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After 20 years of racing RX7's I finally had my first spindle failure of a front strut this last Saturday. And also had my second on Sunday. First the left then the right. Quite a surprise. Thankfully the final failures occurred at slow speed corners T3 and T4 at Willow Springs and the wheels did not totally depart. It would have been much less fun at T8, T9 or T2.
So looking at the failures, it appears that they occurred at the step transition from the parallel part of the spindle where the outer bearing sits to the tapered section. Seems like a bad design with formation of potential stress risers. The materials engineer in our group said that the failures both looked like they had been in the making for a while. My plan is to dye penetrant test frequently (probably every time I repack the bearings). Also considering removing the step so that there is a smooth transition from parallel to tapered.
Although I had never experienced this failure before, I know a couple of people that have. I assume there are plenty of others. Has anyone done anything differently than what I am planning to do that might have better results?
Can you post up some pictures? I would like to know if they fractured in the area that is under the RE Speed big brake bearing spacer that is pressed on my spindles
The failure I am familiar with was also occurring over a long period of time. The owner of the car kept seeing unexplained static camber gain on the side that failed. His also finally gave way in a slow turn.
I think your inspection strategy is sound.
A local racer machined his spindles and fitted Speedway Engineering Mini-Stock Spindles and Hubs (these are Pinto Spindle pins). Mini Stock Hubs and Front Spindle Pin
Last edited by mustanghammer; 05-07-21 at 10:03 PM.
Bending snouts is 100% of why I did the FC conversion ten-twelve years ago. I ran out of strut housings to wreck and there was an FC parts car sitting there.
I never experienced total failure, but I would notice a greater and greater need for bearing re-adjustment, and an eventual inability to get things set up right.
I wonder if a bearing spacer (like what is seen on the pinion bearings) would strengthen things, because it would be putting the main portion of the spindle in tension rather than bending, assuming that you properly torqued the spindle nut.
Here are some pics. They definitely broke at the step transition from parallel to tapered. You can feel the remains of the step on both pieces.
I like the idea of the Pinto spindle and hubs, but I think I will wait on that mod, as it requires a lot of other modifications (brake system) to accommodate it. Considering that these spindles had 20 years of racing on them, for the time being I will go with smoothing out the step transition and testing on a regular basis. I will keep the broken spindle struts though and maybe play around with the pinto parts, in case I have more problems before I retire from racing.
I think the bearing spacer on the pinion gear is only to set the pinion insertion depth properly. I don't think it provides much additional stiffness. Thinking about it, it seems that the hub itself should transfer a lot of the load back to the inner bearing in the manner you were thinking of for the pinion bearing spacer. FYI, I am a chemical engineer not a mechanical engineer, so I probably have no idea what I am talking about! :-)
Carl
Right Spindle/Strut Left Spindle/Strut Departed piece of Left Spindle
The spacer in the pinion bearings is there so the pinion nut has something to tighten against, nothing more nothing less. Some people (including me, in my Ford 9") replace the crush spacer with a fixed shim stack, because that is repeatable to get the preload correct if you are disassembling/reassembling a lot.
These ARE available for RX-7s from some company in Australia whose name I've forgotten. They're also available from Speedway for Mustang II spindles. They are ostensibly so you can lock the bearings into a certain preload, but I see a bigger advantage. The spindle, as delivered on cars by Mazda, is loaded in bending. The end of the spindle is levered up and down by side loads pushing the outer bearing up and down. If you had a spacer between the two inner bearing races, and torqued the nut to some decently high torque, what happens is for the spindle to be able to bend up or down, it will have to grow significantly in length, because of the larger diameter of the spacer. The spindle is now loaded in tension, not bending.
Just some food for thought. Miatas have smaller diameter spindles, but they have ball bearings and a spindle nut that gets torqued to some high value, and they don't have the spindle issues that RX- cars had.
Yes. Those. They look interesting, but they have to be "re-adjusted" every time the hub is replaced, which is every time the rotor is replaced with the standard RX7 rotor/hub. With a separate hub, like a "real" race car, that seems like they could be helpful. I looked up the spindle bearing spacers available for Pinto spindles and they seem to all be adjustable so that they can be adjusted for proper bearing pre-set and seem to be mostly marketed on the basis of making it easier to get proper pre-set so that the hub is more free-turning. No mention of stiffening the spindle. The MSF site does market theirs as providing additional spindle stiffness. They do not appear to be adjustable and look they require shims to get the right bearing pre-set.
Having been a Spec Miata driver for a while, the Miata has waaaay more spindle problems than the RX7. It maybe that the miata problem is more related to the use of ball bearings instead of tapered roller bearings, but broken spindles are pretty common. Mazda actually sells a conversion kit to replace the ball bearings with a tapered roller bearing set-up. It costs somewhere between $600 and $1000, so it didn't really take off.
I may have contributed to my spindle failures by running my hubs/rotors too loose. They nearly always have had play after a race weekend.
Interesting about the Miata spindles. I've never heard of a failure, but have heard of plenty of hub face failures, which is the reason for the aftermarket hubs. Most recently one of my friends tore the center out of a hub while autocrossing on 200tw tires.
That looseness may be part cause but also part symptom of impending failure, or at least the stresses that lead to failure.
i've never heard of a spindle failure in the Miata's either, hubs are a ~6 month wear item though, at best. at worst you bought the really cheap ones and the hub face comes off
Ooops! Very sorry about my mis-information on Spec Miata hubs. I have seen (and heard of) the wheels come off and assumed spindle failure, because that was what I had seen with RX7's. A quick search of Mazdaracers.com quickly convinced me you guys are right and that it was the hubs that were failing and not the spindles. And I agree the front bearings have a pretty short life.
I no longer have my spare miata spindles so was only able to look at pictures on-line and dimensions on RockAuto. It actually looks like the Miata spindle is more robust for a couple of reasons. First, there is not a sharp diameter transition that is in bending stress. The bending stress would appear to all be on a straight shaft that is apparently about 1.1" in diameter. This compares to the RX7 that has a sharp transition behind the outer bearing that will experience a bending moment. The sharp transition is a pretty small diameter change before the tapering to the inner bearing straight section. The sharp transition is where both of my spindles failed and three from friends race cars. Compared to the Miata's 1.1" diameter straight spindle, the RX7 starts at 0.8" at the outer bearing and tapers up to about 1.1".
For now, I am hoping that smoothing out the transition at the beginning of the taper and testing on a regular basis will prevent anymore on track failures. If that doesn't work, I will look at either Pinto spindles or bearing spacers, along with changing to separate hub and rotor. Actually I may look closer at the separate hub/rotor situation and see if I can come up with a bolt on rotor that works with the stock brake system. That could be pretty easy to retrofit.
peejay, when you were having problems with struts a while ago, was it the strut tube that was bending? Very early in my racing, I had the tube bend where it inserts into the spindle block. I tried to jump a small dry creek bed during an off in my novice race and didn't clear :-) Ended up with 7-8 degrees negative camber. I could imagine that the tube gradually bending over time would result in ever increasing negative camber. It is harder to imagine the spindle separating enough to change camber without breaking. Maybe though. I added stiffening at the bottom of the tube and have not had anymore problems with the strut tube bending.
We have seen after market strut tubes (Ground Control) bend/fail were it inserts into the spindle. After that we added a brace from the top of the spindle to the tube on all the ones that we built after that. No issues since that.....I hope I didn't just jinx myself
Similar but my bracing doesn't go as far up the tube.
Here is a side by side of my strut and an OE strut. It appears that the RE Speed spacer is over the area were your spindle broke.
Mine actually both broke at the location identified in the attached picture. I was driving in one that had the same adapter spindle that you have and broke in the same place as these last two. That was prompted by some idiot in an E30 BMW side-swiping me at a ChumpCar race at Las Vegas. It took about 20 laps before the spindle finally failed, but almost guaranteed was prompted by the hit. If you look at the stock spindle (and maybe your modified one) you will see that there is a sharp step, which we are pretty sure acts as a stress riser, making that spot prone to cracking and failure. There is of course a slight radius since the cutting tool cannot be infinitely sharp. In theory at least, smoothing that out should help.
That's interesting because I always thought they broke closer to the strut tube. I think my friend's failure was closer to the thick part of the spindle but I have slept and drank since then.
Where yours failed is where the spindle is bigger on the 84-85 spindle, right? And I see what you are talking about regarding a stress riser.
Regarding the spindle spacers, I like the idea. I saw a set at the local circle track speed shop. They were touting the way they reduce friction and make the hub free wheel over any strength benefits. But they probably need the strength too because I think the Modified guys are running Pinto/Mustang II spindles more than the Chevy stuff.
Regarding the spindle spacers, I like the idea. I saw a set at the local circle track speed shop. They were touting the way they reduce friction and make the hub free wheel over any strength benefits. But they probably need the strength too because I think the Modified guys are running Pinto/Mustang II spindles more than the Chevy stuff.
Isn't there a Chevy application that actually works with Mustang II spindles? Some combination of X rotor and Y bearing equals 5 on 4.75 with those spindles...
Regarding my spindle issues. I wasn't noticing an alignment change. I would notice that I'd lose the ability to keep the wheel bearings adjusted, and replacing the strut housing would solve it. I could tighten the nut to the point that the bearings were tensioned (BAD idea with tapered bearings) and they would still be loose.
Ten seasons later with the same FC spindles, no problems. Although I really do want to get them Magnafluxed just to be safe. I don't trailer, I drive 2-3 hours to events, up to 16 hours each way. I can get 22-ish MPG with the car, I can't justify the expense of buying a truck and a trailer to get 15mpg when the car has license plates and a trailer hitch for my tire trailer.
Yeah they make 5 lug rotors in various patterns to fit the Pinto spindles that are made for IMCA style modifieds. Typical of that racing, all of it is dirt cheap.
What no truck, no trailer!. You're missing out on the second and third order racing money pit.
Any update on your situation? We might be fighting spindle flex causing excessive brake pad knock back so I'm curious if you found a cure. Oh, will you be at ThunderHill this weekend for LuckyDog?
I smoothed out the problem transition on my current strut/spindles. I have not had any problems since. I also have installed shorter Bilstein struts so that I do not bottom out if I go off course and hit a big bump. That was kind of a problem for a while and may have contributed to the broken spindles.
I did have problems with brake pad knock back a couple of months ago. It turned out all the rubber seals/bushings on the calipers (front and rear) were either worn out or missing. That seemed to allow the calipers to wobble around a bit. After replacing them with new from Atkins, the braking problem has not recurred. Although I did have some caliper alignment issues recently on the right rear. That is another story tho that might or might not be at an end. I will find out this weekend. I was planning on going to Thunderhill, but was having difficulty finding a full team, so I decided to save all the driving for another time. Instead I am going to Buttonwillow for an SCCA regional, that also includes a 2 hour enduro. I will see if I can squeeze 2 hours out of the stock '82 tank. I think it is iffy, but.......
Have fun at Thunderhill. One of my other race buddies (Mike) will be up there with his 2nd gen RX7 (white and purple). I will let him know to look you up. Look forward to meeting you and your team one of these days.
05-07-21, 06:00 PM
