Polaris OBark

I just purchased a Park Tool guide and carbon-specific hack saw blade, so I am hoping to do this right.

One thing is obvious even to an idiot like me: if I cut it too short, there ain't no going back.

This is an Enve all-road fork that came with a frame I purchased. It is very long in its uncut state.

Is it better to have a longer steerer tube with lots of spacers, and (say) a 7° upward angle stem, or a shorter tube with fewer spacers, and (say) a 30° stem, to achieve the same bar height? How many is too many spacers (from a physics point of view, rather than aesthetic)?

I suppose now that I own the tools, cutting it twice (choosing the longer option to begin with) isn't out of the question.

SoSmellyAir

The maximum spacer height between the headset and the stem should be listed in the manual for the frame (or the fork), and largely depends on the diameter and wall thickness of the steerer tube (assuming a given load in view of rider weight and tiding style). According to Enve's website (General Fork Installation and Care Instructions – ENVE Support Center Home):

• The spacer stack between the top of the upper bearing and the bottom of the stem must not exceed 40 mm (this distance includes the headset dust cap or tapered spacer on top of the bearing).

Polaris OBark

Thanks!

That link is extremely helpful.

Andrew R Stewart 

I try to talk customers into having a spacer or two ABOVE the stem when installing replacement forks. As one can always go back and cut off another 10mm... I can't remember how many customers I have dealt with over the years that wanted to raise their bars but couldn't because of a short steerer. I can't remember how few I have had that wanted to lower their bars...

Being an old roadie I like a stem with little slope to being horizontal. A stem with a high angle and a shorter steerer will be lighter and possibly measure out as stiffer. Andy

Crankycrank

Here is a chart that compares stem angles+lengths+height which can be useful if you want to get the same riding position from 2 different stems. Stem Comparison Tool | yojimg.net

Koyote

Wise advice. In the past few years, I have raised the bars on a couple of my bikes, and found that it helps on longer rides. I'm not as flexible as I used to be.

Polaris OBark

What is the major concern with having more than 40mm of spacers (in the Enve example) between the headset and the stem? Is it that the compression bolt won't work right, or that the steering tube will have too much strain applied? If the latter, could one use a longer compression plug (if such a thing exists)? For the same reason, I would assume you wouldn't want too much steering tube protruding above the stem -- the compression plug presumably should reside inside the stem region?

SoSmellyAir

The function of the expander plug is to reinforce the carbon fiber steerer against the compression forces exerted by tightening the stem around the steerer. The steerer is designed to flex along its entire length. By using a longer expander plug to reinforce the length of the steerer, you prevent that and focus the strain onto the section of the steerer below the expander plug, which may be worse for the integrity and longevity of the steerer.

But like I said in my prior post, there are certain assumptions made to arrive at the maximum spacer stack height. Maybe you are very light weight. Maybe you will use the shortest possible stem. Maybe you will only ride on the smoothest surface even though you have an all road fork. Maybe you will install the widest possible tires at the optimal air pressure to absorb any bumps you encounter. So maybe you can have a few more millimeters of spacers between the stem and the top of the headset.

SoSmellyAir

Me too! I am not even that old and I like the old school racing quill stem look: Deda Elementi Murex Quill Stem, 26 x 100mm, Silver - Modern Bike

tFUnK

Too much distance between the stem and the headset may allow the steer tube to act as a lever and place too much torque against the headtube.

Polaris OBark

That is consistent with, and a better explanation for, putting the compression plug between the lower and upper part of the stem clamp.

I get it. My wife wants high stack, and I am trying to figure out the best/safest way to achieve that. (Also, she is no longer very light weight. I hope she isn't reading this.)

The Enve fork was provided with the frame, and no instructions, so I am eternally grateful for the link to their very detailed information on their website (and a bit ashamed I didn't look hard enough for it).

SoSmellyAir

Well, if she rides sitting fairly upright she would not be putting that much weight onto the handlebar, stem, and steerer, regardless of her actual total weight. But to be safe (and for warranty purposes), best to use only 40 mm worth of spacers and use a stem with a larger angle.

Andrew R Stewart 

And riser or swept back bars if a riser stem isn't enough hand height. Don't forget that the higher the hands get the more seat set back most riders want. Andy

Polaris OBark

Ok, I have been playing around with measurements (measure twice, saw once), and wouldn't mind a bit of a reality-check, if anyone is interested. The bike frame is fairly similar to my bike, so posted below are the measurements for mine, printed, and hers, in red handwriting. The reach is slightly longer for hers, and the stack significantly lower (-32 mm).

I have 20 mm of spacers on mine, which adds about 19mm to the intrinsic frame stack, resulting in an "effective" stack of 596mm.

Max. 40mm spacers and her intrinsic stack of 545 mm = 583 mm "effective" stack. So I used these as starting points to play around with the stem geometry website input. (I used 72.0° for the fork/head tube angle, as it splits the difference evenly).

Here is what I am getting, using a Ritchey adjustable stem I have set to 45° and it is 90mm center-center:



(I've subtracted 500mm from both heights so that I can see what is going on in the plot grid). Red is my bike, blue is her prediction.

Here are the bike geometry vitals:




Even though I currently have too many spacers in place, the reality is that the measured difference in effective reach is 8mm less than predicted, and, more significantly, the drop (from saddle to bars) appears to be greater in her case, (i.e., the bars sit lower than mine, not higher as predicted). Both saddles are at the same height for simplicity.

I must be doing something wrong, but I can't figure it out.

Briefly, my assumptions are that the most relevant differences are the intrinsic (frame) stack, the angle of the stem, and the length of the stem. (The difference in intrinsic reach is +7mm for her bike, but the total handle bar height is what has me more worried. Currently she has a zero set-back saddle, and mine is set back 10mm).

SoSmellyAir

Polaris OBark Your effective stack calculations check out:

Adjacent (i.e., change in reach) = cosine of 72° x hypotenuse (i.e., 40 mm of headset spacer height) = 12.36 mm
Opposite (i.e., change in stack) = sine of 72° x hypotenuse (i.e., 40 mm of headset spacer height) = 38.04 mm

Trigonometric Functions - Justin Skycak (justinmath.com)

So adding 40 mm of headset spacers effectively reduce reach by 12.36 mm and increase stack by 38.04 mm.

But the height input for the stem comparison tool should not be the effective stack minus an arbitrary 500 mm. Rather, it should be spacer height plus half the stack height of the stem. But I am not sure why you are comparing between two bikes.

Polaris OBark

Just for the record, I did put in 38mm, not 40mm (and similarly 19mm for the 20mm spacers on my bike), as the projection on the vertical axis (calculated as you described).

I'm comparing these two bikes because they are fairly similar in geometry, so I am trying to see where they differ. Since I have one already, I can physically measure, check and verify everything, and I know that it almost fits my wife, so it seemed like a decent reference point.

Subtracting 500mm from the height input doesn't change anything in the calculation (as long as the height difference is accurate), but repositions the figure on the graphic representation so you can see what is going on (try it if you don't believe me).

jccaclimber

Following this theme, if you initially cut the steer tube for a horizontal stem, you can also flip the stem to get height should you need more height than the remaining spacers allow.

Polaris OBark

In this case, 40 mm of spacers sets an upper-bound.

SoSmellyAir

I understand that the 500 mm is just to reposition the graphic.

My point is that the "height" input in the stem comparison tool is for a length at an angle (i.e., the hypotenuse), whereas you had input a length that is vertical by using the effective stack (i.e., the opposite), which may account for the difference you are seeing between calculated and actual.

Polaris OBark

Got it. Sorry.

You are absolutely correct. Fortunately, sin(72) = 0.95