Broken 3 crank arm power meters.... What next?

imafatman

Broke two stages left side power meters. Broke a 4iii left side power meter.

I'm convinced that I'm simply too much of a beast for these puny meters. Clearly the strain gauges were ripped to shreds from my sheer power because they all broke after a sprint effort and then started reporting strange numbers and wouldn't calibrate.

So what else is out there that might be more reliable? For what it's worth I'm now 120kg and I have large legs and I like sprinting.

I have a Ultegra chainset currently and would like to find something I can fit reasonably easily. I don't mind spending but just want a reliable power meter.

joey54321

Are they under warranty? A power meter design for cycling should stop working because of cycling. That is a fault of the manufacturer and you should be able to get it replaced.

imafatman

joey54321 wrote:

Are they under warranty? A power meter design for cycling should stop working because of cycling. That is a fault of the manufacturer and you should be able to get it replaced.

Refunds/replacements are not a problem, my supplier has been accommodating. I want to know if there is a better power meter option which will not be susceptible to the same failures.

Both the stages + 4iiii are identical in the way they are manufactured. Strain gauges are glued onto the left side arm. Both broke after a hard sprint session. Presumably I'm flexing the arms too much due to my weight and the strain gauges are breaking.

The first stages that failed is perhaps a coincidence. The second in the same way is starting to look suspicious. The 3rd one from a different company also failed so it's starting to look like it's my fault.

cougie

Powertap wheel ?

StillGoing

What watts are you putting out to break a power meter when these things are tested to safely hit the watts put out by top cyclists?

imafatman

philthy3 wrote:

What watts are you putting out to break a power meter when these things are tested to safely hit the watts put out by top cyclists?

Last time I tested myself on a watt-bike I was getting about 1600w peak watts. It's not the wattage that is the problem but rather the torque. As a heavier person I will put out much more torque compared to an elite cyclist who is spinning out to 140rpm or more and only weighs like 70kg.

fenix

What are you using the power meters for ? Just recording your max wattage ?

imafatman

Fenix wrote:

What are you using the power meters for ? Just recording your max wattage ?

Heh. I'll ignore the condescending assumption but I actually get a lot of value from having power on my bike.

I use it for structured training to improve my fitness and performance, making excellent progress in improving my FTP. It's also very useful for pacing myself on hard efforts. It's also good to know how many calories I've burned off which takes the guesswork out of my nutrition.

So yes I'd quite like to continue training with power. Right now I'm stuck to using my indoor trainer which is great over winter but it's now perfect riding weather.

sloppyschleckonds

imafatman wrote:

So what else is out there that might be more reliable?

Quarq, P2M & Powertap hubs all proven dependable reliable and accurate PM's.

imafatman

SloppySchleckonds wrote:

imafatman wrote:

So what else is out there that might be more reliable?

Quarq, P2M & Powertap hubs all proven dependable reliable and accurate PM's.

Thanks for the suggestions - hubs are out because I have a couple of wheel sets but the Quark + P2M look very good, unfortunately my supplier doesn't do the P2M so I can't use their store credit but looks like I can pick up a Quark.

Looks like I just have to change my bottom bracket but otherwise a plug and play option with my Ultegra 6800 chainrings.

Cheers will investigate Quark as a serious option.

supermurph09

imafatman wrote:

Fenix wrote:

What are you using the power meters for ? Just recording your max wattage ?

Heh. I'll ignore the condescending assumption but I actually get a lot of value from having power on my bike.

I use it for structured training to improve my fitness and performance, making excellent progress in improving my FTP. It's also very useful for pacing myself on hard efforts. It's also good to know how many calories I've burned off which takes the guesswork out of my nutrition.

So yes I'd quite like to continue training with power. Right now I'm stuck to using my indoor trainer which is great over winter but it's now perfect riding weather.

Have you lost weight to get to 120KG?

joey54321

imafatman wrote:

philthy3 wrote:

What watts are you putting out to break a power meter when these things are tested to safely hit the watts put out by top cyclists?

Last time I tested myself on a watt-bike I was getting about 1600w peak watts. It's not the wattage that is the problem but rather the torque. As a heavier person I will put out much more torque compared to an elite cyclist who is spinning out to 140rpm or more and only weighs like 70kg.

Weight has nothing to do with torque. 1600w is still pretty low compared to the big guys like Kittel, Gripel etc...

They may spin at a higher cadence though. Have you tried that? It might result in more power.

Anyway, I put out a measley 1400w or so on my Quarq every now and again (I don't really train my sprint) and it has survived for several years.

imafatman

Supermurph09 wrote:

Have you lost weight to get to 120KG?

Yes I was at one point nearly 160kg in my life but through cycling, diet and a lot of gym work I have become much fitter and stronger. My goal is to be 90kg and be a strong A group rider. I can just about hang on if it's flat and definitely give them a run in a sprint but the weight is holding me back.

I also want to give track cycling a go because I really enjoy sprinting but frankly it would be laughable at this stage to turn up to the velodrome.

supermurph09

imafatman wrote:

Supermurph09 wrote:

Have you lost weight to get to 120KG?

Yes I was at one point nearly 160kg in my life but through cycling, diet and a lot of gym work I have become much fitter and stronger. My goal is to be 90kg and be a strong A group rider. I can just about hang on if it's flat and definitely give them a run in a sprint but the weight is holding me back.

I also want to give track cycling a go because I really enjoy sprinting but frankly it would be laughable at this stage to turn up to the velodrome.

Well done, that's an amazing effort.

imafatman

joey54321 wrote:

Weight has nothing to do with torque.

It does. Rider weight has a direct impact on the amount of torque that can be transmitted through the pedals.

If you think about two riders standing on the pedals, one 50kg and another 100kg. There will be exactly twice as much force on the pedals for the heavier rider. When cycling in the seated position each rider is still subject to the same physics.

Power is work/time. 1500w at 50rpm is twice as much torque compared to 1500w at 100rpm, because for each revolution of the pedal you have to do twice as much work to make the same power.

So whilst I'm not a world class cyclist, I am heavy. So at the very least when out of the saddle I will be generating more torque which means my crank arms will be flexing that much more.

joey54321 wrote:

Anyway, I put out a measley 1400w or so on my Quarq every now and again (I don't really train my sprint) and it has survived for several years.

Thanks for the update on Quark, I can see the DFour would be a plug and play replacement for me Ultegra....Think I'm gonna pick one of those up and give it a try.

joey54321

imafatman wrote:

joey54321 wrote:

Weight has nothing to do with torque.

It does. Rider weight has a direct impact on the amount of torque that can be transmitted through the pedals.

If you think about two riders standing on the pedals, one 50kg and another 100kg. There will be exactly twice as much force on the pedals for the heavier rider. When cycling in the seated position each rider is still subject to the same physics.

Power is work/time. 1500w at 50rpm is twice as much torque compared to 1500w at 100rpm, because for each revolution of the pedal you have to do twice as much work to make the same power.

So whilst I'm not a world class cyclist, I am heavy. So at the very least when out of the saddle I will be generating more torque which means my crank arms will be flexing that much more.

But you aren't just 'standing on the pedals'. If the pedals aren't moving I can see how extra weight = extra torque.

But if you are 50kg rider pushing down @1500w at 100 rpm, that is the same torque as a 100kg rider pushing down @1500w at 100 rpm, surely.

imafatman

joey54321 wrote:

But you aren't just 'standing on the pedals'. If the pedals aren't moving I can see how extra weight = extra torque.

Absolutely but the point was to highlight the correlation between weight and torque. The same physics apply whilst in the saddle. The heavier rider is technically able to exert more force, assuming they have the legs for it.

joey54321 wrote:

But if you are 50kg rider pushing down @1500w at 100 rpm, that is the same torque as a 100kg rider pushing down @1500w at 100 rpm, surely.

Correct, if they are both spinning at 100rpm and generating 1500w then the torque must be exactly the same.

joey54321

So your issue isn't really the power you're putting out, its that you do it at a very low cadence. So, as I suggested above, have you tried increasing your cadence? Sprinting at 50rpm is likely to be less efficient and generate less power than 100+ rpm.

imafatman

I was just using examples, I do get into the 150-160s when I'm going all out.

I could probably adjust my riding style and be careful about low cadence power but it just feels natural to be able to pull away from a low gear and then keep going. And also getting out of the saddle you always have to drop a few gears and stay reasonably low RPM.

jgsi

I'd certainly be drifting away from Shimano HT cranksets. Glueing on a LH power meter module though is not the issue.
Maybe think of going shorter if possible to 170, even 165 cranks?
Someone's DA crank broke recently at the pinch bolts and posted on here.
I use a Zayante alloy M30 crankset with 4iiii, not much worry of me breaking that crank

redvision

imafatman wrote:

I do get into the 150-160s when I'm going all out.

You hit 160rpm??
How the hell do you do that?
Even pro track sprinters struggle to hit a cadence of 160.

on-yer-bike

Ive seen quite a bit of coverage on Ultegra cranks that have broken. So I wonder if Ultegra is a stiff enough crankset for you and the flexing has damaged the power meter. Maybe spend more on a stiffer crankset?

StillGoing

In my mind, the first thing that should be breaking is the chain as the weakest link (see what I did there). If Shimano cranks are breaking routinely, I'd be questioning the fit for distribution.

cougie

160 rpm is faster than Hoy..

imafatman

JGSI wrote:

I'd certainly be drifting away from Shimano HT cranksets. Glueing on a LH power meter module though is not the issue.
Maybe think of going shorter if possible to 170, even 165 cranks?
Someone's DA crank broke recently at the pinch bolts and posted on here.
I use a Zayante alloy M30 crankset with 4iiii, not much worry of me breaking that crank

Thanks that's an interesting suggestion I have not thought about smaller cranks or trying different ones. By the sounds of it the Zayante's are very stiff so that looks like it could be a good option but I wonder if being crank arm based I might just have the same problem.

The Quarq DZero which I thought would be an easy option has carbon cranks and I'm not sure that's a good idea. If carbon frames and wheels are not suitable for heavier riders then I don't think cranks are either.

cougie wrote:

160 rpm is faster than Hoy..

Well I"m certainly not Chris Hoy..... I'm just reading my cadence figures from my Garmin. Maybe it's not accurate? I'm not really sure what is an unreasonable number to achieve.

ugo.santalucia

Seen a broken Dura Ace one on Facebook the other day... with included details of the injury sustained in the process. I have to say it looks like a fairly flimsy design. The need to incorporate the strain gauge element and the electronics inside the crank arm, means the structural integrity of the unit is severely compromised.
I personally wouldn't buy one... it seems to me a hub power tap or a RSM crankset are more robust alternatives... although probably pricier

philbar72

ugo.santalucia wrote:

Seen a broken Dura Ace one on Facebook the other day... with included details of the injury sustained in the process. I have to say it looks like a fairly flimsy design. The need to incorporate the strain gauge element and the electronics inside the crank arm, means the structural integrity of the unit is severely compromised.
I personally wouldn't buy one... it seems to me a hub power tap or a RSM crankset are more robust alternatives... although probably pricier

that'd be mine. I have to agree, though to be fair to Pioneer they have been exemplary. has to be said I'm probably going to go with something that's solid from now on rather than the dura ace make up. its fundamentally flimsy and also whilst I know I generate a lot of torque, I'm nowhere near elite level in terms of power or torque numbers (peak of 1546 watts and Ftp of 324). the thing literally snapped off, and the mounting bolts ( all 3 (2 pinch bolts and the screw in cap)) were completely unnaffected. I checked their torque settings when I got back and they were all fine... so the outer was completely ripped off.

imafatman

Wow hope you are ok chap. Ever since I heard of these Ultegra/DA cranks snapping I've been worried about it happening to me. Last time I snapped a chain I was in traffic and could have very nearly become a cropper, fortunately it was in Central London and the speeds were slow..

Problem is that it's very difficult to know what to get..... The verve infocrank looks interesting, could be a good option or it could be just as weak.

The cycling world seems to focus on how stiff a crank for better feeling and power delivery but that doesn't tell you the tensile strength. You could have a very stiff crank with a low tensile strength which would just give way. Or a much less stiff crank with a much higher tensile strength.

ugo.santalucia

imafatman wrote:

The cycling world seems to focus on how stiff a crank for better feeling and power delivery but that doesn't tell you the tensile strength. You could have a very stiff crank with a low tensile strength which would just give way. Or a much less stiff crank with a much higher tensile strength.

it's not a matter of tensile strength either... ALL these fractures are down to metal fatigue, which in some way is related to tensile strength, but also not

svetty

JGSI wrote:

I'd certainly be drifting away from Shimano HT cranksets. Glueing on a LH power meter module though is not the issue.
Maybe think of going shorter if possible to 170, even 165 cranks?
Someone's DA crank broke recently at the pinch bolts and posted on here.
I use a Zayante alloy M30 crankset with 4iiii, not much worry of me breaking that crank

Crank length should relate to his height and hip flexibility. Assuming he is a reasonable height I don't think 165mm cranks is a great idea....

You have sound data suggesting that Shimano HT cranks are inherently less reliable than other brands? :roll:

philbar72

Svetty wrote:

You have sound data suggesting that Shimano HT cranks are inherently less reliable than other brands? :roll:

not sure anyone does. but having had what happened to me, its made me think I'd rather go for something that doesn't cause me to get multiple scars and a cracked elbow, simply because I pedalled a little bit hard...