What kind of transmission does a cyclist need?
The perfect drive is decisive for the fun factor while cycling. To make the right decision between the various drive systems one has to make friends with some special terms. Terms like gear display, total gear ratio transmission, gear step and gear number. We are going to start with gear display. It indicates how many meters the bicycles moves forward with one complete crank revolution.
The lowest gear has the shortest display. Less meters per crank revolution mean an easier turn of pedals. Nevertheless, one has to bear in mind with a display of under 1.3m problems with balancing out the bike could occur, because of moving forward too slow.
Table 1 shows how short a display of the lowest gear can be to go up a hill. The table is divided in three columns showing the output: easy riding with 100W (Watt), an exhausting ride with 200W and a sweat-pouring ride with 300W output.
Rider 'A' loves comfortable riding and rides only ascents with max. up to 5%. Should it be an easy riding one would not go beyond a 100W output. A view on table 1 shows with a display of 2m per crank revolution an ascent of 5% and riding 100W is manageable. In case rider A wants to ride an ascent of 7.5% with a 100W output, he will use the lowest gear of 1.3m display. Rider 'B' wants to ride an ascent of 12.5% and is willing to get exhausted.
The table shows he should not go beyond 1.6m display with 200W output. With a display which would satisfy rider 'A', rider 'B' would manage only an ascent of 10%. Rider 'C' is an active bike athlete. For him a sweat-pouring output of 300W is really not a problem. The table shows he can go up-hill a 20% ascent using the lowest gear with a display of 1.5m.
The blue marked areas are showing which combinations should not be used out of balance reasons.
Low gears are not only necessary to ride up-hill. If one of our riders wishes to ride through muddy passages, gravel roads, puddles, roots, gullies, he is advised to choose the lowest gear of approx. 1.5m display, which enables him to ride those difficult passages with ease.
If you shift gears to a higher gear, it leads to a greater display, consequently your bike runs more meters per crank revolution. Thus, the display in the highest or rather fastest gear determines the highest speed you can ride on your bike. Table 2 shows what speed you can achieve with which display. Yet, the crank revolution per minute plays an important role.
Our comfort loving rider 'A' rides 60 crank revolutions max. and is content with a highest speed of 20km/h. Table 2 shows the fastest gear has to have a display of 5.6m/crank revolution minimum to achieve a speed of 20km/h.
Rider 'B' likes to be more athletic and wants to achieve a speed of 40km/h using the help of following wind and riding down a hill. Yet he prefers a frequency of 80 crank revolutions per minute. For this demand table 1 gives the result of an 8.3m display.
Our athletic rider 'C' values high speed very much. 100 crank revolutions per minute means nothing to him. With a display of 7.5m per crank revolution rider 'C' achieves 45km/h.
The next term of importance is total gear ratio transmission. It indicates the relation between lowest and highest gear and is expressed in percent. If the display of the highest gear e.g. 8.3m/crank revolution and that of the lowest gear 1.6m, the total gear ratio transmission is 8.3m/1.6m = 5.2, that means 520%. The total gear ratio transmission tells directly which ascent one can ride on the bike respectively the highest speed which can be achieved. The following directives are valuable for the trekking/touring area of cycle sport:
- Total gear transmission ratio less than 300% = low output (ascents lower than 10%, speed up to 25 km/h, no difficult passages)
- Total gear transmission ratio less 400%: average output (ascents approx. 10%, speed approx. 35 km/h)
- Total gear transmission ratio more than 400%: high output (ascents about 20%, speed over 35 km/h)
- Which total gear transmission ratio do our riders 'A', 'B' and 'C' actually need? The answer is to be found in table 1 and table 2 which state the required highest and lowest gears. Table 3 shows the necessary total gear transmission ratio for our model riders.
Next to gear display and total gear ratio transmission the other important terms are gear step and gear number. The gear step is stated in percent and shows how far the gear transmission relation between the gears is changing. If our rider B rides e.g. in 7th gear and shifts to the next higher gear, he has to lower his crank revolution frequency for about 5%, in case he maintains his speed and the gear step lies at 15%. Shifting in the next lower gear means he has to speed up his pedalling for approx. 15%. In general, gear steps lower than 15% are felt as very comfortable for the rider, because here the drive flow while shifting is undisturbed by small changes of crank revolutions. With gear steps higher than 15% the next higher gear is felt much harder to push, respectively the next lower gear is felt to be pushed too easily. These uncomfortable feelings are enlarged with gear steps even higher than 20%.
- Gear steps lower than 10% are useless, because the resulting change in the gear ratio transmission is not felt at all, excluding street biker racing sports, where the final bits of strength are used, gear steps lower than 10% are preferred.
- A gear step lower than 15% enables a most optimal drive flow, because with the smallest change of drive resistance one can immediately react with gear shifting.
- Gear steps between 15 and 20% lead to a more or less sufficient drive flow, because drive situation could occur where the right gear cannot be found easily (It is too easy or too hard to push the bike).
- Gear steps over 20% are a no go for the sophisticated rider. The differences between the single gears are too big.
The gear number indicates the number of effective gears. An effective gear is distinguished from his neighbour gear with a 10% gear step. All 27 gears of a regular 9-speed chain drive derive from the combination of 3 drive wheels placed at the pedals and nine sprockets at the back wheel. Therefore, 3x9=27 shift combinations are possible. If you take a closer look at the display of these 27 gears, you can easily determine 13 gears of 27 which differentiate by less than 10%. That is why only 27-13=14 effective gears maintain which can clearly be distinguished from each other (see display diagram). With hub drives there are no 'doubled' gears. Thus, a 9-speed hub drive e.g. has nine separate gears following one another directly.
The particular demand of the bicycle rider determines the demand on the gear shifting system. Taken from table 1 and table 2 the lowest and highest gear is determined. The result is a necessary total gear ratio transmission of 500%. And if you accept only gear steps lower than 15%, a bicycle drive of 14-gears is resulting from this demand. If you are satisfied with lower total gear ratio transmission of e.g. 350% and larger gear steps are acceptable, eight or nine gears might be sufficient. With fewer demands the necessary number of gears is reduced.