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Discussion Starter · #1 ·
OK, I've decided to do a modest lowering on my '02 Envoy (w. rear air springs). I've done my research and read all of the relevant threads I could find and still have a couple of questions.
Since it seems to make sense to change the shocks at the same time (50k mi.), the question is: to what? Since there will be less bump travel available, I'm OK with somewhat stiffer shocks so the Bilstein HDs look interesting, but folks here seem to trash them for lowered applications. What I can't find is any explanation of why! I do know that most shocks have bump stops tucked up under the boots and wonder if failure to trim these might be the problem. Received wisdom seems to be that the TBSS shocks are better for this application, but again; why? There is also a post that says that the rear TBSS part number has been superseded by a newer design that is not as good; so where does that leave this discussion? Just to further cloud the issue; when you look on Rock Auto's website for TBSS shocks, they list an AC Delco part number as a retrofit. Has anybody tried those and care to comment? Somehow I'm not feeling any wiser after my research!
Second question: since R&Ring the front shocks is not a job that I care to do twice, are there other parts that should be ordered at the same time? Specifically I'm thinking about the upper spring seat insulator. Since they are about $30-50 a side, I don't feel like buying them just in case. How likely are these to benefit from replacement?
Thanks to all who have contributed their experience and wisdom to this site and have made it an invaluable resource for all of us.
 

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2006 chevy trailblazer_lt
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I have some thoughts and opinions, derived from some work on my truck and on reading and sometimes rereading the relevant threads. I'll synthesize what I have concluded.

I believe that lowering reduces spring travel. As a result, a shock of a given specification will feel harsher/stiffer in an application where there is diminished travel than they will in a stock suspension travel application. So, if that is true, then the HD Bilsteins, which are stiffer than the stock TBSS shocks, will provide a harsher ride. Yes with diminished spring travel you typically want "more" shock to prevent bottoming, but the price you pay is a really harsh ride, and I predict that the HD's will rattle your teeth on a lowered truck.

Further, since most folks tend to overestimate their need for and tolerance of ride stiffness, it is very easy to over-shock a car or truck. Folks think they need more suspension control, and are initially happy with shocks that are stiffer than stock and certainly stiffer than the worn units they removed, but they never compare apples to apples. If their mechanic just put new shocks on their vehicle but didn't tell them what he installed, they would almost always be happy with stock valved shocks. Yes, some do want a harsher ride, stiffer valving, and greater control, but those come at a cost of a much more uncomfortable ride around town and a skittering over washboard surfaces. The best shock for rough roads is not a stiffer one, but rather a more compliant one, in most cases.

I saw the posts about the new TBSS part number, and some questions about whether it was still the same shock. I don't think that was ever answered satisfactorily, and so I did the same Rockauto.com research you did and saw the various part numbers there. I couldn't really make an informed judgment, but suspect that the new number AC Delco units will be pretty darned close insofar as valving is concerned to the old number units. Just an educated guess/opinion.

I had the same "wonder" about the upper spring seat insulator, and even posted a question in one reply in one thread, but never got a response. When I took mine apart at about 148k (replaced both front and rear shocks), I saw that the insulators do indeed wear, and that mine were worn, but I concluded that they weren't worn enough to warrant replacing since I didn't have them handy and since I'm cheap. Would a dealer have replaced them? Undoubtedly. Would yours be worn at 50k? Doubtful. Just my opinion again. If your truck can be out of commission for a few days, just take them apart, examine them closely, and make a decision then.

Finally, I'm certain I wouldn't think of replacing my front or rear shocks at 50k. It woudn't make sense to me. Why? Maybe just maybe they are half worn, but probably not even that. My fronts were shot at 148k for sure (one was totally gone!), and the backs were very worn. [The same is true for the shocks on my Bonneville at 149k, but I haven't replaced them yet.] So my guess is that 100k is a good mileage to replace them. But that too will depend on the kind of driving they have encountered. All highway driving will mean that they will last far longer since they won't be working nearly as much as ones driven all those miles on rough pavement.

Just my thoughts and experiences.

Mark
 

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Discussion Starter · #3 ·
Thanks for your thoughtful remarks. I'll take your advice and pass on the upper insulators this time round. You make a good point too that the shocks probably don't need replacing (there's no real evidence that they do) so the smart move is probably just to leave them for now. Obviously spring rates have to come into consideration too - I didn't bother to bring it up in the original post to try to keep it simpler. Since there is less travel to play with before things start banging together, a lowered suspension should have higher spring rates. Aftermarket lowering springs undoubtedly are although they never say by how much. I doubt that most people (including me) could distinguish between a change in spring rate and a change in damping force, so the shocks may be getting more blame than they deserve. IWith the rear air springs I know that letting air out of the system will increase the spring rate by reducing the compressible volume, but I have no idea whether it's enough to keep off the bump stops without a little help from stiffer shocks. Discussions on this forum haven't been too informative about this but I'm guessing the effect must be approximately correct since Chevy did it with the TBSS. Up front the story is a bit different. If I buy shorter, stiffer springs, there is a good argument for leaving the stock shocks in place. However, the cheaper easier solution is to recut the circlip groove on the shock body to get the drop. In that case there is no increase in spring rate. Some older posts suggest that this is not a problem, but I guess the only real answer is to try it and see if it needs more shock to help keep it from bottoming. I'll have to let y'all know how it all comes out.
 
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