# Dumb question: Tire pressure matters?

A little embarrassed to ask this, but could someone explain how tire pressure on a dumb trainer impacts power output? is it as simple as “Lower tire pressure=higher rolling resistance=lower power” and vice versa? I know i should be able to figure this out without googling it, but i thought i’d start here.

On a smooth surface like the drum on a dumb trainer, higher tire pressure = lower rolling resistance and vice versa, assuming the drum is tightened onto the tire with the same force. This means that at a higher tire pressure, it will require less force to produce a given wheel speed. Therefore, lowering tire pressure could shift the power curve of the trainer “up” and raising tire pressure could shift it “down”.

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Thanks! The more you know…

The trainer calibration will take any to account tire pressure.

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To be kind’ve technical, it doesn’t affect power output. It will, however, affect power transmission.

Let’s assume that you press down on the pedals with the same force and have the same cadence over two identical training sessions. The first session you have 140psi in your tire. The second session you have 70psi. Since power is measured by torque * RPM at the crank power remains the same. Your speed will probably be different though.

If, however, the power were to be measured on the resistance unit, there would be a change because the power is “lost” at the contact point between the tire and the trainer drum.

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That makes a lot of sense. Thanks for the extra clarification!

I’ve got a dumb trainer, so beyond telling TR which one i’m using, i don’t think there’s any calibration.

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Then just set it to a good pressure where you don’t get wheel slippage and don’t worry about it. I’ve experimented with my Tacx Vortex Smart trainer and the difference between 80psi and 100psi barely moves the calibration needle. I mean it doesn’t even move it enough to get off the bike and turn the knob.

In this case what you’re really after is repeatable results.

It doesn’t matter if the computer/TR is telling you that you’re producing 240 watts, 97 gorillas or 6 pounds of butter so long as that number is consistent.

To keep that consistent you need: the same tire, the same type of inner tube, the same pressure and the roller pressing down the same amount on the tire. If you change those variables you might be producing 4 pounds of butter and 93 gorillas.

In reality, those changes are probably under 5% which isn’t a huge deal unless you’re at your top end. 97 gorillas ± 5% isn’t that bad but if you have to do 150% of 97 gorillas and then you have ± 5% you could be in trouble.

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Are you using Virtual Power or a power meter? Both can be used on a dumb trainer.

If it’s the latter, then tire pressure will have little to no affect on your readout.

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Reviving this old topic - I have searched for the answer to my question but cannot find it but believe it it related to this thread.

Can I adjust my tire pressure without disengaging the flywheel on my trainer or will that negatively impact my virtual power? I have always unscrewed the tension on my trainer to remove the flywheel from contacting my tire, checked and adjusted tire pressure as needed, and then engaged my flywheel with my tire (3 full turns as suggested by manufacturer).

I have been thinking - this ensures that my tire pressure remains constant, but is my flywheel to tire pressure truly constant ride to ride? I have marked my dial but still think there might be some wiggle room there that impacts tension. So, if I don’t take my bike off the trainer and keep the flywheel constantly engaged, can I check my tire pressure under these conditions and remain consistent or is my initial approach ideal?

Thanks for any input.

Kris

The trainer doesn’t have some kind of quick release for removing the bike without changing the roller tension? All my wheel-on trainers have had this sort of thing.

I think what I’d do is install your bike, have the roller engaged, check tire pressure (pump to 80psi or whatever works for you), ride and warm up trainer and then calibrate.

Now the next day, you can just top up the pressure to 80psi and then ride. The calibration should be exactly the same assuming temperature in the room is the same.

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