Aluminum rear subframe bushing collapsed?

#14

I think that’s what makes the solution work – that it’s the serrated plates. The only way it’s sliding around is if the bolt loosens enough for the teeth to disengage and jump. From what I’d read from all the threads about it, it sounded like this was still prone to failure, but only because, over time, the bolt loosened enough for the teeth to disengage.

The best solution I’d found (at least, imo) was the castle nut – which I may try if I can get ahold of them quickly enough. But when I found that I’d already welded these nuts onto the bolt heads, drilled, and installed them. Unless the force twisting these bolts is enough to break the weld on the nuts, I don’t see how this would loosen enough to allow the teeth to disengage.

Unless I’m misunderstanding what you mean by “rtab pivot point”…?

Som

#15

Certainly the primary problem with the IE 2nd gen rtab adjusters like you have is threads stripping. I don’t recall a case where someone complained that their 2nd gen IE adjusters moved yet the bolt was still very tight. So give it a try and lets see if it works.

Get some toe plates and check your toe after each event. If it moves, you’ll know it.

#16

Yes, they will move!! I trashed the outside serrated plate and installed a grade 8 nut and bolt, 1/2". One side holds great…will not move or loosen.

On the subframe bushings, they are not designed to be squeezed. They should have a tube that runs down the middle that the nut bottoms on and pulls everything into place. Without this, the bushings get deformed.

Removal…drilling a couple of holes in one bushing will certainly work very well…good idea. I did not do that and ruined mine, plus spending a couple of hours trying to remove them.

#17

Yeah, I’ll definitely get a nut on there, at the least, before reassembling everything.

Chuck, these aluminum bushings don’t have a separate inner tube – the “tube” is a part of the aluminum.

Drilled a couple holes and ended up just using a punch and a 4lb sledge hammer to tap the halves out. Took about an hour to get them out. Might’ve helped that I’d greased the sides of the bushings to help with the install.

Measured the bushings – 63mm tall. Well below the 70mm that it should have been. Hopefully it’ll be an easy conversation with AKG on Monday.

Som

#18

They have changed their design…all the ones I have seen has an inner tube for the nut to bottom on. Good luck Monday.

#19

Called and spoke with Andy at AKG. Here’s how it went:

  • The pieces aren’t designed to meet in the middle.
  • The bolt should be torqued to 20-25 ft/lbs with some Loctite.
  • He had someone else use an impact gun and destroy it too.
  • He’s out of stock right now, but then asked me to hold. He found a couple bottom halves he can send. I had drilled holes in my top halves to get the bottoms out, but fortunately, nothing that really damaged the bushing. 0.01 ounce weight savings!

He got a kick out of it and was chuckling the whole time – which was a little stressful for me at first, but then he was very helpful in terms of finding a way to get me the bottoms. Made me feel much better. They’re going to express mail them out to me.

I was super surprised to hear that they only need to be torqued down to 25 ft/lbs. Wish there were instructions included – everywhere I read had responses that ranged from 50-some ft/lbs to 103 ft/lbs. And it seems like one of the few things not covered in the Bentley. I’m not entirely sure why they wouldn’t just make them meeting and not have to worry about this. Maybe there are slight variations in subframes and it’s more important to have the flanges be flush with the subframe than having them all meet in the middle but sometimes leaving a gap under the flange?

Som

#20

I checked the Bentley and found a value of 56ftlbs but I couldn’t tell for sure if the # was for the front or the rear subframe. After poking around a bit I decided it was likely the front subframe. Then I went to the Service Manual. My copy is in electrons only and moving around it’s links is more miss than hit. I persevered tho and found an unambiguous # of 110NM (81ftlbs).

Thats a helova lot more clamping force than 20-25ftlbs. I’m not sure what AKG is thinking with that design. Sounds non-optimal.

#21

I’m with Ranger, 25 ftlbs is not enough. Buy the Condor subframe bushings…

#22

Yeah, I don’t really understand, either. He said that the factory torque values were specific to the rubber bushings. That the aluminum ones didn’t need nearly that much.

Thinking out loud here, but I’m trying to visualize the forces that are seen at those bushings. Also, I’m not an ME, so please don’t laugh if I say something stupid. :slight_smile:

The springs carry the weight of the car, so the up/down motions (z axis) of the wheel probably doesn’t really translate a whole lot to the bushings. That leaves the x-axis (side-to-side) and y-axis (accel/braking).

Looking at the acceleration (y-axis), that’s going induce a forward force on the control arm, which translates to the subframe, to the bushing, to the bolt. So… effectively… the wheels are pushing the car by pushing the 2 subframe bolts…?

Same deal with braking, but because the fronts are probably braking harder than the rear, probably less actual force applied to the bushing here.

Lateral (x-axis) forces would twist the control arms with one arm being the pivot point and either pushing down or up on the edge of the subframe (depends on the direction the wheel is going).

Don’t think I’ve really revealed anything, though. :frowning: The only thing I can really think of is that the bushing is really only there to transmit lateral subframe forces to the bolt and back. There’s not really much vertical force to contend with. Maybe that changes with rubber – maybe the slight deflection induces more vertical force on the nut that could cause it to come loose?

One other thing to consider is that with 2 rubber bushings, you could have the whole subframe twist around one of the bolts – which could contribute to the bolt twisting and being undone. Maybe this concern doesn’t existing when you have solid bushings as there’s no twist?

I guess this would go back to why bolts are tightened to begin with. Can’t say I know all the physical/mechanical nuances to answer that question. :frowning:

Som

#23

Not saying a guy with 30+ years of experience is infallible, but I have to imagine the guy knows what he’s talking about with something like this, right?

The other thing I was thinking was – what’s the worst that could happen if the nut comes off? The subframe isn’t going to slide out because the bolt comes down from above. The question would be whether those little side supports can support the weight of the rear end (and all the forces applied to it). Assuming they can, it’s not really going to be catastrophic problem, right? But if they can’t, then having the nut loosen could cause the entire rear subframe to fall off the car…

Som

#24

I really like the idea of the tube on the inside to torque against. Of course, deletion of the tube cuts cost and angst…nothing to gaul against. I really don’t see a problem with 25# torque as long as you red loctite the nut. The stresses on that are taken up by the frame above and the plate below. There are minor vertical forces in the downward direction, usually only when the car is jacked up. Follow directions and see if everything works out ok.

#25

Buy the condor bushings and torque them good and tight. I am a mechanical engineer and a subframe bushing that deforms with 25 ftlbs will never be in my race car…

#26

[quote=“Som” post=79684]Not saying a guy with 30+ years of experience is infallible, but I have to imagine the guy knows what he’s talking about with something like this, right?

The other thing I was thinking was – what’s the worst that could happen if the nut comes off? The subframe isn’t going to slide out because the bolt comes down from above. The question would be whether those little side supports can support the weight of the rear end (and all the forces applied to it). Assuming they can, it’s not really going to be catastrophic problem, right? But if they can’t, then having the nut loosen could cause the entire rear subframe to fall off the car…

Som[/quote]

Lol, don’t underestimate how hard racing is on the car. Al K had those two little 8mm bolts loosen up and exit, and his subframe moved all over heck trying to get free. He had to have the whole mount point for that big 12mm bolt repaired.

Racing is so hard on these cars that every bolt that you don’t put locktite on will try to loosen up on you. As the years go by, on many occasions when you are doing something under the car you will find some critical bolt loose and exclaim “holy shit that could have been bad” which is racer-code for kids growing up w/o their father.

Develop a healthy mistrust…that’s not quite the right word. Be over-cautious. Routinely inspect every important part of your car. When you are poking around underneath it doing something, as a matter of course check every fastener you can reach. If something is loose, back it out and put in some blue locktite before putting it back in.

Don’t be too quick to use red locktite. It’s really strong stuff. Often it has to be burned out before the fastener will move. For most applications, blue locktite is sufficient.

If that subframe nut came free you’d prob be one hard lap before your rear end departed company. Then there’s be a shower of sparks for a couple inches Just long enough for your fuel tank to be scraped thru. You’d then be the subject of a thousand discussions each year where newbies were told to be serious about checking their fasteners. You’d be a legend.

#27

+1 from another ME. I have the AKG poly bushings which are pretty much the same design as the Condor: two halves of an aluminum post in the middle surrounded by a solid UHMW polymer. This mimics and improves the stock design, which is to have a solid post firmly attached to the chassis at both ends, with some material doing the bushing between that and the subframe.

I get AKG’s argument that the clamping force isn’t critical: as long as the two halves are together, the bushing will be “solid” enough to keep the subframe and chassis in alignment. However, as Rich said, if this thing can deform in any direction that easily, I don’t trust it. These bushings (along with the differential bushing) transfer all of the forces from the rear wheels to the chassis. The amount of odd-axis forces and vibration they see during racing is high. Following AKG’s directions may make you feel better and put some amount of liability on them in the case of failure, but that isn’t worth a damn during the scene Ranger just described.

For the record, I use lots of AKG’s poly products, and I am very happy with them.

#28

Mechanical engineers always talk like they know everything.

#29

Scott, we do. Stress on the bushing is not vertical. The stresses are parallel to the ground and are resisted by the three subframe mounts. (Which is why you always want all three to be of the same material!) All the nut on the bottom has to do is keep the pieces together and 25 pounds of torque is enough to do this. If you really want to check this out, call Andy and get customer names that are using the new bushings. Do your due diligence before condemning the produce.

#30

The poke at mechanical engineers was just poking fun . All of us in this thread are ME’s. Well, except for SOM. I think he’s a computer geek. The primary target of my jape was myself.

The problem isn’t 25ftlbs keeping the 2 halves pressed together, the problem seems to be that the two halves don’t press together at all. They aren’t tall enough. As a result the openings of the subframe are encouraged to mushroom. Over time they’d open up a bit more and allow movement of the bushing halves. This would allow the whole structure of bushing halves, subframe opening, bolt head up above, and nut down below, to move around a bit. This would further damage all the pieces and they’d all get even more loose.

Even if the 2 halves were tall enough, I’m not comfortable with a torque spec that is 1/3rd of OEM. The way that vibrations can loosen fasteners is tricky business and I don’t understand it well. But the guidance from AKG was 20-25ftlbs. Heck, that’s 10mm bolt territory. To make matters worse, SOM didn’t get that guidance until he called them up. The normal thing to do would have been to treat it like any other 12mm 8.8 bolt and use >2x the ftlbs what AKG recommended.

#31

Once again, I agree with Scott…

#32

Anyone actually know what due diligence is? Has anyone talked with someone who uses these bushings? STFU till you do.

#33

[quote=“cwbaader” post=79694]Anyone actually know what due diligence is? Has anyone talked with someone who uses these bushings? STFU till you do.[/quote] Lol