Suspending a spectre

acetalI’ve still been thinking about suspension bearings. At one point I’d convinced myself that the thing to do was just to cut up the existing wishbones and weld some bushes in into which I could put some rod-ends. I still might do that, to be honest. In fact, I suspect Jeremy would be happy to provide a complete set of new ones.

However, in the meantime I was talking to one of the Baldwin chaps at Silverstone. Ian is one of the main designers of the BDN car that won the championship last year. What’s more, his new car is running well this year although hampered by a late start. I was talking to him about bushes and he said that the new car, as a bit of an experiment really, was using bushes made with Acetal. This is an engineering polymer that is commonly used for similar things. I did some calculations and it does look as though it’s up to the job. (And, to be honest, it’s clearly tougher than bungy rubber or nasty polyurethane anyway.)

So, I bought a stick of it to see how it goes.

Acetal is nice in that it’s machineable and the photo above is of my first experiments with this. In fact, it seems dead easy to machine so I’m going to have a go at making some bushes out of it. I’ll run these around the 16mm diameter shafts that come with the polyurethane bushes so they’re not going entirely to waste. And, I’ll ream those shafts out to 12mm so I can (!) use Jeremy’s M12 bolts.

Interestingly, a whole stack of people came up to me at Silverstone to talk about the Spectre. A common feature was along the lines of “why the heck does he stick with using M12 bolts?”.

The only issue about the Acetal at the moment is working out what tolerance I need to machine them to so that they’re a press-fit into the wishbone housings and an easy runner on the shaft. In time, if the Acetal bushes are not a complete success I could machine out the bushes so that I could insert an Iglidur bush in there instead. As discussed before, this is another engineering polymer and it’s got pretty similar properties to Acetal apart from a much lower coefficient of friction against steel. Consequently it looks ideal for this sort of bearing.

sphericalThe obvious problem with these sorts of bushes, which will rotate nicely but have no other degrees of freedom is the outer mounting on the rear lower wishbone.  (I am, of course, assuming that the mountings in Jeremy’s chassis line up nicely; I’ll probably have to check that at some point.) This outer mounting is the one shown in the photo here. This needs a bit of compliance so that the rear toe can be adjusted. I could just put  a rubber bush in here and put up with it, but I’m actually concerned about excessive movement in the axial direction of these bushes. One solution, again, would be to cut off this housing and weld in a bush into which I could screw a rod end. The alternative would be to mount a spherical bearing within this same housing. The problem is, I can’t work out how to retain such a bearing. At the very least I’ve have to machine a circlip groove, or something similar, in there and that’s going to be hard as the wishbone would be flailing all around the garage. I could just weld some shoulders into that mounting to retain a spherical bearing but that’s a rather one-shot operation. Mind you, it’s the easiest thing to do right now…

Hmm, this needs a bit more thought. Watch this space…

2 thoughts on “Suspending a spectre”

  1. Sorry Tim, I’ve checked my notes the bushes I used are nylon 6, not acetal as I said on Saturday, I did look at using Acetal, if I remember correctly (it was nearly 3 years ago) the final decision on the material was made by the availablity of a suitable sized standard part.

    Ian

    1. 🙂 No problem, the acetal seems fine from the material specs. In fact, what I’m using is Delrin which is an acetal homopolymer. Bob Mortimer tells me that the bushes on the suspension of his Ultima were Delrin so it must be up to the job. As for using a standard part, I’ve got a lathe. Admittedly, it’s going to take ages to make all the bushes but I think I’ve got the process sorted out now…

      Tim

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