Bikes nowadays feature a lot of gears regardless of price range.
Representing a decently priced mountain bike with a market value of 800CAD, my GT Avalanche has the following:
Sunrace CS-M98 – 9 speed cassette
SR Suntour XCM-T414 – 3 speed crank
With 3 in the front and 9 in the back, this in total gives the GT Avalanche 27 gear combinations to play around with. Woah
Representing a cheaply priced mountain bike with a market value of 100CAD, my NEXT Challenger has the following:
Unknown manufacturer – 6 Speed freewheel
Unknown manufacturer – 3 Speed crank
With 3 in the front and 6 in the back, this in total gives the NEXT Challenger 18 gear combinations to play around with. Talk about value!
Woah, what should we do with all those gears?
I bike on the road with cars pretty often, so getting up to reasonable speed quickly is often my #2 concern. (#1 being how to stop effectively in snow) So understanding what these gears mean would help. Since we have numbers, it looks like we can check this out using math 🙂
First step – Rotations per minute (RPM)
It is safe to assume that I can comfortably pedal at ~70 RPM, with a max burst at ~90 RPM. Pro’s can do 120, but hey, I’m not a pro.
Second step – The gear ratios
With different combinations of front and back gears, it will give us different ratios. For example, if the one in front has 22 teeths (the spikey things), and the one at the back has 11 teeths, this gives us a 1:2 ratio. That means, every time we do one full pedal (1 RPM), the wheel at the back will spin two times.
For this exploration, I am going to use the GT Avalanche as the number of teeth in each cog is clearly documented.
Data for the front
Data for the back
All the possible combinations. Ratio = Front / Rear
But hey, the ratios are all over the place, let’s sort them by ascending order
Here we can see that, going with a bigger gear in the front doesn’t always means faster! We can go with a 44 in the front and 36 at the back, giving us a ratio of 1.22. Meanwhile we can go with a 22 in the front and 15 at the back, giving us a higher ratio of 1.47!
*With 22 in the front and 15 at the back, this qualifies as “cross shifting” on my bike. This should never be done if you love your bike. But for the sake of math, we will assume that these are valid combinations.
Third step – Calculating the speed
Speed (km/hr) = RPH x Ratio x Circumference of wheel in km
RPH = Rotation per hour = Rotation per minute x 60 = (90) x 60 = 5400
Ratio is calculated in step 2
Circumference of wheel = 207cm on my GT Avalanche or 0.00207km
Basically, if I pedal at 90 RPM, these are the speeds that I will get with the different gear combinations.
Armed with this information, it is up to your personal preference / ability to determine how you would like to ride.
Personally on the road
I consider 15km/hr the casual bike speed and it looks like 32:24 will get me there
Next I would aim for 20km/hr the reasonable bike speed on bike lanes and it looks like 44:24 will get me there
Next I would aim for 25km/hr the reasonable bike speed on road and it looks like 44:18
At this point I would consider the bike speed to be “road friendly” so going faster from here is optional and dependent on road conditions.
Those gear choices seem fairly arbitrary what was the thought process?
One might ask for 25km/hr why 44:18? Why not 32:15? Both are pretty close. Let’s explore the 20km/hr >>> 25km/hr scenario a bit.
Given the tables:
At 20km/hr, our gear combination would be 44:24. That is gear 3 on front, gear 4 on back.
Going to 25km/hr using 44:18. That means our target combination is gear 3 on front, gear 6 on back. That means two upshifts on the back will get us there. Number of actions = 2.
Going to 25km/hr using 32:15. That means our target combination is gear 2 on front, gear 7 on the back. That means, one upshift on the front and three down shifts on the back. Number of actions = 4.
Every single time you shift are times that you can’t pedal as hard because the gears are “shifting”. I want to accelerate as fast as possible so cars don’t hate me (which they probably already do). To do this we would want to minimize the amount of time shifting so we can spend more time pedaling.
Therefore, I’ve chosen 44:18 as my next gear combination instead of 32:15.
The same logic applies to what was previously proposed…
0 >> 15 – I use 32:24 which are gear combinations 2 and 4.
15 >> 20 – I use 44:24 which are gear combinations 3 and 4. So one upshift on the front will get us there
20 >> 25 – I use 44:18 which are gear combinations 3 and 6. So two upshifts on the back will get us there as discussed above.
If additional speed is required: one upshift on the back (so 3 and 7) will get me to 32km/hr
If I still need more speed: one upshift on the back (so 3 and 8) will get me to 38km/hr
More speed? One upshift on the back (so 3 and 9) will get me to 45km/hr
I might as well be a car @ 45km/hr (Not that I sustain this lol)
TL;DR the shifting makes sense, we minimized shifting time so we can spend more time pedaling.
* On top of that, none of these are “cross shifts” so we are still being nice to the bike 🙂 (3 and 4, might be pushing the definition a little though :P)
Why are there so many gears if all you are going to use is like… 3?
Short answer: different target speeds for different scenarios.
On the road we had very clear goals. 0 >>> 15 >>> 20 >>> 25.
But what if you were off road? Your goals might be 0 >>> 5 >>> 10.
This is a mountain bike and they are supposed to handle a variety of terrains, so all these gears are there for those specific use cases.
Is it necessary to shift though?
Biking around town? No. However, your chain might live longer if you do. The shifter on my NEXT Challenger was left broken for a month or two. I single speeded it all the way. I picked a “middle of the road” gear combination. This resulted in:
A terrible 0-15 acceleration. I’ve stretched the chain a little doing this and my legs are way beefier now (woohoo adaptation :P)
A top terrible top speed. 25km/h? Not possible given my current max RPM.
But did I get around town ok? Sure I did.
Do I miss my gears? Sure I did.