jasonsirota

I bought a new FX3 after upgrading from an FX 7.2. On the 7.2 on the flats, I was riding at about 16mph using 2 on the front, 6-7 on the rear gear

On the FX3 on the flats, 2-7 spins a bit and I'm only getting 16mph when I go to 3 front, 7-8 on the rear. On the downhills, I have to stop pedaling all together.

I called my LBS and they said I'm just pushing the FX3 beyond what its designed for and that I should think about getting an FX S or a road bike.

Does BikeForums Hybrid concur?

hokiefyd 

Hi, and welcome!

I wouldn't agree with your bike shop, and your new FX 3 is just as capable as your old FX 7.2. It sounds like it's just a difference in gearing between the bikes. The sprocket numbers generally have no meaning because we don't know how many teeth they have. The number of teeth on the sprocket determines the gear ratio of that combination.

Both bikes probably have/had a 28-38-48 front crank set, meaning the smallest chain ring has 28 teeth, the middle chain ring has 38 teeth, and the largest chain ring has 48 teeth. From that respect, having the chain in '2' in the front (on the middle ring), should be the same on both bikes.

The rear cassette is probably different. Your new FX 3 has an 11-34 9-speed cassette. The tooth count on this cassette (going from smallest to largest) is 11-13-15-17-20-23-26-30-34. On this cassette, the 7th sprocket (from the top) has 15 teeth. So, the 2-7 combination on your FX 3 is a 38-15. Your old FX 7.2 probably had a 7-speed cassette, and the 7th sprocket was probably had 11 teeth. So, the 2-7 combination on your old bike was probably a 38-11, which is a "taller" gear ratio, meaning you'll be going further with each turn of the crank. This would correlate with your description of the 2-7 "spinning" on your new bike. You'd need to be in 2-9 on your new bike to have the same (or very similar) gear ratio as 2-7 on your old bike.

If you go up to the 3rd (largest) chain ring in the front, then you'd want go jump back to maybe the 5th or 6th sprocket on the rear cassette to keep the gearing about the same.

Your new FX 3 has a 9-speed drivetrain that has more gear ratio range than your old 7.2 FX did, so you should be able to find a comfortable pedaling cadence to get the road speed you desire.

Welcome to the hybrid forum.

mcours2006

I can't believe the gear is that much different on those two bikes. Having check the Trek website, I see that the 7.2 has a 48/38/28 front and 11-32 rear, and the 7.3 has a 48/36/26 front and 11-34 rear. Not drastically different but enough to make difference on on extreme ends.

So on 38/13 you go at 16 mph, but have to go to 48/15. What is your cadence for this?

Don't worry about what gear you're in. Find a cadence you're comfortable pedaling, the select the gear for that. Learn to use the gears to match your cadence.

jasonsirota

Thank you, this is very helpful, let me do some research on gear ratios and how the numbers are are calculated 38-13/48-15 etc. That will help me understand what I'm riding at.

I have not measured my cadence yet although it is on my list of things to do as I ride more often. It's very possible that I am riding a faster cadence on the new bike.

Let me know if you have any more thoughts

Sal Bandini

Calculating gear ratios is easy. Front gear divided by the rear gear gives you the ratio. 38/13=2.9. 48/15=3.2, so about 10% difference. The higher ratio means at a set speed you will be pedaling slower or if you keep the cadence the same as before you will go faster.

jasonsirota

Are teeth always the same distance from each other? i.e. is 15 teeth the same on a 7-speed cassette as a 9-speed cassette?

Sal Bandini

Yes teeth are the only variable when calculating gear ratio.

Sal Bandini

And your LBS is full of it. I have similar bike (Sirrus Sport) and same large ring combo as you (48 front, 9-speed 11-34t rear) and you definitely will not push beyond the limit of this bike.

mcours2006

Don't bother trying to do the calculations yourself. Use this site:
Bicycle Gear Calculator
Plug in the gears, cadence, etc. and see what speed you get.

DorkDisk

Gearing is confusing enough for beginners and Shimano thought adding numbers to OGDs solves this, but in fact adds more confusion.

A "7" means nothing because people have different number of cogs, and different tooth counts at the same positions, and get this: some Shimano OGDs have high numbers as low gears and others use high numbers as high gears.

The chainrings and cogs have the tooth counts marked on them so you don't have to count. On the downhill, try the biggest chainring and the smallest cog and try to go up to 140rpm. A constant 90+ rpm is normal for many riders, a 48x11 is pretty fast.