milkbaby

Just wondering... is there a good online resource explaining how stack and reach are used in bike fit and how to compare frames with these two measurements? When you look at stack and reach numbers of a frame, can you then ignore the seat tube angle (STA) and head tube angle (HTA) in looking at how well a frame might fit and how you balance on the bike between the wheels?

I was wondering because I was tooling around on competitive cyclist looking at the BH G5. I have a Jamis Xenith that I like the fit on, and the BH in XS is lower stack but similar reach to the Jamis, while the BH in S is similar stack but longer reach than the Jamis. Wondering which will be easier to set up similar in fit and will have the same balance between the wheels...

Looigi

Unless you have really wack proportions, balance between the wheels won't be a consideration. Seat tube angle is not a consideration in any case because the position of the saddle can be set as needed by adjustment in height of the seat post and fore/aft adjustment on the saddle rails or with a seatpost with the desired setback. That is a major part of the rationale behind the stack and reach concept as far as comparing frame sizes. Head tube angles vary only slight between bikes and a 1/2 degree difference will only make a mm difference in the fore/aft postion of the bars. Let's say the HT angle differ by 1/2 degree and the bars are 10 cm higher than the top of the head tube. The difference in bar position will only be 0.9 mm.

DaveSSS 

Stack is a great measurement because it elimintates the effects of differing BB drop and fork length, that just comparing head tube lengths does not. You still need to know the minimum headset top section height because some frames might allow an 8mm top and others a minimum 20mm.

Reach is more complicated. You can only compare reach values at ONE stack height. Any variation in the stack between two frames affects the reach. The difference is about 3mm in reach for each 10mm of stack height difference (cosine of the head tube angle is the exact ratio). Take the reach value of the frame with the shorter stack and subtract the proper amount of correction, since that would be the reach if enough spacer was added to make the stacks equal.

For example, is you're comparing two frames, but one has a 20mm taller stack, then the true difference in reach is 6mm more than whatever is posted on the geometry chart.

Seat tube angle still has some value, but only in figuring out how much seatpost setback you might need. In my frame size (51cm), most brands have an STA of 74-74.5 degrees, but there are some like Cervelo that have a 73 degee STA. The difference in setback is (cosA-cosB) x saddle rail height, or a little more than 1cm per degree.

Campag4life

Your best effort is to post your existing frameset geometry here...along with a picture and saddle height. Next post the geometry of the frameset you are interested in. Simple trig will allow you to compare both. STA matters independent of stack and reach. Generally, the right choice of seat post setback and stem will allow you to compensate for any STA or HTA differences if stack and reach are in the same ball park.