Hey everyone,

I'm not entirely sure how to calculate reach on a bike but I just upgraded from a 100mm to 120mm stem and feel a lot more comfy on my bike.

The reach on my bike was 380mm with the 100mm stem. Does adding a 120mm stemvautomatically add 20mm to the reach or is there an alternative formula to calculate reach when increasing stem length?

Thanks for your help!

Reach as published is a frame specification, not a finished bike specification. There is also the idea of finished bike reach, but numbers like that are never published as part of frame geometry. Your number is frame reach and has nothing to do with stem length.

Also because of the angle the stem the length doesn't convert directly to reach. The effect on reach is a little shorter than the length of the stem.

One thing you can do is Google "bike stem calculator". You will get a web site that compares th effects of different length and angle stems. Also takes into account the effect of spacer stack.

And by reach they mean the horizontal distance between the bottom bracket and the top of the head tube (centre to centre). And by horizontal I mean draw vertical lines from the bottom bracket and top of the head tube to the ground and measure the distance between them.

It depends on the stem's rise angle* (if any) Extending a horizontal stem extends the reach by the same amount. But if the stem rises or drops, you have to multiply the added length by the cosine of the rise angle to get the change in reach.

For those who prefer to let the magic of computers handle the math, there are various calculators on line. Here's one.

* rise angle is the net angle from the horizontal based on how the stem and head tube angle add/subtract. Ie. a 73° head tube and 17° (90-73) stem will produce a rise angle of 0 or 34° depending on whether the stem is angled up or down.

Depends on headtube angle, as well as the angle of the stem. For instance, with a 73º headtube:

100mm -7º vs. 120mm -7º: the 120 has 20mm more reach, and is 4mm higher.
100mm 0º vs. 120mm 0º: the 120 has 19mm more reach, and is 6mm higher.
100mm 7º vs. 120mm 7º: the 120 has 18mm more reach, and is 8mm higher.

This is a good tool for comparing stem stack and reach alone: Stem Comparison Tool | yojimg.net

This is a good tool for comparing effective handlebar positions and a lot of other variables, either on different frames, or just with different stem and spacer setups while keeping the same frame: Stack and reach calculator

Frame Reach = Top tube length, forward of the Plumb line thru the BB axis, Set Back is to the rear of that vertical line, to seat tube center.

Ref: C.O.N.I.