Bicycle Trainer Resistance

[Susan Marion]

We are using Tacx bicycle trainers, specifically the Flow, with specialized sport tricycles to train children with Cerebral Palsy to enhance their fitness, leg strength and motor coordination. One aspect of the study is to quantify the "dose" or power at which each child is cycling. To do this, we currently calibrate the subject's torque output using an SRM PowerMeter (crank torque sensor). With the information from our calibration curves, we set the dose of the child's exercise bout by selecting a resistance setting on the Tacx head unit, selecting the appropriate gear, and having the individual cycle at a set cadence (RPM). We use a video game feedback system to try to keep the child at the prescribed RPM.

We would like to both validate and simplify our procedure. It would be helpful if we could quantify each resistance setting on the Tacx head unit. Do any of you know what load corresponds to each of the -4 to +9 settings on the head unit? The Tacx website says that the Flow trainer uses an electromagnetic brake system.

Attached are results from some of the calibrations. As you can see, it appears that the resistance exerted by the trainer changes not only with the level or setting on the Tacx head, but also with the velocity of the rear wheel. Is this normal or expected behavior for this device? Do any of you know how the trainer levels are derived? Is there an algorithm or equation for calculating resistance or trainer level for electromagnetic brake systems?

Is there a better device for maintaining a constant resistance or a better method for maintaining a constant power we should consider?

Thank You!

Susan

[Scott McMillan]

Re: Bicycle Trainer Resistance

Hi,

Most trainers (affordable) increase in resistance when: a) the rear wheel goes faster by pedaling or shifting gears, and b) changing the setting on the trainer. The benefit of ergotrainers is that they have an erg mode. The FLOW (at least the older versions), allowed you to use an erg mode. In the erg mode, you can set the power to a specific level (+/- some error depending on the trainer). In this mode, any changes in the rear wheel speed will be sensed by the internal computer and it will adjust the resistance accordingly. Computrainer is the the best at doing this, they run under $2k per trainer. Tacx is not as precise but good enough for most training purposes. There have been some comparisons been done online between the two. I have not seen peer reviewed comparisons. Computrainer has been peer reviewed against SRMs if I remember correctly. Sorry, I don't have any references handy. For research grade trainers, you want a Velotron and other similiar made-for-lab devices.

Cheers,
Scott McMillan, M.Sc
Consulting and Innovation

https://www.Factor9Coaching.com

https://www.bikebarn.ca

[Mark Corbett]

Re: Bicycle Trainer Resistance

There is growing interest in this area of correlating speed to calculate power and get a 'virtual' power meter on the cheap (e.g. TrainerRoad), but reliability is a big issue. Just searching for 'tacx flow calibration' throws back a load of stuff (e.g. here).
The Tacx Fortius has also been validated:
http://www.ncbi.nlm.nih.gov/pubmed/21380964
I've had some good results with Kurt Kinetic trainers fluid resistance units that are 'calibrated' against speed. I typically get within ~5% of a Powertap. Another mid-range option that does actually measure power is the Powerbeam trainer from Cyclops.
Common with all trainers there are certain things you should do to minimise variations:

• Warm the trainer up thoroughly for 10-15 minutes
• Always use the same tyre pressure (100-110 psi for a road bike)
• If possible use a specific trainer tyre
• Perform a roll-down calibration check (the Flow has this function built in)

Mark
University of Worcester

[Susan Marion]

Re: Bicycle Trainer Resistance

Hi Scott,

Thank you! Your first sentence answered my primary question. We have the Flow T2200. I have not found an erg mode mentioned in the manual or on the head unit. Something that I forgot to mention is that we only use the trainer to change the resistance on the rear wheel. We only change the trainer levels on the trainer head. We do not record any information, other than the trainer level, from the trainer head. We have a MatLab program on a computer that records crank encoder, SRM PowerMeter, and piezoelectric pedal force data from our testing trike. The training trikes are only equipped with crank encoders, however, initially we attach the SRM PowerMeter to the training trikes to generate calibration equations for those trikes. The coefficients from these equations are included in the Power equation in the MatLab program for the training trikes.

I'll take a look at the Computrainer. The Velotron looks like the bike is part of the package. We prefer to have the flexibility to use different sizes and types of bikes with a trainer. We are using two different size recumbent tricycles in our study. The kids like the trikes, they are safe and comfortable, and they ride them. The participants take the trikes home with them and train in the comfort of their own homes for several weeks.

Do you know if there is an equation that relates both trainer level setting and crank rate to crank torque if the pressure of the cylinder against the tire remains constant (e.g. 2 turns of bolt after contact) and the tire pressure is a constant 40 psi?

On a somewhat related topic, your links caused me to wonder, are there cycling training camps for cerebral palsy children? If so, could you email me info?

Have a spectacular day!

Sue

[Susan Marion]

Re: Bicycle Trainer Resistance

Hi Mark,

Thank you for the fantastic links and the checklist for obtaining consistent results!!

As I mentioned in the reply to Scott we only use the trainer to change the resistance on the rear wheel. We change the trainer levels on the trainer head, but we do not record any other information from the trainer head. We have a MatLab program on a computer that records crank angle and torque data from the bikes in the lab. We generate a family of torque versus trainer level calibration equations at different crank rates for the MatLab programs used for the recumbent training trikes that go home with the children.

I'll take a look at the other trainers you mentioned.

I will add specific warm-up times to our protocol. I don't think this was included before. Our bikes have small tires, so the max pressure is 40-50 psi. We've been maintaining tire pressure at 40 psi. I read about the brake calibration in the Flow manual just recently. The person before me may have performed the brake calibration, but we are definitely doing it now.

I was hoping an equation that relates both trainer level setting and crank rate (for a given gear ratio) to crank torque existed. I thought the equation might resemble that for a dashpot or damper: Fc = -c*dx/dt, where Fc is dampening force and c is the viscous damping coefficient. In the case of the trainer, c might correspond to the trainer level setting (-4 to +9)?

Thanks so much and enjoy your day!

Susan

University of Delaware
Shriners Hospital of Philly