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Virtual Tire Slip Explained

What is Virtual Tire Slip?

Virtual tire slip is the sensation that some riders experience at low speeds, in low gearing, and particularly when riding up steep inclines. It is similar to what happens with wheel on trainers, or outdoors on a wet uphill paved road, or a gravel road with sudden torque changes.

What devices experience virtual tire slip?

Virtual tire slip occurs on all Neo and Neo 2 smart trainers. Some owners may testify that they have never experienced virtual tire slip. That does not mean that yours is faulty and theirs is not. Rather, it is an indication they have simply not encountered the conditions where virtual tire slip is likely to occur. For the rest of this article, we’ll use “Neo” to describe both Neo and Neo 2, as the behavior is identical in both, so disambiguation is not required.

What causes virtual tire slip?

The short answer is that whatever you happen to be doing at that moment has exceeded the 85 Newton meter torque threshold of Neo. While that’s an accurate answer, it’s not a very satisfying answer. We’ll take a look at the contributing factors momentarily, but first — another important question.

Why don’t other trainers have this issue?

Neo is the only direct drive trainer that does not make use of a friction component to drive the flywheel. Neo’s resistance comes from magnetic induction, invisible magnetic fields that you, under certain conditions, can overwhelm.

Every other trainer relies on a friction component, typically a belt of some kind. If, for example, you were to loosen the belt slightly, or fail to adjust the belt as it wears over time, the effect would be the same — virtual tire slip under sudden torque changes. With the belt sufficiently tightened (or over-tightened), it may completely snap if you overwhelm the inertia of the flywheel.

The same thing happens with wheel-on trainers as well. Even if you tighten the roller to the point where your tire has significantly deformed, you can (and most likely will) encounter conditions where the wheel is accelerating but the resistance unit prevents the roller from rotating at the same speed. The result is actual tire slip.

In all cases, no matter which type of trainer, slip is most likely to occur at low speeds followed by sudden changes in acceleration.

What are the conditions that cause Neo to slip?

First, I want to be 100% clear: all of the information we’re about to provide has come directly from Tacx. It’s not our opinion, we aren’t guessing, and we’re not making up stories. This is the business, from the folks who know.

Second, it’s important to understand that no single “one” of these elements is responsible for virtual tire slip. It is a combination of these conditions that cause Neo to slip.

  • Low Speed
    At speeds below 10 kilometers per hours, virtual tire slip is more likely, due to the braking force curve that you can see in the graph below that shows the braking force at increasing speeds along a 25% gradient.
  • Gear Ratio
    The lighter the gear ratio you are employing, the more likely you will be to encounter virtual tire slip. This is because lighter gears (granny gears) allow you to accelerate your pedal stroke very rapidly, where larger gears prevent it.

    More on that in a moment, but here is the related statement from Tacx, with respect to both low speed and gear ratio:

    There is a torque limit but you only will experience this on very low speed (below 10km/h) with a ratio of 39 [front chain ring] – 28 [rear cog] or smaller.”

    Translation: change your gear ratio to help prevent tire slip. If you climb in your granniest granny gear to reduce the pedal force required to climb, you increase the likelihood of virtual tire slip. For example, if you’re using a 34T chainring, then slip is more likely in rear cogs larger than 24T.
  • Rider Weight
    Heavier riders are more likely to experience virtual tire slip than lighter riders. When you think of outdoor tires, this seems a little backward. You might think that the force of friction would increase with rider weight, requiring more force to result in slipping between the tire and road surface. However, remember we are not dealing with tires and pavement; we’re only dealing with magnetic induction, and the effort required to exceed its threshold. Heavier riders create more pedal force. This means you have some ability to mitigate the effect by decreasing your weight in whichever application you’re using to simulate cycling physics.
  • Inertia — The Tacx Advantage?
    This may come as something of a surprise, but Tacx deliberately chose to simulate the maximum inertia with respect to rider weight, and gradient, as it applies to the virtual flywheel. Other fixed flywheel trainers allow you to “cheat” a little, continuing to roll forward when gravity should have pulled you to a halt. The effect is that other trainers are simulating a lighter rider weight with respect to inertia. Because Tacx made the decision to simulate, as realistically as possible, all the physics of cycling, your simulated speed drops very quickly when riding on gradients, requiring riders to accelerate every pedal stroke which is a major factor in virtual tire slip.
  • Pedal Acceleration
    Note that we are not talking about cadence. Cadence is your velocity. We are not concerned with pedal velocity, but with pedal acceleration through any portion of the stroke, regardless of the cadence. That may seem confusing, but if your cadence is 90 rpm (the time it takes to complete a revolution), you can rapidly accelerate a portion of that revolution and slow down at others. A smooth, even pedal stroke will practically eliminate virtual tire slip, and is the main reason why some owners report never having experienced virtual tire slip. Their application of force on the pedals is more consistent throughout the entire pedal stroke, reducing moments of rapid acceleration and deceleration.

    This is why lighter gear ratios are also more likely to experience slip. In a lighter gear ratio, you are physically able to accelerate your cranks much, much faster than you can higher gear ratios. This is just like outdoors, where you can spin out your granny gear, but there is no way you’re going to spin out your cruising gear. It’s the rapid change in acceleration made possible by the gearing.

    Putting all that together, here is the statement from Tacx (note – we made some grammatical corrections for readability Dutch -> English, contact us if you want the raw version):

    “The resistance for the gradient is not the problem. At a higher rider weight, the simulation of the full rider weight as inertia can be a problem when the rider does not have a smooth pedal stroke. The high gradient resistance lowers the flywheel speed very fast. The rider will have to accelerate every stroke, and this can go over the max torque — what feels like slipping. Trainers with a fixed flywheel simulate a much lower weight. Reducing the rider weight in the application can solve this and will give a simulated behavior similar to a fixed flywheel trainer. We [Tacx] prefer to simulate the real mass.”

    Translation: Tacx designed Neo to realistically simulate the physics involved while climbing a steep gradient with changes in inertia, but this presents challenges in replicating the outdoor experience. Outdoors, every pedal stroke feels like you’re putting the inertia and acceleration into the pedals, which is almost instantly lost as gravity pulls you to a halt. Trainers that use a weighted flywheel (even the heavy ones) spin more freely which creates unrealistic momentum you don’t experience outdoors, and inadvertently simulates a much lighter rider weight. The trade off for the Neo simulating the physics so precisely is massive torque at low speeds which the brake struggles to contain — and thus the momentary slip.

Additional Information

In September, 2019, Tacx / Garmin began shipping the Neo 2T. The “T” is for torque, because Tacx have increase the torque threshold. The original Neo and Neo 2 had the same threshold: 85Nm.

Neo 2T increases this threshold to 88Nm, and increases the maximum brake force from 250N to 260N.

These seemingly minor changes have essentially eliminated virtual tire slip except in the most extreme circumstances where the intent is specifically to cause virtual tire slip. While normal riding, even in race conditions, both during climbing and sprinting, virtual tire slip appears to be a problem of the past, with near unanimous agreement from reviewers and critics alike.

If you are not able to mitigate virtual tire slip using our suggestions, consider the Tacx Neo 2T. It’s everything you liked about Neo, but less that you didn’t.

Updated on November 26, 2019

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Comments

  1. This is useful, but I think misses one very important point.

    The real world situation where we have experienced this most, is in doing intervals, in transitions from say 200- 550 watts.

    So it’s not down to rider weight, or smoothness of pedalling in this scenario, but type of workout.

    Also, my son, who weighs 65 kg, but does much tougher structured workouts than me, and has much more power (5w/kg),0, experiences it much more then I do, at 88kg and 3 w/kg.

    It seems likely that riders who do structured training, for example vo2 max workouts, are much more likely to experience the issue than those who just ride.

    1. Possibly, but the same rules apply whether your facing an increased gradient or an increased power request in erg mode. To maintain the same cadence and speed, you need to apply substantially more force to the pedals in short order. With that said, it is still very, very individual with respect to pedal stroke.

      Take a look at this example, which is very close to what you were explaining.

      https://analyze.dcrainmaker.com/#/public/03202152-7322-4d23-6f4a-68c53e10f76f

      It jumps from 150 to 500 watts. Look at the cadence line. There are no spikes that might indicate a sudden change in resistance, whereas virtual tire slip can typically be noticed in a graph like this. Because I was the person performing this particular test, I can tell you that the *potential* for virtual tire slip absolutely exists, but it did not for me. This is all down to pedal style.

      Here is the result from someone else whose really struggled to modify his pedal style to prevent slip on the exact same test. Quite a big difference in outcomes. He’s a much stronger rider than I am, but not nearly as smooth, and smooth is what matters with Neo, with respect to virtual tire slip.

      https://analyze.dcrainmaker.com/#/public/79ee8a60-2a39-414d-57c0-e68413e0953a

      I’m not saying you’re wrong. I’m saying that you have more control over the outcome than you may be aware, and it’s worth trying to make adjustments to your pedal style so that your result looks like the first graph, and not like the second graph. Then virtual tire slip goes away like magic.

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