What’s bike geometry? And detailed specs for CHANGE bikes

Biking Skills

June 8th 2023

Buying a new bike that will fit you well can be done simply online; At Flatbike, we focus primarily on stand-over height, overall height, where you want to ride, how you feel about hills, and how your lower back feels about bending over for long periods.

Beyond that–if you’re trying for a specific ride feel, or want to match the exact fit that you already have on another bike–then you’re a whole different level of rider who likely asks about frame geometry. In this article, we’ll answer questions that discerning riders have been asking us for months, while giving others some intro-level guidance as to why these numbers are important.

If you already know all about frame geometry and want to skip the introduction, here is the geometry table for folding CHANGE bikes.

The goal: Represent the design decisions for a bicycle through numbers.

Remember similar triangles in high school geometry class? If all the angles of two triangles are the same, then they both look the same. Similarly, if two bikes have the same angles and other specs, then they will ride much the same.

But on a bike, there are multiple triangles and side lengths. And occasionally, something that used to be a side measurement years ago (like top tube length) is now a virtual distance (top tube effective length) as the shape of bike frames has changed over the years.

But first, a quick, visual review of the different parts of a bike:

Aaron Kuehn’s timeless typogram speaks for itself about what makes a bike. Now we’re going to add terms that generally describe the size of the bike.

In this case, the numbers represent different generally accepted measurements about the bike’s general dimensions. We’ll start with the simple ones:

Basic frame geometry

The bike in the image is a CHANGE Road Warrior folding gravel bike, but could be any bike for this purpose. What’s represented here:

1. Stand-over height. This represents whether you can land on your feet and stand over the top tube when you jump off the seat or if you’re going to have a bad day.

2. Seat tube length. This is the single number often used when listing the size of a bike, even though the overall height to the seat is highly adjustable.

3. Chainstay length. This affects whether the bike is “squirrelly” and responsive or whether it is slower to turn. A longer chainstay, like on touring bikes, also helps keep your heels form hitting your pannier bags.

4. Wheelbase. This also affects the responsive-vs-stable tradeoff, adding one more factor–if there’s more distance to your front wheel, you’re less likely to kick it in a sharp turn.

5. Fork length. This doesn’t affect the ride as much as what replacements are available. And your choices there–steel, carbon, aluminum, shock or not–greatly affect your ride.

6. Head tube length. This, plus fork length, plus the diameter of your wheel, offers a semi-accurate measure of how low your bars can go. We’ll get more accurate with more advanced measurements later. Suffice it to say nobody buys a bike based on this alone.

7. Top tube (actual). Pretty simple. The top tube? This is now long it is. It’s a good indicator of how far you’ll lean forward . . . depending upon how far forward your seat is attached.

8. Top tube (effective). Our first truly virtual measurement. If the top tube were to go straight back horizontally like it used to on older bikes, how long would it be? Great for comparing a new bike to an old bike, or comparing two new bikes with an old measurement.

Advanced frame geometry

Now we’re going to introduce some more virtual measurements (and angles!) to get closer to the numeric representation of the feel of a bike–although wheelbase was a pretty solid indicator of feel. The first thing you’ll notice is some reference comparison lines, such as vertical through the crank, horizontal through the axles, and head tube angle.

What’s represented here:

9. Reach. Given a common reference point of the crank, how far are you reaching forward for the handlebars? That’s the idea, anyway. But there’s the whole idea of what length stem you have and what type of bars at what height. But it’s a starting point for comparison.

10. Stack. How far above the crank before the steering action happens? If your stack is too low, you’ll need to raise the bars at the stem. If too high, won’t have that option.

11. Bottom bracket drop. If your bottom bracket drop is large, the center of your crank is low, and it may interfere with the ground when mountain biking (or when pedaling on sharp road turns). If it’s small you may feel like you are riding up high and need more stack (or angled stem) to get to a more upright riding position.

12. Seat tube angle. In theory, this tells you how much your reach is going to change as you raise and lower your seat. But never having seen anyone yet do the ratio math to figure that out for a bike, I personally consider this one meaningless. Far better to focus on the comfort angle of your actual seat.

13. Head tube angle. This one makes a surprising amount of difference. The higher, or more aggressive, the head tube angle is, the more responsive (and squirrelly) the bike is. The lower, or more “slack” the number is, the more relaxed the handling is. (Ultimate example? Think of someone sitting way back on a long-fork chopper motorcycle: super-relaxed, but it takes half a parking lot to turn around!).

14. Fork rake (also called offset). How far the axle is forward of the line through the center of headtube. The works in cooperation with head tube angle; an aggressive head tube angle that pulls the axle in toward the crank can be softened with a different fork with more offset that pushes the axle back out again.

You may also hear the term trail, which is calculated from three lines–those two intersections where the offset angle and a vertical line thorough the front axle meet the ground? Trail is the distance between them. More offset reduces the trail, which is an interesting inverse math concept, but will this math variable get you to a better ride? I’m not seeing it. (Feel free to passionately argue for its value.)

But there are some geometry measurements that we think have been long underrepresented. How much space does your bike consume when you’re done riding and need to store it?

Storage geometry

This isn’t about the ride at all, but how your bike fits into your life the rest of the time.

How much room are you going to need to allocate in length, height, and width whenever you store or transport your bike?

If it’s too big for the volume of your car, you’re going to be driving around with your valuables hanging on the outside.

What’s represented here:

15. Stored length. Usually, that’s the length of a whole bike. If your bike manufacturer doesn’t provide this, you can easily calculate it as the wheelbase plus the diameter of a wheel (or more accurately a wheelbase plus two radii). This is usually close to five feet long.

16. Stored height. Usually, this is the entire heigh of the bike at the top of the bars. Because most geometry is focused on the frameset, this may not be easy to find. But you can get close by adding the radius of the wheel, the fork length, the headtube length, and whatever is going on above the headset.

17. Bike width at the widest point. This is usually the width of the handlebars, unless the bike folds or another solution is used to flatten the bike (such as the Flatten Your Bike kit of folding THINstem and pop-off pedals).

18. Reduced bike width. This is specific to CHANGE bikes, where a separate wheel can be placed anywhere else in a boat, RV, or plan storage compartment to reduce the overall size.

And with that lengthy introduction, we’re ready to look at the geometry tables…

CHANGE bike and frame geometry.

Specifically, this is the geometry for all three sizes of the CHANGE Road Warrior folding gravel bike. For the other CHANGE models of full-size folding bike, the change from a 700c wheel to a 27.5″ wheel will affect none of the geometry measurements except stand-over height (by about 10 millimeters).

But a different length of fork on other models will change the angle of the frame, affecting everything from head tube angle to effective top tube.

To the many folks who have guided us in the direction of getting and posting these data points–especially Mio in Sweden!–many thanks.

As we have multiple sizes of various bikes out and available in our shop and not immediately going out the door, we will grab frame-specific measurements. Until then, this represents the most authoritative geometry data available on bike and frames from Changebike, LTD.

What did we miss?

Happy bike comparing,

Bob Forgrave