Singlespeed & Dinglespeed Gearing

Crankset and chainrings ready for dinglespeed install.

Singlespeed bikes are great but lack in versatility. Gear them low for climbing up a mountain or gear them high for top speed. You can't have both. I often found myself wishing for a two speed bike. A high speed for riding across town and a low speed for the trails. The trick is having those two speeds without using a chain tensioner or derailleur. The answer was dinglespeed.

For those who know about dinglespeed and don't care about chain length math, skip down a bit...

What's a Dinglespeed:

A dinglespeed setup is like having two singlespeeds. It has two front chainrings and two rear cogs. Large chainring/ small cog for a commuting high speed and small chainring/ large cog for a hill climbing low. It only takes a minute to stop, flip over my bike, pull the rear wheel off, switch the chain and jump back on the bike. What makes the change go quick is having the same chain length for the two speeds so you don't have to adjust your chain tension when you switch.

The Math:

The conventional way to maintain equal chain length is by having the same amount of total teeth. For me this was a 32T chainring with a 17T cog as my high gear (32+17=49) and a 30T chainring with a 19T cog as my low gear. (30+19=49) Simply put a change of 2 teeth between cog and chainring. I figured this out through some trial and error with my existing 32T chainring and assortment of cogs. But I also did the math with an excel tool I made. Download the excel Gearing Calculator.

The high speed of 32/17 should allow me to cruse a respectable 22-27km/h (around 17mph) and provide me with 57.4 gear inches. Still low enough to climb a hill on my commute but high enough to keep up with and pass other cyclists. The low speed of 30/19 should max out around 18-23km/h (around 14mph) and provide me with 48.1 gear inches. This should be plenty low enough to climb up steep single track and yet just high enough to maintain good momentum though rolling hills and valleys.

32/17 vs 30/19 (2 teeth change) calculations.
My dinglespeed gearing of choice.

Aside from calculating my estimated speeds and gear inches, I did all this math because keeping the same amount of total teeth doesn't actually work. As the diameter of the chainring and cog change between speeds so does the length of chain between them. This change of length is caused by the slight difference in angle/length of the tangent line between chainring and cog for each speed. With my chosen gearing of 32/17 and 30/19 I have a calculated difference in chain length of 0.92mm which turned out to be a little too much. If you wanted a bigger change in gear ratio and went with a 34/16 and 28/22, a change of 6 teeth between chainring and cog, the chain length difference would be 2.5mm. Thats not a lot but 2.5mm is enough to make a big difference in chain tension. The chain will be too tight or loose and will have to be adjusted when changing speeds.

34/16 and 28/22 (6 teeth change) gearing calculation.

Disclaimer: The chain length calculation I used is not perfect. A perfect calculation would be adding the length of tangent line between pitch circles to the length of circumference the chain is engaged on the chainring and cog. I calculated my chain length by adding half the pitch circumference of cog and chainring to the distance between the tops of chainring and cog. The way I calculated it is much easier math and is good enough for estimating chain length difference. If this method proves to be unsuccessful, I will develop a more complicated excel tool. Refer to diagram below for an illustrated difference between the perfect and imperfect calculation methods.

If you were skipping over the boring stuff then start reading here...

The Build:

As written above I chose to build my dinglespeed with a 32/17 high and a 30/19 low. Fortunately, I only had to buy one 30T chainring to make that happen. I bought a TA 30T 64bcd aluminum chainring. I bought this one because it had no negative reviews, it was inexpensive and I could get free shipping.

Unboxed TA chainring.

Mounting the new chainring took a bit of time because the entire crankset needs to be removed. It went fairly smooth and gave me a good opportunity to clean some sand out of the hard to reach places of the crankset. After the new chainring was mounted it came time to adjust the chain tension.

Removed crankset and chainrings.

Stock Shimano XT Deore tripple crankset.

Replaced the original 24T 64BCD with the new 30T TA.

On my Jones Plus I adjust chain tension with an eccentric bottom bracket. EBBs are notorious for being a pain to adjust. This pain is the main reason I don't want to have to adjust chain tension on a daily basis. Some trial and error adjusting chain tension finally resulted in some disappointing news. Even with the theoretical difference in chain length of only 0.9mm it is either too tight or too loose. Trying to split the difference by making the 32/17 high gear just loose enough to still let the crankset spin freely (about .25" of chain slop) still makes the 30/19 low gear have about 1.5" of chain slop which is considered too loose by most standards. (Ideally there should be about 1" of chain slop.)

Finished side view.

Conclusion:

I've done a few rides with this dinglespeed setup and will continue to commute across town and climb hills to see how well it works. I haven't noticed any extra wear, friction or drag in the tight high speed. The loose low speed has also been doing well and hasn't been skipping teeth or dropping the chain. I have noticed the extra slop in low gear does allow the chain to occasionally rub against the side of my tire. For now I can live with this setup and see how it goes. However, I may add a chain tensioner in the near future to stop the chain from rubbing the tire and so I can run a slightly wider range of speeds. Something like a 34/15 and 30/19.