What are the hard truths of mountain bike fitting?

Theres no such thing as mountain bike fitting

Being centered up and down

The only unbreakable thing I can come up with is the rider ending in a comfortable position. In my mind everything else can vary a bit depending on how much a rider wants to tailor their fit to pedaling or descending. I think it would be hard to apply cut and dry rules like you can with a road fit.

Yeah, comfort is key. If you're not comfortable, you won't have a good time, which is what most people are after. Optimizing the seated position for power is great if that's the focus of your riding. Similarly, if you're looking to boost jumps all day, setting the bike up for that is more important.

I think there are a lot of similarities to road bike fitting in terms of the very basic question "should you choose the bigger size or the smaller size in the same bike?" I think the two big numbers are height and inseam length, which together determine how much leg you have projecting up and back from the pedals and how much torso you have reaching forward and down to your handlebars, i.e. where do your hips end up above your bottom bracket and how far forward can you reach. But those two numbers don't tell the whole fitting story. There's a great article from Steve Hogg about "functional torso length" that I think does a good job of explaining why some tall people with bad hip or back mobility end up on bikes that are "too small" and love them and why some shorter people with incredible hip and/or back mobility end up happily riding bikes that are "too long."

https://www.stevehoggbikefitting.com/bikefit/2011/05/seat-set-back-for-….

I love Steve Hogg's articles about bike fitting, and recommend them if you're looking to read on bike fitting theory. If you're more of a yougoober I'd recommend the Road Cycling Academy videos with Neill Stanbury. They're excellent:

https://www.youtube.com/watch?v=ifCKYTNoLZA&t=751s

But as others have said, there are a million factors that go into MTB sizing and fitting that are very hard to parameterize, like uphill or downhill preference, steeper or flatter trails, bike fitting history and preference (old school vs. new school reach length), riding technique (forward or back on the bike, vertical or horizontal stance, etc), skill level, etc. I love your project, but I agree with others that it will be hard to define a "perfect fit" for someone without physically getting them on a test bike. And unfortunately, one of the things you'll hear over and over again in those Steve Hogg articles or in the Neill Stanbury videos is that they've been extremely unimpressed by "fit systems" or "fit calculators" that plug people's dimensions into a computer and provide a claimed "ideal bike fit." Essentially, their argument is that bodies are endlessly heterogenous and in road bike fitting there too many factors to ever account for in a computer system, at least in any computer system they've seen before.

On the bright side, I actually think it's easier to fit a person to a mountain bike than a road bike because the body position is so much more static on a road bike that tiny little differences become painful and can lead to overuse injuries in a way that you don't see as often in MTB. So if you're just looking to fit someone on a large vs. a medium of the same brand, I think that's a doable goal. Setting up bar height, saddle height, stem length, saddle tilt, etc seems pretty hopeless on a computer.

I come from the MX side, and I'm still amazed you can buy a bicycle in adult sizes S-XL. If you went down to the local Yamaha dealer and wanted to buy my motorcycle, it comes in YZ250 size. MX riders have to adjust their handlebars and get on with it. 

Yes there is.  While some parts may not apply but the biomechanics of seared pedaling apply to a mtb. 

You don't need to pedal a motorcycle for 4 hours up hills... 

I think that there needs to be a distinction between a correct bike fitting and some fundamental imbalance in bike geometry.

To be specific, I observe that a lot of people downsize because of the understeer tendency of modern bikes (need to push against the handlebar to get a good cornering grip). It could get a better overall experience but it does not correlate with the best fit.

For the fit in itself, I believe the sitting position fundamentals are pretty well understood with the roadside of biking, we just need to correct for the angle of the ground since we are only sitting while climbing in mountain biking (somewhere around 5-6 degrees). 

That being said, I think we could better measure the center of mass ( the weight kinda thing with the knee). A better reference point would be our navel.

The more challenging side of mountain bike fitting is the upright position. The goal is to measure mobility and after that adapt stem length, handlebar height, and overall height of the hand.

For example, a fitting station that could simulate the maximum angle of the trail the customer rides could give information about of his body situate and the freedom of movement he/she still has.

Does it make sense to anyone else?

I agree completely with the last paragraph. Mountain biking is so much more dynamic than road biking that hard/fast rules of fit really don't apply in the same way. In my experience, more people want to size up than down because they are either coming off of an older bike with a crazy long stem and trying to mimic that fit or they are passengers on the bike, not the pilot. I would say there is also a much bigger variability in mountain bike geometry than road bike geometry. In short, I'd say the only key thing is sizing the bike correctly to start with and then making small adjustments with saddle height/fore aft adjustment and stem length/spacer stack/bar height/bar roll/controls position.

Fitting me is utterly impossible according to every "Fitment tool" online.

Canyon's literally says "Are you sure?"
 

5'10" 31" inseam.
Stupid long torso.
Wife is 5'1" and her inseam is the same as mine.

Not ever felt comfortable on anything other than a big DH bike. 

I feel like the most important thing for mountain bike fit is the distance from your feet to your hands. Basically center of BB to the center point of a line drawn from middle of one hand to the other when on the grips.

This is also the method Sam Hill uses to fit and setup bikes.

Once you find the number that works for you, bike setup is so easy. As far as what this distance should "be" is open to interpretation , but Lee has some good thoughts about it. Mtb bikes vary so much in reach and stack, this number can be very helpful for initial setup.

For saddle height/setback and cleat position, using what the roadies do will put you in the most efficient position possible. That has been dialed for many years now.

With the modern steep seat tube angles mountain bikes have these days, I see way too many people waaaay to far forward either ruining their knees or losing massive amounts of energy. Someone 5'0 should not have their saddle in the same position relative to the BB (setback) that a 6'6" rider does. Physics is physics and angles are angles.

I didn't mention this before because it seems so obvious, but it's worth mentioning for those who don't know that optimizing a mountain bike fit for efficiency in a seated pedaling position is completely different than optimizing for downhill performance in a standing position. My bike fit is basically a disaster in terms of cross country pedaling position because it's so single-mindedly focused on standing downhill performance. If I were setting up that bike for cross country instead of downhill, my handlebars would be about 4-6" lower. When I'm pedaling flat and uphill, I mostly just make due with the DH bike fit and deal with it. So that's worth mentioning if you're trying to calculate with "the one bike fit" for any rider: is the rider optimizing for cross country, downhill, or somewhere in the middle?

I have some really f’ed knees and making sure I have saddle height and fore/aft dialed in or I will end up with a bike I can’t pedal for any period of time.  So that is something I will insure the same as if it was a road bike. The rest are less important as far a pedaling for me.  

I'm similar. I am 5 foot 9 and have a ~28inch inseam. I've never used any online fitment tools, but I can't say I have had major issues fitting bikes, so I'm curious what have you been experiencing?

I tend to ride medium-sized bikes with 450-460 reaches and 50mm stems and I don't really have any major problems with my bike fit (other than things like long seat tubes and short droppers), especially compared to my tall, long-legged friends who seem to always struggle with having a seat that ends up over the rear axle.

Ah, same here, but in the other direction.  5'10", 33.5" inseam, 6'2" wingspan. I size bikes by the very uncool metric, "effective top tube". Steep STA is my friend, and there are still bikes that I end up having to slam the saddle forward on. My usual mountain bike size is large. Some mediums kinda fit, but the stack is almost always too low, and I end up smacking my knees on the bars when I pedal around corners. Wife is 5'3" and has around the same torso length as I do. 

A lot of my fit issues with mountain bikes disappeared with the advent of longer/lower/slacker geo, steeper STAs, and 200mm dropper posts.

Off course it makes sense!

On your second paragraph there is something I am pushing towards on my bikes: Longer chainstays. If the bike has a super short chainstay and a super long reach, It demands a kamikaze way of riding. If the bike is balanced, long reach with long chainstays, you are centered and don't need to kamikaze your way downhill. That alone for me is a vital morale boost.

Hip angle won't be a problem as soon as shorter cranks get available. This will get on more and more bikes very soon.

Finally some one who knows the struggle. I’m 6’ and my inseam is 31”.

Ive rode a medium for damn near my whole life so that I could drop my seat low enough.

only with the most modern bikes has that changed some.

The law: disorder prevails. Setting price and availability aside, fitting only arrives after rider proportions, preference, flexibility, and dimensions. Then use case/s and sizing. Then fitting. One fun, expensive thing about mtb is tuning fit with spring rates and curves. DH is easiest because saddle height is minimum only

I love that the bike fit thumbnail photo is a slalom racer (Is that Brian Lopes?) railing on a bike that might be <400 mm reach.

I think someone setting themself up as a mtb fitter today has to be prepared to lead a rider through a series of choices, each of which is going to be a tradeoff. The hardest thing will be escaping the preconceived notion that there is some real but not-yet-known optimum and that every component choice moves the fit toward or away from that optimum. Mtb media for years has gone hard on the use of numbers as a way of symbolizing experiences, like ride quality. Talking about numbers is used as a sort of token of objectivity, where the argument -- whatever it is -- is taken to be somehow more credible because it's sprinkled with these numeric facts. "So-and-so's handlebar width is 755 mm therefore ..." "I have a 35 mm stem and she has a 50 mm stem, so ..." An unkind way to describe this trend is that it assumes a low level of numeracy in the target market. I think the major wrinkle to overcome in establishing a fit system that doesn't fall into these traps is that (unfortunately) the attitudes of people most likely to fall for this stuff are often that they are objectively correct, i.e. well informed and only ready to change their mind if you hit them with even more b.s. numbers. There's a universe of facts that are objective but not useful. A great example was a recent pb article with 20 pros' bar widths and rises, and nobody's body dimensions or head tube heights or RADs or anything else. Useless for answering even basic questions about fit. But the temperature in the comments was around 'some data is better than none.' Unfortunately that's not really true either. They don't stack up in some simple linear way where some is good and more is better. When I hear someone talking about gathering data in mtb I think that's great but also GIGO.

Same. When MTB sizing used to be 13/15/17/19 & 17" was considered medium, shops would try to fit me on 19" bikes. I'd test ride and it sucked.  I was like a damn number 7. Reached the grips but legs straight & couldn't get the seat out of my balls.

Most recently was on a 2018 Kona Process 153SE Medium. 450mm reach felt "good" but still awkward somehow. I still hate full 29er geometry & mullet trail bikes feel argumentative the way they handle.
So I broke the Kona & that lousy, cheap, woke company charged me $1450 for a 2022 frame. It's the same 453mm reach, but with a steeper seat angle.  

But when I start trying to get a good run on descents, I run into the same problem as every other bike in the past.  I'm all over the front w/ my praying mantis torso.  

I'm only ever home on a bigger DH bike.  Riding a size Large mullet right now  and couldn't be happier with it for some reason.

Wanted my next bike to be the GG MegaTrail w/ that 460/470mm swappable reach but now that they've folded, I'm hopeless.

There appear to be a lot of folks who fit oddly like you do.  5'9" with 30" inseam and 6'2" wingspan for me.  I have to have a short seat tower in order to get good dropper action, but size L is my friend.  I like a tall stack.

Your model is pretty much hinged around what a couple of random racers declare is the best (for them) setup. If a formidable racer from america says ride your DH bike front end as high possible, do it. If a survey of enduro racers says run 760 bars, do it. If one person wins on flat pedals, do it. If some french guy tells you to run a crazy long front center and crazy slack head angle, definitely do it. Mullet, 27.5, 29, thick/thin grips, short cranks, big rear rotors, coil/air..... just make sure I can get it in ano purple and it's all good. Here today and gone tomorrow. Essentially what year did you graduate high school/college, is the music you love or the clothes you wear.

Spot on.

One thing I want to bring attention to is that your bars should be at least as high at your saddle*. Larger riders are usually at a disadvantage as larger frames do not have enough stack. More on this - https://www.vitalmtb.com/forums/hub/general-cpitcontact-point-discussion

I like this photo because it shows how ridiculously different the riding position is for two riders on either end of the spectrum. Both bikes are the recommended frame size for their height. Look at how little the bar height changes compared to seat height.

*as a general philosophy but tailored to an individual's proportions. 

Totally agree. One of the hard truths of bike fitting is that bikes are simply not made to fit very tall riders or very short riders, and that's true for road or mountain. The newest generation of 26" kid mountain bikes are pretty cool in that respect, and the new Knolly Endorphin that just released today is pretty cool for the same reason. But the reach/stack situation for big and small bikes is really stupid on MTB, road, and commuter bikes.

Waiting for PVD to chime in on this conversation...

But here's my thought on the original proposed idea: Most customers don't even know exactly how tall they are, let alone their inseam, wingspan, or other potentially relevant measurements. (Men, especially, tend to overestimate their height and ask for a larger frame size than they realistically need. Source: working in shops.) If this theoretical mountain bike fitting law is going to be based on or related to physical proportions, it'll only be as useful/accurate as the customer's measurements are. If those measurements are self-reported, they're probably going to be wrong or inaccurate much of the time.

The one thing to keep in mind about stack is as the headset bearings are spaced further apart the fore/aft "stiffness" of the SC fork is reduced. Obviously, there are a lot of factors as to if or how this actually effects ride feel but, yes, technically stack can change the feel of the fork. I'm not a bike designer but possibly this could explain the non-linear growth of stack vs reach. 

Argument invalid.
Those are mopeds.

"As the headset bearings are spaced further apart the fore/aft "stiffness" of the SC fork is reduced." Wait, what?