12/02/2004: Suspension, Brakes

Over the last month or so, i’ve been reassembling the new subframe for my car.

Some time ago, I put together a page of photos and diagrams of the front suspension for reference. This has been quite useful in determining what colors to paint things and how to reassemble all the parts. Here’s a link for the curious.

These cars use an unusual suspension arrangement involving the use of ‘Spider Trunnions’ instead of modern ball joints. There are two trunnions on each side of the car, at the ends of the upper and lower control arms. Shafts on the steering knuckle (essentially a kingpin) slot into the trunnions. The trunnions have greased bushings that allow the steering knuckles to turn up left and right, and have threads that allow them to swivel in the other direction, so the suspension can move up and down. The picture at the right shows the top trunnion on an assembled suspension, for reference.

Here’s what the trunnion looks like disassembled:

Each end cap is threaded on the inside and the outside. The outer thread screws into threaded openings in the control arms. These are tightened down and do not move. the inner thread engages the threaded shafts on the trunnion, and are lubricated so that the trunnion can rotate along that axis. The two rubber boots are just to help keep the threads greased and keep the dirt off them. Note that although there is a grease fitting on only one side, there’s an internal passage all the way through to the other side, with an opening to the center bushing to lubricate the steering knuckle shaft.

Installing these things is a trick, since you’re simultaneously engaging two sets of threads. In one case (the top one on the right side), I think perhaps the control arm was slightly bent, such that the two sets of threads weren’t in sync completely. It was very hard to get the thing together, but eventually I did get it on. In general, you want to get one side tightened until it just starts to engage, then start the other side, then tighten both a little at a time. I added some loctite on the outer threads to try to keep them from loosening up.

Bottom Trunnion
Top Trunnion

Once tightened down, the rubber boots get pinched against the trunnion bodies, and everything seems quite tight. I think once the car is on the road though, things will break in and start to move more easily.

Note that the trunnion end caps were silver cad plated and the control arms and other suspension is painted with POR-15.

I also test-fit one of the steering knuckles:

I can’t finish putting it together until I get some replacement rubber parts. The old ones I had, while unused, had dried out and gotten hard. While I wait for those to come in from europe, I started to work on the brake rotors.

I have a total of 6 rotors. 2 on the car now, the 2 blue ones that were originally on this subframe, and 2 other old rusty ones. I wanted to get two of them turned down to remove some of the rust pitting and go with those, but before I could do that, state law here requires that they not be turned below the minimum thickness as defined by the manufacturer. Normally, this is stamped into the rotor itself, but FIAT didn’t do that, which made things complicated. It also turns out that no minimum thickness was documented in either of the service manuals I had (the 118H and the 118K one), or anywhere else I could find.

I asked about this on the fiatcabrios list, and nobody knew for sure either. Ron Horowitz had a sedan rotor though, and measured it at .375”, and felt that the cabrio ones were probably the same. The (probably conservative) spec from fiat is that they can be turned .020” under, which means I should shoot for .350”.

I took my two thickest ones to a local shop and had them turned. One of them cleaned up pretty well and came in right at .350”. The one on the right is really no good though- as you can see, it would be necessary to take even more material off to clean it up, and it’s already thinner than I want.

All the others I have are thinner than .350”, so I am currently looking for another decent used rotor. Unfortunately new ones are VERY hard to come from, and the rotors appear to be unique to the 1500 cabriolet, which means it’s unlikely that they’re being widely reproduced.

If necessary, Chris Obert can supply them as a special order item, but they don’t come cheap. So i’m hoping to find a used one in comparable condition that I can work with. I’m not willing to put an unsafe brake rotor on my car, that’s for sure. It’s not worth risking your life to save a few bucks!

I’ve done some googling around, and so far i haven’t found any sources for this rotor (or even one close enough to it to adapt), but i’m sure it can be done if I get really desperate. Unfortunately, FIAT used a different size center hole on these cars than on any of the later cars (which almost all use identical rotors)

The blue discs, while too thin to use, had brand new bearings and races in them, so i’ve separated the hubs from the discs and will be mixing and matching the best of each. I probably should have done this before I had the rotors turned, but oh well..

Separating the rotor from the hub is pretty hard.. What worked best for me was to first clean up any rust and buildup around where the hub comes through the face of the rotor with a 3M surface conditioning disc. Then I screwed in 4 lug bolts and placed the bottom of the rotor surface on a pair of 2x4s to support it. I hammered on the 4 bolts (going corner to corner) until it started to move, then switched to hammering against a wood block placed on the center section until it came apart. I separated 3 of the 4 ones I had (which weren’t on the car) this way. On the last one (the rough one pictured above), the bolt that holds the rotor to the hub was starting to round off, so I decided to just leave it.

I removed the bearing seals from all the hubs and cleaned the bearings in paint thinner. I’ll repack ones from the blue hubs (which appear to be brand new) and install them once the hub and rotor are painted and reassembled.

For paint, i’m planning to use a high-temperature matte black paint. I think originally, these would have been bare (rusty) metal, but I think black will probably look better. I’ll do the calipers in a cast iron colored paint later.