High pivot and pulley bikes are nothing new, but until recently they were limited to downhill and freeride rigs. After great success in downhill racing, they are now pushing into the enduro and trail bike categories. In fact, a fairly high percentage of enduro and trail bikes recently launched use high pivot suspension.
With their rearward-facing axle travel and minimal pedal kickback, they have advantages when driving downhill, especially when it comes to reducing the hardness of angular bumps. But what about climbing? The pulley must create some resistance, not only to turn the bearing in the pulley, but also to pivot the chain pins when the chain (which is under tension) bends around the pulley. Many high pivot bikes also require a lower pulley on the lower chain span to reduce chain growth there, which could add a little more drag. The question is: how much air resistance exactly? And how much slower (if at all) is a pulley going uphill?
Calculating the numbers from Levy’s efficiency test
The first thing that intrigued me about this question was Mike Levy’s efficiency test from last year’s field test. Levy rode all ten bikes – five from the trail bike test and five from the enduro category – up the same hill with an impressive 300W. What caught my eye is the fact that the two idler bikes (the Actofive P-Train and the Norco Shore) came last. The P-Train, which was in the trail bike category and therefore had faster rolling control tires, was slower than all enduro motorcycles without idling.
Norco Shore and Actofive P-Train were by far the two slowest climbing bikes from last year’s field test.
You can forgive the fact that these bikes were the heaviest in the test. However, the rate of climb on a reasonably steep, non-technical climb has a very simple linear relationship to total system weight (system weight is the total weight of the bike plus the rider, in this case Mike Levy, at 70 kg). So if the system weight increased 1%, all other things being equal, we would expect the time to increase 1%. If the climb were less steep, we would expect the time to be shorter as drag would become a factor regardless of weight. Check out bikecalculator.com or this video to learn more about it.
The thing is, Levy’s system weight on the P-Train was 3% heavier than the next slowest trail bike (Ibis Mojo), but the time was 9% slower. Likewise, the system weight of the Norco Shore was 2.7% heavier than the next slowest enduro bike (Trek Slash), but the time was over 9% slower. In both cases there is about a 6% variance in climb rate compared to what we would expect based on the weight difference alone.
I think Mike would be the first to admit that these tests weren’t the most scientific. For one thing, he only climbed one climb on each bike, so we can’t be sure that the results were repeatable and consistent. Nor does it prove that it was the idlers that slowed these two bikes uphill; it could also be a feather bob that steals power by swinging the shock absorber. But high-pivot bikes can be constructed to have very little pedal bob if the spinner and idler positions provide the right amount of anti-squat. So what would happen if you took the pedal bob out of the equation and just focused on the propulsion losses?
Prohibited Dreadnought vs. Privateer 161 climbing test
To find some answers, I ran my own tests on a Forbidden Dreadnought that has relatively little pedal bob. I drove the same 10% increase on asphalt four times at 300W (I couldn’t expect to be outdone by Levy). I did the exact same thing on a bike without a pulley (Privateer 161) with the same wheels and tires set to the same pressure. Both bikes had clean and freshly lubricated chains with the same chain lubricant. I used the same SRM power meter pedals on both bikes and used the lock on both bikes to minimize the effect of the pedal bob, making it a test of drive efficiency rather than overall efficiency including the pedal bob.
On average, the Forbidden was only 0.8% slower than the Privateer in this test. However, the Privateer weighed 0.66 kg more than the Forbidden (16.46 kg or 15.8 kg with pedals). So if we want to know anything about powertrain efficiency, we should take that into account.
Bike Calculator tells us that with a 10% incline and my weight of 86 kg, the Forbidden would ride 0.66% slower if it weighed as much as the Privateer (this is the percentage difference in system weight). If we increase the times of the forbidden by 0.66% to account for the lower weight, the average difference grows to 1.5%.
This is just a rough test in the real world, not real science. The error bars on the 1.5% number are quite wide, so it is not an accurate measurement. But it does suggest the Dreadnought is less efficient than the Privateer when you factor in weight, though the discrepancy is much smaller than that of the two pulleys in the field test. Maybe these two bikes had one crowd from suspension bob, or maybe something on their idlers, was less efficient.
Power meter efficiency test
To get a more accurate and reliable idea of the drag in the idler pulley, I performed another experiment. I borrowed a Wahoo Kickr Smart Turbo Trainer that has a built-in power meter to measure the power transferred to the cartridge. I mounted the Forbidden first, then the Privateer, and used the same SRM pedals to measure my input into the drivetrain. I used the same crank on both bikes so I didn’t have to recalibrate the pedals when swapping bikes, and I also used the same chain (with some links removed for the privateer) to ensure no difference in chain efficiency. I warmed up the power meters before taking measurements. The Wahoo can adjust itself to provide constant resistance which I set to a realistic 230W. This meant that the powertrain output power was exactly the same every time I drove, while if I had been aiming for a certain input power on the pedals, it would be more difficult to keep that constant. I pedaled at a constant 90 RPM and measured the average power on the pedals required to maintain that 230W on the cassette. I measured the average power over two minutes of steady pedaling; it took about a minute for the average performance figures to level off at a constant value.
For the Forbidden, the average power on the cassette was 230 W and the average power on the pedals was 264 W. With the Privateer, my average power on the cassette was again 230W and the average power measured by the pedals was 258W. I repeated this test and got the exact same result.
So in order to have the same output power of 230 W at the wheel, you need six watts more at the crank with the Forbidden. That is 2.3% more power that the driver needs to reach the same speed on the Forbidden; Alternatively, for the same performance, you would go uphill at about the same percentage more slowly. This is a little higher than the number I got from the real test, but given the error bars, it’s on the same level and the indoor test is more likely. This test probably isn’t accurate enough to go in decimal places, so it’s more reasonable to say that the dreadnought is about 2% less efficient.
This test is not meant to be the last word on idle efficiency. You can run this test with a variety of different cadence, power outputs, chain lubricants, and drivetrain wear conditions. The chain in this test was new, clean, and well lubricated; otherwise the performance losses on the idler pulley could be higher than in this test. Different bikes with different idler designs can also have different amounts of drag. Despite all of these variables, I’m still scratching my head as to why the field test bikes were so much slower.
Under the conditions of this test, the pulley drive train was six watts or about 2% less efficient. If all else were the same, you’d have to pedal 2% harder to get the same speed, or about 2% slower for the same effort. By “if all else were the same” I mean ignoring differences in pedal bob (another source of inefficiency) or weight. Some balance bikes may have more pedal bobs and more weight than non-balance bikes of the same category, but this is not always the case.
Is that a big deal? Having to put out 2% more watts is unlikely to immediately notice you in the real world or in a blind test. After the dreadnought pedaled a lot before these measurements, it doesn’t feel like the bike feels noticeably slower than other enduro bikes. On the flip side, if you could ride consistently 2% faster with the same amount of effort, that’s a relatively big deal in terms of the differences between modern bikes. To achieve the same advantage in terms of climbing speed, you would have to reduce the system weight (bike + rider) by 2% – for an average 75 kg rider and a 15 kg bike, that’s around 1.8 kg. And, unlike a weight penalty, the loss of strength has a significant impact both on the flat and uphill. Is a 2% efficiency difference a big deal to you?