Reach and TT length

[sjbraun]

I'm trying to compare frame dimensions between two bikes.

Eff TT Head tube angle Seat tube angle Head tube length Reach
Bike 1. 57.5cm. 72 73 170mm. 388mm

Bike 2. 56.5cm. 72 73 170mm 385mm

How do these frames have almost identical reaches, with TTs that vary by a full centimeter?

Are there any other variables that can impact reach, besides those noted above?

Thanks

[prototoast]

Seat tube angle. A steeper seat tube shortens the top tube without changing the reach.

[tuscanyswe]

Seat tube angle. A steeper seat tube shortens the top tube without changing the reach.

This is true but they both have the same angles both in seat tube and head tube according to the values?


Some manufacturers measure reach differently. One could in theory measure it above and including headset. The other could measure it under or without.

The reach measurement is as such depending on how high up you measure it (stack) So they should be used in conjunction with each other (stack n reach). Hence why i think its easier to just go by effective toptube and headtube lenghts and angles instead.

Does one of the frames perhaps have a different headset solution? Integrated vs regular a head for instance?

I dont see how small differences in where they have measured the reach from would account for a full cm different in toptube lengths. It looks fishy to me tho i guess its 7mm to be exact.

[OtayBW]

Bar geometry (tops and sweep) and reach (and/or) drop,
Placement of the shifters on the tops,
Stem angle
???

[fa63]

Are there any other variables that can impact reach, besides those noted above?
Thanks

Fork length could be a factor. Does one have a much longer fork than the other (for example, a gravel bike vs. a road bike)?

[cmg]

It's the bottom bracket drop along the seat tube angle. More drop moves the bb horizontally. most manuf don't state the drop numbers.

[PacNW2Ford]

Seat tube length, the longer the seat tube, the more top tube is behind the BB. Thus a shorter reach for a given top tube length.

[zmalwo]

headtube length alters where the TT measure starts. Since the ST and HT angles are different by 1 degree, every up and down horizontal tt position alters the relative TT length at certain level. but your frames have identical HT length so that's perplexing

[charliedid]

It's the bottom bracket drop along the seat tube angle. More drop moves the bb horizontally. most manuf don't state the drop numbers.

Seat tube length, the longer the seat tube, the more top tube is behind the BB. Thus a shorter reach for a given top tube length.

I'm liking these two answers.

[Mark McM]

A value that is missing is the Stack of the two frames. Due to the angle of the head tube, as Stack increases, Reach decreases, even if all other frame dimensions remain the same (in other words, as you raise the handlebars, they also move backward). Reach shortens by roughly 3mm for every 10mm increase in stack.

As an example, say you had 2 frames, one with 550mm of stack and 383mm of reach, and one with 530mm of stack and 389mm of reach. The numbers make it sound like the 1st frame is taller, with a shorter reach to the handle bars. But if you were to add 20mm of spacers to the 2nd frame, it would have an effective Stack of 550mm and an effective reach of 383mm - exactly the same as the 1st frame. In other words, just by varying the spacers and/or stem angle, these two frames would fit the same.

[Dave]

If a geometry chart lists the reach, it should also list the stack. If the stack is not the same, you must make the 3mm adjustment in reach for each 10mm of stack height difference. The most common mistake is failing to do this when comparing two adjacent frame sizes.

The seat tube angle should then be compared, only to figure out the difference in seatpost setback that might be required. Every degree that the STA is steeper will require about 10mm more seatpost setback to get the same position, relative to the BB. If more accuracy is desired, the saddle setback is the saddle height, measured along the center of the seat tube, times the cosine of the STA. Run the calculation for both STAs and take the difference.

[Davist]

nevermind… I misread this..