About

This all started with a search for a new bike...AND LED ME DOWN A RABBIT HOLE WITH BIKE GEOMETRY

In the spring of 2017 I was in the market for a new bike. I was riding a full-suspension bike that felt a bit small for me and I was also looking to get back onto a hardtail since I was finding myself on my rigid fat bike with 29+ wheels more often anyway. These were the glory days of plus bikes and these were also the days of major changes from old-school to modern geometry. I hit up a few local demo days and immediately gravitated toward the 2017 Kona Big Honzo DL. One lap and I was sold – this was the bike for me. 

Only trouble was, I’d always ridden medium sized bikes and this demo bike was a large. Was it possible that I had been on the wrong size bikes for years? Or could it be that the new school of “modern” geometry was changing the way we perceived our fit on bikes? I thought I was a medium…time to do some digging, and quick! This one I’d be ordering and building from the frame and I didn’t want to make a sizing mistake again.

I consulted the Kona size chart but it was inconclusive. Way too much overlap in that chart for an average sized person like me. Wanting to overlay the medium and large graphically to visualize the differences in size, I searched the web for a suitable program but came up short. 

ENTER THE GRAPHING PROGRAM

Being handy enough with trigonometry and Microsoft Excel, I decided to make my own comparison geometry table calculator and graphing program to get the tool I was looking for.

I ended getting the medium Big Honzo after justifying to myself through my program that it was a better fit for my size and riding style and I’m glad I did. Job done. The Excel graphing program was perfect…until it wasn’t.

Next thing I knew I was using the spreadsheet to help other people pick out their next bikes and we needed to see more bikes overlaid. 3 bikes. Then 4. Okay, no problem. A few friends and family got some good advice comparing their next bikes for size and geometry. Spreadsheet updated and put away again. 

bike geometry calculator
The Microsoft Excel graphing program in its current form

GEOMETRY MODIFICATIONS

I like to experiment with my bikes and this new one was no exception, I had been seen plenty of people over-forking their Honzos to 140 mm. Yeah, I guess the 68° head angle was a little steep these days. Every 10 mm change is about a half degree right? Next thing you know, the bike has a 140 mm travel fork. 

I decided to throw in a -1.5° angleset too. And yeah, I wasn’t sure I was so fond of the plus tires after all and besides, this bottom bracket drop was a bit much for the 27.5+ wheel size. So I popped in 29×2.6″ wheels instead. Every change I made added a bit more capability to the bike and made it a better all-rounder. But what happened to the bike I bought? What were the geometry numbers on this thing now?

SHORTCOMINGS OF RULES OF THUMB

My spreadsheet wasn’t set up to tell me what these mods were doing to my bike. Some rules of thumb got me in the ballpark. That longer fork taking away about 0.5° on the head angle for every 10 mm? Okay, that puts me at 67°. The angleset? Well, just take -1.5º off the head angle, right? That’s how anglesets are supposed to work. Now I’m at 65.5°. Seat tube angle loses about 1° with the fork length increase but the angleset actually rotates it back a bit, so let’s call it a half a degree on the seat tube angle. Well you know what…? Let’s just run this through an online geometry calculator…yup, those measurements are basically correct. 

I was a little skeptical though. A -1.5° angleset can’t just change the head angle of the bike directly like that, right? Not when the front hub is lifting up with the rotation of the fork and bringing the wheel off the ground with it. I mean it’s probably close but not quite -1.5º. Better check on my own. Time to head back to the graphing program. 

A NEW GEOMETRY CALCULATOR IS BORN

I needed to make a new calculator. This one wouldn’t be focused on comparing sizes or geometry between bikes; no, instead this one would take the bike’s current setup and demonstrate the potential geometry that could be unlocked with a few changes. 

Enter the next phase of the Excel spreadsheet. With fields for changing wheel sizes, changing fork axle-to-crown and offset, and adding anglesets, I had my new geometry numbers. Head tube changed from 68° to 65.92°and seat tube angle from 75º to 74.42°BB drop went from 55 mm to 50.84 mm. Wheelbase increased from 1146 to 1166.26 mm.  

bike geometry calculator
The Microsoft Excel geometry modification calculator

So now what?

Thanks to an untimely injury and an inability to find a hobby that isn’t bike-related, I decided to build a website and share what I had been working on. The heart of this site is the Bike Geometry Calculator. It’s not the only one out there, and definitely not the simplest choice, but it is accurate and comprehensive and should satisfy the curiosity of people who like to experiment with their bikes.

I’ve added a few other calculators that have served me well over the years. If nothing else, have a poke around the site and next time you’re thinking of messing around with your bike or buying a new bike and maybe you’ll find something here that could be useful to you.

Send me a message through the Contact page and let me know what you think. If you see any room for improvement, I’ll see what I can do. I’m new to web design, but I’m usually up for a challenge.

Enjoy the site and keep tinkering!