Sorry that this turned out to be so long...
After 5 weeks and 650 miles, I’d like to offer a follow-up on my BC Coilover adventure.
Pluses: Handling is superlative, at least considering that we are starting with a mass-market sedan. I haven’t tracked it, and have no immediate plans to do so, but it’s a joy to hammer around highway cloverleafs. Cornering limits are higher, body roll is minimal, and you can actually get feedback as to what the front tires are doing. You can get on the power much earlier without worrying about understeering into the guard rail, and when the front tires do struggle for grip under power in a corner it is communicated clearly to the driver. The floatiness is gone when navigating deserted back roads at elevated speeds. The car now communicates to the driver more clearly what is happening. Confidence is greatly increased since you know what the car is doing and aren’t in fear of unexpectedly finding yourself understeering into a ditch. This is all with the stock Michelin tires - I can only imagine how much more grip you’d get with some sportier rubber.
Minuses: I’m having second thoughts about the stiffness of the setup for my daily driver.
About half of the real roads that you encounter here in the northeast have enough vertical motion (from frost heaves, etc.) that it becomes rather fatiguing even for the driver, and moreso for passengers, especially those in the back seat. Many roads in these parts have enough potholes that I find I tense up in anticipation of going over them. I enjoy going places with my kids, and unfortunately riding in my car has become a less than enjoyable experience for them simply because the ride is so rough, even when I tiptoe around at the legal speed limit.
Every once in a while on the highway you go over a bridge overpass expansion joint with an inch or so of vertical misalignment, and such a transition will send a violent hammering impact through the car’s structure. I chose to install with the front camber plates (without rubber isolation) which I’m sure contributes to that, and the added “road buzz” is quite noticeable when going over coarse pavement.
I installed it with ride height set as supplied by BC. It has turned out to be rather too low for my tastes. Again here in the northeast I encountered one back road where a pothole caused the car to scrape a rock in the middle of the road (thankfully I was going at a low speed). This is rather worrisome in any car, but especially in this one where the battery is at risk. The ride height can be increased, but BC says maximum height is still 3/4 inch lower than stock. I guess I’m kind of nervous about that, especially since the under-floor battery doesn’t have an excess of ground clearance to begin with despite the Bolt’s overall “tall” stance.
Regarding the front-end “clunk” that I’ve previously mentioned, I’m happy that BC has agreed to replace the front shock cartridges, but the lead time is rather long and the replacements haven’t arrived yet. The clunking itself tends to augment the fatigue, but hopefully that will be fixed with the warranty replacements.
In the interim: three days ago I put back the stock front suspension (springs and struts), and stock rear springs, but kept the BC rear shocks, as a sort of simulation of what it would be like to keep the stock suspension but with Bilstein or Koni rear shocks. I’ve also kept on the Whiteline rear sway bar. I do miss the immediate feedback and response, but I’ve realized that I had somehow stopped enjoying driving my car since it was too fatiguing, and now I actually enjoy driving it again. The rear sway bar is IMO a shining example of a nearly vice-free improvement in turn-in and balance, so that benefit is still there. The BC shocks still seem a bit harsh on the rear, but I haven’t yet messed with the shock setting (currently mid-range at 15 clicks out of 30). I note that the BC shocks are linearly-valved, while other street/sport shocks (Bilstein, and I think Koni too?) have digressive valving which might be even smoother for street use while still offering better damping than stock.
There were a few other lesser dislikes about the setup. I couldn't use the stock dust boots on the rear shocks because they would hide the adjustment knob, and wouldn’t even fit over the shock body; I worry a bit about that through sandy/salty winters in these parts. The four (per side) adjustment screws in the front camber plates interfere with the hole that the upper strut mount fits through, so I ended up removing two of them, which is not totally confidence-inspiring. And, the clunk is a bit worrisome - I worry that a setup that was engineered by a company in a fair-weather part of the country might not be up to the rigors of sub-freezing temperatures in a daily driver. (I have a first-hand real-world story about that in a different car, which I can share if you’re interested.)
So: I’m not quite sure where to go from here. In one of the threads here, Evolt mentioned going with lower-rate Swift springs, and using the stock rubber-isolated upper strut mounts. That might be a good way to go, but I’m not confident that I’d know what spring rates to select. I don’t mind having spent the money for the BC setup in order to see if it was to my liking; however I’m not sure I’m willing to budget more “experimental cash” without having more confidence that it’ll be something I’ll like. I also worry a bit about having a more limited suspension travel with the coilovers.
I’m tempted to try another short-term experiment: use the stock front suspension, but put the BC front 5kg springs into the back instead of the BC-supplied 7kg springs to see if that mitigates the rear stiffness enough to return to a more comfortable zone. If it does, then perhaps I could order some Swift 4kg front springs for the front and either use the 5kg BC (formerly from the front) or order 5kg Swifts for the rear. But then I’d still worry about ride height and other issues I’d mentioned above, as well as not being certain that it wouldn’t still be too stiff for passengers.
Or, I could step away from the BC setup and just keep the stock springs, and go with Bilstein rear shocks. There is mention in another thread here pondering whether Cruze front Bilsteins could be made to fit by drilling or welding... I’m intrigued by that as well.
I will say that if you’re going to track your Bolt, the BC setup is hands-down the way to go. I can see how it would be a joy to hustle round a road course, and find it credible that you could embarrass a few sports cars in the process. However I think the stock BC setup is just a couple of notches too harsh for me as a daily-driver.