Understanding Stack & Reach

ptlk66

I have read Cervelo's explanation for stack and reach and it all appears to make sense until I started to apply it to plan for the purchase bike number X.

I hope someone can explain where I am going wrong.

I will use a Trek Domane geometry to illustrate my confusion.

A size 52 Domane has an effective top tube of 53.0cm and a reach of 37.1
A size 54 Domane has an effective top tube of 54.2cm and reach of 37.4

The above represents a top tube difference of 1.2cm but only a reach difference of 0.3cm. The reach difference to me appears negligible and I would assume would not even require a change in stem length to achieve the same reach setup on both bikes.

However, I am convinced I have missed something vital because if you take Reach for the smallest Domane (36.8cm) and the largest Domane (38.6cm) - this represents reach range of 1.8cm across the entire range of sizes available for the Domane - does this mean that someone on the smallest size Domane (say with with a 100mm stem) can achieve the same reach setup on the largest Domane by reducing the stem by 2 cm to an 80mm stem???

Totally confused…..

MichaelW

Usually seat-tube angle varies with frame size, getting more laid back with larger frames. I'm not sure why everyone does this, perhaps so they can fit the same sized crank across the range.

When I want to move points of contact from one frame to another (ie setup 2 bikes with identical riding position), I take measurements in [x,y] from a fixed point, using the bottom bracket as [0,0]. This eliminates the angles which can vary.

Angles affect the weight distribution on the frame but not the actual rider position.

neeb

I don't think you have missed anything - it does look as if the reach differences between sizes are quite small.

In the case of the 52 vs. 54, the reach difference is only 3mm despite an effective top tube difference of 12mm because 1) the seat tube angle is a bit steeper on the size 52, and 2) the headtube is shorter on the 52.

But remember, if you were in-between a size 52 and a size 54, you would need to use more spacers on the size 52 to put the bars at the same height as on a size 54. This would reduce the reach including spacers, which is less than the frame reach, because if you add height to the head tube you bring the bars a little closer due to the fact that the head tube is angled.

Also, while it is true that with the saddle positioned in the same place with respect to the bottom bracket, reach is the number that tells you how far away the bars will be (rather than effective top tube length), when you are looking across the entire range of frame sizes you have to remember that people who ride smaller frames will also tend to be more comfortable with a smaller saddle setback in relation to the BB, so the actual distance between the saddle and the bars will be less than the reach figure would indicate.

Basically, there is no single measurement on its own that can tell you how a frame will fit, everything interacts with everything else. The way you get a frame to fit you is by knowing certain variables and holding them constant and then looking at what the other variables will be with those assumptions. E.g. if you know that your saddle setback in relation to the BB isn't going to vary, you can look at frame stack and frame reach to work out "stack with spacers and stem" and "reach with spacers and stem", and if you know what numbers suit you for these you will know what stem length and spacer stack you will end up with on that frame.

You need to plug all of the numbers into something like this: http://bb2stem.blogspot.fi/

One thing I have discovered since I started using that spreadsheet is that fork length often varies quite a bit these days from what used to be a fairly standard 368ish mm. This is very important if you are using head tube length as a guide to how high the bars will be. If the geo charts give stack and reach but not fork length you can plug in numbers experimentally for the latter until you get the correct stack & reach figures.

MichaelW

people who ride smaller frames will also tend to be more comfortable with a smaller saddle setback in relation to the BB

Doesn't the angle of the seat-tube provide for this whatever the frame size?
In the world of custom frames, you adjust seat-tube angle to fit the triangle of upper leg/lower leg/seat-tube/post. It is the proportion of upper-leg/lower-leg that varies between riders and for extremes of ratio, you need unusual angles.

Does that ratio of upper/lower leg differ with the size of the rider?

For one individual rider, selecting beteeen to frames with different sizes/reach and angles, the saddle/BB position will be fixed. The reach will vary but some of that reach is in front of the saddle fore/aft location and some is behind (and hence out of the equation)

majormantra

A useful rule of thumb I learned is that 1 degree change in STA corresponds roughly to 1cm of horizontal TT length. So, for example, imagine 2 frames:

1. STA 73.0, TT 545mm
2. STA 74.0, TT 535mm

With the saddle in the same position relative to the BB, these two frames are effectively the same length. Of course this doesn't account for some of the variables mentioned above, but it's a good starting point for comparison.

neeb

MichaelW wrote:

people who ride smaller frames will also tend to be more comfortable with a smaller saddle setback in relation to the BB

Doesn't the angle of the seat-tube provide for this whatever the frame size?
In the world of custom frames, you adjust seat-tube angle to fit the triangle of upper leg/lower leg/seat-tube/post. It is the proportion of upper-leg/lower-leg that varies between riders and for extremes of ratio, you need unusual angles.

Does that ratio of upper/lower leg differ with the size of the rider?

The ratio of upper to lower leg length will influence ideal saddle setback relative to the BB (and thus choice of seat tube angle in a custom frame) for someone of a given total leg length...

But someone with longer legs will need a bit more saddle setback than someone with shorter legs if they have the same ratio of upper to lower leg length. Basically (as I understand it anyway), the longer your femur, the more setback you are likely to need (all else being equal).

MichaelW wrote:

For one individual rider, selecting beteeen to frames with different sizes/reach and angles, the saddle/BB position will be fixed. The reach will vary but some of that reach is in front of the saddle fore/aft location and some is behind (and hence out of the equation)

No, the point about reach (i.e. "frame reach" as given in geo charts, or total "reach" in this sense including spacers and stem) is that it is all in front of the BB - it's the horizontal distance from a line through the center of the BB to the top of the head tube. So for a given saddle setback from the BB (e.g. for a given rider), and irrespective of seat tube angle, two frames with the same reach will have the same stretch to the bars.

ptlk66

No, the point about reach (i.e. "frame reach" as given in geo charts, or total "reach" in this sense including spacers and stem) is that it is all in front of the BB - it's the horizontal distance from a line through the center of the BB to the top of the head tube. So for a given saddle setback from the BB (e.g. for a given rider), and irrespective of seat tube angle, two frames with the same reach will have the same stretch to the bars.

Yes, exactly, my saddle setback is 6.2cm, so if this is maintained I assume I would only have to adjust stem length to match a fit. I understand how head tube angle and height can effect reach as they are beyond the BB, however by maintaining saddle setback you negate any difference in STA.

However, when using geometry tables to determine frame similarities I struggle to see any correlation between the change in effective top tube and change in reach for two given frames.

As stated earlier I was attempting to use stack and reach to determine a correct frame size based on the assumption the Trek Domane size is correct. My current fit dimensions are:

Saddle Height 70.2 cm
Stem Length 110 mm
Saddle Setback 6.2 cm
Stem Spacers + Headset 2.0 cm

The geometry for the Domane and the new bike are noted below, which of the frame sizes of the new bike would you think is a closer match to the Domane. Also note that the Domane is used as a winter commuter bike, the new bike is for summer and and it to be slightly more aggressive.

Trek Domane New Bike Size1 New Bike Size 2
Seat tube length 475 465 505
Top tube length 530 525 537
Head tube length 145 125 138
Head tube angle 71.3 72 72.5
Seat tube angle 74.2 73.5 73.5
Chainstay length 420 410 410
Wheelbase 1003 967 975
Stack 561 534 546
Reach 371 367 375

thanks

Paul

neeb

Ok, later on I can plug those figures into that app. I linked above. To do it properly it would also be good to have bottom bracket drop and fork rake (offset) - I know that for the Domane at least these are on the geo chart. Also, do you know the angle of the stem (e.g. 8 degrees, 6 degrees), whether it is flipped or not, and the stack height of the stem? (you can just measure the height of the stem clamp).

smidsy

You do not need to understand - just buy a bike that fits

If you only did things that you fully understood would you EVER get involved with women (or men, depending) ! :shock:

neeb

smidsy wrote:

You do not need to understand - just buy a bike that fits

If you only did things that you fully understood would you EVER get involved with women (or men, depending) ! :shock:

Beg to differ. An awful lot of people have bikes that don't fit, you just need to look at people's pictures of their bikes to see that.. If you already know your fit (i.e. you have a bike that fits you), finding another bike that fits exactly is just a case of getting all of the numbers right. It's basic geometry, not rocket science... Unfortunately relationships are not so simple..

smidsy

neeb wrote:

smidsy wrote:

You do not need to understand - just buy a bike that fits

If you only did things that you fully understood would you EVER get involved with women (or men, depending) ! :shock:

Beg to differ. An awful lot of people have bikes that don't fit, you just need to look at people's pictures of their bikes to see that.. If you already know your fit (i.e. you have a bike that fits you), finding another bike that fits exactly is just a case of getting all of the numbers right. It's basic geometry, not rocket science... Unfortunately relationships are not so simple..

I see humour passes you by :roll:

drlodge

MajorMantra wrote:

A useful rule of thumb I learned is that 1 degree change in STA corresponds roughly to 1cm of horizontal TT length. So, for example, imagine 2 frames:

1. STA 73.0, TT 545mm
2. STA 74.0, TT 535mm

With the saddle in the same position relative to the BB, these two frames are effectively the same length. Of course this doesn't account for some of the variables mentioned above, but it's a good starting point for comparison.

I find this the easiest single method to compare frame sizes. I know my frame has 73.5 deg STA, 54.5 cm eTT and 110mm stem. Hence if I'm looking at a frame with a more relaxed 73 deg STA I know I need to add another 0.5cm to the eTT/stem combination to get the same overall "reach" from saddle to bars. The other dimension I then look at is head tube height, most frames are quite short for me here meaning a larger drop from saddle to bars.

My old Condor frame has a 74 deg STA, and I had to get a seat post with a lot of layback to get the saddle in the right position.

neeb

smidsy wrote:

I see humour passes you by :roll:

Ratio of actual funniness to probability that you were being serious was below my joke-detection threshold, sorry..

neeb

drlodge wrote:

MajorMantra wrote:

A useful rule of thumb I learned is that 1 degree change in STA corresponds roughly to 1cm of horizontal TT length. So, for example, imagine 2 frames:

1. STA 73.0, TT 545mm
2. STA 74.0, TT 535mm

With the saddle in the same position relative to the BB, these two frames are effectively the same length. Of course this doesn't account for some of the variables mentioned above, but it's a good starting point for comparison.

I find this the easiest single method to compare frame sizes. I know my frame has 73.5 deg STA, 54.5 cm eTT and 110mm stem. Hence if I'm looking at a frame with a more relaxed 73 deg STA I know I need to add another 0.5cm to the eTT/stem combination to get the same overall "reach" from saddle to bars. The other dimension I then look at is head tube height, most frames are quite short for me here meaning a larger drop from saddle to bars.

My old Condor frame has a 74 deg STA, and I had to get a seat post with a lot of layback to get the saddle in the right position.

Yup, this more or less works as long as the head tubes are roughly the same length. If the head tubes are very different lengths however, this interacts with effective top tube length you need to get the same reach (assuming you want the bars at the same height). If you have two frames with same STA and same eTT but one has a lower headtube, the one with the lower headtube would have a shorter eTT if the headtube was the same height, so the reach will be a bit shorter once you add the spacers.

smidsy

neeb wrote:

smidsy wrote:

I see humour passes you by :roll:

Ratio of actual funniness to probability that you were being serious was below my joke-detection threshold, sorry..

Does that say more about you or me

The smiley was there as a clue though 8)

neeb

smidsy wrote:

neeb wrote:

smidsy wrote:

I see humour passes you by :roll:

Ratio of actual funniness to probability that you were being serious was below my joke-detection threshold, sorry..

Does that say more about you or me

The smiley was there as a clue though 8)

An equal amount (or nothing) about both of us probably.. Let's just call it a humour interfacing error..

neeb

OP - I plugged in the data for the Domane (I got the missing stuff from the published geo charts), but because it has slightly weird sportive geometry it's actually quite important to get a bit more data for the other bike to make a really good comparison. The Domane has a big BB drop (80mm) and a long fork (apparently 382mm, extrapolating from the other data). Also quite a steep STA, shallow HTA and lots of fork offset. If the other bike has more racy geometry then the big differences in these variables could be significant. What model is it?

FYI, assuming the 110mm stem you have on the Domane is -6 degrees and with a 40mm stack height (giving 20 + (0.5 * 40) = 40 for extras), then your current total stack including stem and spacers is 623.25, and your total reach including stem and spacers is 465.69. It is these figures you are ultimately trying to replicate (or tweak if desired) for the new bike, preferably in a way that leaves you with a sensibly sized stem and not too many spacers.

TOP TUBE LENGTH (EFF) 530
HEAD TUBE LENGTH 145
HEAD TUBE ANGLE 71.3
SEAT TUBE ANGLE 74.2
BB DROP 80
FORK LENGTH 382
FORK RAKE 53
STEM LENGTH 110
STEM ANGLE -6
SPACERS + HEADSET + STEM STACK*0.5 40
STACK 561.17
REACH 371.20
STACK WITH SPACERS 599.06
REACH WITH SPACERS 358.38
STACK WITH SPACERS + STEM 623.25
REACH WITH SPACERS + STEM 465.69

neeb

P.S. the only extra data I need for the other bike is 1) BB drop, and 2) EITHER fork offset (rake) or fork length.

ilovegrace

Why not try what an club cyclist wrote in this weeks cycle weekly. form a right angle with your arm press your elboe up to the front of the seat , your fingers should lightly brush the handle bars. Bingo.
I love this type of old school advise , on anything really.Why.because they are usually right.
My local GP says if mother is worried (about her baby's health) i am concerned, if her GRANDMOTHER is worried I am worried.
regards
ILG
PS
Just popping out to the garage to put my elbow to the saddle !!

ilovegrace

Anybody want to buy two bikes ?
Stupid old wives tales !!

neeb

Or you could always sacrifice a chicken and look at the length of the entrails. Don't do it when there's a full moon though or you'll have problems with your knees.

ptlk66

Neeb
I appreciate the effort you have put into responding to my post. I deliberately refrained from posting my height and details of the new bike as i didn't want it to influence the interpretation of geometry. The new bike is canyon cf slx, sizes xs & s, unfortunately canyon do not give bb drop or fork details. I believe XS is the correct size and it will be a racier setup. I have tried the canyon fit system which puts me on S but if my inseam was 1 cm longer i would be M, i do not have a lot of faith in their system as M would be huge.

My original question on geometry also stems from when i tested the 52 and 54 domanes, the 54 felt much bigger but the geometry chart does not appear to represent the frame size difference with a reach difference of only 0.3cm but an Eff TT difference of 1.2. Which is leading me to consider that eff top tube is more reflective of frame size difference than reach.

Also i made an error in my posting i have no spacers only the headset (1cm) below the stem on the trek.

Paul

neeb

No worries Paul, I actually enjoy doing this sort of stuff, in an obsessive/nerdy crossword-puzzle sort of way..

As it happens I was considering a Canyon Ultimate CF SLX myself a few months back, and I emailed Canyon asking about the fork rake. It was like getting blood out of a stone, but they eventually told me it was "41.5mm +/- 1.5mm", which I though was a bit odd. I asked them if they meant that it varied by +/- 1.5mm depending on the frame size, and they said no, they are all 41.5mm, but there is a manufacturing tolerance of 1.5mm... That's very honest of them, as I suspect that there is a margin of error in all manufacturers' fork rakes, but they don't generally admit it. Apart from fit issues, it's worth bearing in mind that with a 41.5mm rake and the shallow head tube angles on the smaller Canyons, you are going to end up with much more trail on the new bike than with the Domane. Ironically, this is going to make the steering more stable and less responsive, despite the Canyon being otherwise a much "racier" bike. So the trail on your Domane is 58mm, but on the size 50 Canyon it would be 66mm, while on the size 53 it would be 63mm. The former especially is going to feel quite noticeably different..

I'm going to assume 70mm for BB drop. It's unlikely to be much different from that on a modern race-orientated bike, and also I found this: http://www.bdc-forum.it/wp-content/uploads/2012/08/CANYON-ROAD-BIKES-2013.pdf
It's a 2013 Canyon catalogue, which (just to inspire confidence..) provides an obviously out-of-date geometry chart, which I assume actually applies to the 2012 models as the dimensions are different from the ones given on the website (the 2014 frames are the same as the 2013 ones as far as I know). But the fork rake is provided on that chart and matches with what Canyon told me for the 2014 bikes, and it also gives a BB drop of 70mm.

So here are the figures. One odd thing is that in order to get the (frame) stack and reach figures to match those on the geo chart, I had to assume slightly different fork lengths for the size 50 and the size 53 - 377mm and 375mm respectively. I'm highly doubtful that the forks are actually different lengths, it's probably got more to do with the way that Canyon are measuring stack & reach. I notice that on their little diagram on the website, the impression is given that stack is measured to the top of the headset rather than to the top of the headtube as it is supposed to be. The apparent 2mm discrepancy between the two frame sizes doesn't really matter, but if they are including the headset in the stack then you are going to end up with a few mm more spacers than you would otherwise have, all else being equal. They also say "Spacer - 25" on the geo chart, with an unreferenced asterisk - I've no idea what this refers to... Maximum spacers? Height of the headset?





So, basically, with the smaller Canyon, assuming you set it up with exactly the same stem and headset as you have on the Domane, and assuming that Canyon aren't doing something weird with their stack numbers in relation to the headset, you are going to end up with the bars nearly 3cm lower and about 4mm closer. Of course that's without any spacers, so you could bring the bars up a bit (dropping an extra 3cm is quite a difference, even if you are looking for a more racy setup). BUT - if you brought the bars up that would also shorten the reach a bit more. E.g., if you added 2cm of spacers to make the bars only 8mm lower than on the Domane, your reach (with spacers and stem) would now be only 458.95, i.e. a whole 1cm less than on the Domane. So you might then want to go with a 120mm stem.

With the larger Canyon (again with same stem and headset as on the Domane), the bars would be about 17mm lower, and 5mm further away. Let's say you added a 1cm spacer - you would now have the bars about 8mm lower than on your current bike and only about 2mm further away.

So, basically, you could probably fit either bike, but the size 53 is going to be a bit closer to the Domane. But if the Domane in any way feels on the large side, you might want to go down to the size 50. But if you do that you may need a few spacers and/or a longer stem.

Also check to see how much seat post you are going to have showing on each size of Canyon, and that it isn't ridiculous. As far as I can tell from the Domane geo chart the measurement they give is right to the top of that extended seat post that goes quite far above the seat post / top tube junction, so comparing figures could be misleading. Simply measure the stated seat tube lengths for the canyons from the centre of the BB on your current Domane - the distance above those measurements is going to be exposed seat tube.

Oh, and any purchasing decisions based on this advice are entirely your own responsibility...

ptlk66

Neeb,

Thanks for all the hard work - it is confirming what I initially thought - that both sizes will fit. However, I note your comment on the issue of trail. Does this mean if i opted for the 50 Canyon that I can expect it to be less responsive than my Domane - as this would be contra to what I want to achieve - I would like the new bike to feel "snappier" at turning than the trek - I would like to be able to flick the bike through corners rather than steer through if that makes sense, and if i have interpreted understanding of trail correctly, it means i am unlikely to achieve this on either size Canyon.

Regards

Paul

neeb

Yes, well, 66mm is quite a lot of trail. Even 63mm is on the high side. It’s a bit of a personal thing, but it does sound as if you might want to find something with less trail... On the other hand, you might not notice it much or adapt, it depends.. If it helps, I can certainly notice the difference between 56mm and 63mm trail, and I much prefer less trail. It’s unfortunate that a lot of manufacturers specify shallow head tube angles on their smaller sizes and then use the same fork right across the sizes. It’s a cost-cutting thing and it’s why you often don’t see the fork offset specified on geometry charts!

All credit to Trek for avoiding this on the Domane – have you thought about a Madone? Seem to have sensible trail figures right across the range on the Series 6 & 7 at least (series 5 has a bit more trail on the smallest size) P.S. I don't work for Trek or anything!

ptlk66

Neeb

The Trek Madone just doesn't do it for me, not saying the Domane is a looker either, but it is a fantastically versatile bike, accepts mudguards, larger tyres and is extremely comfortable. As i only have the entry level 4 series, I may opt for a 6 series project one - i would certainly consider it if i could feel a performance advantage.

I also have a soft spot for Bianchi and still enjoy my Via Nirone…..had a look a geo charts for the Infinito CV, but as you have said, a lot of information is missing. A winter of test rides looks to be in order…..shame about the Canyon as it is fantastic value and I like its shape.

Anyway, the information you have provided has been great and I have learnt a great deal…..thanks.

Paul

freefall_junkie

If you have a soft spot for Bianchi you need to try the Infinito CV. I have just bought one and it is a lovely bike.

I found the stack and reach spreadsheet used above to be invaluable in coming up with a short list of bikes, comparing the fit to my previous bikes. The fit of the 61 Infinito CV is absolutely bang on for my relatively long leg/short torso proportions - it just looks and feels right.

I did consider a Domane, but to my eyes it looked a tad ugly with the hugely long head tube and I wasn't convinced by the the flexy seat post on the test ride - to me the flaw in the design is that different riders on the same bike will get massively different amount of flex depending on a) how heavy they are and b) how high the saddle is above the pivot point. I really liked the Madone 5.9 I tried though, but was a bit put off by the positioning of the rear brake.

Mark

johngti

Got nothing to add here but I want to be able to find it again later. As you were!

tomisitt

I haven't read what Cervelo have to say about stack and reach, but my understanding is this:

The stack-to-reach (STR) ratio is generally considered to be the best guide to how a bike will fit you without necessarily sitting on it. The stack is the vertical distance between the middle of the bottom bracket and the middle of the top of the head tube, while the reach is the horizontal distance between the middle of the bottom bracket and the middle of the top of the top tube.



With these two numbers you divide stack height by reach to get the STR ratio. On a bike which has a stack of (for example) 545mm and a reach of 385mm, you divide 545 by 385, and that gives an STR ratio of 1.41. Anything less than 1.35 represents a very stretched out riding position, while anything over 1.50 represents a pretty upright position. Most builders aim for between 1.40 and 1.50 STR ratio on medium-sized frames.

Generally speaking, different sizes of the same frame will have different STR ratios. Small sizes tend to have lower, more stretched-out STR ratios to avoid things like toe-overlap, while bigger sizes tend to have more higher, more upright STR ratios.

Flexibility is the key here. If you’re a bendy 20 year-old then an STR ratio of 1.35 may be fine, but if you’re a 50 year-old with a bad back, you may need an STR ratio of 1.55 or higher. If you currently have a bike that fits you well and you can work out the STR ratio, you can use that ratio to find other frames that will fit your combination of size and flexibility. I know, for example, that I can fit comfortably on anything with an STR ratio of 1.40 or higher, but anything lower will be (literally) a stretch.

foggymike

ilovegrace, It's elbow on nose of the saddle with fingers straight then your other hand flat at 90 degrees on top of that (so 4 knuckles extending towards the bars off the end of your longest finger). The little finger on the hand across the top should come to the middle of the bars. Done in conjunction with checking that your bars hide your front hub when on the hoods and looking down you can claim to have passed the "old man of the road" reach test

paulmon

I have both the Domane SL (56) and Infinito CV (55) and both are comfortable bikes but the Bianchi is better in just about every dept. The only advantage the Domane has over the Bianchi is the ability to fit mudguards. Fitwise the Domane is slightly longer across the top so I have 80mm stem compared to the 100mm on the Bianchi.