wattage differences: direct drive vs wheel drive trainers

(M MARKO) #1

It has occurred to me that direct drive trainers may be a big advantage in zwift, as they measure watts at the cassette/axle  whereas wheel-on trainers measure it at the tire. Thus the rolling resistance of the tire must be factored in. For the standard trainer tire this is probably 25-30 watts [less if using a higher end racing tire, but who would waste one on a trainer…].

So direct drive cyclists have this wattage advantage, no?

I would add to this, that wheel/ tire inertia must also be overcome by wheel-on trainers, for any acceleration/variation in speed. This requires extra effort, and is independent of the rolling resistance issue.

So it would seem that even with accurately calibrated trainers, there are significant differences related to  the design of the trainers that affect the power and performance that a given rider produces on zwift.


(Paul Allen) #2

What about crank based power meters or even pedal based power meters used for Zwift, are those unfair advantages also?

What is your solution to this so called issue?

(Nigel Doyle) #3

The rolling resistance of the tyre is factored in by the wheel on smart trainer when calculating power. If anything the wheel on smart trainers have an advantage because they are less accurate. Some smart trainers like the Taxc Vortex notoriously overstate the power being generated. People using these for Zwift racing are at an unfair advantage. The direct drive trainers are usually much more accurate if they have been calibrated. Some like the Neo need no calibration.

(Danny Boyd) #4

theres IS a big difference. the Bkool is really bad, becasue it uses a predetermined formula that you dont have any control of or calibrate. At 400W on Bkool, i have it measured on 320W with powermeter, and it gets worse the higher you go - with very little drop off when soft pedalling. 

I since changed to a Kickr, and calibrate every week. Its very accurate, and Im no longer able to ride easily in A races anymore.

I think a lot of trainers are bad, and its very easy to see who has a bad trainer looking at their power profile - whether they know it or not.

(C hris Strub) #5

It’s a good question.  Me, I like my Wahoo Snap.  I can’t remember what conti tire I am using, but it has a super stiff sidewall.  It has shown no sign of wear at all.  I am happy I was able to do something in the winter this year.  I feel stronger now before the season even starts outside.  I feel my avg watts line up with what I did at the end of summer.  Just a little higher now after using the Wahoo.  

Next years upgrade.  The floating wood thing to allow my to rock back and forth.  :slight_smile:

(Steve Copeland) #6

I found it the other way round when I went fro TacX Flux to a Neo.  My power actually dropped off.  I think its a lot more accurate now but I had to work hard to get the numbers back up.  When I try the wheel on trainer in the bike shop I still show bigger numbers.


(Danny Boyd) #7

exactly. the direct drives are far more accurate, and the wheel ons are off, and some of are way off.  the orig post is complete nonsense tbo.

(M MARKO) #8

the accuracy and  reproducibility of computrainer is very high, as good as any direct drive trainer and is calibrated each ride after warmup. I don’t know about other wheel on trainers accuracy, but it seems like they are less accurate than direct drive.

the original post raised the prospect of watt measurement being dependent on where on the drive chain the measurement occurs, with tire rolling resistance being the biggest wattage loss area. computrainer wattage is measured at the tire, so this is a consideration when comparing.

it seems that there significant differences in measured watts  between many trainer types being used on zwift, based on the comments to this query. 


(Shydn Crumph) #9

with the assumption that cyclists use their equipment as intended i don’t see any benefit of one trainer over another. that means wheel-on trainers already account for rolling resistance by using them with prescribed contact pressure or tire pressure.

accuracy of trainers is stated in the technical specifications. example: the wahoo snap is accurate to +/- 3%.

direct drive trainers or powermeters reach higher levels of accuracy, i.e. +/- 1-2%.

so the average technical advantage of one over the other is a matter of 1-2%. if you compare 2 trainers from the opposing end of the spectrum the difference can be 4-5%.

differences bigger than that can only be on account of the trainer not being used correctly or the product being marketed with false accuracy claims.

in my opinion the accuracy issue when zwifting is moot. if i’m not able or willing to spend a ton of money my trainer accuracy won’t be as high as possible. also, there are other ways to tweak displayed wattage, i.e. enter lower rider weight. the result being there will never be “fair” competition due to factors that aren’t being controlled.


(Joe Daknis) #10

“I would add to this, that wheel/ tire inertia must also be overcome by wheel-on trainers, for any acceleration/variation in speed. This requires extra effort, and is independent of the rolling resistance issue.”

Ignoring cassette weights because common to both types:

Given that a typical wheel that one might use on a trainer weighs around 1 kg, plus another 350 - 400g for tire and tube…

Then consider that a typical wheel-on trainer’s flywheel is, generally, smaller and lighter than the flywheels used on direct drive trainers, I don’t think your argument holds any water.

The weight of the flywheel on my old wheel-on trainer (cycleops) was < 3 lb. If you like, go ahead and add another 3 lb (1.4kg) for the weight of tire/wheel/tube above. That gets it to 6lb total.

The flywheel on an Elite Direto is 9.2 lb and a kickr is 12.5 lb. 

So… how’s that an “advantage” for direct drive, again?   






(Steve Copeland) #11

No fly wheel on my Neo!