Choosing the best bike power meter

You want a power meter on your bike.  You know that training with watts will bring your training and racing to the next level.  You may have heard that there can be issues with power pedals and that power cranks are better.  But how on earth are you supposed to choose which power cranks are right for you?

We believe Rotor offer fantastic power cranks, both in terms of the physical product and the data analysis, but wading through the many options can feel like opening a can of worms!!  So we worked with Coach Joe Beer to figure out the most important questions - and answers - to help you find the right option for you.

 

What is the difference between power cranksets and a power spider? 

Rotor has two power cranksets, the INPower and 2INPower and one power spider, the INSpider.

Comparison of Rotor Power Meters | 2INPower, INPower, INSpider

INPower and 2INPower cranksets

The power meter is integrated into the axle and crank arms of the crankset.  The chainrings then fit onto this unit in much the same way as they do onto a standard axle / crank set.  You can choose either a Direct Mount chainring or a spider with separate chainrings to fit onto the crankset.

The cranks are carefully splined into the axle to maximise the transference of power from the cranks, with gauges to collect high-precision power data.  The INPower measures power in just the left leg, whereas the 2INPower analyses both legs independently.

Pros:
  • Integrated unit in the axle, 
  • Analyse torque effectiveness and pedal smoothness
  • 2INPower offers independent left-right measurements
Cons:
  • Cannot be used with Shimano or SRAM cranks
  • Cannot easily be changed from one bike to another for training/racing

INSpider power meter

The INSpider is a power meter integrated into a direct mount spider, which slots straight onto a standard axle and crankset and then your choice of chainrings attach directly to the INSpider.  The INSpider can therefore be paired with your choice of axle, cranks and rings to create a fully-tailored solution, or to retro-fit onto your existing set-up.

The INSpider includes precision gauges, which cleverly collect combined data from both legs - not just the side with the spider & chainset.

Pros:
  • You can add an INspider power meter to an existing standard spider system – just change the spider out
  • Reasonably easy to move to a different bike, although you would probably want a mechanic to do it
  • Compatible with SRAM cranks and chainrings
  • Measures power on both legs
Cons:
  • Cannot analyse left-right leg power balance  

What is the difference between direct mount (DM) one-piece chainrings, and a spider system?  Which do I need? 

If you buy a 2INPower or INPower, you will need to choose which kind of chainring system you want.  Traditionally, chainrings are affixed to the axle by use of a separate 'spider'.  ‘Direct Mount’ chainrings means it’s a one-piece chainring integrating the mounting system.  Either option can be with a single chain ring ( 1x, pronounced ‘one-by’) or conventional 2-ring set up (2x, pronounced ‘two-by’).  

Rotor INPower bike power meter crankset with Direct Mount and spider chainrings

Direct Mount One-Piece

A one-piece Direct Mount means that the spider and chainrings are all moulded together in a single unit, which slots onto the power crankset.

Pros:
  • It is simpler, there are no bolts
  • It is lighter
  • Can be replaced as a whole unit when necessary – if you want different set-ups for different races, you can simply swap the unit out
Cons:
  • You have to choose one of the standard size combinations, but these are ideal for most riders, so not a problem
  • If one chainring of your 2x wears out, you need to replace the whole unit

Spider Chainrings

The spider slots onto the central spindle of the axle, and you then add elements to suit.  Chain rings can be added as singles or doubles to the spider. 

Pros:
  • It is flexible, so you can try out different elements, e.g. different chainrings for different races, Q ring and NoQ (round) ring options, or to try out different gearing combinations as you develop your riding or ride faster/hillier courses.
  • Spiders are available in 4 and 5 bolt options, so they fit many other types of chainrings.
Cons: 
  • There may be many component parts from other chainring set ups that don't fit the Rotor Spider spacing
  • It is slightly heavier

 

What is the difference between single and double chain rings, and which should I choose?

It is all about gearing ratios.  The more chain rings you have, the wider a range of gears you will have available.  But if you have a lot of gears you never use, you are adding weight and increasing the risk of slipping a chain from big to small and small to big chainrings.  Single front chain rings are known as a 1x (pronounced ‘One-By’), where double chain rings are 2x ('Two-by').    

Most cyclists and triathletes will want 2x, to work with the varied terrain, particularly as it is better to work with higher cadence on a smaller gear to get up hills without exhausting your legs for the run. 

Comparison of Rotor Direct Mount single 1x and double 2x chainrings for 2INPower & INPower power meters

However, if you have a wide enough ratio on your rear cassette, you don’t need a front double chain ring option.  With a 1x, you don’t need a front mech, which means saving a few watts for those looking for more easy speed, simpler components with less kit to go wrong, less danger of the chain slipping (1x rings have longer teeth, so you cannot add them to a 2x system) and a lighter set-up.  This is particularly popular on gravel, MTB, time trial and faster triathlon set ups for flatter courses. 

Choose a 2x if:
  • You are riding a ‘normal’ set up in ‘normal’ races. The 2x system is best for most triathlon scenarios.  
  • You are riding a hilly or technical course and will need a wider range of gears
Choose a 1x if:
  • You rarely use the small chainring at the front
  • If you will be riding a flatter course and therefore need less gear range. Usually this is just for Time Trialists

 

How do I work out which size chain ring I want?  What does 56T, 54T, etc mean?

56T, 54T etc literally means the number of teeth (“T”) on your chain ring.  It is normally printed on the side of the chainring, so you don’t need to count the teeth… and if you need to clean the grime off to see it, it’s time to clean your bike!!

Chainrings are normally described in their sets, so if you have a 39/53, it is referring to the two different sizes of chain ring your front chainset (i.e. the smaller has 39 teeth for easier gears and the larger has 53 teeth for faster courses or fast descents).  Standard road bikes all used to be 42/52, then most manufacturers moved to 39/53.

Rotor chain rings in a variety of sizes

39/53 would meet most needs for an average cyclist, so why change? 

  • Go bigger… The faster a cyclist is, the bigger gears they need, or their legs will be spinning wildly, so Time Trialists might use 56/58s as they usually ride fast on the flat.  Triathletes with a 1x system riding at faster speeds may use 54 or 55T.
  • Go smaller… If you are riding steep hills, or are a slow cyclist, you need smaller gears.  Someone with smaller gears may be able to cycle up a hill that they would have to walk if they had a standard range.  A slower cyclist might do better with 34/50, which would give them a smaller range of gears more optimised for their own range. 
  • Where do you ride? Big differences in terrain may also influence your choice – e.g. big descents may need bigger gears so a larger chainring would mean you can keep to the middle gears, reducing strain on the rear mech and chain. 

Remember, you need to work out your front chain ring options together with the rear cassette, as it is the combination of both which gives the range of gears.  If you have a really big spread of gears, you might need a longer reach rear mech, as the chain will need bigger adjustment range of sprockets it has to shift between.

 

How do I know whether I need a 110 BCD or 130 BCD chain ring? 

BCD stands for Bolt Circle Diameter.  If you draw an imaginary circle running through the centre of each chainring mounting hole, the BCD is the diameter of that circle in millimetres.  130 BCD is the old standard, 110 BCD is the new, for both Shimano and SRAM.   Click the image below to print a gauge out (print at A4 to be at the correct scale)Rotor Bolt Circle Diameter (BCD) gauge for bike chainrings

Campagnolo (aka Campag) uses a different system at around 112BCD.  Campagnolo is rarely used by triathletes, but if you do, click here for more details about the Campagnolo system.

 

Do Q rings really work?  When would I want to consider them?

Q rings are oval-shaped, which increases the circumference of the chain ring through the pushing down phase of the pedal cycle.  They are designed to optimise the force applied to the pedal by helping the downward moving leg push a bigger gear, but also reducing the gearing size as the recovering (upward moving leg) approaches the over-the-top phase of the leg circumference (aka Top Dead Centre).

What is the difference between Q-Rings and NoQ Round Rings for a bike chainset

Pros:
  • Q-rings can make it easier if you are on the cusp changing to a bigger/smaller gear – they can avoid the need to change gear then have to change back on seconds later.
  • This makes them particularly advantageous on a course that have steep climbs and or rolling terrain. 
  • You can use the OCP power data to fine-tune the setting of the Q-ring to obtain the maximum possible power from your own personal pedal stroke
Cons:
  • You have to learn how to shift gear, as you have to shift at the right part of the pedal revolution – if you don’t, you can ruin the gear change. So if you struggle to master the gear change it probably isn’t for you – although they might be of most benefit to those people!
 

How do I know if I need a 4 bolt or 5 bolt spider? 

The 5-bolt is the old standard.  Shimano has in recent times moved to a 4-bolt system, so 4-bolt ROTOR spiders can be used with Shimano chainsets. 

Do I need a 4 bolt spider or 5 bolt spider on my Rotor power meter power crank?

There is no advantage to a 4 or 5 bolt set up and it will make no difference to the function, so if you are buying parts for a new chainset, you can choose either.  As chain rings have to be changed quite regularly anyway, there is little advantage to keeping your old chain rings and getting a spider to fit.  However, many people tend to stick with what is already on their bike.  It doesn’t really matter! 

 

What length crank arms do I need? 

This is based on your height and leg length.  For example, a female of 5’4” would probably be on 165mm cranks, a 6ft2” man would use 175mm.  170/172.5 suits the middle range of people. 

Which length crank arms do I need for my Ventum NS1 road bike?

It is crucial to get the right crank length, as having the wrong length of crank changes the biomechanics of your legs.  If cranks are too long, knees lift too high on the upper rotation and legs stretch down much further to go round the bottom of the pedal stroke (aka Bottom Dead Centre).  5-7.5mm might not seem like a huge difference, but if you add that to the full circumference of the feet, it makes a much bigger difference (circa 47mm longer).  

Some people say that shorter cranks are better for triathletes because of some biomechanical advantage for the run.  This is not true - it is a myth – so get the right crank length for your height and pace your bike leg perfectly to get the right run legs as you leave T2.

 

What is the difference between 24mm and 30mm axle and which should I choose?  

This completely depends on the bike manufacturer.  There is a hole through the lower section of the bike frame, into which you insert a ‘Bottom Bracket’ (aka BB).  The axle and supporting bearings then fit into the bottom bracket.  Some bikes have such a small BB hole that you have to use a 24mm option.  If you have a bike with a larger bottom bracket that fits either, then there is no advantage or disadvantage between 24mm or 30mm axle.

Ventum NS1 frameset | What size bottom bracket?

The axle has a crankarm at either end, the right side also including the chainrings. 

Your bike BB construction will dictate whether you need a press-fit or screw-fit bottom bracket.  If you are choosing a new bike and so can choose the bottom bracket, the screw fit is much better:  brands such as Ventum include screw-in BB's.

ALDHU or VEGAST cranks?

ALDHU is Rotor's top end crank set.  The name comes from an abbreviation of Alpe d'Huez, the iconic climb in the Tour de France.  It is lighter weight (347.8g cranks only), made of 7055 Aluminium, and for that you pay a higher price.

VEGAST is the standard Rotor crank.  Still a great, precision engineered piece of kit, but made with forged aluminium which weights a little heavier at 383.8g (cranks only).  

Both are compatible with the full Rotor modular system, with engineered splines to optimise the transfer of power.

Rotor bike cranks | ALDHU vs VEGAST | which is best?

Choose VEGAST if you want an excellent crank that will see you through most scenarios.

Choose ALDHU if those extra few grams of weight are worth the price.  This isn't just for faster riders aiming for the podium - slower riders who struggle to push up hills will also be thankful for the lighter bike! 

Choosing chainring options for Rotor Power meters

To choose your ideal combination of chain ring to go with your power meter, ask the following questions:

  1. What events do you do?
  • If you race to complete and are not bothered about shaving every second off your bike split, you don’t need any aero components (e.g. aero crown for the INspider, or aero chainrings). 
  • If you are riding very flat courses, you may want a 1x system if you would rather have less incidents of changing gears between small and big chainrings.
  1. Do you struggle with climbing?
  • If you are racing hilly courses, whether to get around in one piece or to race your fastest, the weight of the bike is a big factor (e.g. Ironman Wales, Ironman Bolton or 70.3 Lanzarote).  Choose the direct mount chainrings as these are the lightest.  You may also want smaller chain rings to save your legs from grinding low cadences in a climbing gear that is too big.  NOTE: every part of your bike that you can save weight all mounts up and makes climbing easier.
  1. What are your priorities. What do you want to get from it? 
  • Don’t get a direct mount 2x chainring system if you do events with big varieties of terrain. It is worth changing your chainrings to get different gearing combinations for hilly vs. flat courses and a DM Spider is easier to change at home (saving time and mechanics fees).

 Which components do I need for a bike power meter?

Want more help?

Neuff Red has a team of people who can happily talk through your requirements with you, including UK Multisports coach Joe Beer and the mechanics at The Crankshop.  Send an email to sales@neuff-red.co.uk with the answers to these questions:

  1. What events are you doing?
  2. What type of rider are you? Do you go fast downhills / struggle up hill, etc?
  3. What bike do you have? Current set up – how many teeth on the chain ring and cassette?
  4. What height and weight are you and what's your inside leg length? (To the floor not to the ankle bone  - Joe is cycle coach not a tailor.)
  5. How good are you with mechanics / tech? Will you optimise between races, or do you want a single set up?

Send us your answers and we’ll get you answers!

 

Click here to see our range of Rotor Power Meters