Bike Geometry Explained to Measure Right Bike Size

Measure Right Bike Size

Bike Geometry Explained to Measure Right Bike Size

Today’s article is all about bike fit. There’s a ton of trends going on in the bike industry. Since biking, isn’t a one size fits all sport. Some of these trends might be good for some riders.

They might not be good for others. So, I have a few experiments plan today that involve this scale, this whiteboard behind me, and a few other things that will help you demystify some of the terms being thrown around.

There is I’ll help you figure out what’s best for your particular riding style and how these geometry changes affect hardtail choice versus suspension.

Quick Overview

Let’s give a quick overview of some of the terms you use to size bikes nowadays and talk about why some of these changes happen.

If you go back to early mountain bike days, the two ways that you would size a mountain bike were off of a seat tube and off of your top tube, mountain bikes used to be a pretty traditional diamond design and the top tubes were really tall.

People want to know how to measure a bike and here we have gathered some quick useful tips to get the right size.

So, you needed to make sure that you had enough room to stand over the bike. And then you also wanted to know the distance from your seat tube to your head tube.

That would determine the amount of room that you had when you sat on the bike and pedal that down the trail. Flash-forward a number of years.

riders are riding more aggressively refer trails than ever before. They started wanting longer travel. They started wanting bigger wheels and when you have a slack seat tube, long travel, there’s not a lot of room for this big giant wheel to move and have a room without hitting the frame.

So, the most common way to size a bike nowadays is based on reach. Reach is calculated by drawing a vertical line through your cranks and then taking the horizontal measurement from there to the center of your head tube.

If you use your reach, your stem length, and your stack, that’s the vertical measurement from the top of your head tube to your cranks.

If you combine those three things, that’s your cockpit length. Those three numbers will have a pretty big effect on how the bike feels when you’re riding down the trail, whether you’re sitting or standing and riding technical terrain or going super.

There are some terminologies. Now let’s head out on the trail and see how this stuff affects you in the real world.

Seat Tub

So, let’s talk about some of the pros and cons of a real steep seat angle, and a longer each versus a short reach. The number one benefit for a bike with a steep C tube and a long reach is its ability to climb.

You sit in a much more central position on the bike and the days of sitting on the noses saddle hunching over the bars. As riders started going faster and faster steer to angle started slacking out a lot, which put the front wheel much further in front of you.

When you came into a turn, it was really easy to push the front wheel because you didn’t have as much weight on it as you used to.

So, having a longer reach puts more weight on the front tire and allows you to get that traction that you need. Just like anything, every design is going to have pros and it’s going to have cons.

Long Reach Bikes

So, let’s talk about what I perceive to be some of the. Negative points of long reach bikes. First off, if you live somewhere where trails are really tight and twisty, a long-reach bike is just going to be a bigger bike and it’s going to be more of a handful to take through those trails.

So, you’re going to want to give some consideration to how long you want your bike to be. The second thing is something that I hold near and dear to me and that’s the ability to bunny hop a bicycle.

When the reach is too long, the bike could be really hard to bunny hop. And the last negative thing about a long-reach bike, similar to riding singletrack.

It’s longer so doing little jibs and doing one eighties and stuff like that; it’s just going to be a little bit more of a handful on a long reach bike because it’s just more bike to move around.

That’s my quick trail overview. Let’s head back into the shop and dive into it. Full suspension bike pretty much goes right in line with most manufacturers’ size charts nowadays.

But the hardtail is quite a bit smaller when I built my hardtail. Last year, I tried to match all the numbers of my suspension.

To my heart until the reach, the stack. I tried to make them all match on paper. And when I got on those bikes, they felt completely different when you got out on the trail and rode them.

It got me thinking a hardtail is a rigid frame that behaves one way and a suspension bike is a dynamic piece of equipment with a pivot.

That moves everything around and it behaves completely differently. So, I’m gonna use this whiteboard behind me to demonstrate just how that happens.

That’s just with me sitting on it with a proper pre-load I’m going to try and do a seat bounce now and see if I can get it to go even further.

So, you can see before you’re going to preload it. Or do a bunny hop. You’re bringing in your reach quite a bit. This is a pretty big distance from here to here.

If you want to understand how to measure bike size then you need to realize the whole thing.


Now you definitely have front travel compression, but on a suspension bike, when you do that, the bike usually goes down with the rear suspension and the front suspension together, or even on a downhill.

you’re definitely sagging in your rear suspension and front suspension. So, these measurements stay a lot more consistent.

Now I’m going to grab my hardtail and show you the difference between the suspension bike and the hardtail and why a hardtail will always ride, which means feel longer than a suspension bike.

Now it’s time for me to do the same experiment on my hardtail. I’m going to take the marker and put it on top of my grip.

Well, it went from here to here, so I kind of poke through right there, but you can see it arcs from here to here.

That’s about the same distance as the other bike shortens this bike lengthens. So that was actually a pretty interesting experiment for me because I saw that my hardtail has the ability to get as long as my suspension bike has to get short.

Totally makes sense because my suspension bike has the ability to shorten 20 millimeters. And my hardtail has the ability to lengthen 20 millimeters.

That’s why when I get on my hardtail, it almost feels as similar to my suspension bike ready for another experiment. I said I had a scale for something.

talked about bike geometry, we did some experiments and I showed you how sizing for a suspension bike is going to be completely different than a hardtail.

So where does that leave us size charts for bikes are pretty useful, but another great way to figure out what size bike is going to be good for you is to use your rad measurement.

And if you go to a YouTube channel that I’ll link below-called joy. They’ve just done a couple of videos on figuring out your rad measurement.

It’s basically your cockpit on your bike. My suspension bike falls right within the guidelines of what they’re telling you. And I think it’s a great way to measure bikes for hardtails.

My bike is quite a bit shorter and I think these experiments showed you why, as I said, Mountain biking is not a one size fits all sport.

If you’re looking to ride wide open trails and go as fast as possible, those longer bikes are going to give you a ton of stability and they’ll probably work out awesome for you.

But if you’re an east coaster like myself and you want to ride more tight technical trails, do lots of jumping and stuff like that.

When you take into consideration that a hardtail only gets longer like you. It’s always better to lean on the side of a slightly smaller bike. So, there you have it. This video is a ton of fun to make.


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