Friday, September 27, 2013

Quar-ccuity (or Accuiq?) and ANTride


Quarq + Accuity

The Quarq + Accuity crankset is almost ready. Major changes since the Rival based version include:

  1. Zero offset (or “calibration response”) programming has had a major change. Rather than a separate function that takes over control temporarily, it uses the existing code in the main body read the strain gauges. This probably sounds unimportant, but it’s the result of a peculiar issue where current drain was increasing when I tried to zero only the right side. By leaving it to alternatively read left then right sensors like it normally does it maintains currrent draw as is.This improves stability and how “clean” the code is. I think this is the result of the higher impedance wiring used combined with a capacitor issue. Either way, this makes the system more robust over the usable voltage of the battery.
  2. When you use a cycle computer to tell it to Zero offset (or “calibrate”),  it reads back the offset (eg –417 when I zero my Quarq Cinqo). Unlike Quarq, which translates this value to 1/32 N-M, mine is left as the raw ADC values. I’d be surprised if Quarq isn’t doing this as well and it just happens to be “about” 1/32 of a N-M. For this Accuity build each value is 1/20.26th of a N-M for the left and 1/17.50 of a N-M for the right.
  3. This Build uses a different strain gauge setup for the right leg compared to the Rival unit. The S900 crank is using the actual intended setup that was intended for Accuity and not re-working the existing setup of V3. V3 was more sensitive setup, but it’s a pain to wire and difficult to integrate the setup into the board without some lead wire resistance mismatching.
  4. Detection for when the rider has stopped pedalling gives better / “cleaner” restart values. It’s still not perfect, but it generally doesn’t interfere with data. David Johnstone over at has been picking apart my L/R data and pointing out an issue that the values go to 100% left value when you resume pedalling. I haven’t fully nailed this down but I’m almost there (I think).
  5. Reduced power consumption. The Rival crank has a different voltage drop resistor and received the power from the battery directly. Now the strain gauges are powered from the 2.048V linear regulator on the ADS1248. That’s a 1 volt drop plus the same 750 ohm in line resistor. The strain gauges are operating at less than 0.5V. This is important because in the planned sleep mode (ie: not riding) it can turn off the strain gauge power supply.

The next major step (to hopefully happen Sunday) is doing my initial indoor tests. Kurt pointed out to me via twitter that I could use the small cup cone on the lever skewer. I didn’t trust it when I got the trainer so I swapped it. I’ll carefully try it. I’m not 160lbs climber. More Clydesdale.


This isn’t really a real name. I’ve had a lot of traffic thanks to Ray over at due to tweeting in response to the Muin review. He linked to a short post (here) asking for feedback on his week end review.


What is ANTride? The short explanation is that cycle trainers are predictable devices. Things like TrainerRoad and the Lemond trainer have taken advantage of this to create “virtual power”. This is the indoor equivalent of what iBike tries to do outside. Kurt built the inride as an application of their regression curve for average power on their site. However, they were very clever having already tried a basic cycle computer before – they incorporated a spin down test to to measure acceleration and possibly rolling resistance to increase the accuracy. As you can see from the review there are a few people asking about ANT+ since it’s a BTLE only device.

If you’ve used a powermeter then you know that outdoors it can be tough to dial in on a number and hold it since there are so many competing factors such as wind, hills, other riders, etc. Eventually you get better at this but it takes a while. On a trainer this is much easier. If you already own a powermeter then there isn’t a point to this device. If you don’t own a powermeter, or one is out of financial reach than this can be an attractive option to a lot of people. Off season training means training with power.

Why build this if you can indoor train at a computer using a speed sensor. A few reasons.

  • You want to capture that data to your ANT+ cycle computer
  • You don’t train in front of a computer (Garage, basement, spare room, etc)
  • Improved accuracy

That last one is something that Inride does well at a fraction of the price of a direct force power meter. However it comes at a price. Location. In order to determine the acceleration of a wheel accurately a simple magnetic switch sensor doesn’t cut it. This means that none of the Speed interpolators can have that level of accuracy. That is why Inride senses at the roller. It’s higher speed means it can detect accelerations much more accurately.

So going forward there is a choice here. I can achieve the same accuracy as Kurt Kinetic Inride putting it within 2% of a real powermeter but only if I can get more accurate measurements of acceleration. This is where people might say that “all hope is lost”. Hold tight.

There is another way to pull this off with very good accuracy. Enter the MEMs gyro. I discounted this for the powermeter (though I’m thinking about it again, mainly in terms of wanting to offer every possible feature out there at a fraction of the price – and a high speed mode will need it). A gyro attached to the wheel can provide very accurate information.

I’m going to test the magnet sensor on my kurt kinetic and see if I can make the calibration work.

I’m low on available development time so I’m going to think about this some more. In the meantime feel free to leave a comment or email me at

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