gycho77

Can anyone recommend me a cheap(affordable) cadence/speed sensor with computer?
I currently own a Powertap Bluetooth Speed/cadence sensor, but it's slow and cannot read my cadence when I go over 158rpm.

taras0000

Someone has made a homemade PM for under $55.

homemade cycling powermeter

Dalai

Measured deflection. But no way to know if accurate...

carleton

That's essentially how SRM power meters work.

I'd imagine that the creator would have to calibrate the PM with known weights to set the appropriate offset/slope the same way that SRM cranks are calibrated. But, yes, that step is not listed in that tutorial.

On a related note. This is why it's very important to calibrate your cranks before your efforts. Heat (or cold) can change how the metal in the cranks respond to your torque. Same if you calibrate your set the slope of your SRM in the AC of your home then leave the bike in the sun before your effort. Your cranks will be much warmer and flexier and the values that the strain gauges record will be higher.

Dalai

Essentially yes. Both use strain gauges in a Wheatstone bridge arrangement, with the SRM fitting these on the spider and the homemade one on the crank arms.

Always calibrate before and depending on the conditions during.

Being able to calculate the Slope offset is a great feature with the SRM. I have a 20kg +-4g calibration weight just for this very purpose...

taras0000

So the next thing is, does anyone here think they could streamline that thing so it doesn't look all janky?

carleton

That guy has a loooooong way to go. You've got to think about protecting both the power meter and head unit from weather, debris, and shock, too. The hours spent working it all would be worth way more than an off-the-shelf PM would cost.

It would be a great project for sure, but no where near cost effective.

To put it all into perspective, here is the first SRM, which already seems to be ahead of the system linked above. And we know how far the SRM has come.



Stages is probably the closest thing to this. I think they basically glue strain gauges to cranks with known deflection specs. Why they won't glue one to the other side and measure both cranks, I'm not sure.

Dalai

There has been rumours of dual sided Stages out in the wild, but so far these are just rumours.

As to not rushing into the dual sided market, there isn't a need at the moment. Enough people are happy to go with the Stages only measuring one leg. Thinking many are just happy to record their rides and tell everyone at the coffee shop post ride they own a PM...

carleton

I'd guess that they are running into some technical issues. If it were easy, I assume they would offer it by now.

True. I think the #1 argument against Stages by someone trying to sell another brand is that "It only measures one leg." Which is a valid argument. That's one main reason why I wouldn't buy one.

Yeah.

I'll be honest. Even when I did have power meters on my road and track bikes, there were times when I really didn't care to look at the data. "That Sunday ride was just like all the others..."

The Power Meter is like a Piano in the home. Awesome if you know how to use it and are willing to use it. Otherwise, it's just an expensive piece of furniture.

On a related note...did you know that Tennis racquets now have "Smart Sensors" that are similar to what Power Meters are to bikes?


I asked a guy in the tennis pro shop, "So...now that a player has all of this data, what do they do with it?" He kinda shrugged and said, "Heh...I'm not really sure."

Dalai

I like the piano analogy. Very true!

Wasn't aware of the latest in tennis racquets! There are the RPM² insoles for running (and marketed for cyclists) but haven't seen any data or comments yet... https://www.rpm2.com/

queerpunk

Yeah, there are those dual-sided Stages that are being tested. They've been discussed for a few years, but apparently Stages had problem combining data streams from two sensors into one data stream. I think there are pictures of them under Sky.

carleton

Yeah, that's what I assumed was happening. But, what about PM pedals? Didn't they solve that problem?

Dalai

I think with the pedals, one side is just a slave and the data flows through the other master side. Just gluing a Stages sensor on the spider side means you have two masters...

Voodoo76

Thought I would pass on a quirk I see in the Power2Max track (and road). The screen shot attached is from an acceleration. Note that the peak rpm lags speed by a good 3 seconds (peak speed at 40:20, rpm @ 40:23). This has been very consistent at 3 to 4 seconds. It also reads about 200W lower peak than Quarq or Powertap (i would guess related if torque and rpm are offset).

I've discussed this with Power2Max and they state its "normal" and is is essentially due to how they use the accelerometer for cadence. Doesn't really impact steady state efforts. But something to take into account when considering this power meter, if you want to benchmark shorter efforts.

carleton

That's pretty bad.

OK. So that means that it's normal for the data to be wrong. Got it.


This is why it sucks for bike manufacturers to say, "SURE! It'll work on the track, too!". Just because something will fit on a track bike doesn't mean it's designed for use on a track bike.

To be honest, at this point, track racing is more different than similar to road racing. Like comparing Lacrosse to Soccer (get the ball in the net more times than the other team). Similar in concept, but very different in execution and details.

Voodoo76

Yea the data is pretty messed up.

I believe that Quarq has a distinct design advantage here. They use multiple reed switches, so they are updating cadence several times per crank revolution.

taras0000

Hence why I stated earlier that a company should start development with a track powermeter, then dumb it down for road.

Velocirapture

Yes, in a trackie's perfect world.
From a cost/benefit perspective, its not going to happen (for commercially available stuff, that is. Maybe different for hour-record/ olympic-team/ etc type scenarios)

carleton

A lot of the original limitations are now gone. Those mainly being that flash memory was expensive and batteries weren't very good.

Compare a PowerTap head unit from 2007 to what can be made today.

But, Product Owners have to one-up each other. Instead of going for more precise data, they work on things like integrating wifi into a bicycle head unit. GPS drains a head unit much faster than having the option to record data more frequently.

Also, I think the BlueTooth spec needs to be updated.

Dalai

Double reed switch SRM is pretty close to perfect already...

Voodoo76

Do other meters using an accelerometer (eg Stages) have similar timing problems with cadence or is this a Power2Max firmware issue?

queerpunk

Just took a look at my Stages data and cadence and speed track appropriately, without that lag.

Voodoo76

queerpunk, that's very interesting.

This was a little gear, 49x16 with a 700x23 tire. I don't know the exact rollout of this tire but it seems like at 32mph I ought to be closer to 135. The data looks to me like they are averaging.

At 28 mph on the way up I'm at 95rpm, on the way down I'm at 125. Must be some sort of magic hub! I'm going to do a little jump in a similar gear on my road bike (P2Max meter) tonight & see what that looks like.

carleton

Voodoo, if accurate data is important to you, I'd advise looking into a good head unit as well as good cranks. Most road setups round like hell.

If you have an iPhone, the Simulator feature of my app will help you do all of the calculations you need.

https://itunes.apple.com/us/app/trac...926154537?mt=8

You are right that it should be close to 135rpm...134 exactly:

Voodoo76

Carlton, life is too short for bad data! Doubly true at 60, you feel like there aren't too many years left where it will even be worth recording I'll check out the app for sure, appreciate the tip.

As for averaging I agree to an extent. What is displayed is averaged depending on how you set up the head unit. But I've never seen recorded "raw" 1s data look like this. Same head unit (Garmin 510) or an alternate (Powertap Joule) with PT hub or Quarq does not look at all like this.