[R-M-R]

yeah I was thinking say if you had the frames side by side, one with shorter reach, and with stack of A,then the one with longer reach, starting at stack A and increasing from the same stack to B hence making shorter... but thats not right the short one has stack A and reach X and the long one has stack B and reach Y, two completely different points.

Yikes, that's not the easiest description to follow.

Whenever this comes up, I recommend to draw it on paper. Draw the head tube region of a frame using a ruler and pen. Using a pencil over the pen drawing, extend the top tube (this maintains the same stack) and draw a parallel head tube, fork, and down tube. Notice how the fork is now farther forward. Same stack + more reach = longer bike by the exact difference in reach.

Now erase the pencil and experiment with stack. Draw another top tube slightly above the pen drawing. Stop the line directly above the end of the original head tube. This maintains the reach at a different stack. As before, draw a parallel head tube and fork from your pencil top tube. Notice how the fork is again farther forward, but not quite as much. Same reach + more stack = longer bike by about 40% of the difference in stack.

 

[274898]

Reach and stack has everything to do with how your body position and weight will be distributed on the bike.

 

[dimitrin]

Finally, someone who gets it!

To put it another way, here's why stack is not the same as adding or subtracting spacers under a stem:

Adding stack to a frame's geometry moves the front end directly upward, but spacers move along the steerer, which isn't vertical - it's on the axis of the head-tube angle. If you have a 66° head-tube angle, for example, raising the stem 10 mm will also shorten the reach by 4.1 mm.

As LyNx said, a frame with a "long" reach with a low stack is a lot shorter than you think, while a frame with a modest reach and a towering stack is probably a very long bike.

Yikes, that's not the easiest description to follow.

Whenever this comes up, I recommend to draw it on paper. Draw the head tube region of a frame using a ruler and pen. Using a pencil over the pen drawing, extend the top tube (this maintains the same stack) and draw a parallel head tube, fork, and down tube. Notice how the fork is now farther forward. Same stack + more reach = longer bike by the exact difference in reach.

Now erase the pencil and experiment with stack. Draw another top tube slightly above the pen drawing. Stop the line directly above the end of the original head tube. This maintains the reach at a different stack. As before, draw a parallel head tube and fork from your pencil top tube. Notice how the fork is again farther forward, but not quite as much. Same reach + more stack = longer bike by about 40% of the difference in stack.

Reach and stack has everything to do with how your body position and weight will be distributed on the bike.

Does this mean it'd be a wash to add a 10mm spacer under the stem on a 66* HA ?
And that adding spacers to bikes with HAs steeper than 66* will slightly lengthen the effective reach and adding spacer to bikes with HAs less than 66* slightly shorten the effective reach?

 

[R-M-R]

Does this mean it'd be a wash to add a 10mm spacer under the stem on a 66* HA ?
And that adding spacers to bikes with HAs steeper than 66* will slightly lengthen the effective reach and adding spacer to bikes with HAs less than 66* slightly shorten the effective reach?

No, I was referring to the actual frame dimensions. Comparing two bikes with the same reach, but 10 mm different stack, your hands will end up in different locations if you simply add 10 mm of spacers to the bike with lower stack. The point is that spacers or stem rise are not equivalent to an equal change in frame stack.

66° is not a magic number. The exact implications to geometry will change slightly, but the principles remain the same for a head angle of 70° or 60°.

 

[Long slow and fat]

Old thread I know but interesting discussion... If I wanted to increase bar height without effecting reach, rather that add spacers it would be better to do so with riser bars and keep them rolled forward (ie. In the vertical plane)?

 

[mack_turtle]

Old thread I know but interesting discussion... If I wanted to increase bar height without effecting reach, rather that add spacers it would be better to do so with riser bars and keep them rolled forward (ie. In the vertical plane)?

It's going to decrease reach at the handlebar (I've been calling this "effective reach" but I don't think it's catching on) regardless of how you add height to your grips. You need to accommodate by using a longer stem.

 

[Austin-nc]

Old thread I know but interesting discussion... If I wanted to increase bar height without effecting reach, rather that add spacers it would be better to do so with riser bars and keep them rolled forward (ie. In the vertical plane)?

mountain bike geometry is kind of funny in the way there is no way to adjust the cockpit without also changing some other dynamic.
Adding spacers to increase stack height will decrease reach and will also move the bars further away from the front wheel decreasing front end weight

You can instead use higher rise bars while not adding spacers but this will also decrease reach to a point as you really should not be running your bars straight up and down, this is a very common thing but is not a good practice.

When you run your bars in the flat up and down vertical plane you are in effect adding stem length doing this, you want to keep the angle from the front axle to the bars basically in line as much as possible. When riding this keeps all of your leverage and the forces on the front wheel all in one plane. Think of one angle from the front axle all the way through the bars and into the arms to the shoulders.

 

[Long slow and fat]

Yes you are right! So maybe a bar with low or no upsweep would work - to counter the headangle 🤔 ?? Anyway I guess I can go with just a small spacer, a slightly longer stem and roll bars forward just a few degrees. Cumulative effect should do it without having noticeable negative effect on steering!

Would bar width have an impact of your "effective reach" measurement mack-turtle? I need details!

 

[Austin-nc]

Yes you are right! So maybe a bar with low or no upsweep would work - to counter the headangle 🤔 ?? Anyway I guess I can go with just a small spacer, a slightly longer stem and roll bars forward just a few degrees. Cumulative effect should do it without having noticeable negative effect on steering!

Would bar width have an impact of your "effective reach" measurement mack-turtle? I need details!

Yes, I would say a higher rise bar 35mm or so a little past in line with the forks should let you come down on the number of spacers below the stem as well.
I prefer a 40-50mm stem, and I like 790mm bars but I'm pretty tall. I think bar width is more down to comfort and what hand spacing you feel strongest in push up position, I don't think it has a huge effect on stability as long as you're within the normal range of bar width. A 50mm stem will have a more stable feel at higher speed than a 35mm stem though.

 

[Kobeashi636]

As LyNx said, a frame with a "long" reach with a low stack is a lot shorter than you think, while a frame with a modest reach and a towering stack is probably a very long bike.

This only really applies if you intend to run spacers under the low stack bike. If you ran both frames slammed the low stack long reach bike will outreach the other.

 

[Kobeashi636]

At 5'9.5" and a short inseam too much stack and not enough reach is a big issue when it comes to choosing a bike. In my next bike I want at least 450mm reach and 605mm or less stack. 450mm or less seat tube so I can fit a decent dropper as well. I general I still think reach is too short, stack too high, and seat tubes too long on a majority of frames. Tall riders are going to have other fit issues.

I would’ve thought these numbers were nuts a year ago but just went through a fitting session and to get my torso angle where I’d want it for a more XC ride we had to go from a 40mm to a 100mm flipped 7 deg stem. I’m 5’7” and apparently freakish flexibility and now realizing most 29er frames have a hard time accommodating the reach I need without it resulting in too much stack. I’d almost need a large frame and a syntace flatforce stem to reduce stack but it feels absurd to go to a large at 5’7”. It appears my options are either that or running a flipped 80+mm stem on a size medium which baffles me