Lag preventing accuracy in sprints

Apologies if this has been done to death, but I couldn’t find an answer that fits my problem. I have the Wahoo speed and cadence sensors, and am using a ‘dumb’ trainer (Kurt Kinetic Road). I use TR on my iPhone (can’t afford an ANT+ unit right now). There’s currently a horrible lag (10 to 15 seconds) between my shifting cadence/power and this being reflected in the cadence/power data in TR. even more annoying, the figures will creep up over this period, rather than just jumping up after 15 seconds.
On many programs this is tolerable, but I’m now facing programs with very short, intense bursts, and it’ll be impossible to do them at the moment.
Oh, and I’ve switched off ‘smoothing’, and the Bluetooth signal is showing as 90%+ on both units. I’m also using Bluetooth headphones, so my phone is receiving 2 Bluetooth signals from sensors and sending 1 to headphones.
Any suggestions?

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What you are experiencing is an unfortunate “feature” of using Virtual Power. This virtual power estimate is a direct result of the rear wheel speed, as measured and reported from the speed sensor. Unlike a power meter or typical smart trainer, it is “faking” power based on the speed reported from the sensor. As such, the only way for your setup with VP to register a power change is to see and respond to a new wheel speed. This means that it is always a lag, delayed response and reported VP value, because you have to make the wheel spin faster to catch any VP change.

There are a couple of options I can think of:

  1. If you are using a magnet based wheel speed sensor, you can add a second spoke magnet to the wheel. Place them equally around the spokes on the wheel, and then divide the wheel size in the TR app by the total number of magnets.

    • Ex: 2105mm (700x25 circumference) / 2 Magnets = 1053mm

    • This works because you will be catching a magnet 2 times per wheel revolution vs the original 1 per rev, which gives the opportunity to register a speed change twice as fast. Using the divided value will still give the right speed to power ratio in the TR virtual power curve.

  2. The Kinetic inRide is another form of Virtual Power, but it has a couple of benefits.

    • First, it uses a magnet placed in the roller, and as such, it spins by the sensor more frequently since the roller spins many revolutions for just one revolution of the rear tire. This means it has the potential to see a speed change faster than the typical wheel speed sensor that has only one magnet on the wheel.

    • Second, it is a sensor that can be calibrated via the Kinetic app, to potentially give more accurate estimated power.

Thanks - Kinetic Inride is out of stock everywhere (in the UK, at least), so I’ll have to muddle through.
Also, while I take your point about there being a degree of lag on ‘on wheel’ sensors, 10 to 15 seconds is ridiculous! On a rough calculation, my wheel will rotate at 3 revolutions per second at a modest 15mph: I really can’t believe that Wahoo/TR needs 30+ rotations to gauge the rotational speed!

What exact speed sensor do you have?

If it is one of the accelerometer based ones, I expect they may be slower to respond to changes than a magnet based system. It may be amplifying the delay you experience.

Hi >

It’s the Wahoo speed and cadence sensors. They’re not magnetic - not sure if they use. Accelerometer or simple calculate based on how frequently they get turned upside down (the speed sensor rotates on the hub, cadence on the crank)

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Yup, that is what I was guessing based on your comments. I speculate that those devices are slower to respond to changes as compared to magnetic sensors. Total guess, but I had reasonable response times with my Road Machine and simple mag speed sensor, back when I used VP. It was never as fast as power meters or my other smart trainers, but much quicker than the times you mention above.

If you plan to stick with VP, it might be worth the effort to get a mag based design for testing at the very least.

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Agree with Chad, plus:

If you’re able to get ahold of an Inride, you’ll probably be pleasantly surprised. Just a second or two of lag. It is very very good.

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I have the inride3, and there is still a lag (annoying when racing on Zwift and everyone always has the jump on you… makes you have to race smarter and take the initiative). Also the power always dips for like 6 seconds when initiating a sprint. Normally what happens is near the end of the race when the speed is ramping up and I stand to sprint the power not only pauses, but dips… (going from like 400 watts, and when I stand dips down to like 360w for 6 seconds then jumps up to 650W) this happens almost every time. Not ideal.

If I were to do it over again, I would have not bought/sell my kurtkinetic Inride3 (or in your case wahoo speed & cadence sensors) and bought a used left crank power meter (Stages 4i, pioneer) for like $200. It would be like $130-160 more dollars, but I would have an actual power meter that should respond much quicker and I can actually use out on the road. In the past year I trained with power (via trainerroad) then did road races without power. It really would have been nice to have had a power meter on my road races so I could measure my breakaway efforts and just in general to have on my road training rides.