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When doing structured workouts on my Kickr Core, I noticed that it gets extremly hot on the body that holds the electronics and its other internals - to a degree that I can only touch it for a second or two. I bought a home infrared thermometer and measured 80°C on the wheel. There are no obvious negative effects like the device shutting down and the pedaling feel stays consistent.

enter image description here

I am aware that the trainer by design must provide resistance and thus emits heat but I still find it a bit worrying, despite people on the Wahoo forum saying it is "normal". I mostly do medium efforts and my FTP is definitely with the mortals, so I wonder if I should provide some cooling? (Some riders seem to be placing fans in front of their trainer)

Even if the device is seemingly working normal, are there any downsides or long-term effects (electronics, bearings, etc..)?

My workouts are all done in normal room temperature (22-23°C) and I can't place my fan so that both me and the trainer are in the airstream. In ERG mode, I ride in the small ring to keep flywheel speeds low and have the trainer react faster to resistance changes, does this also play a role, i.e. would using the big ring demand less "braking power" and generate less heat?

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    is adding a second fan feasible?
    – Criggie
    Mar 2 at 20:38
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    I think I have one lying around, could check if it makes a difference. My main fan would do but I have to put it close to the front wheel and angle it up due to lack of space, hence no cooling for the trainer.
    – DoNuT
    Mar 2 at 20:51
  • another option is to move the whole thing outside, or into a cooler area like a garage.
    – Criggie
    Mar 2 at 20:59
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    @Criggie Yeah, due to my living situation, unfortunately not. The best I could do is open doors/windows in my living room but that's not practical in winter.
    – DoNuT
    Mar 3 at 7:34

3 Answers 3

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I have a similar Kickr Core. When you do the spindown, the Wahoo app shows the temperature (this temperature is from somewhere inside, though I don’t know where exactly). You can use this to get some idea about the temperature. Mine gets closer to 50 C after going up Alpe du Zwift in a garage where ambient temperature is maybe 10 C. Based on my experience, having temperatures around or slightly above 50 C in ambient 20 C should be quite normal.

UPDATE: Based on the comments, it seems that Wahoo has stopped showing the temperature at the Android app, and has removed the manual spindown function for most trainers. Future updates may remove the temperature info also from iOS and/or manual spindown function from older Cores.

As stated in the other answer, magnets shouldn’t mind such temperatures and most electronics should be somewhere else. And I’d consider it a manufacturing defect if Wahoo would have used bearing grease that can’t handle such temperatures. Bigger chainring gives faster flywheel speed, which could increase the airflow (Kickr has holes in the plastic cover behind the flywheel, indicating that Wahoo has planned some sort of airflow there), but I’m not at all certain how big an effect that would be (though it should be pretty easy to test).

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    Is there some effect on the small ring, though? ChrisH's answer claims no, but lower gearing changes flywheel speeds and torque, right?
    – DoNuT
    Mar 3 at 7:11
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    The power you output is the same no matter which gear you use. And all the power has to be dissipated as heat by the trainer. Pedal at 200W and the trainer becomes a 200W heater.
    – Michael
    Mar 3 at 9:27
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    I have to admit I don’t have facts to support the ‘big ring is cooler’ claim. It was based on a hunch that faster spinning flywheel would cause more airflow, and hence cool the system a bit. I’ve updated the answer.
    – EyeBrown
    Mar 3 at 21:56
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    Nope, now definitely everything at latest firmware and I still get no readings from within the app. It is what it is, just putting my Garmin watch on it and waiting for temperature to climb also gets me 44°C post-ride, so probably around your 50°C if it was showing internal values.
    – DoNuT
    Mar 7 at 17:29
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    Bummer. I tried to see if Kickr would broadcast the temperature over bluetooth in a format that would be easy to read, but it wasn't advertising anything that would've been obviously the temperature. The spindown procedure only requires the trainer to send the time at the end, so perhaps Wahoo is using some proprietary protocol there.
    – EyeBrown
    Mar 10 at 21:36
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For the same exertion, your choice of gearing won't affect how much heat has to be dissipated.

Unless you have two modes of resistance used at once (I don't think the Kickr does properly, I've only used this feature on WattBikes) the flywheel speed doesn't affect where the power goes - it goes into the braking system.

However in some models a small part of your effort mechanically drives a fan that cools the braking system. And that will be geared off the flywheel, so more effective if it's spinning faster. I haven't seen such a setup in pictures of the Kickr, but haven't seen many teardowns either.

Touching the housing for a second or two means it's under about 60 C (if metal). That isn't a problem for most electronics. Of course because heat flows from hot to cold, something inside is warmer than the case. But not necessarily the control electronics. Magnetic coils are not very temperature-sensitive compared to their controllers, and will run hotter as they're part of the braking system.

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    This might be subjective but my impression was the trainer is getting even warmer since I use the small ring, from very warm to the current state. Unfortunately no measurements, my digital (fever) thermometer just tells me I'm dead but 60-ish °C sounds feasible. So, unless electronics start to act up, nothing to worry about, right?
    – DoNuT
    Mar 2 at 18:15
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    I didn't see anything to suggest it's an early sign of impending failure, but I could easily have missed something. And changing other things about your setup or training program could also affect the temperature
    – Chris H
    Mar 2 at 18:50
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    BTW wide range infrared thermometers are really cheap and quite useful. But I say that as an experimental scientist who tinkers with electronics, cooking and DIY, so having measuring kit is in character. I first bought it for measuring the temperature of my bike disc brakes in real time, but it was too useful to dismantle
    – Chris H
    Mar 3 at 8:22
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With my measurements, I also asked Wahoo support and got a short but clear statement:

enter image description here

Temperatures on the flywheel in this area are generally not a problem. As long as the trainer works properly and doesn't make any unusual noises or anything like that, there's no reason to worry.

I kept asking about thermal limits (like a shutdown at a certain temperature), they ignored this but also stated:

enter image description here

70-80° on the flywheel is not a problem. In ERG mode you can also influence this somewhat by using a heavier gear on the bike, then the speed of the flywheel is higher and the magnetic brake has to work less to generate the same resistance .

So, even 80°C seem to be normal and require no cooling. According to Wahoo, gearing ratio respectively flywheel speed also makes a difference. I didn't get a theoretical explanation but that's from their official support.

The latter also matches my observations, I didn't use the small ring until this year, that backs up why I only noticed the problem recently. My trainer always got kind of warm but it gets notably warmer when you use lower gears. Maxing out at 80°C was probably a peak (for me) because the workout went down to 215W @ 60 rpm which requires even more braking.

I would guess for somebody who's able to generate higher sustainable power (like 300W or more) temperatures could even get higher and gearing changes/cooling might be advisable if problems arise at some point.

(Quotes translated from original German replied)

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