Speed sensor for disc wheels

[FlatlandClimber]

Hey there,
Race season resumes and I am looking at a few TTs coming up in the next months.
I am for the first time racing with a rear disc, which is great. However, I can't access the rear hub anymore, so my Wahoo Speed Sensor, that wraps around the hub would have to go on the front. As I understand, that is aerodynamically a rather detrimental place.
Obviously not going to cost me many watts, but I am sure there is a better solution. I have seen options that attached to the outside of the disc. The solutions I saw looked bad to me. I feel like a solution like that "un-true" the wheel.
What solution can you guys recommend?

[okimy]

I think there was a thread like this not that long ago. The consensus was to use GPS when the course is open enough, as it's usually quite accurate nowadays. And judging by the bikes you ride your Garmin/Wahoo/whatever is going to be fairly new. It's the lightest option too, as you don't have to add anything you wouldn't already have on your bike.

[FlatlandClimber]

I think there was a thread like this not that long ago. The consensus was to use GPS when the course is open enough, as it's usually quite accurate nowadays. And judging by the bikes you ride your Garmin/Wahoo/whatever is going to be fairly new. It's the lightest option too, as you don't have to add anything you wouldn't already have on your bike.

Okay, this is the most obvious solution. In training I absolutely hate GPS-based speed. Bounces between 20 and 70 when there is one single tree I ride underneath. Of course I ride based on power/ HR, but speed is a very good indicator of CdA and direction/speed of wind.

[jfranci3]

https://bikerumor.com/2018/05/22/new-ro ... he-market/

https://www.velonews.com/gear/arofly-wo ... wer-meter/ (has speed)

It's DB? mount a magnet on the disc and put the sensor behind the fork leg?
Put a wahoo inside the rim - attach it with silicone or something.
Mount the magnet on the backside of the valve access (or https://www.bike24.com/p2164839.html) , put the sensor on the FD side where the air is dirty anyways
silicone the magnet inside the tire, put the sensor in the frame behind the BB - that spot will get a lot of flats.

[FlatlandClimber]

Okay, a lot of things there. Yes, its a Roval 321 Disc... Disc brake. This "arofly" would be a good option, if it didn't have power meter and cadence sensor. That appears impossible to work properly for me and I found a few very bad reviews on the overall function of the product.
If you take a wahoo sensor and "glue" it to the rim, does it work the same? I mean the rotation is now "side-wards" rather than "forwards".

[Karvalo]

I think there was a thread like this not that long ago. The consensus was to use GPS when the course is open enough, as it's usually quite accurate nowadays. And judging by the bikes you ride your Garmin/Wahoo/whatever is going to be fairly new. It's the lightest option too, as you don't have to add anything you wouldn't already have on your bike.

Okay, this is the most obvious solution. In training I absolutely hate GPS-based speed. Bounces between 20 and 70 when there is one single tree I ride underneath. Of course I ride based on power/ HR, but speed is a very good indicator of CdA and direction/speed of wind.

Get a better GPS. Even my ancient Edge 500 with its notoriously tiny antenna copes just fine outside of dense tree cover and ravines. Newer units are far better again.

And a race isn't really the time to be experimenting with CdA. You either know your position by then or you don't. So when your GPS speed bounces because you rode under that one tree then just... don't react

[Karvalo]

Okay, a lot of things there. Yes, its a Roval 321 Disc... Disc brake. This "arofly" would be a good option, if it didn't have power meter and cadence sensor. That appears impossible to work properly for me and I found a few very bad reviews on the overall function of the product.

Any pure speed sensors prior to the current Garmin and Wahoo generation worked the same way. Spoke magnet (which you can glue to the disc) and fork/chainstay mounted sensor. I can't imagine they're hard to find, like the Wahoo Blue SC sensor is still being made.

[PoorInRichfield]

I've been using GPS-only on my rides this year with a Garmin 830 and it's my experience that GPS is a bit laggy compared to real-time sensors. For racing, I don't think I'd depend on GPS unless "close enough" is good enough. My 2 cents...

[ghostinthemachine]

Bounces between 20 and 70 when there is one single tree I ride underneath.

Your GPS is broken.

All of the ones i've used in the last ~15 years have coped better than that with tree cover, and it gets better every generation.

[mrfish]

Not sure why you would look at instantaneous speed while riding. There are so many confounding variables it just doesn't tell you much. Better IMO to look at 3s power, time, distance and same per lap and then speed afterwards.

[ratef42]

Use an old Garmin GSC-10 speed/cadence sensor and epoxy a magnet onto the disc