Q-ring: Power difference between small and big rings

[IMTriSBR]

Yes, another Q-ring question, but I didn’t see any other posts that was similar to this so I here’s a new one:

Here’s my set up:

• 54 TT Q-ring and a innner 40 Q-ring.
• Big set on 5, Small on 4.
• I am WAY forward on my bike, but that’s just the way I am (even on my road bike)
• Polar Power Sensor as my power meter.

Here’s what’s happening in general.

• There is a huge discrepancy in power when switching from big ring to little ring.

• On a good long climb I can produce anywhere from between 220 - 350 watts in the little ring.

• On the flats, If I’m on the big ring, I have to put in a LOT of effort barely get over 200 Watts.

• This is the weirdest part. When I’m on say a gradual incline, where I’m between the big ring and little ring, and going at the same cadence (I tend to ride in the 90s) if I switch from the big ring to the little ring, I go from ~180 Watts to ~220 Watts instantly. No real change in effort and not real change in speed, but definitely a change in power output.

• 54 x 19 and 40 x 14 is approximately equal, so I tested this on what is basically a flat road. I can literally see my power change as I switch back and forth from these two positions.

• I know what I really have to do to test this phenomenon, but I just don’t have time given that I’m training for an upcoming IM and holding another job. So I was wondering if anyone else has had this “issue”.

Some things to consider:
The aspect ratio of the 54 and 40T rings are different. There is a wider range for the 54T (57 - 51.8) than the 40T (38.4 - 42.2). Since the 40T small ring is “rounder” does this mean that maybe I should be on a chain ring that is less of an oval? I’m also a “spinner” so, might a rounder ring be better for these types of riders?

I’m also questioning the power meter I use. At what rate and at what point in the pedal stroke is the power sensor grabbing data? could it be that the chain tension is artificially low at the point that the power is be sensed, or should i be producing constant power through the pedal stroke such that it doesn’t matter when the power meter is grabbing data?

Anyway, as you can see, I’m profoundly confused by my observations, and would like to see if anyone else is experience what I am. I’m starting to think that my reliance to power is too great and that I should go back to basics and just ride by speed

Thanks in advance fellow ST’ers. I look forward to your responses.

[Andrew_Coggan]

I’m also questioning the power meter I use.
As you should be: the Polar powermeter is notorious for being sensitive to the distance between the chain and the sensor. This problem will only be compounded by the use of non-round chainrings.

[Xavier]

It does sound as though your power meter is ready wrong. I use a Polar power meter with Q-Rings and it’s fine, you have to make sure that the chain passes over the magnetic part of the sensor no more than 2.5cm or so in your usable ring combinations. I used to have a website up with details of how to install one properly but it’s ran out of juice now, I should hopefully get another one up soon!

Xav

[IMTriSBR]

Ah, fascinating…

So I’m wondering why that would make difference in the actual power that it senses. I’m assuming the polar power meter works kind of like a guitar pick-up and senses the vibration frequency of the chain to calculate tension (i.e. higher the frequency, the higher the tension given the same chain weight and length = greater torque).

I can see how if the chain is farther from the sensor, (i.e. a 54 - 25 tooth combination) that the chain to sensor distance would be great and the Amplitude of the signal smaller, but the frequency that it senses should be the same, hence a relatively similar power no mater how far the chain is…

So I thought maybe this is the issue, so I put in an extra spacer just yesturday (any higher and the chain rides the sensor when I’m in the little ring and small cog), but the discrepancy is still very big.

One thing that is intersting is that i ride a 48 cm TT bike with 650c wheels, so the chainstay length is VERY short, therefore there is a huge change in the “Elevation” of my chain when I go from one ring to another, which will kind of support the idea that chain-to-sensor distance is the underlying issue.

May next step would be to ride a computrainer, with my polar power meter and do a little experiment between the two rings and two power systems. I wish I had the time to do an analytical experiment for this issue…

[Former_User]

Ah, fascinating…

So I’m wondering why that would make difference in the actual power that it senses. I’m assuming the polar power meter works kind of like a guitar pick-up and senses the vibration frequency of the chain to calculate tension (i.e. higher the frequency, the higher the tension given the same chain weight and length = greater torque).

I can see how if the chain is farther from the sensor, (i.e. a 54 - 25 tooth combination) that the chain to sensor distance would be great and the Amplitude of the signal smaller, but the frequency that it senses should be the same, hence a relatively similar power no mater how far the chain is…

So I thought maybe this is the issue, so I put in an extra spacer just yesturday (any higher and the chain rides the sensor when I’m in the little ring and small cog), but the discrepancy is still very big.

One thing that is intersting is that i ride a 48 cm TT bike with 650c wheels, so the chainstay length is VERY short, therefore there is a huge change in the “Elevation” of my chain when I go from one ring to another, which will kind of support the idea that chain-to-sensor distance is the underlying issue.

May next step would be to ride a computrainer, with my polar power meter and do a little experiment between the two rings and two power systems. I wish I had the time to do an analytical experiment for this issue…

You’re close in understanding the problem…yes, even though the chain is further away, the frequency seen by the sensor from the chain vibration signal is the same, just weaker. However, since the signal is weaker, this allows the electronics to potentially “lock on” to other signals, such as the signal coming from the pins going past the sensor.

Here’s the thing, if you want consistent readings from your Polar, make sure that the distance from the top of the sensor at the “middle” mark to the chain is no greater than 30mm in your largest “big-big” chainring/cog combo, even if it means that the chain rubs on the top of the sensor when in the small ring and small cogs. You shouldn’t be using such a cross-chained setup anyway, especially with a bike with such a short chainstay. Personally, I prefer to set mine up so that the chain is never further than 25mm.

Also, make sure that the chain ALWAYS passes over an ~1-1/4" square area centered on the “middle” mark of the sensor. That’s where the “bass guitar pickup” lives inside the power module.

Lastly, one thing to remember about the Polar system is that it sometimes acts a bit flaky on trainers. This is a well know issue and may perhaps be related to some of the issues discussed above (i.e. signal-to-noise issues). So, checking it with a Computrainer may be “hit or miss”. I’ve checked my own setup on a friends Computrainer and it behaved reasonably well…but when doing a constant wheel speed check on a fluid trainer, it varied greatly.

Oh yeah…if you want your readings to be the most accurate, ALWAYS physically weigh your chain before installing it. DON’T use the “table values” and don’t even assume that a given chain model is consistent from year to year. I’ve measured a significant weight reduction in DA 9sp chains over the last 3-4 years.

Hope that helps.

[GregX]

i think the big power disparity might be in the measurement error of your meter. but you are on to something here that i discovered quite a while ago.

take a look at these photos. both q-rings are mounted in exactly the same position (and not “q-ring number”, but actual angular ring position relative to the cranks). the bike is also in the exactly the same position, the only thing that is changed is the gear combo selected, the 1st photo is 53 x 11, 2nd photo is 40 x 23. look what happens to the point of peak leverage:

[garytingley]

My suggestion is that for a forward positioned TT/bike, start with the outer Q-Ring OCP #3 setting for two weeks, then try the #4 setting and see how it feels and how your power reads. Each rider has different needs and preferences … one of the best features of Q-Rings is that the OCP system (multi-position bolt holes) are designed to suit your individual riding habits, body geometry, strength and position on the bike.

Setting 5 is an extreme setting, and we only advise this position be used when you constantly stand up directly over the bars on the chainring you wish to use that at setting. Setting 4 would likely be better for this style of aggressive standing riding, including those with a low spinscan and people whose seat is very far forward.

[QR_Sucks]

i’m at 5 and 5 and have absolutely no problem.