Which PM For My MTB?

perryr said:

I have a SRM and I have a PowerTap on my MTB.
Other SRM cons - extremely limited crank selection unless you count trying to use one of the compact cranks/road cranks on your MTB. Odd or large tube frames sometimes have issues installing SRM's. SRM stops reporting data when your RPM's go below 40 or so for an extended period - may not be an issue with your riding style/strength/usual riding terrain. If you live an area where your ride can have wide temperature ranges the SRM offset can vary a signficant amount affecting accuracy for some part of your ride data. Also, the SRM computer mount only likes to go right on the front of your handlebars - I've hit quite a few trees/bushes with that part of my handlebar over the years.

Woofer said:

I have a SRM and I have a PowerTap on my MTB.
Other SRM cons - extremely limited crank selection unless you count trying to use one of the compact cranks/road cranks on your MTB. Odd or large tube frames sometimes have issues installing SRM's. SRM stops reporting data when your RPM's go below 40 or so for an extended period - may not be an issue with your riding style/strength/usual riding terrain. If you live an area where your ride can have wide temperature ranges the SRM offset can vary a signficant amount affecting accuracy for some part of your ride data. Also, the SRM computer mount only likes to go right on the front of your handlebars - I've hit quite a few trees/bushes with that part of my handlebar over the years.

You can also rig your PC5 into a padded, armored saddle bag and mount it to the seatpost. I tried when I was thinking about racing cx.

J

perryr said:

Ergomo, this looks good depending on cranksets available. Not sure of MTB durability. Would I have much hassle dumping data in Cycling peaks and mixing with my PT data?.

The Ergomo has received excellent reviews in regards to durability. Hunter Allen wrote an article about his experience with the device in some incredibly rough MTB stage race in South America - forgot where you can download the article.
Create a new users for Cyclingpeaks if you don't want to mix the data or just select a different workout category (run, swim, etc.) for those rides that you do with the MTB.

TamMan said:

Create a new users for Cyclingpeaks if you don't want to mix the data or just select a different workout category (run, swim, etc.) for those rides that you do with the MTB.

I would like to dump my ride data into cycling peaks under the same user as my Powertap. And have the TSS etc.. calculate and accumulate just like it does with my Powertap. I would like the data from both units to combine and compute accurately to measure training from the same set of legs, just different PM's. I am not so software savey, just wondering if it would be much hassle working with 2 different PM's, frequently alternating from the MTB to the road bike. Would it look pretty seamless?

Thanx for the info, the Ergomo is looking good.

perryr said:

I am not so software savey, just wondering if it would be much hassle working with 2 different PM's, frequently alternating from the MTB to the road bike. Would it look pretty seamless?

That's no problem. I import data from my Ergomo and my Polar. Using a different sport category would just help you separate the two different bikes, i.e. say you wanted to run a report on power values generated by the road bike vs. the MTB. Good luck!

Sorry to brimg this up but the Crank and BB PMs should read higher than the rear wheel PM as it is upstream of the drive train. I believe some modification can be done to lower the Crank and BB units, so you get apples to apples.

TamMan said:

That's no problem. I import data from my Ergomo and my Polar. Using a different sport category would just help you separate the two different bikes, i.e. say you wanted to run a report on power values generated by the road bike vs. the MTB. Good luck!

TamMan said:

The Ergomo has received excellent reviews in regards to durability. Hunter Allen wrote an article about his experience with the device in some incredibly rough MTB stage race in South America.

The Ergomo article can be found here.

It makes a good read for anyone considering an Ergomo.

I've just got one and am waiting for my frame to arrive to use it!

Woofer said:

I have a SRM and I have a PowerTap on my MTB.
Other SRM cons - extremely limited crank selection unless you count trying to use one of the compact cranks/road cranks on your MTB. Odd or large tube frames sometimes have issues installing SRM's. SRM stops reporting data when your RPM's go below 40 or so for an extended period - may not be an issue with your riding style/strength/usual riding terrain. If you live an area where your ride can have wide temperature ranges the SRM offset can vary a signficant amount affecting accuracy for some part of your ride data. Also, the SRM computer mount only likes to go right on the front of your handlebars - I've hit quite a few trees/bushes with that part of my handlebar over the years.

I ride with an SRM on my mountain bike and road bike, and I disagree with most of the cons stated here.

You can get a wide array of chainring sizes on the SRM mountain bike crank. I run mine without the small ring and a 44/32.

You can install the sensor on just about any frame - just take a bit of work to get it just right. The extra magnet with the mountain bike sensor kit makes it work very well and it is sealed to keep out the water.

SRM does not stop reporting data when your cadence drops unless you have it set to do so. You can also set it to only stop recording if SPEED goes to zero for a while, so that it doesn't shut off when you are coasting.

It takes a very large temperature fluctuation to throw off your SRM readings. Even a 20 degree change will probably only throw you off by 1-2%, but if you experience that type of change you can easily reset your zero-offset during your ride. I reset mine a few times each ride.

The mount places the computer in a great place as far as visibility and accessibility - but if you are prone to run head on into trees it is indeed vulnerable. And those are pretty expensive to replace - several hundred bucks I believe.

The biggest limitation to the SRM is that it really doesn't work well on a 73mm bottom bracket shell. It is made for a 68mm shell and if you use a bike with a 73mm you will have to space out the crank arms by several mm. This can lead to very squeaky cranks and poor front shifting. Friction front shifting seems to work much better for this than index. When using a bike with a 68mm shell and the recommended BB (Ultegra Octalink I think it is), the shifting performs quite well.

I compile data from two SRM's into my cycling peaks interface, and assume that the likely difference of 1-2% due to accuracy of the power meters is negligible. I would take the same approach to mixing power tap and SRM data.