Stem stack

Uli

Hi,
I replaced stem on my bike. The old one had 40 mm stack. The new one is 39 mm which leaves the top gap wider. Does that 1 mm makes big difference and should I use more spacers (26 mm instead of current 25) or can I leave it as it is?
Thanks

nicklouse

Well if you can still preload th bearings it does not matter.

If you can not preload the bearings either add more spacers or cut down the steerer.

I know what I would do.

Uli

Well I put everything together and seems to be fine. No movement in the headset. I must go for the ride to be sure.

I know what I would do

I would be delighted if you could share you thoughts?
Thanks

Chunkers1980

Let's just say it wouldn't be cutting the steerer.

Why do you need to go for a ride?

Uli

No, cutting the steerer is not the option. I guess I am going to leave it as it is.
Ride- to put the new setup under some stress to see if it stays preloaded. Field testing
Thanks guys.

nicklouse

Uli wrote:

No, cutting the steerer is not the option. I guess I am going to leave it as it is.
Ride- to put the new setup under some stress to see if it stays preloaded. Field testing
Thanks guys.

If it does not then you have note don the stem bolts up correctly. Spacer have nothing to do with it.

So can you preload the bearings correctly?

If yes job done.

If no add a spacer. Above o below th stem it makes no difference other thn to the bar height.

Uli

OK, so excuse my ignorance in this matter but how do I know the bearings are preloaded correctly? I can tighten the top bolt of the headset to the point where bearings spin without noticeable resistance and there is no side movement in the headset. Then do the stem bolts. Correct?

Chunkers1980

Yep, that's the one.

stubs

Or you could do what a lot of people seem to do. Get the longest allen key they own and tighten the preload till their eyes bulge, knuckles go white and not bother to tighten the pinch bolts properly. Then whine about how shit the bearings are and how I fitted their expensive headset wrong. :roll:

BG2000

Uli wrote:

Hi,
I replaced stem on my bike. The old one had 40 mm stack. The new one is 39 mm which leaves the top gap wider. Does that 1 mm makes big difference and should I use more spacers (26 mm instead of current 25) or can I leave it as it is?
Thanks

That doesn't quite make sense. If your new stem has a smaller stack height, the gap between the stop of the stem and the top of the steering will not be 'wider' as you've stated, it'll be less - in your case, probably 1mm less.

If the difference between your stems is exactly 1mm and there was a small gap previously, then you might need to add a 1mm spacer. But if the gap looks something like below with the new stem fitted, then you probably don't need to do anything:



The reason you need a gap is because stem top caps are often convexed underneath, so it will either hit the top of the steering tube, or the star-nut, which means it won't be compressing the headset bearings when you tighten the bolt, it'll just be tightening the top cap into the steerer tube. A simple test is to rest the top cap on the stem without the bolt and check to see if it's resting nicely on the top of the stem without any gap round the edges. If the steerer/stem gap isn't enough, the top cap will be sitting slightly above the stem, or wobble on top of the star nut. An experience mechanic can easily tell if the gap is enough just by looking at the shape of the top cap. Don't just wrench the top cap bolt really tight, wobble the front end back and forth with the front brake on and assume it's OK. And riding the bike will tell you nothing.

It's also worth mentioning that you do need the steerer as long as possible without actually hitting the top cap. If the steerer is too short, the stem's top pinch bolt won't be clamping on enough of the steering tube, which could be dangerous.