Sizing down - stack and height implications?

I’m looking at a new bike and the general recommendation seems to be to size down if you’re between two sizes.

There seem to be a lot of options for adjusting reach on a smaller frame but what about height and stack (beyond spacers)? I can’t change my leg length :grinning:

I would like to achieve a more upright position on the bike. Would sizing down prohibit this?

If you want to achieve a higher position than would be possible with spacers then I would go for the bigger size. And/or look at a bike with different geometry - endurance geometry or something more all road oriented will typically have more stack. You can achieve higher stack with an angled stem but that’s normally a fix for somebody who has already bought the wrong frame, whereas you still have time to not make that mistake!


Here’s a tool for testing stem length changes;

99 Bikes has a pretty good comparison tool.

There’s was another one I saw a while back that was SUPER in depth. I’ll check my laptop later to see if I have.

One of the super nerds on here probably has a good tool they’ll drop in as well.

Seems if you want a more comfortable position you’d go up a size. Sizing down would be if you want to get low and slam the stem.

When I “wanted” to sit more upright (sorry: I “needed” to sit more upright, two different things) my bike fitter told me to size up because of the headtube length. You can play with stems, bars, controls and saddle setbacks. You can’t change the headtube length or add infinite spacers in the cockpit.

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That really depends on the type of bike, the type of riding you do, and your body.

Also most people want to get more aero, but you have already said you want to sit more upright. Because of that, ignore the “general recommendation”.

A larger frame will have more stack (and reach), so you might not need to use that many spacers. In general I think it benefits the handling when you can have stem length and seatpost within “normal” sizes (not a 170mm stem, for example).


I used this site for comparing two geometries. The graphics are basic but I found it super useful.


I like to have a more upright position due to an old neck injury. On both my road and gravel bike, I’ve picked the bigger of the two possible sizes to achieve this.

You can generally achieve the same fit with both smaller and larger frames.

On the smaller frame (vs the larger), you’ll have a combination of more spacers, and a more upturned and longer stem.

IIRC, for me to achieve the fit I wanted on the smaller frame, I would have needed 3 spacers, and a 17 degree upturned 110mm stem.

Vs on the larger frame, 3 spacers and an downturned 7 degree 80mm stem.

I chose the larger frame cause the setup looks better :). And the bigger frame better accommodates water bottles, frame bags, etc.


While u can adjust ur self on both sizes bigger and smaller, consider that handling will be different…

I was between sizes on Bianchi (55 and 53), and when i tested 55, i had to use 100mm stem… And while it feel comfortable whole handling was not as racing bike should ride… When i shortly test ride 53, i got handling i want (and using there longer stem)…

When i pick size i consider to dont have too much spacers under the stem, in same moment to dont have compromise of shortening the reach with some short stem for a size and to compromise handling…

If i cant achieve this then geometry is not compatible with my body and looking for something else…

But consider another thing… In the past i was out of cycling for a while (job, life…) and when i get back to active cycling lifestyle i had to use spacers under the stem… But soon after loosing some weight and gain back some flexibility i was able to go much lower then when i started biking again… Even if had to buy again probably will go size smaller for one of my bikes…