Worn bushes - effect on geometry

[Andrew]

I've read an opinion on an MX-5 forums that a geometry setup isn't as effective if the bushes on the suspension are very worn.

 

I wondered if Tony has an opinion on this? Mk1 MX5s are now between 10 and 19 years old, so most will have either very old bushes, or will have been fitted with new, probably poly bushes. I wonder how many of the MX5s that Tony sees have had the bushes replaced?

 

On my car (a 1991 MX-5 turbo) the original bushes look like 17 year old bushes, the ARB bushes in particular look pretty tired. I might change some of the bushes at some point this year. I'm pretty sure changing the ARB bushes for e.g. polyurethane wouldn't affect the geometry setup, but I expect changing the bushes on the wishbones would.... Mind you, my wishbone bushes don't look half as bad as the ARB ones, so I'm not sure I'll bother with those just yet.

[LexSport]

As a slight aside to this question - the head of chassis engineering for a major car company posts to another forum I frequent and he has himself built a very rapid E36 M3 track car. He dislikes poly bushes for a number of reasons and attributes the majority of the benefits noticed by people to the fact their OEM bushes were shot and they could have achieved a similar benefit by fitting replacement OEM rubber bushes.

[Tony]

It's a difficult call really... In an ideal world the perfect calibration would have new bushings all round but it's not an ideal world.

 

I see this everyday and my advice to the customer is there are three levels of wear in my opinion

 

1>None/new

2>worn

3>Knacked

 

The worn is the level we live with for the longest until we get to number 3 then we know it's time to replace the bushing due to knocking or squeaks and alike.

 

For me it's easy to see the level of wear because i can measure it, but for the owner all i can suggest is listen to the car.