What makes a bike a "59?" Or a "58?" Or a "57?" Etc…

[Mark McM]

Originally Posted by unterhausen

This is why some people are going with stack and reach, but nobody has gotten used to it yet.

I agree that Stack and Reach makes the most sense - they are direct measurements of the most pertinent dimensions. But Stack and Reach can be confusing for some. Firstly, it isn't a single value. "Small", "Medium", and "Large" are easy to understand, even if you have extended sizes like "Extra Small" or "Medium-Large". If the Stack/Reach concept were applied to T-shirt sizing, you'd have to figure out if you were a Short/Wide or a Tall/Narrow.

But a more serious source of confusion is that Stack and Reach aren't truly independent. If you change head tube length while keeping all other dimensions/angles the same, Reach decreases by about 30% of the Stack increase. This means that, for example, a frame with a Stack/Reach of 550mm/380mm will fit exactly the same as frame with Stack/Reach of 530mm/386mm, if you add 20mm of steerer spacers or a taller stem on the 2nd frame (even though the 2nd frame looks like it should be "longer").

[Mark McM]

Originally Posted by Velocipede

Regarding non-horizontal toptubes on bikes for a long time, true. But fat tire bikes and BMX bikes weren't normally measured in frame sizes like road bikes. Fat tire/cruisers were listed in the tire size.

Which is an interesting tie into the discussion of smaller wheels on women's bikes in the Women Framebuilders thread. For Balloon bikes (and and for children's bikes) there is a very clear understanding that frame geometry is very limited by rider size, and that smaller riders should use smaller wheels. That's why you've got 16", 20", 24", 26" bikes, which describes both wheel size and frame size. I think one of the barriers to the idea of different wheel sizes on "adult" bikes is the perception that full-grown adults should use "full size" wheels.

[BRad704]

Originally Posted by Velocipede

BMX, it was M, L, XL, XXL and that was the length. And most times that was for Mini and Micro or standard styles of racing which also corresponded to the tire size.

I thought very briefly about getting into BMX again as an adult, and if anyone things road sizing is confusing... BMX bikes and sizes totally blew my mind.

[sg8357]

Originally Posted by Mike V

I blame compact, sloping bikes.

BSA sloping tt bikes, 1910, everything old is new again and again..
Note the Road-Racing machine is available sloping or parallel.
NB, the 26in wheels are 597bsd, Brit club size EA1

[Dave]

When trying to decide between two different sizes of bike frame, the most common mistake is not correcting the reach to the same stack height on both frames. If the stack differs by 20mm, I would assume the smaller frame gets 20mm of spacer added, so the reach becomes about 6mm shorter. Then make the reach comparison. If that’s not done, you're comparing two different fits.

Seat tube angle does not affect reach, but it does affect how much seat post setback might be required to get the saddle to the desired position relative to the BB. Seatpost setback is the saddle rail height times the cosine of the STA. At my 67cm rail height, 1 degree is about 11mm in setback difference. I've got a 25mm setback on one frame and a 32mm on the other identical frame. The saddle is better centered on the 32mm, but they both work.

[Mark McM]

Originally Posted by sg8357

BSA sloping tt bikes, 1910, everything old is new again and again..
Note the Road-Racing machine is available sloping or parallel.
NB, the 26in wheels are 597bsd, Brit club size EA1

Indeed, the first MTB also had an upward sloped top tube. But even though they came on the scene 70 years later, the MTB folks were the first to figure out that the correct slope for top tube should be downward.

[Toeclips]

Now that I'm in my 60's
I used to go by center to center seat measurements 58 cm
Now I find myself more concerned about the head tube length 17 to 17.5 cm

I come across a lot of potential ebay finds in 58 cm only to find out the head tube length is less than ideal

Its tricky because 59 or 60cm has the head tube length but stand over height is too high for me

To compensate I use a Nitto stem on my traditional bikes
Serotta Club Special and a Basso
For my Serotta Atlanta I use a quill extender and Ritchey stem

Not slammed at all

[robt57]

Stack spec accounts for BB drop. I think folks forget to adjust for more BB drop to HT length. No to mention adjusting for fork length [axle to crown] for CX and All Road variants of the day.

Example being if the Axle to Crown is 30mm more than your road bike and you get the same HT you think you need, you are likely to have very high bars even slammed.

[prototoast]

Originally Posted by Mark McM

I agree that Stack and Reach makes the most sense - they are direct measurements of the most pertinent dimensions. But Stack and Reach can be confusing for some. Firstly, it isn't a single value. "Small", "Medium", and "Large" are easy to understand, even if you have extended sizes like "Extra Small" or "Medium-Large". If the Stack/Reach concept were applied to T-shirt sizing, you'd have to figure out if you were a Short/Wide or a Tall/Narrow.

And even those sizes are totally inconsistent. There was another thread on here a while back: https://forums.thepaceline.net/showthread.php?t=259409

As I noted in that thread, depending on the manufacturer, my correct size can range from a medium to an extra large.

Ultimately, you really need three points to define bike fit: butt, hands, feet. Typically feet are defined around the bottom bracket (though crank length will come into play) and normalized by the bottom bracket. But that still leaves you with 2 points each requiring 2 coordinates to define. You can do that with x,y coordinates (a la stack and reach) or polar coordinates (a la st angle and length), but you still need 4 numbers for fit. Any size metric that is a single number, or even 2 numbers is insufficient. If you assume a constant ST angle, horizontal TT, and ST = TT length, as was common in the old days, you can fully define the key points with a single number, but once you relax those constraints, any single number will be insufficient.

[Big Dan]

If it fits buy it.

[Clean39T]

Originally Posted by Big Dan

If it fits buy it.

If it fits, you must sits.

.
..
.

I took a chance on a Canyon Ultimate CFR with wildly different geometry than anything I've had before - the reach figure is huge for an "XL", almost 420mm, where (for example) a 59.5 Dogma is around 390mm, and the STA is almost 74-deg vs. the 72.5-deg that is the norm for other bikes this size, at least historically. The Canyon has a 596mm TT and 1034mm WB w 70mm drop. And it fits beautifully. Instead of running a 120/130mm stem on a shorter front-center bike (shorter TT, less reach), the Canyon runs a 110mm stem. I used around 15mm setback and sit much further forward on the Canyon than I have on other bikes in recent years. I'm motoring on it. And I don't think it's just the 6.8-kg weight. Plus, it handles better than nearly any other bike I've ridden in recent years, which I chalk up more to how my weight is balanced on the bike than anything else. I don't get speedwobbles or a floaty front end, which has long been my bugaboo. Anyway, it was a step outside of the standard "62cm ST w 59cm TT and 72.5-STA" box I more or less lived in for years, and I love it. So much so that when I finally get a new custom done, I'll probably just ask to copy this geometry and be done with it.

[tv_vt]

The frame 'size' is just a ballpark figure when you see it on an ad like that IMO. You really need to look at the full geometry to know if it will fit you. As you already have realized, the frame 'size' is only relative to the other sizes a builder or manufacturer makes. Between makers and builders, and even within the same builder's shop, other modifications to the frame, like extended headtube, may make something work that, at first glance at the 'size', you wouldn't think would.

OP XXT, I'm surprised you didn't jump at Dubrat's Stada Bianca. My guess would've been that would fit you.

[mhespenheide]

Originally Posted by Clean39T

I took a chance on a Canyon Ultimate CFR with wildly different geometry than anything I've had before - the reach figure is huge for an "XL", almost 420mm, where (for example) a 59.5 Dogma is around 390mm, and the STA is almost 74-deg vs. the 72.5-deg that is the norm for other bikes this size, at least historically. The Canyon has a 596mm TT and 1034mm WB w 70mm drop. And it fits beautifully. Instead of running a 120/130mm stem on a shorter front-center bike (shorter TT, less reach), the Canyon runs a 110mm stem. I used around 15mm setback and sit much further forward on the Canyon than I have on other bikes in recent years. I'm motoring on it. And I don't think it's just the 6.8-kg weight. Plus, it handles better than nearly any other bike I've ridden in recent years, which I chalk up more to how my weight is balanced on the bike than anything else. I don't get speedwobbles or a floaty front end, which has long been my bugaboo. Anyway, it was a step outside of the standard "62cm ST w 59cm TT and 72.5-STA" box I more or less lived in for years, and I love it. So much so that when I finally get a new custom done, I'll probably just ask to copy this geometry and be done with it.

...and that throws another set of variables into the mix: weight distribution front-rear.

When I picked up my Felt F1PR, I made the mistake of only looking at reach and not the combination of stack and reach. On arrival, the frame was bigger than I thought it was going to be. To get the same fit as I was used to from other bikes, I needed to run a shorter stem than I typically did (110mm instead of 120mm). Then I found the steering to be a bit more stable than I was looking for, and I ended up scooting the saddle a little forward. That, and the addition of a lighter-weight front wheel, helped pick up the steering a touch; enough for me to be happy to hold on to the bike. Not what I would have expected, but it worked.

Fitting, man. It's weird.

[XXtwindad]

Originally Posted by tv_vt

The frame 'size' is just a ballpark figure when you see it on an ad like that IMO. You really need to look at the full geometry to know if it will fit you. As you already have realized, the frame 'size' is only relative to the other sizes a builder or manufacturer makes. Between makers and builders, and even within the same builder's shop, other modifications to the frame, like extended headtube, may make something work that, at first glance at the 'size', you wouldn't think would.

OP XXT, I'm surprised you didn't jump at Dubrat's Stada Bianca. My guess would've been that would fit you.

It would’ve been too small. Otherwise, yeah, I would’ve moved one of my other bikes too make sure it came to the Bay Area.

[robt57]

I will add that reach has to have STA accounted for your fit check list.

Reach of 'XX' on a 73^ STA VS 71.8^ where on the latter you will maybe/more likely use a zero offset seatpost negates a portion of the reach unless you are going to ride the frame with more saddle setback. If you aren't, you got a shorter reach than you thought you choose, effectively a shorter TT.

I think of it as the fore/aft fit window on the bike, being I always start with saddle setback.

I will be sitting in said fore/aft window, even if at the front or rear edge of it. I think of this as fore/aft CG window most importantly, being my girth in said window will effect balance rolling 100%.