How to interpret bike geometry?

Equipment
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I’m currently browsing for a new bike. I would like to get something a bit more upright with a shorter reach than my current bike, which is an Eddy Merckx Mourenx 69 in size small: Geometry Details: Eddy Merckx Mourenx 69 2016

I’ve come across bikeinsights.com that let’s you compare geometries side by side but I’m completely unsure how to interpret the results.

For example, this is my bike against a Specialized Roubaix Compare: 2021 Specialized Bicycles Roubaix Comp 52 vs 2017 Eddy Merckx Mourenx 69 Base Small - Bike Insights,

It looks to me like I’d be in a more upright position on the Specialized, but is that the case and will the reach be shorter?

Or this comparison to a Merida: Compare: 2023 Merida Bikes Scultura 4000 S vs 2017 Eddy Merckx Mourenx 69 Base Small - Bike Insights,

Here it looks like a more upright position but longer reach on the Merida?

Help would be appreciated! Bike recommendations as well :slight_smile: Definitely endurance-style geometry and no racing.

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Stack and reach are easy things to read to quickly compare frames. Yes the Roubaix is taller and shorter than the Eddy Merckx.

Stack is how tall the front end is compared to the bb, reach is how far away the headtube is.
The Roubaix is basically your Eddy with 26 mm of extra spacers and a 5 mm shorter stem. Obviously you can adjust it even further from there with stems and spacers.

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Just to add to that, if you want more explanation, I literally typed this thread title into google and got back several results with good descriptions of things like stack and reach. Here’s the first one, but there were lots more, including some videos. It can be confusing at first, but once you understand, it’s helpful to know. Where it gets confusing for me is that not all brands list the same measurements in their charts.

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Reach and stack are tied together by the inclination of the head tube so if the stack between two bike is different by large amounts the difference in reach doesn’t represent as big an amount as it looks.

Let’s say you have a bike that has a very long head tube, so you take it to a framebuilder to cut it 50mm shorter. Since stack and reach are to the top of the head tube, the bike will now have a lower stack and a longer reach. If you put 50mm of head tube spacers your bars will be exactly where they started.

This is exactly what is happening in the comparison of the Merckx and the Roubaix. Their head tubes overlap in space so switching to the Roubaix would just change how many spacers you have on the bike. If on the Merckx you’ve run out of steerer tube for spacers, or have to use stems that slope up too much for your aesthetic tastes, the Roubaix would otherwise fit similarly.

If you have a guess about how much taller and shorter you’d like the frame to be relative the Merckx, you can use BikeInsight’s search to set a maximum reach and minimum stack, and the appropriate categories to find candidate frames.

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