What do they do?
Steering head bearings form smooth, articulated link between the fork yokes, which grip the fork legs, and the steering head at the front of the frame. Also known as headraces, they must be sensitive enough to allow subtle steering input and feedback, but tough enough to endure kerb impacts, dropped wheelies and braking forces. They are vital to safety and good handling. They keep you in touch with the front end, yet are often ignored at service time. <-- so very true, after speaking to a number of riders in the forum.
Much of the difference you feel between a crate-fresh bike and a 'sloppy' used machine is down to bearing condition. Knackered and unadjusted steering bearings show up as wobbles, clunks and a general 'loose' feeling. Checking out and adjusting bearings takes minutes though replacement can be trickier.
Most bearing maintenance these days consists of checking for play to replacing where necessary, with the expection of steering head bearings which can be adjusted. Advanced bearing wear will show up as a clicking, or clunking of the front end when the brakes are applied - although worn brakes can give similar symptoms. If it is the bearings, they are probably already damaged and will need replacing, so the headraces should be checked routinely every service.
To check for play, hold the forks above the seals, with the front wheel raised, and push to an fro, feeling for movement. It is easy to confuse other play with that at the steering head. If you grab the fork lower you may be feeling play in the fok bushes, or if pushing hard the whole bike may be moving on the stand. Any free play you do isloate to the steering head needs adjustment.
Next, move the steering fully from one side to the other, feeling for 'graunchiness' - where the steering tends to settle facking straight ahead, as is likey to be the case where hitting a kerb or landing many wheelies has dented the bearing surface. If the bearings display any of the above symptoms they need replacing.
Adjustment usually means removing the top yoke to get at the bearing adjuster locknut. If you do it yourself get a manual as procedures vary quite widely from bike to bike.
You're looking for bearings with no play and as little resistances as possible. When all slack is gone, repeat check the steering for smooth movenment. Don't over-tighten bearings to as this is likely to present itself as a very nasty high-speed weave. <-- tank-slapper?
Most aftermarket headrace bearings are of the 'caged taper-roller' type. The rollers are held in place so you don't pull your hair out as you drop them all over the workshop floor, and they handle side-to-side axial loads well. But some riders and designers reckon they offer less sensitivity than the traditional ball bearing system.
Replacing steering head bearings can be a very tricky job, so again check a manual. The whole front end must be removed to get at the lower steering and rebuilding it incorrectly have some pretty sticky consequences.
It's sometimes very difficult to remove old bearings races and it's possible to damage bearing seats if you're clumsy. Replacing seats can be equally tricky - especially with ball bearings. Heating bearings seats with a blow-torch will help get bearings on and off or in and out.
Wheather it's a DIY job or not, re-check the adjustment after your first ride as bearings tend to settle. If you ride around with play in your new bearings, they'll soon be duff ones.
extracts Bike Jan 03