Technical FAQ: 2.6” vs. 2.8” tires; drivetrain friction
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I ride a 27.5 x 2.8″ plus-tire front and rear on my Scott Spark Plus mountain bike. This is OEM.
At my optimal suspension setup, I have a tendency to frequently incur pedal strikes. Increasing the air pressure in the frame shock resolves this for the most part, but I feel I lose some of the benefits of the suspension setup. Likewise, increasing tire pressure will address this, but again I give up the optimal ride characteristics of the 22-25psi I like to run. Looking at many of the new tires coming out, there are several which look great but max out at 27.5 x 2.6. It’s only 2mm, but would reducing tire width also reduce the inflated tire depth at the same pressures, and meaningfully exacerbate my pedal strike problem?
In a word, yes. It, of course, depends on tire manufacturer and model, but at full inflation and no sag, your pedals would be 0.2 inches (5mm) closer to the ground with 27.5 x 2.6” tires than with 27.5 x 2.8” tires on the bike. When running the tires softer, you won’t see this full 5mm difference, but the bike with the smaller tires will still be lower.
Continuing the discussion of friction differences between using the large and small chainrings, ignoring side forces from cross-chaining, I would have expected something like a small chainring combination of 36/18T to have less friction than a large chainring combination of 52/26T. They both give the same gear ratio and, although the large chainring and larger sprocket result in less articulation per chain link, there are proportionally more chain links being forced through that articulation because the chain is moving faster. So, for the chainring and sprocket, the friction losses should cancel out. However, the faster chain of the large chainring combination will put more links through the articulation at the jockey wheels, and the greater tension in the chain would be expected to lead to more friction in the jockey wheel bearings. Has this actually been proven true or false by testing?
Our test in the 2019 VeloNews Gear Guide did not specifically address your question, because we were comparing friction of entire drivetrains, and cross-chaining is a major contributor to the differences in friction between various gear combinations.
That said, the three gear combinations that we did test that had zero cross-chaining were: 48 x 18-tooth (on the 1X system) and, on the 2X drivetrain, 53 x 19-tooth, and 39 x 25-tooth. To get some insight into the ratios you’re asking about and to possibly extrapolate results, the gear ratios of the 53 x 19T and 48 x 18T are very close to the same. It seems to me that your argument would predict that the frictional drag on the 48 x 18T would be less than that of the 53 x 19T, right? I believe you are arguing that the fact that the chain is moving faster with the larger chainring and cog at the same rider speed (since cadence and gear ratio are equal) should outweigh the extra drag due to the increased chain articulation angles mandated by the smaller chainring/cog combination, right?
Well, that was not what we measured on the super-sensitive chain-friction-testing apparatus at Ceramic Speed USA. The energy consumption of the 53 x 19T combination (gear ratio = 2.79) was less, at 9.02 watts, than that of the 48 x 18T combination (gear ratio = 2.67), which was 10.80 watts. As the gear ratios are relatively close, while the difference in energy consumption is quite large, I think it is pretty clear that the higher chain-articulation angles contribute more to the frictional drag than does the chain speed.
From this, I would predict that, in the absence of cross-chaining, the 36 x 18T combination would create more frictional drag than would the 52 x 26T combination. However, to really answer your question unequivocally, one would want to set up the chain-friction-testing apparatus with the chainring lined up straight with the cog in every gear combination.
By the way, you are also arguing, it looks to me, that the chain tension is higher with the larger chainring, but this is demonstrably incorrect. Tension on the upper span of chain is inversely proportional to chainring size — the smaller the chainring, the higher the chain tension. Power output can be simplified to RPM x torque; torque is transferred from the chainring to the cogs by means of the chain tension. Since power output and cadence are constant in this test, RPM and torque are identical in both larger and smaller chainrings. The chain tension (force) with a smaller chainring must be higher, as torque is given by the equation torque = force x radius, and the radius is smaller. Conversely, a larger chainring has a larger radius, and therefore the chain tension — and hence consequent friction — must be lower at the same RPM and power output. When big riders or tandem pairs break freehubs by shearing off pawl teeth or yank spoke heads out of hub flanges, they do it in in the little chainring, not in the big ring.
Higher chain tension increases frictional drag by pulling harder on the pivot pins and by forcing the rollers harder against the teeth of the chainring and cog and by pushing harder on the jockey-wheel bearings. In this case, since chain tension is higher with the smaller chainring and cog, this works against your contention that friction should be lower with the smaller chainring and cog, rather than for it. The bigger the chainring at a given cadence, the higher the chain speed and the higher the articulation rate, yet the lower the articulation angle and the lower the chain tension, and consequent lower frictional energy losses.