[Tjaard]

Basically, if you are over 6'4", here is my flow chart for bike geo selection:

-Look at Asollies chart to see where your proposed bike fits in the world of big bikes. https://public.tableau.com/profile/alexander.sollie#!/vizhome/BikeAnalysis_0/StackandReach if it's not fairly far on the right, and near or above the axis, don't bother.

-Reach must be over ~515mm. Normalize for reach. Of all the bikes you are comparing, subtract 0.4 times the stack difference from the tallest bike from the reach of the lower bikes to give you the comparable reach. In other words, if you set all the bikes up with the grips at the same height, how would the reach compare.

-If stack is low, compare it to a bike you owned to see how high you need to raise the bars.

-Check wheelbase/rearcenter ratio. Is it more than 2.9? Bad choice. Can you modify it (adjustable chaistays, steepening headangle possibly with shorter offset fork)? Is it under 2.8? Wonderful(and rare)!

-What is the claimed effective seat tube angle? What is the actual angle? If the actual angle is slack, our effective angle will be much slacker than the listed one. If so, are you willing to modify it(run an offset seatpost backwards)? Are the chainstays short? The shorter they are, the bigger the problem slack seat angles become.

-Don't exclude bikes because they aren't exactly the right type of bike. We simply don't have that luxury. In other words, if you mostly ride moderate trails, but the best fitting/geometry bike is an enduro bike, you will still be happier on that than on a poor fitting or handling trail bike. Run the suspension a bit firmer, put some faster rolling tires on. Obviously this isn't to say to buy a DH bike for XC or vice versa, just that maybe moving up or down one category is better than a compromised fit or handling.
The one exception would be for Clydesdales. Don't "underbike" especially if you shuttle or lift serve a fair bit, simply because of durability.

 

[Tjaard]

Fork offset

Thanks @tjaard and @alexbn921. So what range offset should I be looking at?

Unfortunately there are few options to choose from. First thing to do is find out what the stock fork is that's used on the bike to get the lifted geometry.

'Standard' offsets are 51mm for 29ers and 45mm for 27.5 forks.
'Short' offset is 42(Sram) or 44(Fox)mm for 29ers, and 37mm for 27.5. Most other brands are similar.

Besides simply changing from a 'standard' to a 'short' offset fork there are a few other options:
You might be able to install the CSU of a 27.5 fork on a 29er fork.
Most 27.5 forks will run 3.0" tires. This size is staring to get close to 29er wheel diameter. So, if you have or want large wheels, maybe a 27.5 fork with 2.8 tire will do the trick for you (note that you will need to have a longer travel fork to make up the axle to crown height).
I did this when the short offset Lyric I wanted wasn't available for my 29er. Instead of the 150mm 29er Lyric I run a 170mm 27.5 fork. Same fork height.

 

[Joules]

Some good info, some not so much:

Asollie's chart, hell yes. More useful info there than every bike publication I've ever seen combined.

Reach, yes. Stack... not so much. Most of the variance comes down to fork travel, almost all bikes have the shortest feasible head tubes so people can 'size up'. If you're installing a new fork, you can get a lot more range by leaving the steerer longer. A legit hi-rise bar would solve the problem totally, but no one makes one now.

wheelbase/chainstay ratio and effective vs actual seat angle - this is huge. The problem is: first, changing the fork offset is going to change the ratio by at most 0.01 - going from a ratio of 3:1 to 2.99:1 isn't worth doing. You aren't the first person I've seen suggest this, but it still doesn't make sense to me. Shorter offset forks are good a lot of the time, but the effect they have on wheelbase is negligible (I think people that suggest this are falling for the fallacy of qualitative thinking - "shorter is an improvement" without considering how much shorter. That's the same logical mistake that gave us 27.5).
Changing the head angle to shorten wheelbase is an utterly stupid idea unless you want your bike to handle like a shopping cart, or want to reminisce about riding in the 1990s (which is to say, riding a bike that handles like a shopping cart).

Type: yes! The more travel, the more stack, if stack is an issue for you, that's probably the only way you're going to solve it. Bigger bikes tend to be built stronger and stiffer, especially if you aren't riding it in it's 'indented' trails (i.e., trail riding on an enduro bike). DH bikes are the exception, and you're not generally going to be able to get the seat to a pedaling position, and many don't have long enough seat tubes to run a dropper. Plus DH wheel spacing and 83mm bottom brackets.

 

[Tjaard]

Some good info, some not so much:
Reach, yes. Stack... not so much If you're installing a new fork, you can get a lot more range by leaving the steerer longer. A legit hi-rise bar would solve the problem totally, but no one makes one now.

My remark about checking frame stack is for two points:

1. Can you even get the grips where you want them (is the fork steerer long enough, are witin a safe marking for the height of the spacers you are using), can you find a bar with sufficient rise?
2. Assuming you CAN raise the grips to where you want them, due to the slack angle of mtb's, you are bringing them back as well. It doesn't matter wheter this is by spacers or risers bars. For every 10 mm that you raise the bars, they come about 4 mm back. This is why stack matters, EVEN when you are able to set the grips at the height you want.

wheelbase/chainstay ratio and effective vs actual seat angle - this is huge.

The problem is:
first, changing the fork offset is going to change the ratio by at most 0.01 - going from a ratio of 3:1 to 2.99:1 isn't worth doing. You aren't the first person I've seen suggest this, but it still doesn't make sense to me. Shorter offset forks are good a lot of the time, but the effect they have on wheelbase is negligible (I think people that suggest this are falling for the fallacy of qualitative thinking - "shorter is an improvement" without considering how much shorter. That's the same logical mistake that gave us 27.5).

I agree that changing fork offset isn't a very significant change, but I figured everyone can do the math for themselves. Mostly the fork offset can go along with:

Changing the head angle to shorten wheelbase is an utterly stupid idea unless you want your bike to handle like a shopping cart, or want to reminisce about riding in the 1990s (which is to say, riding a bike that handles like a shopping cart).
.

Now you are falling for the reverse fallacy: "super steep head angles are horrible (1990's ~71 degrees) so a slightly steep angle (~66 degrees) is horrible too".

I agree that modern bikes with slacker headangles are much better than very old bikes with super step HA's, but much depends on rider, terrain and the rest of the geometry as well. Slacker isn't always better. Somewhere between 62 degrees and 68 degrees is probably a great range for most bikes. Within that range, the optimum depends on so many things.

Secondly, you are ignoring the adjustment to fork offset.
When we say that bikes with steep head angles were bad, we are talking about two(and a half) things:

1. Low trail meant the wheel was easily knocked around and started turning when you didn't want it.
2. Short front center meant you were likely to go over the bars. (Made even worse when #1 caused your wheel to flip sideways on a G-out)
3. 2.5: Short short wheelbase (from front center) and low trail combine for instability at high speeds.

However, if you look at those statements closely, you see that for our modern, super big, bikes they don't apply:

1. Short offset means that even with a (slightly!) steeper head angle, you can have the same or more trail as a slacker HA and long offset
2. with the reach being 40-60mm longer, the front center is as well, even without adjusting the head angle.
3. 2.5: Due to our long front center, and especially if combined with an appropriately long rear center, our wheelbases are more than long enough for stability already.

And finally, the remaining part of poor handling was due to the super long stems, now we have longer reach we can finally use shorter stems comfortably.
(For example, I had a 140mm stem on my 2008 bike. With a 460mm reach that was required for a decent fit. Now, with nearly 520mm reach and wider bars I can finally use a 50mm stem).

 

[Joules]

Assuming you CAN raise the grips to where you want them, due to the slack angle of mtb's, you are bringing them back as well. It doesn't matter wheter this is by spacers or risers bars. For every 10 mm that you raise the bars, they come about 4 mm back. This is why stack matters, EVEN when you are able to set the grips at the height you want.

I say again: 2 frames with the same wheel size and fork travel will have roughly the same stack. It's true that more spacers under the stem moves the bar back, but you could go with a longer stem by the same token.
It is absolutely false that bar rise has the same effect. If you set the rise perpendicular to the ground, as everyone does, it has no effect on reach, only rise.

The rest, I can agree to disagree (though telling people that fork offset will improve front-rear weight distribution is misleading, it will on paper, but to such a small degree that no one one earth is sensitive to notice it). And head angle... sure if you were riding a Pole with an extreme slack head angle, you could steepen it a little bit. Most bikes however are already in the sweetspot, and messing with it is trading something important for the sake of something less important. The only situation I could see that sort of working is if someone up-biked, getting a "enduro" bike for more like "xc" riding, where the bike had an appropriate HA for it's indented purpose, but not what the rider had in mind.

[I'll admit though, I like bikes really slack. I think 68 HA might be appropriate for a road bike these days. In 30 years of riding, I've never sat on a bike and thought "this is too big" or "this is too slack"]

 

[Tjaard]

I say again: 2 frames with the same wheel size and fork travel will have roughly the same stack.

First of all, they don't always in the bigger sizes. Some brands choose to go taller, some shorter. Second, what if you are comparing bikes with diffent fork travel or wheel size?
Third, if the bikes you are comparing are the same, then you are set. I am just reminding you to check the stack, just in case it is different.

It's true that more spacers under the stem moves the bar back, but you could go with a longer stem by the same token.

Yes you could. I am not saying that you can't make it fit, just how to compare it. As your statement points out, a bike with the same reach and HA, but lower stack, once you add spacers to bring the stem to the same height, you are using a longer stem. This is just another way that shows that the comparative reach is shorter. If you are ok with that longer stem, you could have chosen a bike with taller stack and shorter reach.

It is absolutely false that bar rise has the same effect. If you set the rise perpendicular to the ground, as everyone does, it has no effect on reach, only rise.

First off, I sincerely hope no one sets their bars perfectly bolt up right for no good reason. For a very small number of people straight up might be the correct roll, but most people need to roll the bar a bit. Handlebars are not broomsticks, so bar roll will change the orientation of the grips in all planes.

However that isn't relevant to the discussion of frame selection. If you do have your riser bars straight up that that will give you the same grip position as more spacers and a longer stem, which again shows how it does affect reach. It doesn't matter how you get to your grip positions, whether it is stem, spacers or bar or any combination of the three.

The only thing that matters is how far in front of and above the bottom bracket your grips are. (What Lee McCormack calls RAD)

The perpendicular distance from the steering axis to your grips has an effect on handling. Commonly this is talked about as "stem length" but in reality it's only the stems component perpendicular to the steerer, and the bar sweep needs to be subratacted from this.