2020 Update: The cheap hose clamps I used got loose over time and the problem began to reoccur. I replaced them with stainless steel marine hose clamps (about $10 ea.) which tighten only with a wrench and have a lot more clamping power. These worked for a long time. I could tighten them so much that the seat back wouldn’t move at all without significant force. They too loosened some over time, but never getting as bad as before.

Then I found out that C6-version recliners were available and bought a pair.

They have a much larger and stronger spring and some other differences. I just installed them and they feel good. At least one person I know installed these and says they fixed the problem completely. The seatback is much more stable (stiffer) in position in normal driving, indicating that the previous lack of stiffness is due as much to the recliner as to the seatback structure itself. I left off the marine hose clamps to see if this is really a fix by itself. (A friend of mine with a 2010 C6 had exactly the same problem as the C5 but I don’t know when this updated part became available.) I’ll be autocrossing the car the next two weekends and will report back with a further update.

 

FURTHER UPDATE: I’ve done 2 events now, both with a co-driver. One event was Bristol, an extremely bumpy site. No seat-back flopping at all. Seems stiff and stable now.

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One of the most annoying and inexcusably dangerous aspects of the fifth generation Corvette are the seats. Specifically, I mean the tendency of the seat-back to flop to the fully reclined position when loaded. Such as during an emergency maneuver, at the worst possible time during an autocross run, or when pulling out onto a busy highway. Each has happened to me, starting from when I bought the car in pristine condition with 13,800 miles on it.

My seats got progressively worse over the next 40K miles. First, they occasionally let loose at any intermediate position. They gradually worsened until they would not hold any position at all other than fully upright, which is not at all comfortable. At any angle more reclined, one side would slip backwards under normal sitting pressure. Either side could let loose with more load, sometimes both sides at once. Yesterday the seat-back flopped from the fully upright position. I’d had enough.

I’ve searched the internet in vain for anyone who knew how the seat-back locks worked and how to fix them. No luck. Lots of people searching for a solution, but no one finding one. I found various people who had taken their new Corvette to the dealer, supposedly had it fixed with new parts (no longer available) only to have the problem reoccur. Today, I had a little time so I pulled out the seat determined to understand and fix the issue, even if I had to weld it into one spot. Turns out welding wasn’t necessary. Two $1 hose clamps did the trick.

Here’s what you see after removing the seat bottom cushion: two cylindrical mechanisms, one for each side of the seat-back. I call them angle locks but the GM term is recliner. Since they are independent in operation (but actuated together) one can slip and the other hold, creating the common situation where one side falls back and not the other. (The fiberglass construction of the back is very flexible in torsion, so it has no problem twisting until one side falls waaaay back there.

 

The green springs you see in the figure above are what bring the seat-back up to touch you when you actuate the lever. This way, you don’t have to pull the seat up and you then just lean back to the preferred angle and drop the lever. The angle locks are supposed to hold it at your preferred angle. Now, let’s look a little closer at one angle lock device.

The main body of the lock is a steel cylinder that is pinned at the forward end. (It has to rotate a little bit during the seat-back movement.) The cylinder is holding together two split sleeves that are inserted into it. Inside the sleeves is some sort of cam-lock device. I don’t know exactly what it is, but this is the bad-boy that slips. The cam-lock is locking the axial position of a shaft that runs from inside the cylinder all the way back to where it is pinned to the lever arm of the seat-back. (You can’t see it… it’s inside the green spring.) With the shaft locked into position, the seatback is prevented from rotating about the seat-back pivot which is fixed to the lower frame.

To release the cam-lock a cable pulls tangent to the lower edge, causing it to rotate inside the split sleeves. (Two cables are pulled at the same time, actuating both devices simultaneously, more or less.) It takes very little rotational motion, at least on both of mine, to release the shaft and allow it to move in or out.

Some have thought that the weight of the driver pressing down through the foam can deflect the pull cable and release one side. Nice theory, but I don’t think so. The cables do get pinched between the seat cushion support wires and the silver metal shaft you see in the picture above, but the cables have a good amount of slack in them. I tried, but, in spite of how little motion it takes to release the shaft, I could not create any cam-lock rotation and thus seat-back release by deflecting the cables unless I pulled them totally outside the volume of the seat.

I think there’s some sort of spring inside the cylinder that serves to pre-load the cam and thus lock the shaft at all times unless pulled by the cable. It’s theoretically possible that the green springs are doing this job, doing double duty. Maybe the spring(s) get weak? Maybe, but my buddy Glenn and I came to a different conclusion.

We noticed that the gap between the split sleeves wasn’t uniform. The gap was bigger in the middle where the pull cable comes in and smaller at the end where the shaft protrudes and smaller at the other end where the sleeves disappear into the cylinder. It looks like the split sleeves have dimples at the shaft end to lock them to a ferrule of some sort that carries the shaft and holds the split sleeves together.

Our theory is that the cam locks the shaft by squeezing on it. An equal and opposite reaction (expansion) within the split sleeves is therefore required. That expansion may spread the sleeves apart over time, such as during the delivery trip from the factory to the dealership. And they probably weren’t particularly close-toleranced to begin with. So, Glenn suggested that we squeeze the two halves together better. We put hose clamps around them as close to the pull cables as we could and tightened until they cried for their Mamas.

It worked!

The seat-back now locks firmly in any intermediate position, which it would not before. I slam back into it and Holy Toledo Pro-Solo! it holds. Driving the car is so much more comfortable, not to mention much safer.

We’ll have to see if this procedure is permanent, but I can dig deep and afford to put two $1 clamps on each side if I really have to. (Glenn thinks I should market a machined and anodized aerospace-grade aluminum two-piece clamp with thread-lockable screws. What do you think…$25? Hey, maybe titanium. Yeah, that’s it. Titanium! $99.95) If anyone wants to protest me, go ahead and try. I’m not removing those hose clamps! I hereby proclaim this to be the industry-standard repair for a safety issue that’s been vexing Corvette owners for nearly 20 years.

P.S. If anyone has ever cut up or otherwise disassembled one of these angle locks I’d love to see a picture.