Stack / Reach Confusion - Please Help!

I finally got myself along to a bike fit today and, as well as refining my current position, I spoke to the fitter about possible frames that would work for me; however, having gotten home and doing some more research I’ve got more and more confused and so I’m hoping someone will be able to help.

NB: Sorry for the length of this post!

Background: I’m currently riding a GT Sportive Comp, a fairly low-end road bike which I picked up cheap from my LBS a few years ago and always viewed it as having a fairly relaxed geometry. The frame is size 54.5cm and (after today’s fit) I’m running a 100mm stem flipped upside-down (I think it is 7deg) and 15mm of spacers. The fitter did say ideally I could loose another 5mm off the stem but he didn’t think it worth it as I currently feel comfortable in the position (we may redress this at a later date).

At the end of the fit we discussed possible frames (I’m planning to start racing so was thinking Cervélo S3, Trek Madone or a Specialized Venge) the fitter recommended I look at Canyon Aeroad and, after some comments @chad may in a podcast some moths ago I briefly considered a Trek Domane. Now to the confusing part, when trying to workout what would fit well this evening I measured the stack and reach on my current bike and have come up with S555/R385, as I viewed my bike as ‘relaxed’ I was expecting to see aero bikes much longer and lower; however when looking at equivalent frame sizes mine seems to be lower and/or longer than most, I’ve added some data below.

Canyon Aeroad (Small) S 534 (-21) R 386 (+1)
Cervélo S3 (54) S 555 (+0) R 378 (-7)
Madone (54) S 560 (+5) R 381 (-4)
(+/- numbers are difference from my current frame in mm)

My confusion mainly lies around the fitter thinking that a size 54 S3 would be (slightly) too big for me and thus recommending a Small Aeroad but, as I read the numbers, the S3 would be much closer to what I need and the Madone would be best. Full disclosure, a LBS has a 2017 S3 Ultegra Disc on clearance for £2500 and I can get an extra 10% discount so I’m keen to know whether it would fit well.

Looking forward to hear what ya’ll think.

GT’s website has the 54 stack at 555 and reach at 379. Regardless, unless you are pushing the envelop on being too big or too small for a 54, I don’t think it’s as critical as people want to make it out to be. The adjustability of cockpits and seat posts makes it possible to fit a couple different size bikes for most people.

Without knowing more about your dimensions and seeing you in person these things are a bit subjective. Sounds like a good deal on the S3. Great bike.

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Thanks for the reply.

Could you link me to this please? I really struggled to find my bike on their site…

Guess I’m off to look at an S3 :tada:

What @landis said. It’s not as if your new bike can’t be adjusted… you can remove spacers, flip or change the stem, fore/aft seat adjustment, layback or online seatpost, you could even get different crank lengths!

My personal view is that if you are torn between two sizes then size up as it is easier to adjust.

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The clarity on their chart is horrible:

I hesitate to bring this up but, something else to consider if you are close to sizing up/down are the reach (and drop I suppose) of handlebars. Depending on how you place your shifters you can measure everything ten times thinking reach is xxx distance only to have the bar put you 10-20mm more/less. So for me I’ve learned to measure to the top of the hoods.

edit: I understand this is separate from the “reach” of a frame. It will affect your fit however and should be considered again if close to sizing up/down. Or if you know exactly where your saddle needs to be in relation to the BB and how much reach to the shifters you need/want.

I guess I was most surprised by my stack / reach nearly matching that of the S3. I realise having 15mm of spacers and a flipped stem on an aero bike isn’t great; however, I’ve not spent much time on the bike recently so imagine I can work to get that lower.

Thanks for the link, mine is a GTS Comp not GTR, I think it’s the older model with some small frame differences. It used to be on their site but I guess it was taken off.

Understood regarding the bars, I feel as long as the frame is good there is a reasonable amount of after-market bars etc that I could swap out.

You don’t seem to mention handling in all of this, that’s the bit the bike geo will impact (primarily
Wheelbase, front centre, chainstay length, bottom bracket drop, fork rake etc.).

On top of that your seat design has to be considered with seat post (e.g. Specialized Romin seatbone position is way further back than a Toupe).

Then bars all differ in drop and reach to drop (typical these days seems to be is 80ishmm reach and 125ishmm drop).

So you can make a bike bigger or a bike smaller but it all impacts handling on top of how a bike will already be designed to handle.

I am 5’10" and kind of like 380 reach and 550 stack - but to get this and have the bike handling how I like ended up on a 52 CAAD12 with 20mm setback post, Specialized Romin set just back from middle, 25mm of spacers under stem, 110mm stem, 70mm reach/125mm drop/6 degrees flare bars (top of bar dead flat and levers just up front flat).

I could have got same fit using a 54 almost out of the box but this handled better. Way better.

A Cervelo (R3 specifically) in a 54 worked too with a -17 110mm stem but handling was dull.

I dont know if I got exactly the right bike models, but this is a great tool for comparing bike sizes.

You add or remove bikes and sizes as needed.


One challenge I’ve found comparing bikes is that stack and reach don’t give the full picture for a bike. You also need to know other measurements such as seat tube angle and head tube angle to understand how the geometry changes as you move the saddle up and down, and also as you move the stem up and down (albeit smaller effect than seat).

I’ve found a more helpful way to think about bike sizing is contact point geometry, as opposed to frame geometry. This includes the effect of saddle height, stem height, as well as stem length and angle.

Basically, map the relative positioning of the top of the saddle, the front end of the stem, and the bottom bracket (although to be fully comprehensive, it would be the pedal spindle, but bottom bracket is easier).

This allows you to understand the end result of any bike setup, in terms of what really matters - handle bar placement relative to saddle - horizontal reach and vertical drop; plus saddle placement relative to bottom bracket - vertical and horizontal, plus linear distance.

If you have a bike you like the fit on, you can take these measurements directly from the bike (although is a little tricky to get right) and use this to inform a new bike setup - frame, spacers, stem length and angle.

When I did this for the road bike I bought last year, the hardest part was I had to dust off some dormant 25 year old trigonometry skills to run some numbers, as I couldn’t find an online calculator to to this (anyone know of one)?

I wouldn’t obsess over this too much. If you’ve had a proper fit, you’ll have a bunch of measurements you can take to discuss with the LBS when you have a look at the new bike. If they are knowledgable they’ll be able to advise if the bike will fit you well.

Sounds like you’re running a fairly relaxed position on the bike if you have a 7degree 100mm stem flipped upside down with 15mm of spacers. And when he said that you could lose 5mm from the stem, did he mean length or spacers underneath? A 95mm stem on a 54 is short! Things like this can greatly affect the handling.

What’s often more important that the exact measurements of the frame is the measurements between contact points - eg saddle height, saddle set back, nose of saddle to hoods. Something like a bar with a bigger radius could throw the whole fit off even if your stack and reach are the same.

Both, we kept me a little higher as I’m planning on using this bike for a Land’s End to John O’Groats ride next summer. The bike came with a 100mm stem and my fitter suggested that +/-10% from the stock was normally deemed an acceptable range for handling. Ultimately I’ve still got the 100mm on.

Thanks for this, after a quick look some of the sizes seem a bit off but I’ll have a better look on my PC later.

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Yeah, I recommend a double check against the builder sites. Some of the models have different or incorrect data.

But the good stuff is nice for comparing. On stuff I really want to check, I just make a Google sheet and do my own chart. But this is nice for a quick option.

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See if this helps:

It is what I use to compare current fit with different bikes.


I have one bike with two layers of bar tape…makes it feel completely different than my bike with a single layer, even though the bars (and all other measurements) are pretty much the same. It’s weird, and every few months I’ll actually measure both bars cuz I think they must be different!

FWIW this is pretty much the position I’m currently in.