright mount flange / bearing housing. So in the end what is shown (picture 2) is the 'broke from' inner splined 'tube' that is captured in center of the upright mount flange and the 'broke off' wheel flange. Normally those two parts would be one piece.
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Howard Jones

First, As best I could, I tried to get a torque reading when I took it all apart. The retaining nuts did not loosen as far as I could tell. Both the primary and the "lock" nut were tight to at least 100 ft/lbs and the cotter pin was in place. I am 90% sure the nut did not loosen.

If the nut had come off the wheel would have also as the stub shaft was the only thing holding the complete assembly together.

Rob is correct. The wheel stud flange separated from the bearing body through the 90 degree radius in the shoulder that separates the two.

In second and third pictures the bearing housing is one on the left laying flat on its mating surface (to the upright) and the wheel flange is standing on edge to the right.

The point of failure is in the center of the of the two pieces. You can see the splined section is broken into two pieces. One half left in the wheel flange and the other half left in the bearing section.

When looked at closely with a 10X loupe the point of failure appears to have begun on the outer diameter (wall) and progressed to the inter diameter of the splined surface as if it had been stretched. Just as would be expected. You can best see this as the small section of raised jagged material that was the last to tear off in the foreground of the splined hole in the lower bearing flange of pic 1.

Remember, this is a c4 corvette street car part that was never intended to be subjected to these forces. I am running Hoosier R6s 345.30.19s on 12 inch wheels at the rear with about 500hp. The place where this happened is just about the highest lateral g loaded corner on the track. I was adding power at about a 100mph as I left T17 and going towards the exit of this three turn complex (taken as 1 big one) T18. This loads the outside rear to just about as much as can be done on this track.

Broke right where it should have and just like it ought too.


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Is it possible the bearing inner race caused a slight inclusion on the splined hub, setting up a stress, that led to a crack and failure?

Can the bearing be pressed on to the hub “too far”? Ie, into the radius of the hub?

At a minimum it’d be a good idea to have the replacement hub NDT for cracks, both open to the surface and internally.
Is it possible the bearing inner race caused a slight inclusion on the splined hub, setting up a stress, that led to a crack and failure?

Can the bearing be pressed on to the hub “too far”? Ie, into the radius of the hub?

At a minimum it’d be a good idea to have the replacement hub NDT for cracks, both open to the surface and internally.

Very likely given the bearing looks like it has a smaller radius on inner dia which could become critical load point on that splined shaft part of hub where it enters bearing bore when things start flexing under load .. If the bearing radius is small it can ride up on the shaft radius as it is pressed on.... Howard you might want to get Randy to move/copy these posts - 921 thru 928 to other thread so folk can relate to pics etc.
Rob, Ed, Howard, yes got that, have an idea but it will have to wait til weekend to get it down on paper with drawing.
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Very interesting -- do you have a pic of the stub shaft ?
--and can you identify the origin of manufacture of the hub bearing ?
( I know the Chinese bearings are prone to failure)

Howard Jones

Stub shaft.

I am currently looking into a source for stub shafts that will accommodate C5 C6 X Tracker hubs. If I can find or make them then this track use only issue at the rear will have been pretty much solved. If you want to follow that process then it is on the "SLC/hubs race worthy" thread.


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Howard Jones

Pictures at COTA


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Howard Jones

So I upgraded my front bearing hubs to the XTracker design. I got some adapters to fit them to the upright from Strano parts.

Strano Performance Parts - Featured Product

and used these bearings. They are the same spec as the "X Tracker" part.

Search Results for skf br930544 - Free Shipping on Orders Over $99 at Summit Racing

Here are some pictures of the needed minor modification to the uprights, adapter/bearing assembly and the final install.

Important note: The Strano adapter 4 mounting holes (to upright) are threaded 10 X 1.5 mm.


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Howard Jones

So now I began thinking about the rears. After a lot of different possible paths to take including making a all new upright I have settled on re-drilling my existing upright to accept the new C7 33 spline bearing. After a lot of discussion with many people I have faith in, including a member of the 25 hour blue SLC team that has run over a full season including the 25 hours of thunder hill NASA race without a problem, I believe they are plenty strong enough for my use

The issue is the new C7 bearing will not directly bolt on the upright because the bolt circle pattern is larger than the original C4 bearing. A friend gave me this idea to re clock and drill for the C7 bearing on the existing upright. Here is how I did it on mine with a very cheap bench top drill press. All you really need is a 3.5mm drill and a 12mm cap screw counter bore cutting tool. I searched for my tooling on line and ordered the cheapest tooling I could find as I was only really doing this once and the material I was working with was Aluminum. You don't need a $75 cobalt tool for that.

If you have the original un sculped uprights like mine then this is a pretty straight forward fix. If your uprights are then newer type with the sculped sides then check out the thread "SLC hubs/ race worthy for all the info on this topic. So measure many times, setup carefully, and take your time.

When the new 33 spline stub shafts arrive I'll post more on them.


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Howard Jones

The stub shaft arrived from The drive Shaft Shop and I trial fitted them in the bearing/ upright assembly. Looks like a perfect fit. These are the strongest corvette stub shafts they make and they say they are good for 800HP each. That ought to be enough!

They come with new CV joint bolts and the required big ass nut. Not really a bad price at $ 316.99 each.

Next I will put the upright assembly back on the car and see where I am with driveshaft length. I am also looking into a spherical bearing mod to replace the rear ball joints. We will see about that as information is forthcoming.


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Howard Jones

I have received a set of new ball joints from Summit Racing, They seam to be a bit stiff but after I cleaned them out and re greased with new grease they seamed to free up quite a bit. I really like red line products so I use them. I am sure other manufactures products work well also. Just stay away from the 3 dollar stuff. Look for a temp range of at least 400F. This stuff I use is good up to 500F. Why? the heat generated by the rotors is very high and the ball joint is very close to that heat source. When I removed the original ball joints the rubber boots were hard and cracked from the heat and the grease inside looked over heated to me as it was dry and seamed to have been overheated. I will check these after every track weekend from now on.

On to the fitting, The K6136 is about 3/16s taller than the OEM piece. I will make a spacer to fit between the mounting plate and the A arm. This should place it in the original position. I also ordered a 7 degree reamer to clean up the stud hole in the plate on the bottom of the upright. The original placement left about 3/32s inch of threads in the hole. I would like to have the entire stud bearing surface in the hole with maybe a single thread left to be pulled into place as the bolt is tightened. More on this as I final fit the ball joints in the assembly.

I have contacted RCR about the availability of the spherical bearing retrofit piece and would be interested in at least having a look to see if I want to re fit them into my car. More on that if I get any info from RCR.

If anybody has pictures of these pieces please post them.