Tested: How Much Slower Are Idler Bikes For Climbing?

Bikes with a high pivot and an idler pulley are nothing new, but until recently were pretty much confined to downhill and freeride rigs. Now though, following huge success in downhill racing, they're making their way into the enduro and trail bike categories. In fact, a pretty high percentage of the enduro and trail bikes released lately use high pivot suspension.

With their rearward axle paths and minimal pedal-kickback, they have advantages when riding down hill, particularly when it comes to reducing harshness over square-edge bumps. But what about climbing? The idler pulley must create some amount of drag, not just to turn the bearing in the pulley, but also to articulate the chain pins as the chain (which is under tension) bends around the idler wheel. Many high-pivot bikes also require a lower guide pulley on the lower chain span to reduce the chain growth there, which could add a little more drag. The question is: how much drag, exactly? And how much slower (if at all) is an idler bike uphill?

Crunching The Numbers From Levy's Efficiency Test

The first thing that got me intrigued 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 at an impressive 300W. What struck me is the fact that the two idler bikes (the Actofive P-Train and the Norco Shore) came dead last. The P-Train, which was in the trail bike category and so had faster-rolling control tires, was slower than all the non-idler enduro bikes.


You could be forgiven for chalking that up to the fact that these bikes were the heaviest on test. But climbing speed on a reasonably steep, non-technical climb has a very simple linear relationship with total system weight (the system weight is the total weight of the bike plus the rider, which in this case was Mike Levy, at 70Kg). So, if the system weight went up by 1%, all else being equal, we'd expect the time to go up by 1%. If the climb was less steep, we'd expect the time to go up by less than this because aerodynamic drag would become a factor irrespective of the weight. Check out bikecalculator.com or this video to go deeper on this.

The thing is, the system weight of Levy on the P-Train was 3% heavier than the next slowest trail bike (Ibis Mojo), but the time was 9% slower. Similarly, with the Norco Shore the system weight was 2.7% heavier than with the next slowest enduro bike (Trek Slash) but the time was over 9% slower. In both cases, there's about a 6% discrepancy in climbing speed compared to what we'd expect based on the weight difference alone.

Now, I think Mike would be the first to admit that these tests weren't the most scientific. For one thing, he only did one climb on each bike so we can't be sure if the results were repeatable and consistent. It also doesn't prove that it was the idlers making those two bikes go slower up hill; it could also be suspension bob, which robs power by oscillating the shock. But high pivot bikes can be designed to have very little pedal bob if the pivot and idler locations give the right amount of anti-squat. So what would happen if you took pedal bob out of the equation and just focused on the drivetrain losses?

Forbidden Dreadnought vs Privateer 161 Climbing Test

To try and find some answers, I did my own testing on a Forbidden Dreadnought, which has relatively little in the way of pedal-bob. I rode up the same 10% tarmac climb four times at 300W (I couldn't face being out-done by Levy). I did the exact same thing on a bike without an idler (Privateer 161) with the same wheels and tires, set to the same pressures. Both bikes had clean and freshly-lubed chains, using the same chain lube. I used the same SRM power meter pedals for both bikes, and used the lockout on both bikes to minimize the effect of pedal bob, making it more of a test of drivetrain efficiency than overall efficiency including pedal bob.

On average, the Forbidden was just 0.8% slower than the Privateer in this test. However, the Privateer weighed 0.66 kg more than the Forbidden (16.46 kg vs 15.8 kg, with pedals, respectively) so if we want to know about drivetrain efficiency, we should account for that.

Bike Calculator tells us that with a 10% gradient and my 86 kg weight, the Forbidden would go 0.66% slower if it weighed as much as the Privateer (this is the same as the percentage difference in system weight). If we increase the Forbidden's times by 0.66% to account for its lighter weight, the average difference grows to 1.5%.

This is just a crude real-world test, not real science. The error bars on the 1.5% figure are pretty wide, so it's not a precise measurement. But it does suggest the Dreadnought is less efficient than the Privateer once you take weight into account, though the discrepancy is a lot smaller than it was for the two idler bikes in the field test. Maybe those two bikes had a lot of suspension bob, or perhaps something about their idlers was less efficient.


Power Meter Efficiency Test


To get a more accurate and reliable idea of the amount of drag in the idler, I set up another experiment. I borrowed a Wahoo Kickr smart turbo trainer, which has a built-in power-meter, to measure the power being transmitted to the cassette. I first mounted the Forbidden to this, 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 re-calibrate the pedals when I swapped bikes, and I used the same chain too (with a few links removed for the Privateer) to ensure no difference in chain efficiency. I warmed up the power meters before taking any measurements. The Wahoo can self-adjust to provide a consistent amount of resistance, which I set to a realistic 230W. This meant that the output power from the drivetrain was exactly the same each run, whereas if I had aimed for a certain input power at the pedals, this is harder to keep consistent. I pedaled at a consistent 90 rpm, and measured the average power at the pedals required to maintain that 230W at the cassette. I measured the average power over two minutes of steady pedaling; it took around one minute for the average power numbers to settle down to a consistent value.

On the Forbidden, the average power at the cassette was 230W, and the average power at the pedals was 264W. On the Privateer, my average power at 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, to have the same 230W output power at the wheel, you need six more watts at the crank on the Forbidden. That's 2.3% more power required from the rider to go the same speed on the Forbidden; alternatively, you'd go about the same percentage slower uphill at the same power output. This is slightly higher than the figure I came to from the real-world test, but it's in the same ballpark given the error bars, and the indoor test is more likely to be accurate. This test probably isn't precise enough to go into decimal places, so it's more reasonable to say the Dreadnought is about 2% less efficient.

Caveats

This test isn't meant to be the last word on idler efficiency. You could do this test with a whole range of different cadences, power outputs, chain lubes and conditions of drivetrain wear. The chain in this test was new, clean and well-lubricated; otherwise, the power losses at the idler might be higher than they were in this test. Different bikes with different idler designs might have different amounts of drag, too. Despite all those variables, I'm still scratching my head as to why the field test bikes were so much slower.

Conclusion

In the conditions of this test, the idler bike's drivetrain was six watts, or around 2%, less efficient. So if all else was equal, you'd have to pedal 2% harder to go the same speed, or go about 2% slower for the same effort. By "if all else was equal" I mean ignoring any differences in pedal bob (another source of inefficiency) or weight. Some idler bikes may have more pedal bob and more weight than non-idler bikes in the same category, but this isn't always the case.

Is that a big deal? Having to put down 2% more watts isn't something you're likely to immediately notice in the real world or in a blind test. Having done a lot of pedaling on the Dreadnought before taking these measurements, it's not as if the bike feels markedly slower than other enduro bikes. On the other hand, if you could consistently go 2% faster for the same effort, that's a relatively big deal as far as the differences between modern bikes go. To get the same benefit in terms of climbing speed, you'd have to drop 2% of the system weight (bike + rider) - for an average 75 kg cyclist and 15 kg bike, that's about 1.8 kg. And unlike a weight penalty, the power loss will significantly affect riding on the flat as well as up hill. So, is a 2% difference in efficiency a big deal to you?