Suspension Bushes

I have recently been tackling a job that is both time-consuming and repetitive, but is ultimately rewarding and, for me at least, strangely calming; suspension bushes. Now, there are many discussions online about the correct way that these should or should not be fitted, so the method I ended up using is based on a bit of research and my own judgement and, so far at least, things have gone well.

As this build diary is potentially a future reference to others, I thought I would start with a basic guide to bushes and the type of bushes used on the zero, as researching this was very useful for me in the first place.

A suspension bush, fundamentally, allows a wishbone to pivot around a given point, in this case a bolt through the chassis, and therefore move freely up and down in the way suspension should (it's basically a hinge, see diagram below). They also provide a certain amount of isolation from vibration that would be transferred into the chassis from the road, the amount of which is defined by the material used in their construction.

Wishbone

GBS Supplied Bush

The bushes supplied by GBS are of a nylon type, harder than the polyurethane (poly) bushes many people upgrade their sporty road cars to and much, much harder than the metalastic (rubber) bushes used on standard road cars. The major benefit of these is, due to their stiffness, there is little to no movement in any plane other than the one intended (i.e. up and down), which is exactly what is required in a precision fast road/ race car. Another benefit is that nylon doesn't add any "rate" to the suspension, unlike rubber and some poly bushes which deform under load and act like little springs of their own. This make suspension set-up more accurate and predictable. The downsides are an increase in noise and driver discomfort, as vibrations will pass more readily from the suspension arms, through the bush and into the chassis.

Nylon Bush Diagram

The construction of this type of bush is very simple. Two nylon "top hats" are inserted into the wishbone's end, one each side, and a steel crush tube is inserted into the hole through them. Whilst in place, the crush tube remains stationary, held tight by the wishbone mounting bracket, and the nylon insert rotates around it, itself held tight to the inside of the wishbone end by friction. The ends of the nylon bush are in light contact with the mounting bracket, but only with enough pressure to prevent any lateral movement as any more would cause "stiction", which can lead to differing suspension rates on each corner, the amount of which being dependent on how much each bush was being retarded.

My method began by prepping the wishbones. Before the bushes could be inserted, each end needed to be nice and smooth internally. Unfortunately, each one contained a weld line that needed to be ground down and because I have had the chassis powder-coated, the residue from this process also needed to be removed. I happened to have the perfect size grinding stone attachment which took care of the weld and I bought a drum sanding attachment to sort out the powder coating. I also used the sanding drum to make a slight chamfer on the inner edges of the wishbone's end, just to make sure the bush didn't catch on anything.

Tools Used

Sanding Down

The bushes themselves needed a little work before they could be inserted. As they had come straight out of the mold they all had rough edges to one extent or another, with most of them fouling the crush tubes. A little time with a sharp knife or a round file sorted that out. Actually inserting them in the wishbones ended up being much easier than I thought; each bush was lubricated with copper grease, placed in position, and then both sides were slowly wound in with a nut and bolt and some large washers to spread the load. Even if they started slightly wonky, they pulled themselves true as they were tightened up. I don't know if it helped that the wishbones were still warm from the sanding, but I made sure I didn't let them cool too much first.

Inserting Bushes

Nylon In

At this point, I couldn't just stick the crush tubes in and be done with it, as each finished bush was a slightly different width to all the others and the tubes needed to be a spot-on fit. The best advise I found was for each tube to protrude out of the nylon by between 0.1mm and 0.2mm each end, enough for the tube to be clamped by the bracket, but not too much to allow side-to-side movement. My ruler was clearly not up to this level of precision, so I bought a set of Vernier callipers from eBay and got measuring. Santa was also very kind this year and bought me a bench grinder with a sanding belt attachment, which was perfect for slowly grinding down the tubes with a nice flat end. Once prepared, each tube was covered in copper grease and inserted into the bush (I basically ending up covering every mating surface with grease), which was then mounted to the chassis. Each joint requires large washers between the nylon and the bracket, with a couple of them requiring 2 on one side, as the brackets didn't seem to line up quite right (a bit of persuasion with a ratchet strap helped with that). All the bolts (which were also covered in copper grease, notice a pattern?) were the correct length, with none of them touching any other part of the chassis (which would be an IVA fail) and with at least 2 threads showing through the nyloc bolt (also an IVA requirement).

Careful Measurement

All Done

It's also worth noting that the bolts here, and indeed throughout the whole car, have to be high tensile. A metric high tensile bolt is stamped with a steel rating on its end, with 8.8 being the lowest grade considered high tensile, increasing in strength to 9.8, 10.9, 12.9 and beyond. The first digit is its minimum tensile strength divided by 100, so 800 MPa for an 8.8 bolt. The second number signifies that the bolt will begin to yield at, in the case of the 8.8 bolt again, 80% of the ultimate tensile strength, i.e. at least 640MPa. Imperial and stainless bolts are marked with a different system and, again, there are minimum grades of strength that are classed as high tensile.

That wasn't so hard was it? 1 down, 25 to go (if you include the pedals)!

The above diagrams were borrowed from an excellent post on the Westfield Sports Car Club forum, I'm hoping Frosty doesn't mind my use of them.