Bumps in the Road

The other day I got a phone call from a friend with a concern about his Subaru Legacy GT Spec B.  I had helped him improve the handling with some better springs, Whiteline roll center correction kit, and some nice anti-roll bars and end links.  We were both very happy with the results and he had put a few thousand miles on the car since the work had been done.  Now he was concerned because, under specific conditions, the suspension would react with a bang instead of its normal smooth ride.

We talked about the problem over the phone and arraigned for him to bring it by so we could take it out for a test drive and see what was going on.  The day came and he showed up.  He explained that it was most prevalent when he crossed on an overpass and the suspension hit the expansion joint at either end of the bridge.  I took the car out to a shopping area that had some pretty rough perimeter roads.  Sure enough, when an abrupt bump was crossed the suspension would react with a bang.  Then we drove to an overpass that we knew would provide the same situation.  On the way I took the car through its paces to see if it had any other suspension ills.  I was suspecting an errant end link or perhaps a loose nut on a strut.  In spirited maneuvers the car worked very well and behaved as it should with no noise or bad manners, but when we tried the overpass the sound was abrupt and jarring.

It was time to talk to the owner about the problem.  When trying to determine the source of difficult problem it is important to ask some key questions.  My questions centered on how the problem appeared.  Did it happen all of the sudden?  Did it start small and work its way up to the level it was now at?  What changes might have occurred at the same time?

I light bulb went off in my friends head as I went through the questions.  He suddenly realized that it started right after he purchased new tires. 

I remembered he and I had talked about what kind of replacements he should consider and I felt that he should stay with the brand that the car came with, Bridgestone, since his experience had been excellent with them.  We both agreed that it might be the time to just step up to a little more of a performance level and change to the Potenza 760 version. 

Now we had discovered that while the 760’s certainly provided additional grip and were compatible in most driving situations, the sidewall stiffness was totally incompatible with his suspension in that one driving situation.  So it was back to the tire dealer who was good enough to have Bridgestone pro-rate the wear and exchange the tires out for the original 050 version that his car came with.

This is a great illustration of how tires and suspensions can have incompatibilities in one situation and not in another.  A tire that is a real performer on one make and model of a car could turn out to be a gremlin for another.  You see it race cars all the time where the cars are literally designed around the tires and if the team switches to another brand during the season they often have to start completely reengineering the car and suspension.

I have seen some tires that are really great on most cars, yet cause problems like pulling to one side or another because of a difference in the cars’ differentials.  It is important not to forget that tires are sophisticated creations that are an integral part of a car’s suspension and therefore can either live in harmony or in conflict.  As you upgrade to some of the more performance versions of even the most reliable brands you might just find out what my friend did.

So my advice is to do your research.  I personally find that Tire Rack provides a lot of valuable feedback from customers and their own tire testing.  If you read through their customer comments be sure to focus in on the people with a vehicle like your own.  If you have an independent tire installer in your area you might also ask them what they have seen.  After all, they have mounted and balanced hundreds if not thousands of sets of tires and have seen which ones are difficult to balance and true up and those brands that have consistent quality.