As a customer you may have had a shop tell you a certain component on your vehicle was worn out or defective and then had them suggest that you replace that same component on the other side as well. This is particularly common on steering and suspension components and brake components. Why should you replace them in pairs? The most basic answer is to keep the system balanced. 

For instance, if you were to replace a shock or strut on the left (driver) side of the vehicle after 75k miles because it was leaking, it would be a good idea to replace the right (passenger) side as well because the new shock or strut may behave differently than the other side. This could be caused by a couple of different factors:

1) The shock or strut is a different brand. It’s much more likely that you will replace the OE (original equipment) shocks or struts with an aftermarket brand such as Moog, Monroe, Gabriel, or KYB. While these units presumably all meet OE specs, their performance is likely to be slightly different. 

2) One side will be new and one side will have 75k work of wear on it. Shocks and struts wear very slowly and many drivers do not even notice the difference until they replace their shocks or struts. If you replace only one shock or strut, the vehicle will probably handle very unevenly and could even be dangerous!

Another reason you would be well advised to replace suspension and braking components in pairs is because both sides have the same amount of wear on them. Take ball joints and control arms for example.

Every time you make a right hand turn, both wheels turn causing a slight amount of wear on the ball joints. If the left ball joint failed because of lack of lubrication, it is very likely that the right side will follow suit shortly. Honestly, it’s an issue of “Pay me now or pay me later.” 

Not to be upstaged by chassis components, brake parts are subject to the same recommendations. 

A shop is likely to suggest that both calipers be replaced even though only one side may have failed. The reasons for this are the same as above: balance and preventive measures. 

At the end of the day I hope this sheds some light on why your mechanic might replace suggesting replacing to identical parts on opposite sides of the vehicle. If they suggest replacing your water pump while replacing your muffler, perhaps you should find a different mechanic, but most good shops are trying to perform a quality repair, not just trying to dig a little deeper into your grocery budget! On the flip side, as a technician perhaps you should start suggesting replacing the aforementioned parts in pairs instead of only doing one today and replacing the other one when it fails a few weeks or months down the road. 

Check back next week for another Tech Tip Tuesday!

Photographs courtesy of Moog and Brembo.