kosmo886

What is the best way to compare bike geometry and get the key measurements correct? In my case I am looking for a second bike and want it to fit somewhat similarly. I know there are lots of sites that compare specs, but I've found that none have fully complete and consistent specs listed, making the comparison pretty tough. What are the key parts that I should be getting right? I assume stack and reach are pretty important, but those are also reasonably adjustable with different length/angle stems and spacers. Any thoughts on how to go about this would be great.

kosmo886

Here is comparison info from one site as an example.

Branko D

Stack and reach tell you about the distance from the bottom bracket to the stem. Seat tube angle tells you where the saddle is positioned relative to the bottom bracket. If those are similar between the two bikes you are looking at, the fit coordinates will be similar.

Then you have measurements which tell you about how the bike handles - so, chainstay length, BB drop, wheelbase, fork geometry, etc.

https://geometrygeeks.bike/

Here's an example. My road bike is a Fuji Supreme (2019) in size 53, which has a 381mm reach and 538mm stack, and comes with a 74 deg seat tube. I run it with a 120mm - 17 deg stem.

So, building a gravel bike, i found the Ritchey Swiss Cross in size M to have very similar fit coordinates at 388mm reach, 550 stack and 74 deg seat tube. Since it is intended for offroad use, ever so slightly taller and closer handlebars are desirable, and I ordered it sight unseen online together with a 110mm - 17 stem, built it up from various spare parts, cut the excess steerer tube and took it for a ride; fits like a glove. Sure, different geometry handling wise, but the fit is there.
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kosmo886

on the example above, how big a difference is a seat tube angle of 74 vs. 73.7? My sense is that it's not too much?

Elvo

https://www.bikegeo.net/

Branko D

Comes out as the saddle being about 4.5mm forward or aft. Within range of saddle adjustability.

mschwett 

.3 degrees over the length of the seat tube…. 4 or so millimeters forward or back at the height of the seat. certainly within the range you can slide the saddle forward or back unless you’re already maxed out in one direction, which would be a Bad Sign.

Trakhak

After 50-plus years of buying high-performance bikes, I now mostly just look at head tube length and wheelbase. Given those two measurements falling within my preferred range, I know that I can easily make the bike fit correctly.

Iride01 

If you are transferring your fit from one similar bike to the next, then saddle height from the pedal at it's furthest away position and reach to the bars are pretty much all you need.

If you are comparing for purchase, and for some reason want the bike to be just like your old bike, then look at the effective top tube length, seat tube angle and stack. But as alway, try them on. Nothing beats sitting on them, except maybe 10 miles of test ride.

However if one bikes seat tube angle is different then you will still feel that something isn't right. And it probably isn't. You'll have to now find out what bar reach and stem height and maybe even saddle setback you need.

jpescatore

Some of it depends on how sensitive you are to bike "feel" - I have a touring bike, a road bike and now a "gravel" bike (that replaced the touring bike) that have different geometries. But, by keeping the bottom bracket to seat and seat to stem dimensions the same across all three bikes, they are all comfortable to me. I can't tell much difference in handling or feel - I'd make a horrible bike magazine review writer...

Two exceptions to the above:

1. A few years ago I did the Seattle to Portland 2 day ride, and rather than ship my bike to the West Coast I decided to rent one out there. I used the site 99spokes.com to compare my Trek Domane geometry to the rental choices and selected a Fuji Sportif. I brought my seat out, set the BB to seat and seat to stem dimension to my numbers and felt great after 120 miles the first day and 86 on the second day.
2. Since I use it for some loaded touring, I went with a smaller frame on the Jamis, based on the frame stack/stepover height (on a test ride, not just looking at numbers). I wear size 12.5 bike shoes and the smaller frame and my big feet cause more toe/front wheel overlap than I am used to. When moving not an issue, but starting from a dead stop with the bars turned the wrong way causes a problem. Had to learn to think that through when starting up.

SoSmellyAir

You can gauge the effect of the seat tube angle by comparing the effective / horizontal top tube length if the reach and stack are similar, right?

rm -rf

I found it hard to compare bikes, since they don't always measure the same way.

Stack and reach worked great when I bought my new road bike in 2014. Some bikes were 53cm, others 55cm with very similar stack and reach numbers. The old bike fit was pretty good, so I used it to find similar fitting bikes. The goal was to have maybe 20mm of spacers under the stem, to allow later adjustments either up or down. After 30,000 miles, it still is set at 20mm!
Here's an easy way to measure stack and reach for your current bike.

And the first link mentions a nice stem angle comparison tool--very useful. This makes it easy to see the net effect of angles along with stem lengths. Helpful to adjust from an ill-fitting frame with a very short or very long stem to a better frame with a normal 100mm or 110mm stem.

I ignored seat tube angle. I'm guessing that most road bikes seat tube angle differences won't really affect the saddle clamp placement very much.

rm -rf

I wondered how the various seat tube angles affect the saddle placement.

Here's a triangle calculator.
I put in 640 as one side, that's about 25 inches, close to my saddle height above the bottom bracket.
Set that side's base angle=73 degrees, the other base angle 90 degrees, clear out the rest of the fields. click Calc.

It calculates a base distance of 171mm for 74.5 degrees, and 187 for 73 degrees. That base is the saddle setback distance (upside down here). So the 73 degree seat tube moves the saddle back 16mm (0.6 inches), more than I would have guessed.

74 degrees: 176
73.7 degrees: 180. So that's 4mm difference. Like Branko D and mschwett estimated above.

DaveSSS 

Stack and reach tell the story with regard to fit, but reach must be corrected to the same stack height on any group of frames being compared. I always assume that spacers would be used on the smallest frame, to bring the stack up to the same as others being compared. Subtract 3mm from the reach for each 10mm of spacers.

Seat tube angle only affects the amount of seatpost setback that might be required. On the small frames that I ride, the STA is usually 74-74.5 degrees and I can use a 25 or 32mm setback post.

To evaluate the effect of STA with simple trigonometry. For example, my saddle height is 73cm, but the saddle rail height is more like 70cm. The setback at the rail height is 70 times the cosine of the STA. Using 73 degrees, you get 20.5mm and using 74 degrees you get 19.3cm. The difference is 1.2cm or 12mm. This difference is NOT a difference in reach.

razorjack

Reach is useful for MTB, but for road bike I'd look at TTh - as this gives you distance between contact points. (reach doesn't take into account STa).
(1* difference in STa, moves saddle about 11-13mm horizontally at 76cm from BB)
and of course Stack, which tells you how high is your cockpit.
HTa and chainstay are important for handling, but I can't tell you how much (slacker the angle or longer chainstays make bike steadier/slower in response/more boring)

Branko D

The benefit of using stack, reach and seat tube angle is that stack and reach determine where your hands are in relation to the bottom bracket, and hence where they are in relation to your legs. Then it's only a question of whether you can get the saddle in the correct position relative to the bottom bracket, which you can see by looking at seat tube angle (also seatpost setback - especially if it's got an aero seatpost which you can't just change out easily). You do need to look at what (if any) spacers you have and factor that in.

Using a combination effective top tube and stack doesn't do this, so your hands might be closer or further away to your legs depending on the seat tube angle since the intersection of seat tube and virtual top tube is the "starting point" of measuring effective top tube. So, if you've got a bike with a steep seat tube but then a seatpost with a lot of setback, the eff top tube is going to be shorter despite reach and actual position on the bike being potentially the same.

Case in point , compare the the Look 795 in size M and the Giant Propel in size M; the effective top tube is 526,2mm versus 550mm for the Giant, but the handlebar location are very similar (386/552 for the Look and 383/545 for the Giant) and the difference is down to the very steep seat angle of the Look 795 paired with a honking huge setback seatpost, while the Giant has a less steep STA and a much straighter seatpost which places your behind in the same position. If you tried to equate them using effective top tube, you'd be looking at two significantly different sizes.

razorjack

of course you need to look at STa, in both situations

DaveSSS 

STA does not affect reach, if the saddle is placed in the same position relative to the BB. It only affects the seatpost setback that might be needed. There's no need to even look at TT length.

razorjack

as we mentioned before, STa affects your position on a bike, doesn't matter if you look at the reach or TTh, you have to take STa into account as well.

SoSmellyAir

You two are debating semantics. Reach can refer to both:

1. Reach, as in reach vs. stack; and
2. The distance a cyclist, sitting on the saddle, stretches to "reach" the handlebars, i.e., the distance between the saddle and the handlebars;

Seat Tube Angle ("STA") affects #2 but not #1, which is why I said that one can use the Top Tube Effective / Horizontal Length to gauge (but not precisely determine) the effect of STA on #2.

Branko D

Only combined with seatpost setback and saddle fore-aft adjustment, because I'd want to get my seat in the same position relative to the bottom bracket if I was buying a new bike, so, hypothetically, if my new bike had a 74.5 STA instead of 74 which I ride now, that would tell me I'd either need to just slide the saddle further back assuming seatpoat setback is the same.

​​​​​​Then I'd want the same stack and reach as the old bike, or correct it with a different sized stem or a spacer, so I can get my handlebars in the same position relative to the bottom bracket.

So, basically, if I wanted to transplant my bike fit from bike A to bike B, I'd want to know stack, reach, seat tube angle and seatpoat setback (especially on aero bikes where you arw often stuck with the factory one).
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DaveSSS 

As I noted long ago, the STA is only relevant with regard to the seatpost setback that might be required. One degree of STA is worth about 12mm of setback, so IF a frame had an unusual STA or a post with minimal setback, it could cause a problem. I've never bought a frame with a dedicated seatpost that couldn't be changed, but I see some with only a 20mm setback. In my frame size, a 74.5 degree STA is common, but I want a 25-32mm setback to go with it. A 20mm dedicated setback post with a 74.5 degree STA would be a no-go.

Manufacturers list FRAME REACH on their geometry charts, and the STA. They should also list the seatpost setback if the bike has a unique D shaped or aero post that can't be replaced. I like my standard 27.2mm round post frames.