Ask Uncle Dave

Dear Uncle Dave: What's with all this reach and stack nonsense?

Words Dave Tolnai
Date May 28, 2019

Okay...sooooo....oops? I made a bit of a mistake on the original article. I used the wrong fork offset for my Megatower offsets. I definitely searched for this information, but I got it wrong. It's possible that I searched for "Hightower fork offset" and used that? So, really, this is Santa Cruz's fault for their naming convention. Yah...we'll got with that.

Anyhow, drawings are now updated. Text is edited. I've added a couple of other things to better illustrate points. Sorry! (I mean it this time)

Dear Uncle Dave,

I feel lost. I thought I was finally starting to understand geometry. Then I went and demoed some bikes and the first bike I rode reinforced my poorly researched theories. The second one had geometry numbers that were between my bike and this first bike so I thought it would be somewhere in the middle. But no, it felt totally cramped. On the first I felt like I was in a nice aggressive position, but on the 2nd I felt like I was on a 2001 era "north shore" bike with a 180 mm reach, 680 mm stack and 600 mm bb height. I went back to my bike, then back to the 2nd bike and it still felt shorter and taller. I went home and compared the numbers again, but it only confused me even more. It should have felt longer than my current bike but instead it felt shorter. I feel like I can no longer trust reach or stack or seat tube angle are any of these other hocus-pocus ideas. Is it all just made up marketing gibberish or did I miss something? Could it just be this one company posting bogus numbers? Please help.

hugs and kisses,

Geometry is Hard

Dear Huggies:

Your question got a few people around the old NSMB watercooler pretty excited. I wasn't one of them, but I'm always keen to play ball. As I dove in, I start to get a bit more interested as this is actually a pretty complicated question. But of course, as soon as I needed a bit more information from you, you decided that this was more of a "take" than a "give" sort of relationship, so we never did discover the 3 mysterious bikes you were discussing. Which is fine, really. Just fine.

Anyhow, this is something that we've all heard, and experienced, before. Two bikes that look the same on paper, but that somehow feel dramatically different when you ride them. For an example, we're going to use the experience of our own Cam McRae to discuss this phenomenon. If you all remember, Cam discovered the gospel of larger bikes with his review of the Santa Cruz Bronson. And then came back down to earth with his first ride of the Santa Cruz Megatower. And now is gushing about a Large Yeti.

All of these bikes look relatively similar on paper, other than the differing wheel size, travel and intention...And we'll throw the Jeffsy 29er into the mix as well, as that's a somewhat similar bike that Cam has been spouting the XL gospel about as well.

So, I took those 4 bikes, fired up the old Autocad (2002, if anybody is looking to understand what cutting edge tools are necessary for such a scientific analysis) and plotted them out to see what we could find. Prepare for some spectacular graphics.

From this, we learned a few things:

  1. My Autocad skills are amazing! My crop skills are subpar.
  2. If modern bikes were built like old school road bikes, they'd look pretty weird.
  3. I built my models using the classic dimensions (top tube length, bb drop, etc). My reach/stack numbers follow from that, and therefore don't exactly match up with what the manufacturers are stating. This is most likely due to differences in fork length and headset stack...but the numbers here are pretty close to those given by each company. (edit - the intention with doing it this was was to double check that the reach/stack measurements from the manufacturers made sense, and that there wasn't some error there that was the cause of what Cam was seeing. They did, and it wasn't)
  4. From a distance, all four of these bikes look pretty similar.

So, nothing really stands out from completing this exercise. We haven't seen anything that isn't on the geometry chart, but we've at least proven the numbers for ourselves. Let's see what happens when we overlay the bikes (bb fixed on the same plane).

This is definitely a lot more interesting. For me. The Megatower (edit) seems to be the largest (highest stack for sure, and close to the longest reach). The Yeti is the tightest (shortest top tube and reach, plus the steepest seat tube). The Bronson has the slackest seat tube and shortest stack. The Jeffsy floats in the middle. You can start to see how the Megatower might ride larger than the other bikes. All four bikes intersect fairly nicely at the head tube (edit) and wheelbase, and you would think there would be a lot of consistency between how these bikes fit. (edit) Indeed, there is only 0.36 inches (9 mm) difference in reach between the XL Megatower and the shortest bike, and 0.72 inches (18 mm) in bar height (not stack, as the BB heights aren't lined up).

What happens when we do the same comparison with the L Megatower (with everything else kept the same)?

This is also fairly interesting. You can see that the L Megatower shortens up considerably, but (edit) stack comes into better alignment. Indeed, there is now a bit more spread on reach, as the L Megatower is the shortest bike (0.87 inches/22mm between the Megatower and the longest) and the bar heights are much tighter (0.22 inches/5.5 mm from the Megatower to the tallest).

It doesn't slap you in the face that the guy who liked the XL Bronson and Jeffsy would feel more at home on the L Megatower...but you can also just squint a bit and start to see some kind of argument for it. This is why people get angry at one another on the Internet.

So. What is our takeaway from this? Beats the hell out of me. But if forced to make some sense of this:

1 - When we're talking about "fit", we're not really talking about "fit". All the above bikes "fit" Cam, they just ride a bit differently.

2 - If you just shop reach and stack, you might wind up being disappointed. Offset, wheelbase and a whole bunch of other things are going to have an impact on how a bike feels as well.

3a - If you get this "wrong", it's probably not going to be the end of the world, as long as you're within one size of where you think you should be. Probably.

3b - Or...if you get this wrong, it's going to be detrimental, and you're going to ride like crap until you get on the bike that is the right size for you. Maybe.

4 - Even guys who get to ride whatever they want, in whatever size they want, whenever they want, still can't always figure it out exactly.

5 - I'm not totally sure that I believe this myself, but it feels like something that could be true and that maybe needs exploring...but is it possible that the more aggressive the intention of the bicycle, the more likely it is that you might be drawn to a smaller size? The Megatower and SB150 are both pretty aggressive bikes that you need to be on top of...maybe that extra length is a bit too much (for some) when you throw everything together?

6 - It's possible that none of us actually know what we're talking about, so you probably shouldn't worry too much about what people are saying about bike fit. Buy the one that you think is comfortable. Consider that some people are enjoying bikes that are way, way longer than what they used to ride and are getting enjoyment from that. But others are exploring these worlds and retreating to slightly more familiar pastures.


Uncle Dave

Uncle Dave's Music Club

I've been slow on the shows lately, but had a few on the go this week. They both managed to blow the doors off in their own special way.

First up, Kevin Morby, who we've talked about a few times here, and who I'd never before managed to see live. He's a flamboyant showman, in his restrained indie rock sort of way. It was enjoyable, but the new album is...a departure. We'll leave you with this one here, as I haven't listened to it enough to find anything else.

On the opposite side of a similar spectrum is Jenny Lewis. Jenny Lewis, who manages to seductively sing about the darker sides of life, and can quietly and beautifully blow the doors off. It was raucous! I've been meaning to talk about Jenny Lewis for quite some time, and then she went and launched a new album and showed up in town, so it seemed like it was finally time.

Congrats Huggies! This week's prize is a monogrammed prize pack from Lizard Skins. Well - the grips will be etched with the message of your choice (up to 14 characters). Also included is one pair of gloves of your choice and Lizard Skins frame protection. Send us an email and we'll get you hooked up!

Gloves Lizard skins

Lizard Skins Gloves. Pictured left to right, the Monitor SL, Monitor and Monitor HD.

Got a question for Uncle Dave? Send it along to him here. Next week (or the week after), the reader who successfully tickles Uncle Dave's answer bone will win a 10 L Hydration Pack from Hydro Flask's new Journey Series.


Ask Uncle Dave a question, and if he chooses yours, you'll get to choose your colour in one of these 10 litre hydration packs from Hydro Flask. They have an insulated bladder compartment, so your 'water' will stay cold. They're also waterproof, so the rain won't dampen your spirits.

Trending on NSMB


+5 Vik Banerjee AJ Barlas Skyler Primož Resman ManInSteel
fartymarty  - May 27, 2019, 11:43 p.m.

Butt to Bar, Butt to Bar, Butt to Bar.

Know your seat height and measure away.  Your body can adjust to different reaches but can't if the cockpit is too short.  My Krampus has a 440 reach and Murmur has 515 and I can ride both because they feel the same seated.

Joe summarises it pretty well in the video on the Starling www https://www.starlingcycles.com/custom-geometry/


Heinous  - May 28, 2019, 1:16 a.m.

On the music front, Damien Jurado just put a new album out, and it kinda intersects the Morby weirdness and the Jenny lyricism...


+5 Cr4w Timer Mammal Alex D Primož Resman
Vik Banerjee  - May 28, 2019, 6:22 a.m.

People seem pretty hung up on Reach and ignore Stack when comparing bikes. Since Reach depends on Stack to make any sense that leads people to make erroneous fit assumptions. And while Eff TT has fallen out of fashion on the geo chart it's still pretty vital to pay attention to it  to understand how a bike will fit seated.

And I'd add that STAs are getting steep enough that you need to understand how your body feels about them because I think we've gone far enough in the steep direction that they are causing issues for some folks who never paid much attention to STAs in their slacker days. I'm in that camp, but I notice comments popping up on forums expressing the same sentiments so I am not alone. To be fair the same was true when STAs were quite slack. It was just different people who were complaining. I bring it up because if you are one of those people who don't get on with steep STAs and find out you need/want a slacker STA and say install a 1" setback dropper on your bike it's like going up 1 full size in a lot of brands for seated fit.  So that's something you need to factor in when selecting a bike.


AJ Barlas  - May 28, 2019, 7:05 a.m.

Well said, Vik. TT is for sure still important and yeah, people appear more focused on reach and don’t consider stack. I’d like to see geometry charts include the DT length. I find this more important than reach, at least on its own. I’ve been playing with calculating it based on the provided reach and stack but haven’t had enough time on enough bikes with it to see how accurate it is or what works for me.


+1 Primož Resman
Tjaard Breeuwer  - May 28, 2019, 7:09 a.m.

The problem with down tube length is that it depends on fork-crown-height.

If you had two bikes, one with a small wheel and short travel fork, and long headtube, and one with a big wheel and fork and short headtube, they might have identical fit and angles, yet the downtube would be very different.


Perry Schebel  - May 28, 2019, 7:19 a.m.

these days head tubes are all generally short enough that variation is typically not really an issue (at least in my personal experience). ie, i've always got at least one spacer in there.


+1 Velocipedestrian
Tjaard Breeuwer  - May 28, 2019, 7:24 a.m.

What I was talking about here was bikes with taller or shorter headtubes extending down, so with the same stack height.

Imagine for a second two rigid bikes with identical stack and reach:

One with a 27.5 wheel and one with a 29er wheel. The 29er would need a shorter head tube to keep the fit the same, since the crown of the fork is higher above the ground. 

Therefor the 29er would have a longer downtube, even though fit was identical.

That’s all I meant.

If you are comparing bikes with the same travel forks and wheel sizes, then yes, (effective) downtube length is an easy number to grab and compare.


+2 AJ Barlas Velocipedestrian
Perry Schebel  - May 28, 2019, 7:16 a.m.

dt is my favorite measure. an accurate (hard to confirm if manufacturers are always bang on with listed stack & reach numbers) comparitive number you can quickly grab yourself with a tape measure. it's delightful.


Velocipedestrian  - May 30, 2019, 3:27 p.m.

Damn, I just rebuilt (can't even think how many times) my oldest HT frame. I think part of the reason I keep coming back to it is the long reach.

Plumb bob and tape measure gives about 460-470mm reach, this on a 16" seat tube. But as it was designed for a rigid fork it's already sitting high with a 29er rigid at 435mm a-c. 

But - DT is 650mm which is short compared to the shorter reach bikes with longer forks in the house. Still confused, guess it doesn't matter since I'm not shopping for a new frame, just playing lego with the existing machines.


+1 AJ Barlas
Tjaard Breeuwer  - May 28, 2019, 7:34 a.m.

@AJ Barlas said: “’I find this more important than reach, at least on its own.”

I think that is the key. Too many people look at just one or two dimensions and then complain that bikes don’t fit or feel the way they though they would based on that.

Unfortunate there are no shortcuts. If you want to be able to compare all bikes, you have to use multiple dimensions.

There is no single “one and done” number that will tell you everything


+1 Skyler
Tjaard Breeuwer  - May 28, 2019, 7:23 a.m.

I do not recommend paying attention to (listed) effective toptube.

Here is why:

First off, everyone should have their saddle set in a fore-aft position that works best for them. Not at the place where the bike designer happened to put it.

That means that you adjust your saddle position to the same spot on every bike (for a certain use and style of riding).

Let’s take two imaginary bikes, with identical front centers (stack and reach and head angle). One has a steep seat tube angle, one has a slack seat tube angle. Effective toptube will be much longer on the second bike. But when you buy either of the two bikes, you would adjust your saddle position so it ended up the same distance behind the bottom bracket. The seated reach to the bars would be identical.

The thing that is important is effective seat tube angle. To see whether it is at all possible to get your saddle in your preferred position. Unfortunately, it is impossible to calculate effective seat tube angle at your own saddle height, based off the info in a geometry chart.


+1 Tjaard Breeuwer
Skyler  - May 28, 2019, 8:07 a.m.

Yes, and since Uncle Dave based his model off of listed effective seat tube angles, they mostly fail at communicating where your saddle actually ends up.

I can at least confirm that, even slammed all the way forward on the saddle rails of my XL Bronson, my seat doesn't end up anywhere close to a 75° STA. My hardtail has a true, straight-tubed 75° STA, and the horizontal setback from BB to the top of my seat post is 182mm. The 75.3° effective STA on the Bronson actually gives a riding position ~75mm further back over the rear hub at full ride height, due to the ultra slack actually sta. Accounting for suspension sag, those numbers diverge even further.

The only info needed to communicate this in a geo chart are: a) actual seat tube angle, and b) the horizontal distance that this line is offset from the BB.


+1 Luix
Dave Tolnai  - May 28, 2019, 8:18 a.m.

I purposefully built the model using the old school measurements as part of the goal was to check that the stated reach and stacks were accurate, or if that was a part of the problem.  Everything came out pretty darn close, and I attribute any differences as much to headset stack height estimates and wheel size estimates as anything.  Reach and stack don't tell you where your saddle ends up either.


Skyler  - May 29, 2019, 3:33 p.m.

Fair. I know you were mostly comparing reach and stack, but even though people don't give it any credit these days, I think when someone complains about a bike feeling too long, they're actually usually talking about the full cockpit length, from saddle to bar. So, a meaningful way of communicating STA should matter a lot...

Kind of a tangent to your point, so don't take it as criticism.


+3 Kos Skyler Mammal
Michael  - May 29, 2019, 10:20 a.m.

This ^.

How far back ones saddle is relative to the BB is how I determine how that bike will climb steep terrain. If it has a large setback (as a result of my desired cockpit fit) and a short rear I'm not gonna like it. 

I don't really account for reach its a resultant dimension after I account for all the other dimensions in particular my seated cockpit length and stack.


+2 Timer Mammal
Tjaard Breeuwer  - May 28, 2019, 7:05 a.m.

Good point Vic!

There are many people remarking that they don’t understand how bikes fit differently than what they expected based on the geometry chart. I think there are two issues that cause this confusion:

  1. They don’t set up the bike to their own fit. In other words, they don’t set the saddle to the same height and set-back from the bottom bracket, and they don’t set the grips the same distance and drop from the saddle. (And they use different saddles and shaped/widht bars)
  2. They don’t understand the relationship between stack and reach.

With modern mtb’s, the head angle is so slack, that a lower stack makes for a significantly shorter effective reach.

Effective reach’ being the horizontal distance from bottom bracket to the center of the steerer tube at your handlebar height.

Imagine two nearly identical bikes, the only difference being that one has a tall head tube and the other one has a short head tube. let’s say you run no spacers on the tall head tube bike. If you owned the short head tube bike, you would add spacers to get the bar at the same height. These bikes would fit, feel and handle identical. Yet, the second one would have a longer reach, and shorter stack.

If you want to approximate this when looking at geometry charts, you can subtract 0.4 times the difference in stack from the reach of the lower bike.


+3 Tjaard Breeuwer Skyler Jolly_X_Roger
Alex D  - May 28, 2019, 6:14 p.m.

To simplify this, we can normalize reach and ETT for a desired stack and saddle height.


  • Reach = R
  • Stack = S
  • Head Angle = HA
  • Seat Angle Actual = SAA
  • Desired Stack = DS
  • Desired Saddle Height = DH


  • Adjusted Reach = R - (DS - S)/(tan(HA)
  • Adjusted ETT = ETT - (DS - S)/tan(HA) + (DH - S)/tan(SAA)

Adjusted reach is reach for a given stack. Adjusted ETT is ETT for a given stack and saddle height.

If you're doing these calculations in Excel or Sheets, enclose each angle with RADIANS(). If a manufacturer doesn't provide an actual seat angle, you can measure it with a side profile of the bike in Photoshop or equivalent.

Let's compare (for example) the SB150 and the Megatower and assume these desired values:

  • Desired Stack: 665
  • Desired Saddle: 720


  • ETT: 654
  • Reach: 505
  • Stack: 635
  • HA: 64.5
  • SAA: 71.7


  • ETT: 646
  • Reach: 490
  • Stack: 643
  • HA: 65
  • SAA: 67.8


  • Adjusted reach, Megatower: 480
  • Adjusted reach, SB150: 491
  • Adjusted ETT, Megatower: 667
  • Adjusted ETT, SB150: 668

For this hypothetical rider, once the saddle and bars were set up, we'd expect identical seated positions and about 1cm more room standing from the SB150.


Tjaard Breeuwer  - May 28, 2019, 7:02 p.m.

Only question I have:  When you are talking about Seat angle actual, how do you account for the offset of most seat tubes from the bottom bracket?


+2 Tjaard Breeuwer Skyler
Alex D  - May 28, 2019, 7:44 p.m.

ETT accounts for that. It gives us the exact position of the saddle at the bike's nominal stack height. We only care about the effect of SAA on vertical offsets from that position.


Tjaard Breeuwer  - May 29, 2019, 9:50 a.m.

Thanks, I get it now!


ExtraSpecialandBitter  - May 29, 2019, 9:03 a.m.

I'm curious as to what this would actually look like in real life.  

How many spacers would you be adding to each bike?  

Where is the desired reach measured to (center of stem at steerer or at handlebar)?

It's also crazy to me that my saddle height is ~725mm to the rails and I ride a medium...  I must have the proportions of a t-rex.


+1 Mammal
Kos  - May 29, 2019, 5:56 p.m.

Being an (old) engineer and physics geek, I checked this stuff out.

Genius! And extra credit for just using the word "radians" on a bike website.

Might be worth making this easily clickable on the NSMB  site.  Take that PB!

Caveat:  I suppose somebody will prove this wrong, but it sure seems solid.


+1 Tjaard Breeuwer
Alex D  - May 30, 2019, 7:35 p.m.

The caveat is that these are full-suspension bikes. It's not clear to me if geometry charts provided assume a certain sag, or if the profile shots I'm using to gauge the SAA show a sagged bike. And of course, riding is quite dynamic. This approach is really just a guideline for the approximate location of the contact points. It'd be more definitive with a hardtail or a road bike.


+1 Alex D
Jolly_X_Roger  - May 29, 2019, 11:03 p.m.

Your Megatower numbers are off since you used the XXL stack instead of the XL one. Otherwise I like your approach.


Alex D  - May 30, 2019, 7:31 p.m.

Good catch, I updated the comparison.


cxfahrer  - May 28, 2019, 10:44 p.m.

Just wanted to say that the Lizard Skins gloves are a very tight fit, and start disintegrating soon, because of poor seams.

Otherwise they are great.

Sizing and reach-stack was always easy for me. Until now since 1990 every bike was too small for me anyway, so I always chose the largest and had to adapt. That XL Serotta was so tiny, I had to run a 150mm stem. Going OTB on a slight descent was normal. Nowadays I could choose the XXL Jeffsy and still it would not be too large. My XXL Capra is definitely on the small side.


IslandLife  - May 29, 2019, 4:28 p.m.

I've found the best way of navigating through these murky geometry numbers is by watching these "Understanding Advanced Geometry" vids (part 1 and 2) from Vorsprung Suspension:




Kos  - May 29, 2019, 5:51 p.m.

Bigger, not smaller, bikes for more aggressive riding for me.

Jenny Lewis FTW.  Check out her old Rilo Kiley stuff, if you haven't already.


+1 Tjaard Breeuwer
Dave Tolnai  - May 29, 2019, 9:30 p.m.

Not quite what I meant.  The more aggressive the geometry of the bike (slacker, more trail, etc) the more likely you can get away with, or prefer a smaller bike.  Using a yeti sb5.5 as an example, I had to consciously weight the front way more than I was used to.  Had it been a size larger, I’m not sure how it would have worked out, whereas on a less aggressive bike some extra length would be fine.  Like I said, I’m not fully sold on this idea, but if you take Cam’s experience with liking an XL Bronson vs a L mega tower, it sort of makes sense.


Tjaard Breeuwer  - May 30, 2019, 5:23 a.m.

That makes perfect sense.

A slacker bike with more trail will have a longer front Centre and wheelbase(all else being equal). So you don’t need extra reach to create a long wheeelbase for stability. The same goes for over the bars, the front wheel is already further out in front, so OTB is less likely, even with a shorter reach.

Finally, the longer front center reduces weight on the front wheel, compared to a bike with the same rear center and reach, but steeper head angle.


+1 Tjaard Breeuwer
fartymarty  - May 30, 2019, 5:40 a.m.

Hence DH bikes generally have a shorter reach than trail / enduro bikes.


+1 Tjaard Breeuwer
Perry Schebel  - May 30, 2019, 10:09 a.m.

this (having to more aggressively weight the front of longer / slacker FC bikes) is also a side effect of (most) companies using the same swingarm for all size bikes. which is dumb, imo. with a correctly matched RC, all sizes should be able to carve nicely with a more centered riding position. not sure why what norco (and now forbidden) is doing (achieving variable RC's with the same swingarm by adjusting the bb to pivot distance) isn't standard practice.


Mammal  - May 31, 2019, 12:08 p.m.

Fully agree. I give Norco (or Owen Pemberton) a tonne of credit for starting to do this so long ago. Did Norco start that when Owen first arrived, or prior? Can't recall.


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