I have a "XL" (59-60cm) nashbar cyclocross X frame, which is slightly too small. I have to raise the seat up a good bit, and I put in so many spacers (about 20mm) on the handlebar steer-tube, the steertube cracked (I'm assuming from stresses of the longer than usual distance between the headset and stem, resulting in higher than usual torque on the steertube).

In lieu of replacing the fork, I'm looking for a bigger frame (and fork to go with it), going on the theory that my small frame is the cause for the break, not the quality of the fork (although it is a nashbar fork, and cost about $120).

Unfortunately, I stupidly missed out on a number of taller disc friendly bike frames that a lot of manufacturers seemed to be making in 2011. The one I was drooling over in particular was the 63cm Civia Bryant, but they've stopped making it and I can't find anywhere online.

Surly was making the disc trucker in a 64 (wicked), but I can't find it online, and I'm a little wary of Surly, since they have a reputation of making unnecessarily heavy frames, Kona doesn't have any frames over 61cm, Motobecane has a 64cm frame, but no discs, Salsa only makes up to 60cm.

Soma has a 62cm frame that looks promising, but I'm looking for more options (especially taller ones, if they're available).

Does anyone have any recommendations for readily available 62-63cm disc compatible commuter oriented frames?

Universal Cycles seems to have the 63cm Bryant Alfine in stock. If you can manage the hefty price, you could always eBay the parts you didn't want.

You also might want to look closer at geometry, especially if you're buying online. The 60cm Bryant has a 221.5mm head tube, compared to 165 on the XL Nashbar Cylcocross X, so it may work for you.

Cross bikes have a deliberately lower stand over then a traditional road bike. This is to give some clearance on uneven ground. Same deal with Mountain bikes. The important measurement is "effective top tube length" on bikes with slanted top tubes and off road bikes. Next thing I would look at is head tube like was mentioned above.
I ride large bikes as well and most traditional bike 63cm frames are about 60cm Top tube.

That's a good point about the head tube length. I have a suspicion that is the measurement I should look more closely at. I don't know why I didn't consider it before. I'm now putting together a little list of large, disc compatible bike frames with the measurements for comparison.

Also, in the course of looking more closely at measurements, I noticed the measurements of stack and reach, which I think is probably exactly what I want. Besides rec's for large bike frames, anyone with any ideas how to calculate those measurements from online measurements?

Geometry specifications can be frustrating. It seems like there's always at least one number missing from what you'd need to map everything into an arbitrary coordinate space. Beyond that, I've heard that there is some disagreement among sites that specify reach and stack as to exactly what those numbers mean.

I take "reach" to mean the effective horizontal distance from a point directly above the bottom bracket to the top of the top tube. Salsa appears to indicate it this way in their diagrams. Trek's diagram is a little vague, and might be referring to the top of the steerer rather than the top of the top tube. This same problem of point-of-reference exists for effective top tube specifications. But lets pretend that everybody means the same thing and we're measuring to the top tube since that's fixed.

Now brace yourself. I'm about to geek out.



To calculate reach, you'd need to know the distance along the seat tube from the bottom bracket to the point at which the effective top tube measurement is taken. I'll call this distance H. Given the seat tube angle as A and the effective top tube length as T then reach, R, can be calculated as

R = T - [H * sin(90 - A)]

Of course, you don't really know H. Another possibility is if you knew the veritical height difference, S, between the bottom bracket and the top of the top tube (which I believe is the stack). Then,

R = T - [S * tan(90 - A)]

In theory, you can calculate S if you know fork rake, fork length, head tube length and bottom bracket drop. There is a bit of variability based on the headset stack, but you can get close. If you have bottom bracket height instead of drop, you also need wheel size and tire size. The calculations here are messier so I won't post them.

[/geek]

Now, if you aren't currently wearing a plaid shirt with a pocket protector, it's probably much easier to use the rule of thumb that one degree increase in seat tube angle virtually reduces the effective top tube length by about 1 cm. This rule of thumb can usefully compare frames in a way similar to frame reach if you already have a bit whose fit you know. You can, of course, also vary the practical reach by changing stems. Similarly, bottom bracket drop/height, top tube length and fork length together tell you most of what you want to know about frame stack.

You may also be interested in this article wherein Grant Petersen talks about top tube length and the extent to which it matters or doesn't: https://www.rivbike.com/v/vspfiles/as...tr_excerpt.pdf

Also, I ride a 59-60 cm road bike usually, but I ride a 57 Salsa Vaya. Somebody told me they measure the ETT and not the ST length. Might be worth a look, anyway...

I'm curious as to what kind of reach you're looking for. You mentioned the big stack of spacer you used on the Nashbar frame, which tells me you want your handlebars high, but as discussed in the Rivendell link higher bars yield an effectively shorter reach. If you want something with high bars and a long reach, you might want to look at frames designed for flat bars even if you intend to use drop bars. The Surly Karate Monkey comes to mind. Its biggest size has 632mm effective top tube, and while the head tube is a scant 125mm, the 468mm fork will give you all the height you need. The Van Dessel Whiskey Tango Foxtrot is another option which might work for you. On the other hand, if you're happy with the reach on your Nashbar frame, then a riser stem might be what you're after.

In the interest of full disclosure, I'm 5'9" and really have little idea how fit works for tall people. I'm just here for the math.

I'd rethink the fork replacement and look at a stronger fork. I'm 6'6" and race CX on a Vassago Fisticuff 60cm frame. Check out how far jacked up I've got my seatpost and stem. There's 7" of post showing, and that's a 2.5" (63.5mm) stack of spacers below a 110mm stem. That's a lot of steerer torque, and I've never had a single problem with it through a season of commuting, training on equestrian and MTB trails and about a dozen races.

You could take a look at Jamis. https://www.jamisbikes.com/usa/thebik...elite_geo.html

The Aurora Elite has a 62cm and the Bosanova has a 61cm. They also have the Nova Race, which is a cross bike in 61cm with discs.

Andy_K - Great article... Thanks for posting that link.

OP - Not sure about your budget, but check out some of the stock sizes of the Gunnar Fast Lane: https://gunnarbikes.com/site/bikes/fast-lane/

If those don't work - then you can go custom for an up charge.

Not sure where you're located - but if you're tall, (like me), then take some time and get a custom fit done locally. Then consider their recommendation on a frame choice that could work for you. Either stock or custom geometry...

It's always about the fit when it comes to long term comfort in the saddle.

Not sure where you got the impression that Surly frames are unnecessarily heavy. Pretty much standard weight frames for cromo steel. As long as they are in stock, any shop that can order from QBP can get you the frame.

60, 62, 64(!)cm disc trucker https://surlybikes.com/bikes/disc_trucker#specs

I thought the disc truckers were not going to be available until March. Is Surly making the framesets available sooner?

Hire one built , and get it right. Co Motion, is a smaller production shop
in Eugene Oregon.

Lennard Zinn in Colorado builds, some bikes and components for big riders.

Thanks for that stellar explanation, as well as the Petersen article. It took me all the way back to high school geometry (or was it middle school)?

I think I'm going to contact the companies to see what the stack and reach for their bikes are. It would probably be fun to calculate the stack and rise for each bike, but ultimately inaccurate. My main concern is the appropriate stack, especially since that is the value that isn't easily gleaned from published geometry specs, and since I'm not totally convinced that the reach is a more useful measurement of the effective top-tube (the steer tube to seat distance is probably more informative than the steer tube to an arbitrary point along the effective top tube line, which is pretty much determined by the seat tube angle).

Interestingly, with regards to the Petersen article, I noticed I liked the "new" longer TTeff from lowering the handlebars more than when it was more level (and thus with a shorter TT for all reasons listed on the forum so far).

BTW, for wheel sizes, this link has them tabulated https://www.bikecalc.com/wheel_size_math. I'm not sure how accurate they are, since I got a number that was slightly off when calculating a BB drop distance from wheel radius and BB height, then again the tire size wasn't listed on the manuf site, so I had to guess.

You can probably get fairly close. I tried it out with the Salsa Vaya, since they provide all the relevant numbersm including stack. I did it like this.

Let F = fork length
Let H = head tube length
Let A = head tube angle
Let B = bottom bracket drop
Let S = frame stack

S = sin(A) * (F + H) + B

Using the numbers for the 60cm Vaya

S = sin(72) * (405 + 215) + 75
= 664.66

This is in the ballpark of their published stack value, 660.7. If you calculated this way for all the bikes you're interested in, you'd get a reasonable basis for comparison at least.


The idea behind frame reach is that you're going to want to set your saddle in the same position relative to the bottom bracket regardless of seat tube angle. So for a frame with a slacker seat tube angle you'd move the saddle forward and for a frame with a steeper angle you'd move the saddle back, thus making the top tube length relative. Measuring reach relative to the bottom bracket accounts for this change in saddle offset and gives you an equal reference, independent of seat tube angle.


There's generally some variability in tire depth relative to claimed size, so that might be the reason for the discrepancy. The table looks right.

The Soma Double Cross has a very tall headtube and a very long vertual top-tube. It's a great frame.

I worded this backward. An increase in seat tube angle virtually increases effective top tube length because you'll move your saddle back to adjust for it.

At this point I'm probably beating a dead horse and even other geeks are staring at me thinking "what is wrong with that guy?" but I find that basic trigonometry has the same sort of appeal as sudoku, so I worked through the stack calculation to figure out how to get the point where the plane of the steering axis intersects the plane of the wheelbase (the lack of which was the main reason my above stack calculation was only approximate) and in the process figured out why my estimate was close without taking headset stack height into account (the two errors nearly offset one another).

While recognizing that most people won't care, I thought it would be worth recording the results here for the benefit of future Google users.

Using the Pythagorean theorem, it's simple to calculate the virtual fork length (i.e. length along the steering axis) from the actual fork length (F) and the fork offset (O). This gives you a virtual fork length (V) to some point below the plane of the wheelbase. However, the triangle created by V, F and O is intersected by the plane of the wheelbase to form another triangle. The angle where the plane of the wheelbase intersects V is the same as the head tube angle (A) and angle V-O is a 90 degree angle. Using the law of sines, we can use this information to calculate the distance that V extends below the plane of the wheelbase axis thus the distance from the fork crown to the plane of the wheelbase along the steering axis (V').

(hand waving)

V' = sqrt(F^2 - O^2) - O*[sin(90-A)/sin(A)]

Going back to the 60cm Salsa Vaya specifications for a sample, I get

V' = sqrt(405^2 - 45^2) - 45*[sin(72)/sin(18)] = 387.87

Now, going back to my equation for stack above, replacing fork length with V' and including a factor for lower headset stack height (h), we have

S = sin(A) * (V' + h + H) + B

The listed lower stack height for the Vaya's headset is 12mm. This is a pretty good guess for any no-integrated headset, I think.

S = sin(72) * (387.87 + 12 + 215) + 75 = 659.77

This compares favorably to Salsa's claimed frame stack value of 660.7, I think.

So you can calculate stack and reach from the typical geometry specifications.

Since I assume that only my fellow geeks are still reading at this point, I'll mention that while I was doing this it occured to me that it would be possible to calculate the slope of the top tube from actual and effective top tube lengths and the seat tube angle. In the tradition of my favorite math teacher, I'll leave that as an exercise.

that's awesome.

Agreed. The rest of it is beyond my comprehension.

I got a Salsa Vaya a couple of years ago. It has disk brakes and would make a nice commuter if I could still commute by bike.

I'm following along, and all the concepts make sense, but I'm not coming up with the right numbers. Probably a typo. Also, the slight discrepancy between your calculated stack may have to do 1) with the small distance from the center of the top tube to the top of the top tube or 2) because that calculation doesn't take into account the fork rake. Based off of some hand-written notes and wikipedia to remind me what a hypotenuse was, I think the "true" fork length, the length that you would get if you extended a line parallel to the head tube to the level of the axle, is, get ready:
D= fork distance, the desired variable
L= fork length per spec
R= fork rake per spec
H= head tube angle

D = L * sin(H - arcsin(R*sin(180-H)/L))/sin(80-H))

I haven't checked my work yet, and I realize I haven't put my proof in yet, but my level of discomfort from not doing my REAL job is starting to surpass the enjoyment of solving this puzzle.

Hope this is on the right track...

Fork rake and fork offset are the same thing, so I did take that into account. The offset, as I understand it, is specified along a line perpendicular to the steering axis and not along the wheelbase. I wondered about the distance from center to edges of tubes, but I think the specs should take that into account (I could be wrong).

I think that the number of significant digits in each specification is probably the limiting factor, as the calculated result can only be as accurate as the input. In the case of my calculation above with the Vaya, I really should have given my answer as 660mm because I had nothing that would give me 5 good digits. Salsa may have actually measured to get their value.

If you're getting wildly different answers trying to reproduce my results above, it's probably a matter of degrees/radians disagreement in the sine functions.