Piston Slap: Finding The Sweet Spot
James writes:
I have a 1995 BMW E34 530i, and I’m looking into getting it lowered. At first I thought this was a simple affair, that I just needed lowering springs. But some people are telling me that I can’t just put new lowered springs without buying new shocks that are designed to handle these lowered springs, for fear that I’ll wear my tyres out and possibly other components. I thought this might be true for going super-super-low, but these springs will only lower my car 1.5″ at most (Bavauto springs 1.0″-1.5″), if I’m lucky. Is this just my mechanic trying to sell some extra parts, or do I really need new lowering shockers, even for going just a little lower?
Sajeev answers:
Indeed, your mechanic might have a valid reason. Or he might be misinformed. Very few people aside from suspension engineers and super geeks on message boards know the real deal. So I recommend doing two things: ask the manufacturer of said lowering springs (Bavarian Autosport) and jump on a popular BMW forum and solicit help from some of the senior members who tune, tweak and share their knowledge on a regular basis. Both will help you decide if new dampers are in your future.
But shock technology isn’t a black art, it’s common sense: suspension dampers (shocks or struts) operate in a range of motion, just like your arms or legs. They work far more precisely/efficiently in a narrower window of motion: it’s the sweet spot of the shock. It’s like a sandwich: sometimes a bite has with far more bread than anything else. Yuck. But sometimes you get a good taste of everything smack dab in the middle. You always want the sweet spot in dampers, it’s what lowers impact harshness on small bumps and controls float on undulating roads.
Back to lowering your BMW: lowering springs can force the damper to operate out of its factory-authorized sweet spot, and do it enough times to annoy you. But it’s more likely when you go over 1 inch worth of drop. On a BMW, I still suspect a 1.5 inch drop won’t make a difference for many years to come if–and it’s a BIG if–your dampeners are relatively new and you live somewhere with smooth roads. If you don’t know the dampers’ age, replace them now. No excuses!
(Send your queries to mehta@ttac.com)
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Lowering springs will most likely also have an increased spring rate - the stock shocks weren't designed to work with this spring rate, which will quickly blow them out. This is why you should upgrade to at least a Bilstein - something more aggressive than stock, otherwise you'll keep blowing them out.
One thing to remember: going to an aftermarket coil over shock setup pretty much negates anything said here. And I agree with those who've mentioned the usefulness of going to any aftermarket lowering setup for a street car, but that's not exactly answering the question. So I didn't mention it at the time. But kudos, anyway.