pjgreek,

thank you kindly for the explanation above. I look forward to partaking and learning tomorrow.

regards,

-J

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Jehovah and family unity above all.

You have to figure out what length/rise stem will put your pads/extensions in the same spot as they were in during the fit.


Bike Stack + Aerobar Stack = Fit Stack

If he is assuming your aerobar stack is say 20mm, and your bike stack is 500mm then your fit is 520mm stack. You can also do 490 + 30, 480 + 40, 520 + 0. Same for the reach.

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Ride Scoozy Electric Bicycles
http://www.RideScoozy.com

I've have my stack and reach measurements from a Rotul fitting. My measurements don't fit any frame perfectly, only approximately. I assume the perfect match is made when a bike's stack and reach are close to mine, and then adjustments to the saddle height, and raising the aero bar up or down and forward or backward, brings the fit close to the perfection point. Generally, now much adjustability do tri bikes have? (I've just read the review of the Quintana Roo CD 0.1, but as far as I can tell, the reach is closest to the small frame, but the stack is closer to a ML.)

x2. I know the definition of S/R and can measure them on my bike. What I'm missing is the so what, now what?

Thanks for taking the time to walk us newbies through it!

So what body measurements do we need to do to determine what kind of bike is right for us (e.g., low and long or narrow and tall)?

Is there perhaps a chart or something that would show you what bikes match up the best for your individual body measurements based on their stack and reach?

You're suggesting that I can measure my road bike, perhaps also using a level and a plumb bob, and work from there to decide what tri bike numbers would work. I'm guessing I'd add in adjustments to both stack and reach for the different seat tube angles since that rotates the point where I'm reaching from. I imagine I'd also adjust so I wouldn't need the shortened stem they used when fitting me to aerobars on the road bike. Then I'd guesstimate another adjustment for any changes I want from my current fit. Is that right so far?

Are there any other adjustments to account for? I don't design bikes. Is there an adjustment to the frame sizes because of various fork or aerobar configurations, maybe because of the different weight distribution on a tri bike? Anything else? Seriously, could I work out useful values from my road bike measurements like this? It's not obvious to me that this would work.

"Could you please define: reach?"

draw a vertical line thru the bike's bottom bracket (where the crank attaches). draw a vertical line through the center of the head tube top (where your fork's steer column sticks up above the frame). the distance between those two lines is the bike's reach.

"Could you please define stack"

draw horizontal lines through these same two places on the frame. the distance between these lines is stack.

what is important about these two metrics? nothing, per se. what's important are these two places: the bottom bracket, and the head tube top. this, because you must attach to your frame at these two spots. look at it this way: you have to clip into the pedals, right? and the pedals are attached to the crank, and the crank is attached to the bottom bracket. you must attach to the bike here. no getting around that. (for you wiseacres, we're going to omit any discussion of drop-center cranks or side-mount pedals.)

now, same sorta thing at the head tube top. you grab the handlebars, and they're attached to the stem, which is attached to the steer column, which comes up thru the head tube at the head tube top. if you hop aboard a bike - any bike, road, mtb, or tri - and the bike seems too short, you do what? you put a longer stem on the bike, right? that's the quickest, most obvious solution. if the bike's too long, you put a shorter stem on. this is because you can't move the bike's head tube top. you can only normalize for that spot on the bike being too close to the BB, or too far out in front of the BB. an overlong stem is just a bad proxy for a top tube that's not long enough.

what if the handlebars are too low? you put spacers under the stem. or you pitch the stem up at an angle. if the handlebars are too high, and you need them lower, you take spacers out, or you pitch (angle) the stem down.

are we clear? (((yes))). are we clear!! (((crystal))).

well, let's hope we're clear! but if we're not don't be afraid to say so. you've got to get this part down. assuming you're with me...

at a certain point, you just can't put a stem any shorter on a bike that's too long. or a stem any longer on a bike that's too short. you don't want a 5cm stem on your bike, and you don't want a 15cm stem on your bike. right? you want a stem length that's something in between, like 10cm or 12cm.

likewise, you don't want 7cm of spacers underneath your stem, do you? if this is what you've got underneath your stem, you've got a bike that's got a head tube that's sitting too low to the ground. you need a taller frame. if your stem is 5cm, your bike is too long. if you have a 15cm stem on your bike, your bike is not long enough, and you're making up for the lack of your bike's length thru a stem that's a bad proxy for the length you frame lacks.

what we're really saying here is that your head tube top is not in the right spot, spatially, relative to the bottom bracket. it's either not high enough above the BB, or it's too high above it. or it's not far enough in front of the bottom bracket, or it's too far in front of it.

realize this: frames today are much stronger, lighter and aerodynamic than headset spacers or stems. that's why you need a frame that places the head tube top in the correct spot, so that your stem isn't too long, too short, pitched up, or sitting atop a bunch of headset spacers.

so, a bike that fits right has the head tube top strategically positioned correctly relative to the bottom bracket. this is the very first requirement. do not pass go, do not collect $200, until and unless your prospective bike's head tube top is placed at the correct spot relative to the BB: the right height above it, the right length in front of it (the right "stack" and the right "reach").

why did we need to come up with stack and reach? wasn't the bike business doing just fine for a century with the metrics it had? well, consider this graphic:



each of these bikes above has the same top tube length. the top tube has been historically the measure that determines a frame's length. the problem is this: tri bikes have a lot more seat angle disparity, one to another, than do road race bikes. so, the two bikes above have the same top tube length, but, their reach measures vary significantly. example: kuota kueen k (size L) and an orbea ordu (size 54cm) each have a top tube length of 55cm. but the kueen k has a reach of 43.9cm, while the ordu has a reach of 40.9cm.

so, these bikes have the same length top tubes, but one is actually 3 full cms longer than the other. why? because the ordu is built with a 74° seat angle, and the kueen k with a 78.5° seat angle. the kueen k's steeper seat angle "pushes" its top tube further in front of the bottom bracket.

"but!" you might counter, "if i just take the parts off the ordu and put them on the kueen k, the length of the bike is exactly the same!"

well, yes, if you just hope aboard each bike the way each bike is designed. but we don't think bikes are ridden that way. we say that you have a set of "fit coordinates" that are specific to you. and by "fit coordinates" we mean that, for example, your saddle height is the same regardless of what bike you're aboard. so, if you move from one bike to another, and the seat tube of one bike is 56cm tall and the new bike's seat tube is 57cm, you don't just ride these bikes with the same amount of seat post sticking out. you'll build your new bike with one less cm of seat post sticking up, because what matters is the overall distance from the bottom bracket to the top of the saddle. you "normalize" for the new bike's taller seat tube by shoving that seat post in a little bit.

likewise, it's not only your saddle height that's specific to you (regardless of what bike you ride), your saddle position fore/aft is also specific to you. so, if you are aboard a tri bike that fits you well, your saddle nose is a given distance in front of or behind the bottom bracket. if you move to a new bike with a shallower seat angle, you'll have to push that saddle a little bit forward on the rails to "normalize" for that shallower seat angle, and this will get your saddle nose back to its right fore/aft spot relative to the BB.

back to that kueen k and the ordu: if you pull all the parts off your kuota kueen k and stick all these parts on a new orbea ordu, you're going to need to move that saddle waaaay forward on its rails, to normalize for the very slack seat angle on the ordu (compared with the kueen k). this will result in the ordu being too "narrow" for you, that is, you'll have a short cockpit, you'll feel scrunched up. if you had a 10cm stem on your kueen k, now you'll have to put a 13cm stem on your ordu in order to enjoy the same fit coordinates on your ordu that you had on your kueen k.

probably we better stop here and let this sink in.

let's not talk right now about how you use this data. right now, we're just talking about stack and reach, what these metrics refer to, and why stack and reach are a more precise set of metrics than, for example, top tube length and head tube length. once we get this down, we can tackle the next step.

ask what questions you need to ask now about stack and reach, or, just tell me you're an ignorant slut and you still don't get it. which is fine. but, if you can drill down on what precisely you don't get (if you're having any trouble with what i've written above) please tell me, and i'll attack this from a different angle.

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Dan Empfield
aka Slowman

I have to admit, I've been too "lazy" to dig into the subject because I *thought* it would be overwhelming. The manner in which you've described stack/reach here, makes everything crystal clear.

Thanks for taking the time to do this. I look forward to the continued lesson.

Drew

Good stuff Dan. I, for one, had zero clue what stack and reach were until I read this. 6 years doing tris, and every time I'd see a stack and reach thread, I'd just skip over it since I knew nothing about it. I'm learning now. Thanks!

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My business-eBodyboarding.com

It's like reading Ishmael.

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____________________________________________________
Dream of racing again some day...

I'm no expert at all on S&R, but I can make sense of that bike diagram for sure. Stack is vertical. So when you say "only travel so far back" referring to that horizontal line, that line has no meaning except to join to the top of the headtube. It doesn't matter how long it is. It just creates a horizontal reference from which to measure down to the lower line which is even with the BB.

Waiting for the next post from Slowman manana...

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____________________________________________________
Dream of racing again some day...

Dan, I am an ignorant slut.

I can pick out stack and reach on a diagram. I understand why using stack and reach is more precise than saying top tube length. I get how using the stack and reach numbers I would have to buy different parts to get the kueen k and the ordu to match up. (although I freely confess that if I was to do that, I would likely go to the lbs and say "Make these the same like they say on Slowtwitch")

What I do not know is why I would want them to match up. Specifically, why do I care about these numbers as they relate to ME? How do (or do they) these numbers make me better on the bike? I get geekdom for the sake of geekdom, but does this metric mean anything outside of the rarified world of bike builders?

I don't have any questions but I do have two observations.


Observation #1: Stack and reach depend on things that aren't part of the frame. If you take the same frame and add a different fork (or, for that matter, just a front tire with a different rollout) the stack and reach will be different.


Observation #2: Stack and reach don't depend on some things that are part of the frame. For example, seat tube angle. Two frames can have the same stack and reach but very different seat tube angles.

Without getting too complicated:

it is these stack and reach numbers on a frame that help determine what size bike you would purchase, based on an "optimal" aero position. This position is relatively athletic, but as Dan puts it, achievable.

So, through the course of fitting, you would find out what your optimal stack and reach numbers would be. Then, if the fitter is also your bike seller, they would know which frames are close to those numbers, so that you can wind up running with the fewest number of spacers and other modifications that hurt the aero profile.

Hope that's not as confusing as it looks, and I'm sure Dan will do a better job than I...

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Editor-in-Chief, Slowtwitch.com | Twitter

I have a question regarding moving from one bike to another. I know the stack and reach of my P3SL, but when you throw into the mix it's outfitted with a -25* stem how does one start to account for those things when moving to a different frame?

In a perfect world my new frame would have a significantly lower stack, correct?

I know the measurements of my 3 points in space down to the mm, but I don't know how to take those points and make them useable in a search for a new frame one day?

SIR! YES SIR! CRYSTAL CLEAR, SIR!!

Dan your explanation is easy enough to comprehend. I haven't absorbed it all, but most of it. I will read your post a few more times and follow up with any questions/concerns I might have by todays end. However, your explanation was thorough enough that I understood how you can make some adjustments with two different bike geometries and make them work for you by understanding stack and reach. What I don't understand is why you feel that the stem shouldn't be shorter than 100 mm or exceed 120 mm? I'll just take your vast knowledge of "what works well" and accept it as truth, since that is a different topic altogether. I have also added "narrow" to my vocabulary. I now understand that it stands for a short cockpit. I think and would humbly suggest/request that we put up a sticky for such words and their definition. I realize you have plenty on your plate and probably don't have much time for much else, but I hope you will entertain the idea or maybe put it in the back burner in the event you find yourself with some spare time. I believe having an understanding of the lingo will eliminate a lot of the questions we ponder, but to embarrassed to ask about. Again, professor Dan, thank you for the explanation..........understand clearly. [you should be charging for this]

With a great deal of respect and sincerity,

-J

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Jehovah and family unity above all.

"why do I care about these numbers as they relate to ME?"

these numbers have a very limited utility. they are ONLY useful when trying to discern what bike(s) on the market will best fit you, or when you're comparing one bike to another, or maybe when you're just trying to determine what STYLE of rider you are and then matching your style to a bike of that same geometric style. you do not have your own stack and reach. only bikes have a stack and reach.

look at it this way. bikes have head tubes, right? and they have top tubes. the only time these metrics matter to you is when you go to buy a new bike, and you want a bike with a head tube that's tall enough (without being too tall). then you want to make sure it has the right top tube length because you want it to be long enough (without being too long).

stack and reach are just more precise terms for a frame's height and length. they are height, normalized for bottom bracket drop and wheel size, and length normalized for seat angle.

stack and reach do perform a service, they are useful, and i'll tell you when and how they're useful in another installment later today.

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Dan Empfield
aka Slowman

"If you take the same frame and add a different fork (or, for that matter, just a front tire with a different rollout) the stack and reach will be different."

that's true. stack and reach are simply a different way of measuring the bike. so, as you say, if you put a different fork on the bike, or a tire that's got a higher profile, you now no longer have the same seat angle on your bike. however, the difference is not typically a lot. having said that, when i was a bike maker and kestrel started selling its EMS fork to OEMs, we used to have to make two different frames: one for our standard steel forks, one to accept the EMS fork, because the distance from the center of the brake hole to the crown race seat was 8mm greater on the EMS fork than was evident on our own fork crowns.

but this doesn't matter much for our purposes. most bikes sold today have proprietary forks that aren't going to get changed, and, if a tire of a different profile is placed on the frame it's going to raise the front end of the bike (v the bb) what? 2mm? for the purposes of bike fitting, this is inside the range of acceptable noise.

"Two frames can have the same stack and reach but very different seat tube angles."

absolutely. but seat tube angles are irrelevant as long as you can get the saddle where it needs to be. for example, an orbea ordu in 54cm has a stack and reach of 520mm and 409mm. a trek speed concept in size M has a stack/reach of 517mm and 408mm. these bikes are almost size-identical in terms of how you're going to fit aboard them. however, one has a 74° seat angle, the other a 78° seat angle. the only thing that matters is: if you're going to ride an ordu at 78° or 79°, can you get the saddle forward enough? and the answer is yes, as long as you get it with the 78° seat post (the steepest of the 3 available posts for that bike).

but this is a good way of looking at the utility of stack and reach as metrics. the orbea described above has a top tube length of 55cm. the speed concept's is 51.8cm. yet these bikes have almost identical lengths. the orbea's seat angle slackens, and the top tube has to "grow" to follow it back to 74°. then you pitch the saddle forward again using that 78° dogleg seat post. if you draw a line from the seat clamp on that post directly down to the bottom bracket, and measure from where it bisects the top tube, and measure from that junction forward to the head tube, you get something very much like the speed concept's 51.8cm top tube.

but don't decry the ordu's slack angle. plenty of bikes have been built with these slack angles. cervelos, for example. if you go back and look at the old P3SLs, with their flippable seat post heads, the actual seat tube angle of those bikes was pretty slack, like 76°, maybe 76.5° at most. but they were designed to be ridden clamp-forward, and what matters is the posture of the bike below the saddle. the bikes were built with a lot of reach, which made them fit very badly when the clamps were facing backward, and very well when they were facing forward.

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Dan Empfield
aka Slowman

OK, you want to draw out the completely clueless - at the risk of being hung out to dry in this forum, I'm clueless. How to you measure stack? Creating two 90 degree angles is easy, but how do you determine how far to measure, in distance, from the bottom bracket forward before beginning the vertical (90d) angle?

What are you getting from the measurement of the stack that you aren't getting in the measurement of the reach.

No, these aren't softball questions from a knowledgeable poster, I'm truly an idiot. Now I'll duck and cover.

"I know the stack and reach of my P3SL, but when you throw into the mix it's outfitted with a -25* stem how does one start to account for those things when moving to a different frame?"

we'll get to that.

"In a perfect world my new frame would have a significantly lower stack, correct?"

well, maybe. it depends on the aerobars on your current bike, and the headset top cap on your current bike, and of course we're assuming that you and your bike are fitted correctly to each other.

but in broad strokes, all things equal, yes, your ideal bike might be slightly lower in stack. however, we'll talk about that some, because i don't know that that's the case for you.

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Dan Empfield
aka Slowman

Dan:

I've printed and digested your series of articles on FISTing, and have used the calculations in the articles to help set-up some existing bikes, including my own. To that end, I've found that my current bike, a (used) P3 purchased before I did any of this, actually turned out to be a bit too long and a bit too low for me. So, with seat slammed forward and a single spacer under the stem, away I go. The next bike will have a bit better fitting frame.

In all this, it seems that the FIST method seems to start at Step 2, rather than Step 1. I can approximate a bike to fit someone after I know the saddle height and saddle -to-bar length from their road bike (to start the process of determining an appropriate bike based upon S&R). The problem I've ALWAYS had is how to measure the PERSON (step 1), to begin to consider the bike (step 2).

There are some general guidelines that have been tossed about: long-legged, short torso; short-legged, long-torso, average build, limited flexibility, etc. But, these have never been quantified or put into a measurable system.

THAT is the part where I flounder, but perhaps outside the scope of this discussion.

FWIW, I watched Dan do a bike fit out at the ranch during a tri camp. He put the rider on an adjustable bike fit device, then started measuring and adjusting until the numbers aligned, (reach, stack, knee angle, etc, etc). But then, he asked the rider to comment on COMFORT - what changes to the base position were more comfortable. The rider then departed from the 'ideal numbers' to something more personal. If I recall, he preferred a more 'relaxed' position, he was a big guy, a Police Detective, and didn't like to be in that perfect aero position for longer races. Once Dan normalized the numbers based on rider input, he then looked in his bike geometry database and listed the various frames that would work best for this rider (P2C was top of list).

It was very cool, I didn't get most of it, but the most critical input was the actual rider's departure from the 'ideal'.

I think this is an incredibly important factor when deciding on the perfect geometry for a new bike purchase - few riders are actually comfortable riding in a position dictated purely by the numbers.

Yea it's an odd thing for me. I'm that guy that emailed you about my midfoot cleat and my huge move forward and down in saddle changes. I **think** I need a ton of reach, but also less stack than a guy who would typically **need** that much reach(due to my low saddle height from the midfoot cleat). I have some very odd things going on and I realize that. 165 cranks and midfoot cleats opened up my hip angle so much it's was crazy. I was instantly and easily able to get quite low, but found I had moved forward so much I began to scratch my head on 'perhaps I have hit the other end of the spectrum'. I had to make these moves with my saddle in order to close my hip angle back down to where it feels most powerful. It **felt** great, but realized I was venturing into fit issues very few ever delve into. The crank change was made post knee surgery, loved the results. The cleat change was made post Achilles rehab, loved the results.

I looked at a 55cm P3SL b/c the stack was much more appropriate, but then I had to give away reach which would have put me in the ridiculous area of stem lengths.

As always, many thanks.

"How to you measure stack?"

let's try a new diagram:



is this better?

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Dan Empfield
aka Slowman