There seems to be a plethora of choice for replacing suspension bushes on the MX-5, from OE to various polyurethane (PU) brands ranging in price from under £100 to a few hundred quid. It's fair to say that PU bushes are generally seen as 'hard', 'stiff' or 'race bushes', and with a general lack of information and understanding of the products available all PU brands seem to be tarred with the same brush. So, is there a clear advantage in moving to PU and is there a significant difference between your sub-£100 eBay brand to SuperPro that will cost 3 times as much?

Understanding the basic principles of PU

A clear advantage of PU is the resistance from the elements - elements that eventually deteriorate rubber bushes and lead to less than ideal control of the suspension arms. Two design options are available: a copy of OE with a bonded crush tube (technically, a bush) and a bush with a separate crush tube that 'floats' or free pivots inside the bush (technically, a bearing). PU does not have the same twisting properties of rubber, and therefore having a PU bush with a bonded crush tube results in a number of issues. In order for the suspension to work effectively, the control arms must be allowed to move through their whole travel. With the OE bush, the amount of movement in the control arms is a function of the rubber's Shore hardness (there's a reduction in twisting properties the greater the hardness) as it is required to twist to allow the suspension arm to move, but if you were to replace the rubber with a compound that does not allow twisting you essentially stop the suspension from moving. If you think of it as a spring, a OE rubber bush will have a low spring rate where you can move a control arm by hand to some degree whereas a PU copy will have a very high spring rate restricting movement of the suspension arm. This design causes issues with the longevity of the bush where it can be forced to twist in the housing instead, or break the bond of the crush tube. In essence, a PU replacement bush should be designed as a bearing.

Because a bearing, in theory, allows free movement of the suspension arm the PU should be designed to do something different to what the rubber is designed to do - keeping the suspension arms in their correct position. Preventing unwanted movement of the suspension arms enables rigid control of the alignment.

What makes SuperPro Bushes different?

It's important to point out that SuperPro (SP) are firstly suspension engineers, rather than bush manufacturers. Understanding suspension dynamics enables them to engineer and design replacement bushes for the proper function. A bush is not designed until the OE suspension is measured and tested in-housing using their rig. A brief understanding of the process is seen in the first half of their YouTube video.

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