Tag Archives: Gear Inches

Matching gear ratios on two different bikes

I have a fixed gear bike with a gear ratio of 48:16. I have a road bike with standard 53/39 front chain rings and a 11/28 rear cassette. My coach has given me some fixed gear workouts for my training, but I’m hesitant to put my fixed gear on the trainer. So, the challenge is to find out what combination of gearing I need to run on the road bike in order to simulate the gearing of the fixie.

“What’s the big deal,” you ask. “That is easy!” Well, that is easy for you to say. I am mathematically and mechanically challenged. I even had trouble counting the teeth on my cassette.

T-shirt

Wow! What is the gear ratio on that thing?!

Approaching this problem, I first thought I should count the total number of teeth from the different combination of rings. So, that would mean my fixed gear would have a total of 64 teeth. To match up my road bike, I would need to run a gear ratio of 39:25. Of course, that doesn’t take into account the laws of physics. My brain was telling me that was way too simple, and getting on the bike and spinning confirmed it.

There are several things to consider when you are measuring the gearing on your bike. 1) One obvious component is the gears, but it doesn’t end there. 2) You also need to take into account the wheels. 3) If you want to get really technical, you can throw in crank arm length. We need to know information about these three things — or at least two of them — to properly match up the bikes.

All normal road bikes have overdrive gearing. That means that the rear wheel always will turn more revolutions than the crank. The higher the gear ratio (say 53:11) the more times your wheel will spin around per pedal revolution. At my lowest gear ratio (39:28) the wheel will spin fewer times per revolution, but will still turn more times per crank.

The way this works is the rings on the cassette make the rear wheel seem bigger or smaller based on the number of teeth. Though the wheel remains consistent in circumference, the rings alter how large the wheel appears to the front crank. The laws of leverage make it so that turning a large gear connected to a small gear will make the smaller gear turn multiple times per each revolution of the larger gear.

That is why just counting teeth won’t work. A front ring of 32 teeth connected by a chain to a rear ring of 32 teeth will spin the rear wheel only once per pedal revolution. Take those 64 teeth and divide it 39 and 25 front to rear and suddenly you’re getting a lot more wheel turns per crank revolution. That means more speed, but also more power needed to turn it.

It then becomes a little complicated figuring out how to match up the gears since I don’t have a 48 front ring on my road bike. I would need to find the combination of gears using a 39 front ring and one of the rings on my rear cassette. So, I turned to the Internet and my local bicycle shop for answers.

I found calculators and such stuff on the Internet, but I wasn’t exactly sure how to use them. None of the charts and explanations dealt directly with trying to match up gearing between two bikes without common gear ratios. I was coming up with numbers, but not sure how to apply them.

Mike at Sunshine Cycle Shop helped me out. He pointed out that you have to take the wheel into consideration. For instance, if one wheel of a bike is smaller than the wheel of a second bike, even running identical gear ratios would not give equal speed or power readings.

1) Make sure your wheels are the same circumference. To be technical about it, you should make sure the tires are the same type.

Mike went on to explain that you must now consider the amount of times your wheel turns per pedal revolution. So, rather than concentrating on the number of teeth on or size of your rings, you should focus on the ground. That is what will allow you to measure all of the components — wheel, front ring, and rear ring.

2) Mark a spot on the floor. Put the valve stem of the front wheel of your fixed gear on that mark. Then make sure that your right crank arm is pointing straight up. Now you start rolling your bike backward or forward. Guide the bike in a straight line until the crank arm returns to the upright position. Mark the spot where your wheel stops.

What you will find is that your valve stem will make several revolutions before the pedal makes a full revolution. You will now find a distance between two points marked out on the floor. The measurement of this distance is called “gear inches.” It is the distance your bike travels using a particular gear ratio.

3) Using the same process, now get your road bike and experiment with the gear ratios that produce the closest number of  gear inches to those arrived at with the fixed gear.

For me that means the closest I can come to matching up my road bike with the fixed gear is to run a 39:13. This gets me verrrrry close to an even match. As a matter of fact, if you use Sheldon Brown’s Bicycle Gear Calculator, it gives you the same results (78.8) for 39:13 and 48:16.

So, if I want to use my road bike on the trainer to do my fixed gear workouts, I can just keep the gearing at 39:13. I’m looking forward to trying it tonight. I have another fixed gear workout then.

I did get my fixed gear on the trainer. I really had to extend the locking bar and I’m not sure I want to put the stress on it that would probably come if I tried to ride with much power. However, just for the fun of it, I do think I will see if I can figure out how to make the iBike power meter work on the trainer. Word is you should be able to do it. I might even try a workout as long as it is an endurance ride and not a power workout.

Anyway, if there is anyone out there trying to match up their fixie gearing with their road bike, I hope this will be a help. Do you have a simpler way? Lay it on us.