Whats the max that you can fit spacer wise under the stem ?
I've always been the reluctant type when it comes to cutting down new steerers ?. Mainly cos i ride XL and that always is the largest headtube length, and because i chop and change bike frames you dont always get consistency in that length, so i would normally put about 20-30mm under the stem(though usually about 25mm.
.
Ive another fork I need to cut the steerer on, but I might swop it between frames and theres about 15mm between them. So if i cut it too short its to short really to be comfortable for me on the other frame, and if i cut it too long then i worry theres going to be too many spacers under the stem to be safe.
.
#Also whats the score on having spacers above the stem. I've done this before but only by about 5mm, and between the bikes I've got to accommodate them all would probably mean having 20mm on top which to my thought at least does seem a bit high.
It's more about how it looks than engineering. If you have your bars at the same height, it doesn't make any difference to the stresses on the steerer and frame whether you use riser bars, a high rise stem, or spacers under the stem. First thing is to find a comfortable height for your bars. I start with the bars level with the seat and then adjust from there. Once you have that settled, cut the steerer.
I’m similar. And have been able to reuse a fork on frames with different lengths.
Jack Moir is known for running a big stack of spacers. An old shot of one of his bikes:
Doesn't look pretty, but there's nothing fundamentally unsafe about it.
If you have a look on the fork specs/warranty, a lot used to say max spacer stack was 40mm before stem. It may have changed over the years, but its a rule i still stick to.
Just had a quick google, fox say 30mm and ive seen that quoted for top end forks with carbon steeters by rockshox as well, with warnings of catastrophic fork failure, so well worth checking against what you have and then making a personal risk assessment
It’s more about how it looks than engineering. If you have your bars at the same height, it doesn’t make any difference to the stresses on the steerer and frame
Not sure how you come to that conclusion, if you have a 100mm headtube and a 150mm headtube and the stem sits at 200mm on both then there will be a bigger moment acting above the shorter headtube (no, the bearing doesn't clamp it but it does offer radial support).
The forces coming from the front wheel are far greater than the forces coming from the bars.
It shouldn't really matter, I'm lanky so run a lot of spacers. I only usually cut a small length of steerer off. I've probably got at least 50mm of spacers under my stem on one bike. Spacers above the stem is also fine but don't look good and can be seen as dangerous if you were to have a crash and it impact your chest.
It’s more about how it looks than engineering.
LOL, it's all about the engineering.
Too much unsupported steerer above the pivot/bending point (top bearing) leads to excessive flex, shortened HS bearing life (both top and bottom) and potentially premature fatigue failure of the steerer (or catastrophic failure if you've got a really bad stack of conditions and a carbon/lightweight steerer)
My general rule is if its a carbon steerer I have a shorter stack. If its an Ali steerer im happy to make it a higher stack.
Too much unsupported steerer above the pivot/bending point (top bearing) leads to excessive flex, shortened HS bearing life (both top and bottom) and potentially premature fatigue failure of the steerer (or catastrophic failure if you’ve got a really bad stack of conditions and a carbon/lightweight steerer)
If the bars are at the same height, the bending moment on the steerer is the same. If you raise the bars by using riser bars or a stem with more rise, you will increase the bending moment on your steerer. However, as TJ pointed out, the stress put through the steerer by the front wheel is much greater. (e.g. when you do and emergency stop and the back wheel leaves the ground).
Not sure how you come to that conclusion, if you have a 100mm headtube and a 150mm headtube and the stem sits at 200mm on both then there will be a bigger moment acting above the shorter headtube (no, the bearing doesn’t clamp it but it does offer radial support).
Ahh, yes, if you have a frame with dramatically different headtube lengths, it will change the stresses on the steerer. However, for the same length head tube and the bars at the same height, it won't make any difference whether you put spacers under the stem, use a riser stem, or use riser bars. The bending moment is purely a result of how high the grips are above the top headset bearing.
I usually leave about 30-35mm extra for spacers for reselling purposes, but I currently have 10mm under the stem so about 20mm above, not a great idea having once crashed onto it stomach first just below the naval I can confirm it hurts like hell, I think the issue is the cost of a steerer replacement being so ridiculously high and overpriced, it needs to come down to make it a more viable popular option, the cost of CSU units are also steep af, I've literally bought a Yari for less than the cost of replacing the csu. I prefer the good old days of bolt on single crowns, much cheaper and user friendly replacement option.
but I currently have 10mm under the stem so about 20mm above
Just make sure if it's a carbon steerer the fork bung is long enough to reach the stem so it has something to support it when you tighten it. Otherwise, it might crack.
There’s about 10cm (100mm) of steerer spacers between the headset and the stem on our tandems. Don’t overthink things, it will be fine
Is it a Steel fork?
As for running "lots" of spacers above the stem, you probably need to be a lot more careful with carbon steerers, as the internal bung support may end up nowhere near the stem clamp bolts.
In that respect I'm lucky my Cube Attain fork isn't carbon, when my lower back isn't moaning I often run 30mm of spacers above the stem to not be a total aero brick (610mm stack), but when it's fragile I can resort to 30mm under the stem.
