After debuting at Interbike last fall, 4iiii’s crank arm based power meter has become smaller, using a new casing to give it a lower profile for better frame clearance.

They also gave it tabs for a rubber strap to hold it in place and apply continuous pressure while the bonding agent cures and permanently affixes the unit to your crank. That aids in self installs, but for now they’re doing factory installs where you send your crank arm in and have them apply it.

Why? Because they’re refining the epoxies to make it as simple as possible to insure you’re able to install it perfectly. In the meantime, they’re including the back and forth shipping of your crank in the $399 price.

It’s left only for now, right side will be ready in June. It’ll have a slightly different shape and plastic because it needs to be located behind the chainrings, which has a tighter clearance and needs to address battery accessibility.

While it may be a bit smaller now, those tabs on the unit for the straps aren’t exactly pretty. So, technically they’re not necessary once the unit is affixed and fully cured and could be shaved/grinded/sanded off without harming anything (except maybe the warranty).

The 4iiii started shipping about three weeks ago and they’re filling pre-orders now.


  1. Can someone tell me why it needs to be bonded on? I’ll take a stab at it ad say it needs absolute stability to be accurate, but I’d like to know more.

  2. @Mike They have to bond it to the crank arm so the straingauges can measure the “flex” of the crankarm. Kind of hard to do if they are not permanently attached to the arm.

  3. Durianrider i don’t think so. A P2M is 599 crank included now wit renown precision and reliability. I really see no bragain in making a 50$ economy at the risk of messing with an almost diy solution like this.

  4. I got my 4iii’s precision on friday and now have three rides on it. So far it has worked flawlessly. @399 its 200 less than the closest competitor. That and I get it on an Ultegra crankset instead of an lower end FSA crankset like the P2M.

  5. I fail to see how this device could accurately measure power at the crankarm. Wouldn’t the flex on any given crankarm vary due to material/design, resulting in differing measurements?

    That aside, I hope the first photo is showing only a prototype unit installed as the quality of the work is pretty terrible.

  6. Michael,
    The manufacturing precision of any major alloy crankarm is such that variations from unit to unit are insignificant with respect to mechanical properties. The material (Al alloy) is almost universal across the industry, with respect to mechanical properties. I am sure 4iiii did calibration testing to prove out their accuracy.
    The effect of crank arm length can be accommodated via software.
    The only real issue is precision in placement.

    This is simple strain gauge tech. 4iiii is finally giving it to people at a price more inline with the actual component prices (which are dirt cheap).

  7. @ Michael,

    Yes, as Alex said, there is a calibration step after the unit is bonded to the arm. The have a dummy spindle which is mounted to the crank arm in place of a pedal, and then a weight of known mass is hung off the spindle in a few different locations along its length. This allows them to determine the crank arm deflection characteristics under known loads, which can then be used to compute power. Apparently, it is still up in the air as to if this method will work on carbon cranks though, as they may not behave as consistently across the spectrum of foreseeable loads or extended time periods.

  8. @JF

    For all practical purposes, the install of the measurement pod to the crankarm is permanent. The pod is epoxy bonded to the aluminum crankarm, just like the Stages power meter used by Team Sky. The strain gauges, or associated electronics in the pod, would likely be damaged or destroyed if you tried to remove it. You might get lucky, but I sure wouldn’t plan on it.

    If you are using compatible cranksets (for example any Shimano Hollowtech II crankset), you could simply transfer the left crank arm to another bike. This is a very easy process.

  9. I hardly think this will force Quarq and P2M “shares” lower, especially when P2M already has a product that measures power from both legs for just a bit more than the price of the 4iii. Likewise, Quarq has dropped its prices. Not everyone is satisfied with power measured on only one leg.

    SRM, however, is going to see its market share drop if it doesn’t respond to all the new, lower priced power meters on the market and all the price reductions from established brands. As for SRM’s alleged better accuracy, well, it’s not that much better at all than P2M, Quarq, and PowerTap, all of whom offer significantly lower prices. Even Rotor is offering lower prices, although the jury is still out on Rotor PMs’ accuracy.

    Of course this is all good for the consumer.

  10. @Psi Squared… Last I checked, this is no different than P2M, Quarq, Stages (for a start)…all of these (like the 4iiii’s) have the strain gauge on one side…any L vs R is a calculation/estimate, not necessarily true power. This would be where Vector, Pioneer, Look,( to mention a few) have some advantages (for additional $$) ….You want true L/R…wait a few more months, it sounds like 4iiii will address that for less money than anyone else.

  11. Mick – P2M and Quarq are mounted on the Right, but measure between spider and crank. 100% of power is measured. You don’t get “Left” vs “Right” power, but you can get “12 to 6” vs “6 to 12” power, which is slightly different.

  12. @Mick: as Greg said, P2M, Quarq, and SRM are measuring strain in the spider which is created by each leg, so you’re getting power measurement from both sides.

What do you think?