Shock Mount for Dummies (me), please.

Decided to change the shock on my Trek Top Fuel 9.8. 2018. Currently a Fox Float Performance with remote lockout. Hoping to fit a Fox Float Performance DPS in its place. Free up some space on the bars, lose some cables etc., is the intention. Mounts are the same on both shocks so I thought it would be fairly straightforward, but no.

Removed the old shock, trunnion type so lower mount only to worry about – two poly bushes, shaft through the bushes, two o rings, two (unequal) spacers and the bolts that go through the rocker link to hold it all together.

The spacers and o rings come off easily enough but the centre shaft is very tight in the poly bushes. So tight I don’t see how it can rotate freely, which surely it needs to do. Surely the shock doesn’t rotate on the long through rocker fixing bolt? Can’t believe it should.

The new shock has the IGUS bushes already fitted but if anything the old centre pin/shaft is even tighter. I could force it in but it then wouldn’t rotate freely.

In a previous life I’d have made my own bushes/shafts, or bored/reamed out the bushes to give a good slide fit, but that option no longer open to me.

So can someone explain to me how it is supposed to work, and if the centre shaft should be free to rotate on the poly bush or not? If so, how do you open it out a thou’ or two without access to proper equipment? A flap wheel and a Dremel? Sandpaper and a stick?

Help!

last shock i fitted was literally a press fit shaft into the bushes. i can only imagine that the amount it rotates, and the actual torque it experiences is a enough not to matter so much?

There’s far more force applied through the suspension linkage than you’ll ever manage by hand.

It needs to be tight otherwise it’ll rattle when you ride.

When my bushes are worn, the pin will usually press out by hand but once the new bushes are in, it’s a vice and socket job to get the pin back in.

The material that the bushes are made from is slippery enough that an interference fit with the pin still allows the shock to pivot.

If you’re just removing the pin to change the shock and not because they’re worn, I’d expect them to still be nice and tight.

If you look at a new bush next to one which is fitted into the shock, it’ll look bigger. Once the bush is pressed into the shock, the ID closes up to make sure it’s nice and tight around the pin. The pin will probably rotate freely in a new bush on the bench but won’t once the bush is in the shock.

I reckon you’ll be fine.

What bens says is my experience as well. Though some bushes are definitely tighter than others from new.

Not trying to be rude but that sounds complete nonsense to me. The centre pin/shaft won’t easily fit into the poly bushes even when they are removed from the shock. If they close up when fitted they will be even tighter on the shaft. Limits and fit, by definition, have to be precise and hammering things together with a socket and a vice is not my idea of precision. It’s bollocks.

If they are designed to rotate on the shaft then that’s what they should do. Folk fret about the shock settings, fine tuning, damping, volume blah blah but tif he shock is pretty well seized up before you start, what is the point. Admittedly the rotational movement is very small but any resistance to rotational movement is wrong. There’s got to be a better way.

I’ve got a spare pair of bushes, I’ll experiment.

I’ve rarely been able to push the axle through the poly bushes without using a press. But with the leverage of the swing arm/rocker, and large amounts or torque(?) put through the system with your weight on it, it always moves super smoothly. I’ve used the same bike with bearings and standard bushes. Obviously the bearings spin super freely by hand. But once everything is installed, I dont notice any difference at all to the suspension performance or how easily the bike gets into its travel by just pushing down on the saddle.

The bushes are cheap bits of plastic that need to eliminate play between the shock and the pin and allow the shock to rotate. The ability of the shock to rotate comes from the fact that the bushes are made from a low friction material that allows the pin to rotate even though it’s an interference fit in the bush. Not because they’re a perfect marvel of engineering that slides together perfectly with absolute precision.

The bushes are a tight fit into the eyelet, the pin is then a tight fit into the bush. It’s all needs to pressed together. It’s not bollocks. That’s how it works.

Put in fridge and hope the centre pin shrinks slightly?