When looking at the stack and reach numbers (awesome resource btw) how close should I be looking to get to the numbers, obviously spot on would probably be the best fit, but how far is too far away? i.e. 2cm, 5cm, 10cm, etc.
Looking for a bike for my wife, from the stack and reach numbers the Plasma, Transition and Blue T14 are almost spot on but trying to see how far out I should be considering.
the way i understand stack and reach (and someone may correct me here), is that it can be quite variable based on the components you’re using.
for example, stack is used as a indicator of seat height. but, it also must account for saddle-armrest drop. so, if you find a bike you like that’s a bit too ‘tall’, you could mount a low profile aerobar - this way, the drop is at the correct distance. obviously then, with a ‘short’ frame pick a high profile aerobar to make up for the extra exposed seatpost over the top tube.
reach can also be accomodated by stem length. this could be a bit more difficult with integrated aerobars, but it’s not impossible. the one consideration you must take though, is that an overly long or short stem will affect steering/balance (as will extreme saddle adjustment fore/aft).
I would think you could get away with 1-2cm either way, but I haven’t taken measurements of my equipment to check that either. but, consider going from a 110 to 90 stem. or the switch from a ‘top loaded’ aerobar (pads on top of extensions, on top of base bar) to a ventus/vision type.
personally, i like slowman’s approach of finding the parts you ‘need’ to have (saddle, cranks), then the parts you really, really want (frame, or aerobars, etc), then building the bike from there to meet your geometric needs. check out his article about duplicating his P3C to the slice for more info (he built the P3C based on wanting the ventus bars, then built the slice based on the parts he needed to match the P3C. pretty cool system)
“for example, stack is used as a indicator of seat height. but, it also must account for saddle-armrest drop.”
i’m not sure that’s a helpful statement. let’s recap:
stack and reach are very simply the X and Y axis measures from the BB to the head tube top. as such, they really don’t relate to saddle height. i know that in a roundabout way you’re right, but, i run into a lot of people these days who use the terms stack and reach to mean things they don’t mean.
also, you’re right that these metrics can’t be looked at in a vacuum. a frame’s stack should be higher or lower based on whether the aerobar you place on the bike has a greater or lesser distance between its armrests and its handlebar clamp.
in answer to the OP’s question, i would fit based on reach more than on stack, because there are a lot of things you can do to make a bike taller or lower but very few things you can do that impact a bike’s length. so if i needed a bike with 43cm of reach assuming a stem length that was about 2cm shorter than what i would ride on my road race bike, i would look for a frame that was about 15mm of that reach, one way or the other. i would try to keep it within that 3cm, the closer to 43cm the better.
but if you have a set of components you prefer, best to keep those in mind when figuring stack and reach. for example, i really like this new cannondale slice i’m riding around on, but, i’m lucky, because i also like the new Felt Devox bar that i placed on it, and, that’s a low profile aerobar. riding that bike with a higher profile aerobar would be tough for me, because of my fit coordinates. if you don’t have a range of aerobar options, then you lose a prime weapon for changing a bike’s stack.
Is there any database listing armrest height for various aerobars? Are there any specific bars that offer more upward adjustability than others?