Stack and Reach For Mountain Bikes = BS

[joeg26er]

Ok - got into a discussion with a triathlete friend / roadie.

They said that

“The great thing about this sizing method is that anyone with a piece of string with a weight on it and a measuring tape can accurately measure their current bikes. The rider can then compare their current bike to any brand using Reach and Stack and know exactly how it will fit without even riding it.”

I said - BS - that only applies to Road Bikes and Tri bikes. My position is that using only stack and reach is not sufficient and ETT is more important.

Stack MIGHT be OK (there are issues with using that for MTB) but I can assure you that sizing an MTB by Reach is BS and should NOT be used to pick MTB fit. Effective TT is best in that regard.

Why? Well, MTB's these days can run the gamut of seat tube angles. Since Reach is defined vertically from the BB it does NOT take into account a super slack seat tube angle or one that might be steeper than normal.

Reach might be useful for MTB sizing IF one never sat down when riding OR IF like in road or tri bikes, the seat tube (and headtube) angles were 99% consistent.

Stack height is easily obviated in importance since MTB's have forks that can have a super wide range of axle to crown measurements AND sag.

Thoughts?

[Marin]

He's right.

Your position over the BB is what counts, and saddle setback will be used to correct for the weird seat angles some MTBs have.

[savechief]

The purpose of stack and reach is to be able to easily compare bikes between manufacturers using two defined, common points. Effective top tube can be measured differently between manufacturers. Some measure starting at the center, top of the head tube. Others measure starting at the center of the head tube, but from where the top tube intersects that center line. Since head tubes are angled, this will give two different starting points and two different numbers on the same exact bike.

Based on your lower body geometry, there is an optimal point in space for you to sit in relation to the bottom bracket. Once you know that point in space, you should maintain it from bike to bike, regardless of seat tube angle. You can accommodate different seat tube angles through choice of seatpost (setback or straight) and moving your saddle on the rails. The fact that Reach ignores the STA is a *good* thing, not a bad thing as you indicate. When you get to the point that the saddles are set up identically in relation to the bottom bracket, the ETT measurement is not relevant, and Reach is what will indicate how far away the handlebars are when you are seated. Standing on the pedals, seat position relative to BB and ETT both become meaningless, and Reach is really the only thing that will determine how "long" or "short" the bike feels.

Yes, the Stack measurement is dependent on the axle-to-crown length of the fork, but typically manufacturers provide the A2C length they used when measuring Stack, so compensating for a fork form a different manufacturer with a different A2C measurement is easy. But again, Stack and Reach are really to compare frames from different manufacturers with all other variables being equal. It's the best way to determine what size you should buy of a different manufacturers bike if you are riding a bike in which you're happy with the current setup. What stem length will I need? How many spacers will I need? Will I be able to get my handlebars equally as low as they currently are? Does that manufacturer even have a size that will allow an identical position? Those are all questions that Stack and Reach can help answer. ETT cannot.

[boots2000]

I will say that both of you are somewhat correct and somewhat incorrect.

Stack and reach will give you a general idea, as will ETT.

But where they both fall flat in terms of sizing an mtb is what happens when you sit on it (especially for a full suspension bike).
Different suspension systems go in different directions and by different amounts when you sit on them.
So what you might think is a 73.5 degree seat angle might be a 71 degree seat angle when you are sitting on the bike.

So- setting up an mtb requires finding the seat position where you have your ideal setback- and then deciding what stem you need to put the bars in the correct place.

The other thing that is making mtb fit funny is the "idea" that short stems and wide bars are needed to make the bike "ride" better.
Stem length should be fit driven- If you put a 30mm stem on your bike it could likely affect your fit in a negative manner.

I know the trend in bar width is for wider bars- I am not even going to get into that. But- the wider bar will change your needs as far as stem length goes.

[joeg26er]

1) MTB riders are never as static on their bikes as road/tri riders, where they obsess about a mm cleat position or exactly where their knee bone is over the pedal axle or behind or in front of it by a few mm. We (MTBers) are much more dynamic on the bikes. Heck, some of us even use platform pedals the size of small frying pans.

2) MTB's have funny angles and HT/ ST angles are not within the narrow framework that road and Tri bikes are in

What I am taking exception to is the stack / reach dogma as has been outlined above. Those people don't seem to realize that ETT is much simpler and DOES take into account the funny and variable angles to a greater extent than stack which TOTALLY ignores what's behind the vertical line above the BB axis.

Not buying into the straw man argument that ETT is not standardized. It's always or should be center of HT to center of ST and even if it was measured not at the top of each tube that would only change by a few mm.

Here are two imaginary Mountain bikes- tell me that both will fit / ride the same as they have the same stack and reach?
The first red line is the seat tube. The second red line is the down tube. The blue lines are stack and reach.

Yes, neither is perfect but ETT tells more about what the bike will be like as far as the cockpit fit both sitting and standing/moving around.

[joeg26er]

The other thing that is making mtb fit funny is the "idea" that short stems and wide bars are needed to make the bike "ride" better.
Stem length should be fit driven- If you put a 30mm stem on your bike it could likely affect your fit in a negative manner.

I know the trend in bar width is for wider bars- I am not even going to get into that. But- the wider bar will change your needs as far as stem length goes.

Well I am going to get into that - wider bars are for more leverage against steering forces. Have you tried wider bars? I used to be a narrow bar rider and I can tell you when I switched to wider bars, technical sections got much easier due to the physics of leverage. Certainly, one can only go so wide in proportion to their physical dimensions. But the physics advantage of going wider on one's bars is not disputable. Anecdotally, going to wider bars did not change my stem length.

Stem length is fit AND steering dynamics driven. Each bicycle has a certain weight distribution that is what the designer wants. The individual can fine tune that as they see suits themselves. If somebody must use a super short or super long stem due to their personal prefs or due to purchasing the wrong size and it is outside a certain tolerance, handling will suffer and definitely will be outside the envelope the specific geometry was designed for.

[boots2000]

Your argument on ETT holds no water. This is because the part of the triangle behind the bb is moving with a full suspension bike.

I have tried wider bars- I agree that we can ride wider bars than was the style 10-15 years ago, but they still need to be in line with body size. Many riders ride with bars that are way too wide.
Yes, this gives you leverage, but at the cost of a relaxed upper body.
Also depends on where you ride. For me, if I go wider than 675mm I am hitting trees and there is one trail where I consistently hit the rock wall on one side of the trail.

I disagree- handling will suffer as much on a bike with too short of a stem as too long of a stem.
A short stem and super wide bars just serves to handcuff the rider.
Here is my formula- reach should be longer than your reach to tops on a road bike, shorter than reach to hoods. Drop is variable, but a good place to start is half of road bike drop.

The other thing that is making mtb fit funny is the "idea" that short stems and wide bars are needed to make the bike "ride" better.
Stem length should be fit driven- If you put a 30mm stem on your bike it could likely affect your fit in a negative manner.

I know the trend in bar width is for wider bars- I am not even going to get into that. But- the wider bar will change your needs as far as stem length goes.

Well I am going to get into that - wider bars are for more leverage against steering forces. Have you tried wider bars? I used to be a narrow bar rider and I can tell you when I switched to wider bars, technical sections got much easier due to the physics of leverage. Certainly, one can only go so wide in proportion to their physical dimensions. But the physics advantage of going wider on one's bars is not disputable. Anecdotally, going to wider bars did not change my stem length.

Stem length is fit AND steering dynamics driven. Each bicycle has a certain weight distribution that is what the designer wants. The individual can fine tune that as they see suits themselves. If somebody must use a super short or super long stem due to their personal prefs or due to purchasing the wrong size and it is outside a certain tolerance, handling will suffer and definitely will be outside the envelope the specific geometry was designed for.

[mattr]

Is the op getting confused between the on trend DH gnar core point and shoot sleds.
And XC bikes.

I suspect so.

[bm0p700f]

My position on my xc bike is similar to that on my road bike while on the hoods. I pick a frame for that position. Yes we move about more but to ride a mtb quickly your position still has to resdonably aero. I ride rigid bike though and i dont move about that much. With xc ridking (the way it used to be) i find i sit alot.
However i pick the stem length e.tc in conjuction with the frame size because i knew it would work. So i would agree you pick stem seat post set back e.t.c in conjuction with the frame geometry but not to make the bike fit but so it works. You pick the right frame to get the right fit. But this is what is done with road bikes. It is pretty obvious really.

For a traditional xc riding ett length is quite important. My position does not work for rocky dh decents or the way trail riding has evolved. This is why fitting some one for an mtb is more complex.

I am not sure ehy thi

[DamonRinard]

Stack and Reach for mountain bikes are not BS. They're just numbers on a geo chart that represent two useful dimensions.
ETT is still there on the chart too because it's still useful for choosing the right size bike.
It's not one or the other.

For MTBs especially, OP is right in that weight distribution matters more than on road, so STA should also be taken into account.

[Calnago]

Well said Damon

[TheKaiser]

My position is that using only stack and reach is not sufficient and ETT is more important.

Stack MIGHT be OK (there are issues with using that for MTB) but I can assure you that sizing an MTB by Reach is BS and should NOT be used to pick MTB fit. Effective TT is best in that regard.

Why? Well, MTB's these days can run the gamut of seat tube angles. Since Reach is defined vertically from the BB it does NOT take into account a super slack seat tube angle or one that might be steeper than normal.

Stack height is easily obviated in importance since MTB's have forks that can have a super wide range of axle to crown measurements AND sag.

Thoughts?

One of the primary benefits of reach is because of those crazy seat tube angles on some bikes. Reach allows you to ensure you have adequate cockpit length when standing pedaling (so you aren't slamming your knees into your shifters). It also ensures that once you have adjusted your saddle to your desired position you still have an appropriate cockpit length, rather than being fooled by the "long" ETT on a bike, but not taking into account that it has a 70 degree Effective Seat angle, (or the inverse for one of the new enduro bikes with a 76 degree seat angle). If sizing by ETT on one of those bike, you may find that once your seat is adjusted to your desired position it doesn't work as you expected it would. Desired saddle position, for most ergonomically oriented riders, exists as an ideal totally independent of the seat angle that a given bike happens to have.

The fact that MTBers move around on the bike more has nothing to do with it, assuming that you still have a preferred saddle position and agree that the standing reach on a given frame could be either to long or too short.

The measurement of Reach ensures that you know how a bike will fit while standing and, for most normally proportioned riders, who desire a saddle position that falls in a reasonable range +/- of KOPS, then you can almost assuredly achieve your desired saddle position with, at most, a seat post swap. If you size only by ETT, then you could conceivably end up with a bike that has your desired cockpit length when seated but at the expense of your desired standing fit and/or saddle position relative to BB.

ETT becomes valuable too if you prefer a short reach when standing but like to stretch out when seated, or some sort of unsual "dual personality" to the fit of the bike, but again, that assumes that you are cool with letting desired cockpit length drive your saddle to BB relationship. You don't even need ETT to figure that out though, as you could simply look at the reach and the STA to figure out how much seated reach capacity the bike will have.

I think that it is valuable to consider a few more points:

1. There are people who don't seem to understand that (all else being equal) a slack seat angle with a straight post and a steep seat angle with a layback post will equal each other. As long as the same rider position is achieved, there is no difference.

2. There are people who still seem to think it is justified to adjust cockpit length by moving the saddle, rather than adjusting the saddle in relation to the BB to achieve desired pedaling mechanics, and then using the frame/stem/bar selection for cockpit adjustments. People in the former camp often don't see the pure beauty of Stack and Reach as they don't hold accurate pedaling mechanics with the same sort of reverence as people in the latter camp.

If you think it is a good practice to let STA drive saddle position as long as the ETT is in your desired range, then feel free, but that is putting yourself at the mercy of any given bike designers whim, and will result in a very different actual fit depending on if, for example, you buy a Knolly vs. a Mondraker (particularly so if you have long legs and run the saddle quite high).

[KWalker]

Stack and reach help you compare two frames relative to each other on paper. How you ride the bike and what your setup needs from there differs. Suspension designs and tube angles may influence how the bike rides, responds and turns and might influence your stem stack and reach as well as your weighting over the bike.

For a hardtail it is slightly easier, but it is the effective STA with tires on that matters WITH the front suspension sag taken into account. I found that using a plumb line that the saddle setback with and without weight on a hardtail changed the effective nose behind BB position. With a FS bike it is even more so.

My solution was to use the good old fashioned and much ridiculed static and dynamic knee over pedal so that I take the mechanical orientation from my road bike and mimic it on the MTB. I'm not directly over the spindle on either, but the amount relative to the spindle is the same on both. That gives you a starting point that you can refine by riding, climbing, etc.