So I got the new headset fitted to the tandem after having the cups turned down as the insertion was too much ( now 15 mm insertion) Its a cartridge bearing headset with angular contact bearings

The problem is that the headset does not run freely turned to full lock. It is fine 45 Degrees either side of centre but as it turns further than that there is a progressive resistance – it still feels smooth but feels over tight.

any clues? I have had it all apart twice and resembled it without the extra seals. I can't see anything catching or fouling. Everything looks to be installed properly.

Ta

Have you actually ridden it yet? It might bed in when you do….

Somethings not sat parallel on the installation would be my guess, thus as you turn it binds. If its a cartridge unit, have you made sure they're sat square and the mating up with the races squarely?

Headtube need facing?

Bearings running OK when out of the cups?

Cups must be slightly out/slightly ovalised/butting something internally. It dosn't have to be out by much to bind a little. Knock the cups out and reinstall them with plenty of coppa slip.

How much cleareance has the turning down given you to the stiops inside the headstock? Has the machining slightly ovalised them?

Bent steerer tube?

Ta chaps – I take it cartidge bearings have less tolerance of stuff being askew?

Bearings running fine when no load applied

Clearance of 1mm from the end of the cups to the end of the stops inside the headstock. Not ovalised.

Bent Steerer – I hope not – its a reinforced steel one

Ok – its a strip. clean of all the mating faces and reinstall I guess.

My cartridge bearing headset has specific top and lower bearings.

When I first assembled it I didn't notice the tiny difference in size between the two – result same problem as you describe.

Quick swap and all is kosha.

If all components are in their correct geometric condition, not bent or deformed, then its an alignment issue.

As Science Officer says, the rotational axis of the bearings should be absolutely parallel to axis of steerer tube (yes there will be a tolerance, but it'll be very small), otherwise you will experience symptons similar to those you describe.

I've had this very issue recently on a frame. All looked true to the eye, but once I got some facing done, the difference was clear.

Good luck, let us know what you discover about this.
😉

Luminous

Why would you need it to turn more than 45 degrees?

I doubt a clean alone will sort it Jezza, facing may be needed.

Wonder if cups could have been damaged in lathe?

Why would you need it to turn more than 45 degrees?

Barspins? 😀

Want to bring it in for the boys to look at TJ?

I might end up doing that TBC.

I shall have a further tinker first. I take it the cartridge bearings are less tolerant of misalignment than cup and cone?

I'll remove and reinstall everything – sounds like its possible something is not seated properly.

Ta

I pressed a headset into my 853 Inbred (my first build) and it was fine so when my new frame came along I did the same. The minute I applied even the smallest pre-load it bound with particularly bad sticking points at certain places in the rotation. Everything looked fine to the eye but I ended up getting the headtube faced and reamed and now it's absolutely fine. I was just lucky the first time I guess.