If the gear ratio is too low, I will spin out too easily at cruising speeds. On the other hand, if the gear ratio is too high, it will be tough to start off from a stand still, and even normal slopes will be too difficult.
Based on my experience riding the Focus Paralane all-weather road bike, I usually move off in the 6th gear, which has a gear inch of about 57. Then, I would cruise leisurely in gear 7 and 8, which are 64 and 72 gear inches respectively.
Therefore, I will aim for a single speed gear range of perhaps around 60-65 gear inches, since I need to find an ideal balance for my own riding style and fitness. It will neither be a fast bike nor a climbing bike. Note that everyone will have different preferences, so you need to try and decide for yourself, and not just follow others because it works for them.
I have already decided to use a 52T Stone chain ring on this United Trifold, so I just need to decide the sprocket size to use.
Table shows the gear inches if different chain ring and sprocket sizes are used. Based on a 52T chain ring, I can use either a 13T or 14T sprocket to get the gear inches that I want.
With a cadence of 80-90 RPM, a 13T sprocket will give a speed of 25-28 km/h. A 14T sprocket will give a speed of 23-26 km/h.
I decided to try the more relaxed gearing first, which is with the 52T chain ring and 14T sprocket. This will give a gear inch of 59.4 and a speed of 23-26 km/h, at a cadence of 80-90 RPM.
Coincidentally, the single speed gear ratio that I have chosen corresponds exactly to gear 5 on the stock United Trifold 7 speed internal hub setup. By the way, I think the stock gear range is well chosen with a suitable low and top gearing.
Although you can technically use any 14T sprocket from a cassette, the sprockets on a multi-speed cassette have shorter teeth to facilitate shifting. This may cause chain drop, especially if the chain line is not ideal. Also, sprockets from a cassette are thinner, which means less durability, although it is not a concern for me. The concern is that thinner sprockets are more likely to gouge aluminium freehub bodies, especially if you use that sprocket 100% of the time. There is even more stress if you stand up to pedal when going up slopes, since you can't spin smoothly up slopes when there is only a single speed.
I decided to get a dedicated single speed 14T sprocket, since it is not expensive anyway. As shown below, the gear teeth are tall and thick, although it still fits 11 speed chains. This type has an internal spline shape that matches standard Shimano freehub bodies.
14T single speed sprocket weighs 26 grams.
Chain is able to enter the inner width of the 11 speed chain, although there is not much free play.
11 speed chain is able to rest fully on the sprocket teeth. The tall teeth improves chain engagement.
Next, I need to place the sprocket somewhere along the freehub body, to best match the chain line of the front chain ring. This means that I need a lot of freehub spacers to clamp the single speed sprocket on the freehub body. This is not so easy, as there are a couple of requirements for the spacers.
1) Spacers should allow fine adjustment steps to best align the chain line with the front chain ring.
2) Spacers + sprocket need to stack up to a height just beyond the thread on the freehub body.
Too short, and the lock ring will bottom out on the freehub body.
Too tall, and the thread engagement will be reduced, which might strip the threads on the freehub body.
I bought a set of spacers from Taobao, which are specifically designed for single speed conversions like mine. However, I doubt that the spacer heights and chain line will be perfect, as there are just 3 spacers, which does not allow for fine position adjustments.
Set of spacers and lock ring for single speed conversion using standard freehub bodies.
The lock ring in this conversion kit has a larger flange (right side), but for what purpose?
The flange on the cassette lock ring (left) has a smaller flange diameter, as it needs clearance with the chain when it rests on the small 11T sprocket.
A lock ring for single speed conversion will rest on the spacer instead, which has a larger diameter. The flange diameter thus needs to be larger to rest properly on the spacer.
Next comes the trial and error process of choosing the appropriate spacer heights and best chain line position. This United Trifold frame that I have (unlucky or applies to all?) is quite warped, so I think any measurements are not useful, best to just try it out.
The trial and error process basically involves choosing the spacers that allow the lock ring to be tightened correctly (without bottoming out, and with maximum thread engagement). Then, arrange the sprocket among the spacers to align the chain line with the front chain ring. A straight chain line is the best for efficiency and chain retention. This involves repeated lock ring tightening and loosening, and repeated installation and removal of the rear wheel from the frame. Repeat this process until all the requirements are satisfied.
Final spacer arrangement and sprocket position, with a chain line that looks best aligned to the front chain ring.
I used an aluminium spacer just under the lock ring, to prevent damaging the resin spacers during lock ring tightening. This is the ideal chain line and spacer stack, for future reference.
With the ideal spacer stack established, I weighed them to enable accurate weight tracking for the bike.
Another view of the single speed setup (bike is shown upside down), before installing the chain tensioner.
It was fun to calculate and choose the appropriate single speed gear ratio for this United Trifold. Whether it is appropriate or not will need to be tested, once the whole bike is assembled. If the gear ratio is too high or too low, I can always change to a larger or smaller sprocket.