How do the smart trainers calculate your road speed?

Discussion in 'Virtual Training: Bkool, Zwift, CycleOps & others' started by jimbo2112, 1 Mar 2018.

  1. jimbo2112

    jimbo2112 Active Member

    I've bought a Kickr Mk III and am using Bkool app for my turbo rides. I do like it, but I am curious about some aspects of the science behind the hardware and software and whether there's an opportunity to improve them both.

    First question, which I think will lead to more in this thread! Is the road speed calculated using the smart trainer flywheel? If not, why not? From what I see on screen, I doubt it. Is it due to different trainers having different gearing?
     
    Whorty likes this.
  2. Whorty

    Whorty Yes, the doctor did sign off the TUE!

    Location:
    SW Wiltshire
    We believe that the in game speed is derived from the watts you're producing (on a power meter trainer such as a Kickr or Neo) plus the gradient of the road plus your weight. So, it says, you're 14 stone, riding up a 3% incline at 250 watts it derives a speed of 30 kph (for example).

    It seems to be different on the BKool Pro trainers which do not have a built in power meter - here the speed in game does seem to be directly related to the wheel speed, then in game the software derives the watts you would need to hit that speed, based on the resistance on the trainer and the weight of the rider.

    It's pretty much accepted that BKool's own trainers have an advantage of anywhere up to 20% on none Bkool trainers when ridden in the Bkool simulation software.
     
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  3. CXRAndy

    CXRAndy Veteran

    Location:
    Lincs
    if you use Zwift the speed is calculated from power alone. Zwift have a decent algorithm, so speed is very comparable to outdoor.

    Bkool for their own trainers (like whorty) uses speed from the trainer flywheel then calculates a power and speed figure. It is wildly over optimistic in speed and power

    if you compete in Bkool with your Kickr, you quickly notice that riders producing less power will be quicker than you. That is normal in Bkool for non Bkool trainers. Try out Zwift and you find your power and w/kg will place you correctly within race classification
     
    Last edited: 2 Mar 2018
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  4. smutchin

    smutchin Cat 6 Racer

    Location:
    The Red Enclave
    This is interesting. I assumed that my Bkool Smart Pro + Zwift was calculating speed from the actual speed sensor on my bike. If not, that might explain some of the anomalies I’ve experienced - ie a particular cadence/gear combo not giving the same speed as I’d expect on the road.
     
    Last edited: 5 Mar 2018
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  5. CXRAndy

    CXRAndy Veteran

    Location:
    Lincs
    The Bkool trainer is a decent trainer, but it is still a budget product in features and in comparison to the likes of a Tacx Neo, Wahoo Kickr and top end Cyclops or Elite smart trainers lack the climb resistance, max power and accuracy
     
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  6. ibooij

    ibooij Senior Member

    Location:
    the Netherlands
    In BigRingVR, just as in Zwift, in-game speed is calculated using power, incline of the road, weight (ride and bike) and air resistance. BigRingVR does not use the speed that is communicated by the trainer.

    Many trainers, especially the more budget ones, have limited brake power. They start to overestimate speed at about 5%-6% grades. Calculating speed from power helps keeping things real. Unfortunately, the power meters in most trainers are pretty bad, so values can still be off, but that's life.
     
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  7. Whorty

    Whorty Yes, the doctor did sign off the TUE!

    Location:
    SW Wiltshire
    How do you calculate the speed and power of a trainer like the Bkool Pro where there is no in built power meter.
     
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  8. ibooij

    ibooij Senior Member

    Location:
    the Netherlands
    @Whorty the Bkool Pro does report power over ANT+ FE-C. We use that reported power.

    That said, the power reported from Bkool trainers seems to be at least a little unreliable. Especially in the higher power ranges (> 300W), power seems to be overreported by these trainers.
     
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  9. LBHIFI

    LBHIFI Über Member

    Location:
    Liseleje
    It makes sense. They probably used to calculate power in Bsim from roller speed and applied resistance. Should be a no-brainer moving the calculation to the unit itself. I’m guessing others do same. They just do it better .
     
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  10. OP
    OP
    jimbo2112

    jimbo2112 Active Member

    OK, so this seems to show what I had suspected. I did not know that Bkool as an app underreports my power on my Kickr, but I have noticed how rubbish I seem to be against most everyone else!

    I'm surprised and disappointed that my flywheel speed is not taken into account as this is a pretty good absolute reference point? Surely if they used the flywheel alone there would be less need to take other variables into account? The app knows the gradient so knows how much resistance to put into the turbo to try and hold me back. Therefore if I can manage 15mph on that 6% incline then so be it? The Wattage is then just a byproduct to report on rather than use as a variable?

    I may be getting this horribly wrong and welcome being schooled on this. I'm pretty sure that it way more complicated and there are hoards of data scientists working on the algorithms to create that sweet spot of accuracy.

    It remains, though, that I get a little miffed when I know my bike speed is nothing like that which is being shown on screen on the Bkool app.

    Fire away!
     
  11. Flint

    Flint Active Member

    Same thing for Veloreality, we calculate on-screen speed based on power reported by either trainer or power meter if you have one. Other parameters like road grade, weight, air resistance etc.etc. are also taken into account. Actual wheel speed is ignored as in most cases it does not correspond to input conditions.
     
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  12. ibooij

    ibooij Senior Member

    Location:
    the Netherlands
    The main issue is the following: Even if the wheel speed is completely accurate, when the calculation inside the trainer is completely the same as in the apps, problems will occur when the trainer is taxed beyond its limits. Many budget trainers can reach 5%-6% incline at maximum (dependent on user weight as well). If the simulation sets an incline of 10%, the trainer won't be able to give enough resistance. Wheel speed will then be higher than it should be, leading to a very unrealistic simulation. The same goes for when you are descending in the simulation and the trainer does not have the ability to "push" (very few trainers can do that). Wheel speed will then be lower than it would be on a real descent.

    It seems, though I'm not sure, that Bkool BSim uses the wheel speed for Bkool trainers, or maybe even for all trainers. On ascents, the riders on weaker (Bkool) trainers will have an advantage, because their wheel speed will be too high on any steep slope.
     
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  13. OP
    OP
    jimbo2112

    jimbo2112 Active Member

    So the science is largely based on the limitations of less expensive gear? That's a shame as my Kickr claims to work up to 20%. I work with guys that speak algorithms as a mother tongue so may quiz them on the complexities, but I would like to see (at least on my Kickr) a combination of road speed calcs combined with better account taken for momentum, especially downhill, and the actual speed of the flywheel.

    Although, having written this, and thinking about it, I assume that additional hardware & firmware changes would be needed if no account of the flywheel is currently taken into account, so never going to happen on my kit?!

    Next conversation is creating my own custom rocker gear... but that will be a new thread!
     
    Whorty likes this.
  14. Flint

    Flint Active Member

    There is no need to "speak algorithms as a mother tongue". Just go to http://bikecalculator.com , set road grade to 20%, set your weight and other parameters and then you can play with power/speed. For example if total weight (yourself and your bike) is 90 kg the trainer should be able to dump 318 watts at the speed of 6 kph. Very easy to check.
     
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